Sedimentary Facies and Depositional Framework of the Tertiary West Crocker Formation around Kota Kinabalu, Sabah

The Oligo-Miocene West Crocker Formation in Sabah has been interpreted as part of an extensive, unconfined submarine fan system, but a detailed description of the sedimentary facies are lacking. This paper describes the sedimentary facies and their relationships, based on a study of eight major outcrops of the West Crocker around Kota Kinabalu, Sabah. Seven sedimentary facies have been identified. Facies A - pebbly, medium-to coarse-grained, amalgamated sandstone. This facies consists of poorly sorted, massive (structureless) sandstone beds with thicknesses ranging from 1 to 38 m. This facies generally have a very high sandstone-to-shale ratio. The bases of sand commonly have flame and load structures. Mud clasts and rip-up clasts are common, with rare carbonaceous and sandstone clasts. Facies B - fine- to coarse-grained sandstone, generally massive, moderately to poorly sorted, and has post-depositional dewatering structures, e.g. dish structures and pipe marks. It has a lower sandstone-to-shale ratio compared to Facies A. Rip-up mudclasts and sole marks, such as flute and tool marks, are common at the base of this facies. (iii) Facies C - composed of sharp-based, graded beds, which may form complete Bouma sequences. The sandstones are commonly laminated or crosslaminated. (iv) Facies D - parallel laminated, very fine- to medium-grained sandstone, and shale. It may be organized into Bouma sequences, but with the Ta division absent. (v) Facies E - fine- to coarse-grained thinbedded sandstones, occasionally with climbing ripples, lenticular bedding, and intraformational rip-up clasts. Trace fossils are common, and include Nereites, Spirorhaphe, Megagrapton, Paleodictyon, Cosmorhaphe and Helminthoida. (vi) Facies F - “chaotic” shale-rich units with slumped beds and syn-sedimentary folds; and (vii) Facies G - consist of fine-grained, pelagic and hemipelagic deposits. These facies are organized in facies associations that represent different deposits of the submarine fan system. These are: (i) slope deposits – dominated by facies F and G, with subordinate facies A and B; (ii) channel-fill deposits - generally showing upward-thinning and upward-fining packages, in which facies A, B, C and D are dominant, with minor occurrences of facies E and F; (iii) levee or overbank deposits - dominated by shale with very thinly bedded sandstone, showing thinning-upward packages and dominated by facies E and G, with minor facies F; and (iv) lobes or sheet sands – comprising coarsening-upward packages, 15-30 m thick, and are characterized by facies A, B, E, and G, with minor Facies F.