Page 255 - Carbonate Facies in Geologic History

242 Permo-Triassic Buildups and Late Triassic Ecologic Reefs

Orton ella is known. Flat, encrusting red algae are as usual on the outer side of the
reef and in detritus where they constitute 5-18% of the poorly-sorted bioclasts.
Ott pointed out that the sponge morphology of segmented, chambered, and
vesicular structure is probably a reef growth adaptation. The segmented character
results from rhythms of growth but also permits flexibility for waving in moderate
currents. The fine and delicate growth forms, beaded and pipe-like form of larger
organisms, and the lack of knobby, onkoidal or encrusting growth of sponges,
hydrozoans or corals indicates carbonate buildup in water of some depth and
moderate current. Optimum depth for the particular sponges is given as 4-18 m
and for the corals as deep as 20 m. Such depths may be at or below wave base in
inland seas. The width of the reef rim is not known but its outer fauna is clearly
normal marine and its interior biota shows considerable salinity variation. A few
zones of oolite may indicate very shoal water or even islands at the margins.
Two areas of dasycladaceans mark facies within the banks themselves. The
Teutloporella herculea zone is positioned in the near backreef on both sides of the
bank but may range 10-15 km into the bank. The algae stalks may be oriented.
Codiacean and red algae also occur in this facies. The sediment represents muddy
lime sand (packstone) at the crest of the profile or in a more protected environ-
ment just behind. The interior of the bank formed a considerable lagoon and tidal
flat complex. It consists of 5-10 m thick cycles with a thick subtidal zone grading
upward to mm algal laminites with reddish geopetals and fenestral fabric. These
beds are dolomitic. Sarntheim (1967) listed grain aggregate lumps, fecal pellet
mud, pure lime mud, and bioclastic muds with abundant broken pieces of Diplo-
pora annulata as characteristic of bank interior facies, particularly in the subtidal
part of the cycle. Some of these beds are shoal grainstones.
In summary, the Wetterstein limestone may be interpreted as consisting of
major carbonate banks with margins mostly of bioclastic accumulations which
contain downslope scattered encrusting and low-growing forms such as sponges
and organ pipe (dendroid) corals. Growth forms and general biological composi-
tion indicate accumulation just at or below wave base in only mildly agitated
water (20 m). The author classes such profiles as Type I, downslope mud accumu-
lations. The maximum height of the Wetterstein banks above the Partnach sedi-
ments in the basin is believed by most German and Austrian researchers to be
only 100-150 m which is the extra thickness of the Raibl where it overlies the off-
bank Partnach facies. One has, however, only to look at the undisturbed strati-
graphic relations in the Dolomites to speculate on the possibility of much greater
relief than this. On the other hand, the lack of talus and typical toe-of-slope lime
mudstone facies (belt 3) might indicate more subdued relief of the Wettersteinkalk
banks.

Later Triassic Deposits in the Dolomites

After the growth of the great banks in the South Alps was essentially complete,
the intervening basins were filled with brown pyroclastics resulting from intrusion
and eruption of basic volcanic material (plagioclase-pyroxine porphyry). Volcanic
ash and pillow lavas are mixed with abundant terrigenous clastics and plant