Page 361 - Carbonate Facies in Geologic History

Chapter XII

Summary

This final chapter reviews the stratigraphic principles set down in Chapter II. It
generalizes and describes in some detail nine standard facies belts developed
along typical carbonate shelf margins; it characterizes several types of shelf mar-
gin profiles seen in the geologic record, and lists the common facies and trends
displayed by individual buildups of both the basins and the shelves. At the end an
attempt is made to interrelate the tectonic, hydrologic, climatic, substrate and
organic parameters which control carbonate sedimentation. The prevalent tec-
tonic settings of carbonate deposition are outlined, reviewing parts of Chapters II,
IV,and VI.

Stratigraphic Principles

1. There is only a single common carbonate facies pattern. It is formed essentially
from in situ accumulation of biochemically precipitated sediment. It consists of an
inner, shallow, low-energy zone (undathem or shelf) and an outer, deeper, low-
energy zone (fondothem or basin) which is separated by a shallow high-energy
zone (clinothem or shelf margin).
2. This pattern is the natural development of maximum carbonate production
and accumulation downslope from and peripheral to a positive element. Such a
sedimentary pattern is typically initiated following a marine transgression and the
inundation of a shelf. In initial stages a carbonate ramp may develop with a high
energy zone close to shore but a platform with a seaward margin rapidly forms,
dividing the facies into three parts.
3. The general arrangement of the above three belts may be subdivided environ-
mentally into at least nine standard facies belts, described later in this Chapter.
4. The facies belts vary in width and uniformity, being narrower and more regular
where the platform margin is steep. Where subsidence has been less and tectonic
elements are not so well defined, the belts are wider and more diffuse.
5. Progradation: Thickness and stratigraphic patterns are the result of balance
between subsidence and the rate of sedimentation. Calcium carbonate accumula-
tion is very rapid when conditions are optimum. This results in prevalent regres-
sive patterns even where considerable thicknesses of platforms and offshore banks
attest to a great subsidence. Generally, the constructed depositional relief in-
creases at the shelf margin as a basin and adjacent shelf subside.
6. Preserved sedimentary records formed during transgressions are known but
are rare. Transgressive units generally are thin. Independent evidence exists that
transgressions may be relatively rapid.