Page 320 - Carbonate Facies in Geologic History

Summary-Comparison with Permian Reef Complex 307

behind the Dachstein reef rim (Fig. X-19). This interior area remained relatively
undrained even at low stands of sea level when it became essentially a complex of
tidal flats, sabkhas, and salinas. The best-developed diagenetic features, which
mark the typical Lofer cycle, are seen in the better-drained, near backreef Dach-
steinkalk, closer to the reef rim. This area was also more exposed to marine water
of the splash zone.

Summary-Comparison with Permian Reef Complex

Combining Fischer's (1964) description of the Loferites with Dunham's observa-
tion in the Permian Reef Complex (1972) the following general diagenetic charac-
teristics may be recognized. See also Smith (1974 b).
1. Common dolomitization (see following section).
2. Large scale development of aligned shrinkage pores, fenestral fabric or "birdseye"
sparry blebs develo~d partly through desiccation in algal mat mudstones with or without
Girvanella tubules, in peloidal or homogeneous lime mudstones.
3. Prism cracks-large scale desiccation polygons formed in muddy sediment or in hard-
ened rock resulting in tepee structures formed through several meters of sediment. The
process is clearly one of sediment expansion resulting in upward curling and breakage of
edges of mega polygons. Vadose sediment may fill in the cracks.
4. Sheet cracks parallel to bedding, or oblique zebra cracks in host sediment. Such
cracks, like the prism cracks, are generally filled with coarse calcite druse and/or internal
sediment. Dunham believes this type of druse in the Permian Reef Complex may result from
precipitation from meteoric water. Other researchers maintain a marine origin for this ce-
ment. Folk (1974) has pointed out that its coarse drusy crystal habit could indicate that its
calcite mineralogy was pseudomorphed from aragonite. In the Dolomites many years ago
bulbous and veinous forms of this cement were described as a fossil Evinospongia.
5. Neptunian dikes. Megafissures cutting down through many meters of strata and filled
with sediment of either marine or meteoric water origin or an alternating combination of
both. The origin of the dikes is complex. They may be in part tectonic as a result of a slump-
fissuring, but may also represent karst solution along joints at edges of giant polygons (tepee
structures). The oblique "zebra cracks" in sediment filling some of these may be caused by
dewatering of fme lime-mud sediment, or possibly by slumping at the same time. Many of the
dikes are filled with marine sediment bearing brachiopods in the Triassic and cephalopods
and crinoids in both Triassic and Permian.
6. Red soil and caliche horizons are particularly common in the Dolomites and Permian
Reef Complex. Breccias, peloidal fabric with reverse grading, and downward oriented crinkly
lamination are common hallmarks of this structure.
7. Large coated particles have been interpreted as "vadose pisoids" in strata of the
Permian Reef Complex. See Newell et al. (1953), Thomas (1965), Dunham (1969a), and
Kendall (1969) for prevailing views. The downward elongation and fitting of pisoids, perched
silt grains, drusy coating which envelop groups of individual balls, and smoothly curving
laminae indicate that many of the particles are not original depositional entities but may be
concretions. These are identical to some of the concentrically coated particles described as
algal balls of Sphaerocodium in the Triassic of the Dolomites. Vadose zone caliche, marine-
salt spray evaporation, concretions caused by fresh water seepage in coastal swamps, and
cave-pearl formation have all been considered as alternative explanations for many concentri-
cally coated lumps which have formerly been considered onkoids.
True onkoids are also common features of these cyclic deposits. Kendall (1969) proposed
a multistage origin for such particles in the Permian facies of onkoids, deposited in beds which
are later additionally cemented by passage of vadose zone water which may be either of
marine or meteoric origin. The process may have been reversed in places: concretions may
have been eroded out, coated, and redeposited in a marine environment. To date no exact