Turbidite, debrite, and hybrid event beds in submarine lobe deposits of the Palaeocene to middle Eocene Kapit and Pelagus members, Belaga Formation, Sarawak, Malaysia

Abstract

Turbiditic flysch units of the Rajang Group form a large crescentic belt of deformed strata in central Borneo. Unfortunately, our understanding of its bed type characteristics and depositional setting is very poor. Here, we present a detailed bed type analysis of the Palaeocene to middle Eocene Kapit and Pelagus members of the Belaga Formation, based on detailed investigations of recent road-cut exposures around Sibu, Sarawak, Malaysia. Five bed types are identified from the studied sections, representing deposition from turbidity currents, debris flows, and flows that show turbulent and laminar characteristics. The Belaga Formation deposits are divided into four bed type associations based on bed type assemblages, bed geometry, degree of bed amalgamation, vertical grain size, and bed thickness trends, for example, lobe axis, lobe off-axis, lobe fringe, and slump. Hybrid event beds (HEBs) are not restricted only to the lobe fringe but are also a common element of the lobe axis sub-environment in the Belaga Formation system. Their common occurrence in a proximal location in the lobes is probably due to enhanced seafloor erosion and rapid deceleration due to loss of confinement at the channel lobe transition further up-dip. Evidence such as bidirectional ripple cross-lamination in thin-bedded turbidites and variable palaeocurrent orientations suggest a complex depositional topography, which also suppressed flow turbulence and promoted deposition of hybrid event beds in proximal locations through deflection and deceleration of incoming flows by confining counter slopes. Similar bed type assemblages within the slump and lobe deposits indicate a local origin for the slumps. Localized failures can form on low gradient slopes developed in tectonically active basins with complex topography.