--> Fine-Grained Deposits from Hyperpycnal Flow in Deepwater Basins: Impact on Top Seals in Turbidite Traps, Analogs and Examples

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Fine-Grained Deposits from Hyperpycnal Flow in Deepwater Basins: Impact on Top Seals in Turbidite Traps, Analogs and Examples

Abstract

Abstract

Hyperpycnal gravity currents rapidly transport sediment across shore from rivers to the continental shelf and deep sea during flood stages of rivers. In an Holocene analog to the deposits from these currents, called hyperpycnites or hypopycnites, the Santa Barbara Basin, California Borderland has had considerable study for over 50 years for the Holocene. Surface plumes of fine-grained sediments have been interpreted from LANDSAT data over a large area of deep sea and hemipelagic basin. Shallow water hyperpycnal currents and their deposits have been studied by the USGS, UC Santa Barbara and the University of Southern California on the continental shelf, where they produce shallow water submarine fans in water depths of less than 80 m. In deeper water at the anoxic basin center, laminated sediments alternate with gray “flood layers” which are caused by unusually wet, greenhouse climate events in this Mediterranean climate. These flood layers are likely formed by hyperpycnal flow of mud and fine silt in multiple “nepheloid layers”—layers in the water mass where density stratification traps flood sediments until they settle out slowly into the basin center. These flood layers display very plastic behavior in geotechnical measurement of piston cores and geotechnical cores and would have the characteristics, when lithified to form very effective seals. Additional potential seals in this deepwater setting are very organic rich layers, up to 9% TOC, which form in non-flood years. Fine-grained turbidites, dominantly silt in texture are often associated with these hyperpycnites, and likely are formed when sediment stored on the shallow shelf from shelfal hyperpycnal flow reach the continental shelf break and are transported in normal, bottom-hugging turbidite flow. During a sandier, Icehouse Earth setting, or with high tectonic activity, these silty turbidites would be replaced by sandy turbidites, excellent petroleum reservoirs deposited in deep water, as in Southern California Tertiary petroleum provinces. In the Pliocene Ventura Basin, a giant oil field province, thin seals between numerous stacked pays of turbidite sandstone are likely one of the best ancient analogs to the Holocene depositional model. Hyperpycnite plastic shales are important sealing beds in both convetional and unconventional reservoirs.