High-frequency depositional cycles in the Muschelkalk (Middle Triassic) of southern Poland: Origin and implications for Germanic Basin astrochronological scales

Abstract

Abstract High-frequency depositional cycles are often used for long-distance correlations within sedimentary basins and constructing high-resolution chronostratigraphic scales calibrated to the orbital clock. Such an approach has been used for the tropical, epicontinental Muschelkalk Sea covering most of Europe in the Middle Triassic. This paper aims at establishing the spatial extent and internal architecture of shallow-marine Muschelkalk cycles within a small (120 by 50\u202fkm) area of the Germanic Basin, to identify their local vs. regional distribution and cycle-forming mechanisms. Three groups of cycles are distinguished: 1) peritidal cycles representing a tidal flat-lagoon system which experienced longer periods of emersion; 2) shallow subtidal cycles formed around the fair-weather wave base; and 3) deep subtidal cycles developed around the storm wave base. The three cycle types differ markedly in the composition, facies homogeneity, sedimentary trends, hierarchical organization, continuity, and degree of lateral changeability and complexity, which collectively translates into the ease of cycle recognition. Despite the differences, the defined cycles do not extend laterally beyond the studied area and have laterally variable facies heterogeneities, sedimentary trends, and thicknesses. This variability resulted from more or less mosaic infilling of accommodation space, reflecting local bathymetry, physicochemical conditions, and carbonate production and accumulation rates on the platform. Accommodation space evolved by the interplay of eustatic sea-level fluctuations regional subsidence, and synsedimentary tectonic-block movements, but the contribution of these factors changed over time and was to varying degrees masked for each of the three cycle types by intrinsic sedimentary processes. The study shows that Middle Triassic cycles of the Germanic Basin have a local to sub-regional distribution and thereby are inappropriate for basin-wide correlations and chronostratigraphic inferences. It also strengthens the opinion of several workers that correlation of idealized cycles is unrealistic without checking the horizontal continuity of facies and key surfaces directly in the field.