Geological Society of Malaysia Annual Geological Conference 2000 September 8-9 2000, Pulau Pinang, Malaysia
Stratigraphic Position of the Rangsi Conglomerate in Sarawak
ISMAIL CHE MAT ZIN
Universiti Teknologi Petronas Bandar Sri Iskandar, 31750, Tronoh, Perak Abstract The Rangsi conglomerate that outcrops in the Tatau Horst area in Sarawak has for a long time been regarded as the basal unit of the Tatau formation. The interpretation was, however, primarily based on the succession of the geological formations, and no detailed stratigraphic work was done to support the interpretation. A study conducted in this area using seismic stratigraphic technique shows that the Rangsi conglomerate is much younger than the Tatau formation. This conglomeratic rock unit is possibly equivalent to the Balingian formation that is of late Miocene age. Furthermore, the area that is called Tatau Horst, seismically does not represent structural feature such as a "horst" of extensional tectonics. Instead, it is characterized by positive flower structure, suggesting that the structure was formed as a result of a transpressional strike-slip tectonic episode, during early to late Miocene times.
Kedudukan Konglomerat Rangsi Dalam Stratigrafi Sarawak Abstrak Konglomerat Rangsi yang tersingkap di kawasan Horst Tatau di Sarawak telah lama dikenali sebagai batuan dasar formasi Tatau. Tafsiran ini hanya berdasarkan jujukan formasi geologi dan tidak terdapat kajian stratigrafi terperinci untuk menyokong tafsiran tersebut yang telah diterbitkan sebelum ini. Kajian yang dilakukan di kawasan ini dengan mengunakan teknik stratigrafi seismik menunjukkan bahawa konglomerat Rangsi terletak lebih muda berbanding formasi Tatau. Konglomerat ini berkemungkinan sama dengan formasi Balingian yang berusia Miosen akhir. Tambahan pula kawasan yang dinamakan Horst Tatau secara seismik tidak menunjukkan fitur struktur 'horst' daripada tektonik regangan. Sebaliknya, ia disifatkan oleh struktur bunga positif, mencadangkan pembentukan struktur basil daripada episod tektonik gelinciran jurus transpressi semasa Miocene awal hingga akhir.
INTRODUCTION unit in Sarawak. It is also aimed at describing the tectonic history of the study area that may contribute towards a In the Tatau Horst area (Figure 1), the Rangsi better understanding of the regional tectonics of Sarawak. conglomerate that unconformably overlies the Belaga formation (Figure 2), has been regarded by many workers MATERIALS AND METHODS as a classical example of the geological contact between the Tatau and Belaga formations. The interpretation was, Overseas Petroleum and Investigation Corp (OPIC) however, primarily based on the generalized sequence in was the former petroleum exploration company operating the succession of the geological formations. No detailed in this area referred to as SK 12. In its five-year exploration biostratigraphic work has been publish to support and period, OPIC drilled several wells and acquired extensive determine the age of this particular conglomerate unit. In seismic coverage of the area. Two of the regional seismic the area adjacent to this particular outcrop, (Figure 2), it is lines passed through the Tatau Horst area (Figure 3). The known that the age of the Tatau formation is mainly of late present study used the information from these lines together Eocene- early Oligocene and the Bawang member of Belaga with many other lines in the onshore area as well as data formation is predominantly of Eocene age (Yin, 1992). from all the wells drilled in the area. A seismic stratigraphic study on the regional lines from The two regional seismic lines were tied to the other this region reveals that the Rangsi equivalent belongs to a lines, which have been interpreted and calibrated earlier younger sequence, compared to the equivalent unit of the with the well data from the area (Ismail, 1996). The Tatau formation. In addition, the area that has been called interpretations were carried out using seismic sequence Tatau Horst (Figure I) seismically does not seem to represent stratigraphic techniques, i.e. to correlate the unconformities a structural feature such as a "horst" of extensional tectonics. and their correlatable conformities that formed as bounding This paper is aimed at discussing the new interpretation surfaces for a sequence, and to analyze the internal character of the stratigraphy of the Rangsi conglomerate that has of the packages in addition to the structural interpretation. previously been described as the representative unit of the All the subsurface unconformities in the area have also basal part of the Tatau formation, the oldest sedimentary been dated earlier. 132 ISMAIL CHE MAT Z IN
...... ' :N~. ·. .' :• ~ .' : ·.·.···a ... ·...... ' . '. ' ...... ' .
Eelaga •
Figure I: Map of the study area showing the location ofTa tau Horst and the geology oftheMukah-Balingian-Tatau and Bintulu area. Sketch based on Yin (1992). The abbreviations used are Ta:Tatau, Bu:Buan, Bl:Balingian , Ny:Nyalau , Bg:Begrih, Li:Liang formations. Black is granodiorite at Bukit Pi ring (BP), andesite and rhyolite lavas at Arip and andesite at Bukit Mersing.
RESULTS OF THE SEISMIC Tertiary Two Sequence (T2S) STRATIGRAPHIC STUDY the sequence shows a tremendous thickening northward. Strong reflectors mark the lower boundary with Five regional unconformities have been recognized in mild truncation on T1S. The upper boundary is marked by this area (Figure 4) . The geoseismic section (Figure 5) a very clear angular unconformity at the base of T4S. This depicts the nature of reflection termination at the upper and means that the T3S, which is preserved in the offshore area lower boundaries of each of the sequences. By comparing of Balingian (Ismail, 1996), has been totally eroded in this the number of sequences preserved in this area with the area. proposed stratigraphic scheme of Sarawak (Mat-Zin and The internal character of T2S is of parallel very Tucker, 1998) in Figure 6, all the main sequences are continues low frequency and high amplitude reflectors. It present except Tertiary Sequence Three (T3S). changes to shingle and clinoform towards the upper part of the preserved sequence. Based on the seismic Tertiary One Sequence (T1 S) configuration and data from nearby wells, it is interpreted The sequence does not show any thickening in this that T2S was also deposited mainly in a shallow marine area. The lower boundary is marked by strong reflectors environment. that differentiate between the reflective intervals of Sequence One, and the overlying the chaotic reflectors of Tertiary Four Sequence (T4S) the Belaga formation. Similar to T2S, the sequence shows tremendous Tl S is internally characterised by parallel, sub thickening northward with divergent seismic packages, continuous, low frequency and high amplitude reflectors. suggesting a basin fill deposition, which probably developed Based on the seismic configuration and data from a nearby during T4S times. The lower boundary is marked by a well, it is interpreted that T1S was mainly deposited in a strong angular unconformity and strong reflective zone and shallow marine environment. mild truncation marks the upper boundary at the base of TSS. Geological Society of Malaysia Annual Geological Conference 2000 STRATIGRAPHIC POSITION OF THE RANGSI CONGLOMERATE IN SARAWAK 133
where the Rangs i conglomerate outcrops (Figure 2) a close appearance is observed. This similarity can be described by the presence of an angular unconformity, at the base of the Rangsi conglomerate. There are also several higher order unconfonnities that occur within the conglomerate unit. Judging from this appearance on the seismic package and its close location to the outcrop of the Rangsi conglomerate, it is intepreted that the highly reflective seismic package represents the same rock unit outcrops in the Tatau Horst area, i.e. the Rangsi conglomerate (Figure 2). Tertiary Five and Six Sequences (TSS and T6S) These sequences thicken northward. The lower Figure 2: Photograph of Rangsi conglomerate showing the contact boundaries are marked by gentle onlap features and mild between what is said to be "Tatau" and Belaga formations. truncation on the upper boundaries. However, the whole part of the two sequences have been truncated by an unconformity at the base ofT7S that leaves no preservation of the two sequences in the Tatau Horst area (Figure 5). The intemal characteristics of the two sequences appear to be similar. It is characterized by mainly discontinuous high frequency , high amplitude reflectors. On this basis and other evidence from nearby wells, it is interpreted that the two sequences were mainly deposited in a coastal plain environment.
STRATIGRAPHIC POSITION OF THE RANGSICONGLOMERATE
The stratigraphic scheme by Ismail , 1996 (Figure 6) that has been adopted for this seismic stratigraphic study, allows the age determination on all the identified sequences. The stratigraphic scheme for the onshore formation by the Petronas Research and Scientific Services (PRSS), with offshore Cycle Stratigraphic Scheme (Ho, 1978), allows a Figure 3: Map showing the orientation of seismic lines passing through the Tatau Horst area. more accurate correlation between the geological formation and the identified sequences in the study area (Figure 7). The internal character of T4S in the basinal area is By referring to the merged stratigraphic scheme (Figure subparallel high frequency and low amplitude reflectors . 7) , the TIS is equivalent to Tatau, Buan, and the lower part The reflectors are continuous at the basal section and become of Nyalau, Tanggap and Subis Limestone. The T2S is discontinuous towards the upper part of the sequence. Based equivalent to the upper part of Nyalau, Tanggap, Subis on the seismic configuration and data from nearby wells , it Limestone and the lower part of Setap Shale. The T3S that is interpreted that T4S in the basinal area was deposited may be of equivalent age to Lambir, the upper part of the mainly in a shallow marine environment, which changed to Sibuti and Setap Shale formations, is totally missing in the a coastal plain environment towards the upper part of the study area. sequence. All the identified unconformities as per Ismail (1996) can be seen to correlate very well with the unconformities Rangsi Conglomerate in onshore formations. Nevertheless, the unconformity The base T4S unconformity can be traced landward, between the Nyalau and Buan formations (PRSS, 1991) to obtain an anomalous reflective seismic package that cannot be recognized from the seismic package. extends over an area of about 4 km wide. The lower unit of The Rangsi conglomerate that is interpreted to be the package appears to clearly truncate the underlying situated within T4S, is of late Miocene age. It is perhaps chaotic reflector, while the upper package with a steeper equivalent to the Balingian formation (PRSS, 1991). In angle truncates the flatter lower package. comparison to the previous interpretation (that of basal By comparing the seismic package that occurs at a Nyalau formation) the Rangsi conglomerate is some 25 depth of about 4 m in the northern flank of the Tatau Horst, million years younger than its earlier proposed age.
September 8-9 2000, Pulau Pinang, Malaysia 134 ISMAIL CHE MAT ZIN
Figure 4: Seismic section passes
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ showingilirough Rangsithe nature conglomerate, of reflection ~ terminations between sequences and tectonic nature of "Tatau Horst".
NW SE Figure 5: Geoseismic section RANGS/CONGLOMERATE along Tatau-Balingian area showing the nature of reflection termination and internation seismic character of every sequence. It also shows the pos ition of Rangsi conglomerate with respect to the Tertiary sequence unit and the structural style of Tatau Horst.
TECTONIC AND SEDIMENTATION The cessation of tectonic movement during the early HISTORY to middle Miocene times in the Tatau area is marked by a severe angular unconformity at the base of T4S. The basal The NW -SE geoseismic section (Figure 5) shows the unit of this unconformity is represented by the conglomerate relationship between the sequences and the tectonic nature unit that is known as the Rangsi conglomerate. Several of the study area. It clearly shows that the area referred to episodes of tectonic movement took place that post dated as Tatau Horst is characterized by a positive flower structure. the deposition of the Rangsi conglomerate, resulting in the It is believed to have formed as a result of a highly deformed deposition and erosion of both TSS and T6S during the transpressional strike-slip episode that post-dated the Pliocene period. deposition ofT2S during early Miocene times. This peeriod of tectonic unrest may represent the most active Tertiary CONCLUSIONS tectonic period in the Tatau area. As a result of these movements, the whole ofTlS was perhaps moved laterally, This study concludes that the Rangsi conglomerate is away from this area, while the southern portion ofT2S was of equivalent age to the Balingian formation. It forms the uplifted and severely eroded. proximal unit of the T4S, possibly of alluvial fan or braided
Geological Society of Malaysia Annual Geological Conference 2000 g ;; 5:::::::- Sequence ;2 Strat. Z, ~ ~~ J] ~ ~ :~~nkt~:~~c ;!!!. !!> F.~ g~ Scheme SUBSIJRF~(;E c .i'>c L ONSHOREFORMAnONS 2 § .o• ~ e·~ -;~~ SEiltlt=Nr.F<: e r::~ ~.S! ]~ ~ .... 5] ~ (Ismail, 1996) EPOCH ~:.~~~~ ~ it~ :§.~~ MUKAH-8AUNGIAN TATAU- BAUNGGIAN u~ ~ ~ z~ 5~ ~~g oCi'iS ~ ~~ QUAT ..m1. SN23 VII UANG UAHO T7S _E_ S900 Pv2 582 NNI9 Vlll "'r"~"~ I- 22 VD 3481 2- 2-J G'·""'"";' I~~!' 21 T6S .._ VII ~ ~ :::~!~:~~::~:~==:~·~=~~rnTnl11:t>-- ...... --~1riiJr1ripTfJCI!BEJ:G):RI:rHX1~:q~ .. _- ..:... :---~ NNl5 3- '"'U 2ll en!!. PUOCENE 4- G<.mq;ril>c NN IJ-14 '12ll VI TSS 4- NN I2 18 f--- 52 L'~~12:.~i::I!!OC~'-i-----l MIRI ~ - 1\J\/VVVV\, 5.2- SN17 (/) 6- -1 Ckdutcr1ci f--- 17 m SA35 .... -~~--- BAUNOIAN T4S 1\JKAU :0 ~ NN II NN II 1- l> v -1 8- r... ~ _, -;, 16 :::!!!![ SN16 ...... 630 8- ~" ~~ ~~-A CON~ , ~~ Gi T4S - 1Q4 NN9 i:J!Ill: :0 9- NNIO INN9 ~ 9- .J''Ji~ l> 62ll v Ill- v "0 Gr.k:ng:acnsil f--- NNS Ill- -~ I =-... ~ - ~ NN7 SN12 Ill ~ ll- 0 NN9 SA300 1-- 11.5- !!! IV NN7 ~ ~ -'stm "'() 12- 6 10 12- G•.ook""m ~ ::& - 'sN1o 0 12 (f) NN6-8 "',_, 13- ::& NNS~ " NN6 IV 0 ... =i 1--- IV 0 , !-16.3-;;~ Ill ssoo PoS SOS SN7 6 ts- !"'·'""'~«•"' NNS ~ 51 15- SI!TAP SHALE NNS 10-9 TJS z f--- 16- ~ ..... NN3 SNI 0 w Ill ~ 1-- 'Tl Gt..11 1canus ""38 17- NN4 I ~ u 17- T2S -1 0 I 18- 1--- 18- '*'2 SN5 NN3 ~ m h- Po379 D 19- G.bWicnsis NN3 ~ 19- f--1-----l :JJ 1--- l> 2ll- f- f-- 23.3 ~ SN4 s z S300 ll TIS ~ G) 2~ NN2 21- 1--- Gr.kugk:ri NN2 Phc88 (f) NP2S SN3 22- f--- 22- 0 23- 23- 0 o~n;; 1--- NYA1AU NN I 4 NYALAU z 2k 1--- 24- G) NP24 w r f--- 1--- ..... 2s- 25- 29.3 0 t- T1S s:: 26- NP'-1 :no I 26-- m NI'2S 3 ~ BUAN :0 27- f--- Pall45 21- NP23 "- Gr.iocn::l~ "' ~ 211- ~ 28- m TIS z "" TATAU NP2A I 29- l
Figure 6: Composite stratigraphic table with the previous schemes used for the Sarawak Basin Figure 7: A combination of the stratigraphic scheme of onshore Sarawak (PRSS , 1991 ) and the and the proposed Sequence Stratigraphic Scheme by Ismail , 1996. stratigraphic scheme for Sarawak subsurface sequences (Ismail, 1996) showing the position of the Rangsi conglomerate in Sarawak' s stratigraphy. w ()1 136 ISMAIL CHE MAT ZIN stream deposit. Therefore, this conglomerate is believed to be about 25 million years younger than previously interpreted, where it was reported to be the basal unit of the Tatau formation. The study contributes towards better understanding of the tectonic nature and history of the study area.The area called "Tatau Horst" was not formed as a horst that is normally associated with extensional tectonis. Instead, it is formed as a result of strike-slip movement. Therefore, the term "horst'' that describes the interpreted geological origin of the structure should be replaced by another name such as "Tatau Transpressional Area", which reflects the true tectonic origin of the structure. Although it is well understood that the onshore geological formations are diachronous, this study provides an understanding of the correlation between surface geological formations and subsurface Tertiary sequences. This will contribute towards better harmony between the interpretations of field geologists and subsurface geologists who utilise different databases. ACKNOWLEDGEMENTS The author would like the thank Mr. George Cheah for his comments, and editorial work, and the management of Universiti Teknologi Petronas for allowing the presentation and publication of this paper. REFERENCES
Ismail Che Mat Zin, 1996. Tectonic evolution and sedimentation history ofSarawakBasin. Bulletin ofthe Geological Society of Malaysia, 41:41-52. Mat-Zin, I.C. and Tucker, M .E., 1998. An Altemati ve Stratigraphic Scheme for the Sarawak Basin. Journal of South-East Asian Sciences (1998), 1-18. PRSS, 1991. Geological field-guide, Sibu-Miri traverse, Sarawak by Haile, N.S, and Ho, K.W, 1991 (Unpublished) Yin, E.H., 1992. Geological map of Sarawak, 2"d ed.Geological Survey of Malaysia. Scale 1:500,000.
Geological Society of Malaysia Annual Geological Conference 2000