Danced

january 2002

Author: Assoc. Professor Dr. Felix Tongkul Geology Program, School of Science and Technology, Universiti Malaysia Sabah, Locked Bag 2073, MY 88999 Kota Kinabalu, Malaysia Phone: + 60 88 320000-5756 Fax +60 88 435324 E-mail: [email protected] '

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I F. Tongkul Geology Program School of Science and Technology I Universiti Malaysia Sabah

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I January, 2002

II Summary

In an attempt to understandthe evolution of Maliau Basin a structural geology study within and outside the basin based on secondary information, satellite and radar imageries,aerial photographs, primary field and laboratorydata was carriedout.

The study has documentedthe natural architectureof Maliau Basin, unravelled the tectonic evolution of the basin and detenninedongoing geologicalprocesses. These are importanttowards the managementof Maliau Basin.

Maliau Basin is made up interbedded layers of sandstoneand mudstone, approximately 7500 metres thick, which were deposited, in an ancient deltaic-coastal environment, between 9-15 million years ago. The layers at the base of the basin consist mainly of mudstones reaching up to 2000 metres thick. Near the rim of the basin, thick sandstone interbedded with thin mudstone and coal seams occur. Towards the centre of the basin a series of sandstone-dominated and mudstone-dominated strata of various thicknesses occur. The youngest sediment is located near the Camel Trophy Field Station, whereas the oldest can be found near Lake Linumunsut. The basin sits on older sedimentary rocks, also comprising of thick layers of sandstoneand mudstone, with slight unconformity.

The evolution of Maliau Basin was structurally controlled. Faults trending NW -SE and NE-SW together with the structural trends and topography of the underlying sedimentary I rocks played important roles in the development of the basin. The underlying sedimentary rocks, which began its deposition about 20 million years ago on a large elongate basin trending NE-SW was subjected to NW -SE compression between 14-15 million years ago. The tectonic compression resulted in the gentle folding of the underlying sedimentary rocks forming the initial concentric shape of Maliau Basin. The newly formed concentric-shaped basin was subsequently filled by Neogene sediments until about 9 million years ago when the basin was uplifted above sea level due to continued compression in eastern Sabah. The compression resulted in the gentle folding of the sedimentary layers and at the same time accentuated the concentric shape of the basin, through the reactivation of old fault system. About 5 million years ago Maliau Basin, and the surrounding areas was probably uplifted to its present height with a slight tilt to the Southeast. Following the uplift, Maliau Basin and surrounding areas were subjected to intense weathering and erosion that continued up to this day.

The presenceof different lithological units, orientation of layers and fracture planes affects the ongoing geological processesthat shapethe unique landscapeof Maliau Basin. It is importantthat thesediverse lithologies and geologicalstructures be takeninto accountwhen developingany part of the basinto ovoid geologicalhazards. It is equally important to control any activities that can drastically alter the existing geological processeswithin the basin. Future studies may look into the possibleoccurrence of an ancientlake inside Maliau Basin and a systematic characterisationand evaluation of the waterfalls apart from detailedsedimentological characteristics of the sedimentarylayers.

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Summary ...ii Table of Contents ...iii List of Figures/Table/Photographs ...iv Acknowledgements. ..v

INTRODUCTION Background... I Objectives/Scope. Methodology. ..I

II. REGIONAL GEOLOGICAL SETTING.. 5 Tectono-stratigraphicUnits... 5 Major Structures...8

ill. GENERAL GEOLOGY OF MALIAU BASIN AREA 8 Stratigraphy. ..8 Lithological Units. ..11 Environment of Deposition ...19

IV STRUCTURAL GEOLOGY. ..19 Geometry ...19 I Lineament ...19 Bedding /Strata ...19 Fractures/Joints... 26 Faults ...26 Deformation/Stress. ..26

v. BASIN ARCHITECTURE AND EVOLUTION 26 I Architecture (Structural Plan of the Basin) ...26 Evolution (How the Basin Formed) ...31

VI. ONGOING GEOLOGICAL PROCESSES Drainage System Evolution... 37 ' I Waterfall Development ...38 i Landslide Occurrences. ..38 Soil Development... 38

VI. MANAGEMENT IMPLICATION .45

YD, FUTURE STUDIES... 45 Sedimentological and Paleontology. ..45 Occurrence of an Ancient Lake ...45 Intrinsic Geological ResourcesDevelopment... 45

References 45 Enclosures

-111- I List of Figures/Table/Photographs

Figures

1. Locationmap of the Maliau Basin. ..2 2. Routeand samplelocation map... 4 3. Regionalgeological map of centraland SoutheastSabah ...6 4. Regionalstructural cross-sections in centraland SoutheastSabah ...7 5. Simplified stratigraphyof the Maliau Basinarea... 10 6. Geologicalmap of the Maliau Basin ...13 7. Depositionalenvironment for the Maliau Basinarea... 20 8. Satelliteand radar imagesof the Maliau Basin ...21 9. Morpho-structuralmap of the Maliau Basin... 22 10. Rosediagram showing negative lineament pattern within the Maliau Basin 23 11. Geologicalcentre of the Maliau Basin... 25 12. Rosediagram showing fracture pattern within the Maliau Basin... 29 13. Schematictectonic evolution of the Maliau Basin ...32 14. Schematictectonic modelto showdevelopment of the Maliau Basin ...35 15. Evolution of Maliau Basindrainage system ...39 16. Schematicmodel to showriver undercuttingone valley... 40 17. Schematicmodel to show developmentof gorges,benches and waterfalls. 40 18. Schematicmodel to show detachmentof sandstonebeds ...41 19. Schematicmodel to showhow different rock typescontrol soil types. ..41

Table 1. Results of biostratigraphical studies on mudstone samples ...9 I Photographs

1. Temporarycamp site ...3 2. Crossingthe Maliau River... 3 3. Compositionof sandstone...12 4. MudstoneUnit ...14 5. MudstoneUnit ...14 6. Mudstonewith Thin SandstoneUnit ...15 7. Rare coal layer ...15 8. Sandstoneand MudstoneUnit... 16 9. Sandstoneand MudstoneUnit with ripple mark... 16 10. Gastropodfossil... 17 11. Vertical trace fossil... 17 1\ 12. SandstoneUnit ...18 13. SandstoneUnit showingcross-bedding. ..18 14. Gently dipping layers ...24 15. Nearly horizontallayers ...24 16. Two setsof fractures...27 17. Fracturesperpendicular to river flow... 28 18. Fracturesparallel to river flow... 28 II 19. Normal fault ...30 20. Shearedzone ...30 21. River downcuttingutilizing existing fractures... 42 22. Lateral erosionon weakermudstone layers... 42 23. Hard sandstonelayers produces structural benches and waterfalls 43 24. Rockfall due to headwarderosion... 44 25. Landslideon a steepside of a river valley... 44

-IV- Acknowledgement

The project could never have happened without the kind invitation of Dr. Waidi Sinun and Dr. Tony Greer. Similarly the project could never have been realised without the generous financial and technical support from Yayasan Sabah and DANCED, Denmark and logistical and technical support from Universiti Malaysia Sabah.

During the early part of the research, aerial photographs were extensively used. This would not have been possible without the full co-operation of the Aerial Photograph Section of the Land and Survey Department, Sabah. The research would have been incomplete without the biostratigraphical analysis of mudstone samples by Shell-Miri, led by Pedro P. Barbeito with contribution from Ajeng Imang Baya, Lau Ping Kiong, Musa Musbah and Voon Hian Fah.

Throughout the research, Chang Fui Khiong, my research assistant, gave full support and drafted the geological map. My colleague, Dr. Marcus Jopony gave useful suggestions along the way. The field work was made easier with logistic support by staffs and students of Universiti Malaysia Sabah (Mohamad Yusuf, Sanin, Cyprian, John and Peter), field staffs of Yasasan Sabah (Jadda Suhaimi, Norhaidah Maral, Andy, Hasan, Shapie and Chak) and friends (Epip, Phillip, Vitalis, Abel and Melvin).

The Geological Team. From left—Melvin, Epip, Sanin, Chang, Chak, Hasan, Shapie, Author and Andy.

- v - I. INTRODUCTION

Background

The Maliau Basin is one of a series of saucer-shaped basins found in Central and Southeast Sabah (Fig. 1). These basins are depositional sites of ancient sedimentary rocks. The peculiar shape of the basin has puzzled many people--scientists and non- scientists. Earlier regional geological studies indicate that the development of these saucer-shaped basins are controlled by deep-seated geological structures (e.g. Lee & Coong, 1989; Tongkul, 1993). Recent local geological studies within the Maliau Basin by Tjia et al., (1990) and nearby Malibau Basin by Allagu (1997) also support the above interpretation. However, the exact mechanism as to how the geological structures controlled the shape of these saucer-shapedbasins is not clear. This is basically due to the lack of geological and structural information on these basins, including the Maliau Basin. In other to obtain a deeper understanding on how the Maliau Basin developed a more detailed and systematic study on its structural geology was carried out.

0 b j ectives/Scope

The aim of the study was to produce a geological and structural map of the Maliau Basin; to determine the architecture (structural plan of the basin) and tectonic evolution of the basin; and to understand the ongoing geological processesoperating within the basin.

I Methodology Aerial photographs together with satellite and radar imageries were used to determine the regional structure of the basin and surrounding areas. Based on secondary information and from aerial photo interpretations several field works were carried inside and outside the basin. Four fieldtrips were carried out inside the basin (23-30 October 2000, 18-23 February 2001, 4-17 April 2001 and 10-18 May 2001), whereas two fieldstrips were carried out outside the basin (12-18 April 2001 and 15-22 October 2001. Field data collected included lithological types, orientation of strata, fractures and faults. Several mudstone and sandstone samples were taken for laboratory analysis. The sandstonewas used for petrographic studies whereas the mudstones for its microfossil content. A total of 40 mudstone samples were sent to Shell in Miri for paleontological analysis. The fieldwork was mostly carried out along the tributaries of the Maliau River (Photos. 1 & 2). The field route and sample location is shown in Figure 2.

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II Figure 1. Locationmap of Maliau Basin

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II I Photo 1. Temporary campingsite while doing fieldwork alongthe Maliau River.

Photo2. Crossingthe Maliau River.

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Figure 2. Field traversesand mudstonesample locations

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II Tectono-Stratigraphic Units

The Mesozoic basementrock representsthe oldest rock group (estimated to be more than 65 million years old). The basement rock is made up of igneous, metamorphic and sedimentary rocks associations. The igneous rock consists of serpentinite, gabbro, basalt and granodiorite whereas the metamorphic rock consists of gneiss, amphibolite and schist (Yin, 1985). The sedimentary rock consisting of cherts and red shales are closely associated with the basalts. The rock units included here are the Crystalline Basementand I Chert-Spilite Formation (Leong, 1974). The basement rock is interpreted as remnant of an old oceanic lithosphere obducted over East Sabah (Hutchison, 1988). The basement rock has undergone intense deformation and its present regional structural trend is NW- SE.

Lying unconformably on top of the basementrock are intensely deformed Paleogene(25- I 65 million years old) sedimentary rocks comprising mainly of sandstone,mudstone and minor occurrence of limestone. The rock units included in this group are the Sapulut Formation, Labang Formation and Kulapis Formation. The rock group has a regional structural trend oriented NW-SE and SW-NE. Steeply dipping beds and overturned beds are common.

The third rock group is a Neogene (18-22 million years old) chaotic deposit or melange deposit which lie unconformably on the basementand Paleogene sediments. The deposit is characterised by the occurrence of chaotic mixtures of blocks of a single lithology or different exotic lithologies in a grey or red mud matrix. The rock unit is represented by the Kuamut Formations. This rock unit is interpreted to representdeposits due to regional sliding and slumping.

The fourth rock group is a moderately deformed Neogene sedimentary rock comprising of sandstone,mudstone with minor occurrence of conglomerate, limestone and coal. The Neogene sedimentary rock can be divided into an older and younger rock group. The older rock group (16-18 million years old) which is comparatively more deformed include the Kalabakan and Tanjong Formations, whereas the younger rock group (5-15 million years old) which shows little deformation include the Kapilit and Simengaris Formations. These rock group occurs as sedimentary depressions, characterised by their circular- and ellipsoid-shaped basins.

The youngest rock group comprised of Quaternary (less than 2 million years old) sedimentary rocks and volcanic rocks. The sedimentary rocks comprised of gravel, sand,

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Figure 3. Regional geological map of Central and SoutheastSabah I

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~rn ] ~ rn ro ..c § ~ ~ Q) oS rn rn 8 ~ ~ 0 u Q) rn I rn rn 0 1-4 U I ] ~ r/) -.:t Q) ~ I ~

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7- I I silt, mud and coral fragments.The volcanic rocks consistmainly of andesiticand basaltic lava flows.

Major Structures

The structural lineaments which representsbedding strikes on the sedimentary rock units shows various trends in central and Southeast Sabah (see Fig. 3). The Paleogene sediments show NW-SE and NE-SW structural lineaments. In contrast, the Neogene sediments show concentric (circular and ellipsoid) structural lineaments. The circular structural lineaments clearly seen on the Maliau, Malibau, Bangan, Kuamut and Bukit Garam Basins trends NE-SW. The ellipsoid structural lineaments clearly seen on Sesui, Luis and Silimpopon Synclines trend NW -SE.

Two prominent sets of faults trending NE-SW and NW -SE occur in central and Southeast Sabah. The two sets of faults are closely associated with the all the rock units here. Within the vicinity of the Maliau basin area, two prominent NE-SW trending faults I occur. The Pinangah and Lonod Fault identified by Collenette (1965) occur at the Northwest and Southeastof the basin, respectively. The Lonod Fault, which appearsto be a horizontal fault, separatesthe Maliau and Malibau basins. Another fault, named the Lombunan Fault occurs Southeastof the Pinangah Fault.

III. GENERAL GEOLOGY OF MALIAU BASIN

Stratigraphy

The age and distribution of the rock units in the Maliau Basin and surrounding areashave not been fully resolved. Previously, the sedimentary rocks within the Maliau Basin has been designated as the Tanjong Formation and tentatively assigned an Upper Miocene age by Collenette (1965). Based on sparse paleontological evidence Tjia et al., (1990) suggested a Lower Miocene age for the sedimentary rocks within the basin-the name Tanjong Formation maintained. Based on a recent extensive sedimentological and paleontological study of the sedimentary rock units in the surrounding areas of the Maliau Basin a revised stratigraphy of the area has been proposed by Allagu (1997 & 2001). Allagu proposed that younger (Middle-Upper Miocene) and less deformed rocks in this region, previously assigned as Tanjong Formation be changed to Kapilit Formation whereas older (Lower Miocene) and relatively more deformed rocks, previously assigned as Tanjong and Kapilit Formations be changed to Tanjong Formation.

In this study, following the revised stratigraphy proposed by Allagu, rocks that fomled the Maliau Basin is now assigned the Kapilit Fomlation (see Fig. 3). The reason for this is that the rock units here are of Middle Miocene in age and relatively less defomled compared to rocks in the surrounding areas. Biostratigraphical analysis results of

-8 - mudstonesamples from the Maliau Basin(Table 1), althoughnot very good, gavean age range from 10 to 16 million years old. Based on available information, a simplified stratigraphyof the Maliau Basinand surroundingareas is constructed(Fig. 5).

Table 1. Summaryof Resultsfrom BiostratigraphicalAnalysis by SarawakShell Bhd.

Sample Number Nannoplankton Palynological Planktonic Zone Zone Foraminifera Zone ~.~..l Barren MB2 Barren MB3 MB4 MB5

Delta front-River mouth

Barren Coastal River mouth

MB23 Barren I P400? Barren Barren P400? --' ,J Barren P500/610 Barren Barren Barren LLI NN4-NN5 ,-- ..,. LL2 ..,. " SNI Barren Barren SN2 Barren Barren SN3 Barren Barren SN4 Barren Barren Barren Barren SN6 'T~-' P400 SN7 SN5-SN13 SN9 Barren Barren Note:~ P400 (Late Lower Miocene: 16-17 Ma), P400/500 (Early Middle Miocene: 14-15 Ma), P500/61 (Late Middle Miocene: 11-13 Ma), SN5-SNI3 (Middle Lower Miocene to Late Middle Miocene: 12-22 Ma), NN4 (Late Lower Miocene: 16-17 Ma), NN5 (Early Middle Miocene: 14-15 Ma), NN6 (Middle I Middle Miocene: 12-13ma), NN7 (Late Middle Miocene: 11-12Ma).

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I Figure 5. Generalstratigraphy of the Maliau Basinand surroundingareas

I -10- Lithological Units

The Kapilit Formation in the Maliau Basin and surrounding areas consists of interbedded sandstone and mudstone of varying proportion with minor occurrence of coal beds. The sandstoneand mudstone range in thickness from a few centimetres to several metres. The prominent ridges are made up of thick amalgamated sandstone beds reaching up to 30 meters thick. Most of the sandstonesare fine-grained and consists mainly of quartz and chert grains. The grains are cemented together by clay minerals (Photo. 3). The mudstones are dark greyish in colour and contain high organic materials. Several coal beds have been found associated with the mudstone. The mudstone also contains microfossils (agglutinated forms and calcareous benthonic forms) and macrofossils such as gastropods and bivalves.

The Kapilit Formation is estimated to be about 7500 metres thick. Based on the dominanceand thicknessof sandstoneand mudstonesequences, the Kapilit Formation has beendivided into four mapablelithological units. The distributionsof theseunits are I shownin the geologicalmap (Fig. 6 & Enclosure1). Mudstone Unit

The first lithological unit consistsmostly of mudstone(95%) with minor occurrenceof fine-grainedsandstone beds (5%). The muddy unit generallyappears dark grey in colour I and is found at the baseof the Kapilit Formation. A good exposureof this unit can be seenalong Sg.Namatoi, near Lake Linumunsut(Photo. 4 & 5).

Mudstone with Thin SandstoneUnit

The secondlithological unit consistsof grey mudstone(80%) interbeddedwith thin fine- I grainedsandstone (20%). The unit can be found towardsthe centreof the basin,near the Maliau Falls area(Photo. 6). Thin coal layersare alsoassociated with this unit (Photo.7).

Sandstoneand Mudstone Unit

The third lithological unit consists of approximately equal amount of sandstone (50%) I and mudstone layers (50%) showing various thicknesses. This unit can be found near the rim and at the centre of the basin (Photo. 8). The sandstonelayers usually shows cross- bedding and ripple marks (Photo. 9). Blocks of sandstone from this unit contained gastropod macrofossil (Photo. 10). The mudstone layers shows vertical burrows left by certain organism (Photo 11).

SandstoneUnit

The fourth lithological unit consists of thick amalgamatedsandstone (80%) and mudstone layers (20%). The rim of the basin is mostly made up from this unit (Photo. 12). Cross- bedding is the most common feature seen in this unit (Photo. 13). I

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Photo. 3. Mineral composition of sandstoneconsisting mostly of quartz grains (0.3-0.5 mm) cemented by clay minerals.

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Figure 6. Geological map of Mali au Basin -13 - I

Photo4. MudstoneUnit. The unit consistentirely of mudstonewith someconcretions. Located at Sg.Namatoi near Lake Linumunsut.

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Photo5. MudstoneUnit. Thin samdstoneinterbeds within the I mudstone.Located at Sg. Gaharunear Lake Linumunsut.

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Photo6. Mudstonewith Thin SandstoneUnit. The sandstonebeds usually pinch-outlaterally. Locatednear Rafflesia Camp.

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Photo7. Rarecoal bed found within the Mudstoneand Thin I SandstoneUnit. Locatednear Simpang Tiga, Sg.Maliau.

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I Photo8. Sandstoneand MudstoneUnit. Approximatelyequal amount of sandstoneand mudstone layers. Located near Giluk Fall.

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Photo 9. Sandstoneand Mudstone Unit. Ancient ripple marks on top of sandstone.Located upstream of a river tributaries near Raffiesia Camp.

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Photo 10. Gastropodfossils embedded in sandstonefrom the Sandstoneand MudstoneUnit. Locatednear Lake Linumunsut.

Photo 11. Vertical trace fossils within the mudstone layers from the I Sandstoneand Mudstone Unit. Located near Rafflesia Camp. -17- Photo12. SandstoneUnit. Amalgamatedsandstone beds. Locatednear Giluk Fall.

Photo 13. SandstoneUnit. Cross-bedding. Located near Maliau Fall.

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I Environment of Deposition

Based on the sedimentary structures and fossils (macro and microfossils) found within the four lithological units, the Maliau Basin area appears to have been deposited in a deltaic-shallow shelf-slope area. The Mudstone Unit was probably deposited as marine muds in a prodelta setting. The Mudstone with Thin Sandstone Unit may have been deposited in lagoon, bay or mangrove swamp areas. The Sandstone and Mudstone Unit together with the Sandstone Unit was possibly deposited as tidal channels, inter- distributary channels, river mouth bars, tidal sand bars and storm sands (Fig. 7). I IV. STRUCTURAL GEOLOGY

Geometry

The Maliau Basin exhibits a nearly circular shape with steep slopes on all sides (Fig. 8). The elevation of the basin is affected by the slight tilt of the basin to the Southeast. Overall, the elevation of the basin is about 1500m at the rim and drops gradually to about 800m at the centre of the basin. Except for a narrow opening in the Southeast it is enclosed on all sides. The size of the enclosed basin is about 390km2 with a maximum diameter of about 25km. The basin is carved by a set of radiating tributaries of Sg. Maliau, leaving behind erosional ridges and peneplains.

Lineament

The positive lineaments (ridges representing bedding strikes) and negative lineaments I (river valleys representing fractures) shows the overall structure of the Maliau Basin area (Fig. 9 & Enclosure 2). The positive lineaments clearly outlines the circular rim of the basin, especially the northern, western and easternrim. The positive lineaments are fairly continuous at the rim and along the Strike Ridge. In other parts of the basin the positive lineaments are segmented by the occurrence of river valleys, producing V -shaped lineaments, referred to as flatirons. The negative lineaments dissect the basin surface at I various angles. However four negative lineament directions, NW -SE, NE-SW, NNW- SSE and WNW-ESE appearsto stand out most (Fig. 10).

Bedding/Strata Structural field data shows that the orientationof bedding generallyfollows the semi- circular shapeof the basin.The dip of beddingvaries slightly inside and outsidethe basin (seeEnclosure 2). Outsidethe basin, at the foot of the basinnear Agathis Campand Lake Linumunsut,the bedding dips between40-45 degrees.The dip becomesgentler towards the rim of the basin. At the rim of the basin, the bedding dips between15-25 degrees (photo. 14). Towards the centre of the basin, the bedding dips gently between3-10 I

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I Fig. 7. Schematic model to show the depositional environment of the Maliau Basin area

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Figure 8. Satellite (top) and radar (bottom) imageries of the Maliau Basin and surrounding areas

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GEOLOGICAL CROSS -SECTION IA -B) (He;ght in metre)

SSW(A) SOuthernRon C._I Trophy S Malia" Strike Rklge Northern Rim lake linumunsul + 1 1

Figure9. Morpho-Structuralmap of Mali au Basin

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w E w~ ~E

~ N = 969 -.- N= 138 s s I B ~ E -'-N

w I-E

N = 397 N=40 s s

c --'-N F --'--N

w E w rE

N=182 -,- s s N=40

F N I

w~ ~E

N=1766 s

Figure 10. Rose diagram of Maliau Basin negative lineaments (river system) : A) Order 1, (B) Order 2, (C) Order 3, (D) Order 4, I ( E ) Order 5, F ) Order 6 and ( G. All order.

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Photo14. Gentlydipping layers.Located near Simpang Tiga Long Ridge River.

Photo 15. Nearly horizontallayers. Located near Alin Fall.

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Figure 11. Geologicalcentre of the Maliau Basinlocated near Camel TrophyField Centre

I -25- degrees(Photo. 15). Basedon the convergenceof dip direction, the geologicalcentre of the Maliau basinhas beenlocated near the CamelTrophy Field Station(Fig. 11).

Fractures/Joints

Sub-vertical to vertical fractures of various scales (some reaching tens of meters long) are commonly seen on the rock (Photos. 16, 17 & 18). The fractures, which usually occur perpendicular and parallel to the strike of bedding, show various orientations (Fig. 12). Four fracture orientations predominate, NW-SE (extensional fractures), NE-SW (tensional fractures), NNW-SSE (left shear fractures) and WNW-ESE (right shear fractures).

Faults

Fault occurrence is quite rare inside the basin. During the present study no fault was seen. Previous study by Tjia et al, (1990) recorded a few minor normal faults trending NW-SE. Outside the basin, minor normal faults have been recorded near Lake Linumunsut, trending E- W and Southwest of the basin trending NE-SW (Photo. 19). The presence of sheared zone characterised by brecciated sandstone blocks south of the Maliau Basin, along the road to Belian Camp (Photo. 20) possibly indicate field evidence for the Lonod Fault.

Deformation/Stress

The gentle dip and the lack of faulting and folding within bedding suggests that the Maliau Basin sediments have undergone a relatively mild deformation compared to other rock units, such as the Labang Formation in the surrounding area. The concentric bedding strike pattern suggests that at least two main compression directions occurred here. Based on the orientation of major faults, such as the Lonod Fault, the fracture pattern and the slight tilt of the basin towards the Southeast, a NW -SE and NE-SW compression direction is interpreted.

I v. BASIN ARCHITECTURE AND EVOLUTION

Architecture (Structural Plan of the Basin)

The Maliau Basin is made up of about 7,500 metres thick sandstoneand mudstone layers I deposited in a deltaic-coastal environment. The layer at the base of the basin consists mainly of mudstones reaching up to 2000 metres thick. Near the rim of the basin, thick sandstoneinterbedded with thin mudstone and coal seam occur. Towards the centre of the basin a series of sandstone-dominated and mudstone-dominated sequences of various thicknesses occur. The deposition took place during the Middle Miocene epoch (10-15 I million years ago), spanning about 5 million years. The youngest sediment is located near

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(b) Photo Ifa & b. Two setsof fracturestrending NW-SE and NE-SW on sandstone.Located near Ginseng Camp.

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Photo17. Fractureperpendicular to river. Locatednear Giluk Fall.

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Photo 18. Fractureparallel to river. Locatednear Rafilesia Camp.

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I 116"45'E 116"50'E 116"SS'E 117'OO'E

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I Figure 12.Rose diagram showing fracture pattern

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Photo19. Normal fault trendingNE-SW cutting througha coal layer (70 cm thick). Locatedoutside Maliau Basin (Southwest)

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Photo20. Shearedzone characterized by disruptedlayers. Possibly a I manifestationof the Lonod Fault. Locatednear Belian Camp. -30-

I the Camel Trophy Field Station, whereas the oldest sediment can be found near Lake Linumunsut. The basin sits on older sedimentary rocks, also comprising of thick layers of sandstoneand mudstone, with slight unconformity. The semi-circular shape of the basin generally follows the underlying structural trends of the older sediments. The E- W trending strike ridges of the older sedimentary rocks bounds the basin on the northern side whereas the NE-SW trending Lombunan-Pinangah and Lonod Faults bounds the basin on its western and eastern side. Based on the structural geometry of the basin it is envisaged that the present southwestern rim of the basin originally extend for a few kilometres further south.

Evolution (How the Basin Formed)

The evolution of the Maliau Basin, together with the other Neogene sedimentary basins was structurally controlled. Deep-seated structures such as faults found within the pre- Neogene rocks are directly or indirectly manifested in the overlying Neogene sediments. The topography and structural trends of the underlying rocks controlled the initial development of the Neogene basins. However, subsequentextension and compression in the region controlled the shape of the basins as we seetoday. It is envisaged that the early development of the Maliau Basin was closely related to the deposition and deformation of the older underlying sedimentary rocks.

The evolution of the Maliau Basin, together with the other Neogene basins are illustrated I by the tectonic models in Fig. 13 & 14. The evolution of the Maliau Basin is traced back from the early Miocene epoch, about 22 million years ago. After the Pliocene epoch, about 5 million years ago, the architecture of the Maliau Basin has not changed, except for the effect of weathering and erosion on its surface.

During the early Miocene most of Sabahas we know today was still submerged under the sea. The land mass during this time mostly occur in western Sabah, where the Crocker Range and Trusmadi Range is today and several islands in east Sabah, where the basementrocks near Lahad Datu occur today. The Pre-Neogenerocks forms a fold-thrust belt trending NE-SW in western Sabah and swinging E- W in northern and eastern Sabah (Fig. 13a & Fig. 14a). The fold-thrust belt resulted from compression of the pre-Neogene rocks as the Australia-Pacific Plates collided with the southeasternmargin of the Eurasia Plate. The coastline and sedimentary basins possibly follows the regional structural trend of the pre-Neogene rocks. The uplifted pre-Neogene rocks provided sediments for the newly formed basins in western and easternSabah.

After the development of the fold-thrust belt, the easternpart of Sabah began to subside and formed a huge elongate basin trending NE-SW where older Neogene sediments began to accumulate (Fig. 13b & Fig. 14b). The subsidenceof the basin, accompanied by regional normal faults, was probably related to the ongoing NW -SE extension in this region due to the opening of the Sulu Sea Basin located further north. The major NW -SE horizontal fault south of Sabah possibly forms the southeastern boundary of the extension. As the region continued to subside more and more sediments accumulated. In certain unstable areas, such as slopes, large submarine slumps and slides occurred

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Most of easternSabah still underwater

Regional subsidence in I easternSabah due to NW -SE extension

Fig. 13 a & b Schematictectonic evolutionof Maliau Basin I -32- I

SCS -South China Sea Basin SS -Sulu Sea basin CS -Celebes Sea Basin

Extensivedeposition of Neogenesediments

Initial depositionof Maliau sediments

I Fig. 13c&d Schematictectonic evolutionof Maliau Basin 33 - End of sedimentdeposition in the Maliau Basin

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Circular shapeof the Neogenebasins established

Fig. 13e&f Schematictectonic evolutionof Maliau Basin

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Fig. 14a, b & c. Schematictectonic model to showthe I developmentof the Maliau Basin

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Fig. 14 d, e & f. Schematic tectonic model to show I the development of the Maliau Basin

-36- I I producing the Neogenemelange deposits. The relative motion of horizontal faults and normal faults may have initiated the segmentationof the huge elongatebasin into several smallerbasins (Fig. 13c& Fig 14c).

By early Middle Miocene, between 14-15 million years ago, the eastern part of Sabah was again subjected to NW -SE compression as tectonic plate interaction continued in this region. Active volcanism in eastern Sabah during this time was probably related to this tectonic force. The compression resulted in the gentle folding of the older Neogene and concentric shape of the smaller basins, with the faults acting as their boundaries (Fig. 13d & Fig 14d). At this time, most of eastern Sabah was still under the sea-mostly very shallow waters.

The concentric-shaped basins were subsequently filled by younger Neogene sediments continuously until about 9 million years ago when the basins was uplifted above sea level due to continued compression in eastern Sabah. The compression resulted in the gentle folding of the sedimentary layers and at the same time accentuated the concentric shape of the basins, through the reactivation of old faults (Fig. 13e & Fig 14e). The intrusion of I Mt. Kinabalu in West Sabahoccurred during this time.

As a result of the crustal shortening of the region due to the continued tectonic plate collision in this region, the whole of Sabah, including the shallow coastal area we see today, was fully uplifted above sea level by the end of Miocene time, about 5 million years ago (Fig. 13f & Fig. 14t). The Maliau Basin and the surrounding areas were probably uplifted to its present height with a slight tilt to the Southeast. Following the uplift, the Maliau Basin and surrounding areas were subjected to intense weathering and erosion that continued up to this day.

VI. ONGOING GEOLOGICAL PROCESSES

Apart from knowing the evolutionof the basin, which highlights the intrinsic geological value of the basin, the study provides a better understandingof the ongoing geological processesoperating in the area, which is important for managementpurposes. The distribution of various lithological types,the orientationof beddingplanes and fractures have direct relationshipto the drainagepattern, headwarderosion, and occurrenceof waterfalls,landslides and soil typesof the basin.

Drainage System Evolution

The saucer-like surface of the Maliau Basin and the slight tilt of the basin to the Southeast control the flow of the rivers. The orientation of the inward dipping strata, which is reflected on the surface, controls the speed at which the surface water moves. Near the rim of the basin, where the surface slope is higher the water moves very fast, whereas at the geological centre of the basin, near the Camel Trophy Field Station, where surface slope is nearly horizontal, the water moves relatively slower. The slow movement

-37- of water resulted in water clogging at the geologicalcentre of the basin. Where the speedof wateris higher,the rate of downcuttingand headward erosion is faster.

Lines of geological weakness, such as fractures assist river downcutting (Photo. 21) and the fonnation of gorge-like valleys. The four major sets of fractures on the strata, trending NW-SE, NE-SW, NNW-SSE and WNW-WSE directly controls the drainage pattern here (Fig. 15). The downcutting of the rivers produces deep gorges and leave behind narrow ridges. Where the river has taken advantage of weak strata, such as the mudstone-dominated sequences,it migrates laterally (photo. 22), undercutting one valley side to produce an asymmetrical valley (Fig. 16). Where the river has cut down on the hard sandstonelayers and soft mudstone layers, selective weathering and transport on the I valley sides produces structural benches or terraces with each hard stratum. As the river flows over these structural benches,waterfalls develop (Photo. 23).

Waterfall Development

The Maliau Basin produces some of the most spectacular waterfalls in Malaysia. During the study 29 waterfalls showing heights more than 5 metres were recorded (Enclosure 3). The density of waterfalls is extremely high. For example, in an area of about 10 Km2 at the geological centre of the basin (near Camel Trophy), several spectacular waterfalls such as the Giluk, Noh, Mempersona, Takob-akob, Epip and Alin, occur. Some of these waterfalls, like the Giluk Fall are made up of several steps. The high density of waterfall I here can be attributed to the right combination of rock types (hard and soft layers), geological structures (vertical fractures and gentle dipping layers) and geological processes.The common occurrence of multi-storey waterfalls is related to the repetitive occurrence of resistant sandstonelayers and weak mudstone layers (Fig. 17).

Landslide Occurrences

Severallandslides have beenobserved inside and outsidethe basin.The steepside of the basinis naturallyprone to landslide.Inside the basin,where the slopesare quite gentle, landslidesare not as common.However, the orientationof the bedding planes and the occurrenceof fractures assist in the developmentof landslides.The fracturesprovide readymade fracture planes allowing the rocks to fragmenteasily (Photo. 24). Easyaccess of groundwatercausing increase pore-water pressure can add load to slope that can lead to slope failure (Photo. 25). The downwarddipping beddingplanes allow the detached massesto slip easily (Fig. 18). The thick sandstonebeds can slide easily over the mudstonebeds. The steepand narrow valleys, createdby lateral erosion are prone to landslideoccurrence. I Soil Development

The different rock units (parentmaterial) recognised in this study affect the distribution and variability of soil types within and outsidethe basin.The sandstone-dominatedstrata producesoils that can be easily leachedout leavingbehind beach-like white sand.On the I

-38- I Fracturescreated by NW -SE tectonic compression

ExtensionalFrac:tures Tensional Fractu-es --She. Fractures

Fractures provide weak planesfor stream I downcutting

YO,.,. Early Drainage System 1 PreseN Rim of Basin

Headward erosion dowcutting creates deeper and longer river valleys

"'-\" MotureDrainage System -,J PresentRim of Sasin I Figure. 15 Evolutionof Maliau Basindrainage system

-19 I

Fig. 17 Schematicmodel to showriver undercuttingone valley side in the Maliau Basin

I

Fig. 18 Schematicmodel to showthe developmentof narrowgorges, structuralbenches and waterfalls in the Maliau Basin

-40-

I I

Fig. 18 Schematicmodel to showdetachment of sandstonelayer down dip alongweak mudstone layer in the Maliau Basin

I

Fig. 19 Schematic model to show how different rock types control the distribution of different soil types in the Maliau Basin

-41 - I I

Photo21. River downcuttingutilizing existingvertical fractures.Located near Ginseng camp.

I

I

Photo22. Lateral erosionby river on weakermudstone I layers.Located near Simpang Tiga.

-42-

I I

(a)

Photo by C.L.Chan (b)

Photo 23 a & b. Hard sandstonelayers produces structural bencheswhere waterfalls develop. Located at Maliau Fall.

-43 - Photo24. Rock fall dueto headwarderosion on fractured sandstone.Located near Giluk Fall.

I

Photo25. Landslideon a steepside of a river valley. Located nearRafflesia Camp. I -44-

I I other hand the mudstone-dominatedstrata, which are less porous, leave behind mangrove-likemud (Fig. 19).

VII. MANAGEMENT IMPLICATION

The occurrence of V -shaped valleys, deep gorges, numerous short streams, gullies and waterfalls indicate that the Maliau Basin is still in its early geomorphic stage of evolution. Headward and lateral stream erosion will continue to the change the shape and course of the Maliau River and its tributaries. The rate of erosion would dramatically increase should more rainwater runoff enters the river as a result of surface disturbance. To ensure that the present fragile landscapeand spectacular waterfalls of Maliau Basin is maintained, any activity that may drastically alter the ongoing gomorphological processes (weathering, erosion and deposition) here should be banned.

I Due to the occurrenceof fracturesand unfavourableorientation of rock stratasome areas within the Maliau Basin are more prone to landslideand rockfall occurrences.To ovoid suchgeological hazards a proper geological evaluationof future developmentsites and activities within the basinshould be carriedout.

VIII. FUTURE STUDIES

Sedimentology and Palaeontology

I To obtain a much deeper understanding of the ancient depositional history of the Maliau Basin, a detailed study of the sedimentary strata throughout the basin is still required.

Occurrence of an Ancient Lake

I After the uplift of the Maliau Basin, it was probably filled up with rainwater, where a huge lake existed, before the present drainage system developed. It might also be possible that seawater was trapped inside the basin. Evidence for the existence of such a lake in the past, near the geological centre of the basin would be interesting to study.

As the lake drained out its water and sediments out of the basin a huge alluvial fan deposit may have developed at the opening of the basin, near where the Belian Camp is located. A detailed study of the alluvial sediments may provide support for this. Both studies would provide important clues as to the nature of the existing ecosystemsof these sites.

I During the course of the present study numerouswaterfalls were encountered.The characteristicsof thesewaterfalls were not documented,as this was not the focus of the study.To properly evaluatethe potentialof thesewaterfalls for eco-tourismdevelopment a systematiccharacterisation of eachwaterfall is required.

References

Allagu Ballaguru, 1997. Sedimentologyand structural developmentof the Malibau Basin, Sabah.Tectonic Stratigraphyand PetroleumSystems of Borneo Conference,UBD (abstract:65).

Allagu Ballaguru, 2001. Tectonic evolution and sedimentation of Southern Sabah, Malaysia. PhD. Thesis(unpublished), University of London.393 p.

Collenette, P., 1965. The geology and mineral resources of the Pensiangan and Upper Kinabatangan area, Sabah,Malaysia. Geol. Surv. Borneo Region, Mem. 12, 150p.

Hutchison, 1988. Stratigraphic-tectonic model for East Borneo. Bulletin Geological Society of Malaysia 22, 135-151.

Lee, C.P. & Coong, T.K., 1989. Circular basins of Sabah. Geo. Soc. of Malaysia PetroleumGeology Seminar (abstract: 40).

Leong, K.M., 1974.The geologyand mineral resourcesof the Upper SegamaValley and I Darvel Bay area,Sabah. Geological Survey Malaysia Mem. 4 (revised),354 p. Tjia, H.D., Komoo, I, Lim P.S. & Surat, T., 1990. The Maliau Basin, Sabah: Geology and tectonic setting. Bulletin Geological Society Malaysia 27, 261-292.

Tongkul, F., 1993. Structural control on the development of Neogene basins in Sabah. I Bulletin Geological Society Malaysia 33, 95-103. Yin, E.H., 1985. Geological map ofSabah. 3rdEdition.

Enclosures

1. Geologicalmap of Maliau Basin 2 Structuralmap of Maliau Basin 3. Waterfallsof Maliau Basin I

I WATERFALLS OF MALIAU BASIN CONSERVATION AREA

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