Page 246 - Carbonate Facies in Geologic History

The Middle Triassic of the Dolomites 233

The sponges (Sycons, Sphinctozoans or beaded forms) dominate and are the
only organisms larger than a few cm. Corals are practically absent in the latest
Permian "organic reef'. Fragments and whole shells of normal marine (stenoha-
line) forms such as brachiopods, bryozoans, crinoids, and cryptostomate bryo-
zoan are abundant in places and have, in general, delicate forms adapted best to
life in water at or below wave base. Fractures or large cavities filled with thick-
shelled nautiloids and echinoids occur. Much lime mud and ca1cisilt (up to 50%
of volume) occurs in the "organic reef'. The micrite matrix, the low-lying encrust-
ing habit and rather delicate form of the sponges and bryozoans, and the variety
of normal marine biota in the upper part of the massive limestone, indicate that
very special conditions prevailed to form what Newell et al. (1953) considered to
be typical "reef' rock. Much more detailed study of vertical zonation, taxonomy,
and growth forms of these organisms is necessary before the full story on environ-
ment of deposition can be adduced. The limited variety and primitive character of
the sessile encrusting fauna in much of the organic reef may result from a spill-
over of saline water through the barrier bar sands. The rapid increase downslope
of abundant, normal marine fossils (bryozoans and brachiopods) indicates water
of open marine salinity. Practically all the well-known Permian faunas including
the sessile "reef-dwelling" brachiopods, Leptodus, and the richtofenids, come from
these slope deposits.
The evidence seems compelling that waves crashing on a massive organic
barrier would not have resulted in the type of sedimentary and organic record
now seen in the Capitan "reef-rock". More probably the sponge-algal micrite
represents a quiet-water accumulation. Was this across a shallow flat extending
out to a slope, dropping off into a sea with low wave action? Or did the slope
decline rapidly into water a few tens of meters deep seaward of and below an
exposed ridge of sand bars-a slope where sponges and other encrusting organ-
isms struggled for survival in a lime mud substrate? Profile studies would indicate
the latter is the more probable but more detailed field study is necessary.

The Middle Triassic of the Dolomites

The spectacular scenery of the Dolomites in the southern Alps (South Tyrol of
Italy) has long inspired and fascinated mountaineers, skiers, and geologists. Huge
vertical masses of blanched limestone rise like spires and immense castles from a
dissected plateau of deep green meadows and dark soil. The steep vertical relief of
these great masses is as great as 1000 m; much of it is due to original deposition,
for these giant crags are mostly structurally intact remains of carbonate buildups
of Middle Triassic age. The Dolomites have been studied as models of fossil
"reefs" for almost 100 years. Mojsisovics' work, "Die Dolomit-Riffe von Stidtirol
und Venedig" (1879) still stands as a classic in the history of geological investiga-
tion of such phenomena.
The Dolomites are located in northern Italy, east of the Adige Valley, just
southeast of the Alpine Brenner Pass separating Italy from Austria (Figs. VIII-10
and VIII-1l). Structurally, they belong to the little deformed South Alps and are
separated from the main Alpine deformation belt by the Giudicaria lineament, an