Evidence to support near-steady flows (?) in an alluvial sandy system: the plateau formation Bako National Park, Sarawak

Johansson, M. and Stow, D. A. V. (1999) Evidence to support near-steady flows (?) in an alluvial sandy system: the plateau formation Bako National Park, Sarawak. Bulletin of the Geological Society of Malaysia, 43. pp. 433-438. ISSN 2637-109X

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Abstract

The sandstones at Balm are interpreted as having been deposited in a sandy braided stream environment. This system is characterized by variable discharge, high bedload, complex bar forms, high lateral mobility with low preservation of fine material. The dominant sediments, concurrent with this interpretation are large-scale cross-bedding and parallel laminations. However, other sediments, particularly thick units of structureless sands commonly liquified are thought to be anomalous to this type of setting. Three methods of ‘massive‘ sand deposition are recognized (i) bed amalgamation, (ii) ‘near steady‘ turbulent flow, and (iii) liquifaction. A turbidity current is thought to only be able to form during the 100 to 1,000 yr. storm event where a high concentration flow with high turbulence passes over a hydraulic jump. The unison of these three factors may allow the process of ‘continual aggradation‘ to occur. Although this is thought to be rare these mega-storms have a high capacity to deposit large amounts of sediment, highly preservable in the rock record. However, with the benefit of extensive outcrops it can be recognized that the majority of structureless sands located at the Bako Peninsula are formed from the amalgamation of sands in the channel thalweg and the liquifaction of climbing ripple destroying the primary depositional fabric.

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