Page 241 - Sedimentology and Stratigraphy

228 Shallow Marine Carbonate and Evaporite Environments

Fig. 15.3 Different groups of organisms
have been important producers of carbo-
nate sedimentary material through the
Phanerozoic; limestones of different ages
therefore tend to have different biogenic
components.

completely surrounded by deep water and therefore numerous other organisms form a chlorozoan
do not receive any terrigenous clastic supply. A car- assemblage. In restricted seas where the salinities
bonate atoll is a particular class of carbonate bank are higher only green algae flourish, and form a
formed above a subsiding volcanic island. Three chloralgal association (Lees 1975). Temperate car-
morphologies of carbonate platform are recognised: bonates formed in cooler waters are dominated by
they may be flat-topped with a sharp change in the remains of benthic foraminifers and molluscs, a
slope at the edge forming a steep margin, either as a foramol assemblage (Wilson & Vecsei 2005). Ooids
rimmed or non-rimmed shelf, or they may have a are most commonly associated with chlorozoan and
ramp morphology, a gentle (typically less than 1˚ ) chloralgal assemblages.
slope down to deeper water with no break in slope
(James 2003).
15.2 COASTAL CARBONATE AND
EVAPORITE ENVIRONMENTS
15.1.4 Carbonate grain types
and assemblages 15.2.1 Beaches

The range of types of carbonate grain is reviewed in The patterns of sedimentation along high-energy
Chapter 3. The relative abundance of the different coastlines with carbonate sedimentation are very
carbonate-forming organisms has varied considerably similar to those of clastic, wave-dominated coastlines
though time (Fig. 15.3) (Walker & James 1992), so, (13.3). Carbonate material in the form of bioclastic
in contrast to terrigenous clastic facies in shallow debris and ooids is reworked by wave action into
marine environments, the characteristics of shallow- ridges that form strand plains along the coast or
marine carbonate facies depend on the time period barrier islands separated from the shore by a lagoon
in which they were deposited. Most significantly, (Tucker & Wright 1990; Braithwaite 2005). The tex-
the absence of abundant shelly organisms in the Pre- ture of carbonate sediments deposited on barrier island
cambrian means that carbonate facies from this time and strand plain beaches is typically well-sorted and
are markedly different from Phanerozoic deposits in with a low mud matrix content (grainstone and pack-
that they lack bioclastic components. stone). Few organisms live in the high-energy foreshore
The skeletal grain associations that occur on car- zone, so almost all of the carbonate detritus is reworked
bonate platforms are temperature and salinity depen- from the shoreface. Sedimentary structures are low
dent. In low latitudes where the shallow sea is always angle (3˚ to 13˚ ) cross-stratification dipping seaward
over 15˚ C and the salinity is normal, corals and on the foreshore and landwards in the backshore area.
calcareous green algae are common and along with Barrier islands formed of carbonate sediment form in