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Depositional History And Stratigraphic Architecture Of Lofted Hyperpycnal Flows In The Santa Barbara Channel, Southern California


Gravity flows in the Santa Barbara Channel (SBC) of Southern California have the potential to damage pipelines leading to offshore petroleum platforms, yet their recurrence intervals and initiation mechanisms remain unknown. The recent discovery of seven fans on the continental shelf of the SBC poses an important question; namely, are the SBC fans deposited by sediment gravity flows capable of damaging shelf infrastructure? Here, we present grain size trends, radiocarbon dates, and overall stratigraphic architecture of two fans in the SBC from eight cores and nine grab samples collected in October, 2013. The Tajiguas Creek fan and Refugio Creek fan lie on the northern shelf of the SBC in 30 m to 70 m water depth. The fans lie directly offshore from small mountainous creeks that exceed the suspended sediment threshold of 40 g/L required for plunging at the fluvial-marine interface. Both fans contain approximately seven elongate lobes with abrupt terminations. A transgressive ravinement surface (TRS) is present at or near the base of five cores from the Tajiguas Creek fan. This layer is interpreted as a TRS based on its abundant shell fragments of a diverse assemblage and because radiocarbon dates from shells directly below this layer correlate well to local relative sea-level curves. Above the TRS, lobe-forming subaqueous gravity flows deposited structureless fine- to medium-grained sands that are slightly graded. Radiocarbon dates indicate that flow events occurred as early as 8500 Cal years BP and may still be active today. The snub-nosed geometry of each lobe implies en masse freezing of deposits, but the incorporation of shell material and evidence of erosion in shallow seismic profiles indicate an initially turbulent flow regime. The location and geometry of these features suggests that the fans in the SBC are the result of lofted hyperpycnal flows. A small shoreface slope break found immediately up-dip from these features and the absence of a similar slope break in areas without fans suggests that seafloor geometry may play a key role in their deposition.