Geology Reference
In-Depth Information
•
Member A:
Marl, silty marl, dolomitic marl; argilla-
ceous mudstone, unfossiliferous lime mudstone (Pl.
140/1); lime mudstone with quartz grains; peloid
wackestone; wackestone with sparse skeletal grains
(e.g. ostracods); intraclast wackestone and floatstone
with clasts consisting of reworked microbial mats;
poorly sorted fine-grained breccia with black pebbles
and other clasts; intraclast wackestone and floatstone
with reworked material from Member C (Pl. 140/2; Fig.
16.2D). In addition, variegated-colored micrites within
solution cavities and fine-bedded micrites are common
features of member A.
•
Member B:
Unfossiliferous lime mudstone; fenes-
tral mudstone/bindstone (Pl. 17/3; Pl. 140/2) including
micritic, peloidal (Fig. 16.2C) and intrapelmicritic
loferites; laminated mudstone/bindstone, stromatolitic
mudstone/bindstone (Pl. 46/2; Pl. 140/3); peloid pack-
stone; intraclast wackestone and floatstone with clasts
consisting of reworked microbial mats; fine-grained
polymict breccias containing material both of Member
B and Member A; peloidal, intraclastic and bioclastic
grainstone (shedding from subtidal areas; Fig. 16.2A,
B).
•
Member C:
Lime mudstone with solution cavities;
mudstone and wackestone with sparse tiny shell de-
bris; mudstone and wackestone with benthic foramin-
ifera; peloid wackestone; bioclastic wackestone and
packstone with low-diversity dasyclad algae and small
gastropods; bioclastic wackestone and floatstone with
bivalves (Pl. 140/4), in places shell coquinas; bioclas-
tic wackestone and bafflestone with corals (Pl. 140/5);
oncoid wackestone and floatstone with cyanoids formed
Plate 140 Cyclic Carbonates: The 'Lofer Cyclothem' of the Alpine Triassic
The Late Triassic Dachstein Formation of the Northern Calcareous Alps in Austria comprises an up to 1,500 m
thick sequence of bedded limestones, deposited within a several kilometer wide lagoonal belt behind a shelf-
margin reef zone (see Fig. 15.29). The Dachstein facies is widely used as a reference example of meter-scale
cycles (Lofer cycle named after the Loferer Steinberge) produced by high-frequency sea-level oscillations. The
cycles form meter- to decameter-thick successions. The
ideal Lofer cycle
(Fig. 16.1) as defined by Fischer
(1964) comprises a subtidal Member C with normal marine biota, an intertidal Member B, and a supratidal
Member A. Both the intertidal and the supratidal members are characterized by fenestral laminites and stroma-
tolites. The supratidal member A may or may not have a terrestrial horizon on top. Contacts between the mem-
bers are gradational or sharp. Lithologies and microfacies reflect a broad spectrum of rapidly changing environ-
mental conditions.
1
Argillaceous lime mudstone. Member A.
The micritic and pelmicritic matrix contains spar-filled voids some of which are
infilled with (red) calcisiltite (1). Large horizontal and vertical voids (2) and micritic and pelmicritic areas bounded by
crinkled spar-filled cracks indicate pedogenic overprint. Late Triassic (Dachstein limestone, Norian): Breithorn, Steinernes
Meer, Salzburg, Austria.
2
Limestone conglomerate
(1) overlain by
fenestral mudstone
(2 to 4). The conglomerate consists of subangular and rounded
micrite clasts embedded within a calcisiltite matrix. This floatstone texture corresponds to the 'intraformational conglom-
erate' attributed by Fischer (1964) to the
Member A
and regarded as an indication of supratidal reworking. The lower part
of the fenestral mudstone (2) contains limestone clasts, too. This zone is overlain by characteristic loferites (3) with
aligned spar-filled voids, assigned to
Member B
and considered to be intertidal in origin. Larger geopetally filled cavities
are interpreted as shrinkage pores (4) within this zone and overlying it. Same locality as -> 1.
3
Laminated loferite
consisting of flat and undulated micritic and sparry layers. The densely set micritic layers (1) consist of
peloids forming a packstone fabric. The more widely set sparry layers (3) exhibit porostromate calcimicrobial fabrics in
some areas. The center of the picture (2) shows stacked, spar-filled structures, probably tepees (arrows). The samples
corresponds to the crinkled 'algal mat loferite' distinguished by Fischer (1964) and the 'laminated loferite' B1 described
by Goldhammer et al. (1992).
Member B.
Same locality as -> 1.
4
Bivalve floatstone
with recrystallized megalodontid shells (compare Fig. 10.40). The matrix is a fine-grained packstone
with variously sized peloids, aggregate grains and a few benthic foraminifera. Small white sections are ostracod and
gastropod shells. The fine-grained matrix indicates a protected platform interior environment.
Member C.
Subtidal part of
the Lofer cycle. Same locality as -> 1.
5
Coral floatstone.
Dendroid corals (
Retiophyllia
) embedded within a bioclastic packstone matrix containing abundant
fine-grained shell and coral debris. Note the difference in the composition of the matrix as compared with -> 4, indicating
the existence of small coral patches in the outer zone of the platform.
Member C.
Subtidal part of the Lofer cycle. Late
Triassic (bedded Dachstein limestone, Norian): North of Riemannhaus, Steinernes Meer, Salzburg, Austria.
1-5: Thin sections courtesy of E. Samankassou (Fribourg).
