Page 263 - Carbonate Facies in Geologic History

250 Permo-Triassic Buildups and Late Triassic Ecologic Reefs

1. The Hohe Gall, a Reef Knoll Ram~Type II
The Hohe G611 carbonate facies complex lies in a thrust sheet north of the
Torrener Joch, high above the town of Berchtesgaden (Fig. VIII-18). Its southern
boundary and original depositional slope is obscured by faulting so that no direct
observation of original relief is possible. Zankl (1967) offered faunal evidence for a
moderate depth (200 m) of the intervening basinal Hallstatt facies. He believes
that only a gentle slope was present, 1 or 2 degrees into the basin. Part of this can
be observed on the Hohes Brett just south of the GOl!. The detrital slope facies has
pockets of Hallstatt red limestone.
Much is known about the biological composition of the Hohe G611 margin,
(Zankl, 1969, 1971). See Fig. VIII-19 and VIII-20. The ramp marking the southern
and open-sea side of the great buildup is made up of a series of reef knolls, each
characterized by one or two framebuilding species and with varying groups of
organisms as subsidiaries. Biocoenoses cannot be well delineated; clearly their
sequences in time and space are rather irregular. It is known that: (1) sponges
along with the red alga Solenopora generally form lamellar crusts on reef sides;
(2) "1hecosmilia", a large organpipe dendroid form, exists on more protected
back-reef sides but also in quieter water basinal beds; (3) reef knolls are domi-
nated (75% of total framework) by the corals Astraeomorpha, "1hecosmilia" and
calcareous sponges. The coral colonies may be up to 10 m high. Other coral
genera are the large solitary Montlivaltia, colonial 1hamnastrea, and Palaestrea.
Encrusting spongiostromes (blue-green algal crusts), stromatoporoids (spongio-
morph hydrozoans), bryozoans, encrusting foraminifera, and Cheiloporites to-
gether make up the other 25% of volume of boundstone. Zankl has demonstrated
how multiple generations of these encrusters grew intermittently with marine
cementation. The interior of the knolls contains mainly calcarenitic-calcisiltite
debris and not much micrite. Constructed voids filled with cement are present
toward the tops of the knolls. This mayor may not be marine or splash-zone
cement. A vadose origin is possible.
The bulk of the outer platform strata are not reef knolls; 90% is estimated to
be composed of bioclastic detritus. Most of the knoll-flanking material is bioclas-
tic grainstone but resedimented particles, lithoclasts, are also abundant. The fore-
reef detritus is mostly subangular and poorly sorted; more resedimented parti-
cles are present down the slope, and the matrix becomes more micritic (up to
30%). The biota is highly diverse, particularly with corals and encrusting organ-
isms derived from the tremendous organic growth at the knoll-reef centers farther
up the gentle slope. The immediate back-reef and reef crestal sediments in places
contain very coarse rubble. Back-reef cycles of the Dachsteinkalk, which are
present just shelf ward of the bank margin, are described in Chapter X.

2. Rhaetic Organic Reef Rims-Type III
During very latest Triassic times a large scale separation of Rhaetic shale
basins and intervening reef-covered swells occurred in the northern area where
earlier the Hauptdolomit platform had existed. Fabricius (1966) gave a paleogeo-
graphical analysis and map of one of these basins and its reef margins. The
argillaceous K6ssen facies spread into the troughs and contained a benthonic
fauna with characteristic mollusks. About half of the Dachstein reef species are