Abstract

Recently acquired multibeam bathymetry and backscatter data, high resolution sub-bottom profiles, and piston cores from the deep Makassar Strait provide new insight into classic questions of deep marine sedimentation. This paper examines two features that are particularly well imaged by these data: a modern basin floor fan, and upslope-migrating deep sea sediment waves.

A large, 2500 sq km, low relief basin floor fan has been discovered at a water depth of >2000 m in the Makassar Strait and is very well imaged with quantitative multibeam backscatter. The Makassar fan shows a systematically varying backscatter response that paints a picture easily identified as a deep sea fan. Channels are straight, bifurcating, and show low sinuosity. Lobes are generally 1-2 km wide and 2 to 6 km long. The sediment lobes are more elongate in the along-channel direction (2 to 6 km long by 1-2 km wide) as compared to the typically equidimensional "textbook" lobes. Relic channels, presumably from lowstand times, can be seen almost to the distal limits of the fan and appear to feed the most distal sediment lobes. The channels incise restricted portions of the upper fan system, but do not appear to show significant levee formation adjacent to the channels.

Large sediment waves (with wavelengths of approximately 1-3 km and heights of about 10-30 m) have been observed on the modern sea floor and within the Neogene strata of the Makassar Strait. A nearly universal feature of these bedforms is that the upslope limb shows thicker strata than the correlative downslope limb, and the crest of the waves migrate landward as you go up-section, indicating that the bedforms grow or migrate up-dip (antidune direction for down-dip flow). High-resolution sub-bottom profile data show that the strata comprising the sediment waves are thickest on the landward-facing (stoss) side and thinnest on the more steeply dipping seaward-facing side. Sediment cores (~6 m length) from the stoss side and wave crest are composed of interbedded very fine sand and mud. Cores from the lee side are entirely mud. Similar bedforms in other basins have been interpreted to be formed by contour currents. Based upon their spatial distribution and orientation, we speculate that the Makassar sediment waves form by intermittent non-channelized down-slope sediment transport by hyperpycnal flow and/or turbidity currents associated with floods or storms in major river catchment areas.