Saltern, mudflat, and dry playa: playa basin types of a retreating epeiric sea (Keuper, Germany)

Abstract

Playa basins contain saline lakes and the evaporitic mudflats around them. However, playa basins can be different depending if they are fed by marine incursions, dominated by perennial lakes of brackish or fresh water, or dominantly dry. A discrimination of playa types, which originated under such different conditions, can be difficult in the rock record. The present study contributes to our understanding of these settings by reconstructing the evaporitic facies evolution in the German Keuper Basin after the retreat of an epeiric sea. The Middle to Late Triassic Grabfeld Formation (ca. 237–233 Ma) consists of weathered gypsum and dolomitic marl at the surface. Non-weathered, anhydrite and dolomitic marl successions were studied at the tunnel face and on drillcores for the Stuttgart 21 construction project around Stuttgart. From oldest to youngest, the following facies were recognized: (1) The Grundgipsschichten facies represents an environment of large, shallow, sulfate lagoonal lakes (“saltern”). Aren itic sulfate was deposited in very shallow waters, comprising many tepee-like structures, enterolithic folds and numerous truncation surfaces. (2) The Bochingen-Horizont facies is characterized by greenish gray laminated mudstone and dolomite deposited in perennial saline lakes. Anhydrite was locally altered by brackish or fresh water. (3) The Dunkelrote Mergel facies contains the sediments of a dominantly dry playa basin, red mudstone and nodular anhydrite. Sheetfloods resulted in ephemeral lakes under a monsoonally controlled climate. Sedimentation was sparse with numerous truncation surfaces. Large, festoon-like structures on the tunnel face were interpreted as paleo-gilgai structures. The study concludes that an evaporative environment of a coast (“coastal sabkha”) can never be undoubtedly distinguished from a playa basin in the sedimentary record.