Page 153 - Geology of Carbonate Reservoirs

134 DEPOSITIONAL CARBONATE RESERVOIRS

Supratidal

Intertidal

Shallow Subtidal

A B

Figure 5.9 Ideal depositional succession for the tidal - flat environment in (a) humid and (b)
arid climates. Tidal - flat successions can be divided into three main parts: the uppermost

supratidal marsh, the alternately wet - and - dry intertidal belt that is usually marked by tidal
channels and ponds, and the always - wet adjacent lagoon or shallow marine environment. The
model for this succession is the Andros Island area of the Bahamas (see Figure 5.4 ), an iso-
lated, low - energy, tropical shelf. The sebkha or arid climate tidal - fl at succession is similar to
the previous example except that evaporites such as gypsum and even anhydrite characterize
the supratidal zone. Chicken - wire (nodular) and enterolithic (intestine - shaped) anhydrite are
common in the upper parts of sebkhas along the Trucial Coast of the Arab Emirates (see
Figure 5.3 ), the model area for this succession.

coarser material washed from the beach and nearshore marine environment. Typical
shallow subtidal environments are covered by wackestones and mudstones but
patch reefs may also be present, either on ramps or rimmed shelves. Exceptions to
the general rule are grainstone – packstone accumulations on bathymetric highs that
may punctuate the otherwise monotonous, mud - dominated subtidal environment.
The shelf - edge (slope - break) succession may consist almost entirely of skeletal reefs
with minor, interbedded grainstones, or it may consist of nearly equal parts of grain-
stones and skeletal buildups. Some slope - break successions — some of those on the
Great Bahama Banks, for example — consist only of grainstone buildups. Drawings
of slope - toe successions do not include slump blocks but instead focus on debrites
and turbidites. Distal turbidites, along with laminites and rhythmites, are the default
basinal deposits that occur on both ramps and shelves.

5.3 PALEOTOPOGRAPHY AND DEPOSITIONAL FACIES

The seven idealized depositional successions and their variants portray typical
deposits that characterize environmental cells across ramps and shelves. The under-