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THE   GEOLOGY   OF  THE   MELINAU  LIMESTONE  OF   THE   GUNONG  MULU NATIONAL   PARK – Barry Webb

The Mulu Formation   (shales  and sandstones),   the Melinau Limestone and the Setap Shale constitute  the bulk of  a suite  of Tertiary rocks  in northwestern Borneo.     They were  deposited in an outer arc basin associated with subduction along  the,  now inactive.  Northwest Borneo Trench. The Melinau  Limestone  is  a  lenticular  reef  complex of massive  fine  grained limestone with a maximum thickness  of  about  15OO metres.     It is more  intensely  folded and faulted  than previously thought which  explains  the greater  thickness  and anomalous  outcrop pattern deduced by previous workers.     The major  fractures  in the  limestone are  related to the  folding  and are  the main control on cave passage development.     In addition,   zones of recrystallisation associated with the  folds  and  faults  have  influenced the  limestone hydrology.

The Regional Geological Setting

The  three major  rock formations   in  the Mulu  area   are,   in  ascending order,   the Mulu Formation,   the Melinau Limestone   and  the Setap Shale   (Fig.l).     The Mulu Formation,   comprising  sandstones  and shales,   is  named  after Gunong Mulu where it crops  out   in  the  core  of   a  regional  anticlinal  structure.     The Melinau Limestone crops  out along  the  northwestern  flank of  this  structure  forming  a  spectacular line of hills.  Along the northwestern edge of the hills a broad alluvial plain is developed over the limestone and this is bounded further northwest by gentler hills of Setap Shale.  These three formations constitute the bulk of a suite of Tertiary rocks which filled the Northwest Borneo Trench.

Fig 1 - The geology of the Gunung Api area.

Hamilton (1979) proposed that from Eocene to late Pliocene times the South China Sea floor was being subducted southeastwards beneath Borneo.  As sediments were scraped off the sea floor and accreted onto the Borneo plate, the subduction zone was shifted northwestwards to its present, inactive position nearly 200 km northwest of the island.  Most of the rocks in the Northwest Borneo Trench appear to have been deposited in an outer arc basin developed over the wedge of accreted sediments.
The Mulu Formation, the oldest deposit in the basin, has never been examined in detail.  Near its contact with the overlying Melinau Limestone it is a relatively undeformed, shallow water, sandstone/shale sequence. Elsewhere in Northwest Borneo its correlatives appear to be part of the accretion complex underlying the basin.
As the outer arc basin deepened, its axis migrated to a position north-west of the Mulu area.  Setap Shale was deposited in the deeper regions but in the shallower water around Mulu the Melinau Limestone reef complex was developed. Eventually, as subsidence outpaced carbonate build-up, the reef complex was overwhelmed and buried beneath the shales.
In its later stages the basin was undergoing compression since pene-contemporaneous folds affect the distribution of the youngest sediments.  This compression reached a peak in the late. Pliocene orogeny when the main northeast-trending folds and faults were developed.  In the Mulu and Setap Formations, folds are tight and, locally, even isoclinal.  This suggests they may-have initiated at an early date while the sediments were still poorly consolidated. In the Melinau Limestone the folds are open periclines associated with high-angled reverse faults.
A variably developed fracture cleavage related to the folding is normally present in the Mulu Formation and locally in the Setap Shale.  Similar effects in the limestone are represented by a dominant northeasterly-trending joint set.
After the erogenic uplift of the late Pliocene, deposition in the Mulu area has been limited to superficial fluvial sediments.  The dominant geological processes throughout Pleistocene and Recent times have been erosional

Previous Work

The major published sources of information on the geology of the Melinau Limestones are Liechti et al (1960), Wilford (1961) and Adams (1965).  They all reproduce a map taken from an earlier, unpublished report to Sarawak Shell Oilfields Ltd by Shepherd in 1954.  There has been no major revision of this map and later work has mainly involved more detailed subdivision and correlation of the limestone on the basis of its prolific fauna of foraminifera.  Unfortunately, there is a minor inconsistency in Shepherd’s map and section which has caused problems for more detailed studies of the area.
Shepherd’s map indicates moderately intense folding of the limestone in the northern part of its outcrop but his section in the Api area shows only a single fold pair.  On the basis of this simplified structure, later workers have calculated the maximum thickness of the limestone at around 2000 m, although Shepherd’s own estimate (Wilford, 1961, Fig. 9) is only around 1000 m.  In addition, the simplified structure fails to explain the large invaginations of Mulu Formation into the limestone belt in the Melinau Gorge, Hidden Valley and Garden of Eden (Fig. 1).  Geomorphological evidence indicates that the limestone hills are to some extent an original topography exhumed from beneath the shale cover.  Consequently, after the R.G.S. Mulu Expedition of 1977/8, it was suggested that the sandstone/shale outcrops in the invaginations might be younger deposits filling original gorges in the limestone and that similar deposits might be present beneath the alluvial plain.

The Limestone Outcrop

During the Mulu ‘80 Expedition, Shepherd’s original boundaries of the Melinau Limestone were confirmed.  Field work was limited to the area south of the Melinau Gorge but the entire outcrop was examined from aerial photographs. The photogeological expression of the three major formations is very marked and it is clear that Shepherd’s map is primarily photogeological.  Exposures of limestone on the alluvial plain are quite common and there seems no reason to assume that any other lithology is present.  The sandstone/shale sequences in the Melinau Gorge and Garden of Eden are seen clearly to underlie the limestone and to be identical to and continuous with undoubted Mulu Formation further east.  Their occurrence in the invaginations is the result of previously unrecognised folds and faults.

Lithology and Environment of Deposition

The Melinau Limestone is a lenticular reef complex of Upper Eocene to Lower Miocene age.  It reaches a maximum thickness of around 1500 m in the Melinau Gorge area but thins out to both north and south.  It is predominantly a massive, pale grey calcilutite or fine calcarenite, locally recrystallised to marble. It is very pure with insoluble residues less than 1% and dolomite content normally only 2-20%.  Lithological variations are not very marked and are further obscured at surface exposures by a white patina due to weathering.  The macrofauna, including corals, bryozoans, bivalves and gastropods is very sparse and subdivision has depended on the abundant microfauna of foraminifera.  Both the massive, uniform lithology and the microfaunal species indicate a back-reef, lagoonal environment predominated over the area.
Actual reef structures are notoriously elusive in complexes of this age. They are indicated in the present area by rapidly varying, initial, depositional dips in the more thinly bedded, basal limestones.  This facies is well exposed near the plunge pool of Nasib Bagus where algal balls are abundant and small algal patch reefs may be present.  The poor development of this facies and the lack of any fore-reef facies indicates that the main reef front must have migrated rapidly across the area to a more stable position farther northwest. This, in turn, suggests that the reef was probably of the barrier type, similar to Miocene reefs described from the Philippines (Carpzzi et al, 1976).
Partial emergence of the reef complex during the Oligocene is indicated by an intraformational unconformity in the northern part of the outcrop.  This progressively cuts out the Oligocene strata as they are traced northeastwards from the Melinau Gorge.  The intense, secondary dolomitisation of the Upper Oligocene strata at Bukit Berar is probably related.

Structure

Shepherd’s simplified structure of the limestone belt involves a major, open, anticline/syncline fold pair with a reversed fault, the Melinau Fault, along the common limb.  In the north, along the eastern flank of Buda, he recognised a subsidiary fold pair which he did not continue southwestwards, although minor folds are again implied by inliers of Mulu Formation southwest of Deer Cave. The present investigation shows that the subsidiary fold pair again affects the limestone where it extends farther eastwards between the Hidden and Paku valleys (Fig. 1).  In addition, a further fold pair was found immediately northwest of the other.  The syncline of this second fold pair runs close to the eastern boundary of the limestone, beneath the main ridge line.  The associated anticline is intensely reverse faulted along its northwestern limb, the fault zone running roughly along the axis of the hills.  It is this folding and faulting that causes the Mulu Formation to crop out in the Melinau Gorge, Hidden Valley and Garden of Eden.
The newly discovered folds have the regional, northeasterly trend with wavelengths a little over 1 km.  Their style is fairly open with broad, rounded anticlinal hinge zones and more abruptly ‘pinched’ synclines.  Limb dips are generally up to 70° to the northwest and 40° to the southeast but appear to decrease farther northwest.  Bedding is locally sub-horizontal in the fault zone and although somewhat variable along the western flank of Api, seldom exceeds 40° to the northwest.
A variably developed fracture cleavage of pressure solution type is related to this folding in the Mulu Formation and similar pressure solution effects are seen in the limestone.  With post-orogenic stress release this has resulted in a steeply inclined, close spaced joint set in the limestone trending parallel to the fold axes.
The failure of the limestone to crop out in the core of the most easterly syncline where its trace cuts the ridge north of Hidden Valley suggests that the folds must plunge.  This is confirmed by bedding/cleavage intersections in the underlying Mulu Formation which indicate axial plunges of up to 20° to the northeast and southwest.  No other related structures have been seen in the Mult Formation but a sub-vertical, roughly east-west trending joint set is developed in the limestone.  It appears to be developed preferentially near plunge culminations (Figs, l and 2 ) and rare elsewhere.
Two belts of intense recrystalisation of the limestone are related to the structure.  One is associated with the rather tight syncline along the eastern edge of the hills and the other with the fault zone along the axis of the hills (Fig. 1).  The latter is by far the more pronounced and affects the limestone up to a high level.  The link with the structure suggests that the recrystalisation is related to pressure solution effects, although these normally promote the development of the main joint set whereas in the recrystalised zones, joints are very poorly developed.

The Geological Controls on Cave Development

The extremely low porosity of the Melinau Limestone necessitates the existence of open fractures for the entry of water and subsequent cave development.  The major fractures noted in the Api area include bedding, the main northeasterly trending joint set, the east-west joint set and minor joints, perpendicular to bedding (Fig.2 ).

The major sets of open fractures in the Melinau Limestones

The bedding in the limestone was effectively sealed during diagenesis but reopened by flexural slip during folding.  This is most around the Snake Track area and along Revival.  Its effects are relatively minor and it may be more ubiquitous than it appears.
Strong control by the main joint set in the anticlinal hinge zones is shown by the Green Cave/Deer Cave system and by Prediction Cave.  Originally, the latter probably connected with high level caves above Batu Nigel.  It lies almost at the base of the limestone and as the Paku lowered its base level the water moved eastwards, down dip, aided by the local east-west joints, to form the Nasib Bagus system.  Sarawak Chamber is developed in the thinly-bedded basal limestones.  It is floored by Mulu Formation and roofed by limestones of the massive lagoonal facies.
In addition to these features directly promoting water movement, it is clear that features inhibiting its movement have also influenced cave development in the Api area.  The zones of recrystalisation form the most important hydrological barriers but there is evidence that the major anticlinal culminations could inhibit throughflow.
The main zone of recrystalisation along the axis of the hill line clearly inhibited the westward flow of the Melinau Paku. and thus encouraged the development of the Green Cave/Deer Cave system.  This was probably further aided by a marked hydrological gradient towards the Tutoh.  The water from Hidden Valley probably took a similar course prior to its capture by the Prediction/ Nasib system.  A deep surface depression may mark this course between the Hidden Valley and the Paku.  It runs along the same anticlinal crest flanking the recrystalisation zone as the Green/Deer Cave system.
Northwards flow of water from Hidden Valley and southwards flow of water from the Melinau Gorge may have been inhibited by the updoming of Mulu Formation in the anticlinal culmination north of Hidden Valley.  Apart from Wonder Cave no major systems have been found in this part of Api and along the Melinau Gorge it is, at present, a minor resurgence area for percolation water.  The Melinau may have flowed northwards into Benerat but there is no evidence that it flowed southwards until it breached the main recrystallization zone and promoted the development of the Clearwater system.
This study of the geological controls over cave development was necessarily limited but it serves to illustrate an important relationship.  Although the cave systems are guided by well defined structural weaknesses in the limestone, their actual locations are more closely controlled by hydrological barriers.pronounced on fold flanks so that bedding plane fractures are most important in those regions. They allow water movement both up and down dip and along strike.
The main joint set is the dominant fracture system and common everywhere except in the zones of recrystalisation.  Although ubiquitous, the fractures tend to be more open along anticlinal crests and somewhat more poorly developed along synclines.  They allow water to move both vertically and parallel to the fold axes.
The east-west joint set is limited to belts corresponding to plunge culminations.  It allows vertical and east-west movement of water.
Minor joints perpendicular to bedding commonly occur on fold flanks.  They facilitate water movement perpendicular to bedding and along strike.
In general, cave passage initiation will take place at the intersection of two or more of these fractures.  It will then develop preferentially along one c other of the fractures depending on local geological and hydrological controls.
Clearwater Cave, developed along a fold flank, is dominantly controlled by bedding and the main joint set.  Control by the east-west joint set is notable near the Edge of the World and in the main river passage (Fig. 1).  These coincide with plunge culminations seen farther east, to the north and south of Hidden Valley.  Control by the joint set perpendicular to bedding was noted around the Snake Track area and along Revival.  Its effects are relatively minor and it may be more ubiquitous than it appears.
Strong control by the main joint set in the anticlinal hinge zones is shown by the Green Cave/Deer Cave system and by Prediction Cave.  Originally, the latter probably connected with high level caves above Batu Nigel.  It lies almost at the base of the limestone and as the Paku lowered its base level the water moved eastwards, down dip, aided by the local east-west joints, to form the Nasib Bagus system.  Sarawak Chamber is developed in the thinly-bedded basal limestones.  It is floored by Mulu Formation and roofed by limestones of the massive lagoonal facies.
In addition to these features directly promoting water movement, it is clear that features inhibiting its movement have also influenced cave development in the Api area.  The zones of recrystalisation form the most important hydrological barriers but there is evidence that the major anticlinal culminations could inhibit throughflow.
The main zone of recrystalisation along the axis of the hill line clearly inhibited the westward flow of the Melinau Paku. and thus encouraged the development of the Green Cave/Deer Cave system.  This was probably further aided by a marked hydrological gradient towards the Tutoh.  The water from Hidden Valley probably took a similar course prior to its capture by the Prediction/ Nasib system.  A deep surface depression may mark this course between the Hidden Valley and the Paku.  It runs along the same anticlinal crest flanking the recrystalisation zone as the Green/Deer Cave system.
Northwards flow of water from Hidden Valley and southwards flow of water from the Melinau Gorge may have been inhibited by the updoming of Mulu Formation in the anticlinal culmination north of Hidden Valley.  Apart from Wonder Cave no major systems have been found in this part of Api and along the Melinau Gorge it is, at present, a minor resurgence area for percolation water.  The Melinau may have flowed northwards into Benerat but there is no evidence that it flowed southwards until it breached the main recrystallization zone and promoted the development of the Clearwater system.
This study of the geological controls over cave development was necessarily limited but it serves to illustrate an important relationship.  Although the cave systems are guided by well defined structural weaknesses in the limestone, their actual locations are more closely controlled by hydrological barriers.

First published in the Transactions of the British Cave Research Association.   June  1982.