Clastic Shelf Deposits
South Brazilian Wave Dominated Coast
Contributed by Abdulrahman AlJarad while a student of USC taking the course Marine Sediments 553
Continental margin - Siliciclastic sediments.
The continental shelf, continental slope and continental rise, collectively form continental margins, particularly where they form the trailing edge to continents. The sediments accumulating on these continental margins vary between siliciclastic and carbonate, with most areas accumulating siliciclastics. Here webpages focus on siliciclastic sedimentary The accumulation of carbonate are the focus elsewhere on the site where it deals with carbonate sedimentary record.
The siliciclastics of the continental margin are sourced from the shoreline; their character a product of the local physical processes and the geology of the source terrain. Whatever the latitude, grain size tends to decrease with distance from shore, as distances of transportation and depths of water below wave base increase and mechanisms of sediment transportation vary. Sediments accumulating offshore at higher latitudes may reflect glacial processes, their associated fluvial systems, the local hydrodynamic and oceanographic conditions. Sediments to mid and lower latitude shelves similarly will be products of the local geology, local processes and conditions.
The Continental margin morphology – Subdivided into:
Continental shelf – Areas of shallow, gradually sloping seafloor extend from the shoreline to where there is a marked changes slope into ocean depths down continental slope. This seaward boundaformry of the continental shelf averages about 130 m in depth (see figure). Continental shelves compose around 8 percent of the entire oceanic area and are underlain by continental crust, and slope seaward at an average slope of about 0.1º, or about 2 meters per kilometer. Their geology is often similar to that of the adjacent exposed portion of the continent. The width of continental shelves at the present sea-level stand varies from a few kilometers to more than 400 kilometers. Throughout geologic time, the width of continental shelves has varied greatly with the rise and fall of eustatic sea level. During periods of lower sea level, rivers may have flowed across the inner continental shelf accumulating sediments that were later reworked by waves and submarine currents and are known as relict sediments.
Continental slope – here the ocean floor extends from seaward edge of continental shelves. In some places, such as south of Aleutian Islands, slopes descend directly to ocean deeps. In other places, such as off eastern North America, they grade into somewhat gentler continental rises, which in turn lead to deep ocean floors.
Continental rise - In some regions the base of continental slope is characterized by a gentler continental rise, leading downward to deep ocean floor.
Problems associated with siliciclastic shelves:
- It may be hard to determine the age of the sediments since their post-depositional history may have involved extensive re-working during their exposure to the last major sea level position.
- Data collected on the shelves is gradually increasing. Recently researchers have been more interested in what is going on the slopes and the occurrence of mass transport of these sediments to forms, etc.
Wednesday, March 11, 2015