Page 295 - Carbonate Facies in Geologic History

282 Shoaling upward Shelf Cycles and Shelf Dolomitization

modify somewhat the basic cyclic patterns. In addition to local tectonism men-
tioned above, these include: changes in degree of restriction of water circulation
over the shelf, tidal variations, over-all amount of shelf submergence, climatic
changes, degree and frequency of periodic drops of sea level, and variation in
bathymetric relief, i.e., height and steepness of carbonate platforms or shoals.
Chapter II has outlined the causal mechanisms for cyclic sedimentation and refers
to current works on cyclic sedimentation.
Observations of many such shelf carbonate cycles shows that three general but
not completely distinctive types of upward shoal sedimentary cycles are present.
1. Oolite or grainstone cycles:
These contain a major grainstone body, commonly oolitic, at or near the top;
in some examples lagoonal deposits occur above. Generally a hard ground sur-
face occurs at the top.
2. Lime-mud cycles with essentially low-energy carbonate deposition
throughout:
The top is commonly thin-bedded tidal flat carbonate or sabkha (salt-flat)
evaporites.
Cycles 1 and 2 are formed on wide, shallow cratonic shelves and may grade
into each other, the first type being more toward the open sea side of the shelf.
Nevertheless, on a wide regional basis there is a tendency for the two distinct
types to exist individually and examples of both are described below.
3. Platform interior cycles with intense early diagenesis:
Such cycles may resemble the other two types but commonly have coarse,
poorly sorted Bahama-type peloidal, grapes tone- and onkoid-bearing sediment and
abundant fenestral fabric, druse veins, tepees, pisoids, and other strong imprints of
vadose diagenesis. Large offshore banks or atolls in rigorous, seasonal, evaporitic

Table X-I. Outline of recurring environmental sequences in upward-shoaling carbonate cycles
Standard facies belts Subenvironments Phase of cycles

9 Marine hard ground or surface of Terminal phase
subaerial exposure
Sabkha-pond evaporite
8 Tide flat Clear water carbonate-
shoaling phase
Lagoon and tidal channels and shelf
mud mounds and low banks
6-7 Immediate back reef shoals and tidal
bars
2 or 7 Widespread marine water of depth Normal marine, open
sufficient for circulation, at or circulation phase
approaching wave base
Below wave base, terrigenous Beginning terrigenous
influx < subsidence clastic phase
Lagoonal- littoral, terrigenous
influx> subsidence