Page 266 - Carbonate Facies in Geologic History

Examples of Banks and Reefs 253

also found in the Rhaetic reefs. Figure VIII-21, from Fabricius, provides a useful
synthesis of the whole gamut offacies.
Steep shelf margins with reefs were also studied in detail by Ohlen (1959) at
the well-known Steinplatte-Sonnenwande complex above Waidring near Lofer,
Austria (Zankl, 1971). The Steinplatte reef rises about 100 m above a Kossen dark
shale basin and forms the northwestern border of an extensive carbonate bank
whose preserved exposures trend to the east behind it along the Sonnenwande
(Sunshine Walls). The growth history and biological evolution of the Steinplatte
has been rather clearly worked out (Fig. VIII-22). Apparently reef knolls devel-
oped first in the argillaceous Kossen beds on piles of bivalves such as Gervillia,
Oxytoma, Avicula, Ostraea, terebratuloids, and crinoids. The knolls grew by devel-
opment of heads of Thecosmilia and the stromatoporoid-like hydrozoan Stroma-
tomorpha rhaetica colonies in lime mud (Plate XXVI B). These early mounds
formed in water of a depth of 30m or more if one assumes a stable sea level. The
adaptable Thecosmilia commonly exists in micrite matrix. It is found both in
Kossen basins and in backreef areas. A middle phase of Steinplatte reef growth
occurred when Thecosmilia proliferated and spread shelfward with the continued
Rhaetic transgression. A diverse algal assemblage occurs with this large organ-
pipe coral and includes Solenopora and Sphaerocodium. Sponges and spongio-
morph hydrozoans are also abundant. Many varied forms existed on the forereef
slopes at this later stage: the foraminifera, Cheilosporites, Labyrinthia, Tetrataxis,
crinoids, certain sponges, Solenopora, and the corals Thamnasteria, and three
other genera (Plate XXIV A). The last stage of reef growth reached 100 m above
the sea floor and depositional slopes of almost 30 degrees can be traced from it
down into the Kossen beds. The front of this last stage is mostly brecciated rubble
and the reef mass is exclusively of Thecosmilia clathrata, sponges, and spongio-
morph hydrozoans. Sediment equivalent to this last stage is largely calcarenite,
typically in backreef "Lofer cycles" (Fischer, 1964), spread over a wide area along
the Sonnenwande. Four or five species of dasycladacean algae occur, as well as
the foraminifera 1riasina, Angulodiscus, Tetrataxis (closer to the reef), rotalids, and
Labyrinthia. The lime grainstone facies contains large megalodont bivalves and a
variety of gastropods. Fenestral laminites are also common.

3. Rhaetic Bioherms
In addition to the bank-edge framebuilt reef rims described above, protected
Rhaetian shale basins of the Kossen Formation contained large bioherms such as
those found in the Salzburg area at Adnet, Rotelwand, and Feichtenstein. Several
of these have been studied in detail. They developed much like the small reef
knolls formed early in downslope position at the Steinplatte, probably originating
on piles of bivalves which accumulated on the argillaceous lime mud bottom. This
type of basal deposit may be seen at the Rotelwand bioherm. Initial relief above
the sea floor was not great for these mounds but they built up in deep water by
means of a framework of the ubiquitous dendroid Thecosmilia. Sieber (1937)
described the Rotelwand as being a composite of small micritic reef knolls which
accumulated to wave base but not in the zone of agitation. The top of the Rotel-
wand, however, contains abundant void-filling calcite, some with stromatactoid
shapes and many encrusting organisms forming boundstone as well as large