Page 319 - Carbonate Facies in Geologic History

306 Shoaling upward Shelf Cycles and Shelf Dolomitization

Rapidly foiling •• 0 1 _____ 11
HAUPTOOlOMIT OACHSTfINKAlK leh"" to A
f;lCh.,'. Lof.r cycle.
C
Unit

o 0

J
o
~~~~~:'
A
. ' .· . . ...
. .~ .... "( .. 'r •• y ••
A

A turton A
. . • 0 II!) Unit

Fig.X-19. The formation of Hauptdolomit-Dachstein cycles, northern Limestone Alps of
Austria. Possible explanation of Lofer cycles developed in Dachsteinkalk at bank edge posi-
tion and their relation to bank interior Hauptdolomit cycles. The pattern is very idealized;
the cycles may be very irregular because of numerous shoals and channels. Major deposition
takes place near to reef during transgressive (high sea-level stand) phase during formation of
unitC

brecciation, and soil formation. Many cycles cannot be traced more than a few
hundred meters laterally indicating irregular topography on the surfaces of un-
derlying C units. In some sequences a disconformity also occurs at the top of the
laminated tidal flat B unit.
Sections farther away from the Dachstein shelf edge in the Hauptdolomit
show persistent sedimentation without the erosional surfaces. The Hauptdolomit
consists primarily of only two facies: (1) A light-colored thick-bedded, bioclastic,
burrow-mottled, dolomitic wackestone with foraminifera such as Angulodiscus
and Permodiscus, dasycladacean particles and scattered megalodonts in living
positions. The unit may contain siliceous streaks and laminae. (2) These beds
alternate with thinner units of tidal flat mm laminite, both planar and wavy
(Plates XIA, XIIB). In places the laminae are torn into irregular crusts. The
laminae are probably algal stromatolites but well-developed algal heads are rare.
The unit is dark in places. Breccia beds are common, probably representing both
residue of evaporite solution and some pebble conglomerates formed in shallow
tidal channels. Fenestral fabric is not so well developed in the Hauptdolomit as in
the Dachstein limestone which formed at the periphery of the bank. In some
places pockets of stagnant marine water occurred on the platform and were filled
with dark, highly bituminous laminated limestone with well-preserved fish re-
mains, worms, and plant fragments (MUller-Jungbluth, 1968).
A plausible relationship between the Dachstein and Hauptdolomit alterations
may be explained by a periodic rise and fall of sea level in the wide lagoon far