Spatial variation in geometry of fine-grained sediment waves: An outcrop-scale example from the Upper Miocene Halang Formation submarine-fan system, West Java

The annual meeting of the Sedimentological Society of Japan

Osaka and Kyoto, March 2009

Spatial variation in geometry of fine-grained sediment waves: An outcrop-scale example from the Upper Miocene Halang Formation submarine-fan system, West Java

Mukti, M.M.*, Ito, M.*

* Chiba University

Abstract

Outcrop-scale sediment waves were recognized within a lower part of the Upper Miocene Halang Formation turbidite system in the Bogor Trough, West Java. The turbidite system is a mud-rich, longitudinal system from the west to the east along the basin axis, and is characterized by channel-and-overbank deposits in association with frontal-splay and crevasse-splay deposits, indicating a mud-dominated submarine-fan system. The Halang Formation submarine-fan system is also represented by high-sinuosity channels with average depth of 7 m and average width of 400 m. The present example of sediment waves were found in overbank deposits, and are characterized by the average wavelength of 14 m and by the average wave-height of 0.2 m. The orientation of the crest of the sediment waves is nearly parallel or oblique to the axis of a submarine-fan channel. Sediment-wave deposits are dominated by mudstone with intercalations of very thin-bedded, graded and/or laminated, very fine-grained sandstones and siltstones. Fine-grained sandstones in sediment wave deposits are slightly coarser in the proximal area to the channel margin compared to the intercalated sandstone beds in a distal overbank environment. Thickness of the sediment-wave intervals and sandstone percentages in the sediment-wave deposits were, respectively, thicker and higher in the stoss sides compare to the lee sides. Mudstones in the sediment-wave deposits are structureless or locally laminated with siltstone laminae, and is also characterized by clay fabric here in named ‘granular structure’ that is remarkably different from random structure in hemipelagic muddy deposits. Spatial arrangement of the sediment-wave deposits exhibit climbing forms that indicate upstream migration. The aspect ratio of the present example is largely similar to that of seismic-scale sediment waves from modern submarine-fans: Length, height, and wave-form index of the present sediment waves progressively decrease away from a margin of submarine-fan channel deposits. Furthermore, the Halang Formation sediment waves are asymmetry in a proximal portion of overbank deposits and change to being symmetry in a farther distal overbank environment. These proximal-to-distal geometrical change in the present sediment waves is interpreted to have been a response of a downflow variations in velocities, thickness, and the Froude number of turbidity currents that spilled over from the submarine-fan channels. The present outcrop-scale example of sediment waves can be useful for better understanding of formative processes of turbidity-current-induced sediment waves.