EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 1 1 , 5 3 3 5 4 3 (1986)

CAVE SEDIMENTATION IN THE NEW GUINEA HIGHLANDS

DAVID GILLIESON

Departmen! of Geography. Australian Defence Force Acodemy, University of New Sowth Wales, Canberra, Australia, 2601

Received 18 January 1985 Revised 9 September 1985

ABSTRACT

This paper investigates the nature and processes of sedimentation of allogenic cave deposits in the high relief, everwet karsts of montane New Guinea. Under the high intensity rainfall regime, episodic mass movements in small karst catchments provide a wide range of sediment textures from clayey gravels to fine clays. These allogenic sediments are deposited into pools of water within the caves, giving sedimentary structures analogous to turbidites. Diamictons within the cave relate to episodic mudflows in the catchment. These deposits move as fluidized masses in a manner similar to some esker deposits. Cross-stratified sediments are formed by dumping of pulses of sediment laden water into deep pools. Extremely fine-grained clays and muds accrete parallel to underlying surfaces following flood pulses. These deposits represent the last phase of catchment instability, when a small amount of slopewash occurs. Catchment processes are dominated by solution and episodic mass movements. When the thick root mat which masks the ground is disrupted, some slopewash occurs but it is not a major component in catchment processes.

KEY WORDS Cave sedimentation Tropical karst New Guinea Sedimentary structures

INTRODUCTION

Cave sediments have long attracted scientific interest, principally for their included macrofauna and microflora. Thus the contribution to Quaternary studies is well established (Butzer, 1982 79). However, study of cave sedimentation processes, and their implications for surface environments, have been relatively neglected. In this context the works of Brain (1967), Bull (1980,1981) and Renault (1968) are outstanding. In particular, the latter author’s examination of the interrelationships between cave sedimentation and genesis provided a major conceptual advance which was, however, neglected in the decade following its publication. The widespread unctuous clays of cave deposits have been studied by Bull (1981) and Frank (1975), and a ‘settling tank’ mechanism invoked for their deposition.

In contrast the sediments of humid tropical caves are virtually unknown. Beck (1 975) provides one of the few accounts of tropical cave sediments, but this work concentrates on allogenic gravels. The fine sediments of tropical caves noted in general speleological literature are not considered, although recently Frank (198 1) has presented results of analysis of archaeological cave sediments in tower karst.

This paper investigates the nature and processes of sedimentation of allogenic cave deposits in the Highlands of Papua New Guinea. The caves lie in extensive, high relief karsts which for the most part are cloaked in primary rainforest. Rainfall is high, from 3500 to 9000 mm annually; the region is everwet. As a result, sinking streams have wide fluctuations in discharge, even diurnally. The magnitude of peak flows makes sampling of mobile sediments difficult, as the samplers would almost certainly be destroyed. Reconstruction of processes must therefore rely on inference from sedimentary structures and from laboratory analysis of sediments.

Underlying the extensive karst plateaux of the Hindenburg and Muller Ranges are large and long cave systems (Figure 1). The caves have formed in multiple overthrust Miocene limestone sheets, up to 600 m thick,

01 97-9337/86/050533 -1 1$05.50 0 1986 by John Wiley & Sons, Ltd.

5 34 D. GILLIESON

/Escarpment

River

rn Settlement . Cave

1 Selrninurn Tern

2 Bern Tern, lbulup Tern, Ok Kltkil Tern

3 Atea Kananda

Figure 1 . The Southern Fold Mountains of Papua New Guinea, with study sites

which are sandwiched between glauconitic sandstones and shales. Thus within the contributing catchment of each cave there may be fluvial sediments which range in texture from fine clays to fine sands and gravels. Small stocks of granodiorite contribute resistant pebbles and boulders.

The large canyons and tubes of Selminum Tem Cave have carried the past and present drainage of the Hindenburg Plateau to large springs at the base of the Hindenburg Wall. Five levels of cave passages extend over a vertical range of 150 m and have a total surveyed length of 20.5 km. In some passages spongework and solutional rock pendants, with an absence of scalloping, suggest erosion by slowly moving groundwater. Subsequent phreatic flow regimes have resulted in the formation of tubular passages 10 to 20 m in diameter developed down and aslant the dip. Deep sediment fills in these passages are capped by calcite speleothems which give a minimum age for the cessation of water flow at 18 OOO to 22 OOO BP. These passages formerly fed springs at the Hindenberg Wall, and flow against the dip is indicated from the perched phreatic tubes of the Hole in The Wall, a cave 265 m above the base of the Hindenburg Wall. Later vadose incision formed a large canyon, up to 30 m wide and high, with broad channel incuts and deep sediment accumulations. A detailed account of the cave’s geomorphology is given by Gillieson (1983, 1985).

Atea Kananda underlies the Muller Plateau and has 305 km of surveyed passage developed on four levels. The passages extend on a gradient down the northern limb of a syncline and are of nothephreatic (Jennings, 1977), dynamic phreatic and vadose origin. Some floodwater mazes have developed near active streamsinks, while in the highest levels of the case there are spongework mazes with solutional rock pendants and an absence of scalloping. This suggests that these uppermost passages were eroded by very slowly moving groundwater. At lower levels there are phreatic tubes and joint-guided passages which have been formed by fast moving water.

CAVE SEDIMENTATION 535

One such tube can be traced for over 6 km from the highest part of the cave system. Relict sediment banks in this passage are sandy muds and clays, and previously it was nearly blocked by them. The dominantly phreatic passages have been invaded by the Yu Atea River, whose discharge ranges from 4 to 20 m3 s- I . The Yu Atea has excavated large vadose canyons, perched on mudstone interbeds in the limestone. A thin parallel-accreted stratum of organic sandy clay coats the walls of all presently active passages, and can be seen in some abandoned passages.

Near the Telefomin patrol post, the Bem Tem caves form a series of past and present karst resurgences. Four passage levels are present and they are accordant with the upper surface and erosion terraces of late Pleistocene fan deposits which fill in the Sepik valley (Gillieson and Hope, in press). A stalagmite capping deep laminated sediments in Bem Tem gave a uranium series date of 12900+4OOO B.P. The speleothem was badly contaminated with detrital thorium, and this result must be regarded as a maximum age only. It suggests that the incision of the alluvial fan deposits, and related sequence of cave passage development, is of late Pleistocene age.

THE CAVE SEDIMENTS

The sediment sequences within the caves studied (Selminum Tem, Atea Kananda, Bem Tem) are made up of a number of discrete depositional events, each the result of a pulse of sediment laden water entering pools of standing water in the cave passage. The sedimentary structures present indicate a range of energy conditions from high (associated with mudflows) to low (associated with slow settling from standing water).

Diamictons are widespread in the Highlands of PNG, and the mudflows which produce them may move at high velocities for several kilometres down stream channels (Pain, 1975). Two diamictons are present in the main passage of Selminum Tem. One attains a maximum depth of 5 m in Coprates Canyon, a 30 m high vadose canyon. The section in Figure 2 is located 1.5 km from a streamsink by which it entered the cave. Underlying the section is limestone breakdown on a stream incut; the diamicton fills the interstices between the limestone boulders. The diamicton is a silty fine sandy pebble gravel which is matrix supported and unstratified. Within the deposit are small lenses of gravel and smudges (Kruger, 1979) of laminated muds. These imply that the diamicton overrode existing deposits during deposition. Above the diamicton is a lens of unstratified silty gravel with an erosional contact to the underlying unit. The silty gravel is itself overlain unconformably by planar cross-stratified pebbly mud. Alternate red oxidized and blue grey reduced laminae are the result of textural difference: the oxidized laminae are fine to medium quartzose silts, while the reduced laminae are very fine clays. There is no evidence of post depositional deformation. Numerous porphyritic clasts in this diamicton suggest that the source is likely to be the Kauwol stock on the upper reaches of the Ok Tedi (Figure l), some 15 km away. Within the cave this diamicton can be traced for a further 2 km, at which point the deposit is lost under roof fall blocks. The upper surface of the cross-stratified pebbly mud, upon which one walks, has been sculpted by water flow into large mounds. Some of these, especially near Orion Cavern, have surge marks up to 1 m long. Many of these are dendritic.

Also in Orion Cavern is a 4 m section in which a diamicton is exposed. At the base of the section the diamicton rests on a bedrock bench. It appears to have been deposited as foreset beds into standing water. Beds of avalanche front cross-stratified muddy fine sandy pebble gravel are graded into cross-stratified silts and clays. Thin smudges of gleyed clay are present. The diamicton is unconformably overlain by low angle cross- stratified fine sandy mud, with a reversed inclination in the beds. The sequence is in an embayment of the passage, which here is a 20 m diameter tube. Some shift or eddy in flow direction is likely. Elsewhere reworking of the diamicton by small percolation streams has produced lenticular stratification within the diamicton, analogous to scour channel infills.

A second major diamicton up to 6 m deep is exposed by entrenching at Warp Drive, 3 km from the streamsink (Figure 2). Again it is a matrix supported fine silty pebble gravel but it lacks porphyry clasts, implying a different source. Thin beds of horizontally stratified fine sandy muds occur near the present top of the section. They have an unconformity with both underlying and overlying diamictons.

Diamictons are polymodal, therefore poorly sorted and reflect mobilization of a range of sediment textures (Figure 3) in a mudflow. Whereas the Selminum Tem diamictons are clayey cobble gravels to clayey granular

536 D. GILLIESON

TEXTURE & STRUCTURES 0

1

2

M

3

4

5

COLOUR

7.5YR5/6

matrix 2.5Y6/2

micro crass-stratified and low angle cross-strotified pebbly mud, with oxidised and gleyed laminae

lens of unstratified silty gravel

mudflow deposit - silty fine sandy pebble gravel, poorly sorted and unstratified, with slumped lenses of fine mud ond gravel

lens of unstrotified silty grovel

I imestone breokdown

ST34 COPRATES CANYON

COLOUR

5Y6/3

2.5\/6/2

2.5\/6/4 2.5Y6/4 to 10 Y R6/8 10YR5/6

2.5\16/2

TEXTURE & STRUCTURES

flowstone and stalagmites U/Th age 11 700 +/- 4300

cross- stratified slightly granular fine sandy mud

low angle cross-stratified fine silty well sorted sand

low angle cross-stratified and slumped slightly granular fine sandy mud to granular mud with oxidised and gleyed laminae I en t i cular s t rat i f ied granul or mud

graded, steeply cross-stratified muddy fine sandy pebble gravel

ST18 ORION CAVERN

Figure 2. Stratigraphy of two diamictons in Selminum Tem cave

gravels, those present in Bem Tem are derived from fine grained sedimentary rocks and are thus clayey sands to clayey granule gravels. For both types, the quartz sand grain micromorphology demonstrates the fluidized transport mode of mudflows. The grains are well rounded, with small fractures due to intergrain abrasive contact (Figure 4). Abraded edges, grooves and chattermarks (Bull et al., 1980) support inference of a matrix supported deposit moving as a fluidized mass. These features are absent on quartz grains from the limestone, sandstone, or shale, which either show angular fractured grains or angular grains with surfaces modified by

CAVE SEDIMENTATION 537

O I y i - - r - - n -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 1010.5

GRAVEL 1 SAND I SILT 1 CLAY @

Figure 3. Particle-size distributions of diamictons from caves in the PNG Highlands

diagenetic etching. The possibility of inheritance cannot be entirely discounted, however. The penetration of a cave passage by a mudflow for up to 3 km requires explanation, as a tendency to blockage of the passage might be expected.

Saunderson (1977) has proposed a mechanism for esker formation in subglacial pipes which may be appropriate to caves. He suggests that the sediment is emplaced as a sliding bed, in which the whole body of sediment moves along the channel bed at the pipe-full (phreatic) stage. The driving force for the movement is the pressure gradient through the fluidized sliding bed, and the shear stress at the surface, while frictional drag on the walls of the tube tends to retard movement. If the bed stops sliding, the water flow is confined to the space between the top of the bed and the pipe roof and this increases the pressure gradient and re-initiates movement. The sliding action is therefore self-maintaining until the pipe enlarges or pressure loss (along a distributary) is possible.

The sedimentary structures formed by sliding-bed processes have been studied experimentally by McDonald and Vincent (1972). Dune forms produced by transport of glaciofluvial sand in an acrylic pipe have foreset beds which become finer-grained downslope and these are capped by stoss laminations. Slump structures on the upper foreset are preserved as discontinuities. Natural esker sediments display similar cross- stratified, parallel laminated beds. However in this situation esker sedimentation is accommodated by upwards melting of the ice roof, presumably under pressure. Dardis and McCabe (1983) describe essentially similar sequences formed in subglacial channels or tubes preserved in drumlin cores. Gravity flow deposits, in this case matrix-supported diamictons, are present in the sequence with parallel-laminated silts and clays. Analogous avalanche front cross-stratified pebble and cobble gravels are described. Beds of this type are formed by avalanching of sediment on oversteepened slopes into standing water, as in glaciolacustrine sedimentation (Allen, 1970).

The range of sedimentary structures in the cave deposits is comparable with both the experimental and esker sequences. It therefore seems likely that sliding-bed processes are involved in the transport and deposition of

538

A

B

D. GILLIESON

Figure 4. Micromorphology of (A) diamicton and (B) fluvial quartz grains from PNG caves. Magnification in (A) is 100 x , in (B) it is lo00 x

allogenic sediment in these tropical caves, given passage-full flow. The latter is indicated by the dynamic phreatic bedrock passages which make up the caves. Although diamictons in caves have been described before (Drew and Cohen, 1980), this study presents the first evidence for sliding-bed facies in caves.

Most of the clayey silts and sandy clays which underlie diamictons in the cave passages show horizontal discontinuous stratification, suggesting high flow velocities in the cave conduit (Picard and High, 1973). Textures associated with these bedforms are more strongly unimodal in the fine sand sizes (Figure 5).

CAVE SEDIMENTATION 539

100

90

80

70

60 z

; 50 V

I

5 c

40

30

20

10

0 -

GRAVEL I SAND 1 SILT I CLAY

Figure 5. Particle-size distributions of basal fluviatile silts and clays from PNG caves

Other sediments, derived from rapid mass movements in the karst catchments, are deposited on sloping surfaces in water impounded by the diamictons. A range of cross-stratified structures are present. The dumping of sediment from pulses of water into standing pools produces sedimentary structures analogous to turbidite sequences. Thus at site ST18 a thick cross-stratified clay is made up of couplets of oxidized silt and gleyed very fine clay, each couplet comprising a pulse of sedimentation. Quartz sand micromorphology from these sediments shows v-pitting characteristic of aqueous transport with grain collision, and numerous small fractures due to high velocity grain impacts (Figure 4). Particle-size data show a wide range of fine-textured modal grain sizes (Figure 6). At other sites, principally side passages, low angle cross-stratification suggests lower energy of deposition into standing water.

The final sediments to be deposited at the end of each cycle are fine clays to muds. These produce a range of horizontally stratified deposits, some graded as a result of major flood pulses. Considerable hiatuses in deposition are indicated by desiccation cracks and interbedded calcite flowstones, often made of floe calcite. An example from Selminum Tem shows several phases of deposition, each starting with a coarse sediment and terminating with extremely fine clay (grain size 13 to 14 phi). In Atea Kananda these deposits are widespread and may attain depths of 8 m. There the mean grain size is coarser (4 to 5 phi) as the source rock is a siltstone. Intergranular very fine clay makes these sediments extremely cohesive.

Extremely fine clays to clayey silts may be deposited from standing water, either due to backflooding of cave passages or impoundment behind deep sediment banks. These deposits behave as colloids and accrete parallel to underlying rock or sediment surfaces. They are in some ways analogous to the ‘cap muds’ of Bull (1981). However, the PNG deposits are finer textured (Figure 6) and relate to slow settling from flood waters in the cave, instead of fine sediment entering from roofcracks. In the PNG karst, these sediments result from very fine slopewash clays mobilized after rapid mass movements in a catchment.

540

70 -

60 - z

D. GILLIESON

I ,YAOlC ;/: /‘I

90 1

Figure 6. Particle-size distributions of fine-grained backflooding sediments from PNG caves

The range of sedimentary structures present can be related to trends in depositional energy and the nature of water flows (Figure 7). Most of the sediments relate to dumping into standing water, with various water velocities and depositional slopes. There appear to be depositional clines between standing pools and discrete cave streams, with variations due to depositional energy level.

Detailed examination of one sediment sequence in Selminum Tem (Figure 8) shows several peaks in mean grain size which may relate to phases of catchment erosion. A granular fine sandy silt forms the lowest stratum

LOW ENERGY *HIGH ENERGY

STANDING parallel accretion inclined cross-stratification diamicton with WATER stratification with flame structures avalanche front

cross-stratification

horizontal stratification

horizontal graded stratification

low angle cross- stratification diamicton

micro and festoon cross-stratification

lenticular stratification within diamictons

cross-stratified gravels horizontal discontinuous stratification J.

CAVE STREAMS

Figure 7. Relationships between water flow type and depositional energy as expressed in cave sediment structures

CAVE SEDIMENTATION

laminated slightly granular fine san&j mud f lmtone interlaminated with sediment two graded strata. lower fine silt to fine.clay. upper slightly granular fine silt to fine mud thin flmtone sheet dessication c r d s laminated raded strata, slightly granular fine sandy c~aypto fine clay thKk flowstone with intetlarninated mud uoss stratified graded unit, from pebMy fine mud at base to fine clay at top

lens of pebMy mud infilling channel in lower unit slightly granular fine sandy silt

-

lmestone bedrock 2-

54 1

_ _ ----__--___-___

- - - - - - - - - - - - - _ _ _ - _ _ __ - -. - - - - - - - - - - - - - - - - - - - - - - _ _ - - - - __ - - - _ _ - - - - - - - - - - - -

COLOUR

7.5YR516

7.5YR414

7.5YR616

7.5YR414

7.5YR516 10YR4/4

0 25 so .Z10-6G Oe-’~rn-~g-’

Figure 8. Stratigraphy, granulometry and non-directional magnetic measurements for sediments in the Upper cave of Selminum Tem, PNG. Mz+ = phi graphic mean, ui = inclusive graphic standard deviation, Ski = inclusive graphic skewness, K, = graphic kurtosis, C % = % organic carbon (Walkley-Black). x = mass specific susceptibility, IRM (+ 3000) = isothermal remanent magnetization,

measured at 3 kOe, xq = quadrature (frequency dependent) susceptibility

and appears to correlate across the passage. This unit appears to be a fluviatile sediment and may represent the bedload of the active upper phreatic tube.

An overlying mudflow deposit was derived from the immediate area above the cave. The velocity of flow was sufficient to erode the underlying deposit. Above the mudflow is a cross-stratified graded unit, which is most probably the result of rapid pulses of sedimentation into standing water impounded by the mudflow. When compared with surface sediment samples of known origin, this unit may represent at least two colluvial slumps and a debris avalanche, each succeeded by slopewash sediments. At the top of this unit, slopewash sediments are overlain by a thick, impure flowstone which represents a hiatus in sedimentation. Above the flowstone, at least two phases of colluvial slumping and subsequent slopewash, deposited as graded stratification, occur. Desiccation cracks in the upper part of this sequence suggest slow drying out of the standing water in the passage. A thick flowstone overlies this and is itself overlain by two graded strata, representing the results of two colluvial slumpslopewash units. The uppermost allochthonous sediment is a slightly granular fine sandy mud, interpreted as the product of slopewash and capped by flowstone dated by uranium series at 21 500 k 900 B.P. The deposition of this flowstone appears to mark the end of active sedimentation in this phreatic conduit, and is supported by a similar date (21 700 f 1200 B.P.) for capping flowstone on the other side of the passage. A U/Th date of 18 800 k 1100 B.P. for a stalagmite resting on a comparable sediment in nearby Trinity Cave

542 D. GILLIESON

supports the notion of abandonment of the Upper Selminum Tem-Trinity Cave phreatic tube between 20-22 OOO B.P.

The trends in granulometric statistics for the section (Figure 8) suggest overall fining upwards of sedimentation as flow velocities declined in the passage. Mean particle size decreases upwards, as do sorting and skewness; kurtosis remains fairly constant throughout the sequence, reflecting its insensitivity as an indicator in poorly sorted sediments. Organic carbon values are overall low, less than 0-5 per cent; this may reflect a general loss of organic matter in suspension. Much of the observed variation in organic carbon is due to experimental error, but values for slopewash facies seem to be higher than those for the basal fluviatile sediments, or for colluvial slump material.

Some inferences about the nature of these sediment sources can be gained from an examination of non- directional magnetic measurements. Values of specific magnetic susceptibility ( x ) show peaks coincident with coarse mean grain size. This suggests that the peaks are due to influx of coarser subsoil material with a higher proportion of primary magnetic minerals. These peaks are greatly enhanced when saturation isothermal remanent magnetization (IRM), measured at 3 kOe, is plotted. A number of peaks due to erosion of mineral subsoil are identifiable and correlate well with mean grain size. The ratio between IRM and x shows high values consistent with a primary magnetic assemblage in the sediment, while this interpretation is further supported by particularly low values of frequency dependent or quadrature susceptibility (xq). A high value (8.83) may relate to inclusion of finer grained material in a rapid mass movement deposit.

The sedimentary sequence in the cave is therefore made up of a number of discrete events, each starting with an inferred mass movement (mudflow, debris avalanche or colluvial slump), followed by a fining upwards sequence of slopewash facies, and ending in a phase of flowstone deposition. If this sequence reflects events in the small catchments feeding the phreatic passage, then hillslope erosion is largely accomplished by periodic phases of instability, resulting in pulses of sediment moving out of the catchment. This accords with the observations of Pain and Bowler (1973) and Simonett (1 967).

CAVE DEPOSITS AND SURFICIAL PROCESSES

The processes of transport and deposition of the waterlain cave sediments have been determined by granulometry, micromorphology and comparison with surficial sediments. For Selminum Tem, Atea Kananda and Bem Tern, the sediments are predominantly the result of rapid mass movements within the cave catchments. The specific processes identified are mudflows, debris avalanches and colluvial slumps, forming a cline in decreasing depositional energy. Slopewash sediments, although present and therefore not lost in suspension, do not make up a high proportion of the deposits. This may be due to the role of a thick root mat protecting the soil surface from rainsplash erosion under the primary montane forest. It is therefore likely that hillslope processes in these continuously wet montane karsts are dominated by limestone solution and episodic rapid mass movements. The trigger factors responsible for the latter are prolonged, intense rainfall and earthquakes. From the Selminum Tem stratigraphy, the frequency of mudflows in the catchment is about one every 5000 years, while the debris avalanches and colluvial slumps appear to occur once every 400 to 900 years. Pain and Bowler (1973) have estimated a return interval of 200 years for these processes in catchments near Madang, a seismically active area. The above estimates from cave sediments stratigraphy are therefore of the right order of magnitude.

Much research on karst denudation has concentrated on steady state processes such as dissolution of limestone, though the role of flood events has been stressed. The landscape of the Southern Fold Mountains appears to be in a state of dynamic metastable equilibrium, where periodic catastrophic events affect the long term trend in landsurface denudation. Although rapid mass movements appear catastrophic on a short timescale (101-102 y), they are clearly normal events in the long term (lo3-lo4 y) evolution of the continuously wet karsts of the region.

ACKNOWLEDGEMENTS

This work was supported by a Commonwealth Postgraduate Research Scholarship and the University of Queensland. The uranium series dates were produced by Profs. D. C. Ford and H. P. Schwartz of McMaster

CAVE SEDIMENTATION 543

University, Ontario, while Prof. F. Oldfield and Mr. A. Krawiecki provided the non-directional magnetic measurements. For hospitality in the field I thank Mr. Neil Ryan of Mt. Hagen and the people of the Tifalmin area in the Star Mountains. Lastly I thank Ms. Jill Landsberg for her help and support both above and below ground over several years.

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