Page 184 - Carbonate Facies in Geologic History

Special Organic Communities Forming Carbonate Buildups 171

rim many of the uplifts lying seaward of the terrigenous sediments and form
narrow carbonate shelf margins. Shelf areas are wider in the east, e.g., the Kansas
shelf, the north central Texas outcrop region, and the eastern shelf of the Midland
basin. These contain cyclic sequences of mixed clastics and carbonates and have
been extensively studied by many geologists and are discussed in Chapter VII.
Climatic differences, as well as tectonics, left a strong imprint on Pennsyl-
vanian sedimentation. Most of the seas were warm and tropical and of normal
salinity as indicated by various types of calcareous algae and large foraminifera,
as well as by the varied higher invertebrates which flourished in them. Pennsyl-
vanian evaporites are known only in three western areas, along the axis of the
Transcontinental arch which forms the crest of the southwesterly extended cra-
ton. Tidal flat conditions and stratigraphically extensive dolomites are unknown
except in these evaporite troughs which are, from northeast to southwest: the
northern Denver, central Colorado, and Paradox basins. The evaporites are gen-
erally of Desmoinesian age and coincide with the maximum limestone deposition
of this same age farther southeast in Texas and New Mexico. Somewhat later,
Wolfcampian evaporites, dolomite, and tidal flat strata formed in the more east-
erly basins. These lie in the subsurface of the northern midcontinent (Pine Salt of
Montana and the Big Blue series of Kansas) subsurface and the Earp Formation
of the shelf north of the Pedregosa basin. In most basins and shelves, however,
open marine circulation prevailed despite intermittent ingress of large amounts of
fine terrigenous material. Pennsylvanian strata are, in general, not very dolomitic
in this whole area. A more humid climate is indicated farther east. Coal appears in
Kansas and north central Texas and becomes important in the Appalachian
foredeep. Sediments derived from the cratonic uplifts west from Kansas and
Texas tend to be red. The easternmost traces of coal occur in these states and coal
becomes more prevalent northeast to Pennsylvania. Paleomagnetic studies situate
the Pennsylvanian equator diagonally across North America from West Texas to
Newfoundland, placing the coals along this equatorial belt and the evaporites and
red beds in the northern hemisphere desert. The extensive sand sheet of the
Tensleep-Wells-Quadrant Formations also falls in the northern desert area and
includes some sand dunes as well as marine sands.

Special Organic Communities Forming Carbonate Buildups
in Late Paleozoic Strata

The special microfacies recognized within cyclic shelf sequences and described in
Chapter VII are equally applicable to the strata surrounding Permo-Pennsyl-
vanian carbonate buildups. In addition, at least a half dozen special assemblages
of organisms exist which were capable of forming impressive and distinct masses
of carbonate sediment during this time. These organisms belong mostly to quite
different groups from those seen commonly in the geologic record before and after
the Late Paleozoic. Some of them are as yet poorly known biologically and most
have no Recent analogs in present carbonate areas. Whereas none of the forms
seems capable of construction of an impressive organic framework, their prolific