Page 130 - Geology of Carbonate Reservoirs

DEPOSITIONAL ENVIRONMENTS AND PROCESSES 111

directly into the sea are called attached because they are extensions of the mainland.
Those separated from the mainland by lagoons are detached beach – dune complexes,
or simply barrier islands. Dunes are usually present immediately updip from beaches
in carbonate environments but they may be absent if the sediment supply is limited,
if the eolianites migrate inland, or if the winds are not strong enough on a regular
basis to move sand off the beaches. Some reservoirs in ancient beaches and dunes
have probably gone unrecognized because of a lack of knowledge about their depo-
sitional characteristics. A variety of papers describing carbonate eolianites ranging
in age from Paleozoic to Holocene can be found in Abegg et al. (2001a) . Kerans
and Loucks (2002) describe the attached beach in the Lower Cretaceous Cow Creek
Formation of Central Texas and present useful interpretive illustrations of beach
anatomy and sedimentary structures.
Processes that create beaches and dunes have been studied in modern environ-
ments where cause – effect relationships between processes and products — funda-
mental rock properties — are reasonably easy to identify. The studies have shown
that the beach environment has three distinctive subdivisions (from bottom to top):
the lower, middle, and upper shoreface divisions. The environmental characteristics
of these divisions depend on the hydrodynamic regime and the prevailing climate
at that geographic location. Open ocean waves are the main engine that drives
shoreline sedimentary processes. Waves are formed by momentum transfer from
wind to water as wind shears across the water ’ s surface. Winds, of course, are part

of the global atmospheric circulation system, but they are also influenced by regional
climatic events such as tropical storms associated with low - pressure circulation and
the “ northers ” that accompany atmospheric high - pressure fronts. The amount of
momentum transfer from wind to water and the resulting wave climate depends on
the distance (fetch) that the wind shears over the water ’ s surface, the length of time
the wind blows (duration), and the velocity of the wind. A storm wind that blows
over a long fetch for a long time at high speed will generate very large waves that
can rise to great heights in the breaker zone. In deep water, ocean waves (known
as swells in the open sea) take a form in cross section that is similar to a sine curve.
These swells, called Airy waves after the mathematician George Airy, have little
effect on deep sea sediments, but as they approach shallow water, they undergo a
shoaling transformation that changes oscillatory motion in the water column beneath

each wave to surging solitary waves, to breaking waves, and finally to longshore
currents. Over - steepened solitary waves break in the surf zone and run up the
sloping beach face. Water from breaking waves that does not infiltrate the beach

sands returns downslope as backwash. Water returning to the sea after passing
through the breaker zone is shunted laterally in a stepwise fashion forming a long-
shore drift or longshore current that runs parallel to the beach face. Sometimes this
drift accelerates to the extent that rip currents are produced to move water perpen-
dicular to the beach face.
Beach environments extend to water depths where fair - weather wave motion just
moves sediments by oscillatory motion generated by swells as they just begin to feel
the bottom and interact with loose sediment. The depth at which this oscillatory
motion reaches the bottom with enough velocity to move sand - sized sediment
depends on grain size, wave climate, and beach slope, among others. In the NW Gulf
of Mexico, where the fair - weather wave period averages about 4 seconds and waves
average about 4 feet in height, fine terrigenous sand is moved by oscillatory motion