Sains Malaysiana 36(2): 159-163 (2007)
An Integrated Geophysical Study of the Quaternary Basin at Olak Lempit – Banting Area, Selangor, Malaysia (Kajian Geofizik Bersepadu Lembangan Kuaterneri di Kawasan Olak Lempit – Banting, Selangor, Malaysia)
Abdul Rahim Samsudin, Umar Hamzah & Zuraidah Ramli School of Environment and Natural Resource Sciences Faculty of Science & Technology, Universiti Kebangsaan Malaysia 43600 UKM, Bangi Selangor, Malaysia
Received: 1 March 2007 / Accepted: 6 April 2007
ABSTRACT
An integrated geophysical study was conducted to investigate the subsurface regional structure and the presence of a Quaternary sedimentary basin in the Olak Lempit - Banting area of Selangor, Malaysia. A regional gravity survey and the high resolution reflection seismic were employed to determine the thickness and areal distribution of the alluvial sedimentary basin as well as to investigate the depth and topography of the bedrock in the study area. The sedimentary basin hosts one of the most important coastal alluvial aquifer which was used to cater the shortage of domestic water supply during the worst water crisis that hit the state of Selangor in 1998. The surface geological map shows that in general 70% of the study area is covered by Quaternary deposits of Beruas, Gula and Simpang Formations which overlie the sedimentary bedrock of Kenny Hill Formation. The Beruas Formation consists of mainly clay, sandy clay and peat of Holocene fluviatile-estuarine deposits, whereas the Gula Formation represents Holocene marine to estuarine sediments which mostly consists of clay and minor sand. The Simpang Formation (Pleistocene) is a continental deposit comprising of gravel, sand, clay and silt. The underlying Kenny Hill Formation consists of a monotonous sequence of interbedded shales, mudstones and sandstones. The rock is Carbonaceous in age and it forms an undulating surface topography in the eastern part of the study area. A total of 121 gravity stations were established using a La Coste & Romberg gravity meter and the elevations of most of the stations were determined barometrically using Tiernan-Wallace altimeters. The high resolution seismic reflection using the common mid point (CMP) or roll along technique was carried out using a 24 channel signal enhancement seismograph and high frequency geophones. A total length of about 1.7 km stacked seismic section has been acquired in this survey and a nearby borehole data was used for interpretation. A relative Bouguer anomaly map shows an elongated zone of low gravity anomaly trending approximately NW-SE which is interpreted to be the deposition center of the Quaternary basin. The interpreted gravity profiles running across the central area of the study area show that the basin has thickness varies from tenth to several hundred meters with maximum depth to bedrock of about 275m. A gravity profile which passes through the eastern edge of the basin was modeled with depth to bedrock of about 178m below ground which agrees very well with those obtained from the interpreted seicmic section and borehole data. The stacked seismic section shows several high amplitude parallel to sub-parallel reflection overlying discontinuos and low reflection pattern. Reflections on the eastern part of the section is much shallower than the one observed on the western part which clearly indicates the presence of basinal structure with a total interpreted depth to bedrock of about 200 meters.
Keywords: Quaternary basin; seismic section; gravity modeling; subsurface structure
ABSTRAK
Penyiasatan geofizik bersepadu telah dijalankan untuk mengkaji struktur rantau bawah permukaan dan kewujudan lembangan sedimen Kuaterneri di kawasan Olak Lempit – Banting Selangor, Malaysia. Survei graviti rantau dan seismos pantulan beresolusi tinggi telah digunakan untuk menentukan ketebalan dan keluasan lembangan sedimen aluvium serta mengkaji kedalaman dan topografi batuan dasar di kawasan kajian. Lembangan sedimen ini merupakan akuifer aluvium pantai penting yang digunakan untuk menampung bekalan air semasa berlakunya krisis air terburuk di negeri Selangor pada tahun 1998. Peta geologi menunjukkan kawasan kajian terdiri daripada 70% endapan aluvium kuaterneri daripada Formasi Beruas, Gula dan Simpang yang menindih batuan sedimen Formasi Kenny Hill. Formasi Beruas ini kebanyakannya merupakan lempung, lempung berpasir dan gambut daripada sedimen fluviatil-estuarin yang berusia Holosen, manakala Formasi Gula pula merupakan sedimen marin-estuarin yang kebanyakannya terdiri daripada lempung dan sedikit pasir. Formasi Simpang pula (berusia Pleistosen) merupakan endapan benua yang terdiri daripada gravel, pasir, lempung dan lodak. Batuan dasar Formasi Kenny Hill terbentuk daripada selang lapis batu syal, batu lumpur and batu pasir. Batuan ini berusia Karbon dan membentuk topografi bawah permukaan beralun di bahagian timur kawasan kajian.Sebanyak 121 stesen graviti telah diukur menggunakan meter graviti La Coste & Romberg dan ketinggian kebanyakan stesen ditentukan dengan menggunakan barometer Tiernan-Wallace. Survei seismos pantulan beresolusi tinggi menggunakan kaedah titik tengah sepunya (CMP) atau kaedah anjakan kesisi telah dilakukan dengan menggunakan seismograf peningkatan isyarat 24 saluran dan geofon berfrekuensi tinggi. Sepanjang 1.7 km keratan seismos telah diperolehi dalam survei ini dan data lubang gerudi yang berhampiran telah digunakan untuk pentafsiran. Peta bouguer anomali relatif menunjukkan zon anomali graviti rendah yang membujur dengan arah kurang lebih baratlaut-tenggara yang ditafsirkan sebagai pusat pengendapan sedimen lembangan Kuarterner. Pentafsiran profil graviti yang memotong bahagian tengah kawasan kajian menunjukkan ketebalan sedimen berubah daripada puluhan ke ratusan meter tebal dengan kedalaman maksimum batuan dasar sekitar 275m. Profil graviti yang melalui bahagian tepi timur lembangan telah dimodelkan dengan kedalaman sekitar 178m dan sesuai dengan hasil tafsiran yang diperolehi daripada keratan seismos dan lubang gerudi. Keratan seismos timbunan menunjukkan ciri pantulan selari dan sub-selari berfrekuensi tinggi menindeh corak pantulan berfrekuensi rendah yang tidak kontinu. Pantulan di sebelah timur kelihatan lebih cetek berbanding pantulan disebelah barat kawasan kajian yang jelas menunjukkan kewujudan struktur lembangan dengan kedalaman tafsiran batuan dasar sekitar 200 meter.
Kata kunci: Lembangan Kuarterneri; keratan seismos; permodelan graviti; struktur bawah permukaan.
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