Page 245 - Carbonate Facies in Geologic History

232 Permo-Triassic Buildups and Late Triassic Ecologic Reefs

The distinction between submarine cementation and meteoric cementation in
the strata of the Permian Reef Complex is a problem currently being resolved in
the light of studies of Holocene subtidal cementation of reefs and other littoral
deposits. One must be cautious about interpreting the downward extent and
influence of meteoric water within the shelf-margin strata until clearer relation-
ships are established between the type and sequence of cementing carbonate
crystals and the diagenetic environments (Schmidt and Klement, 1971). Coarse
bladed or drusy void-lining cement is spectacularly developed in the Permian
sponge-algal micritic facies. It is commonly an expanded and displacive fabric. In
places it is dark and cloudy owing to inclusions. Remnants of tiny marine bio-
clasts may be included in it. It is generally later than the fine wavy lamellar rinds
which line the cavities. The coarse fibrous-bladed druse could be calcite pseudo-
morph after aragonite. Masses of the latter are reported by Ginsburg and James
(1974) from slopes 80 m below the surface of modern barrier reefs off British
Honduras. The micritic lamellar wavy rinds could be the Mg calcite marine
cements reported by Land (1971) and Ginsburg et al. (1971) from Jamaica and
Bermuda. But both types of cement in the Permian Reef Complex, as well as in the
Dolomites, are similar to certain cave deposits as pointed out by Dunham (1972).
Perhaps the inference is that the diagenesis affecting these sediments is littoral.
Wave splash zone, marine water saturation from high tides, interspersed with
meteoric ground water and rainfall in a strongly seasonal climate may be the
explanation. Caliche structures as well as thick rinds of cement may result from
precipitation from marine, as well as from fresh water. Purser's and Loreau's
discovery (1973) of extensive splash zone aragonite cementation along the Trucial
coast may be important in evaluating the types of cement seen in the Permian
Reef facies.
Problem of biologic composition of "organic reef': The book of Newell et al.
(1953) is invaluable for description of the organic composition of the Permian
Reef Complex. The uppermost lime sands of the latest Permian, interpreted to
represent the crest of the shelf margin, are made up almost exclusively of piles of
dasycladacean algae and oriented fusulinids with abundant scattered mollusks,
principally large thick-shelled gastropods. Shelf ward of the lime sand the biota is
confined to a few mollusks, ostracods, calcispheres, and stromatolitic algae. This
biological composition indicates conditions of abnormal and/or fluctuating salin-
ity and water temperature. With the exception of the fusulinids, which probably
lived slightly downslope from the crest and were washed up into sandbars, the
fauna is euryhaline. The abundant dasycladaceans probably indicate extremely
shallow water.
In many places, the organic reef is also dominated by a specialized sessile
benthonic biota of primitive forms which could withstand great variation of
marine conditions. These include abundant Solenopora, green algae, hydrozoans?,
Tubiphytes, and sponges. Primitive encrusting forms of alga and/or foraminifera
make up patches of boundstone and help to line cavities later filled with lami-
nated crusts and coarse, drusy spar. The role of blue-green algal stromatolites in
the "organic reef' is hard to evaluate because many structures in the past assigned
to algae are probably inorganic encrustations resulting from vadose processes or
even marine micritic calcite cement.