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So why are the caves of Mulu so spectacular and what makes them so significant? This guide explains how the caves formed and why they have developed into the magnificent landscape we see today. But first, we need to know a little about the geology and geomorphology of the Park and how caves form.

A guide to the karst geomorphology of Mulu

Dr A R Farrant

Geology of the Gunung Mulu National Park

The geology of the area is crucial to understanding why the Gunung Mulu National Park is so special. The rocks consist of a sequence of sandstones, limestones and shales which lie on the western side of a large anticline or upfold (the ‘Mulu uplift’ or dome), so the rocks dip steeply at between 40° and 70° to the north-west. Three main rock units can be identified. In order of deposition, these units are the Mulu Formation, the Melinau Limestone and the Setap Shales Formation.

The oldest rocks in the National Park belong to the Mulu Formation, a series of coarse sandstone and shales, 5-6 km thick of Late Cretaceous to Eocene age (c. 40- 60 million years old), which forms the high ground of Gunung Mulu. The sandstones form very steep ‘V’ shaped valleys separated by narrow ridges which culminate in the summit of Gunung Mulu at an elevation of 2377 m. Landslides, usually in the form of long narrow ribbon slides, are common on the steep slopes and often occur during heavy rain.

geol map for Matt with moonshadow reduced
Geological map of the Mulu area

Above the Mulu Formation lies the Melinau Limestone Formation. This unit consists of a 2.1 km thick sequence of massively bedded, strong, pale grey limestones of Upper Eocene-Lower Miocene age. These were laid down in a shallow sea between 20 and 40 million years ago. Because of the steep dip to the west (at around 60-70°), the Melinau Limestone now outcrops to the west of Gunung Mulu, forming a narrow, lenticular outcrop culminating in Gunung Api. The limestone also underlies the alluvial plain to the west where occasional steep sided limestone towers (batus) project through the alluvium.

Overlying the Melinau Limestone is the Setap Shale Formation, which consists of a sequence of mudstones with occasional marly bands and thin sandstone beds. These rocks were lain down in deeper water during the Middle Oligocene – Early Miocene period, some 20-30 million years ago. These outcrop to the west of the park, forming a small escarpment. Although generally younger that the other two rock units, the softer nature of these rocks means they have been preferentially eroded to form the low-lying area to the west of Gunung Api.

However, it is the Melinau Limestone which makes the Gunung Mulu area deservedly famous. As limestone is soluble, the action of water over time creates a ‘karst’ landscape, with many classic features peculiar to limestone terrains, including caves, disappearing rivers, sinkholes, springs and pinnacles. Four limestone massifs, from Gunung Buda and Gunung Benerat to the north, to Gunung Api (1692 m) and the Deer Cave Massif in the south dominate the landscape. These massifs rise steeply above the alluvial plains, often in sheer cliffs over 300 m high before levelling slightly to culminate in the summit. The surface is pockmarked with a dense network of closed depressions, collapse sinkholes, pinnacles and dry valleys, especially in the south of the Api massif. The limestone supports a very rugged, mature karst topography, riddled with some of the largest and most extensive cave systems in the world.

Cave Formation

Caves form by the dissolution of limestone. Rainwater picks up carbon dioxide from the air, and more especially from the soil (where micro-organisms and the decay of organic matter generate high levels of carbon dioxide), combining to form carbonic acid. Limestone, made of calcium carbonate, is soluble in this weak acid, and so as the acidic water percolates through the limestone, it gradually enlarges the bedding planes, joints and fissures within the rock, eventually creating caves large enough to be explored.

The high rainfall, coupled with the tropical forest ecosystem and a strong, but soluble limestone, combines to produce a situation perfect for cave development. Many of the cave systems are formed where ‘aggressive’ acidic water draining off the higher impermeable sandstones of Gunung Mulu, sinks underground on meeting the limestone. Over time, this water has etched out the major underground watercourses we see today. Most of the streams draining west off Gunung Mulu sink at least partially underground. Water flowing down the Melinau Paku sinks underground to either flow into Clearwater Cave or Cave of the Winds. Similarly, water flowing into the Garden of Eden traverses the limestone outcrop underground via Deer Cave. Further north, some of the water passing through the Melinau Gorge finds its way into the Clearwater River, while the Hidden Valley stream flows through Good Luck and Cobra Caves on route to the Melinau Paku.

The exceptionally strong Melinau Limestone means that any caves that do form can grow to very large size before collapsing. Furthermore, the relatively small number of joints and fissures within the massively bedded limestone means that cave development is focussed on a small number of highly significant fractures. In addition, these processes have been ongoing over a long period of time, uninterrupted by glaciations or prolonged periods of dryness, allowing the underground rivers to carve out some of the largest cave systems on the planet.

Underground water flow

Water flowing though the limestone will follow the line of least resistance, which generally means along bedding planes, joints and fault lines. Over time, the water generally enlarges the major joints and bedding planes within the rock to form a cave.

Local geology also has an important role to play in determining the style and type of cave formed. In the Gunung Mulu National Park, most of the water enters the limestone at the contact between the Mulu Formation sandstone and the younger Melinau Limestone, or as percolation from the surface. However, because the limestone dips quite steeply to the west, any water draining from the sinks in the east to the resurgences on the western side of the limestone ridge must rise stratigraphically up through the limestone sequence. To do this the water must cut through each of the limestone beds, either along joints or faults. This causes the passages to develop a characteristic looping profile. These ‘phreatic loops’ are formed where water descends down-dip along a bedding plane and then has to rise up a joint or fault to regain a higher bedding plane.

The number and depth of the phreatic loops depends on the number of fractures and fissures within the bedrock, or how laterally continuous they are. Put simply, the more joints and bedding planes there are, or the longer they are, the more likely the cave will develop at or close to the water-table. This is encapsulated in the ‘four state’ model, which outlines the various ‘states’ of cave development that may occur:

fourstatediaglan

  • State 1. Bathyphreatic (deep phreatic) systems. These develop where fractures are widely spaced and cave systems are compelled to follow deep flow paths because no shallower routes are open.
  • State 2. Phreatic with multiple loops. These occur when the frequency of penetrable fractures is significantly higher. Caves with deep phreatic loops separated by short stretches of vadose cave develop
  • State 3. Mixture of phreatic and water-table caves. With increasing fracture frequency, caves with a mixture of shorter, shallower loops and quasi-horizontal passages developed at or near the water-table, form.
  • State 4. Water-table caves. Where fracture frequency is sufficiently high, or single fractures are aligned along the direction of regional water flow, low gradient, very direct routes to the spring can develop, at or close to the water-table.

Where the water is draining at right angles to the dip (ie along strike), then water can follow a single bedding plane to the resurgence, giving rise to water-table caves. Where water flows in the direction of the dip, then the water will flow down bedding planes, but then have to rise up through the limestone sequence to gain the next higher bedding plane on the way to the resurgence.

In Gunung Api, the resurgence for the water is at the southern end of the limestone ridge. In this case, much of the cave is oriented north-south along the strike of the rocks at right angles to the dip. Passages oriented along strike will generally be quasi-horizontal as they can follow one or two bedding planes to the resurgence. This explains why many of the major passages in Mulu are horizontal, because they run north south along the strike of the limestone. Good examples of these can be seen in Clearwater, where both ‘Revival’ and ‘Inflation’ follow the same bedding plane for several kilometres. Similarly, Hyperspace Bypass is another strike orientated passage.

However, because the bedding planes in the Melinau Limestone are widely spaced, any passages trending parallel with the dip are generally State 1 caves, with deep phreatic loops. Many of the ‘ramps’ are the down dip limbs of these deep phreatic loops, while the pitches of Ronnie’s Delight and Deep Thought in Clearwater are superb examples of the joint aligned ascending limb of the loop.

Where major faults or joints are approximately in line with the direction of water flow, these may be utilised by the conduit, enabling the passage to cut horizontally through the limestone sequence. In practice however, any given passage will follow a complex route along a combination of different bedding plane – joint intersections. Thus many of the caves in the Mulu National Park are a complex mix of State 1 and State 4 caves, depending on whether the water flow is along strike or with the dip of the rock.

Controls on base level.

The level at which a cave develops is controlled by the lowest available point in the limestone where water can escape. In the case of Gunung Api, this occurs at the southern end of the ridge at the entrance to Clearwater Cave. For Gunung Benerat, the topographic gradient is to the north, so the water emerges from the lowest point at Terikan Rising Cave.

Once a cave system has developed, it will determine the level of the water-table within its own system. This is fixed at the level of crest of the higher phreatic loops, rather than the water-table determining the elevation of the cave. However, the water-table has an important effect on the type of cave passage formed. ‘Vadose’ passages are formed above the water table, and can be easily distinguished from sub-water-table ‘phreatic’ passages by their morphology. Vadose passages tend to be canyon shaped with small irregular scallops (dissolutional flow markings), while phreatic passages have an elliptical or circular, smooth, sculpted morphology, with large rounded scallops. Furthermore, flow in vadose passages is always downhill, whereas in a phreatic passage, in which the water flows under pressure, uphill segments can occur, although the passage always follows the hydraulic gradient.

If the base-level to which the cave is graded falls, due to valley incision and erosion downstream, the cave system reacts either by developing new passages graded to the new, lower, base-level (leaving the original conduit high and dry as a relict passage), or by cutting down the floor of the original passage. Thus over time, as base-level falls, a vertically stacked series of cave levels will be formed, with the oldest caves preserved at the highest level, and progressively younger caves at lower levels, with the modern active system at the base.

It is possible to estimate the age of the higher relict caves in several ways, either relatively or directly by dating cave sediments and speleothems.

Cave Deposits

Once cave has been formed, then it may become partially or totally infilled with cave deposits. Two major types of deposit are commonly seen in the Mulu caves; cave sediments, mostly gravel and clay, and calcite stalagmites.

Stalagmites and stalactites (‘speleothems’)

In the tropical environment, the growth and decay of vegetation and the high levels of bacterial and microbial activity in the soil creates enhanced levels of carbon dioxide in the soil. Much of this is generated by root respiration and bacterial decomposition. Rainwater percolating through the soil will absorb this carbon dioxide, becoming more acidic in the process. This enables the percolating water to dissolve more of the limestone bedrock. On reaching the underlying cave atmosphere, which has generally lower levels of carbon dioxide, the percolating water degasses and by doing so, becomes supersaturated with calcium carbonate, which is deposited as speleothem. A variety of different types of speleothem can develop, but the majority are composed of calcite. Near the entrances to many of the caves, evaporation of the drip waters can enhance stalagmite deposition, which is why many of the caves are almost choked by calcite near their entrances.

Cave sediments

Many of the passages in the Clearwater system have thick deposits of a coarse, sandstone gravel. These gravels are often overlain by a thick deposit of silt and laminated clay (known as ‘cricket muds’). Where undisturbed, these muds are finely laminated, but often the muds have been disturbed by burrowing cave crickets (Rhaphidophora oophaga). The gravels were deposited by major underground rivers bringing in coarse sediment from the slopes of Gunung Mulu. These are mostly of sandstone and siltstone, but the composition of the gravels changes with distance from the source. Downstream, the relative percentage of sandstone gradually increases as the weaker siltstone is broken down. Thus the ratio of sandstone to siltstone can be used in the high level relict passages to estimate the distance to former stream sinks. For example, the relatively large amounts of siltstone in gravels in the abandoned high level Armistice Series in eastern Clearwater suggests the passage was fed by the Hidden Valley stream, rather than water from the more distance Melinau Gorge.

Today, the modern Clearwater River is carrying very little sediment. This is because it has no major stream inputs bringing in sediment from Gunung Mulu, and hence has no coarse sediment bed load.

The climate has a big effect on the amount of sediment in transport. In today’s warm, wet ‘interglacial’ climate, the hillsides are rapidly eroding, washing much sediment off Gunung Mulu into the Melinau and Melinau Paku rivers. The amount of sediment is too much for the river to transport, so it deposits it as a large sediment fan at the mouth of the Melinau Gorge and in the Paku valley. The modern Melinau fan extends as far south as Long Lutut, but remnants of earlier fans preserved as river terraces prove that the fans have been much more extensive in the past

As the fan builds up over time, (a process known as aggradation), then eventually the fan progrades far enough to block or submerge of the resurgence. When this happens, it causes ponding within the cave system leading to the widespread deposition of the fine grained, almost ubiquitous cricket muds throughout the cave systems affected.

Effects of sediment influx.

The most distinctive effect of sediment aggradation within the caves is the formation of dissolutional wall notches. Many passages in Clearwater have a well defined horizontal notch etched into the wall and generally associated with a gravel fill. In cross-section, the roof and floor of the notch are essentially horizontal with a curved back wall. These notches are up to 4 m high, 2 m deep and extend several kilometres along a single passage. An outstanding example is seen in the Clearwater River Passage. The floor of the notch is often covered in gravel capped by a mud layer 1-5 m thick. Furthermore, there are many notches preserved at different elevations in Clearwater and two or three notches may exist in a single passage, e.g. Hyperspace Bypass. Twenty-four separate notches have been mapped, the highest occurring over 240 m above the resurgence.

The notches are formed when large amounts of coarse gravel are transported through the cave system. This has the effect of protecting the passage floor from further downwards erosion, so instead dissolution is concentrated along the passage sides, etching a notch level with the top of the gravel deposits. The level of the notch is level with the gravel fill which is controlled by the level of the resurgence.

The aggradation of the Mulu alluvial fans is controlled by changes in climate. Tectonic activity and the uplift of Gunung Mulu provides the potential for alluvial fans to develop, but it is climate that triggers the formation of the alluvial fans. Independent evidence from palynological, planktonic, foraminiferal and stable isotope data in offshore sediment cores, onshore vegetational and palynological data, and from climate modelling suggests that the climate during glacial periods was drier. If this hypothesis is correct, then the most active sediment transport and fan aggradation should occur during interglacial periods such as the present day. Thus each notch seen in the caves should correspond to an interglacial episode when there was active sediment transport through the cave.

If this is the case, then the age of each notch should match with an interglacial period. This can be tested by estimating the age of each notch using a variety of dating  techniques.

Dating of the caves

Cave deposits, both sediments and speleothems can be dated using a variety of techniques, the most common of which is the Uranium Series method. This using the radioactive decay of the uranium (234U) into thorium (230Th)

Uranium is soluble in water, whereas thorium is not. Thus when stalagmites form from water percolating through the rock, the drip-water will contain uranium, but no thorium. Once the stalagmite has formed, the uranium isotope 234U gradually decays into the thorium isotope 230Th at a known and constant rate. Thus as time passes, the amount of thorium in the stalagmite increases. Thus, by measuring the ratio of 234U and 230Th using a mass spectrometer, the age of the stalagmite can be calculated. This method works for stalagmites up to half a million years old. In stalagmites older than this, the thorium also begins to decay and the U/Th ratio can no longer be relied upon. The ratio is measured using a mass-spectrometer.

An alternative method of working out the relative age of a cave is ‘palaeomagnetic’ analysis. This technique utilises known changes in the Earth’s magnetic field. Through time, the Earth’s magnetic north pole has ‘wandered’ around by several degrees, periodically ‘flipping’ from north to south and back again. These fluctuations in the magnetic field have been independently dated using other methods. The last time the magnetic pole reversed was 780,000 years ago, and before that, approximately 910,000 years ago.

These changes in the Earth’s magnetic field are recorded in the sediments found throughout the caves. Clay particles deposited in still water will preferentially align themselves to the prevailing magnetic field at the time of deposition. Thus by taking carefully oriented cores of fine grained, clay rich sediment, it is possible using a magnetometer to determine the direction (and thus polarity) of the magnetic pole when the sediment was deposited. The most recent sediments will be aligned towards the current North Pole. However, older sediments preserved in higher level relict caves since abandoned by the river may preserve evidence of former pole positions. By taking a suite of ‘fossil’ sediment samples at progressively higher elevations above the river, it is possible to work back through time to when the sediments were deposited during the last period of reversed polarity.

All the sediments sampled from below about 142 metres above the present Clearwater River have a normal ‘north’ magnetic polarity. Above this level, the sediments have a reversed polarity, corresponding to the last reversal in the magnetic field, 780,000 years ago, until around 182-186 m, when the sediments become normal again, probably corresponding to the next polarity change 910,000 years ago.

The dating of the cave sediments and speleothems indicates that the highest level passages in the Gunung Api massif (Nilong’s Cave), some 440 m above the modern resurgence, is probably around two million years old. From this it is estimated that the average rate of erosion and base-level lowering is around 19 cm per thousand years. The palaeomagnetic dating also demonstrates that the notches do correspond to interglacial periods, proving that the alluvial fans aggrade in response to climatic events, rather than just pure uplift of the mountains.

Summary

The caves of the Gunung Mulu National Park are some of the most spectacular in the world. Not only does the Park contain some of the largest caves on the planet, these caves are also superb natural laboratories for studying climate change and past events. The extensive network of well-surveyed caves, especially the Clearwater system, but also those under Gunung Benerat have taught us much about the history of the area over the last 2-3 million years. But it has also provided us many more questions, and there is much, much more to discover and learn. We now know the caves began to be formed over two million years ago, and that the rate of base-level lowering has been roughly constant at 19 cm per thousand years over this time. But does this hold true for recently discovered systems under Gunung Benerat? How will climate change affect the Park in the future. What more secrets can these cave systems tell us?

Without the work of the many park guides, cavers and scientists over the last 30 years, we would literally only be scratching at the surface of Mulu. There are still many more kilometres of cave passage yet to be found throughout the area, with many wonders still undiscovered.

What will the next 30 years bring?


Dr Andrew R Farrant is a Senior Geologist with the British Geological Survey, based in Nottingham, UK. He was part of a British Cave Research Association expedition to Mulu in 1991, where he undertook research into the caves as part of his Ph.D thesis under the supervision of Prof. Peter L. Smart at the University of Bristol, UK.