Geology Reference
In-Depth Information
Carbonate Platforms: Broad Categories
Rimmed carbonate shelves (Ginsburg and James 1974): Shallow periconti-
nental flat-topped platforms. Outer wave-agitated edge characterized by mor-
phological rims (barrier reefs, shoals, islands) that absorb wave action and
a pronounced break of slope into deeper waters. Rimmed shelf margins
comprise accretionary, bypass and erosional margins. High-energy facies
occurs predominantly at the outer shelf margin. Widths typically a few to
about 100 km. Modern examples: Queensland Shelf off eastern Australia
with the Great Barrier Reef, South Florida Shelf, Belize Shelf, Gulf of Suez.
Non-rimmed carbonate shelves: Shallow platforms without a pronounced
marginal barrier. Characterized by open water circulation. The term includes
open carbonate platforms as well as carbonate ramps. Widths a few to
more than 100 km. Modern examples: western Florida, Yucatan, Brazil. At-
lantic, common in cool-water settings.
Homoclinal ramps (Read 1982): Shallow
epeiric and pericontinental platforms charac-
terized by gentle uniform depositional slopes,
passing downwards from a shallow nearshore
high-energy facies offshore into a more
muddy deeper-water facies without a marked break in slope. The angle of
slope is commonly less than 1
°
(of the order of a few meters per kilometer),
but steeper dips may occur. Width between 10 and > 100 km. Modern ex-
amples: Trucial Coast of the Arabian Gulf, Shark Bay of Western Australia.
Distally steepened ramps (Read 1982): Simi-
lar to homoclinal ramps, but with a distinct in-
crease in gradient in the outer, deep ramp re-
gion. Width between 10 and 100 km. Modern
examples: Northeastern Yucatan, western
Florida.
Epeiric platforms (Shaw 1964): Very exten-
sive, flat, cratonic areas covered by shallow
seas. Dominated by shallow subtidal-intertidal
low-energy facies and tidal flat cycles. The
oceanward margin can be gentle (ramp-like) or steep (shelf-like), and can
be rimmed. Width between 100 and 10 000 km. No good modern examples
of epeiric platforms exist.
Isolated carbonate platforms: Isolated or detached shallow-water platforms,
offshore from continental shelves surrounded by deep water. Platform mar-
gins partly with reefs and sand shoals, platform interior with low-energy
facies and tidal flats. Most isolated platforms have steep margins and slopes
into deep and very deep water. Widths between 10 and 100 km. Modern
examples: Bahama Bank, Glovers reef/Belize.
Oceanic atolls: Formed on extinct, subsiding volcanoes that rise
several hundreds to thousands of meters from the deep ocean
floor. Characterized by raised reefal rims, steep outer slopes and
low coral islets encircling shallow and deep lagoons with pin-
nacle reefs. The circular or elliptical structures are surrounded
by deep water of the open sea. Atolls range in diameter between
less than 1 km to more than 130 km. Modern examples: Maldive
Islands, Indian Ocean. Common in the western and central Pa-
cific Ocean.
Drowned carbonate platforms (Schlager 1981): Rapid sea-level
rises, strong subsidence, or drastic reduction in carbonate pro-
ductivity as result of environmental stress can cause complete
or incipient drowning of rimmed shelves, ramps and isolated plat-
form. Pelagic platforms are drowned fragments of an ancestor
shallow-marine platform. Modern examples: Atolls in the Pacific
and Indian Ocean, Blake Plateau north of the Bahamas.
Fig. 2.6.
Carbonate platforms: Broad categories.
