Shallow-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 698

4.2 Triassic Epicontinental Carbonates: Germany
The Middle Triassic (Anisian-Ladinian), epicontinental carbonates and evap-
orites of the Muschelkalk Group in the Germanic Basin were deposited bet-
ween continental siliciclastics of the Buntsandstein and Keuper groups. The
carbonates of the Lower and Upper Muschelkalk contain a moderate to rich
ichnofauna, whereas the evaporitic Middle Muschelkalk is principally devoid
of trace fossils, except in its lower and upper transition zones ( Knaust, 2007b ).
In contrast to the storm-dominated homoclinal carbonate ramp of the Upper
Muschelkalk, the Lower Muschelkalk resulted from deposition on a partly
detached and distally steepened carbonate ramp with influence of synsedimen-
tary tectonics along the basin axis ( Knaust, 2000 ). Sedimentation of the Lower
Muschelkalk Group (Anisian) took place in a restricted, dysoxic, and evapo-
ritic environment with starved basin conditions. Its ichnology is briefly out-
lined below.
The vast portion of the low-energy, lime-mud-dominated Lower Muschel-
kalk Group belongs to the Cruziana Ichnofacies, with intercalated high-energy
skeletal and Oolite Beds of the Skolithos Ichnofacies, and omission surfaces
demarcated by the Glossifungites Ichnofacies and more rarely Trypanites Ich-
nofacies. Ichnodiversity is moderate, but only a few ichnotaxa repeatedly occur
throughout the succession.
The Lower Muschelkalk is built of numerous stacked depositional
sequences, most of them showing a shallowing-upward trend (e.g., Knaust,
1998 ). Such an idealized sequence is typically between 0.5 and 10 m thick
and shows a generally upward-decreasing amount of bioturbation from high/
moderate to low or zero. It consists of characteristic lithofacies and trace-fossil
assemblages reflecting different depositional environments on a prograding car-
bonate ramp. Depending on the stratigraphic and regional position within the
Germanic Basin, different variations of carbonate ramps can be recognized, such
as a homoclinal ramp with a very low gradient and siliciclastic influx in parts, a
distally steepened ramp with a considerable amount of deformed and reworked
sediment, and a detached carbonate ramp with a low gradient and two or more
belts of shoals. Taking the detached ramp as an example (as it occurs in the
region of central Germany), five lithofacies and related trace-fossil associations
occur from the top of the sequence downward ( Figs. 7 and 8 ).
4.2.1 Grainstone/Packstone (Rudite, Arenite): Rare Bioturbation
Beds of bioclastic (bivalves, brachiopods, echinoderms), intraclastic (flat-
pebble conglomerates), or oolitic limestone occur on top of shallowing-
upward cycles and may contain elements of the Skolithos Ichnofacies. These
regional marker beds occur above lagoonal or intertidal deposits. Large-scale
cross-bedding and vadose diagenetic fabric indicate an origin as high-energy