Unrimmed Shelves
Characteristics of unrimmed shelves:
1) a 10m to 300 km wide seafloor that gently slopes offshore from a continental area
2) facies belts of variable width that closely parallel bathymetric contours
3) gradual transition of facies belts from inner, shallow shelf to outer, deep shelf to basin
4) lithofacies are generally grainy
5) high-energy, carbonate sands in the wave and/or tide agitated inner shelf
6) skeletal muddy sands to muds in quiet deeper outer shelf that are only periodically
affected by storms
7) localised patch reefs and sand shoals, but no continuous reef trends
8) no protection from onshore waves
Holocene-warm water
Modern warm water unrimmed shelves are sites of low or moderate energy processes controlled by waves, tides or currents. High-energy conditions result from occasional hurricanes. Typically in these environments the sediments are not in equilibrium with present day shelf hydrodynamic processes.
Example 1: Trucial Coast
The Trucial Coast is located in the Persian Gulf. The composition of the sediments differs throughout the region; on the Arabian side there is very little siliclastic input so the sediment is carbonate rich while on the north side there is and abundance of terrigenous sediment that originated from the Agros mountains. The lithofacies include a back ramp with microbial intertidal flats that pass landward into an evaporitic basin and skeletal-pelleted sands to pelleted lime muds in protected lagoons, a shallow ramp with high energy skeletal/oolitic sand shoals, beach barrier systems and coral reefs, a deep ramp that is transitional from aggregate/skeletal sands dominated by molluscs and foraminifera to skeletal muddy sands dominated by mollusc debris, and a gradual transition into bivalve rich marls of deeper water. Offshore sediments have gradual increase in mud content as you go seaward. Sediment distribution is controlled by shamals, which generate large, powerful waves.
Example 2: West Florida Shelf
The West Florida shelf is the eastern
boundary of the Gulf of Mexico, a low-energy sea. It's facies parallel the shoreline
and include an inner shelf that is composed of siliclastic sands, quartz, and
molluscan sands, the outer shelf of coralline algal sands, the shelf edge of
oolitic, peloidal, and lithoclastic sands, and the slope of planktonic foraminiferal
oozes (up to 30% clay minerals). The landward shelf edge contains small carbonate
buildups covered by corals, bryozoans, and Halimeda. Sediment distribution is
controlled by waves, tides, and hurricanes.
Example 3: Campeche shelf
The Campeche shelf is located on the western side of the Gulf of Mexico(as shown in the picture above), opposite from the West Florida shelf. The facies include inner and outer shelves composed of molluscan rich sediments, the shelf edge of benthic and planktonic foraminifera along with oolitic, peloidal, and lithoclastic sands, and the slope of planktonic foraminiferal oozes and sands. Isolated organic buildups occur at the shelf edge on top of antecedent highs. Sediment distribution is controlled by waves, tides, and hurricanes.
Ancient-warm water
Example 1: Jurassic Smackover Formation in southern US
This particular formation is
characterized by the following facies; ooid grainstones, packstones, and sandstones
in the nearshore belt, peloidal/bioclastic wackestones and foraminferal mudstones
further offshore, and organic rich, laminated mudstones in the deepest regions.
Holocene-cool water
Example 1: Gulf of Gabes
The Rottnest shelf is located off
the southwestern coast of Australia, a high-energy site. The facies include
a wave-rippled cross-stratified sandy inner shelf, an outer shelf composed of
bryozoan sands and sandy muds which are subject to extensive bioturbation, and
a muddy planktonic carbonate slope. Overall the sediment is dominated by bryozoans,
calcareous red algae, molluscs, foraminifera, echinoderms, pteropods, ostrachods,
and brachiopods.
Ancient cool water
Ancient unrimmed shelves with cool water conditions are very rare due to the problems associated with the identification of ancient temperate water carbonates.
Example 1: Eucla Platform (Mid Cenozoic)
The Eucla Platform is located off
the southern coast of Australia. It is composed of limestones which consist
of skeletal grainstones with bioclasts (bryzoa, bivalves, brachiopods, corals).
The lower cross-bedded and overlying thin bedded facies may represent subaqueous
dunes and interdunes that originated in much deeper waters.