Page 389 - Carbonate Facies in Geologic History

376 Summary

subsidence of the major tectonic elements. Classifications presented by Krebs
(1971) and Krebs and Montjoy (1972) and Wilson (1974) have more or less been
expanded for wider application. A related classification has been presented also
by Heckel (1974). As in Chapter II, the numerals on the following outline are
chosen to agree with those of Krebs and Montjoy.
1. Basinal buildups in areas of great subsidence.
a) In marginal cratonic basins or miogeosynclines:
(1) Major offshore banks.
E.g., Central Basin platform of the West Texas Permian basin, Cretaceous of central
Mexico, Dolomites of south Tyrol.
(2) Linear trends along basement faults.
E.g., Leduc-Rimbey Late Devonian buildups, central Alberta basin, Canada.
(3) Pinnacle reefs either in belts or random patches.
E.g., Michigan basin Silurian; pinnacle buildups paralleling the edges of the basin;
the Williston basin and Zama area in northern Alberta have mostly random
patches; Pennsylvanian ofthe Midland basin, north central Texas, banks parallel to
eastern shelf margin; Frasnian (Late Devonian) buildups of Dinant basin, Belgium.
b) Within geosynclinal troughs on or around volcanic uplifts. E.g., Rhenish trough of
Rhenish Shale Mountains, Germany.
c) Major offshore banks under oceanic influence. E.g., Great Bahama Banks and related
complexes.
2. Buildups at edges of major platforms and ramps developed off cratonic blocks; areas of
great subsidence.
a) Linear buildups along shelf margins of these platforms. E.g., the Mosul Cretaceous
platform of northern Iraq; Permian Reef Complex surrounding the Delaware basin and
built out from the eastern side of the Pedernal uplift in New Mexico; Devonian trends
such as the Pres Qu'ile and around the Peace River high of northern Alberta; and the
Lennard shelf fringing the Canning basin, Western Australia. Shelf margin Types I, II,
and III.
b) Complexes of individual banks and reefs widely spread along and upon such shelf
margins. Shelf margin Type II. For example, Middle and Late Devonian buildups
within the external shelf of the Rhenish trough, southeast of the Old Red Continent,
northwestern Germany; the eastern edge of the western Morroccan shelf (Devonian);
Silurian shelf (low clastic belt) north of the Illinois basin, U.S.A.
c) Major but narrow fringes off faulted edges of cratonic blocks or orogenic ridges. E.g.,
the Smackover Late Jurassic trend around northern Gulf of Mexico; British Honduras
(Belize) coast.
3. Buildups on ramps or platforms in areas of moderate subsidence; built out into shallow
intracratonic basins; on shelf areas with scattered mounds and patch reefs. Shelf margin
Type II most common.
a) Micritic mounds and masses with very slight relief and in shallow water, ordinarily with
only sponges or bryozoans and an algal stromatolite cap. E.g., Lower Ordovician of
southwestern U.S.A., Middle Ordovician (Chazyan) of Adirondack shelf.
b) Simple mud-mound accumulations with calcarenitic flank beds. E.g., Waulsortian
mounds in Early Mississippian strata, Sacramento Mountains, New Mexico developed
below shelf margins at edges of basins.
c) Boundstone reefs atop banks or mud-mound accumulations. Silurian of northern Mid-
continent, U.S.A; Spanish Sahara Devonian patch reefs.
d) Faros or patch reefs of pure boundstone. E.g., Silurian and Devonian over wide areas of
the U.S.A. and Canada; Devonian of Eifel west of Rhine River; Silurian of Gotland;
modern patch reefs of Florida strait and British Honduras.
e) Upward shoaling cycles across ramps from the shelf to the basin. E.g., the Mississippian
Madison Group of the Williston basin.
4 and 5. Low fringing buildups or halos of cyclic character around local positive areas on
shelves. E.g., Callovian bank, southeast Paris basin, north of Morvan massif and Ed-
wards (Cretaceous) limestone around Llano uplift, central Texas.