Page 268 - Carbonate Facies in Geologic History

Similarities and Differences between Permian and Triassic Reef Complexes 255

southern Rockies. The configurations of the great banks of the Permian of the
southwestern U.S.A. and the Dolomites are well-preserved compared to the struc-
turally distorted geography of carbonate buildups in the Northern Limestone
Alps.
Individual major banks developed over earlier platform carbonates, the Leon-
ardian (Middle Permian) paleotopography being somewhat analogous to the
Lower Triassic Anisian. This common phenomenon is described in Chapter XII.
Basins in between the banks were starved of sediment, were somewhat euxinic
and probably had between 700 and 1000 m of water. In both areas basins were
later filled-in the Triassic with volcanics and in the Permian with sands and
minor volcanic ash contemporaneous with deposition and, in later Permian time,
with 700 m of gypsum. In both cases exotic blocks tumbled off the 25-35 degree
slopes and rest in fine clastics or volcanics well out into the basin.
The edges of the banks contain carbonate accumulations with specialized
organisms which are very similar when Late Permian and Middle Triassic are
compared and which contrast with those of the Late Triassic. Most of the faunal
changes recognized at the end of the Paleozoic were in environments other than
those of the reefy carbonate shelf margins. Similarities between organisms in these
sediments are partly obscured by different identifications by workers in various
parts of the world who publish in different languages.
For example, microscopic work shows that the bell-shaped chambered fora-
minifer Tetrataxis and encrusting tubular foraminifera are common to all the reef
masses. Permian Reef Complex organic rich ("reefy") micrites have not been
studied paleontologically in thin sections to the same degree as the Triassic in
Bavaria and Austria. When this is done probably other similarities will result, as
well as a few differences. Many microtubular organisms are known from the
Triassic which have not yet been seen in the Permian. Additional faunal similari-
ties between the buildups such as red algae, crinoids, and brachiopods on the
foreslopes, ammonoids in basinal sediments, euomphalid gastropods and dasycla-
daceans in shelf marginal positions are common to many other carbonate banks.
The faunal differences between Permian and Triassic assemblages are mainly in
the lack of reefoid bryozoans and brachiopods and fusulinids in the Triassic and
the significant addition of two groups of organisms-the mollusks and corals
(Table VIII-2). Mollusca are fairly common in the Permian, but are important in
Triassic reefs both as a foundation for boundstone and as contributors to frame-
work. Specialized pelagic bivalves occurring in the Mesozoic basinal sediments in
great abundance are absent in the Late Paleozoic. In addition, unique heavy-
shelled megalodont bivalves form a common Mesozoic backreef biofacies not
seen in the Permian. Corals are represented only by small solitary forms in the
Permian forereef sediments; neither are they very common in early and middle
Triassic strata in the Dolomites. They become generally more diverse, colonial,
and very abundant in the Late Triassic. Late Permian and Mid-Triassic bank
margins are held up principally by encrusting forms which trapped and protected
vast quantities of fine-grain sediment mainly in down slope areas (Type I). The
advent of abundant large coral colonies, plus dominance ofhydrozoans in Norian
and Rhaetic beds, resulted in development of reef knolls at the frontal margins
(Type II) or in true reef rims (Type III) in places.