Asymmetry of an epicontinental basin—facies, cycles, tectonics and hydrodynamics: The Triassic Upper Muschelkalk, South Germanic Basin

Abstract

The Middle Triassic–Upper Muschelkalk deposits of central Europe form an excellent, “laboratory‐scale” outcrop analogue for subsurface hydrocarbon reservoirs in epeiric basins. This study provides a detailed sedimentological analysis at microfacies to basin‐wide scales within a well‐constrained biostratigraphic and sequence stratigraphic framework. The database comprises 31 outcrops and 27 drill cores in the southern part of the Triassic Germanic Basin, from Luxembourg, eastern France, northern Switzerland and southwestern Germany. The Upper Muschelkalk comprises 21 lithofacies types which are grouped into eight distinct lithofacies associations encompassing coastal sabkha, peritidal, backshoal, shoal, shoal fringe, foreshoal to offshoal environments. A 1D sequence stratigraphic framework was established on different cyclic scales ranging from depositional sequences, cycle sets, cycles to mini‐cycles. Regional 2D correlations based on 1D cycle analysis and biostratigraphic conodont/ceratite zonation reveal the temporal and spatial distribution of reservoir bodies and the morphological variance of 1D cycles across the carbonate ramp. A basin‐wide W–E ‘coast to coast’ cross‐section shows an asymmetric basin architecture with significant differences in facies distribution, cycle morphology, and consequently sizes of potential reservoir bodies. This distinct asymmetry is controlled by the palaeo‐tectonic setting (quiet versus more active) and the hydrodynamics (windward versus leeward, inflow of freshwater versus marine waters) which differs from coast to coast. Such asymmetric patterns will have to be considered in exploration scenarios of subsurface hydrocarbon‐bearing epeiric basins in the Middle East and elsewhere.

Facies and depositional environments of the Upper Muschelkalk (Schinznach Formation, Middle Triassic) in northern Switzerland

“…Stacks of up to five 5th-order sequences compose 5-13 m thick 4th-order cycles (Koehrer et al 2010;Palermo et al 2010;Warnecke and Aigner 2019). These cycles have been interpreted as representing sedimentation resulting from long-eccentricity (400-kyr) periods (Warnecke and Aigner 2019) and are regionally correlatable based on biostratigraphic constraints, stratigraphic marker beds, lateral facies correlations, changes in regional facies development, the presence of proximal shoreline outcrops/boreholes and through the multitude of previous regional studies in the southern Germanic basin (Koehrer et al 2010;Palermo et al 2010;Warnecke and Aigner 2019). In the case of the northern Swiss Upper Muschelkalk, 4th-order cyclicity is likely present but its reliable identification must await the availability of more information.…”

“…Small-scale cycles with thicknesses of 0.2-7 m are recognized throughout the German Upper Muschelkalk (Aigner 1985;Braun 2003;Koehrer et al 2010;Warnecke and Aigner 2019). These units have been interpreted as 5thorder cycles, after the hierarchal classification of Vail et al (1991), and interpreted to reflect the short (100-kyr) orbital eccentricity period (Aigner et al 1999;Koehrer et al 2010).…”

“…Cycles generally begin with a hemicycle consisting of mudstone facies that grade upwards into higher-energy facies, followed by the next hemicycle that shows facies grading upwards back into low-energy mudstone facies. Most cycles correlate laterally across northern Switzerland, however, ''missed beats'' (Goldhammer et al 1990) occur due to difficulty in identifying cyclicity in homogenous mudstones and due to erosion of shallowwater facies during regressive phases (Warnecke and Aigner 2019). This particularly affects the top of the Upper Muschelkalk, where an unknown amount of sediment and cycles are missing due to the erosion associated with the Lettenkohle unconformity (Warnecke and Aigner 2019).…”

“…Oolitic shoals of the Upper Muschelkalk of southern Germany are up to 30 km in length and 15 km in width and developed as progradational bodies during regressive sequences (Palermo et al 2010). They occur in the same locations as the underlying crinoidal banks of the transgressive ramp and were influenced by the subtle relief created by crinoidal banks or by minor palaeohighs that rimmed the shorelines of the Vindelician High (Aigner 1985;Braun 2003;Geyer and Gwinner 2011;Warnecke and Aigner 2019). However, palaeotectonic maps of northern Switzerland indicate that no NNE-SSW trending palaeohighs are present in Northern Switzerland (Madritsch 2015).…”

“…Sediments of the last few cycles of the Upper Muschelkalk, which were not eroded away at the top of unit, reflect the increasing restriction and sea-level fall of the Upper Muschelkalk Sea. These sediments, replete with palaeosol, microbial laminates, brecciation, shrinkage cracks and extensive evaporite deposits and their dissolution vugs, have been found from the Netherlands to southern Germany along the palaeoshorelines of the Vindelician High and Rhenish Massif (Schauer and Aigner 1997;Braun 2003;Pöppelreiter et al 2003;Koehrer et al 2010;Warnecke and Aigner 2019). In Switzerland, however, the thickest Upper Muschelkalk evaporites were deposited in the central parts of the basin, in locations [ 100 km seaward from the Anisian/Ladinian palaeoshorelines, as seen in the boreholes of Pfaffnau and Schafisheim (Fig.…”