CHANGES IN FLOW DIRECTION AT A POINT CAUSED BY OBSTACLES DURING PASSAGE OF A DENSITY CURRENT

Abstract

The flow pattern and sediment character of laboratory density currents partly blocked by bed topography is used to demonstrate how changes in paleocurrent direction through a turbidite at a single site can be explained. These experiments confirm that a relatively small obstacle may affect bed thickness and paleocurrent patterns at considerable distances away from obstacles. Density currents generated by a lock-exchange mechanism spread radially across the tank floor and were partially blocked by a wedge-shaped obstacle with steep face (scarp) facing upstream. When the body of the current impinged on the obstacle, a thickened region of the flow developed upstream of the scarp, with increased entrainment of ambient fluid above it. The upflow edge of the thickened region was at a small angle to the obstacle front and propagated rapidly upflow and to the sides of the obstacle. Internal flow vectors observed with thread flags were radial from the lock before the head reached the obstacle. After the head passed the obstacle, flow vectors in the region of thickened flow upstream and to the sides of the obstacle were reoriented nearly parallel to the obstacle scarp. Thus flow vector at a point rotated by as much as 45degrees during passage of the current. The deposits of sediment-laden density currents thickened in the region of observed flow thickening because of an increased particle flux to the bed caused by velocity reduction. The upflow limit of the influence of the obstacle on deposition is marked by an abrupt increase in the thickness of the deposits. This area of thickening occurs in an area where the flow is accumulative.