Page 294 - Carbonate Facies in Geologic History

Chapter X

Shoaling upward Shelf Cycles and Shelf Dolomitization

Three types of sedimentary cycles are characteristically developed during periods
of pure carbonate sedimentation and are remarkably widespread features of
broad platforms and interiors of major offshore banks. Most shelf or "backreef'
strata contain such cycles even though they may not be obvious if terrigenous
beds are rare and do not serve to accentuate the stratification. Cycles consisting of
mixtures of terrigenous and limestone units are discussed in Chapter VII.
Pure carbonate shelf cycles almost invariably are asymmetric and consist of
upward shoaling lithologic sequences, most of the deposition having occurred
during periods of marine regression following through the environments repre-
sented by facies belts 6 through 9. It is as if a relatively rapid rise of sea level
occurred repeatedly on a steadily subsiding shelf and was followed persistently by
sedimentary progradation and a fill-in of the inundated area over some period of
time. A rhythmic or hemicyclic pattern would logically stem from this process,
but the same effect could result just as easily from episodic and abrupt shelf
subsidence as from independent regional sea-level fluctuation.
Such cycles tend to multiply over tectonically "neutral" areas. In locations far
within the shelf, or near its landward side where uplift is persistent, exposure
surfaces are prominent and cycle members are incomplete through non-deposi-
tion. On the outer edges of shelves, subsidence is too continuous and water is too
deep for the effect of sea level fluctuation to be reflected in the sedimentary record
(Fig. X-I).
Carbonate shelf cycles usually belong to a general regressive earth history;
they tend to multiply upward in a major stratigraphic sequence, becoming thin-
ner, more restricted marine in character, and less regular.
Shelves with pure carbonate deposition are hardly affected by terrigenous
influx and the variations in clastic sedimentation cannot be part of the cause for
cyclicity in such strata. Nevertheless, several other interrelated controls operate to
complicate the fundamental mechanism of relative sea level fluctuation and to

,
Off.horo.opon Cyelic .holl dopo.ih \ Do'omit.xed lupratidal deposits
Mori,.. sh.1f .011 doveloped \ Many ommilion IUffaee.
3 2
Cyel .. di.oppoo,ing '\ , Cyel .. ob.curod

grainstone shool.

Fig.X-l. Three cyclic patterns from shelf to basin showing best developed cycles in the
intermediate area of moderate subsidence