Three small headlands in the sea cliffs west of Gaviota Beach, California, are the remnant fill of three discrete submarine gullies incised into the late Miocene submarine slope environment. These promontories provide excellent, three-dimensional exposure of the gully fill in outcrop, permitting documentation of their complex internal stratigraphic architecture. Detailed study of these exposures elucidates the sedimentologic processes that occur in the filling of slope gullies, guides interpretation of the acoustic records of otherwise unsampled modern gully systems on continental slopes, and provides insight into the heterogeneity that may characterize slope gully petroleum reservoirs.
We develop a comprehensive facies scheme to describe the variability within these intercalated coarse- and fine-grained deposits and use two-dimensional photopans to interpret the overall depositional system. Defined by internal bedforms, sedimentation units, and sediment size, each facies records sedimentation under different hydrodynamic conditions and can be genetically related to a discrete depositional mechanism. We differentiate channel axis, margin, and overbank sub-environments within the gully fill, which together define overall crudely braided, low-sinuosity channels within the gullies. Like many modern gully systems, the Gaviota gullies probably initiated through local slope oversteepening that led to slope failure, slumping, and initiation of sediment flows. Erosion and sedimentation from these high-density turbidity currents formed the primary depositional process for the Gaviota conglomerate units. Once initiated, the gullies acted as sediment conduits from the shelf and possibly onshore regions.
Despite their prevalence on modern upper slopes and their pivotal role in shaping shelf margins and transporting sediment to deeper water, the details of submarine slope gully formation and filling remain obscure. The Gaviota gully complexes provide valuable insights into long-term gully fill attributes not easily obtained from modern slope gully systems and rarely preserved in the rock record.