Page 363 - Carbonate Facies in Geologic History

350 Summary

Nine Standard Facies Belts
in an Ideal Model of a Carbonate Complex

As indicated in Chapter II, recognition of a consistently recurring pattern of
limestone facies in the ancient geologic record constituted an important advance
in carbonate stratigraphy and sedimentology. The studies were further enhanced
by using Holocene sediments to interpret these geologic examples. The evolution
of the concept of carbonate facies belts has been in progress since about 1950 and
has resulted in the development of essentially a single ideal model. This is applica-
ble, with variations in thickness and regularity, to all the tectonic settings outlined
in the latter part of this chapter. Figures 11-5 and XII-1 depict this ideal sequence.
The reasons for development of the sequence have been given in Chapter II.
The following section discusses the nine facies belts in more detail and should
serve as a summary of the whole range of facies described in Chapter IV through
XI. This model is patterned across an ideal shelf margin. Figure XII-2 illustrates
profiles of some actual examples using this schema (see also Armstrong, 1974,
Fig. 10). Types of basinal and deeper water carbonates are especially detailed in
the following section. A latter part of this chapter describes the origin and facies
sequence of carbonate mounds found principally along the open marine facies
belts 2 and 7.

Belt 1 A. Turbidite and Leptogeosynclinal Deep Water Facies (F ondothem)

The deeper water facies belts described below are conveniently divided into those
in: geosynclinal troughs (a) filled with continuously and rapidly deposited lime-
stone turbidites, (b) quiet deep water, slow sedimentation with intermittent debris
flows (leptogeosynclinal deposits of Trumpy, 1960).
a) Limestone turbidites: Geosynclinal troughs filled by allochthonous car-
bonates are not common in the geological record but they are impressive and
several have been well described, e.g., Thomson and Thomasson (1969) on the
limestone of the Marathon basin, West Texas; McBride (1970) on the Maravillas
of the same area; The Flysch Calcaire and rhythmic limestones of the Alps
(Lombard, 1956, Beaudoin, 1970), and the Apennines (Carozzi, 1955; Scholle,
1971) fall within this category. Meischner (1965) coined the useful term allodapic
limestone for those basinal sequences of breccias, microbreccias, and lime sands
derived from contemporaneously formed shelf and slope carbonate particles.
These are commonly interbedded with calcareous pelagites and argillaceous
strata. Thicknesses of such beds may vary but are often great. The sequences may
contain exotic boulders and unusually coarse sediment. The allochthonous mater-
ial may be emplaced by turbidity flows, by mass movement of debris, or even by
volcanic eruption. Geosynclinal subsidence and sediment instability are apt to
result in a thick and continuous record of deep-water environment with all the
sedimentary structures and textures of terrigenous flysch. The troughs may be
narrow and contain rapidly changing facies.