# Major structural unconformities across Malaysia: geodynamics and basin development of Lawin, Perak

Noor Liana Mahmud, Benjamin Sautter

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Noor Liana Mahmud, Benjamin Sautter. Major structural unconformities across Malaysia: geodynamics and basin development of Lawin, Perak. IOP Conference Series: Earth and Environmental Science, Jul 2020, Kuching, France. pp.012011, 10.1088/1755-1315/1003/1/012011. [hal-04886627](https://hal.science/hal-04886627v1)

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# Major structural unconformities across Malaysia: geodynamics and basin development of Lawin, Perak

To cite this article: Noor Liana Mahmud and Benjamin Sautter 2022 IOP Conf. Ser.: Earth Environ. Sci. 1003 012011

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# Major structural unconformities across Malaysia: geodynamics and basin development of Lawin, Perak

Noor Liana Mahmud¹, Benjamin Sautter²

¹,² Department of Geosciences, Universiti Teknologi PETRONAS, MALAYSIA.

E-mail: liana_19001427@utp.edu.my, benjamin_sautter@utp.edu.my

**Abstract.** The first deposits overlying an unconformity may give important insights on the tectonic vertical motions and erosion processes that erased previous records. The Early Cenozoic Deposits in Peninsular Malaysia are reported to be isolated and lie unconformably on Paleozoic basement rocks. Denudation pulses from the Late Cretaceous to Eocene left the Cenozoic deposits in scarce. Stratigraphic and sedimentology studies were restricted to the availability of the resources. This study aims at providing information on the development of the early deposits from a structural perspective with the emphasis on source to sink dynamics. We interpreted the structural settings of Lawin using Digital Elevation Model (DEM) data and satellite images. Structural interpretations were cross-checked with published studies. Field observations were conducted and have been assessed through chemical and petrographic analyses. Evidence of compression and shearing are found to support interpretation. We learned that Lawin had experienced a compression and stages of extensions in its history and developed during extension. Lawin is associated to a major fault that reactivated several times, and from XRD analysis, we have illite-mica which has a close-relationship to a frequent tectonic activities and kaolinite is due to weathering. Petrography analysis were performed from SEM images and thin sections to support minerals found. Evidence of mineral fragments, iron-rich sedimentation and clay fillings were common with few kaolinite pouches. Sedimentology studies were modelled as alluvial fans. Based on DEM analysis, two different lobes are observed and on outcrop, two different angles of tilted bedding between the two lobes were measured, thus we modelled them as incised alluvial fans where the recent fan we see on DEM has extended further from abandoned lobe. Due to its tilting towards the west, we modelled them following an extensional regime.

**Keywords:** SE Asia regional unconformity, Cenozoic rifting, basin study, conglomerate.

## 1. Introduction

A regional size of unconformity was reported throughout the Western Sundaland [1], [2]. The Late Cretaceous to Paleogene sections were missed and resumption of sedimentation only occurred from the Eocene onwards [2]–[4]. New models were illustrated to reconstruct the Western Sundaland from the Cretaceous to recent million years [5], [6]. Evidence was found to demonstrate the activities contributed to the regional unconformity [3], [7], [8]. Offshore of the Western Sundaland was stretched in the Cenozoic and filled with Tertiary sediments, thickened by rapid subsidence during the Mid-Late Tertiary [3], [9]. The subsidence had occasioned to relatively uplifted the continental core of the Peninsula [3], which has been exposed to surface since the Jurassic time [10] and unprotected from intense weathering due to tropical climate and heavy rainfall. Records here are incomplete due to limitations of study area and age constraint to uncover the missing episodes, both onshore and offshore areas [2]. The absence of fossilized materials makes it harder to add more information, thus previous works were done from sedimentology and lithostratigraphic point of view.

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The Malay Peninsula has exposed to surface since the Jurassic [10], thus has been subjected to intense weathering ever since, contributing to the erosion of missing sections. However, stratigraphic records from offshore basins also reported having similar conditions where these basins have direct contact with the Paleozoic basements and missed sections from the same period [2]. This may reflect a regional deformation to influence an extensive unconformity. It is crucial to constraint the development of onshore basins in order to see the correlation with offshore basins. More importantly, dating the early deposits will constraint the time of erosion of the Late Cretaceous to Paleogene and the beginning of the deposition of Cenozoic basins. Across Malaysia, several Tertiary onshore basins were identified in the literature [11]. Correlations were relative, hence proper dating and in-depth study on these basins are needed.

One correlation made on Cenozoic deposits was using marker of coal lignite seams sandwiched between sandstones/silt which was reported across the Peninsula [11]. They were in the composition of lignite, signifying a shallow deposition of peat. Age was approximately from the Eocene to Oligocene [12]. Overlain the coal seams reported conglomeratic beds which referred here as the Early Deposits. These Early Deposits are the remnants of the missing sections during the Late Cretaceous to Paleogene. In literature, there are eight (8) localities of the Early Deposits out of fourteen (14) basins in the Cenozoic stratigraphic record. In this study, Lawin locality will be examined from its structures, sedimentological evidences from field and laboratory analyses to add information in the study of geodynamics and basin study and later on to make connection to the study of the SEA Regional Unconformity.

## 2. Methodology

### 2.1. Regional Structural Analysis

Regional structural analysis is performed using input data from Shuttle Radar Topography Mission (SRTM) and first underwent DEM analysis mainly on hill shading and contouring to produce an elevation model. Structural analysis was aided by data from satellite images. A chronology was made from interpretation of structural features and offset correlation honouring the movements of the associated the Bok Bak Fault [13], [14] and interpreted strike-slip duplex [3], [7]. Interpretations are represented on the elevation model and cross-sections are gathered to support the observations.

### 2.2. Field Observations and Measurements

Five (5) outcrops were explored and each recorded with logs and cross-section as well as structural measurements. In each locality, sieving analysis and sedimentological studies were done. In this study, we are classifying the conglomerate and understanding their origins. Matrix were sampled to perform petrography and chemical analyses.

### 2.3. Identification of Minerals

Two matrix samples were sieved to powder size and a 100g of powder for each were sent for X-ray fluorescence (XRF) analysis to determine the elements from powder and X-ray powder diffraction (XRD) step scan analysis. Results from both were interpreted using PANanalytical X'Pert Pro diffraction software and using a search and match method to find identical minerals, conditioned to elements found in XRF analysis, its d-spacing, and positioning.

Raw samples of the matrix were seen under the Scanning Electron Microscope (SEM) and examined using back-scattered electron imaging and X-ray dispersive (EDS) analysis. The samples were also seen under the microscope after been impregnated using epoxy resin and hardener and cut into thin sections.

## 3. Preliminary Results and Discussion

### 3.1. Geodynamic and Basin Development of the Lawin Conglomeratic Basin

Lawin conglomeratic deposit was seen in the morphology of alluvial fans on the Digital Elevation Model (DEM) (Figure 1). It was characterized as debris flows from fallen scarps [15] and spread 4 km in lobe shapes. Lawin conglomeratic beds rested on Bintang Granite following N-S and NE-SW normal fault directions and associated with the Bok Bak major fault. Its underlying rocks believed to be the Silurian- Ordovician Baling Group [11].

We have mapped our observations of fractures, faults and interpreted shear motions to the development of the Lawin basin and assembled the evidence according to the chronology of events following to their
relationships. Locally, this basin is structurally-controlled by faults [11]. A timeline of episodic deformations is grouped in this analysis:

<figure><img src="image_5.png" /><figcaption>**Figure 1.** Map of Northwest of Peninsular Malaysia. Three localities of Early Deposits are shown taken from Raj et.al. (1998); Lawin (this study), Kuala Kangsar and Pengkalan Hulu-Betong Transect Area (Nenering). Information on the strike-slip duplex from Sautter et.al. (2017,2019), and Morley (2004), the movement of the Bok Bak Fault from Harun (2002), and the Bentong-Raub suture zone from Metcalfe (2011).</figcaption></figure>

### 3.1.1. *Episode of Compression*

The age of Bintang Batholith was dated 216 ± 2 Ma [16], and the Bok Bak Fault was interpreted to move in left-lateral strike-slip motion in the Late Cretaceous [13], [14], [17]. The Bok Bak Fault was attached
to a nested strike-slip duplex with huge offsets on the Main Range Granite [7]. This duplex was due to the Late Cretaceous – Paleogene transpressional activity resulted in a large wrench fault from the north of Peninsular Malaysia to Singapore, and broad folds and thrusts [3], [7]. We interpret two indicators to add up to information. A thrust-fault with horsetails which connected to the Bok Bak Fault and branched out to N140, thrusting on the granitic body and a left-lateral ductile shear strain observed at the edge of the alluvial fan seen on DEM (Figure 2).

<figure><img src="image_6.png" /><figcaption>**Figure 2** Compression in Lawin. a) An overall conjugate fracture following to NE-SW and NW-SE orientations on the rose showing an E-W compression. Horsetail features and a left-lateral sedimentary deformation (white box), b) NW-SE and NE-SW measurements on Bintang Granite</figcaption></figure>

### 3.1.2. Basin Development of Lawin Deposits

The mechanism behind the formation of the Lawin basin is interpreted to be tectonic-related. Previous publications had it discussed to be bound by structures [19], [33], related to the Bok Bak Fault. Mokhtar (2016) mentioned in a journal that it was formed in a pull-apart basin however with no great details [19]. In this study, we interpreted Lawin to experience extension in the Eocene during the rightlateral movement of the Bok Bak Fault.

<figure><img src="image_7.png" /><figcaption>**Figure 3** Phases of extension in Lawin. a) The early phase of extension can be seen by the localized pull-apart structure, gave rise to the Bintang Granite, b) Continuation of regional E-W extension opened the Perak Valley, c) Field evidence of N-S quartz vein and N30° sheared feldspar</figcaption></figure>

The extension started with a localized extensional bend that gave rise to Bintang Granite in a pullapart structure. An elongated spindle-shaped pull-apart basin with a width of 10 km at the centre (Figure 3a). A deep-faulted opening, as observed on cross-section (Figure 4). It was further extent regionally in the E-W direction following a N-S compression which could be due to the extrusion of the Southeast Asia after the collision of India and resulted in N-S normal faults and subsidence in the area of the area of
Perak Valley [20] (Figure 3b). Along the N-S dip-slip fault which formed space for accumulation, the low region had deposited the Lawin conglomeratic series. The deposits were formed in an extensional regime thus, having rift basin structures with syn-depositional sediments [15]. We measured the beddings with gentle dip and facing to the west (Figure 5). From the satellite view, the lobe we studied was amassed as debris flow dominated along with an inactive lobe. This also seen in its lithological architecture of depositional system.

<figure><img src="image_8.png" /><figcaption>**Figure 4.** Development and accumulation of Lawin deposits. Deep-rooted faults which give rise to the Bintang Granite, E-W extension following a N-S rifting provide an accommodation space and followed by the accumulation of Lawin Conglomeratic Series at the low region.</figcaption></figure>

## 3.2. Field-based Observations

Localities L1 (5°18'06.0"N 101°01'29.3"E) and L2 (5°18'03.2"N 101°01'33.3"E) are our proposed boundary between granite body with sedimentary deposits. On L2, the boundary was made based on observation where on the hillslope of L2 and facing to the North, to the west was the granitic body and to the East found boulders and conglomerates and they are separated by a steep terrace following to the NNE-SSW and N-S orientations which follows to the interpreted normal fault. L3 (5°17'52.1"N 101°02'13.0"E) is located on a lobe which believed was incised by another lobe where L4 (5°17'29.0"N 101°02'24.2"E) and L5 (5°17'23.6"N 101°02'46.7"E) deposited on (Figure 5). Structural evidence in L3 shows well-stratification with a dipping angle of ~34° to the West with nearly vertical fractures. On the other hand, L4 and L5 both are gently dipping ~15° facing west. From cross-section, these beddings are represented in fan-shaped deposition following to the E-W extension (Figure 4).

<figure><img src="image_9.png" /><figcaption>**Figure 5.** Field observation and measurements. a) Fan-lobes of Lawin alluvial in map view with different dipping angles for titled beds on each lobe, b) Locality L3 cross-sections and recorded coarsening upward facies variations indicating an uplifting.</figcaption></figure>

## 3.3. Sedimentary, Chemical and Petrographic Analysis of Lawin

From the sedimentary analysis, we found Lawin is dominated by non-cohesive debris flow. Evidence from sieving analysis defined a sand matrix-supported conglomerates or paraconglomerate and getting
well-sorted going farther. Clasts are mainly granite, sandstone, quartz, and claystone, ranging sizes from boulder to pebbles accordingly. We found two different types of granite rocks on L3 (abandoned lobe); a feldspathic granite and tourmaline granite while the clasts of granite in the active lobe are only the feldspathic type. Other clasts believed to come from older rocks. Four facies were determined, massive and thin lamina matrix-supported, clast supported conglomerates and conglomerate sandstone of detrital. Lawin showed a reverse-gradation of accumulation which related to uplifting [15] (Figure 5).

In the matrix samples, we acquired analyses from petrography and chemical experiments. Thin sections displayed microfractures on grains, filled with clays, dissolved minerals especially feldspar as they leached, and kaolinite pouches. SEM images showed dissolved feldspar, platy kaolinite, and commonly inherited feldspar (Figure 7).

<table><caption>Table 1. XRD analysis of Lawin</caption><thead><tr><td>Mineral Present</td><td>Percentage (%)</td></tr></thead><tbody><tr><td>Quartz</td><td>25</td></tr><tr><td>Feldspar</td><td>51</td></tr><tr><td>Illite-Mica</td><td>10</td></tr><tr><td>Kaolinite</td><td>14</td></tr></tbody></table>

<figure><img src="image_10.png" /><figcaption>**Figure 6.** XRD analysis of Lawin showing presence of Illite-Mica and Kaolinite in clay minerals based on its positioning, d-spacing and FWHM. Illite-Mica as by product of kaolinization and related to frequent tectonic activity. Kaolinite is the stable clay mineral in area with intense weathering.</figcaption></figure>

Chemical analysis listed minerals of kaolinite, illite-mica, quartz, and feldspar are present with no indication of other phyllosilicates such as smectite nor chlorite (Table 1). The kaolinite formed due to the intense weathering derived from leaching of potassium feldspar. Platy kaolinite seen in SEM images implied a long and slow process of transformation (Figure 7a). Few elemental oxides are found from XRF analysis where they accompanied the kaolinite filling in microfractures. Illite-mica is mainly due to the low-grade alteration of mica as they as the by-product during kaolinization. This might be related to their setting of having frequent tectonic activities as they were associated with major fault and gone through stages of extension and reactivation [21]. The peaks in XRD analysis showed that they are poorly ordered and the potential to have more detrital clays than diagenetic clays (Figure 6).
<figure><img src="image_11.png" /><figcaption>**Figure 7.** Petrography analyses of Lawin matrix. (a) SEM image of Kaolinite in platy shape showing a long, and slow process of kaolinitization. (b) Microfragments of minerals (Quartz and Feldspars) with clay fillings surround and in between microcracks.</figcaption></figure>

**4. Conclusion**

The development of the Lawin Conglomeratic basin is governed by E-W extension associated with the movement of the Bok Bak Fault. Lawin in chronology first experienced an E-W compression followed by an E-W extension. Lawin was developed as a pull-apart basin followed by a N-S normal fault to accumulate Lawin Conglomeratic Series. The pull apart structure and N-S opening of the Perak Valley were similar to structures that influence most of the offshore basin. In literature, Lawin is timeequivalent to other Tertiary Basins (Batu Arang, Bukit Arang, Layang-Layang, Chenor-Paloh, and Bukit Besut), however with no direct evidence from fossil. This study aimed to date the clay minerals from basins to constrain the timeline. More works are in-progress.

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