Clastic Marine Shelf Systems

(continental shelf/shallow marine/offshore)

Shelf Settings

Clastic shelves in the stratigraphic record mostly represent two settings, pericontinental (continental passive margin) and epicontinental (continental interior).

Shelf Processes

The interaction between waves (fairweather and storm), tides, and contrasts in water density creates a complex set of processes operating to transport sediment on the sea floor.

Geostrophic Currents

Geostrophic currents are set up by pressure gradients that cause flow along an isobar that curves away from the shoreline in response to the Coriollis Effect. In deeper water, they often move as a nepheloid flow of suspended sediment derived through hypopycnal flow, hyperpycnal flow, and storm surges.

Photo by W. W. Little

Storm-dominated Shelves

The interaction between waves (fairweather and storm), tides, and contrasts in water density creates a complex set of processes operating to transport sediment on the sea floor.

Photo by W. W. Little

Photo by W. W. Little

Wave/storm-dominated shelves ideally exhibit a transition from sands in the lower shoreface, to alternating sands and muds below fairweather wave base, to muddy facies below storm wave base.

Shoreline to Offshore Model

Wave-base

Wave base is the depth to which waves make contact with the sea floor.• Fair-weather wave base varies from approximately 5 to 15 m depth.•Storm wave base ranges from around 15 to 30 m depth.

Clastic marine shelves are flat and slope gently basinward, producing a graded profile in which deposits become finer-grained and less susceptible to wave activity away from the shoreline, reflecting an overall decrease in energy. The general pattern can be complicated by tide and submarine currents. Often, depositional relics remain from earlier base-level conditions.

Simplified Facies Model

Shanmugam 2008

Shanmugam 2000

More Complex Facies Model

Shore to Shelf Transition

Shelf sediment transport can be accomplished through quiet-water settling, storm waves, storm swells, tides, submarine currents, and bioturbation. The relative importance of these processes varies with distance from the shoreline, water depth, basin geometry, and sediment supply (type and abundance).

Heckel 1977

Photo by W. W. Little

Laminated mud forms by suspension settling and is preserved in abundance only below storm wave-base.

Laminated Mud

Photo by W. W. Little

Bioturbated Mud and Sand

Bioturbation is common in marine shelf deposits and can be expressed in a variety of forms that are indicative of water depth.

Hummocky Beds

Thin storm beds are scattered throughout the proximal portion of the marine shelf, becoming finer-grained basinward. Storm beds typically have erosional bases. Graded bedding, hummocky cross-bedding, gravel intraclasts, and shell concenrations are common structures.

Hummocky Cross-bedded Sand

Hummocky cross-bedded sand is produced during major storms and typically forms thin beds scattered through a predominantly muddy succession.

Photo by W. W. Little

Photo by W. W. Little

Photo by W. W. Little

Shell Beds

Shell beds accumulate during large storms. They typical have a sharp, erosional base and grade upward into overlying sand.

Photo by W. W. Little

Megaripples

Cross-bedding in storm beds is related to the migration of megaripples.

Large scale cross-bedding

Large scale cross-bedding diagram

Photo by W. W. Little

Limestone

Thin limestone beds are often scattered through the succession. These represent condensed intervals (unconformities) formed during sea-level highstands.

Photo by W. W. Little

Large-scale Architecture

Offshore marine deposits are often found in cyclical coarsening-upward successions with shoreline deposits. The marine deposits typically consist of monotonous laminated and bioturbated shales with increasingly common interbedded storm beds in the upward transition to shorface sediments.

Photo by W. W. Little

Offshore to Shoreface Succession

Photo by W. W. Little

Photo by W. W. Little

Offshore Zone

Photo by W. W. Little

Photo by W. W. Little

Offshore to Transition Zone

Photo by W. W. Little

Transition Zone to Shoreface

Photo by W. W. Little

EaES 455-6 34

EaES 455-6 35

Storm-dominated Shelf Profiles

A typical succession consists of interbedded quiet water and storm deposits. Quiet water sediments are consist mostly of bioturbated and laminated mud that can be glauconitic. Storm “beds” are typically erosional at the base and fine upward from intraclastic gravel, through trough cross-bedded sand to hummocky cross-bedded sand.

Shanmugam 2000

Tide-dominated Shelves

On shallow shelves in areas with high tidal ranges, subaqueous tidal currents can transport sand to create a variety of bedforms, depending upon tidal strength (current velocity) and sediment supply. These range from mud-draped dunes to sand waves to longitudinal ribbons to erosional surfaces with increasing current velocity. Tidal bedforms can be tens of meters high and several kilometers wide where sand is abundant. Storm beds can be interbedded with tidal deposits, and bioturbation can be well-developed.

Tidal Dunes

Tidal dunes can be complex, showing evidence of bidirectional flow internally, while maintaining their asymmetrical morphology demonstrating prevailing sediment transport direction. Reactivation surfaces are common.

Tide-dominated Shelf Profiles