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Chapter 10: Siliciclastic Marine Environments The Shelf Environment Major subdivisions and regions of the oceanic realm Subdivisions of the Continental Shelf Structural Barriers that form the seaward margins of continental shelves Sediment transport processes operating on the continental shelf Orbital motion of waves and particle movement Fair-weather waves: waves generated locally by wind movement across the shelf from deeper water onto the shallowwater inner shelf. Swells: low-relief, long-period, long-wave-length waves generated by storms that originate far out to sea. Storm waves: stronger more energenic waves that accompany storm activity on the shelf. They erode the beachface and upper shoreface. Wind-forced currents: unidirectional currents generated by wind-shear stress as wind blows across the water surface, gradually putting into motion deeper and deeper layer of water (Ekman transport). Ekman transport Ekman transport on shore With downwelling a geostrophic current is formed. This current initially moves obliquely offshore, but thanks to the Coriolis force, it assumes a direction roughly parallel to the shoreline. Sediment plumes: sediment discharging from river mouths into oceans. Hypopycnal flows can reach the middle shelf and Hyperpycnal flows typically stay within the inner shelf as turbitity flows. Nepheloid flow: suspended sediment reaching height of several hundred meters above the seafloor that slowly flows seaward as a density flow. Dust storms off the Sahara Desert supply sediment to marine systems. Storm bed deposits Tide–dominated shelves Maximum Spring tide velocities Continental shelves affected by intruding ocean currents Ancient Siliciclastic Shelf Sediments are distinguished by the following characteristics: •Tabular shape •Extensive lateral dimensions and great thickness •Moderate compositional maturity of sands •Generally well developed, even, laterally extensive bedding •Storm beds in some shelf deposits •Wide diversity and abundance of normal marine fossil organisms •Diagnostic association of trace fossils More specific characteristics are related to deposition under tidedominated or storm-dominated conditions. Ancient siliciclastic shelf sediments Continental margin review Deep-sea sediment processes & deposits Sediment transport to the deep ocean Submarine fan deposition Graded volcaniclastic turbidite with Bouma divisions marked. Distribution of dominant deep-sea sediments in the modern ocean Ancient Deep-sea Sediments (Rhythmites) Rhythmically-bedded turbidites of Canning Formation; found in ANWR.