Summary

Natural turbidity currents are difficult to observe directly, which means that we are heavily reliant on modeling techniques to predict the character of their deposits. We have used some experiments to model turbidity currents and spatial variations in their deposits. These experiments used scaled turbidity currents with a mixed grain-size sediment analogue. The thickness and the grain-size distribution of the deposits were measured, and estimates of deposit porosity calculated using an empirical algorithm. The deposits of unobstructed flows form elongate lobes, that thin and fine both downstream and laterally. In subsequent experiments, the currents were confronted by radially symmetrical obstacles placed directly in the path of the flow. When the obstacle height was significantly greater than the flow thickness, the flow deflected around it, creating a thicker deposit in proximal areas, two lobes alongside obstacle, and sediment shadow behind it. When the obstacle height was significantly less than the flow thickness, the shape and thickness of the deposit were similar to those of an unobstructed flow. However, the obstacle had a considerable effect on the grain-size and sorting of the deposit. This effect is due to grain-size stratification within the flow; the coarser grains are concentrated near the bed, where they are more susceptible to topographic influence, whilst finer grains are more evenly distributed through the flow. We can postulate the effects in other topographically influenced turbidite systems. These techniques can be used to predict the distribution of primary porosity in the subsurface wherever we can reconstruct the syndepositional topography.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah