Turbidite, Debrite or Something in Between: Re-Thinking the West Crocker Formation

Abstract

The Oligo-Miocene West Crocker Formation (WCF) of West Sabah is often referred to as a sand-rich turbidite system, and has been the subject of detailed sedimentological studies during the last few years. The essential features of the WCF sediments can be observed at outcrops scattered within driving distance from Kota Kinabalu (Fig. 1). For the most part, thickly bedded facies, representing the high-density, sandy turbidites is found in most of the outcrops studied, with the exception of Taman Maju, Sepangar (Fig. 1), where there are more of the “classical” flysch-like, thin-bedded turbidites. In general, the Crocker is sand-rich and very thickly bedded (> 1 m), commonly 1.5-3 m thick, while some may be up to 35 m. Based on the presence of subtle scour and amalgamation surfaces, these thick beds were formed not by a single flow but multiple flow events. Internally, the thick beds, most of which are poorly sorted despite the overall normal grading, are characterized by faint low-angle laminations, resulting from traction, passing upwards into contorted bedding due either to deposition from a slurry, or to soft-sediment deformation and dewatering. Deposition from slurry involves rapid dumping of a dense muddy and water-saturated mass of sediment. Hence, in these types of beds at least, there is strong evidence for some form of high-density (sandy) turbidity flows, slurry, or both. Erosion at base of the flows, indicated by flute casts and various other sole marks are common. Well-developed load structures, including large ball-and-pillow (or “jam roll”) structure due to loading and sinking of these dense flows into water-saturated muddy substrate are indicative of the scale and dynamics of these flows.