Page 141 - Geology of Carbonate Reservoirs

122 DEPOSITIONAL CARBONATE RESERVOIRS

therefore both inner and middle ramp settings could be included in the shallow
neritic environment. Evidence of wave and current action, presence of photozoans,
including reefs, in tropical environments, and high taxonomic diversity are all char-
acteristic of the middle ramp setting. Much of the outer ramp extends below the
depths that are commonly affected by surface waves and currents, below ample light
penetration, and usually below the zone of optimum carbonate production. The
outer ramp environment may be in the bathyal rather than the neritic environment,
although some basins such as the modern Persian Gulf are so shallow that they do

not reach the required depths of more than 200 m to fit the strict defi nition of
bathyal. The outer ramp represents the deep - water transition to the basinal environ-
ment. In that sense, it can be somewhat similar to the upper slope environment on
open shelves, but the outer ramp environment is very unlike the slope environment
on rimmed shelves because ramps lack pronounced slope breaks and their tendency
to submarine mass wasting that gives rise to slumps, rock falls, debris fl ows, and
similar gravity - driven slope deposits. Ramps and some open shelves lack those types
of deposits.

Hydrological characteristics that influence carbonate sedimentation on ramps
and shelves are (1) the hydrodynamics of the environment, mainly the amount of
wave and current energy, (2) nutrient content, (3) salinity, (4) temperature, and (5)
water clarity. The shallow subtidal environment exists on both ramps and shelves
but the facies on rimmed shelves and ramps are different. Inner parts of ramps and
open shelves are unprotected from incoming ocean waves and storms while rimmed
shelf interiors are protected by the rim at the slope break. Except for the beach
environment, fair - weather wave action in the neritic environment has only limited
effect on bottom sediments; that is, the swells pass through the neritic environment
and form oscillation ripples on bottom sediment, depending on wave period and
water depth. If the water is too deep, passing swells have no effect on bottom sedi-
ments. Currents are driven mainly by wind and tides, although as waves pass through
gaps in shelf rims, they are diffracted and translated into currents. Density currents,
turbidity currents, and geostrophic currents are uncommon in the shallow neritic
zone. Tidal effects are most pronounced in areas where tidal currents are focused
by natural inlets and around abrupt changes in bathymetry. Storms may affect the
entire shallow ramp or shelf, depending on storm severity. Storms can cause massive
movement of bottom sediments, shoreline erosion, and strong currents at all depths
across platforms, especially ramps and open shelves. As powerful storms pass inland

and the storm surge recedes, strong return flows may manifest as subtidal density
currents that transport shoreline sediments considerable distances seaward. These
deposits are distinctive and usually easily recognizable as anomalies on the other-
wise muddy neritic seabed.
Restricted interiors behind shelf rims are subject to wide swings in temperature
and salinity during seasonal changes. Shallow waters on rimmed shelf interiors are
also protected from oceanic currents, tides, and waves. In contrast, ramps and open
shelves are swept by incoming waves, currents, and tides that fertilize and oxygenate
the water while maintaining normal marine salinity and temperature. These more
favorable environmental qualities make the neritic environment on ramps and open
shelves better suited for greater rates of biogenic sediment production than the
sheltered interiors of rimmed shelves; therefore, from the reservoir geology view-
point, shallow interiors of both ramps and open shelves are likely to exhibit patch