Page 258 - Carbonate Facies in Geologic History

Examples of Banks and Reefs 245

MF 4. Biosparite with bioclasts encrusted on all sides. Onkoidal grainstone. Subdivision
into two types possible based on homogeneity ofthe bioclasts. SMF-13.
MF 5. Fine debris biomicrite, detritus variably sorted, principally molluscan. Two subtypes
based on whether fabric is homogeneous or bioturbated. SMF-9, microbioclastic
variety of bioclastic wackestone.
MF6. Micrite or biomicrite with monotonous microfauna of foraminifera or ostracods,
subdivisions based on biological grouping. SMF-23, a variety of lagoonal wacke-
stone and mudstone with restricted marine fauna.
MF 7. Unfossiliferous micrite. Subdivisions based on presence of grains consisting of micrit-
ic lithoclasts, biogenic micrite crusts, or intraformation pebble conglomerate and
pellets. SMF-24, SMF-16, peloidal wackestone.
MF 8. Algal mats with fenestral fabric. SMF-19 and SMF-20 (Plates XII, XIV).
MF9. Onkoidal micrite to onkoidal sparite with large alga-foraminiferal balls and litho-
clasts. SMF-22 and SMF-13 (Plate IX).
MF 10. Pelsparite and biopelsparite with large peloids and non encrusting green algae and
foraminiferal particles. Two subtypes distinguished on presence or absence of frame-
building organisms. SMF -18, foraminifera-alga grainstone.
MF 11. Intrasparite with preponderance of bahamith particles, grapes tone lumps. SMF -17.
MF 12. Oosparite, subdivided on basis of associated bioclastic grains. SMF -15.

Examples of Banks and Reefs

From east of Innsbruck to west of Salzburg, the Norian-Dachstein forms a great
reef complex hundreds of meters thick. According to Fischer (1964) there existed a
barrier reef along the southern margin of the Northern Limestone Alps, parts of
which are now located far north of the original southern margin in tectonically
transported thrust sheets. Zankl (Fig. VIII -17) reconstructed the paleogeography
as a series of basins surrounded by a number of thick limestone banks, rimmed on
the south and southwest by narrow reef belts. The foreslope talus of these bank-
rimming reefs disappears regionally to the south and on all sides away from
individual banks. The off-bank facies consists of thin, reddish ammonite-bearing
Hallstatt facies with Halobia coquinas interpreted as a deep-water starved basin
sediment. Other basinal facies are known which include breccia beds, dark cherty
limestones with turbidites, and shallower more argillaceous limestone with ben-
thonic faunas.
The highest Triassic in the Northern Limestone Alps also displays significant
facies changes. The Rhaetic parts of the Dachstein pass northward into the marly
bituminous Kassen beds, evidently a sediment deposited in several slightly deeper
basins rimmed in late Rhaetic time by small barrier and patch reefs. The height of
individual bioherms within such basins and the relief at the front of the Stein-
platte reef which borders a platform north of Lofer and Waidring in Austria,
indicates a depth of at least 100 m for some of the basins.
Three forms of organic buildups may be delineated in these Late Triassic
strata in which corals and spongiomorph hydrozoans appear in abundance. Two
types of linear shelf margin buildups occur: Type II margins are composed of
small reef knolls of patchy frame-building corals and spongiomorphs on gentle
slopes described by Zankl (1969) on the Rohe Gall above Berchtesgaden in
Bavaria, and Type III, true sharp reef rims at the Steinplatte described by Ohlen
(1959) and by Fabricius (1966) in Rhaeto-Lias beds farther north in Bavaria. In