Page 139 - Geology of Carbonate Reservoirs

120 DEPOSITIONAL CARBONATE RESERVOIRS

another if there are differences in climate, physiography, and, to a lesser extent,
differences in hydrologic regimes at the different places. For example, subtidal
(shallow neritic or lagoons behind barrier islands) facies typically contain mixtures
of in situ marine and allochthonous lagoonal allochems along with constituents
derived from the intertidal and supratidal zones but that were swept off the fl ats by
strong ebb tides or storm currents. Hurricane - strength storms can produce grainy
washover fans that are incorporated into the otherwise muddy, burrowed, lagoonal
sequence. Subtidal successions exhibit muddy depositional textures typical of low -
energy settings and they exhibit marine phreatic diagenesis because the environ-
ment is always wet. Subtidal facies may have differences in taxonomic diversity
because open marine settings have higher diversity than lagoonal or restricted
marine settings. Rates of sedimentation vary depending on the amount of mud
imported or produced in place and on the volume of allochthonous constituents
brought down from the adjacent intertidal or supratidal regimes. The volume of
imported sediment varies with storm frequency, tidal range, and local hydrologic

regime. Subtidal sequences adjacent to tidal flats have many distinctive character-
istics that make them relatively easy to recognize regardless of age or location.
Intertidal facies vary greatly but predictably. Incised tidal channels may vary in

number and dimensions across tidal flats but they are always marked by their sharp

erosional bases and fining - upward channel - fi ll sequences. Channels only a few tens
of centimeters deep may not display obvious fining - upward sequences. Repeated

successions of shallow channels fi lled with fl at - pebble conglomerates and intraclas-
tic packstones are characteristic of the Cambro - Ordovician rocks around much of
the Transcontinental Arch in North America, for example (Lochman - Balk and
Wilson, 1958 ). The form of these pebble - fi lled zones is sometimes hardly recogniz-
able as channels. Larger channels like those on Andros Island in the Bahamas are

flanked by levees or curvilinear ridges covered with desiccated algal mats, tunneled
with bioturbation, and blanketed with pelleted micrite. Fecal pellets are common

constituents on tidal flats of almost all geological ages. The pellet producers are
typically high - spired browsing gastropods that ingest organic - rich mud and excrete
compacted pellets. Ponds between leveed channels may vary in size but they nearly
always contain extensive deposits of pelleted micrite and relatively continuous algal
mats that have not been cracked by desiccation. In dry climates, ponds may be
ephemeral and the once - wet hollows may be filled with evaporites such as gypsum

or halite. Evaporitic pond deposits on the interior of broad, shallow shelves may
extend for many square kilometers. Ultimately, these thin, widespread evaporite

beds can be barriers or baffles to fluid movement in the subsurface.

Supratidal facies are characterized by desiccated algal mats, flat - pebble intra-
clasts derived from fragmented algal mats and mud cracks, stromatolites, bioturba-
tion, root traces, fenestral porosity, and crusts of evaporite minerals. In arid climates
such as the modern Persian Gulf, the supratidal zone, commonly known as the
sebkha , is partially covered with siliciclastic sands blown off the adjacent Arabian
desert. Interstitial evaporites are common in the shallow subsurface of the upper
intertidal – lower supratidal zone, or sebkha. If subsurface precipitation of the evapo-
rites, mainly gypsum and anhydrite, continues for a long enough time, the growth
of evaporite crystals displaces the surrounding carbonate sediment to form peculiar
and characteristic deformation called enterolithic structures. Gypsum crystal rosettes
may continue to grow at the surface and in the shallow subsurface until the rosettes