Page 267 - Carbonate Facies in Geologic History

254 Permo-Triassic Buildups and Late Triassic Ecologic Reefs

terebratuloid brachiopods, sponges, corals, and hydrozoans. Most of these
Rhaetic bioherms are unbedded and surrounded by shale. Many of them possess
fissures filled with reddish Liassic sediment. Perhaps these represent an old karst
topography. It is easy to imagine that the bioherms grew to wave base and later
came out of the water. Against this hypothesis is the observation that in places the
reddish Liassic sediment contains fossils of deep water or open marine environ-
ments (brachiopods, crinoids, ammonoids). If subaerial exposure did occur, very
rapid and extreme subsidence must have followed. The fissures in the bioherms
may be tectonic and not solution in origin.

Similarities and Differences between
Permian and Triassic Reef Complexes

A comparison between Late Permian and Triassic carbonate shelf margins per-
mits an evaluation of the effect of a rigorous evaporative climate on sediments of
great offshore banks. Our only well-studied Recent model for such a bank, the
Bahamas, exists in a tropical climate. Also of interest is the chance to observe the
slow evolution of reef-building, particularly of frame-constructing organisms dur-
ing this time and the steepening effect this gradual change in organisms had on
depositional profiles.
The Permian of the southwestern U.S.A. and the European Triassic are strik-
ingly similar. Even the regional settings resemble each other. North from the
Triassic Tethys, throughout western Europe, existed restricted marine shelf and
shallow basin deposits of the Germanic Trias (Bundsandstein, Muschelkalk, and
Keuper). These redbeds, evaporites, and extensive restricted marine limestones
much resemble the north central Texas and Oklahoma back reef shelf deposits of
Middle and Late Permian age. This is considered to result from similar climates.
In each case the clastic-evaporite deposition on the shelves was interrupted by a
major marine transgression (the San Andres-Grayburg of West Texas-New Mex-
ico and the Muschelkalk of Europe).
Tectonic history is similar in the basins south of the evaporitic shelves. Here in
both instances great carbonate banks developed. Both the basin areas were sub-
jected to tensional or perhaps directional stresses leading to normal or strike-slip
faulting which resulted in horst and graben structure. This seems to have localized
bank development during a period of extreme subsidence which followed soon
after tectonic fragmentation. The Permian banks developed over a fragmented
foreland lying north of an active geosyncline (Ouachita-Marathon) whose oro-
genic development climaxed in the Carboniferous. The Austroalpine Triassic
banks developed over or around uplifted fragmented continental blocks, proba-
bly over roots of a Hercynian (Carboniferous) orogenic belt, not over its foreland.
Subsidence in both cases was great. The Permian bank thickness totals close
to 1000 m and the Triassic several times that. Later orogenic history was rather
different in the two examples. Both bank complexes were caught in Tertiary fold
belts but the compressive Alpine deformation in the northern part of the Tethyan
trough was much more extreme than the block-faulting and open warping of the