Page 372 - Carbonate Facies in Geologic History

Belt 9. Platform Evaporite Facies 359

d) Bedding and sedimentary structures: Medium to platy bedded; burrowing
and pelleting of sediment common. When clay is admixed, ball and flow compac-
tion structures are present, as well as nodular and wavy bedding.
e) Terrigenous clastics: When present, generally in well-segregated beds inter-
calated with limestones.
o Biota: Fauna may be abundant with mollusks, sponges, arthropods, forami-
nifera, and algae particularly common. Patch reefs are present; abundant marine
grasses and trees play important roles in trapping and stabilizing fine sediment in
shallow water. Organisms requiring normal marine salinity are present but may
be rarer than in the open sea, e.g., brachiopods, cephalopods, echinoderms, and
red algae.

Belt 8. Facies of Restricted Circulation on Marine Platforms

This facies includes mostly fine sediment in very shallow, cut-off ponds and
lagoons which have restricted circulation and hypersaline water. Geographically
the lagoons may be classified into those behind or between barrier reefs, those
formed behind coastal spits, or those within atolls or faros. They are generally,
but not exclusively, shallow. This belt also includes the highly diverse and well-
known intertidal environments. Lime mud is characteristic of these deposits
which form on levees, intertidal flats, ponds, and marshes. Coarser sediment exists
in tidal channels and local beaches. The highly variable conditions here include
fresh, salt, and even hypersaline water; areas of subaerial exposure; both reducing
and oxygenated conditions, and vegetation of both marine and fresh-water
swamp areas. Wind-blown clastics may contribute significantly in some places.
All of these constitute a stress environment for organisms. Diagenetic effects are
strongly marked in the resulting sediment.
a) Prevailing rock types: Generally lime muddy sediment with much dolo-
mite.
b) Color: Light.
c) Grain type and depositional texture: Highly varied, most sediments consist-
ing of lime mud; grainstone rare except for thoroughly pelleted sediment. Chan-
nels contain lithoclastic grainy sediment; clotted, peIIeted mudstones and wacke-
stones are most common.
d) Bedding and sedimentary structures: Much laminated mudstone, fenestral
(birdseye) fabric, algal stromatolite, small-scale graded bedding, dolomite and
caliche crusts. The channel sands show cross-bedding.
e) Terrigenous clastics: Rare except for wind-blown material; where present,
usually in well-segregated layers.
o Biota: Very limited fauna und flora, mainly gastropods, algae, foraminifera
(miliolids), and ostracods. These organisms may occur locally in great abundance.

Belt 9. Platform Evaporite Facies

The supratidal and inland pond environment of the restricted marine platform
developed in an evaporative climate-the areas of sabkha, salinas, salt flats. In-