Page 131 - Geology of Carbonate Reservoirs

112 DEPOSITIONAL CARBONATE RESERVOIRS

from surface waves to average depths of about 10 m. This lowest division of the
beach succession is called the lower shoreface zone and it is dominated by oscilla-
tory motion from Airy waves passing overhead and from burrowing by bottom
dwellers. The middle shoreface environment is dominated by unidirectional currents
rather than oscillatory wave action. The currents are formed indirectly by waves and
are known as longshore currents. They create bar – trough topography in water
depths that vary from about 1 to 4 or 5 m, depending on the power of the wave
climate and its resulting longshore currents. In the NW Gulf example the middle
shoreface environment extends from knee - deep to about 2 - or 3 - m depths. Finally,
the upper shoreface environment is the zone of breaking waves where the inclined
beds of the beach face are formed. The physical processes that dominate the upper
shoreface zone are breaking waves with run - up and backwash that move up, across,
and down the sloping beach face. Storms and accompanying high tides and large
waves can create a storm berm landward of the upper shoreface surface and in front
of the adjacent dunes, if dunes are present.

Carbonate dunes are usually confined to coastal zones adjacent to beaches
because beaches are their source of sediment. However, dunes can migrate tens of

kilometers inland when deflation exhumes or rips up carbonate grains and supplies
them to the shifting dunes (Abegg et al., 2001 ). In contrast, siliciclastic desert eoli-
anites can occur in the middle of continents where they are not dependent on a

coastal sand source. Even though deflation can produce particles for dune growth,
the land surface must be amenable to erosion. It is unlikely that indurated mud-
stones and wackestones would yield much carbonate sand to winds blowing over
their surfaces. Availability of sediment is a primary limiting factor for dune construc-
tion because carbonates are produced in always - wet environments. As we have
already mentioned, carbonate particles are not easily transported over long dis-
tances because they are soft and there is no aqueous carbonate sand factory in
deserts. This usually limits carbonate eolianites to coastal settings. Depositional
porosity in beaches and dunes is probably greater than that in almost any other kind
of carbonate succession, but because early cementation is limited in beaches and
especially in dunes, they seem to be particularly susceptible to early compaction and
attendant porosity loss (Abegg et al., 2001 ).

5.2.2 Depositional Rock Properties in Beach –Dune Successions
Fundamental rock properties include texture, grain types (composition), sedimen-
tary structures, and fossil content (diversity and type). Beach – dune textures depend
on grain types in most cases. Carbonate beaches in modern tropical environments
are composed primarily of nonskeletal grains such as ooids, pellets, clasts, and
peloids, but beaches in modern temperate climates generally consist of skeletal
grains because the carbonate factory produces different constituents in temperate
and tropical environments, as was first pointed out by Chave (1967) , Lees and Buller

(1972) , and Lees, (1975) . Oolitic and peloidal beaches are typically fi ne to medium
sand sized, but bioclastic dunes may consist of fine, medium, or coarse sand - sized

particles. Sorting is usually very good in beaches and dunes, particularly in the dune
and upper shoreface sectors where breaking waves, swash, and backwash continu-
ally abrade, sort, and polish constituent grains that are supplied to beaches and
dunes alike. Beach and dune textures are grainstones for practical purposes. The