AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
U-Shaped Slope Gully Systems on Low-Sediment Supply Passive Margins: Examples from Gabon (West Africa) and Timor Sea (Indonesia)
(1) Earth Science and Engineering, Imperial College London, London, United Kingdom.
(2) Niko Resources, Jakarta, Indonesia.
The hydrocarbon industry has primarily focused on high sediment-supply parts of passive margins with canyon systems that feed sand-rich submarine fans. In contrast, the low-sediment supply parts of the same passive margins exhibit a different range of less well-understood sediment routing systems. Here we describe suites of sub-parallel slope gullies, defined by seabed bathymetric expression in low sediment-supply systems from the Gabonese continental margin and the Timor trough, NW Australian continental margin. The Gabon gullies were mapped on 3D seismic data whereas those in Timor were mapped on high-resolution multibeam bathymetry data and 2D seismic data.
The Gabon gullies initiate at the shelf-edge break in 150-1500 m water depths on a slope that has a gradient of 5° decreasing to 2°. The Timor gullies start at 15 km distance from the Australian shelf-edge break in water depths of 800-1900m on a slope with a gradient of less than 1°. The gully sets persist laterally for distances of up to 50 km (Timor) and at least 40km (Gabon). The gullies are u-shaped, with a relief of 5-30 m (Gabon) and 18-191 m (Timor), and widths of 50-400 m (Gabon) and 300-500 m (Timor). Those on the Gabon margin become narrower and shallower with distance down the slope, whereas the Timor gullies show little down-axis variation. The Timor gullies appear to have resulted from simultaneous aggradation along inter-gully ridges and non-deposition along the adjacent gully floor. Hence, they formed mainly in response to deposition, rather than erosion. In the Gabon area both erosional and aggradational gullies are found.
Upslope migrating sediment waves occur in close proximity to the gullies on the Gabon margin. The erosional gullies appear to cross-cut sediment waves but sediment waves are observed to migrate up-slope locally within the aggradational gullies.
Evidence is lacking for any slumping or headward erosion at the headwall of the gullies which may have generated sediment gravity flows capable of simultaneous floor erosion and flank deposition. Hence, it is proposed that the gullies and related sediment waves were formed by diffuse, sheet-like, mud-rich turbidity currents. Instabilities in the turbidity currents led to regions of faster and slower flow. It is inferred that gully axes experienced flow velocities that mainly exceeded the settling velocity of the sediment in suspension. Lower flow velocities on gully flanks promoted sediment deposition.