Deep-Sea Fan-Valleys, Past and Present
Comparison of ancient and modern marine sedimentary environments can yield better interpretations of both. Extensive exposures of modern sediments are not available, but detailed time correlation over large areas and much environmental data are absent from ancient sediments. The development of deep sea fan-valleys and the processes leading to their eventual filling have been studied through detailed comparison of two contemporary fan-valleys with one of Miocene age now exposed on land. Results from narrow-beam near-bottom echo soundings and seismic reflection profiles from a deep-towed instrument package were coordinated with information from bottom sampling on La Jolla and San Lucas fans, off southern California and Baja California respectively. A Miocene fan-valley exposed in sea cliffs near Dana Point, southern California, was used for the comparison. In all three cases, a fan-valley has been cut into horizontally bedded muds and silts of the surrounding fan. Terraces form on resistent horizons cropping out on the valley walls. Irregularly bedded sands with mudlumps and conglomerates in some sections fill the main parts of channels. Later deposits within channels are finer grained and more regularly bedded. La Jolla fan is cut by a single fan-valley; San Lucas fan has a braided system of valleys. These two different patterns may result either from differences in the rate of deposition and grade of sediment or from different amounts of incision in the feeding canyons following the post-Pleistocene rise in sea level.