Page 100 - Geology of Carbonate Reservoirs

CARBONATE DEPOSITIONAL PLATFORMS 81

current velocity is high. In the case of rimmed shelves, the rim environment has the
highest “ energy ” regime (greatest amount of hydraulic power) on the platform.

Sediments on the seafloor adjacent to the rim are subject to more winnowing,
sorting, and transportation than those in other zones on the shelf. S helf edges — the
rim and its immediate surroundings — exhibit depositional facies generally charac-
terized by grainstones and framestone/bindstone reefs. These accumulations lack
mud matrix and can have high depositional porosity. Inboard from the rim, the
character of the depositional facies changes progressively from grainstones and
reefs to mud - rich facies of the shallow subtidal and tidal - flat domains. This predict-

able facies pattern enables the explorationist to predict the location and dimensions
of facies with highest and lowest depositional porosity. In fact, it is possible to sub-
divide the entire platform into “ cells ” or zones where each cell is represented by
depositional facies that exhibit a limited variety of textures, grain types, sedimentary
structures, and biota. This idea was developed by Wilson (1975) as the “ standard

microfacies ” concept and modified by Ahr (1985) to form three - dimensional depo-
sitional models on ramps and shelves. If enough is known about the age of the shelf,
it is also possible to predict the kinds of fossils that will be present in the various
paleoenvironmental cells.
Open shelves have a pronounced slope break accompanied by facies changes,
but they lack shallow - water rims that interact with fair - weather waves, tides, and
currents. The absence of rims may be the result of cool water temperature that
prevented reef growth and inorganic carbonate precipitation. Most reef - building

organisms, particularly hermatypic corals, thrive in water temperatures above 18 ° C
and large - scale inorganic precipitation is limited to warm waters. In other words,
most reefs and grainstone accumulations form in tropical environments instead of
temperate or cool ones. Another reason for the absence of rims at shelf edges is

that the breaks in slope may occur at a depth too great for prolific growth of reef
organisms or for inorganic carbonate precipitation. The environment where bio-
genic and chemogenic production of carbonate sediments is at its maximum pro-
ductivity is sometimes referred to as the “ carbonate factory. ” Sediment production
in tropical waters is dependent largely on photozoan organisms; consequently, sedi-
ment production typically decreases with water depth at a rate depending on the
depth of light penetration, among other environmental variables. For tropical envi-
ronments, the optimum depth for carbonate sediment production is about 10 meters,
according to Wilson (1975) . Production may occur at depths of 90 m and more in
temperate environments (Fornos and Ahr, 1997, 2006 ; James and Clarke, 1997 ), or
where heterozoan organisms (those not dependent on photosynthesis) are the
primary contributors. Finally, the zone of maximum sediment production on open
shelves may not coincide with the zone of sediment retention. Sediment produced
in shallow water may be transported to deep water by powerful waves and currents.
In some high - energy, temperate environments both production and retention of
carbonate sediments occur at water depths exceeding 100 meters (James et al.,
1992 ). In such settings, the edge of the open shelf may occur at tens of meters to
over 100 - m depth and still be accompanied by laterally persistent differences in
sedimentary characteristics (facies) above and below the slope break. On open
shelves with extremely vigorous wave climates, such as present - day southern
Australia, there may be little or no sediment accumulation in shallow water because
powerful waves and currents erode and transport it offshore (James et al., 1992 ).