THE COMPLEXITY OF SEDIMENTATION IN A SUBMARINE CANYON: THE HEAD OF EEL CANYON, NORTHERN CALIFORNIA

Submarine canyons are identified by their complex morphology of multiple entrants, meandering channels and steep walls.  Few studies have addressed depositional patterns and transport processes within distinct microenvironments.  Precision sampling with the ROV Ventana in August 2001, based on direct video observations coupled with a detailed bathymetric chart created using high-resolution multi-beam Hydrosweep (15 kHz) data, facilitated this study into small-scale spatial variability of microenvironments within Eel submarine canyon.  22 long pushcores were x-rayed and subsampled to construct grain-size and radioisotope profiles (²¹⁰Pb, ¹³⁷Cs).  Spatial variability of sedimentation in the head of Eel Canyon is addressed on a range of scales: between and along entrant channels (km), across channels (100 m), and at dive sites (10 m).  Preserved physical stratification in the thalwegs of the northern entrants suggests they are receiving more sediment than their southern counterparts.  Cores collected on channel walls generally exhibit elevated degrees of bioturbation with varying levels of preserved stratification.  In contrast, cores from channel floors tend to have reduced levels of bioturbation and well preserved physical stratification.  These patterns correlate with observations of sediment accumulation rates, which are significantly greater in the channels (>1 cm/y) than on the walls (<1 cm/y).  Large amounts (>10%, up to 50%) of sand and wood debris were discovered in cores from the four entrant channels.  This material corroborates evidence from sedimentary structures (e.g., graded bedding, cross-bedded sand layers) that indicate frequent occurrence of gravity flows.  The combination of intense sedimentation from nepheloid layers and gravity flows within a complicated morphologic system leads to clear distinctions between microenvironments, but also some recognizable and unifying trends in sedimentation.