Outcrop-scale acoustic facies analysis and latest Quaternary development of Hueneme and Dume submarine fans, offshore California

The uppermost Quaternary deposits of the Hueneme and Dume submarine fans in the Santa Monica Basin have been investigated using a closed-spaced grid o f boomer seismic-reflection profiles, which give vertical resolution of a f ew tens of centimetres with acoustic penetration to 50 m. Acoustic facies i ntegrated with geometry define six architectural elements, some with discre te subelements that are of a scale that can be recognized in outcrops of an cient turbidite systems. In the Santa Monica Basin, the relationship of the se elements to fan morphology, stratigraphy and sediment source is precisel y known. The width of upper Hueneme fan valley has been reduced from 5 km since the last glacial maximum to 1 km at present by construction of laterally confin ed sandy levees within the main valley. The middle fan comprises three main subelements: thick sand deposits at the termination of the fan valley, low -gradient sandy lobes typically 5 km long and < 10 m thick, and scoured lob es formed of alternating sand and mud beds with many erosional depressions. The site of thickest lobe sediment accumulation shifts through time, with each sand bed deposited in a previous bathymetric low (i.e. compensation cy cles). The lower fan and basin plain consists of sheet-like alternations of sand and mud with shallow channels and lenses. Variations in the rate of late Quaternary sea level rise initiated changes in sediment facies distribution. At lowstand, and during the approximately 11 ka stillstand in sea level, the Hueneme Fan was fed largely by hyperpycn al flow from the Santa Clara River delta, depositing high sediment waves on the right hand levee and thick sandy lobes on the middle fan. At highstand of sea level, most turbidity currents were generated by failure of silty p rodelta muds. In contrast, the smaller Dume Fan was apparently always fed f rom littoral drift of sand through a single-canyon point source.