Download Chapter 10: Siliciclastic Marine Environments The Shelf

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.