Sedimentology and Significance of Linked Turbidite-Debris Flows in the West Crocker Fm (Oligocene-Miocene), NW Borneo, Malaysia

Zakaria, Abdullah Adli¹, Howard D. Johnson², Christopher A.L. Jackson³, Felix Tongkul⁴, Paul D. Crevello⁵ (1) Imperial College London, London, United Kingdom (2) Imperial College, University of London (3) Imperial College, (4) Universiti Malaysia Sabah, Kota Kinabalu, Malaysia (5) Petrex Asia Reservoir & Stratigraphy Group, Kuala Lumpur, Malaysia

Exposures of the West Crocker Fm in NW Borneo, Malaysia indicate a common upward transition from well-sorted sandstones interpreted as turbidite deposits to either (1) mud-rich sandstones or (2) clast-rich mudstones interpreted as the products of debris flows. The sandstones are fine to coarse-grained, have sharp, erosive bases and are up to 5 m thick. Internally the sandstones are massive or weakly-graded, and may contain convolute or sub-horizontal stratification and water-escape pipes. The contacts between the turbidites and overlying debris flow units are sharp but may be undulose due to the development of clastic injections sourced from the turbidite sandstones. The overlying debris flow units consist of either mud-rich sandstones or clast-rich mudstones which are up to 0.44 m thick, are rich in shale, sandstone and/or coal fragments and may be capped by current-rippled fine sandstones. Based on the depositional facies, their vertical relationships and the nature of the unit contacts, the deposits are interpreted as being sourced from the same flow event with the debris flows transported and emplaced on the just deposited, still liquefied sandstones. Palaeogeographic considerations suggest that debris flows units within such flows may have been transported >100 km down the slope and across the basinfloor. Quantitative analysis of the thickness of the turbidite and debris flow components of the gravity-flows indicates that no significant relationship exists between the thicknesses of the units, suggesting fluctuations in the type of sediment incorporated in the parent flow or the manner in which material was segregated during transport.