Bulletin of the Geological Society of Malaysia, Volume 60, December 2014, pp. 45 – 53
Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia
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Abstract: The Kampung Opak limestone, which is considered part of the Sibuti Formation, represents an approximately 44 m thick succession of regularly interbedded limestone and marl. The Kampung Opak limestone is mainly composed of mudstone and wackestone. These relatively thicker limestone beds (~20-50 cm) are interbedded with thin marls (~1-5 cm). Three facies were identified based on the percentage of quartz grains in the limestone: (1) Facies A with more than 5 % quartz, restricted to the lower part of the succession; (2) Facies B with 1 - 5 % quartz and forming the middle part of the succession; (3) Facies C with less than 1% quartz, forming the upper part of the succession. The facies generally form a fining upward succession.
The depositional environment is interpreted as a shallow marine shelf, in waters less than 40 m deep, based on sedimentary facies, ichnology and palaeontology. The fine grained facies indicate a low energy setting. The presence of the Cruziana ichnofacies in the limestone suggests a shallow marine environment on the shelf. The abundance of pelagic foraminifera suggests an open sea with high productivity. D/T ratio of Amphistegina sp. specimens from the limestone indicates a water depth of less than 40 m.
Twelve species of planktonic foraminifera were identified from the Kampung Opak limestone: Cassigenella chipolensis, Globigerina venezuelana, Globigerinoides altiapertura, Globigerinoides immaturus, Globigerinoides obliquus, Globigerinoides subquadratus, Globigerinoides trilobus, Globoquadrina dehiscens, Globorotalia mayeri, Hastigerina aequilateralis, Orbulina bilobata and Orbulina suturalis. The microfossil assemblage gives a Middle Miocene age (Globorotalia peripheroronda zone – Globigerinoides subquadrata zone, Tf stage). The Kampung Opak limestone probably represents the upper part of the Sibuti Formation. This further extends the age range of the Sibuti Formation from Early Miocene to late Middle Miocene age.
Keywords: Kampung Opak Limestone, Bekenu, Middle Miocene, planktonic foraminiferal zonation, Sibuti Formation