Page 182 - Carbonate Facies in Geologic History

Chapter VI

Pennsylvanian-Lower Permian Shelf Margin Facies
in Southwestern United States of America

Coal measures of the Appalachians and cyclothemic deposits of the midcontinent
Pennsylvanian are well known, but equally characteristic are several types of
carbonate buildups produced in the clear and shallow seas around the southern
extension of the North American craton. The sections containing these buildups
remain lithologically varied because this was a time of general tectonic instability
and considerable terrigenous sedimentation. Many of the carbonate-producing
organisms which formed these offshore banks, shelf-margin buildups and shelf-
interior mounds are unique to the Late Paleozoic. They appear in the Pennsyl-
vanian and evolve continuously in the early Permian and even to the Triassic.
They principally include types of algae and foraminifera, with several algal-like
forms and certain sponges and stromatoporoids. Very few large frame-building
orgamsms occur.

Paleotectonic Setting, Geologic History, and Climate

Widespread Late Paleozoic tectonic activity was as characteristic of North Amer-
ica as of the rest of the world (Fig. VI-l). The huge triangular extension of the
craton which extends southwest through the U.S.A. stretches from the present
Great Lakes region to New Mexico, Arizona, and Sonora. This was bordered
throughout Paleozoic time by geosynclinal troughs along which sporadic folding
and uplift proceeded in various places. Foredeep basins formed persistently be-
tween the active orogenic belts and the craton, e.g., the central Appalachians, the
Black Warrior, Arkoma, Fort Worth, Valverde-Marathon, and Chihuahua-Ped-
regosa basins. Many thousands of meters of terrigenous sediment poured into
these basins and, at intervals, out of them across the shelves of the cratonic
interior. Similarly, the mid-Paleozoic Antler orogenic belt of the northern U.S.A.
Cordilleran geosyncline remained somewhat active well into Pennsylvanian time
in Idaho and northwestern Utah. Apparently, however, orogeny was not so vio-
lent here as in the southern and eastern perimeters of the shield.
The North American craton itself records equally impressive orogenic activity
through the Late Paleozoic, but of a very different style. After general marine
regression in Middle Mississippian (Visean) time terrigenous clastics of the Mera-
mecan and Chesteran Series began to spread across the craton from the south and
east, coincident with orogenic activity in the southern geosyncline. Only in the
deeper tectonic basins adjacent to the southern trough did continuous sedimenta-
tion occur. Cratonic uplift removed part of the thin argillaceous Late Missis-
sippian strata of the continental interior and the rising of isolated blocks contin-
ued coincidentally with Pennsylvanian marine transgression. The tectonic pattern