Page 371 - Carbonate Facies in Geologic History

358 Summary

e) Terrigenous clastics: Essentially absent.
f) Biota: Colonies of sessile framebuilding organisms mayor may not domi-
nate. Growth form is determined by the water energy. The forms may also be low-
lying and encrusting. Ramose or dendritic forms exist in more protected places.
Communities of organisms, dwelling in various ecologic niches, may form beds of
abundant accessory organisms (e.g., layers of brachiopods, molluscs and crinoids).

Belt 6. Winnowed Platform Edge Sands
These take the form of shoals, beaches, offshore or tidal bars in fans or belts, or
eolianite dune islands. Sites of such marginal sands range from well above sea
level to 5 or 10 m deep. Much clean sand is winnowed and deposited by waves,
tidal, or longshore currents of 1-2 knots. The salinity is normal marine because of
good circulation. The environment is well oxygenated but not hospitable to ma-
rine life because of the shifting substrate.
a) Prevailing rock type: Cross-bedded calcareous or dolomitic lime sand.
b) Color: Light.
c) Grain types and depositional texture: Rounded and fairly well-sorted grain-
stones. Some are coated and oolitic. Others are merely rounded bioclasts.
d) Bedding and sedimentary structures: Marine sands with medium to small-
scale festoon cross-bedding. Eolianites have large-scale cross-bedding with dips of
more than 25 degrees. Surfaces representing stratigraphic hiatus are common in
both subenvironments. Eolianites have preserved old soil horizons and root casts.
e) Terrigenous clastics: Quartz sand may be present with the calcarenites.
f) Biota: Worn and abraided coquinas of benthonic animals living on reef and
foreslope are common. Few indigenous organisms occur because of the shifting
substrate. Large bivalves (megalodonts) or gastropods are common, as are frag-
mented remains of large dasycladacean algae and certain foraminifera. Such
forms are prevalent in these strata throughout the geologic column.

Belt 7. Open Marine Platform Facies (Shallow Undathem)
Geographically such environments are located in straits, open lagoons, and bays
behind the outer platform edge. The general term shelf lagoon is applicable.
Water is shallow, generally a few meters to tens of meters deep. Salinity varies
from essentially normal marine to somewhat higher; circulation is very moderate.
Water conditions are favorable for organisms but often the stenohaline forms are
excluded. The sediments are texturally varied but contain considerable amounts
of lime mud.
a) Prevailing rock types: Variable limestone and in some cases lenses and thin
beds of land-derived clastics.
b) Color: Light and dark.
c) Grain types and depositional texture: Great variety of textures, grainstone
to mudstone, e.g., lenses of lime sands commonly of shelly and angular fragments
or lumachelles (coquinas of essentially whole shells), beds of bioclastic wacke-
stones; mounds and lenses of organically produced and trapped sediment; bio-
stromes.