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.

The Sedimentary Record of Paleogene Sequences in Sarawak, Malaysia

“…Rajang Group is mostly based on outcrop data, but unfortunately outcrops are scattered over large out by various authors to characterise the facies and sandbody types for a better understanding of the Rajang deep marine depositional system (e.g., Zainol et al, 2007;Galin et al, 2017;Kuswandaru et al, 2018). In the southernmost corner of Sarawak, geologically called the Kuching Zone (Haile, 1974), we find again Paleogene sediments, but here in fluvial facies.…”

“…The member has since been redefined Wolfenden (1960), Liechti et al (1960) and Heng (1992), as presented by Henning-Breitfeld et al (2020). ( Bottom In detailed sedimentological studies, the turbidites and debrites in the Belaga Formation were interpreted as the deposits of basin floor submarine fans in the Cretaceous-Eocene at the northern margin of Sarawak (Zainol et al, 2007;Kuswandaru et al, 2018). A recent summary of the Rajang Group deposition is also offered by…”

Facies and bed type characteristics of channel‐lobe transition deposits from the Oligocene‐Miocene Tajau Sandstone Member, Kudat Formation, Sabah, Malaysia

“…Subdivision of depositional lobes (axis, off axis, fringe, and distal fringe) can also be identified based on the bed type associations observed. Tripartite/hybrid beds are helpful indicators for frontal and lateral lobe fringe settings (see Haughton et al, 2003; Haughton et al, 2009; Kuswandaru et al, 2019; Prélat & Hodgson, 2013; Spychala, Hodgson, Flint, & Mountney, 2015; Spychala, 2016; Spychala, Hodgson, Prelat, et al, 2017). Bed type association and geometry in the two main depositional environments provide evidence that the facies distribution in the Tajau Sandstone Member is mainly controlled by the following factors: (a) sediment supply rate and calibre; (b) flow processes and rheology, including spatial and temporal flow transformations; and (c) seabed morphology and topography.…”

Geochemistry and paleogeography of the Rajang Group, Northwest Borneo, Malaysia

“…3f). Both the Kapit and Pelagus members contain facies indicative of gravity flow deposition, including turbidites and debrites (Bakar et al, 2007;Galin et al, 2017;Kuswandaru et al, 2019). Unit 3, seen only in the Metah Member, is dominated by shale, slate and mudstone with less thick sandstones compared to the Pelagus Member (Fig.…”