Chapter 10: Siliciclastic Marine Environments. The Shelf Environment

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 shallow-water 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 tide-dominated 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.