Page 373 - Carbonate Facies in Geologic History

360 Summary

SEAS
I LAGOO'lAL T I DE
CYCLIC SEDIMENTS FLATS
FORESLDPE TAWS
PROFILE - 2° TO 25°
~ .'
SEAS OUIET TO MODERATE ~. ~::.-. :' ~_
lAOODNAL CYCLIC SEDIMENTS

IT LOWER PROFILE

ROUGH SEAS
LAGOONAL MUD TIDE
FLAT
ill STEEPEST PROfiLE-UP TO 45° OR MORE

Fig.XII-3. Three types of carbonate shelf margins: I, downslope lime-mud accumulation; II,
knoll reef ramp or platform; III, organic reef rim. From Wilson (1974, Fig. 1), with permission
of American Association of Petroleum Geologists

tense heat and aridity is common, at least seasonally. Marine flooding is sporadic.
The addition of gypsum or anhydrite, formed from evaporative concentration of
sea water in the sediments, is both depositional and diagenetic. Precipitation in
and replacement of the original sediment occurs as the dense brines move down
through the sediment or are pulled to the surface through evaporation of intersti-
tial waters. The precipitated sulfate is unstable and may deform through crystal
growth, intake of water of crystallization, or compaction.
a) Prevailing rock types: Nodular and wavy anhydrite, or gypsum interlami-
nated with dolomite. Such rock types are commonly associated with redbeds.
b) Color: Highly variable, red, yellow, brown.
c) Grain type and depositional texture: Very fine grain carbonate sediment;
gypsum/anhydrite crystals often form a felted mat of tiny lath-shaped crystals or,
when secondary, are large, bladed, and poikilotopic.
d) Bedding and sedimentary structures: Laminate, both wispy and planar
types, mud cracks, stromatolite and spongiostrome structure, gypsum rosettes
and selenite blades (pseudomorphed by anhydrite), and syngenetic, diagenetic,
and deformational structures such as nodular and chickenwire (flaser) structure,
enterolithic (contorted) folding. Also diastem surfaces and caliche crusts are com-
mon.
e) Terrigenous clastics: May be very common, redbeds and windblown sedi-
ment.
f) Biota: Almost no indigenous fauna except for blue-green algal stromato-
lites, brine shrimp.