5. Sea Level Processes and Effects

of Sea Level Change

Importance of sea level positions Sea Level processes and indicatorsCoastal morphology and the Recent rise in sea levelIce-driven sea level fluctuations Tectonically driven sea level fluctuations

Importance of sea level positions

Important to know the position of sea level Depth of deposition dominates the major facies patterns of the material accumulating on it

Also …the size distributions of clastic sediments, the chemistry of biogenous and authigenic matter, the distribution of benthic organisms.

Two kinds of sea level fluctuations

Global (eustatic)

Regional (tectonic)

Global (eustatic) Fluctuations

produce contemporaneous transgressions and regressions on the shelves of all continents

originate from changes in the volume of ocean water or in the average depth of the ocean basin

Examples are changes in ice volume, or changes in sea floor spreading rates

Regional (tectonic) Fluctuations

consist in transgressions and regressions on one particular shelf

produced by regional sinking or uplift of the shelf.

Effects on a smaller scale

Where the sea level intersects the continental margin, physical, chemical, and biological processes are of high intensity

Waves, tides, and currents show maximum activity

The productivity is great

Sea level is the baseline of erosion and deposition: exposed areas are eroded and submerged areas build up; determine the coastal morphology.

On a larger scale

1) determines the degree to which shelves are submerged Submerged shelves absorb much more sunlight than exposed ones. Climate is mild during times of high sea level, harsh during times of retreat of the ocean into its basins. closely tied to paleoclimatic evolution.

2) the rates of erosion of the continents and the sites of deposition in the ocean How sediment is transported to the deep ocean is controlled by sea level. During high stands, the transport of sediment by turbidity currents will be reduced. Turbidity currents depend on supply of mud to the outer edge of the shelf. This supply is highest when sea level is low, and is greatly reduced during high stands when the submerged shelves trap the material delivered by rivers. Thus, the types of sediment bodies found at the base of the ocean margins should depend on the history of sea level fluctuation.

Fig.5.1 Sea level fluctuations; calendar of geologic history. The diagram shows a  section of the late Paleozoic cyclothem successions in Kansas, which consist of alternating marine and nonmarine sediments. Description of facies to the left; interpretation in terms of depositional environment to the right. The interpretation uses "Walther's Rule" introduced a century ago, which states that only those sediment types that are neighbors geographically will be neighbors in stratigraphic succession.[R. C. Moore, as drawn by J. C. Crowell, 1978, Am J Sci 278: 1345]

Fig.5.2 Wave-cut terrace at Enoshima, Pacific coast of central Japan[photo E. S.]

5.2 Sea Level Processes and Indicators; Wave Action

wave-cut terraces and beach deposits

sea cliffs as the result of wave action

In southern California, rates of cliff retreat are between 1 ~100 feet per century.

Sea Level Processes and Indicators; Wave Action

Waves transport and rework the sediment, changing its texture by sorting and influencing its structure (ex. ripple marks)

Thick enrichment of heavy minerals (placers) on a beach

Well-sorted skeletal remains (shell pavements or coquina)

Fig. 5.3 a-c. Ripple marks as wave indicators. a Oscillation ripples. wave length about 5 cm, with Arenicola fecal strings; tidal flats. North Sea. b Oscillation ripples, wave length 30cm. Top of Sylvania Sea Mount near Bikini Atoll, depth 1500m, calcareous ooze. c Internal structure of current ripples, sandy tidal flats. German North Sea Coast. Frame is 22 cm wide

Wave Action

Waves usually influence the sea floor only down to about 10 to 20m.

Responsible for producing certain types of sediments such as polished beach shingles and calcareous oolites.

TidesThe tides, those long waves made by the Moon's daily passage The amplitudes of the waves are highest close to the coast. If the distribution of their frequencies and amplitudes can be reconstructed for the geologic past, we can obtain important clues about changes in the Earth-Moon system, and about basinal morphology. The classic sea level indicator facies is represented by the whole of the deposits of the intertidal zone, which is alternately submerged and exposed by the tides. Sinking coasts with high sediment supply tend to have large intertidals, and hence accumulate large intertidal sediment bodies (wadden)

Diurnal & Semidiurnal Tides

Diurnal Tide

Semi-diurnal Tide

Fig.5.4 Tidal records from harbor cities. At bottom the dates (march 1936) and the phases of the moon.

Tides and storm action: the intertidal zone

Small sea level changes can quickly lead to submergence or emergence. Storm play an important role in producing such change. The storm flood created new access for the tides into the salt marshes and moors lying behind natural barriers, inland from the intertidal flats. Slight changes in sea level, and the action of heavy storms, bring marked changes in the type of sediment deposited.. The record which is preserved on the slowly sinking floor consists of an intercalation of peat, salt marsh deposits, marine muds, and the sands and shells of the beach. Storm deposits are common within the intercalation.

Fig 5.5 a, b. Desiccation cracks in algal mats, in a lagoon on the west coast of Baga California near San Quintin. a View from a sand dune, toward inner lagoon. Marsh vegetation, stressed by high salinity from evaporation, gives way to algal mats  toward the intertidal. b Detail of algal mats(section 2 m wide). The mats separate into polygonal pieces and curl upon drying. Upturned rims collect evaporites.[Photos E. S.]

Sedimentary structures in intertidal flats Desiccation cracks

rain drop impressions

pseudomorphs of cubic halite crystals

precipitation of gypsum,

tracks of land animals on sediments with marine organisms.

PhotosynthesisPhotosynthesis can only proceed when sufficient light is available.

The light intensity drops to 1% of the surface value anywhere between 10 and 200 m water depth, depending on the clearness of the water.

Sessile plants, such as the geologically important calcareous algae and algal mats, generally occur no deeper than 100 m or so.

Animals living in symbiosis with algae also indicate shallow water.

large foraminifera, stone corals, and certain molluscs.

Photic zone & aphotic zone

Fig.5.6 a, b. Sea level rise during deglaciation. a Various hypotheses of how the sea level is supposed to gave risen during deglaciation (about 15000 to 9000 years ago), and aftrerwards during the Holocene. Dates are based on radiocarbon determination.[E. Seibold, 1974, r. Brinkmann (ed). Lehrbuch der allgemeinen Geologie. wol 1, 2nd ed, F. Enke, Stuttgart]. b Results from thorium-uranium dating on corals (Acropora palmata off Barbados) suggest that these radiocabon dates are too young, by about 10% for 10000 years, and 20 % for 1500 tears. Two major pulses of sea level rise are indicated

5.3 Coastal Morphology and the Recent Rise in Sea Level- General effects of recent sea level rise

The recent rapid rise in sea level which began about 15000 years ago and lasted till about 7000 years ago.

The sea level rose in response to the melting of glacial ice.

The maximum addition of water to the ocean occurred between 13000 and 9000 years ago.

General effects of recent sea level rise

The sea level changed by approximately 130 m on the whole; that is, it rose from the average depth of the shelf edge to the present position.

In temperate humid regions, coastal peat bogs grew upward and became flooded by salt water and covered by marine sediments.

Dunes were eroded away by the approaching surf.

Resistant matter left from the erosional process collected as a transgression conglomerate.

The basal conglomerate typical of many marine transgression sequence in the geologic record.

Fig.5.7 a, b Sea level rise and East Coast morphology. a Drowned river valleys, Cape Cod. Barrier beaches migrate into the entrance of the estuary, as spots, and restrict access to the open sea.  b Closed-up of a migrating spit, Buzzard's Bay. Migration is to the left. note forced detouring of the tidal channel

Example for the effects of the rise of sea level

A coast of drowned rivers. Longshore sand transport tends to close off the estuaries which fill with sediment to make marshlands. Relict sediments

Fig.5.8 Subaerial delta shapes as a function of sediment supply and distributional energy (waves, current).

River Mouths

• Why a delta forms and not an estuary depends of many factors.

• for instance, low tidal activity as in land-surrounded seas (Mississippi and Nile delta), high sediment load due to strong seasonal rains, and high erosion rates in the mountain hinterland (the Indus, Ganges, and Irrawadi deltas).

Tidal motion can reach far into the estuaries.

Salt water intrudes along the bottom.

This intrusion of the marine realm brings marine sediments upriver.

Why then are the estuaries not filled in?

River floods, tidal action, and especially the young age of estuaries are the cause.

The drowned river valleys produced by the transgression have not yet come to equilibrium with the sediment load.

Fig.5.9 a-f. Schematic cross-section through a birdfoot delta. a Inter-levee lowlands, marshes (levees are ridges confining the distributary channels); b delta front; b delta front; c prodelta; d open shelf floor; e older base(may or may not be deltaic); f distributary channel with fill and levees. A Delta builds up and out, the sea retreats, the sedimentary sequence is  " regressive" (a across b, b across c, etc); B The sea level rises and the delta retreats, the sedimentary sequence is "transgressive“ (a below b, below c, etc). Note that the regressive  or transgressive nature of the sequence cannot be seen on the surface, but only by studying the sequence of sediments within the delta.

Fig.5.10 The bird-foot delta of the Mississippi. The subsidence of the delta allows the sea to re-invade the areas of abandoned distributaries[J. Gilluly et al., 1968, Principles of geology. W. H. Freeman. San Francisco, after H. N. Fisk]

Recent Rise in Sea Level

If sea level falls, the regression is accelerated.

Conversely, a rise of sea level or a sinking coastline will ideally produce a transgressive sequence.

The recent rise of sea level resulted in the predominance of transgressive sequences on the top of the deltas of the world.

Lagoons and Barriers

A facies zonation parallel to the coastline. Offshore bars, or barrier islands occur together with a sand beach. The beach may be backed by dunes, which the wind pile up using sand from the beach. This beach-dune complex may form a barrier for interior lagoons.

Fig.5.11 Typical geomorphic features of a barrier-type coast. [H. E. Reineck, I. B. Sin 호 , 1973. Depositional sedimentary environments. Springer, Heildelberg, based on a diagram by C. D. Masters. 1965]

Fig 5.12 Overwash fans on St, Joseph's Island, Texas, produced by storm waves, Open Gulf to the left, lagoon to the right.[Photo courtesy D. L. Eicher]

Lagoons and Barriers Rivers emptying into the lagoons may cut on or several channels through such barriers, especially when flooding, chopping them up into a series of barrier islands.

From the seaward side, storm waves may break through the barrier, forming overwash fans on the lagoon beach.

Tidal action keeps such channels open, building deltas on both sides.

Fig 5.13 Architecture of Galveston Island. Regressive sequence below the barrier beach near Galveston, Gulf of Mexico. Advance of  barrier island due to high supply of sand and relatively stable sea level. The dates (1200, 2000, 3500), are based on the 14C content of shells.

How does barrier-and-lagoon morphology reflects changes in sea level?

The balance between the rise of sea level relative to the land and the supply of sediment to the coast is the crucial factor.

Since the sea level stabilized, about 6000 years ago, supply of materials has been important.

Fig5.14 a, b. Mangrove vegetation on Bimini (Bahamas). a Rhizopora at low tied. The roots are covered at high tide. Plant about 2m wide. b Avicennia with air roots. Width of view ar proximal edge 2.5m

During the recent rise of sea level, the various facies zones paralleling the coast migrated landward. Landward invasion of the mangrove swamps. Expansion of mangrove swamps during deglaciation due to a rising sea level, must have produced an organic-rich layer on many tropical shelves. Such layers will coalify.

5.4 Ice-Driven Sea Level Fluctuations- The Würm Low Stand

Sea level has been constantly changing over the last several hundred thousand years as a result of the waxing and waning of large continental ice masses. About 17,000 years ago, the last major ice age (the Würm in Europe, the Wisconsin in North America), enough water was tied up in the continental glaciers to depress sea level by some 130 m. Large shelf areas fell dry as a result of the ice-caused regression of the sea. Rivers crossed the shelves and entered the sea at the shelf edge, cutting backward into the shelf. Their sediment load was dropped in a narrow zone at the upper slope, became unstable there and slid, starting turbidity flows which rushed down the undersea canyons.

Fig.5.15 a, b. Fluctuation of δ18O in the planktonic foraminifer Globigerinoides sacculifer,  western equatorial Pacific. a "Brunhes": present normal magnetic epoch, since 790000 year ago. "Matuyama": previous epoch, during which the Earth's magnetic field was reversed. Eight isotope cycles are visible within the Brunhes; the average duration is therefore 100000 years. The temperature of surface waters is nearly constant through rime, in the area where the core was taken. Hence the isotope fluctuations are produced largely by the build-p and decay of northern ice masses, as shown in b.[Core data from N. J. Shackleton, N. D. Opdyke; 1973. Quat Res 3; 39]

5.4 Ice-Driven Sea Level Fluctuations- The Pleistocene Fluctuations

Major transgressions were much more rapid than any of the regressions.

The build-up of ice (and the sea level drop) proceeded at a slower rate than the melting of ice. The evidence for this concepts rests on the oxygen isotope record of pelagic foraminifera.

The ratio of the two isotopes in the shells is in equilibrium with the ratio of the isotopes in the water in which the shells grew.

If the 18O/16O ratio in the water changes, it will change similarly in the shell.

Every time the sea level drops, the 18O/16O ratio of the water increases, because the glacial ice is made of water which is impoverished in δ18O.

Seawater is enriched in δ18O during glacials.

The effect of temperature on the isotopic signal can be neglected in the present context.

Fig.5.16 Daly's Glacial Control Theory of coral reefs. The time sequence is from 1 (124000 years age) to 5 (the present). The ring shape of atolls is due to the more favorable situation for coral growth at the edges of the island (due to cleanness of water, and high food supply). The knolls in the lagoon grow up on slightly elevated, mud-free ground.[R. A. Daly, 1934 The changing world of the ice age. Yale Univ. Press, New Haven]

Effects on Reef Growth Each highstand of the sea level resulted in a build-up of reef carbonates, while the lowstands resulted in erosion.

Results show that the uplifted corals grew during several high sea level stands, namely 124,000 years ago, at 103,000 years, and at 82,000 years.

The strength of the sea level fluctuations increased at 6 million years ago (in the latest Miocene) and again about 3 million years ago, when northern glaciation set in.

5.5 Tectonically Driven Sea Level Fluctuations

Sea level fluctuated considerably all through the Phanerozoic, even during periods when apparently no ice was present.

As to the present sea floor, fluctuations since the Jurassic are of special interest.

The thick sediment stacks in the "passive" continental margins have been much studied for economic reasons.

When a margin sinks more or less continuously, coastal sediment bodies must reach great thickness, provided the sediment supply keeps pace with subsidence, and deposition remains locked in to the sea level.

The significance of the sand bodies. They can retain great quantities of water, petroleum, or gas. In as much as the sea level variations within this geologic period were not driven by the growth and decay of ice caps, they were not reflected in the isotopic composition of seawater. We cannot, therefore, find them in the isotopic composition of the foraminifera in the manner indicated earlier. How then can we measure these fluctuations?

Reconstruction of sea level changes

The intensive world-wide exploration of continental margins by seismic profiling.

The sediment-stacking patterns in margins of different ocean basins are quite similar.

They must be due to global sea level variation.

Vail and Mitchum derive sea level fluctuations from the geometry of sediment layers, as recognized on seismic reflection records.

During a fall of sea level (regression) the reverse occurs, and erosion sets in on the shelf.... hiatus.

Reconstruction of sea level changes

Fig.5.17 Effect of sea level position on depositional pattern at the continental margin. Highstand and lowstand of sea level. [P, R. Vail. et al., 1977, Am Assoc Pet Geol Mem 26:49]

Fig.5.18 Relative changes of sea level as deduced from the geometry of margin sediment bodies. The underlying model (solid line) is that gives in Fig.5.17.

Fig.5.19 Reflection seismic line offshore Northwest Africa. The Triassic (TR). Jurassic (J), and Cretaceous (K) Supersequences are separated by dotted lines, Arrows indicate marine on- and offlaps. During the Jurassic six seismic sequences are separated by minor unconformities. With the beginning of the Cretaceous, carbonate sedimentation including reef building at the shelf margin (e. g. j2.1) ends. The subsiding continental margin after the opening of the Atlantic has been covered afterwards by thick sequences of deep-water shales (K1.2) and deltaic sandstones. They were truncated by erosion with the beginning of Tertiary (T). Complicated Tertiary sequences. Faults marked by half arrows The fundamental pattern is similar to that observed in the Gulf of Mexico

Reconstruction of sea level changes

The "Vail sea level curve" is a useful tool for the correlation of seismic stratigraphies of continental margin sediments. To what extent this curve reflects the true global sea level variations is not known. One problem is that the rate of sediment supply must play an important role in controlling the geometry of the sediment bodies. Another is that the rate of sinking of the margins must be considered.

Fig.5.20 Relationship between volume of Mid-Ocean Ridge and spreading rate. Upper cross-section of ridges spreading at 6 cm/yr (left) and at 2 cm/yr (right); lower cross-sections of the same ridges 20 million tears after a change from 6 cm/yr to 2cm/yr (left) and a change from 2 cm/yr to 6cm/yr (right). The changes in ridge volume must produce corresponding changes in sea level.[W. C. Pitman. 1979, Am Assoc Pet Geol Mem 29:453]

The Causes of ChangeThis can be done by increasing the total mass of new lithosphere formed per year, that is, by increasing sea floor spreading rates or by increasing the length of the Mid-Ocean Ridge and the trenches, or both

There are indications, from the magnetic stripes on the sea floor, that global spreading rates in the Late Cretaceous may have been much higher than now.

It has been proposed that the high sea level stand of the Late Cretaceous were caused by such a fast spreading rate.

Mountain-building with shallow ocean crust, or with continental crust, is one way Shallow crust (which is stacked up within mountain rages) is removed and replaced by deeper sea floor which will cover itself with a thicker layer of water, drawing down the general sea level.

The quickest way to change sea level is to fill or to empty an isolated ocean basin. The Mediterranean dried up intermittently between 5 and 6 million years ago. The water had to go elsewhere: global sea level was raised by about 10 m whenever the Mediterranean dried up. Conversely, when ocean water rushed in to fill an empty basin, global sea level must have dropped by the same amount.

Other Causes

The postage stamps reflect the international concern about the fate of Venice. They picture the Palace at the Canale Grande and San Marcus Cathedral threatened the sea.

5.6 Sea Level and the Fate of Venice

Fig.5.21 Amount of subsidence of ground in the vicinity of Venice. Upper left Location Map of Venice and surroundings; Right subsidence profiles along the lines A B C (upper) and D B E (lower). Difference in elevation (in mm) refers to period between 1952 and 1968. Treviso (E) is taken as fixed point for the geodetic measurements; hence its change in elevation (if any is defined as zero. Note high rates of subsidence (about 7.5 mm/yr) in the industrial region of Marghera (near B) and in the Po delta near Chioggia (D). The center of the city of Vencie subsided by about 80mm. i. e., 5mm/yr. [ Data from R. Frasetto, 1972, CNR-Lab Stud Din Masse Tech Rep no 4.]

Fig.5.22 Recent rise of sea level in two regions. a Europe and Africa. b West coast of North America. Points are regional averages of the annual sealevel anomaly. Long-term trends fitted by eye (note difference in slope). [T. P. Barnett, 1984; J Geophys Res 89; 7980; trends added]

Fig.5.23 The flooding of San Marcus Square by more than 1m in November 1966. [Photo courtesy A. Stefanon, Venice]