Dynamic stratigraphy of an evaporite-to-red bed sequence, Gipskeuper (Triassic), southwest German Basin

Abstract

The dynamics of evaporite accumulation are studied in the Gipskeuper (Triassic) of the southwest German epicontinental basin based on a hierarchical, three-level stratigraphic analysis:(1)Among the various stratification types, the following are most important: (a) thin carbonate intervals with marine faunas; (b) massive sulfates with selenite, indicating very shallow subaqueous gypsum precipitation; (c) thin-bedded gypsarenites with a variety of sedimentary structures suggesting clastic evaporite accumulation during high-energy events (“gypsum tempestites”); (d) laminated gypsum with spectacular tepee structures, testifying inter- to supratidal settings; (e) green-grey and reddish claystones with nodular gypsum indicating evaporitic playa mudflats.(2)Facies sequences occur in several varieties and commonly form 1–4 m thick transgressive/regressive cycles with shallowing-upwards trends. The long recognized carbonate “marker beds” represent short-term trans- or ingressions into the almost flat Keuper Basin. Highly disturbed tepee horizons form widely correlatable regressive peaks. These sequences form evaporite equivalents to the ubiquitous shallowing-upward cycles known from carbonate systems.(3)The basin-fill consists of a small-scale cycles being vertically stacked upon each other to form the basic building block of the layer-cake architecture of the Gipskeuper succession. The stacking of small-scale cycles is superimposed onto a larger-scale, regressive cycle, which records a general shift from restricted marginal marine to hypersaline marine to the more continental red bed deposition of the overlying Keuper. This hierarchy of cycles probably reflects various orders of eustatic sealevel fluctuations.The Gipskeuper thus exemplifies dynamics of evaporite accumulation and allows us to understand some of the processes that produce a “layer-cake” stratigraphy. Its cycle hierarchy is a common motif in the entire southwest German Basin fill, and appears typical for epicontinental basins.