6.3 SEDIMENTARY MODELS
249
Fig. 6.53. Map showing the present-day sediment distribution of the gulf between Arabia and Iran. The gently
sloping Arabian shelf is a modern example of a carbonate ramp. (From Emery, 1956. American Association
of Petroleum Geologists Bulletin, AAPG 9 1956, reprinted by permission of the American Association of
Petroleum Geologists whose permission is required for further use.)
Pleistocene limestone (Newell and Rigby, 1957; Purdy, 1961, 1963; Imbrie and Buchanan, 1965; Ball, 1967). The central part of the Great Bahama Bank is a site of fecal
pellet mud deposition. Tidal flats are present around land areas, like Andros Island.
These deposit algal stromatolites and dolomitic muds, analogous to those of the Arabian coast. Evaporites are largely absent, however, because of the higher precipitation.
Rimmed carbonate platforms may often be controlled tectonically. The platform margin is often a fault and may mark a change from continental to oceanic crust. However,
this is not always the case. Benthonic carbonate organic growth is most prolific where
the photic zone impinges on the seabed. This is because nutrients from the open ocean
will be removed by the beasties along this zone, so beasties on the landward side will
suffer malnutrition. Thus carbonate shoals or biogenic reefs may prograde basinward.
A drop in sea level will cause the erosion of a sea cliff into newly cemented limestone.
When sea level rises, optimum organic carbonate growth will take place at the crest of
the drowned sea cliff. If this process is repeated a rimmed carbonate platform margin
of atectonic origin may form (Fig. 6.56). This evolutionary pattern has been noted on
modern carbonate shelf margins where sedimentation takes place over a paleokarst topography of drowned Pleistocene and earlier limestones (Purdy, 1974).
Carbonate shelf sediments are of great economic importance, but it may be more appropriate to discuss this topic after looking at "reef" models. Reefs are a particular constituent of carbonate shelf facies of great economic interest.
6.3.2.8 Reefs
The reef is one of a spectrum of marine shelf environments. Such is its importance,
however, that it merits special attention. A "reef" has been defined as "a chain of rocks
at, or near the surface of water: a shoal or bank" (Chambers Dictionary, 1972 edition).
249
Fig. 6.53. Map showing the present-day sediment distribution of the gulf between Arabia and Iran. The gently
sloping Arabian shelf is a modern example of a carbonate ramp. (From Emery, 1956. American Association
of Petroleum Geologists Bulletin, AAPG 9 1956, reprinted by permission of the American Association of
Petroleum Geologists whose permission is required for further use.)
Pleistocene limestone (Newell and Rigby, 1957; Purdy, 1961, 1963; Imbrie and Buchanan, 1965; Ball, 1967). The central part of the Great Bahama Bank is a site of fecal
pellet mud deposition. Tidal flats are present around land areas, like Andros Island.
These deposit algal stromatolites and dolomitic muds, analogous to those of the Arabian coast. Evaporites are largely absent, however, because of the higher precipitation.
Rimmed carbonate platforms may often be controlled tectonically. The platform margin is often a fault and may mark a change from continental to oceanic crust. However,
this is not always the case. Benthonic carbonate organic growth is most prolific where
the photic zone impinges on the seabed. This is because nutrients from the open ocean
will be removed by the beasties along this zone, so beasties on the landward side will
suffer malnutrition. Thus carbonate shoals or biogenic reefs may prograde basinward.
A drop in sea level will cause the erosion of a sea cliff into newly cemented limestone.
When sea level rises, optimum organic carbonate growth will take place at the crest of
the drowned sea cliff. If this process is repeated a rimmed carbonate platform margin
of atectonic origin may form (Fig. 6.56). This evolutionary pattern has been noted on
modern carbonate shelf margins where sedimentation takes place over a paleokarst topography of drowned Pleistocene and earlier limestones (Purdy, 1974).
Carbonate shelf sediments are of great economic importance, but it may be more appropriate to discuss this topic after looking at "reef" models. Reefs are a particular constituent of carbonate shelf facies of great economic interest.
6.3.2.8 Reefs
The reef is one of a spectrum of marine shelf environments. Such is its importance,
however, that it merits special attention. A "reef" has been defined as "a chain of rocks
at, or near the surface of water: a shoal or bank" (Chambers Dictionary, 1972 edition).