Geology Cafe

Offshore: From the Continental Shelf To the Deep Sea Basin

Marine Canyons, Abyssal Fans, Submerged Mountains, and Abyssal Plains
The offshore regions have a rich, complex geologic history hidden from view beneath the ocean surface. The ocean bed is an important place to study sedimentary processes because many rocks found on land have their origins at sea. Sea level changes associated with the climate changes responsible for continental glaciation periods are reflected on both land at at sea. The continental shelf break occurs in most places in California many miles offshore. The shelf break marks the general boundary of the California coastline when sea level was nearly 350 to 400 feet lower than present. This ancient sea level corresponds to the time when continental glaciation was at it peak worldwide. Glacial max in California
The Continental Margin: This combined topographic and bathymetric map of the greater Monterey Bay and San Francisco Bay demonstrates that the offshore region along the continental margin are extremely geologically active, and that sedimentary processes (erosion and deposition) are occurring and responsible for large-scale changes. See large and larger versions. 3D
Abyssal fan deposits: These sand and shale layers are exposed along the coast in San Mateo County. These rocks, called "turbidites"reflect a long history of underwater landslides that carried sediments from the continental region to the deep ocean basin offshore.
Distal fan deposits: These deposits originally accumulated far offshore, but are now exposed high in the Santa Cruz Mountains. The alternating layers of sandstone and shale formed as submarine turbidity currents (underwater density current storms) carry sediment far out to sea. The coarser sediments settle first, leaving layers of sand, whereas the finer sediment settle later, forming layers of shale.
Click on thumbnail images for a larger view.
Ribbon chert: These folded layers exposed in the Marin Headlands consist of thick bedded red chert layers with thin red shale partings. The chert formed from siliceous marine ooze, whereas the shale probably represents dust layers from distant continental dust storms. These cyclic bedding reflects deposition during the Jurassic period far offshore in deep water of the Pacific Basin.
Ribbon chert: These folded layers exposed along the road to Mount Diablo Peak. The are similar to the layered shale and chert beds exposed in the Marin Headlands. Although originally flat-lying layers of siliceous ooze and mud, these have been partially metamorphosed and subjected to tectonic deformation during their 150 million year plate-tectonic voyage from the deep ocean basin to the high peaks of the California Coastal Ranges.
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8/28/2012