Page 142 - Geology of Carbonate Reservoirs

DEPOSITIONAL ENVIRONMENTS AND PROCESSES 123

reefs, bioclastic buildups, or offshore grainstone banks but the subtidal interior of
rimmed shelves may not. Finding the potential reservoir facies is then a matter of
identifying antecedent topography on platforms because relict highs are generally
sites for high - energy deposits and relict lows will be sinks for mud - dominated
sediments.
Carbonate productivity and taxonomic diversity depend on bathymetry, climate,
and hydrological characteristics. Optimum carbonate production (the carbonate
factory) in tropical climates extends to a depth of about 10 m and the maximum
depth of sediment production is about 200 m (Wilson, 1975 ). In temperate oceans
carbonate production extends to depths of as much as 100 m because productivity
in the temperate carbonate factory is based on heterozoans , a term coined by James
(1997) to identify cool - water associations such as echinoderms, bryozoans, foramin-
ifera, and red algae, among others. In contrast, photozoans — organisms that require
sunlight — occupy sunlit zones in clear - water, tropical carbonate environments. The
depths where the carbonate factory is in full production in both tropical and tem-
perate settings are about the same as the depths where most waves, persistent cur-
rents, and storms occur. It follows that the inner ramp environment in tropical and
temperate climates will have high grain/mud ratios (grain - rich sediments), large
numbers of benthic organisms per unit area, good potential for patch reef growth,
and generally high taxonomic diversity. In traditional oceanographic terminology
the zone from high tide to about 60 - m depth is known as the eulittoral environment,

where sessile organisms and plants flourish. This depth range corresponds roughly
with the zone of optimum carbonate production on temperate, low - energy ramps
(Fornos and Ahr, 1997, 2006 ). Taxonomic diversity is an important clue to recognize
when trying to distinguish neritic successions formed in restricted environments
(rimmed shelf interiors) from those formed in open marine environments (ramps
and open shelves). Restricted environments are stressful to most normal marine life
so that few organisms, especially stenothermic and stenohaline (low tolerance to
temperature and salinity change) benthic organisms, can flourish under those condi-

tions. This means that rimmed shelf interiors commonly have depositional succes-
sions with high mud content, low taxonomic diversity, or even low skeletal grain
content. The combination of arid climate and restricted circulation usually leads to
hypersalinity, and under extreme conditions, this can result in evaporite precipita-
tion. The shallow interior of restricted shelf environments is also subjected to low
water temperature and hyposalinity in wet, temperate climates. This will result in
low taxonomic diversity and perhaps in low carbonate productivity, especially the
aragonitic constituents that require tropical conditions.

5.2.6 Depositional Rock Properties in Shallow Subtidal Successions

Standard facies descriptors such as depositional texture, sedimentary structures,
constituent composition, and taxonomic diversity are determined by the interac-
tions among hydrological characteristics, bathymetry, and productivity. Hydrological
characteristics, as we have noted, include wave and current activity, water tempera-
ture, oxygenation, salinity, nutrients, and water clarity. Environmental quality is a
way of describing hydrological characteristics based on whether the hydrological
regime is favorable to carbonate productivity and taxonomic diversity or not.
Bathymetry is simply seabed topography. Bathymetry in the ordinary neritic