Titel: Combined architectural-element and microfacies analyses in carbonate sedimentology: the intra-basin shoal complex of the Lower Muschelkalk (Rüdersdorf Formation, Anisian)
Fabian Käsbohrer, Matthias Franz
University of Göttingen, Göttingen, Germany
Veranstaltung: ECMS 2021
The layer-cake strata of the epeiric Anisian-Ladinian Muschelkalk are devoid of large-scale architectural elements, e. g. steep clinoforms of a progradional platform slope. This resulted in a simplified approach in facies analysis, in which results of classical microfacies analysis (microscale) are used directly to conclude on depositional models (macroscale). In coastal settings, depositional cycles (mesoscale) are often defined in an intermediate step to visualize progradational or retrogradational patterns of shorelines or shoreline-detached shoals. Apart from such large-scale stratal pattern architectures, a conceptual approach to integrate the depositional architecture of individual beds and sets of strata is missing.
The intra-basin shoal system of the Anisian Rüdersdorf Formation shows a complex pattern of fore- to backshoal facies (Schaumkalk) interfingering with mixed clastic-carbonate facies of an epeiric Inland Sea (Wellenkalk, Jena Formation). Thus, it represents an ideal case study on depositional architectures of shallow marine carbonates. Our data are based on a detailed bed-by-bed outcrop study in the Rüdersdorf open pit mine to the East of Berlin. In addition, we included data from exploration wells situated in and near the open pit mine to reveal lateral facies changes. Consecutive observations from micro- to macroscale led to a classification of lithofacies types, architectural elements and lithofacies associations. The vertical succession exposed in the outcrop shows gradual transitions from bioturbated marly mudstone lithofacies (Wellenkalk) to thick cross-bedded oolitic/oomoldic grainstone lithofacies (Schaumkalk). Local 2-D correlation schemes implementing exploration wells reveal vertical and lateral facies shifts associated to the repeated formation, migration and drowning of an intra-basin shoal complex.
The vertical and lateral facies shifts are associated to subaqueous sediment gravity flows, which redistributed carbonate grains from the shoal complex to foreshoal and backshoal environments. The bed-by-bed outcrop analysis revealed distinct architectures of sheet-like gravity flow deposits with decreasing internal complexity in down-current direction. The architectural elements of the medium- to low-energy transition zone are in noticeable contrast to those of the high-energy shoal fringe and top. The architectural elements observed so far enable an improved reconstruction of the physical processes leading to deposition of the Rüdersdorf Formation. If properly defined, the implementation of architectural-element analysis (mesoscale) in carbonate sedimentology may become the consistent intermediate step from microfacies analysis (microscale) to depositional models (macroscale).
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