Flood-Derived Sand Lobes on the Shelf of the Northern Santa Barbara Channel, Southern California

Abstract:

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? 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. 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 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 hybrid hyperpycnal-debris flows, likely during floods associated with El Niño Southern Oscillation. A small shoreface slope break found immediately up-dip from these features suggests that seafloor geometry may play a key role in their deposition. The absence of a similar shoreface slope brake in areas without fans suggests a relationship between fan formation and shelf gradient.