Page 315 - Carbonate Facies in Geologic History

302 Shoaling upward Shelf Cycles and Shelf Dolomitization

Table X-2. Sequence of sedimentary features in the upper Clearfork, Flanagan field, Texas
(Lucia, 1972)

Interpreted Sedimentary structure Fossils Particle size
sedimentary
environment

Supratidal Irregular laminations, Rare Lithoclasts to
(Facies lithoclasts, desiccation Thin-shelled small lime mud
Belt 8-9) features, quartz silt beds forams, ostracods,
molluscs

Intertidal Distinct burrows, churned- Veryfew Fine sand-size
(Belt 8) to-wispy, mottled struc- Thin-shelled small pellets to lime
tures. Quartz silt beds. Algal forams, ostracods, mud
stromatolites. Discon- molluscs.
tinuous fractures Filamentous algae
Current-laminated rocks, Very few Fine sand-size
cross-bedding Echinoids, pellets to mud
small molluscs, with some
lith oclasts
Infratidal Churned rocks, Locally abundant Coarse sand-size
(Belt 7) burrowed rocks Echinoids, large pellets to lime
fusulinids, molluscs, mud
algal-forams (?)
bryozoans

sedimentologic study by Shinn et al. (1969) described the environments of such a
cycle formed in the last 5000 years on the vast mud flats west of Andros Island,
Bahamas. Fine sediment derived from the Bahama platform was brought east-
ward and trapped against the lee shore of Pleistocene eolianite on the island. The
mud flat has prograded westward (toward the source of sediment) for 10--15 km at
a thickness of 5 or 6 m in this brief period. The same process is clearly seen in the
Persian Gulf lagoons around Qatar and the Trucial Coast where the prograding
sequence is capped by sabkha anhydrite and gypsum.
Many great carbonate sequences of the geologic column consist of such cycles.
They have the following special characteristics:
1. They tend to occur on wide shelves and across shallow intracratonic basins
particularly where far removed from major shelf margins. In certain cases, how-
ever, major carbonate ramps extending into miogeosynclines may also accumu-
late such deposits.
2. Micritic textures are general throughout. The only common packstone or
grainstones are found in thin channel and natural levee deposits.
3. The earliest beds in the cycles are generally open marine or partly restricted
sediment. The upper part is always a tidal flat sequence whose sedimentary struc-
tures generally hold the key to environmental interpretation-specifically algal
stromatolites marking the ancient level of high tide. Evaporitic climates com-
monly result in a cycle with a capping sabkha anhydrite sequence; tropical cli-