Page 38 - Carbonate Facies in Geologic History

Standard Facies Belts 25

Standard Facies Belts

In the following discussion the pattern is conventionally portrayed as a profile
across a gently sloping shelf atop a platform with an abrupt shelf margin. Meth-
ods used in identifying the nine facies and their interpreted environments noted
below (Fig. 11-4) are reviewed in the next Chapter and their lithologic characteris-
tics are summarized in the final part of the book.

1. Basin facies-starved or filled basin (fondothem): Water is too deep and
dark for benthonic production of carbonate, and deposition is dependent on the
amount of influx of fine argillaceous and siliceous material and the rain of decay-
ing plankton. Euxinic and hypersaline conditions may result.

2. Shelf facies (deep undathem): Water with a depth of tens or even a few
hundred meters, generally oxygenated and of normal marine salinity. Good· cur-
rent circulation. Deep enough to be below normal wave base but intermittent
storms affect the bottom sediments.

3. Basin margin or deep shelfmarginfacies (clinothem): Formed at toe of slope
of carbonate producing shelf from material derived from the shelf. Depth, wave
base conditions and oxygen level about that of facies 2.
4. F ores lope facies of carbonate platform (marine talus .. clinothem): Generally
the slope is located above the lower limit of oxygenated water and from above to
below wave base. Material is debris deposited on an incline commonly as steep as
30 degrees, is unstable and varies greatly in size. Bedding contains slumps,
mounds, wedge-shaped foresets, and large blocks.
5. Organic reef of platform margin: Ecologic character varies dependent on
water energy, steepness of slope, organic productivity, amount of frame construc-
tion, binding, or trapping, frequency of subaerial exposures, and consequent ce-
mentation. Three types of linear shelf-margin organic buildup profiles may be
discerned: Type I is formed by downslope carbonate mud and organic debris
accumulations. Type II consists of ramps of knoll reefs, organic framebuilding
organisms in isolated clumps or encrusting sheets or organisms growing up to
wave base and stabilizing debris accumulations. Type III are frame-constructed
reef rims like modern coral-algal assemblages with sessile forms growing up
through wave base into the surf zone.
The present author (Wilson, 1974) applied these three types of profiles to the
shelf margins discussed within the book. The summary Chapter reviews them and
offers additional examples.
6. ffinnowed platform edge sands: These take the form of shoals, beaches,
offshore tidal bars in fans or belts, or dune islands. Depths of such marginal sands
range from 5 or 10 m to above sea level. The environment is well oxygenated but
not hospitable to marine life because of shifting substrate.
7. Open marine platformfacies (shallow undathem): Environments are located
in straits, open lagoons and bays behind the outer platform edge. Water depth is
generally shallow, a few tens of meters deep at most. Salinity varies from essen-
tially normal marine to somewhat variable salinity. Circulation is moderate.