Page 140 - Geology of Carbonate Reservoirs

DEPOSITIONAL ENVIRONMENTS AND PROCESSES 121

coalesce to form a feature commonly called “ chicken - wire ” fabric. In the subsurface,
at depths of 3000 feet or more according to Hardie (1967) , the chicken - wire fabric
consists of anhydrite (CaSO 4 ); at the surface it consists of the stable mineral gypsum
(CaSO 4 • 2H 2 O). In wet climates such as Andros Island in the Bahamas, large depos-
its of interstitial evaporites are absent, but thin dolomitic crusts have been found
around elevated hummocks in the supratidal zone (Shinn et al., 1969 ).

5.2.5 The Shallow Subtidal (Neritic) Environment
The shallow subtidal or neritic environment extends from the lowest low tide line
to an arbitrary depth of 200 m. On large platforms, the shallow subtidal zone can
cover very large areas and the sedimentary characteristics of this vast expanse can

vary considerably. Unlike the tidal - flat environment, where variability is predictable
within the three main subdivisions of the zone, the shallow subtidal zone may be
full of surprises for the uninformed explorationist or engineer. Shallow subtidal
environmental characteristics and resulting depositional rock properties depend on
the following main factors: (1) bathymetry, (2) hydrological regime, (3) carbonate
productivity, and (4) taxonomic diversity.
The shallow subtidal environment can be defi ned bathymetrically as the always -
wet zone that extends from lowest low tide to the slope break on shelves and from
lowest low tide to the outer ramp environment on ramps. Because there is no phys-
iographic marker on ramps to denote the lower limits of the neritic environment,
an arbitrary water depth or a significant change in facies character is commonly

used to mark its end, but facies characteristics, trace fossils, and taxonomic diversity
may vary markedly from the inner ramp to the outer ramp environments. Ocean-
ographers traditionally put the upper and lower boundaries of the neritic environ-
ment at the low tide mark and about 200 m, respectively. The pelagic or open water

zone deeper than 200 m is classified simply as the oceanic environment, although

some texts refer to the benthic zone that extends from 200 to 1000 m as the bathyal
or archibenthic environment. The benthic zone below 1000 m is classifi ed as the
abyssal or abyssal - benthic environment. These are arbitrary boundaries that are not
detectable in the rock record; consequently, they are not very helpful for reservoir
studies. Most geologists now divide carbonate ramps into inner, middle, and outer
ramp environments (Burchette and Wright, 1992 ). Those authors describe the typical
sedimentological and stratigraphic characteristics of each environmental subdivi-

sion. The beach – dune, tidal - flat, and lagoonal environments discussed previously are
part of the updip margin of the inner ramp environment. There is no depth designa-
tion that distinguishes inner from middle, or middle from outer ramp. Those distinc-
tions are made on facies characteristics and biota, which change gradually as depth
increases. The changes are gradual so that the transitional zones between inner - to -
middle and middle - to - outer ramp have overlapping characteristics. Depending on
the ramp slope angle and environmental hydrodynamics, the shallow neritic envi-
ronment can include the entire inner ramp and all or part of the middle ramp. The
middle ramp environment differs from the inner ramp setting in that it is character-
ized by lower taxonomic diversity, lower grain/mud ratio, and fewer patch reefs
(except where antecedent topographic highs present suitable habitat). But middle
ramp facies commonly include organisms, sedimentary structures, and depositional
textures that are typical of the shallow, open ocean rather than the slope or basin;