Page 138 - Geology of Carbonate Reservoirs

DEPOSITIONAL ENVIRONMENTS AND PROCESSES 119

evaporite deposition but favor accumulation of lime mud. Tidal currents move water
and lime mud onto the intertidal zone between the always - wet and almost always -
dry (supratidal) zones. Ebb tides allow the water to drain off leaving the mud

behind. Over time, the tidal flats where mud accumulates in this fashion can build
up and outward. In other words, they are progradational.

The ebb and flood of tidal currents typically scour channels between the upper
reaches of the intertidal zone and the adjacent subtidal zone. Depending on the tidal
prism (the volume of water exchanged during the tidal cycle), the channels may be
less than a meter in depth. If the tidal prism is large and current velocity in the
channels is great, tidal channels may be considerably deeper than one meter. The
greatest channel depth is nearest the subtidal zone and channels become smaller
landward. As with all natural channels, meanders form if the channeled zone is large

enough in area. Meandering tidal channels superficially resemble delta distributary
channels with small point bars, cut - banks, and levees. Levees are the highest topo-
graphic feature along the channels, they may be above the level of high tide, may
be subjected to desiccation, and may be extensively burrowed and vegetated.
Between channels and levees are almost always - wet hollows commonly called
ponds. Ponds are receptacles for lime mud brought in during flood tide. Pond

margins, depending on climate and tidal range, may be intermittently wet and dry.
Especially adjacent to levees, desiccation may produce algal stromatolites, mud
cracks, and fenestral porosity.
Finally, above the intertidal zone is the nearly always - dry, supratidal zone where
desiccation is the main fair - weather environmental process. Periodic fl ooding occurs
during storms, exceptional tides, or during heavy rains. Because this zone is usually
above the reach of the mud - carrying tides, the sedimentation rate is low, vegetation
and algal mats may grow where moisture permits, and in dry climates where evapo-
ration causes interstitial waters to become supersaturated, evaporite precipitation
occurs. Gypsum and rarely anhydrite are precipitated in the upper sedimentary
layers of the intertidal – supratidal transition zone. Mud cracks, fenestral pores, and
stromatolites are products of periodic wetting and drying in the upper intertidal and
lower supratidal zones. High tides or storms erode the mud - cracked surfaces to

produce intraclasts, or “ flat pebbles. ” These clasts, along with fossil shells and pellets,
are the principal grain types found on modern and ancient tidal flats. Flat - pebble

conglomerates and intraclastic grainstones or packstones with modest amounts of
intergranular porosity exist on tidal flats but their volume is usually too small to be

considered seriously as exploration targets. In most cases, tidal - fl at reservoirs
produce from diagenetically enhanced porosity rather than from depositional poros-

ity. Altered tidal - flat deposits are among the most common examples of reservoirs
described in the literature. Roehl (1967) discussed an example of an altered Paleo-
zoic tidal fl at in the Williston Basin, Montana. He drew parallels between this sub-

surface reservoir and the modern facies array on the tidal flats of Andros Island in
the Bahamas.

5.2.4 Depositional Rock Properties in Tidal Flat –Lagoon Successions

Tidal flat – lagoon depositional successions reflect the three main environmental

subdivisions we have already discussed — namely, the subtidal, intertidal, and supra-

tidal regimes. Depositional successions in tidal flats may vary from one location to