Observations from Multibeam Bathymetry and Backscatter Data, Sub-Bottom Profile Data, and Piston Core Results That Bear on the Transportation and Deposition of Sediment in the Deep Water Makassar Strait, Indonesia

John Decker¹, Philip A. Teas¹, Peter Baillie², Daniel L. Orange¹, and Widjanarko^3
1Black Gold Energy, Jakarta, Indonesia
²TGS-Nopec, Perth, Australia
³MIGAS, Jakarta, Indonesia

INTRODUCTION

Ongoing marine hydrographic and coring studies in Indonesia have provided remarkable new data on sediment transport and deposition from the shelf edge to deep marine basins. These data both support and refute current gospel, but more importantly provide insights into previously unknown phenomenon. The Makassar Strait is a north-south trending, 150 km wide strait between the islands of Sulawesi and Borneo. This is the main conduit for the Indonesian Through Flow where the Pacific Ocean drains into the Indian Ocean. The swift current with horizontal and vertical mega eddies add yet unresolved complications to our understanding of sedimentation and erosion in the Makassar Strait. In addition, the high resolution bathymetry picture of the Makassar Strait is still incomplete, but we do now have enough data to make some interesting observations and some initial conclusions. The purpose of this paper is to report on 2 spectacularly imaged depositional features of the Makassar Starit; a large basin floor fan and large landward migrating sediment waves; the depositional products of different shelf to basin transportation processes.

MAKASSAR FAN

The most striking feature in the Makassar Strait, from a sedimentological prospective, is a large, 2500 sq km, low relief basin floor fan. Fan components are very well imaged with quantitative multibeam backscatter which shows a systematically varying response which can be correlated with lithologic changes observed in piston core samples. Channels are straight, bifurcating, and show low sinuousity. 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. Proximal lobes occur within or immediately adjacent to proximal channels and are interpreted to represent deposition during modern highstand conditions. The channels incise portions of the upper fan system, but do not appear to show significant levee formation adjacent to the channels.

SEDIMENT WAVES

Large sediment waves are particularly well imaged with multibeam bathymetry and backscatter, and on subbottom profile data. The waves have wavelengths of approximately 1-3 km and heights of about 10-30 m. A nearly universal feature of these bedforms is that the the crest of the waves migrate landward 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.

INTERPRETATION

The Makassar Fan was first recognized in a 2003 multibeam survey, but its origin at that time was unknown. New data acquired in 2006 and 2007 by TGS Nopec interrogates the sediment feeder system for the fan. The fan is fed from a single deeply incised feeder channel but this channel is not connected directly as a visible bathymetric channel to any shelfal source. Rather the incised channel seems only to connect an upper basin to a lower basin. It appears that sediment is deposited in the upper basin and drained via a single channel to the lower basin. There are 3 sources of sediment into the upper basin; one (possibly 2) river fed channels, slope originating debris flows, and non-channelized (line source?) sediment apron composed of large-scale upslope migrating sediment waves. At this time we do not know the relative contribution of each source type but we do have cores from the 3 sources. Slope sediments from the Makassar (west) side are generally clay with discrete mm scale silt layers, the channel contains sand and small pebble gravel; levees are interbedded ripple sands and mud, and the sediment apron oddly contains pits with mud interbedded with sand and small pebble gravel, the sediment waves themselves are predominantly clay with mm to cm scale sand-silt graded beds. The Makassar Fan is fed from the upper basin although there may be an indistinct but direct conduit to the Lariang River drainage of West Sulawesi.

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.

AAPG Search and Discovery Article #90079©2008 AAPG Hedberg Conference, Ushuaia-Patagonia, Argentina