Page 64 - Carbonate Facies in Geologic History

Causes of Cyclicity in Shelf Strata 51

(1967). Microfacies and environmental interpretation of several carbonate shelf
cycles are discussed in Chapter X. As Coogan (1972, p. 7) has observed, most
shallow shelf or platform cycles are asymmetric or rhythmic, containing a thin
transgressive record, often in sharp contact below with beds representing the top
of the underlying cycle. The balance of the sequence is gradational upward and is
regressive, culminating in a shoal-water phase.

Causes of Cyclicity in Shelf Strata

A tectonically balanced wide platform built just to sea level is widely affected by
repeated marine transgression and regression. But what are the mechanisms and
underlying causes of cyclic sedimentation over such platforms? The repetition of
rather complex sedimentary sequences argues strongly for some systematic
causes. These have been discussed in so many scientific papers and textbooks that
they are not treated in detail here (see references above). The various hypotheses
which may apply to the common patterns observed include:
1. Steady subsidence plus an independent external (out-of-basin, world wide)
mechanism for causing repeated sea level fluctuations. Subsidence added to
independent eustatic sea level rise results in relatively rapid transgression.
(a) Eustatic sea level changes due to glacial periods such as in the Pleistocene
and Late Paleozoic.
(b) Eustatic sea level changes caused by periodic large-scale movements of the
earth's tectonic plates.
2. Steady subsidence and stable world-wide sea level, plus an external mechanism
for causing repeated periods of sedimentary fIll-in with a built-in mechanism
for stopping sedimentation usually by completely filling up the basin.
(a) Alternately widening and narrowing of marine source area for lime mud by
outbuilding of carbonate sheets.
(b) Periodic climatic changes controlling development of reefs or carbonate
banks causing restricted circulation and more evaporite conditions in the
basin.
(c) Periodic climatic changes acting on land source areas for terrigenous clas-
tics, and/or periodic shifting of deltaic distributaries to furnish alternately
distant and nearby sources for clastics.
(d) Tectonism in distant land masses to change stream gradient and furnish
clastics to the basins.
3. Episodic subsidence of shelf with gradual fill in of steadily produced carbon-
ate-evaporite sediments or a combination of episodic subsidence and episodic
production. Tilting down of the shelf and up of the hinterland, "seesaw action"
along a hingeline.
4. Episodic "bobbing up and down" of shelf with a total effect of subsidence (to
preserve the record) and a more or less continuous sedimentation.
These processes are not necessarily mutually exclusive. This lies at the heart of
the complexity of the phenomenon and the difficulties in its explanation. Prevail-
ing views on the causes of shelf cycles usually favor an eustatic mechanism instead
of purely local tectonic causes. One cogent reason for this is that shelf cycles occur