The Great Crocker Submarine Fan: A World Class Foredeep Turbidite System

Abstract

Abstract The Crocker Formation represents a classic, 'unconfined' basin-floor submarine fan complex that was deposited in an accretionary foredeep basin. The dimensions of the foredeep basin and thickness of the Crocker are difficult to reconstruct because of tectonic shortening and inability to resolve true stratigraphic thickness; net accumulation is proposed to range between 1000 to > 10,000 meters. What is evident is that the upper Crocker exposed in the vicinity of Kota Kinabalu, Sabah, consists of multiple stacked sand complexes over 300 meters thick, which forms a major submarine fan complex. The Crocker submarine turbidite fan system covers more than 25,000 sq km. Its aerial extent rivals those of the classic great turbidite systems described from modern and ancient settings throughout the world. Its tectonic-stratigraphic setting implies that some thrust tectonics and shortening should have influenced sediment dispersal patterns. In Sabah, the Crocker is exposed in thrust sheets and folds, therefore smart geology is required for stratigraphy and sedimentology studies. Several depositional elements are recognized in the Crocker turbidite complex: leveed channels which are comprised of channel axis and margin facies, proximal levee and distal overbank levee facies, splays and avulsed channels, clay plugged channels, and distributary lobe-sheet splays. Slumps, debris flows and shock beds, or seismictites, are present but rare. The depositional elements are arranged into coarsening-upward, fining-upward and stacked amalgamated successions, or stacking patterns, representing progradational lobe and/or splay deposits, channel-levee migration, and stacked multi-story channels, respectively. Facies reconstructions and the lateral and strike extent of the Crocker Formation supports an extensive turbidite complex of off-lapping, unrestricted channel-lobe fans. Individual channel complexes contain up to 30 to 60 meters of massively stacked sandstone, which was deposited by individual turbidity current flow deposits that rarely exceeded 3 meters of net sand. These channel axis deposits were filled by actively scouring high-density flows; here shale beds are rare. An inverse correlation exists between proximal indicators (sand thickness and sedimentary structures) versus shale thickness. Thick channel axis sands typically lack appreciable shale, except for abandoned channels. Confining levees range up to 35 m in thickness; here again, the inverse relationship holds where thin sand with distal Bouma facies corresponds to intervals of thick or numerous shale beds. Some splays and inter channel deposits contain mega-shale clasts localized in the tops of high-density deposits. The Crocker sediments are immature and moderate to poorly sorted. Sedimentary structures of the sands indicate proximal to distal flows deposited in closely associated sedimentary facies of a levee-channel, overbank and distributary splay complex: sands vary from massive, ungraded Ta and S1-2, locally large-scale scour and traction bedding and graded Ta-c and Tc-e. Channel axis sands are typically coarse-grained to granular in the channel-base, mega-flute scours structures. These sands represent deposition from highly turbulent, high-density currents confined within channels. High-density and low-density, dilute turbidite currents are mixed in channel margin deposits, while low-density currents characterize distal levee overbank and distal fan deposits. Flow stripped climbing Tc ripples deposited by rapid deceleration and sediment fall-out provide a key criterion for proximal levee deposits. The composition of the turbidite sediment in the Crocker suggests a continental-derived source. Entry points are unknown, but the nearest sediment source may be the more proximal Kelabit - Long Barawan sub basin to the immediate south of the Crocker system, or a more distant source from the Kuching High. The most distant and likely source may have come across the Natuna Shelf from the paleo Mekong Delta (Hutchison, 1996), since only oceanic basement rocks and limited sand and shale-prone lithofacies make up the accretionary prism. An eastward sourcing is discussed but not considered sufficient to provide the large volumes of sediment required for the Rajang and Crocker sediments. Important lessons can be learned about this and other turbidite systems of Borneo, especially when the observations are put into an accurate tectonic/structural, stratigraphic and sedimentologic frame-work. The Crocker outcrops can be used as subsurface analogs to help understand modern and ancient turbidite systems throughout SE Asia.