Page 109 - Carbonate Facies in Geologic History

Chapter IV

The Advent of Framebuilders In the Middle Paleozoic

Whereas the preceding Chapters discussed the fundamentals of carbonate sedi-
mentology, stratigraphy, and petrography, this second portion of the book de-
scribes a variety of carbonate facies complexes given more or less in order of
geologic age. These examples serve as documentation for many of the ideas
expressed in the first three Chapters. The facies occur in many diverse tectonic
settings and are found in shelf cycles, mounds and patch reefs on shelves, buildups
at shelf margins, basinal banks, and pinnacles. Well-studied outcrop areas as well
as subsurface petroleum reservoirs are included. The examples cited may serve as
models for interpretation ofless well-known facies patterns.
Considerable changes in the biota of carbonate accumulations occurred at the
beginning of the Middle Ordovician, just prior to the Carboniferous, in the Late
Triassic, Cretaceous, and early Tertiary. All of these changes are significant in
terms of size and amount of framebuilders, prediction of biofacies patterns, trend
and shape of bodies of sediment, and susceptibility to diagenesis. Similarities in
growth form appear from System to System in the geological record. "Carbonate
buildups are like Shakespeare; the plays go on-only the actors change" (Gins-
burg).

The Earliest Buildups

The first of the critical biologic changes affecting carbonate reefs and mounds
occurred in the Middle Ordovician. This was the development of lime-precipita-
ting coelenterates which were capable of encrusting each other, branching, and
forming a framework. Before this time, from middle Precambrian through earliest
Ordovician, carbonate buildups were formed mainly by calcareous algae in the
form of tiny bushes or wiers or by stromatolitic laminae forming bulbous or
lamellar-undulose layers. These minute barnes or gummy layers acted as traps
and permitted accretion of considerable quantities of carbonate mud which was
carried to them by gentle tides or currents. This process has been well studied in
modern tidal flat areas. The only other sessile marine organisms known to partici-
pate in the buildups formed by such trapping of mud are sponges and related
forms whose fossils appeared first in Cambrian strata. Low banks of lime mud
were formed by these archeocyathids in Lower and Middle Cambrian time.
Regional studies, principally by the Canadians Aitkin (1966) and Hoffman
(1974), have demonstrated that the limited and simple algal flora was capable of
stabilizing massive carbonate accumulations at shelf margins. The process was
rapid enough to form considerable relief on tectonic hinge-lines (Fig. IV-I). Black
shale basins, such as the Burgess (Middle Cambrian of Alberta) and Middle