The stratigraphic and geochemical record of Playa Cycles: monitoring a Pangaean monsoon-like system (Triassic, Middle Keuper, S. Germany)

Abstract

The combination of high-resolution stratigraphy with high-resolution geochemistry allowed monitoring the cyclicity of a playa system in two dimensions, time and space. The playa cycles of the Middle Upper Triassic Steinmergel-Keuper, located in southern Germany, were studied both in terms of lateral facies shift variations and long-term cyclic patterns. The lateral variation was expressed by investigating a time equivalent transect that spanned the Kieselsandstein horizon within a 130 km distance, from marginal to mid-playa position, whereas the long-term cyclic pattern was investigated in core Malschenberg, where a time record of ca. 5×10 6 years was encountered. Mudstone/dolomite cycles, with dolomites representing the wet and mudstones the dry phases, reflect cyclic playa lake-level changes, indicating varying intensities of monsoon-like circulation. Moisture transport from the Tethys Ocean into the German Basin is implied by δ 18O depletion trends of isochronous dolomite beds from the margin to mid-playa position, suggesting dominantly Rayleigh-like circulation patterns. Distinct covariance of δ 18O/δ 13C is used as indicator of lake-level stands. The K/Al curve, with inverse covariation compared to δ 18O, reflects fluctuating weathering conditions on the fan and the Vindelician High. A hierarchic cycle stacking pattern composed of metre-scale to tens of metre cycles is interpreted to be a result of changing Pangaean monsoon-like intensities in the Milankovitch frequency band. Spectral analysis shows that the power spectra are dominated by precession and eccentricity.