HIGH- VERSUS LOW-DENSITY HYPERPYCNAL FLOW: DEPOSITS AND RELATIVE TIMING WITHIN A 4TH-ORDER GENETIC SEQUENCE, CENTRAL TERTIARY BASIN, SPITSBERGEN

Sediment concentration at the river mouth is a first-order influence on the energy of hyperpycnal flow. The variability in peak discharges and hydrograph shapes of river floods leads to a broad spectrum of hyperpycnal flow and resulting deposits because discharge is a major control on sediment concentration. Low-density hyperpycnal flow is defined as those flows which are not energetic enough to erode channels. These flows are therefore unconfined and not capable of long-distance runout. High-density hyperpycnal flow has the potential to become self-confined by creating channels and are more likely to transport sediment over great distances.

Channelized slope deposits in the Central Tertiary Basin of Spitsbergen record sedimentation from sand-rich, high-density hyperpycnal flow discharged from shelf-edge delta distributaries during relative sea-level lowstand. The presence of an attached basin-floor fan indicates bypass of sand also occurred through the slope channels. The high-density hyperpycnites consist of medium-bedded, fine- to medium-grained sandstone dominated by plane-parallel lamination and current ripples with occasional ungraded beds. Non-channelized slope deposits are thin bedded and heterolithic with crude wavy lamina and current ripples. They are interpreted as forming from low-density hyperpycnal flow, and likely formed lobate geometries due to their unconfined nature. Low-density hyperpycnites dominate the observed downslope deposits of aggradational late lowstand shelf-edge deltas. Channelized, high-density hyperpycnites, as well as the basin-floor fan, are largely downdip of non-aggradational, early lowstand shelf-edge deltas. This relationship suggests that sediment storage within the floodplain may have played a large role in determining the initial energy and bypass potential of hyperpycnal flow.