Page 383 - Carbonate Facies in Geologic History

370 Summary

A variety of processes act upon the biochemical system. These effect the
various carbonate facies and bodies of sediment discussed in this volume. The
following section outlines the inter-relationships of the basic sedimentary controls
of these processes: tectonics, climate, eustatic sea-level fluctuations, hydrology,
organic composition, and character of substrate. Table XII-1 is an attempt to
organize these in a meaningful way. Combinations of these parameters control
degrees of depth, water agitation, circulation, salinity and substrate character and
frequency of subaerial exposure. In turn these act upon sediments within the
general tectonic settings represented by (1) wide shallow-shelf deposits,
(2) carbonate shelf margins, and (3) basins and geosynclines. Table XII-2 outlines
many of these relationships within the framework of the tectonic settings of
prominent carbonate depositional patterns. A glance at the tables shows that the
relationships may be complex, and that, unlike the case of terrigenous sediments,
a tectonic classification of carbonate sedimentary patterns is not so useful in
generalizing and arranging natural groups of rock. Some of the interrelationships
are discussed below. Others are charted on the Tables.

Tectonism

Tectonic activity operates to control carbonate sedimentation at various levels.
Its basic control is in the rate and continuity of subsidence without which sedi-
mentation is negligible. Rapid and continuous subsidence results in extraordinar-
ily thick sections in certain carbonate producing areas. As pointed out in Chap-
ters I and II, tectonic subsidence when balanced by or slightly overcome by rate of
sedimentation may result in carbonate upbuilding or sedimentary progradation
just as impressive as in clastic deltas.
The tectonic framework of a region may directly affect the water circulation
along its coasts. The complex morphology of the Arabian side of the Persian Gulf
attests to this. Orientation of contemporaneous fold belts and fault trends oblique
to the edges of basement blocks control the regularity of coastlines and embay-
ments and hence exert control on the degree of restriction of circulation of marine
waters. On the other hand, structural orientation normal to prevailing wind and
wave direction encourages rapid organic growth causing barrier and fringing
reefs. Local structural uplift, when drowned, may form patch or pinnacle reefs as
is happening today in the Persian Gulf and in the Gulf of Mexico.
Moreover, slight subsidence of a basin may influence diagenesis by permitting
the buildup of carbonate platforms even with the level of the sea.

Eustatic Sea-Level Changes and Substrate Effects

Both world-wide glaciation and megatectonic shifting of oceanic and cratonic
plates may induce eustatic sea level changes, a process operating either with or in
opposition to local tectonic subsidence. Where uncompensated by tectonic sub-
sidence, eustatic sea-level lowering may bring about an unusual drop in the water
table, improved subsurface drainage, and stronger diagenesis in carbonate shelf