Are Hyperpycnal Flows “True” Hyperpycnal Flows?

Piret Plink-Bjorklund
Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO

In the Eocene Central Basin of Spitsbergen, the hyperpycnal flows were generated episodically. Between these episodes mouth bars (MB) prograded, indicating hypopycnal conditions. The hyperpycnal episodes were associated with significant erosion of the MB, caused by the downdip accelerating hyperpycnal flows. As the flows reached the toes of the MB, flow thickening and deceleration occurred, causing them to drop some of the coarse sediment load. These hydraulic jump deposits are volumetrically much smaller than the eroded portion of the MB. Thus, the hyperpycnal flows gained extra sediment by MB erosion. This suggests that rivers that produce hyperpycnal flows regularly (although episodically) maintain their MB by flow erosion rather than by downslope collapse. Should the flows, generated by direct river effluent, but contributed by MB erosion be still called hyperpycnal flows? As erosion and entrainment is natural behavior of turbidity currents - yes.

The MB build-out and episodic erosion ismost active during the falling and lowest relative sea level (RSL). Thus, hyperpycnal flow systems will have a specific stacking pattern in their basin-floor fans. As the rivers are gradually approaching the shelf edge during a RSL fall, the hyperpycnal-flow-generated turbidites on the basin floor will be reaching gradually further into the basin, in accordance with the slopeward progress of the river mouth. This creates an initial, gently prograding deposit in the lowermost part of the fan. At the time when the river mouth is at the shelf edge, and the hyperpycnal flows can debouch directly onto deepwater slopes, the most sand-rich and thickest beds accumulate. At the time when the RSL starts rising, pushing the river mouth landward, and decreasing the contribution by MB erosion, the turbidites on the fan are heterolithic and their seaward pinch-outs step landward (retrograde).

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas