Subaqueous clinoforms created by sandy wave-supported gravity flows: Lessons from the Central California shelf

Subaqueous clinoforms are an important shelf feature. Their origins are typically associated with subaerial deltas but recent work has identified similar features in settings without a significant fluvial source (Mitchell, 2012; Patruno et al., 2015; Patruno and Helland-Hansen, 2018). These other studies have shown that such subaqueous clinoforms, also known as infralittoral prograding wedges (IPWs), are created largely by wave-induced processes. This study uses geophysical, sedimentological, and radiocarbon data to determine the sedimentary characteristics and genesis of a shore-parallel subaqueous clinoform developed far from any significant river on the central California continental shelf; a sedimentary shelf buildup known locally as the Cross Hosgri Slope. Sediment cores through the clinoformal sedimentary unit reveal that it is composed of beds with an erosive base, overlain by a thin (∼ 15 cm) coarsening upward sequence of shelly fine sands transitioning to a fining upward sequence marked by alternating parallel and ripple cross laminated very fine sands. The sedimentary succession is often capped by fine silts that are commonly interbedded with thin very fine sand beds. Radiocarbon dating of shells collected just above the erosive base indicate the subaqueous clinoform initiated progradation ∼7 ka, nucleating on an older Younger Dryas relict shoreface. We suggest the Cross Hosgri Slope was created by winter-storm waves mobilizing sands in water depths up to ∼70 m that transitioned into wave-supported gravity flows. The wave-supported gravity flows traveled downslope to water depths of up to ∼80 m, corresponding to the foot of the subaqueous clinoform. They did not travel beyond this depth as wave influence at these depths is negligible and the shelf gradient is insufficient to maintain movement of the load alone. Our work suggests that wave-supported gravity flows can entrain very fine sands and silts and build subaqueous clinoforms, even in the absence of a significant river source. Furthermore, we provide a facies model for sandy wave-supported gravity flow deposits.