The Posidonia Shale (German: Posidonienschiefer, also called Schistes Bitumineux in Luxembourg), is an Early Jurassic (Toarcian) geological formation in Germany, northern Switzerland, northwestern Austria, southern Luxembourg and the Netherlands. Posidonia Shale got its name from the fossils of the oyster-related bivalve "Posidonia bronni" that are commonly found throughout the formation. However, this name is technically incorrect, as Posidonia is an invalid genus. The name Posidonia Shale has been used for more than a century, until two independent revisions of the Early Jurassic lithostratigraphy in Germany and Switzerland proposed a name change. The Posidonia Shale is currently known as the Sachrang Formation in Germany and as the Rietheim Member of the Staffelegg Formation in Switzerland. The Posidonia Shales is still considered a valid common name for the lower Toarcian Black Shales in this region. Posidonia Shale, which was deposited under strongly reducing conditions during the Toarcian oceanic anoxic event, trapped and exceptionally preserved complete fossils of marine fish and reptiles. The formation consists of oil shales formed of fine-grained sediments and bituminous limestones in fine, alternating layers. It crops out in several locations in southwestern Germany, although most remains are near the village of Holzmaden and Dotternhausen. The European oil shales deposited on a sea floor during the Early Toarcian in the ancient Tethys Ocean are described as being deposited in an anoxic deep water environment, although the details of the depositional environment are the subject of debate by researchers.
Geology
The Posidonia Shale was originally referred to as the Black Jurassic. It was first covered by the Franconian Jura, which borders the northeast on the Obermainisches Hügelland and the Oberpfälzisch-Obermainisches Hügelland, and is tectonically part of the Faulkschollenland. The cover of Franconian Jura rocks lies west of the Saxothuringian basement and borders the Franconian Line. Throughout Germany, it is covered by the early Toarcian Central European Epicontinental Basin (CEB) that gradually evolved from a low topography tidal flat, to floodplains, to a shallow shelf sea. The sea intermittently connected with the Viking Corridor and the Proto-Atlantic Ocean in the north, and with the Tethys Ocean towards the south. This filled the area between the Netherlands and Tyrol with seawater and subdivided it into several sub-basins with differing conditions and biotas. The CEB also established a relatively shallow transcontinental seaway between the Tethyan and the Boreal Arctic Sea biota, thus mixing cold and hot waters. The Mesozoic was marked by the breakup of Pangaea during the Late Triassic, which led to the early Atlantic Ocean connecting with the Boreal waters of the Panthalassa Ocean, and several local marine and continental sub-basins. The CEB was part of the Laurasian continental-marine shelf that rested over the Keuper and slowly opened towards the southeast into the deeper Tethys Ocean. This Early Jurassic marine flooding lead to the development of paleocurrents. Towards the west is the north–south Kilberg Fault of the Keilberg Rift, the main fault in the Regensburg Basin. Developed by the sinking of the southern German Jura Plateau during the Miocene, it separates the higher, older crystalline Moldanubian Basement from the Lower Jurassic chalk complex of the eastern Franconian Jura. During the late Pliensbachian, the zone became a relatively narrow, flat deposit area that flooded during the early Toarcian. It reemerged during the Bifrons substage with a changing coastline, thanks to rhythmic uplifts and subsidences of older Paleozoic and Triassic siliciclastic deposits from the east. The granites and gneisses produced by the slow magma crystallization were eroded from the Paleozoic exposures on the east, and the resulting siliciclastic sediments were deposited on the Jurassic prograded alluvial nearshore sandstone that evolved gradually into the Bajocian layers. The slopes of the area are partially covered by till, soliflucted rubble and loess from the Würm glaciation.
Stratigraphy
The German and Swiss stratigraphy systems were developed independently, which explains the different names used for the Posidonia Shale in these two countries: Sachrang Formation in Germany and the Rietheim Member of the Staffelegg Formation in Switzerland.
Sachrang Formation in Germany The Sachrang Shales were originally cited in the revision of the study on the Alpine Upper Black Slate, which is composed of dark grey, somewhat sandy, disintegrating marl that forms large, thin plates and overlies Pliensbachian breccia. The definition of the Sachrang Shales has been convoluted throughout the history of studies at the location, with works on the North Alpine Mesozoic preferring to call these deposits the Sachranger Shale to provide a brief, distinct diagnosis. On the Unken syncline near Lofen, basin deposits with abundant aragonite and calcite help determine the major Jurassic basin geometry, as the posterior Alpine deformation made dating complicated on some layers. Correlated Unken and Diessbach basins developed mostly during the Toarcian, with deposits of abundant material from the near-emerged landmasses. On the Unken syncline, the breccias associated with the normal faults were deposited until the Oxfordian age.
Rietheim Member of the Staffelegg Formation in Switzerland In Switzerland, after a revision of the Early Jurassic lithostratigraphy, many reference names have changed. The Rietheim Member of the Staffelegg Formation is now considered the lateral equivalent of the German Posidonia Shale in Switzerland. Therefore, the name Rietheim Member of the Staffelegg Formation replaces the former name Posidonienschiefer in Switzerland, while the German classification system remains unchanged. The German and the Swiss lithostratification classification systems are developed in an independent way and should not be mixed. In the German system, the former Posidonia Shales are at the formation level (Sachrang Formation), while in the Swiss classification, they are at the member level (Rietheim Member).
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