physiography, foraminifera and sedimentation in the dovey estuary (wales)

PHYSIOGRAPHY, FORAMINIFERA AND SEDIMENTATION IN THE DOVEY ESTUARY (WALES)

JOHN HAYNJ3 AND MAX DOBSON

A study of the chemistry, circulation and sedimentation is followed by an outline account of the living and dead communities (particularly foraminifera) characteristic of the main physiographic subenvironments. On the basis of the relationships established these results are used in an interpretation of the vertical sequence.

Une etude de la chimie, circulation et skdirnentation est suivie d'un compte-rendu sur les communautks vivants et kteintes (en particulier les foraminif6res) caracteristiques des principaux subenvironments physiographiques. Sur la base des relations ktablies, ces rksultats sont utilists dans une interpretation de la sequence verticale.

Eingangs wird die Chemie, Swomung und Sedimentation behandelt ; es folgt eine Beschreibung der lebenden und toten Gemeinschaften (byonden die der Foraminiferen), die charakteristisch fir die wichtigsten, geomorphologischen Tellmilieus sind. Die festgestellten Verhiiltnisse bilden die Grundiage fiir die Ergebnisse, auf die sich eine Dantellung in vertikaler Folge stiitzt.

1 . INTRODUCTION (a) Scope of paper

Although estuaries and salt marshes have received considerable attention the relationship between the complex physical variables in this environment and the fauna, especially microfauna, remains obscure. Indeed, according to one author (Phleger 1965) the evidence so far collected is insufficient to establish whether causal relationships between physical factors and faunal distribution actually exist. The Dovey estuary offers an ideal opportunity for testing possible relationships because it is unpolluted and shows well marked but restricted living communities. Also, it is known (Adams and Haynes 1965) that the variations in foraminifera1 faunas through the latest deposits apparently reflect the physiographic changes associated with cyclic development of the marshes. For these reasons a detailed physiographic study has been made. The results, which emphasize the importance of salinity and pH, are of general importance because the estuary is a classic area in the study of marsh development and associated sedimentation (Chapman 1960). No information on these factors or currents was published prior to the present study, so this work fills an important gap in knowledge. Although some of the details may be unique the basic pattern revealed may serve for comparison with other shallow, well mixed estuaries in the temperate zone. Salinity is shown to be a factor intimately connected with the pattern of sedimentation. The characteristic features of estuaries can therefore only be understood when salinity variations and tidal currents are taken into account, together with sedimentation and the ecology of marsh communities. An attempt is made to distinguish the main physiographic zones and the results are used in an interpretation of the sequence of sediments penetrated in two boreholes. A detailed account of foraminifera distribution will be given in another paper.

Geol. J. Vol. 6, Pt. 2,1969 217

218 J. HAYNES AND M. DOBSON

POSITION OF SEDIMENT SAlVpLa

M L W S M O M LOW Walw Sping Tad= MH W S M u n Hipn Water Spr8C-a Tide I - x M r s h Trzmect

Fig. 1. Location map showing position of samples in the Dovey estuary. Land above Mean High Water Springs shaded. Artificial embankments hachured.

' @) General physiography, tidal characteristics and bed morphology

Most British estuaries display a broadly similar physical form, possessing an outer part consisting of a succession of banks and channels shaped by tidal currents and an inner part dominated by a main channel meandering in a loose bed and associated with a series of short, blind channels. These short, blind channels are termed flood barbs and are equivalent to the flood channels of an outer estuary (Robinson 1960). The Dovey estuary on the west coast of Wales, Lat. 52" 33' N, Long. 4" 00' W (see Fig. I), is 5 miles (8 km) long, 1+ miles (2.4 km) wide near Aberdovey and tapers to less than 1 mile (1.6 km) at Frongoch before passing into a tidal river above Gogarth. A railway embankment restricts the estuary to the south and the seaward entrance has an effective width of only half a mile due to northward growth of the Ynyslas spit. The entire unshaded portion of Fig. 1 is covered at high spring tide, whilst at low water only the main channel is covered with a maximum depth of 2 fathom (3 -7 m) near Aberdovey Jetty (PI. 17). The estuary as it exists today is almost totally infilled and large areas have been reclaimed; it therefore represents a late stage in the accretionary cycle. The estuary discharges directly onto an open coast where long- shore drift inhibits the evolution of banks and channels. The estuary mouth thus shows a restricted flood-ebb circulation pattern.

The pronounced landward shallowing of Cardigan Bay produces a strongly asymmetric tidal wave with the mean spring rise being achieved at Aberdovey in only 5+ hours. Extreme spring tides at Aberdovey reach 16 ft (4-88 m) [calculated from standard port, Holyhead, chart datum, -8 ft (-2.44 m) O.D.] and the difference between extreme springs and neaps is about 7 ft (2-13 m). The mean spring rise is

PLATE 17

Aerial photo of the Dovey Estuary at low tide showing main channel and attendant flood'barbs. Scale: 1'65 x Fig. 1. Channels as in October 1962 when sample collection was started. Aberdovey jetty is clearly visible, as is also the limit of Sptrrtiria growth and the scarp between the high and low marsh, approx. M.H.W.S. Note also the form of the shoals in the estuary mouth, reflecting the dominance of incoming currents on the south side and outgoing currents on the north.

Geol. Journal, Vol. 6 PLATE 17

PHYSIOGRAPHY, FORAMINIFERA A N D SEDIMENTATION, WALES 219

TABLE 1 : MEAN MONTHLY A N D SEASONAL FLOWS AT THE DOVEY GAUGING STATION, 1966, WITH MEAN MONTHLY RAINFALL FIGURES FOR ABERDOVEY.

Monrh

January

February

March

April

May

June

July

August

September

October

November

December

Rainfall in inches

1.41

4-51

2.33

3.62

2 -97

4.92

3.09

2 .oo

1 -95

5-51

3 -22

6.51

(35.6)

(114.3)

(58 -3)

(9 1 -4)

(75 .O)

(124.8)

(78.1)

(50.8)

(49 '9)

( 1 39 '7)

(8 I -3)

( I 65 '1)

Cusees

554

1,439

63 5

896

509

769

333

450

482

594

917

3,372

(56,100)

(143,000)

(64,200)

(90,700)

(51,800)

(78,500)

(33,600)

(45,900)

(48,900)

(60,500)

(93,400)

(345,000)

Season

Winter 876 cusecs (88,700 m3/hr)

Spring 724 cusecs (73,900 ms/hr)

Summer 422 cusecs (42,800 m3/hr)

Autumn 1'628 cusecs (163,000 m3/hr)

14 ft 9 in. (4-49 m) and the mean neap rise 11 ft 3 in. (3.42 m). This gives an average spring tide input of about 50,000 cusecs (5,100,000 m3/hr) and an average neap tide input of 10,000 cusecs (1,019,000 m3/hr). By contrast the average fresh water flow at Dovey Bridge (upstream from Glandovey) is about 800 cusecs (81,600 m3/hr). (Cusecs = cubic feet per second; m3/hr = cubic metres per hour.) Tidal domination of the estuary is therefore pronounced. The input of tidal water has been computed from the variable cross sectional geometry and velocities observed at Aberdovey Jetty (Station 29) through a neap flood tide, 10 ft (3.05 m) and a spring flood tide, 15 ft (4.57 m). Figures for Dovey river discharge (upland flow) were supplied by the Gwynedd River Board (Table 1) who maintain a gauge at Dovey Bridge, Machynlleth, upstream from Glandovey, off map, Fig. 1.

Current measurements made 2 hours before high water at selected stations in the estuary mouth show that a positive linear relationship exists between the log of the velocity and the tide height. This means that there are greater differences in salinity and velocity between 13 ft (3.96 m) and 15 ft (4.57 m) springs than between lo f t (3.05 m) and 13 ft (3.96 m) neap to ordinary tides. Salinity generalisations based on mean springs are therefore inadequate and must take extreme conditions into account.

The estuary faces west and the main channel tends to the north bank, as might be expected in an estuary of this type with the development of anticlockwise circulation (Cameron and Pritchard 1963), but the effect of the northward moving littoral drift may be more significant than the effects of the Coriolis force. It is broad, shallow and characteristically sinuous and no significant increase in width occurs towards the sea. Short side channels branch off the main channel in the upstream direction. These blind channels or flood barbs are deep at their seaward ends and shoal landwards.


They result from the tendency of flood waters to evade the ebb flow down the main channel. The development of anticlockwise circulation is well seen at low water in the estuary mouth where the flood begins on the south side while a seaward flow continues at Aberdovey Jetty on the north side.

Five physiographic subenvironments are recognised. The salt marshes constitute the first two subenvironments and were the subject of the early ecological enquiry of Yapp, Johns and Jones (1916; 1917) whilst the marsh sediment accumulation rates were investigated by Richards (1934). The marshes are mainly developed on the south side of the estuary and extend from the point reached by extreme spring tides, approximately the line of the Cambrian Railway, down to the limit of Spartina growth (Figs 1, 14 and Pls 18-20). The other subenvironments comprise the main channel and the marsh channels, well shown on PI. 17, and the open sand flats which cover the area from the limit of Spartina growth down to the main channel (Fig. 1 and P1. 18a). The freshwater fens of Borth Bog, not dealt with here, are mainly developed south of the Cambrian Railway line.

2. WATER CHEMISTRY AND CIRCULATION

(a) Salinity of the coastal water - Inner Cardigan Bay

Published data on salinities in Cardigan Bay appear to be restricted to those of Lee (1960) which show that values fell from 34.5%, in St. Georges Channel to less than 34%, in the eastern part of Cardigan Bay in March 1953. Measurements were not taken within the 10 fathom (18.3 m) line. To remedy this deficiency surface samples have been taken by bucket and full profiles by Knudsen bottle out to the 10 fathom (18.3 m) line off the Dovey Estuary during October, November and December 1962; April, July and October 1963 and February 1964; also in February, May, July and October 1965. Analysis of these samples. show that salinities vary between 31%,, and 34%, falling below 30%, near river mouths.

@) Salinity and currents in the estuary

A series of maps (Fig. 2) have been prepared to show salinities and currents in the estuary at critical intervals during a low neap tide and a high spring tide. The isohalines are based on samples collected at half-hour intervals at 18 stations in the estuary from Ynyslas foreshore to the railway crossing above Glandovey, approximately 800 samples in all. These samples were taken on the high spring tide, 15 ft 7 in. (4.74 m), of 13 October 1966 and the neap tide, 11 ft 9 in. (3.58 m) of 19 January 1967. The currents indicated on the maps are interpolated from springs, 14 ft (4.27 rn) and 15 ft (4.57 m) and neap tides, 10 ft (3.05 m) and 11 ft (3.35 m) studied in the spring and early summer 1966.

PLATE 18

a

b

Mega ripples with associated swirl pits developed on the open sand flats south of Station 27. Looking east. Metre rule right foreground. River Leri in background. Marsh sequence exposed in bank of the River Clettwr. Dense Sporrinierrrm of low marsh with laminated low marsh sequence below. Typical marsh creek left. The Fossil Forest horizon is just below water level.

Geol. Journal, Vol. 6 PLATE IS

b


High water. It will be seen from Fig. 2A that at high water of the spring tide, marine coastal water (water of more than 30:L0 salinity) penetrates about 4 miles (6.4 km) up the estuary, as far as Frongoch Point. Salinities decline rapidly in the upper estuary and fall to 25%,, in the tidal river at Glandovey, and to 20Z0 below the railway bridge. Water is fresh (less than 1%,) 2 miles (3.2 km) upstream. At this time the high marsh Jmcetum is covered by 1 ft (0.3 m) or more of water. Currents fall to zero both at the surface and on the bottom at Aberdovey Jetty, while currents up to 0.8 knot (0.4 m/ sec) at 215" continue on the south side of the estuary mouth at the Inner Buoy. At the same time the ebb has already begun in the Leri Mouth with a strong flow, 1 *2 knots (0.61 m/sec) at 150", directed towards the north shore. The flood still continues in the upper estuary with currents at Abertafol up to 0.5 knot (0.25 mlsec.) at 295" at the surface and 0.3 knot (0.15 m/sec) at 295" at the bottom [at the equivalent hour of an ordinary tide 13 ft 4 in. (4 m), 17 June 19661, and with a strong upstream flow of freshwater above the railway bridge. Salinity stratification does not, occur, the difference between top and bottom being less than 05%, in the deepest parts of the main channel, 30 ft (9.14 m) at the Jetty and also at Frongoch, 20 ft (6.10 m). The tendency for freshwater to be ponded back against the north shore is shown by the figures for stations near Abertafol with values lower than expected from the position of the 34%, isohaline. Low values in the throat of the estuary are also due to ponding back as well as freshening by streams.

At high water of neap tide coastal waters penetrate only into the estuary mouth, as far as Trefri, 2 miles (3 km) upstream. Values fall to lo%, at Frongoch Point and to zero in the throat of the estuary. The High Marsh and the upper part of the Low Marsh remain uncovered at this time. Salinity stratification does not occur and weak surface flood currents continue in the estuary mouth, while bottom currents fall to zero. I hour after High Water. Spring tide coastal water continues to move into the upper estuary and salinities above 30%, can be measured up to Gogarth. Strong outgoing currents now occur at the Jetty, both at the top, 1.7 knots (0 -87 m/sec) at 53" and the bottom, 1 -5 knots (0.76 m/sec) at 60", and in the Leri Mouth, 1.6 knots (0.81 m/sec) at 150", and 1.2 knots (0.61 m/sec) at 164".

The neap tide isohalines show little change except that the 32%, isohaline is slightly farther up the estuary. Weak outgoing currents now occur at the Jetty, 0.2 knot (0.1 m/sec) 70", at the top and 0.1 knot (0.05 m/sec) at 70" at the bottom, with stronger ones in the Leri Mouth up to 0-6 knot (0-3 m/sec) at 154", at the surface, 0.5 knot (0.25 m/sec) at 160", at the bottom. 2 hours after High Water. The isohalines now show an approximate shift of half a mile (almost 1 km) downstream with rather more rapid freshening in the tidal river at Glandovey. The large amounts of high saline water draining off the marshes tend to retard freshening the lower estuary. Currents are now nearly maximum, over 2 knots (1 m/sec) at the surface at the Jetty and in the Leri Mouth with surges to almost 3 knots (1 -54m/sec). Bottom currents reach 1 -6 knots (0.8 m/sec). In the "Big Bend" (Fig. 2C) currents reach 1.2 knots (0.6 m/sec) at 23" at the top, 0.8 knot (0-41 m/sec) at 33" at the bottom. Neap tide currents reach over 1 knot (0.5 m/sec) at the surface, in the estuary mouth, but are uniformly low at the bottom, up to 0.3 knot (0.15 mlsec). 3 hours after High Water. Spring tide ebb currents are now at a maximum with surges over 3 knots (1 -5 m/sec) both at top and bottom at the Jetty. At Abertafol currents reach 1 *8 knots (0.92 m/sec) at 50" at the top, 1 a3 knots (0.66 m/sec) at 37" at the bottom. This is reflected in the rapid shift of the isohalines for the spring tide with the 34X0 line moving to Aberdovey. Salinities are now below lo%, in the tidal river but water is fresh above Glandovey. The neap tide currents in the estuary are again

222 J. HAYNES AND M. DOBSON approximately 1 knot (0.5 m/sec) at the surface with very low bottom currents and the salinity values decline more slowly than for the equivalent period of the spring tide. 4 hours after High Water. Spring ebb currents continue strongly in the estuary mouth, up to 1 -6 knots at the surface at the Jetty with bottom currents fluctuating between 1 and 1-5 knots (0.6 and 0.76 mlsec). High salinity water has now withdrawn and water more saline than 30%, has fallen back to the "Big Bend". Salinities are now less than 5X0 in the tidal river. Neap tide water samples were not collected for this hour, but neap tide ebb currents at this time show a continuing fall in strength to less than 1 knot (0.5 m/sec) ,with very low bottom strengths, 0.1 to 0.2 knot (0.05 to 0.1 m/sec). There is now no movement of water south of the shoal as the entrance to the south pass is closed by bars (Pl. 17, see Fig. 12F for 1966 configuration) which now become emergent. 5 hours after High Water. Spring ebb currents have now fallen to less than 1 knot (0.5 m/sec) in the estuary mouth [although they continue at Abertafol and in the Big Bend at speeds greater than 1 knot (0.5 m/sec) both at the top and the bottom]. The 30%, isohaline can now be drawn at Pen Helyg and the lo%, line just above Frongoch Water samples were not collected at this hour for the neap tide. Neap tide ebb currents are nowvery lowin the estuary mouth, jetty readings being 0.3 knot (0.15 m/sec.) at the top and 0.1 knot (0-5 m/sec) at the bottom, although currents of 0.6 knot (0.3 m/sec) continue in the mouth of the Leri. 6 hours after High Water. Spring ebb currents are now very low at the Jetty, 0-3 knot (0.15 m/sec) at the top, 0.2 knot (0.1 m/sec) at the bottom, while continuing up to 1 knot (0-5 m/sec) in the Big Bend, and at 0.5 knot (0.25 m/sec) at Abertafol. Salinity is now less than 30%, at Aberdovey, and the lo%, isohaline can now be drawn below Panteidal. Salinity sampIes were not collected for the neap tide. Low Water: (5; hours before High Wafer). Spring ebb currents, although slight, continue at the Jetty, but fall to zero at the Inner Buoy. Ebb currents also continue at almost 1 knot (0.5 m/sec) in the Big Bend, 0-9 knot (0.45 m/sec) at the top at 24", 0.7 knot (0.36 m/sec) at 24" at the bottom. Salinities are now less than 28X0 at Aberdovey and less than 5%, above Panteidal. Water samples were not collected for the neap tide. 4; hours before High Water. For almost an hour after low water, slight spring ebb currents continue at the Jetty and along the north shore while the flood begins on the south side of the estuary mouth. lncoming currents begin a short time after low water on the south side and this causes the water level to rise at the Jetty, while salinities are still falling. Currents reach 1 knot (0-5 m/sec) at the surface at the Bar Buoy and 0.5 knot (0.25 m/sec) at the Inner Buoy after 1 hour. The current is then reversed at the Jetty but is stronger at the bottom, 0-2 knot (0.1 m/sec), than at the top, 0.1 knot (0.05 m/sec), leading to weak salinity stratification with bottom water up to 2%" higher than the surface water. The situation during the young flood of the neap tide is closely similar and the water at the Jetty remains almost stationary and falls in salinity while rising in level.

In 1962 when the configuration of the channels was as shown on the salinity maps the young flood undoubtedly pushed in south of the shoal. At the present time (Fig. 12F) the ebb-flood circulation cell (Robinson 1960) appears to have been moved north of the shoal. 33 hours before High Water. By the end of the second hour of the spring Rood the current direction is reversed in the Big Bend while the ebb continues above Frongoch with concomitant crowding of the isohalines in the mid estuary. Surface current strengths both in the estuary mouth and in the Big Bend reach 1 knot (03 mjsec) and bottom currents are almost equally strong. [Note the strong currents into the Leri

.

PHYSIOGRAPHY, FORAMINIFERA AND SEDIMENTATION, WALES 223

“1

.---..-.-- \ ,.----- . uimim ---. . . _--a__

\ \--, Winit), ,..-- y-..

r; a. c 20

up(ca$flow 3346uK+ \

10

2HA

3H* 1 HA

B


Mouth from the north-east direction, 1 knot (0.5 m/sec) at 45".] As the flood waters reach Abertafol and surface ebb currents fall to zero strong salinity stratification can be measured in the deepest part of the channel, 13 ft (3.96 m); scour depression, 14-2%,at the bottom, 1 *6%, at the top (equivalent period 13 ft 4 in. (4.06 m) tide 17 June 1966. Neap tide currents are still weak at this time in the estuary mouth, 0-3 knot (0.15 m/sec) at the surface, 0.2 knot (0.1 m/sec) at the bottom, and the movement of the water at the Jetty remains too small to measure with the Direct Reading Current Meter, less than 0.1 knot (0.05 m/sec). Nevertheless, the salinities at the Jetty show a steady rise of 4X0 in the hour with bottom water remaining up to l:!, higher. This indicates a strong lateral salinity gradient out to mid channel at this point. 2& hours before High Water. The spring tide is now at maximum flood with currents up to 2 knots (1 m/sec) at the surface and 1 a6 knots (0.8 m/sec) at the bottom in the estuary mouth. At the Big Bend currents reach 1 -5 knots (0.76 m/sec) at 255" at the surface and 1 knot (0.5 m/sec) 255" at the bottom. Upstream flow is not so strong at Abertafol, 0.6 knot (0.3 m/sec) at 280" at the surface and 0.5 knot (0.24 m/sec) at 290" at the bottom. Currents strengths for the equivalent period of the neap tide are much lower and exceed 0.5 knot (0-25 mlsec) only at the surface south of the shoal. The much slower penetration of marine waters during the neaps is shown by the position of the isohalines in the Big Bend as compared with the hour before. I t hours before High Water. On the spring tide the strongest currents now occur in the upper estuary and at Abertafol reach 2 knots (1 m/sec) at 275" at the surface and 1 -5 knots(0.76 m/sec)at 275" at the bottom. Marine waters now push up the estuary to Abertafol, and the 20%, isohaline can be drawn at Frongoch. The currents in the estuary mouth showa slight slackening compared with the previous hour, but continue at speeds gieater than 1 knot (0.5 m/sec). Bottom currents now exceed surface currents at the Jetty and the Inner Buoy and the flow into the Leri Mouth is now from the north-west over the submerged spit. On the neap tide the currents in the estuary mouth are at maximum, 0-8 knot (0.4 m/sec) surface south of the shoal, but are again strongest at the bottom at the Jetty and at the Inner Buoy. Marine waters now move up the main channel at the entrance to the Big Bend.

The salinity data for these tides is also shown graphically (Fig. 3A, B). The graph for the spring tide illustrates very well the dominance of water above 30%, salinity at high water with smooth curves indicating even withdrawal during the early ebb with high saline waters running off the marshes. Marine salinities are maintained in the estuary mouth until the fifth hour after highwater. After this time water below lo%, salinity reaches the mid-estuary, shown by a marked bend in the isohalines. The ebb continues in the upper estuary while the flood begins producing a steep salinity front, well shown by the curves for 3& and 2+ hours before high water. The graph for the neap tide also shows these features but to a less marked degree. It also shows the more feeble penetration of marine waters and their lesser persistence in the estuary mouth. The curve for 2 hours after high water approximates to a position between the 5 and 6 hour curves for the spring tide. These measurements were made during

*

PLATE 19 a General view of marshes looking north-east from transect towards Frongoch Point. Juncetum

in foreground, sward in middle distance beyond group of people, open mud and Spartinietum in background. Taking small core in Juncetum, near Station 5. Dense Spartinietrrm in background. Pan in foreground. Looking north-east.

b

Geol. Journal, Vol. 6 PLATE 19

b


I

Fig. 4. Maps showing salinities at different tides.


autumn and winter, but similar results were obtained for tidal cycles analysed in the summer. Readings of salinity, velocity and tide height through 24 hours at the Jetty are also graphed (Fig. 3C). Tides analysed were neap tides over the period 12/13 July 1966 predicted heights 1 1 ft 3 in. (3 -42 m) and 10 ft 5 in. (3.2 m) and spring tides over the period 20/21 July predicted heights of 14 ft 1 in. (4.24 m) and 14 ft (4-27 m).

The graphs show very well the decided asymmetry of the tidal wave with a flood period of 5+ hours and a rate of rise of up to 54 ft per hr in the case of the springs. The curves for salinity reflect this asymmetry, again greatest for the springs with a steeper rise in values on the flood and slower decline (with greater persistence of marine salinities) on the ebb. The lowest salinites are shown at low water neaps. Maximum velocities tend to occur about 2 hours before and after high water with highest speeds recorded on the ebb. Currents at the Jetty were too small to measure on the D.R.C.M. less than 0.1 knot (0.05 m/sec) on the flood of the neaps, but speeds of up to 1 knot (03 m/sec) were recorded on the neap ebb with maxima rather later than on the springs. Surface currents generally exceeded bottom currents, but when maximum currents were recorded during surges they were the same at the bottom. Bottom currents slightly exceeded surface currents just before high water.

(c) Variation in the pattern of salinity

The maps and graphs give a good idea of the salinity and current ranges through both the diurnal and lunar cycle. However, it must be kept in mind that salinities expected on a particular tide will be modified by rate of upland flow, weather and variation in initial salinity of the coastal water. Meteorological effects are chiefly felt in the upper estuary where tides of equivalent predicted height show great differences in penetration apparently as a consequence of differences in upland flow.

Figures kindly supplied by the Gwynedd River Authority show the following mean monthly flows (in cu. ft per sec) in the River Dovey for 1966 (Table 1).

There is considerable daily variation from 162 cusecs (16,300 m3/hr), during periods of low run off to 13,345 cusecs (1,330,000 m3/hr) during flood conditions.

Upland flow on 13 October 1966, the day of the 15 ft 7 in. (4-74 in) spring tide mapped Fig. 2 was 334 cusecs (34,100 m3/hr), less than one quarter the seasonal mean. This tide showed extreme penetration of marine waters, almost into the throat of the estuary with salinities of 20%, immediately below the railway bridge. In contrast on the equivalent spring tide of 26 April 1963, 15 ft 8 in. (4.76 m), marine waters penetrated to Abertafol only and salinities fell to lo%, in the throat of the estuary, and to less than 5%, at Glandovey (Fig. 4A). Upland flow on this occasion was 694 cusecs (70,800 m3/hr), twice the previous amount, and near to the spring seasonal mean. A 15 ft 7 in. (4-74 m) spring tide studied by the Water Pollution Research Laboratory at Aberdovey Jetty, Panteidal and the Railway Bridge on 5 April, 1966 when upland flow was almost the same, 724 cusecs (73,900 m3/hr), shows comparable high water figures with salinities remaining below I%, at the Railway Bridge.

PLATE 20

a Scraping surface sample in the sward (Armerietum), Station 11, marsh transect; elongate, branching pans with invading Spurtinn at the edges. Scarp separating Juncetum from sward in background. N.B. Railway embankment in far background, looking east. Isolated clumps of Sporrina in open mud of the low marsh. View from Station 111 towards Station IV, marked by rod. Spartinietrrnr in background. Aberdovey far left background, looking north-west.

b

Geol. Journal, Vol. 6 ._. ..

PLATE 20

b


' Salinity proti18 %. Nmap Tide IO'Id 17/5/61 9-L5a.m.

$91 12.6 10.9 . 1 2 a c - 12- - - 13.25 12.7 12.13 - - - - - - - -*--

----- - _ _ - - - - - -__

-2' * _------ - 5 -

-10-

-15'-

. C 0 E G ' t t y I

~ b b retarded Flood retarded Flood Ebb I- loo yards -1 A

Salinity profile -1.. Spring lid. 14'7" 25/5/67. 5-35p.m.

6 39 7.02 - ---A' 7.15 6.73 6.05

.a . \

-___- - - - - . . . . . .*'\I0 / -10' J \

I C b F i

Flood Ebb Bl 1- I00 yards + Fig. 5. Profiles showing salinity stratification in the Main Channel at Aberdovey Jetty, Station

29, looking west. Measurements made at points B-G, 70 yards apart along a traverse from the south to north bank. Vertical scale in ft O.D. as shown. A. first hour of a neap tide. B. first hour of a spring tide.

Upland flow on 19 January 1967 the day of the 11 ft 9 in. (3.58 m) neap tide mapped was also low, 434 cusecs (44,300 m3/hr), just over half the seasonal mean. It is, therefore, probable that penetration was near maximum. Penetration shown by an equivalent tide, 11 ft 3 in. (3 -42 m) on 17 May 1963, upland flow 476 cusecs (48,400 m3/hr), was slightly less (Fig. 4B). The tides measured at the Jetty during 12/13 and 20/21 July 1966 coincided with a period of very low run-off, 237 cusecs (24,000 m3/hr), and 162 cusecs (16,300 m3/hr), and the salinity curves reflect maximum penetration with minimum freshening on the ebb, falling only 9 % to 23 % at low water spring tide, compared with 14x0 at L.W.S. on 5 April 1966.


Penetration of coastal water upstream and fresher water downstream is then strongly influenced by rate of upland flow. From the seasonal flow figures maximum penetration of coastal water would be expected to occur in summer, minimum penetration in autumn together with maximum freshening of the lower estuary. To some extent this will be counteracted by higher spring tides occumng in Autumn and Spring. Also the head-waters of the Dovey system are short and run-off rapid. This is indicated by the rather erratic monthly means which shows that periods of low run-off can occur at all seasons. Again, summer rainfall is variable and in 1963 the minimum occurred in winter.

(d) Salinity stratification As has been shown only weak vertical stratification was discovered during the

investigation of the tides mapped, reaching 2%, between top and bottom on the early flood of the spring tide at the Jetty. Similar results were obtained for other tides measured in the estuary mouth in the spring and summer of 1966. Under average weather conditions with dominant westerlies, tides enter the shallow estuary mouth with strong turbulence creating a homogeneous vertical profile. However, under certain weather conditions including calm weather and high run-off, with weak tides, strong underthrusting of saline water occurs as shown by the profile taken at the Jetty during the first hour of the flood, 10 ft 10 in. (3.3 m) neap tide, 17 May 1967. Upland flow reached approximately 1,100 cusecs (112,000 m3/hr) in the previous 12 hour period (Fig. 5A). The flood begins on the south side while the ebb continues near the Jetty; this is well shown by the salinity curves, surface values falling to less than lo%, at G. The surface to bottom salinity difference reaches 13X0 in the deepest part of the channel, and the highest values are reached nearest the surface at C where the flood is strongest. On the south side therefore, the flood is strongest at the bottom while on the north side the ebb is strongest at the top. The upcurve of the isohalines near the Jetty shows that the ebb is retarded by the banks and the downcurve along the south shore indicates that the flood is retarded in the same manner. Five traverses were made at 20 minute intervals showing that this stratification was maintained for at least 14 hours. Measurements taken during the first hour of the flood of a spring tide under similar weather conditions, 14 ft 7 in. (4.47 m), 25 May 1967, upland flow reaching approximately 2,100 cusecs (214,000 m3/hr) in the previous 12 hour period, show only weak stratification (Fig. 5B).

(e) Salinities on the marshes As the intertidal marshes are uncovered early during the ebb they are not much

affected by less saline waters from upstream during the late ebb. The waters left behind in thin sheets on the muds of the marshes thus tend to show high tide salinities. Salinities measured in this residual water, after the time of the full ebb of the spring tide, 15 ft 7 in. (4.74 m), 12 May 1964, are shown in Fig. 4C. Upland flow on this day was 1,057 cusecs (102,000 m3/hr). It will be seen that these figures correspond reason- ably well with figures for high water of'a spring tide of low penetration showing values between 26%, and 31x0 in the lower estuary and 3%, to 17%, in the upper estuary.

To discover how the salinity of residual water varies with weather conditions and exposure during neaps, monthly measurements have been made at ten stations along a traverse from the high marsh Juncetum down to the main channel. This traverse which transects the marsh zones at right angles is located just upstream from the Clettwr mouth at approximately the point reached by waters of coastal salinity, under


Y

W c

20

W n 10 r

0

30 1

SEA MAIN CHANNEL MARSH

Fig. 6. Graphs of temperature through 1963 in Cardigan Bay, in the main channel and on the marshes.

mean upland flow conditions (Fig. 1, I-X). Samples were taken each month from March 1963 until April 1964, after the time of full ebb. Results are given in Table 2.

High marsh Juncetum-Station I , Table 2. Here salinities show diluted high tide values ranging from 13.9 to 29-4%,. Complete drainage occurs during neaps in dry weather. It is noteworthy that the lowest salinity recorded, 13.9%,, represents the effect of June rainfall following complete drying and mud cracking during late May and early June. This indicates that dilution and drainage outweight evaporation so that relatively little salt accummulates at the surface of the mud. When the pools fill up with rain-water salinities are therefore low. The low salinities for March and April reflect the heavy rainfall for those months. It should be pointed out here that the present marshes are enclosed within a railway embankment so it was not possible to extend the transect to include the transition to freshwater fen.

Low marsh and high marsh sward-Stations II-VII, Table 2. In this part of the marsh, measurements taken after the highest springs, as in April and October, tend to show high tide values. The effect of heavy rainfall during the ebb is shown by the figures for March. Half an inch fell during the time these samples were collected and the values are correspondingly lower, by about 4%,, than those for April. As is to be expected the greatest modification occurs during neaps. The effect of wet weather during neaps is shown by the figures for May, with reduction to 17%, at one station on the open mud. The effect of evaporation during neaps in hot weather is shown by figures for June when salinity at Station I1 reach 36%,. On 1 August, following a week of hot weather, all the stations became dry, except Station I1 which showed a salinity of 30%,. These figures show that residual water tends to drain away before extreme salinities are reached, as in the Juncetum. This is possibly made easy by the quartzose nature of the muds. Salinities may also be prevented from rising or falling to extreme values by movements of the underground marsh water. Chapman (1960) has described how the water table in marshes tends to rise and fall with the tides and to this must be added the effect of drainage from the freshwater fens which would be greatest during neaps. Winter figures, January 1964, are lower and range between 26%, to 29%,. As December and January were very dry these figures presumably reflect previous high, ordinary tide figures and are closely comparable with those for February, taken after a 13 ft (3.96 m) tide. Higher figures for small tides in spring and autumn, September 1963 and March 1964, presumably reflect greater evaporation.

TA

BL

E 2

4

zm

m"

$g

N

17.8

31.5

31.3

31.5

31.7

31.5

27.9

28.0

28.5

13.7

0.17

5.00

16

* 32 2-

--

20.0

18.4

20.6

17.0

Dry

Dry

Dry

18.4

4-0

2.8

0.22

--

--

--

--

--

.__-

--

--

-__.

--

2.80

12 --

I-"

-

13.9 17.5

29 -4

31.8 - 27.1

31.2 -

25-4 25.4

25.2 20 -5

30.3 31.2

28.4 29.4

23.7

33.9 32.0

31.2 26.6

26.4

29.0 32.8

31.2 27.5

34.3 33.3

32.6 27.6

28.9

Dry

Dry

32.2 D

ry D

ry

Dry

32.0 31.4

Dry

29.3

30.5 31-4

26.2 26.5

Dry

Dry

10.8 D

ry

12.7 12.3

9.0

Nil

0.08 N

il N

il N

il

-----

-----

-----

--___.--

-----

__

-__

__

__

-

-----

-----

-----

-----

3-52 2.96

0.76 1.62

1.62

12 16

15 11

13 ___----

A. I.

Pool, Juncetuni

50 11. Pool, Sw

ard

50 111. Thick Spartina

50 IV. O

penMud

50 V. O

pen Mud, Sandy

50 VI. O

penSandyMud

-

-

-

100 VII. T

hick Spartinu

100 VIII. Pool, edge flats

-

28 *3 36.1

27-9 - 24.7

26 -4 - 30.0

Dry -

29 -5 32.6

35.5 -

28.3 29.1

32.3 -

26.5

27 *5 . -

30.2

27 -3

26.6 -

28 -6 -

31 -9 31 -7

200 I IX

. Sandshoal 26:3 -

13-0

Nil -

3.46

400 I B. X.

Edgem

ainchannel I

0 52

4.25

15 -

0.09

4 -25

13 -

-

Nil

1 49

15 __.

Nil

3 -77

11 -

-

on day R

ainfall ininches

month

Tide height in ft

13 -

16 -

Results of m

onthly determinations of salinity along a traverse from

the 'Junceturn to the main channel in the D

ovey Estuary, A

-B, Fig. 4C.

Monthly rainfall figures refer to calendar m

onth. The figures given in the colum

n under Tide C

over give the tide height requlred to submerge

the particular physiographic zone.

PHYSIOGRAPHY, FORAMINIFERA AND SEDIMENTATION, WALES 23 1

Salinities on the low marsh and high marsh sward, therefore, tend to reflect the tidal regime, fluctuating by about lo%, with weather variations. Along the line of the traverse the range is 17 to 36%, and the average of the readings 29.4%,. This average figure may be compared with the results of Milne (1938) who found that in the mid estuary of the Tamar, Devon, salinities ranged from 27.5 to 29-5%, at the point covered by mean high water springs.

Open sand $ats-Stations VIII to X, Table 2. Here the range is much greater, 2-31%,. On any particular day salinities are found to be reduced when compared with those on the marshes, representing freshening during the early ebb by water from upstream. However, salinities tend to be quite high compared with the fresher water in the main channel at full ebb. (Variation at Station IX is concerned with a small tributary stream continually changing its course.)

(f ) Turbulence

During the course of velocity logging in the vicinity of Aberdovey Jetty, the con- tinuously recording Kelvin Hughes Current Meter repeatedly indicated that there existed a strong fluctuating motion superimposed on the general water movement. Fluctuations in velocities during measurements by current meters in estuaries and the sea are well documented (Bowden 1962) and phenomena of this type are considered to be due to turbulent motion expressed as shearing and eddying. Both vertical turbulence and horizontal turbulence are recognised, but in this study vertical turbulence was not directly measured. As it may be considered that vertical turbulence is a function of the distribution of the mean properties of the water, an analysis of salinity, temperature and velocity was made. There is no significant gradient with respect to salinity and temperature in thevicinity of Aberdovey Jetty, and thus vertical turbulence must derive its main impetus from the considerable velocity gradient.

During periods of high upland flow more distinct salinity gradients exist. Tempera- ture profiles may also vary, especially where the sand banks after long exposure to insolation raise the temperature of the covering water. The thermocline phenomena that appear impersistently in Cardigan Bay during the summer have no influence on the estuary, for in the vicinity of the bar the gradient is destroyed.

As has been indicated, velocity profiles are based on readings from three positions, 2 ft (0.6 m) from the bed, mid depth and 2 ft (0.6 m) from the surface. Unfortunately this was insufficiently detailed to establish if a log/law relationship for the velocity and depth existed. Nevertheless, bottom friction so severely modifies the profile that vertical turbulence must be considerable.

Two types of turbulence were recorded, short fluctuations with periods of 2 seconds andamplitudes of 0.3 knot (0.15 m/sec) and long fluctuations with periods of 2 minutes and amplitudes of 1 knot (0.5 mlsec). Considerable amplitude variation occurred through the profile, implying marked shearing especially in association with the long fluctuations. The short fluctuations tended to disappear at depth. Whilst shearing phenomena could be recorded, the pronounced eddying that was a marked feature of the water motion during the period of maximum velocity could not be recorded.

It can be inferred that the marked vertical and horizontal turbulence experienced in the Dovey Estuary is due to physical rather than chemical factors. There is sufficiently intense turbulence at all depths to insure almost complete mixing under average conditions as demonstrated by the salinity results. Whilst bed friction must be the main cause of the vertical velocity profile distribution, the wind, particularly when funnelled down the estuary, must also have a significant effect.


(g) Some conclusions regarding circulation

Estuarine circulation patterns based on overall salinity structures have been described by many authors including Day (1951), Rochford (1951), Stommel and Farmer (1952), Ketchum (1953) and Pritchard (1955). More recently a review by Cameron and Pritchard (1963) and a study of the Mersey by Bowden and Sharafel Din (1966) have emphasized the significance of the dynamics of the system. The significance of these salinity structures lies in the relationship that exists between salinity, river discharge and tidal variations. Results indicate that longitudinal and vertical salinity differences increase with increasing river discharge, a fact that has been demonstrated for the Mersey by Bowden and Sharaf el Din (1966) where it is considered that river discharge has a greater effect on stratification than the vicissitudes of the tides.

The existence of vertical eddy diffusion and horizontal shearing during spring tides on the Dovey has been noted, and these factors, frequently aided by the.prevailing wind, greatly influence and distort the vertical density gradient. Response to high river discharge is especially rapid where a coincidence with neap tides occurs whilst vertical stratification in response to high river discharge during spring tides is frequently obscured. Significantly, horizontal salinity response to low discharge during springs is slower but may be maintained longer, for horizontal advection may be total with high salinity values penetrating almost to the tidal limit. Bunching of the isohalines in mid estuary and the location of the 30%, line must then be considered a factor of discharge control. The variation of salinity during the tidal period is such that times of highest salinity occur half an hour after high water, and those of lowest salinity several hours after low water. Lateral north-south variation appears to be a marked feature of the early flood predominantly reflecting the geostrophic effect, as the velocity gradient variation during the flood is directly reflected in the vertical salinity gradient. Horizontal advection by tidal control, and vertical diffusion as the result of velocity controlled turbulence are the two processes controlling the salinity distribution in the estuary, whilst the river discharge influences the concentration limits.

Any attempt therefore, to define or classify the Dovey Estuary as it exists today must be considered of limited value only. The physical variables at times create a moderately stratified type (Cameron and Pritchard 1963) but a vertically homogeneous form with restricted lateral variation is more usual.

3. OTHER FACTORS (a) Temperature

Average sea temperatures off Aberystwyth during 1963 are shown in graph form (Fig. 6) where they are compared with temperatures measured monthly on a traverse across the marsh through 1963 and main channel temperatures taken in 1963 and during salinity work in 1966. 1963 was unusual in that January and February were abnormally dry and cold. Temperatures remained below freezing for approximately six weeks and the low marsh became piled with ice flows. Normally, freezing temperatures are maintained only overnight or at most for a few days and a winter low of about 4°C is reached in the sea in February. There is then a steady rise to 15°C in July, and values of this order are maintained until early September. This is followed by a slow decline until early November, when a more rapid fall takes place. During the summer, surface water temperatures of about 17°C are achieved during warm spells.

1100.

ZIZOO. 9

A A 0 a

0

CALCIUM MAGNESIUM VALUES As DETERMINED 0 BY ATOM1 C ADSORPTION 4'0

AA SUMER MAGNESIUM OO 0 SUMMER CALCIUM WINTER MAGNESIUM WINTER CALCIUM

A 0 M A

0 A 0 00 A

A 0

0

a

Fig. 7. Diagram showing winter and summer values of dissolved calcium and magnesium in Dovey Estuary water, 1967.

Main channel temperatures at full ebb are lower than sea temperatures during winter, and up to about 3°C warmer in spring and summer. The maximum temperature recorded was 19°C. Autumn temperatures tend to be close to sea temperatures.

Temperatures measured during the ebb in thin sheets and pools on the marsh show the greatest extremes, from 0°C to 25"C, and again values are lower than those in the sea during winter. The marsh temperatures tend to be close to air temperatures and day temperatures show a rapid rise in early spring and 15°C was recorded in early April (as compared with July in the sea). As temperature is an important control of reproduction in foraminifera this factor alone may explain the abundance of such species as Ammonia beccarii batavus on the intertidal fiats compared with their rarity, or absence, living in Cardigan Bay. This is also the period of the first diatom bloom. On particular days there are interesting variations in temperature in relation to shelter and vegetation cover. On warm still days the temperature at open stations may be several degrees higher than at stations in the Spartinietum, or in pools with high banks. On windy days, especially in winter, this tends to be reversed.

The headwaters of the Dovey system drain boggy moorlands and lakes with very low pH, and values as low as 5 occur in the river, and 6.5 just above the tidal limit. In the estuary, due to the dominance of coastal waters, values between 7 and 8 tend to prevail.

234 J. HAYNES AND M. UOBSON

+\- 0 MILE t 0 IILOYErnE i A


I 1 I

-19 0 +1( +29 +34 .i

MZ i

Fig. 9. Scatter plot illustrating relationship of Mz to ap in 1-40 series. Sinusoidal.trend adapted from Folk and Ward (1 957) and Caston (1 966).

Measurements taken in mid Channel over the high spring tide of 10 July 1967, within one hour, show the following values; Aberdovey Jetty 7.7, Trefri 7.9, Panteidal 7.9, Frongoch 7.6, Gogarth 7.6; values on the marsh transect were: I1 7 -6, I11 7 -6, IV 7 -5 and VI 7 3. More detailed measurements on the marsh transect taken in July 1966 at low water were 17.6, I1 8.1, I11 7.8, IV 8-0, V 8.0, VI 7-8, VII 7-9, VIII 8.0, IX 8.1 and X 8.1. These seem to indicate lowest values in the Juncetum pan and at low marsh stations in dense Spartinietm. Low values would also be expected at the surface of the Juncetum. The figures for the transect are similar, but higher than measurements made by Stevenson and Emery (1958), in the Sulicomietum and Spurtinietum at Newport Bay, California and discussed by Bradshaw (1961), but with the same tendency for the low water values to be greater. Bradshaw’s experiments (1961) show that living, calcareous, brackish water foraminifera species, such as Ammonia beccarii are highly resistant to pH changes, tolerating values as low as 2 for up to 25 minutes. Empty tests on the other hand may be dissolved at values below 7.

(c) Dissolved oxygen

The Water Pollution Board has measured variation in amounts of dissolved oxygen through a number of tides at different stations in the estuary, and their figures show 97-110% saturation in the upper estuary with rather lower values at Aberdovey Jetty, 87-93 % saturation, lowest at low water.


(d) Calcium and Magnesium Determinations of both calcium and magnesium concentrations were made on 43

water samples collected from stations extending from Ynyslas to Gogarth. Thirty of these samples were selected from the salinity survey samples collected on 19 January 1967; the remaining 13 were collected on 10 July 1967.


The results of these analyses are presented in Fig. 7. Calcium plots for both summer and winter display a simple linear relationship of solubility to salinity, and as might be expected small inflexions occur where salinity concentrations are below 5%,. A slight summer trend may be observed in the calcium plot with values approximately 25 ppm higher than the winter figures. The results for the magnesium determinations appear complex; however two trends are evident. The lower figures from the summer samples plot at a lower angle compared with the higher figures from the winter. The range between the trends widens from 100 ppm at 23%, to 250 ppm at 32%. Within the magnesium winter trend are plots of four summer samples; these were collected from the marsh transect during high water slack, and display an increase of 250 ppm magnesium compared to other summer values. This pattern is not repeated in the calcium results.

Other workers including Carpenter (1957) and Van Andel and Postma (1954) have recorded simple linear relationships for both calcium and magnesium .in estuaries. Murray (1966) in a paper on Christchurch harbour reports 'calcium deficiencies' in summer bottom water, a feature he attributes to organic activity. Although our work on calcium and magnesium can only be considered preliminary it will be noted that although the marshes are certainly zones of high biological activity no calcium deficiency is discernible.

4. THE SEDIMENTS

A limited textural investigation of the sediments of the estuary has been made to define more clearly the degree of grain size distribution and to establish whether there exist textural trends that can be related to the energy of the environment or to pro- venance, and whether these trends affect the distribution of the flora and fauna. The investigation involved analysis of 40 samples covering the estuary and analysis of monthly samples from a marsh transect.

Textural data reported by Moore (1964) on the sediments of Cardigan Bay have been added to the estuary sample data. The positions of the sample stations are indicated in Fig. 1 ; the marsh transect is marked by Roman numerals; the 1-40 series is seen to cover the entire area, and where they coincide with the sample stations of Moore the latter are bracketed. The stations are displayed on the 1962 configuration map as 1962 was the time of collection of the original samples that formed part of the Cardigan Bay study. The 1-40 series were collected in May 1964. The results of the textural enquiry are presented on three charts (Fig. 8) and a diagram (Fig. 11).

The Mz measure used in Fig. 8A, was introduced by Folk and Ward (1957) where Mz = (g16+g50+g84)/3: it spans more than two-thirds of the sediment distribution and provides an immediate indication of the nature of the deposit. A contour interval of lp, has been selected to conform with the Wentworth grade scale limits.

The sorting parameter ap, = (q8+16)/2 of Otto (1939) and Inman (1952) has been plotted on Fig. 8B. The samples are either very well or well sorted with a range of 05%, or very poorly sorted with a deviation exceeding lg. Only four of the samples have deviations in the range 0.5 to 1 .OOq. The scale for sorting has been adopted from Folk (1966).

Skewness Fig. 8C is reported as ag = (916+984)-(2 'g50) Inman (1952) which g84-~16

whilst geometrically independent of the sorting is not sensitive to sharply skewed sediments as is the Inclusive Graphic Skewness of Folk (1966), nevertheless, it was considered adequate for this investigation.

238 J. HAYNES AND M. DOBSON (a) The marsh transect

Monthly samples were collected from a marsh transect between February 1966 and January 1967 at times indicated on Fig. 11 and at positions shown on Fig. 1 (see also Fig. 13).

Care was taken to ensure that only the surface veneer was sampled. Size analysis was conducted by the methods described in the appendix; material coarser than 9, was sieved and the finer material was analysed by the pipette method after removal of the vegetable material.

(b) Sediment interpretation

Considerable doubt surrounds the validity of interpreting the textural statistics of samples analysed by several methods (Whitehouse et al. 1960), and it must also be observed that the transportation and deposition of much of the finer material is by aggregates (flocs) whereas size analysis has been conducted only after total disaggrega- tion.

So that the effects of analysis methods on the one hand, and any significant trends on the other might be more easily appreciated, plots of Mz against oy and Mz against ap have been included in this interpretation. In broad terms the sediment pattern consists of medium to fine sand, 1.5 to 3rp,

covering the greater part of the estuary and very fine sand and silt 3rpexisting as a broad belt from Ynyslas to Frongoch (Fig. 8A). This picture is complicated by the existence of two concentrations of medium to coarse cobbles, one on the foreshore between Aberdovey and Pen Helyg and the other a remnant spit which extends from Station 27 westwards to Ynyslas beach.

The chart of skewness, as presented in Fig. 8C, demonstrates the fluctuating nature of this parameter when applied to the estuary. The significance of skewness is a difficult aspect of textural interpretation. Duane (1964) considers that skewness is significant after -0.1 and +O.lcp. In the estuary the sand mode concentrated about 2-sCp fluctuates between positive and negative, and higher silt values in samples from the high sand flats import strong positive skewness. Negative skewness reappears in some of the sediments from the high marsh.

As Mason and Folk (1958) concluded that skewness is sensitive to environment it is significant that a well defined zone of negatively skewed sand occurs at the estuary mouth. This may be the result of direct winnowing in an area of high turbulence, but is more likely to be the result of Cardigan Bay control. In the immediate bay area, the sediments are noticeably negatively skewed, and it is considered that it is this external influence that is reflected in the skewness values. Thus, in the Dovey, skewness studies also show that the bay environment extends at least as far as Trefri, with lower energy values radiating outwards and being represented by a wide zone of variable skewness, characteristic of estuaries. Duane (1 964) finds negatively skewed sands in sounds and estuary mouths, with little skewness preference from channels to banks. He also notes a positive negative association within estuaries, which he similarly attributes to energy fluctuations (Fig. SC). The greater part of the marsh tends to be positively skewed, but negative skewness appears as a restricted feature of a few high marsh deposits with high fine sand values.

Folk and Ward (1957) demonstrated the existence of interrelationships between significant textural parameters, showing that plots of arp and skewness against Mz form sine curves with polymodal sediments. Folk and Ward (1957 24 fig. 19) indicate the theoretical extension of the curves particularly towards the finer modes.


VI I

VI

V

IV

111

IIS

II

IS

i

F M 'A M j j A S O N D J

m CLAY L g SILT I-1 FINE SAND

< 0.0ormm D 002-@01rnrn 0.02- 0 . 2 m

Fig. 1 1 . Monthly variation in clay, silt and fine sand at stations along the transect across the Dovey Marshes. See Fig. I 3 for transect profile I-X, Fig. 1 . 0


The plot of Mz against og, is shown on Fig. 9 and has superimposed on it the theore- tically extended standard sinusoidal curve. The medium to fine sands have a standard deviation of 0-5p and a plot very close to the curve, as do many samples from the higher sand flats and the low marshes with deviations in the range 1 to 2 ~ . However, a high proportion of the very fine samples plot well to the right of the curve, a feature that may be attributed to the method of analysis.

The plot of skewness against Mz (Fig. 10) reveals the extent of the positive negative fluctuations, and also serves to show that the majority of the samples are positive, only the well sorted 2.59 sands of the estuary mouth being clearly negative. The weakly negative very fine-grained samples display a confusing scatter, possibly reflecting the method of transportation. This feature of transport style will be seen to militate against skewness analysis but encourages an examination of the marsh sediments in terms of clay/silt ratios.

With the exception of the upper high marsh the sediments from the transect, reported as clay/silt/fine sand indicate a 3 : 1 relationship between the silt and the clay fraction, for each station. The upper high marsh ratios are between 2:1 and 1 :I. The 3:l ratio is poorly defined at IV, the associated high sand content being attributed to the fact that this Station is at the head of a gulley network. Silt clay ratios of 1 :2 have been reported by Van Straaten (1959). Evans (1965) also plotted sand silt clay diagrams from the Wash, where there is a tendency for a 3:1 ratio. Bouchon Vaseauz (Fluid Mud) values, particularly for the Thames estuary, display a ratio range from 1 :I to 3:l. Thus similar environments appear to contain differing silt/clay ratios, a fact which is probably due to local supply. The Dovey river normally carries very little suspended material; it is only during flood that fine material is supplied in any quantity. This may be responsible for clay and silt values being higher during Spring and Autumn when rainfall is particularly high. By contrast the strong variations that occur (high clay/silt values in the western marshes, and low in the east) must be attributed to salinity values during neap and middle tides being too low to effectively flocculate the fine material on the eastern marsh.

(c) Sedimentary petrology

The petrology of the surface samples from the estuary has already been examined by Moore (1964) who made a petrological appraisal of the sediments, especially those in the upper estuary and fluviatile zones by means of photomicrographs of thin sections (Moore and Garraway 1963; Moore 1968).

The fine-grained quartzose sands are restricted to the estuary as they are not found above the tidal limit. The mobile sediments up river consist of slate and shale with a lack of quartz sand; further down stream the lithic fragments are smaller in size, but the overall composition of the sediment remains the same. Station 159 is within the tidal influence and here the sediment contains a small proportion of quartzose sand grains giving a bimodal distribution to the sediment; at Station 163 the textureis sand size with quartz sand dominant. This dominance is strengthened in the middle estuary as demonstrated by Station X. Station 26, in the estuary mouth, has a ratio of lithic fragments to quartzose sand that compares well with that found throughout most of Cardigan Bay and would be classed as sub-greywacke (Pettijohn 1957).

Mineralogically the Bay material consists of quartz, muscovite and chlorite with two feldspars and calcite. This is qualitatively identical to the Llandovery sediments of the area, the Aberystwyth Grits which are a turbidite sand/shale sequence (Wood and Smith 1959) and consist basically of quartz, muscovite and chlorite with some alcite. This simple tripart petrology is repeated in the clay size material of the


estuary marshes. The marshes also appear to display a limited quantitative differentiation, in that on the low marshes the samples contained a higher proportion of chlorite than samples from the high marsh; this may be due to the greater mobility of muscovite flakes. The ratio of lithic fragments to quartz grains is approximately twice that of the bay material. In the Bay the total lithic fragments average between 10 and 15 %. In places in Tremadoc Bay the total exceeds 30% (Caston 1966); and the Dovey estuary has percentages of total lithic fragments around 25% although, as has been seen, this rises considerably in the upper sections to as much as 50 %, and feldspar may account for as much as 8 %, mica 5 %.

5. CYCLIC MORPHOLOGICAL CHANGES

Figures for the input of tidal water as well as average river discharge (upland flow) have been given in the introduction. Clearly, there is significant tidal control, for the volume of water passing the jetty on the neaps exceeds 10,000 cusecs (1,019,000 m3/hr) whilst for the spring tide the figures reach 50,000 cusecs (5,100,000 m3/hr). Tidal penetration expressed by the salinities demonstrates even more clearly the degree of domination by marine waters. It is significant that whilst all this water enters the estuary during the flood period the main morphological features of the estuary are ebb and river discharge oriented. This is considered to be due principally to two factors:

(1) Strong tidal asymmetry which allows a seaward flow of at least 7 hours at the Jetty and even longer upstream (See diagram on velocity and salinity).

(2) Concentration of ebb flow in the main channel. The ebb orientated main channel dominates the morphology and at the time of

writing (1967) is in the form of two loops. The larger and more significant is developed opposite Trefri; the second located at Frongoch repeats the details though not the size of the first. Both have attendant flood barbs, drainage channels and mega ripple belts, all similarly located. This detailed repetition of a hydraulic pattern is more commonly witnessed in outer estuaries.

As the diagram (Fig. 12) shows, the course of the main channel has fluctuated. Surveys from 1886 and 1900 and aerial photographs from 1946, 1950, 1962 and 1966 demonstrate that the last expression of maximum curvature of the Trefri loop occurred in 1946. The 1950 survey indicates that this loop was abandoned in favour of the more direct Trefri channel route as in 1886. The Frongoch Channel being more confined and less well developed modified its course later, and took longer to adjust, the new course being well established by 1962. The fact that the disposition of the present day features compares well with 1946 suggests that the long term cycle of change has a periodicity of 20 years. There is little indication that it is due to silting and it is more likely the result of over extension of the meanders. The position of the channel at Aberdovey is conspicuously stable, although west of the Jetty the line of the main channel together with the position of the bar changes with a greater frequency than the morphology of the inner estuary. The abandoned loops and channels are frequently adopted by drainage channels prolonging their existence, whilst the course of the Len is frequently modified by their scouring effect.

6. ESTUARINE ENVIRONMENTS

The following physiographic units can. be recognised in the Dovey Estuary between high and low tide limits: (a) high marsh and pans; (b) low marsh; (c) open sand flats; (d) main channel and associated shoals; and (e) marsh channels and minor tributaries.

DOMlNANl

LIVING

SPECLES

DOMINANT

OEAO

SPECIES

Fig. 13. Diagram showing marsh transect and sample stations, I-X, Fig. 1 , with histograms of foraminifera abundance, April 1964. The histograms are based on total numbers in 10 ml of sediment.

!A Jadmmina macmscms II Rn.lphidium depnuulum

IIA hdammina rnacr.sc.ns 611 Prntmlphidium dqnswlun

I* Pmldphidium d*penulum v Prolmlphidium dqxessulum

I t Elphidium excwalum [3 only] w Ammonia beccarii balavus

* M I ProWvhidium dcpr*rlulum 12 only1 I Jadammina rnacrmicms

I* Jad-m macrescms Ammonia bmccarii balavus I1 11 Pmlmlphidlum dmprmssuulurn Ammonia beccarii balwus 1

IIA Jadarnmina macr.scmr I# Protdphidkm d-subm

Prolalphldium d*pmssulum V Proldphidium depmsulum

w Ammonia bmccarii balavus w Elphidurn smlsmymnsm

w Ammonia bucarii balavus

These environmental units relate generally to tidal cover and currents. Salt marshes are developed most extensively on the south side of the estuary and formerly passed gradually landwards into the freshwater fens of Borth Bog. At present, due to extensive reclamation, marine marsh is largely confined to the seaward side of the Cambrian Railway line. On the north side of the estuary the most extensive development is on the seaward side of the railway line between Gogarth and Dovey Junction (Fig. 1).

PHYSIOGRAPHY, FORAMINIFERA AND SEDIMENTATION, WALES

(a) The high marsh and pans

The high marsh is covered by water only during spring tides and the highest parts only by exceptionally high tides. It is often separated from the low marsh by a small cliff which approximates to the mean level of spring tides. This part of the marsh is characterised by dense vegetation, plant associations 2-5 of Yapp, Johns and Jones (1916). These are from the highest to lowest:

243

(5 ) Juncetum maritimi (4) Festucetum rubrae (3) Arrnerietum maritime (2) , Glycerieturn maritimae (Puccinellietum)

Associations 2-4 make a close, short sward on the lower, high marsh (Pl. 20a) while the vegetation on the upper, high marsh in the Juncetum is about 18 in. (0.45 m) high (Pl. 19a, b). In areas of present or recent erosion the sward is separated from the Juncerum (as also from the low marsh) by cliffs about 2 ft (0.6 m) high. Salt pans occur only occasionally in the upper high marsh and are large, up to 5 yds (4.5 rn) in diameter, I8 in. (0.45 rn) deep and with up to 1 ft (0.3 m) of water-in them after high tides. They tend to be isolated, not connected to others by channels, and round to oval in shape. These pans are usually bare of vegetation and may dry out with mud cracking during neaps. The pans in the lower, high marsh sward are more irregular and smaller, 2-3 yds (1.83 m-2.7 m) in diameter and tend to be connected by channels with each other and with the low marsh. During the last thirty years the low marsh has been extensively colonised by Spartina toiinsendi. This vigorous hybrid has penetrated into the sward along the connecting channels and is now well established in many of the pans, especially round the margins (PI. 20a). The photographs in Yapp, Johns and Jones (1916) are now of considerable historic interest as they show the marshes before this development.

The deposits of the high marsh unlike those from similar environments in the Wash (Evans 1965) are, macroscopically, structureless clayey silts with high amounts of fine sand (0-25-0-0625 mm), in addition the sediments are usually rich in plant debris. Results from four marsh traverses show that clay values (0.002 mm) frequently exceed 15 % which must be considered a high figure in an otherwise sandy estuary. The high marsh deposits within the zone bounded by Stations 18 and 5 (Fig. 1) are '

very similar and display no obvious lateral trends; small fluctuations occur in the clay/ silt ratios, the range being from 1 :2 to 1 :3, whilst the silt content is steady at 45 "/u. No obvious seasonal variations occur in the sediment, the possible exception being that less clay appears to reach the high marsh in summer.

The hydroxide, monosulphide, and bisulphide zones discussed by Van Straaten (1954) were displayed in the short 60 cm cores collected both from the sward and the pan. Plant debris was usually absent from pan cores, with the hydroxide layer correspondingly thin by contrast to sward cores which were always rich in vegetable material. The upper high marsh cores frequently penetrated well digested Phragmites peat, and this occurred not only at the transect, but also at Stations 5 and 10 where it is well exposed in the high marsh cliff. At a comparable depth of penetration on the lower marsh the monotonous clayey silt is underlain by fine sand with shell fragments.

Throughout most of the high marsh area the flora appears too dense to allow depositional structures to develop, except in the pans where structures are limited to burrows, trails and bird prints.

The fauna on the high marsh surface is extremely limited and observations support Evans' (1965) description of salt marshes in the Wash as a zoological desert. Mollusca appear to be absent and foraminifera are dominated by a small number of arenaceous


I SAMPLE STATIONS

- . - - pi xo - - .x. : . .

CON TI NU ED

bd fl la 2 - 1 SPECIES

Fig. 14. Frequency April 1964.

diagram of foraminifera recovered at stations along the marsh transect

species, including Haplophragmoides. subinvolutum, Jadammina macrescens, Miliam- mina fusca and Trochammina injlata. At the river end of the estuary no other living species have been found, but seawards of Abertafol increasing numbers of the living calcareous species that dominate the low marsh appear on the lower, high marsh sward, together with Leptocythere pellucida. The foraminiferal fauna of the Juncetum remains almost exclusively arenaceous (Figs 13, 14).

As can be seen from the frequency diagrams the highest total of specimens occurs on the sward, being exceeded at only one other station, in the low marsh. The total of living specimens on the sward, Station IIA (mainly J. macrescem) is, in fact, the highest recorded. It has been pointed out by Phleger (1966) that records of completely arenaceous marsh foraminiferal populations may be due to methods of collection, preservation or study. In this case, samples were skimmed from the top 0.5 cm of oxidized sediment and washed, stained and dried on the same day. There seems little doubt that the clear predominance of arenaceous forms is real, especially in the Juncetm, and not due to in sitri or post collection solution of the calcareous tests.

The absence of a varied fauna from the high marsh surface is presumably connected with the long periods of exposure and drying between high springs. The


Juncetum in particular is covered only occasionally, less than a total of 100 hours in a year. On the other hand, tidal inundation occurs often enough to exclude land and freshwater mollusca, such as Limnea which is common in the freshwater and alluvial meadows. Oxygen and pH probably fall to low levels below the surface layer of decayed vegetation.

The pans of the sward show a much more varied living fauna because of channel connections with the low marsh. These pans represent an extension of the low marsh environment into the sward, together with its fauna, including the dominant calcareous foraminifera described under 6 @). The deeper pans often contain green algae and marine fauna including shrimps and fish swept in on high springs. Salinity and pH are not significantly different than on the low marsh. The isolated pans of the Juncetum, on the other hand, show mainly dead foraminifera, probably derived from the nearby marsh surface. These pans, probably due to their tendency to dry out during neaps, show the lowest dead and living totals and are also largely barren of other microfauna, apart from temporary immigrants such as gnat larvae and beetles. Lowest salinities, 13%,, and pH 7.6, on the transect were recorded here. This may explain the dominance of Efphidium excavatum in the live fauna whereas Jadammina macrescens dominates the dead fauna.

@) The low marsh

The low marsh is generally separated from the high marsh by a small cliff which approximates to the mean level of spring tides. This part of the marsh is completely covered by all tides except the lowest neaps. This zone corresponds to the higher mud flats of Evans (1965). It is characterised by open vegetation on soft muds correspond- ing partly to plant association 1 of Yapp, Johns and Jones (1961), Saficornietum europaeae (PI. 20b). Since the investigations of these authors fifty years ago, this association has been largely replaced by rice grass, Spartina tonmendi, which has also actively invaded the edges of the open flats. The lower limit of the low marsh may now be conveniently taken to coincide with the approximate limit of the Spartinietum. Over much of the low marsh, Spartina now occurs as almost pure stands, up to 3 ft (0.91 m) high, but areas of mud are still present, bare of vegetation except for Enteromorpha and occasional Saficornia, with the first colonists appearing as isolated clumps, as between stations 111, IV and V.

The deposits of the lower marsh display a considerable range in composition from clayey silts with a high sand content, more characteristic of the higher marsh, to sediments which are largely fine sand ; generally however, silty sands predominate. The average clay concentration is 7 % giving a clay/silt ratio which is firmly established at 1 :3, even so it is within this environment that the greatest phi deviations occur, the deposits are, in fact, poorly sorted.

Analyses of samples from the four previously mentioned traverses clearly demonstrate a strong lateral variation, although the 1 :3 ratio of claylsilt remains. Towards the tidal-limit clay values are 5 and silt 15 %. Whilst at the estuary end, particularly in the vicinity of Station 16, the values are 16 % for clay and 44% for silt; seawards of Station 16 a gradual change to fine sand deposits occurs. Seasonal variations as in the high marsh demonstrate that the deposits are laminated fine sands and silty .sands. The thin laminae of silty material are frequently spaced as widely as 8 in. (20 cm), this is particularly so at Station VII. At Station V the laminations disappear at depth, being replaced by structureless fine sand with shell fragments. All the cores taken from the lower marsh had fine-grained silty sand at the top often in contrast to


the rest of the core, a fact which must reflect the efficiency of the Spartinietum as a sediment trap. The only plant debris noted in the cores were Spartina roots, otherwise the cores were clean.

Bands of iron deposits, which are probably the result of bacterial activity occur within the lower marsh deposits; other banding noted may be the result of the alga Enteromorpha which is effective in trapping fine sediment.

The high sand values found in these deposits reflect the carrying competence of the currents. Depositional structures are a feature of this environment especially in areas where the Spartina is not developed, and particularly where gullies cross the marsh, the most common structure being the assymetrical ripple marks which have long crests with low amplitudes 0.4 in. (1 cm) and wavelengths 4 in. (10 cm).

The low marsh tends to be waterlogged and between tides a film of water, some- times a few inches deep, often covers the surface away from drainage channels. Partly for this reason this environment has the richest fauna in the estuary. Gasteropods include Littorina littorea, which travel over the mud and also climb in the Spartina, reaching a density of about one per sq. ft in summer and also Hydrobia ulvae which burrow into the surface and reach a density of more than one per sq. in. in certain areas. Sub-surface, deposit feeding pelecypods include Scrobiculariaplana and Mya arenaria. Worms are common and the rosette patterns of Nereis, together with mollusc trails are characteristic sediment markings. Amphipods are very common and burrows of Corophium occur at densities of up to 3 per sq. in. (6.45 cm2). Foraminifera and ostracods also reach their greatest abundance on the low marsh. The dominant species are the calcareous Protelphidium depressulum and Elphidium excavatum. Miliammina fwca is co-dominant at the river end of the estuary and tends to replace them there. Other abundant living calcareous species include Ammonia beccarii and Elphidium selseyense. Small cores taken each season, January, April, July, October 1965 and January 1966, show that living Ammonia beccarii and Protelphidium depressulum can occur down to 1 in. (2.5 cm) depth, though possibly they are swept down burrows accidentally. The dominant living ostracod is Leptocythere pellucida, almost to the exclusion of all others.

As shown in Figs 13 and 14 the low marsh, in contrast to the high marsh, shows a large total number of calcareous species, over 40 at Station V. Only about 15 species have been found actually living in the estuary, so the long tail of relatively infrequent species shown on the frequency diagram for the middle levels of the marsh undoubtedly represents dead fauna swept in from Cardigan Bay, including Globigerina. Diatoms, pollen grains, seeds and echinoid spines occur and the swept-in plankton includes copepods and coccoliths. We have found from study of the sinking rates of Quinqueloculina seminulum and Ammonia beccarii that the tests of smaller calcareous foraminifera tend to behave hydraulically like sand grains a grade smaller on the Wentworth Scale. This explains the tendency for concentration of the exotics (of average fine sand size) in the sandy silts of the low marsh and their rarity in the medium to fine sands of the open flats and main channel, i.e. a higher species diversity. Comparison of the histograms of per cent sizes (Fig. 13) illustrates very well the tendency of the dead tests to behave mechanically as a fraction of the sediment. On the high marsh, where we can assume little, if any sorting, the numbers on the 0.076 mm (200) sieve tend to be equal or exceed the rest. On the low marsh the numbers tend to be greatest on the 0.15 mm (100) sieve. On the open flats and in the main channel the numbers are highest on the 0-15 mm (100) sieve, but those on the 0.25 mm (60) sieve equal or exceed those on the 0.076 mm (200) sieve. Total number as well as number of kinds falls off in IX and X and, significantly, the dominant forms are the heavier, more robust and ornamented species,


The percentage of living forms at transect stations is shown on the right-hand of Fig. 14. High percentages relative to dead forms have been used as an indication of the relative rates of deposition by Phleger (1960; 1966). There are difficulties in applying this idea in an area where dead tests are moving with the sediment, but it is significant that the figures give the highest rates for the low marsh (with lower figures for the open mud flash) and a low rate for the Juncetum with lowest rates of all for the upper, high marsh pans.

(c) The open sand flats

The open sand flats are the most extensive physiographic unit and consist of winnowed sands too unstable to support vegetation of any kind (Pl. Ha). Although the open flats are covered by all tides (except for the tops of the highest shoals which remain exposed during the lowest neaps) drainage is rapid and on occasion the dried surface is moved by wind. Only a few macroscopic life forms are adapted to this hostile environment, chiefly worms, especially Arenicola and the active, filter feeding pelecypod Cardium edule. Except in a few limited areas foraminifera1 faunas consist entirely of dead, current swept specimens derived both from the marshes and also from Cardigan Bay. Many of these shells are damaged and worn. Dominant forms are Ammonia beccarii var. batavus, Elphidium selseyense, E. crispum and Eoeponidella mamilla.

At the estuary mouth the open sand flats pass into the sands of the foreshore with a living intertidal beach fauna dominated by Donax vittatus and Venus gallina together with Ensis ensis, Lutraria lutraria and Natica catena. The shells of these species are swept into the estuary mouth and accumulate in the shell gravels at the end of the Ynyslas sand spit. They occur together with estuarine species and dead mollusca swept in from inner Cardigan Bay. The mollusca include Cyprina islandica, Ostrea edulis, Chlamys tigerina, Mactra corallina, Meretrix incrassatus, Spistila solida, Cardium echinatum, C. norvegicum and the gasteropods Buccinum undatum, Turritella communis, Ctathrus clathrus and Trivia europea.

The sediments in this environment are medium to fine-grained sands containing abundant shell fragments. Throughout most of the area covered by the open sand flats the deposits are fine sand with a high proportion of the grains falling in the 2-39, range. Towards the estuary mouth the sands are more effectively winnowed, producing a well sorted to negatively skewed medium sand, the bulk of grains being concentrated in the 2-3q range.

Where the main channel has eroded into the sand flats depositional structures are exposed, usually in the form of crude laminations. Mixed wave and current activity has produced a great complex of ripple forms which appear to completely cover the sand flats when viewed at low water springs. The most frequent form is the asymmetric small scale ripple, which displays long straight crests, low amplitudes 0.4-0.8 in. (1-2 cm) and wavelengths of 4 in. (10 cm).

As observation is restricted to low water most of the bed slopes are ebb-orientated; however, where the rising tide has a flow course not diametrically opposite, the falling tide, interference patterns develop. Modification of flood-orientated ripples by the falling tide may be directly observed, the resulting structures often being difficult to distinguish from oscillation ripples which are also present on the flats. Current oscilla- tions appear to be responsible for modifying the crests of asymmetric ripples by creating troughs, usually 1.5 in. (4 cm) wide and 0.6 in. (1 a 5 cm) deep, that can be traced along the original crest line of the ripples. Linguoid ripples appear infrequently on the sand flats and only near the main channel margin. The large scale ripples, here called mega ripples even though their amplitudes rarely exceed 39 in. (1 m), are located


along the margin of the main channel, particularly on the Trefri Shoal and on either side of the Leri Mouth; they cease to be present on the sand flats above Panteidal.

-Mega ripples which may be flood-orientated in the vicinity of the Jetty are well developed throughout the length of the Main Channel, at least as far up as Frongoch. On the Trefri Shoal, the structure is extensive, and trains of 50 are not unusual. The mega ripples are well developed near Station 27, where they have amplitudes of21.6 in. (55 cm) and wavelengths of 9.6 ft (6 m), they are orientated to the ebb, and have a bearing of 335". In detail the form is complex with swirl or scour pits developed at the base of the lee slopes and frequently associated with a ridge that runs forward over the next stoss (Pl. 18). Small scale asymmetric ripples co-exist with the mega ripples at all times, but are restricted to the stoss side where they are frequently arranged in curves associated with scour pits. Linguoid ripples displaying an en echelon pattern, being associated with turbulent water (Allen 1963), tend to occur near the mega ripple crest. The physical limits and their actual location are controlled by the hydro- dynamic conditions, the critical ones being flow depth and flow velocity. On the Dovey, the well developed trains are located where a significant change in bed slope exists, which must cause a local increase in transport rate creating a mutually accen- tuating condition (Bagnold 1956). Large ripples with associated small scale ripples are features of the low flow regime (Simons and Richardson 1960) where the computed Froude number is between 0.3 and 0.5. The orientation of the mega ripples at Station 27 indicates that they formed duringthe first 2 hours of the ebb, where the water depth on a spring tide 14 ft-16 ft (4-27-4-88 m) would be between 3-28 and 6-56 ft (1-2 m) and the velocity between (0-8-1 m/sec) 1 -6 and 2 knots. (Fig. 3C.).

(d) Main Channel (and associated shoals)

In the main channel 'environment water cover is continuous except over the associated shoals at the full ebb of extreme springs. Movement is at a maximum over a bottom of unstable sands. The sediments are the same as those on the sand flats, consisting of fine to medium sand usually well sorted and frequently negatively skewed. The structures are also a continuation of those noted for the sand flats. Mega ripples cover the entire length of the main channel from Aberdovey to Frongoch and are comparable in scale to those seen at low water on the sand flats (PI. I8a). Associated with the mega ripples, particularly in the shoal areas on the inside of beds, are the asymmetrical ripples.

Vegetation is absent and the macro and microfauna consist of current-swept dead forms derived both from the marshes and Cardigan Bay. Where the main channel sweeps against the rocky north shore at Abertafol and near Aberdovey green and brown algae, including Funcs and Enteromorpha, have established themselves and at Penhelyg Point there are strong colonies of Mytilus edulis. Other fauna include Patella vulgata, Serpulid worms and Balanus sp. Associated with the algae and mussel banks is a meagre foraminifera1 fauna including Quinqueloculina seminulum and Miliolinella subrotundum. Dead specimens in this zone are often white and opaque due to leaching, especially in the upper estuary.

(e) Marsh Channels (and minor tributaries) The lower courses of the channels and creeks draining the marshes and the tributary

streams such as the Clettwr and the Leri. resemble the main channel with strong movement and unstable sand bottoms inimical to plant and animal life. The sediments are very poorly sorted and cover the complete range of sizes from clay to fine sand. In


Fig. 15. Map showing the general distribution of holocene sediments above high tide mark in the Dovey Estuary area, the location of auger holes A-K (Adam and Haynes 1965) and the two deep boreholes, 2 and 3, immediately behind the blown sand and shingle of the storm beach at Borth. The dotted line shows Mean Low Water Spring Tide level (not shown in the estuary).

the vicinity of the railway, especially at Stations 38 and 39, the Clettwr has a muddy, poorly sorted sediment covering the bed; downstream the sediments more closely correspond to sand flat deposits. Where the channels pass through the low marsh they are often choked with banks of shells including Scrobicularia and Mya. These genera can be seen in position of growth in the exposed silts of the creek walls (PI. 18b). Some of the shell banks are colonised by Mytilus.

Many of the creeks show active head water erosion, and one has cut back across the line of the marsh transect since work began, scouring the surrounding muds and building prominent levees on either bank. Older channels cutting back into the high marsh are up to 8 ft (2.44 m) deep. Fauna is limited, but living EIphidium excavatum and Protelphidium depressulum occur sparingly in some of the pools and also as far up the Clettwr as the tidal limit.

Sediments near the creeks tend to retain air as a result of the disparity in the ratio of rise of the water table to the rise of the tidal water; this results in cavities forming within the sediment.

The resemblance of the tributaries and gullies to the Main Channel only holds for the lower parts of their courses where they cross the sand flats. The sediment at Station 24 is a well-sorted fine sand as is VII A on the transect, resembling closely


samples from the Main Channel. Further up the gullies towards the marsh bimodal features appear and where the channels cross the low marsh the deposits become poorly-sorted fine sands with high silt values, Station 39 on the Clettwr and Station 30 on the Leri are strongly bimodal muddy sands; Station 38 similarly has high silt values attesting to the effect of local high velocities and reworking of the marsh deposits. The gully streams in particular with their meandering courses rework and redeposit the marsh sediments frequently at high angles (Evans 1965).

(f ) Some general conclusions concerning environmental controls The penetration of marine water into the estuary exerts a physical and chemical

control over sedimentation as indicated by the different clay/silt ratios that occur on the marshes towards the estuary mouth compared with the river end and by the build up of the marshes largely on the south side. Sediment analysis has established that skewness reflects the dominance of marine conditions in the lower estuary demons- trating that statistics can be used to infer environment. Living assemblages (particularly foraminifera-almost confined to the marshes) show a dramatically different distribution compared with dead assemblages. This is related to the measurable physical variables, water chemistry and circulation. The different factors are related but salinity, currents and pH appear to be the most important. There appears to be a direct causal relationship between the location of living foraminiferal assemblages on the marshes and the maintenance of high tide salinities there. Calcareous foraminifera are characteristic of the low marsh, an area covered by ordinary tides with salinities generally above 20%,. Agglutinating species tend to replace them towards the river end and on the high marsh, an area which is generally exposed with lowered salinities, below 20%,, and with lowered pH. The lack of living forms in the main channel can be ascribed to extreme salinity variation with high turbulence and low pH. During the ebb, fresh acid water flows down into the upper estuary and on the spring tide when salinity stratification is weak or destroyed by turbulence salinities below lo%, can occur as far down as the estuary mouth. Currents are strongest in the main channel, reaching up to 2 knots (1 m/sec) 2 hours before high water springs and 3-4 knots (2 m/sec) after highwater, with strong turbulence and formation of mega ripples. As might be expected current sorting of dead faunas is most marked in the main channel and on the open sand flats, and foraminiferal tests hydraulically equiv,alent to the silt fraction are winnowed from the sands and deposited upon the marshes where currents fall to 'zero', particularly at high water slack.

7. BOREHOLE EVIDENCE OF THE ESTUARINE SUBCROP Auger bohngs taken through the present salt marshes (Adams and Haynes 1965)

show that the deposits of the different environments are sheets reflecting cyclic development with the building out of the high marsh over the low marsh. This is emphasized by the close similarity between the frequency diagrams of foraminifera for the marsh transect and the diagrams previously published for the latest marsh cycle. Auger borings into the Scrobicularia clays below the Fossil Forest showed that these deposits also represent a cycle of marsh development that occurred in late Boreal and early Atlantic times, the Forest Layer having an absolute date of 6,000 B.P. (Godwin and Willis 1961). Since this early work we have been able to study samples from two deeper borings drilled at Borth, in one case to 95 ft (28.96 m), which allow a reassessment of the microfauna of the Scrobicularia clays and also investigation of the thick sequence of underlying grey silty sands. The location of the boreholes, strati- graphical sequence and foraminifera distribution are given% Figs 15, 16 and 17.


c-

0 0 0 0 a a

.O .

.

.

. - . - . I . . s

. . - 0 @@am 0 oocn a .am

.I . -. I . .

m . .. m m . . -. m- . .

0 . - I

I . . . . .

m I .V P

252 5. HAYNES AND M. DOBSON

pL ~ p o t " 1

35' ;?2 . . . . . . . . . . . . . . . . . . . . .>( . . . . . . . . . .

BARREN ZONE

.I--- C9Q a 6 SQ =>68 SYMBOLS 0.1 - -= 2-5 "6-20 0-20-50 ()=51-X10 .=201-500 .->W ,

Fig. 17. Frequency diagram showing distribution of sediments and foraminifera in a borehole near Tinks Shop, Borth, Borehole 2 on Fig. 15.

The sediments were analysed by the same procedures as those adopted for the marsh transect samples. It has been possible to indicate from the results of the analyses, and by using the same interpretation criteria, the particular subenvironments of deposition. Subsequent microfauna analysis has been used to verify and define more clearly the subenvironments present. Two subenvironments occur below the peat bed, high marsh represented by the Scrobicularia clay and low marsh as grey silty sands. The two subenvironments are clearly defined in borehole 3, even though sample D7 has a composition that places it as lower high marsh, and D8 from the same borehole has an upper low marsh composition. In borehole 2 the sediment ratios in the grey silty sand are closely similar to ratios found on the high marsh. In this case therefore, it reflects a higher position on the marsh profile.

The term Scrobicularia clay has been retained because of the stratigraphic significance of the term, even though in this case it is a clayey silt. It is a blue grey clayey silt that is distinguishable from the underlying grey silty sand. The clay values compare well with the present day high marsh values, being between 15 and 20%. Fine and coarse silt values are high with little fine sand, fine silt (6 to 9p) accounts for 30 % of the sediment, whilst fine sand values (4q) rarely exceed 10%.

Apart from a horizon of shale fragments found in borehole 2 and considered to be hill wash, the rest of the material, particularly that found in borehole 3, is a remarkably uniform deposit of grey silty sand. The stable composition is particularly well displayed in the very fine fractions, material finer than 9p represents 7 % of the sediment and only increases to 10 % at the base of the borehole. Similarly between 8 % and 9 % of the fine silt (6 to 9p) is present. Coarse silt and fine sand account for the rest of the sediment, and again the percentages are stable. Borehole 2 has a similarly uniform display, but with higher silt and clay values.


The charts showing the frequency distribution of foraminifera in the boreholes confirm the results of the auger borings into the Scrobicularia clays showing an entirely arenaceous fauna immediately below the Forest layer, identical to the higk marsh fauna of the transect, passing down into a low marsh fauna dominated by Ammonia beccarii var. tepida (lumped with var. batavus in 1965), Protelphidium depressulum, Elphidiurn excavatum and Elphidium selseyense (E. orbiculare of 1965), together with Buccella frigida. N.B. the counts are based on foraminifera retained on the 100 sieve from samples of 50 g original weight. This must be borne in mind when comparing them with figures for the transect based on foraminifera retained on the 200 sieve from 10 ml original weight. This results in a lower total of rare species for the borings, but the essentially similar low marsh character of the fauna is still revealed. The samples are from cased borings and include penetration samples taken with open spoon. The foraminifera1 fauna in the underlying grey silty sands is the same as that in lower part of the Scrobicularia clays and is essentially of low marsh character to the bottom of each boring, being interrupted only in borehole 2 by the barren zone of hill wash. Borehole 2 differs slightly in the greater persistence of arenaceous species indicating the proximity of the high marsh front, as is shown by the character of the sediments. This means that for a long period sediment accumulation kept pace with marine transgression. I t is known from pollen analysis (Godwin 1956) that the main post glacial sea rise took place in the Boreal, pollen zones V and VI, and had reached about 60 ft (18.29 m) below present sea level in the Swansea area by the middle upper Boreal, zone VIb (Godwin 1940). It is therefore, probable that the grey silty sands, which accumulated near sea level, represent most of the late Boreal from about 8,000 B.P. until the standstill at 6,000 B.P., represented by the Forest layer. Further evidence is provided by the foraminifera which although essentially the same as on the present tidal flats 6f the Dovey include Ammonia beccarii var. tepida as a dominant form with A. beccarii var. bntaws, the dominant living subspecies at present, making up less than 1 % of the plexus. Bradshaw (1957; 1961) has shown that var. tepida is a warm water subspecies with optimum reproduction between 25°C and 30°C. The lowest temperature at which reproduction occurred was 20°C and at this lower limit specimens took four times as long to reach reproductive maturity. As shown by the graph (Fig. 6) temperatures on the intertidal flats rarely rise to the optimum range for reproduction of this subspecies and presumably for this reason it is replaced by the temperate subspecies batavus.

Considerably higher maximum summer temperatures are, therefore, indicated during the time of accumulation of the grey silty sands. Godwin (1956) has summarized floral and faunal evidence to show that mean summer temperatures were 2.5”C higher during the late Boreal and early Atlantic pollen zones VI-VIIIa. This would have been sufficient to bring summer temperatures on the intertidal flats into the optimum range for reproduction of repidn. This species has recently been recorded living in N. France (Rouvillois 1967). The fauna thus supports the idea that the grey silty sands accumulated during the Climatic Optimum. As pointed out earlier by Adams and Haynes (1965), the pre Fossil Forest faunas also differ from the present marsh faunas in the relative abundance of Buccella frigidn and scarcity of Bulimina gibba. It is also significant that Macfadyen (1942) in a large fauna from equivalent deposits in Swansea Docks recorded specimens hitherto known only from the West Indies, Nonion grateloupi and N . sloanii, among 24 species “more a t home in warmer waters”. These presumably represent swept in exotics as the dominant species are intertidal, including Protelphidium depressulum, Elphidium excavatum and Ammonia beccarii. A similar but meagre marsh fauna was described by Jones and Chapman (1896) from deposits of the same age at Barry Docks. The chief difference

’

254 J. HAYNES AND M. DOBSON between the sequence in these areas, and that in the Borth boreholes is in the presence of numerous intercalated peat beds. The boreholes are on the seaward edge of the estuary presumably at locations always below high tide level though the Boreal. Borings put down more to the east, on the raised bog, may be expected to reveal a sequence more like that studied in South Wales.

If it be accepted that the grey silty sands accumulated during the late Boreal, which commenced approximately 8,000 years ago, then the sequence in borehole 3 was built up in 2,000 years representing an average sea rise, and sedimentation rate of 4 ft (1 -22 m) a century. Towards 6,080 B.P., the sea rise fell below the sedimentation rate, allowing high marsh to build out over the sites of the boreholes; the cycle ending with the passage to a forest climax during the post-Flandrian marine standstill. Accretion on the present Dovey marshes varies from a mimum of 1 ft (0-3 m) in 60 years at the river end to 1 ft (0.3 m) in 30 years at the sea end, an average of 1 ft (0-3 m) in 45 years (Richards, summarized in Chapman 1960). The rate indicated for the grey silty sands is thus about twice the present average and greater than at the seaward end of the marshes. Foraminifera frequencies tend to lend support to this idea. The charts presented in 1965 show that frequencies are generally much higher in the deposits of the latest marsh cycle, compared with the Scrobicularia clays. Frequencies are similarly lower for the boreholes, a sign of greater dilution by sediment. The 1965 charts also show that the frequencies in the Scrobicularia clays as well as in the deposits of the latest marsh cycle increase northwards towards the present estuary. This can be explained as due to diachronous out building of the marshes towards the north, with fall in sedimentation rates as silting up occurred along the banks of the main channel.

Cardigan Bay has been an area of relative crustal stability during postglacial times and marsh building in the Dovey estuary can be regarded simply in relation to the eustatic sea rise with standstills at 6,000 and 1,500 B.P. On the other hand Churchill (1965) has shown that whereas the Forest layer at Ynyslas is now at -2 ft (-0.61 m) O.D. equivalent deposits in Swansea Bay are at -10 ft (-3.05 m) O.D. This is explained as due to a relative crustal upwarping of about 10 ft (3-05 m) since 6,000 B.P. in the Cardigan Bay area. A sea rise of 20 ft (6.1 m) is indicated for the latest marine transgression if we assume the position of the Forest Layer to be undisturbed. If 10 ft (3.05 m) must be added to allow for a countervailing upwarping of the crust, then we must assume that at 6,000 B.P. the shoreline was at the present 5 fathom (9.1 m) line. Some of the apparent sea rise might be accounted for by com- paction, but against this is the lack of Boreal peats in the boreholes.

Acknowledgements. The authors wish to thank the Staff and Technicians of the Geology Department, particularly Dr. R. C. Whatley and Dr. A. V. Bromley for their valuable assistance. Information from the Gwynedd River Authority and the Water Pollution Research Laboratories is gratefully acknowledged.

APPENDIX-METHODS A modified N.I.O. salinity bridge was used to measure the salinities of all the samples, except

where only very small amounts of sample were collected, as when analysing the thin sheets of water left behind on the marshes between tides. In this case the Harvey Method was emp!oyed, in which 10 ml of sample are titrated with a solution containing 27.25 g of silver nitrate per Iitre. The volume in ml of silver nitrate required roughly equals the salinity of the sample; potassium chromate is used as an indicator of the end point.

Detailed vertical salinity and temperature profiles were made at Aberdovey, on a traverse extending from Aberdovey to the south shore, and at other stations as far up as the Glandovey Rail Bridge. The direct reading instrument was a Model RS5-3 portable salinorneter made by Industrial Instruments Inc. with an accuracy of kO.3 %. This instrument was calibrated before and after use in standardised Aberystwyth sea water.


A variety of methods for salinity determination have been used, including titrations on samples stored in different kinds of containers. 800 water samples were collected, 600 in glass bottles and 200 in polythene bottles. For this reason, charts and diagrams have been drawn to the nearest part per thousand only.

A portable Pye pH meter was used to measure the marsh water and because of the possibility of fluctations this field meter was regularly checked against a Pye Dynacap laboratory pH meter. This laboratory meter was further used to measure the pH from samples collected as part of the salinity investigations.

Because of the high concentrations of magnesium and calcium in sea water, these elements were determined by methods employed in rock analyses (Riley1958). Interfering elements, except manganese, were removed by extraction of their oxinates at pH 4.9-5.1 with chloroform; the magnesium and calcium were then determined in the extracted aqueous solution by titration with E.D.T.A. with Eriochrome Black T as indicator. Eight determinations were completed for each sample, four for calcium with magnesium, and four for calcium alone. No correction for manganese interference was included in the final calculations. All samples were subsequently analysed with a Unicam SP 90 atomic adsorption spectrophotometer. The diagram (Fig. 7) only contains data from this second method, as results from the first method were comparable.

A Direct Reading Kelvin Hughes Current Meter with a direction accuracy within 14' and a velocity accuracy of 2+ % was used for all current measurements. The velocity investigation was concentrated around theestuaryentrance wheresurges of up to 3 knots (1 -5 m/sec) made it necessary to have the instrument weighted at all times. Where possible bottom, mid and top readings were taken.

The sediment samples (surface skims) were collected by hand and by the use of Normalair aqua- lung equipment in the deeper parts of the main channel and estuary mouth. Cuts of these samples for microfaunal analysis were either examined immediately or stored in 10 % formalin buffered with borax at 20 g per litre. Rose Bengal stain at concentrations of 1 g per litre was used to enable living foraminifera to be distinguished. Small cores were obtained by driving 3 in. (7.6 cm) plastic tubes (Erinoid) 2 ft 6 in, (0.76 m) into the sediment with a belaying pin. Water was then poured into the top of the tube which was then corked and made air-tight to allow withdrawal without loss of the core.

The sediment samples were analysed by wet-sieving with B.S. sieves at +p intervals and any material finer than 4q1 was analysed by the Andreason pipette technique, sodium hexameta- phosphate being used as a dispersal agent. The thin sections were made as described in Moore and Garraway (1963) and the results are largely derived from the work of Moore (1964).

A Philips X-ray diffractometer with cobalt filter Cu K radiation was used throughout. Runs were made from 4" to 80" 26 at a rate of 1" per minute. The X-ray samples were smears made during the pipette analysis and evaporated on prepared glass slides.

Temporary levelled tide boards were erected at Aberdovey Jetty and at Panteidal to assist in the general tidal studies. All mid-estuary work was carried out with a Dunlop rubber boat and 18 hp outboard.

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GEOLOGY DEPARTMENT, UNIVERSITY COLLEGE OF WALES, ABERYSTWYTH.

physiography, foraminifera and sedimentation in the dovey estuary (wales)

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