The Posidonia Shale (, 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).
Detailed description of the stratigraphy in Germany The bituminous facies overlie the Pliensbachian and consist mostly of clay marl and marl shales with a total organic carbon content of over 2% by weight. Some levels are referred to as "bitumen-free" or "bitumen-poor". The lowest sequence of layers is buried. It is made of seagrass shale (Seegrasschiefer), approximately 15–20 cm thick, with clay marl stone appearing in darker browns and greys. These first layers have abundant foraminifera and ostracods in a medium to light grey color. The next layer above features blue-grey marls of the uppermost Pliensbachian spinatum zone, being medium grey, pyrite-rich clay marl stones. They are also a part of the Amaltheenton Formation, which gradually wedge out to the east in the area of Aalen-Wasseralfingen. The middle includes thin, disturbed bituminous layers found throughout southwest Germany. Gradually, the bioturbation of the seagrass shale merges into the subsequent ash-grey marls without a sharp facies boundary. The ash-grey marl marks the start of the Posidonia Shale, made by dark grey marl, abundant in pyrite with bituminous marl shale intrusions. This layer marks a sea transgression, as it extends from Asselfingen/Wutach to Aalen-Reichenbach and then wedges out to the east from Aalen-Wasseralfingen. This initial sequence is overlaid by extremely thin (2–5 cm), unnamed bituminous, seagrass shale clay marl, followed by darker layers with the same lithology. The next are the "Koblenzer-Hainzen" clay successions of the upper semicelatum subzone, initially poorly layered and bituminous with abundance of Steirmannia radiata and Dactylioceras semieelatum. This section is well-layered and black-brown in color. It is marked by pyritized lagerstätten fossils, which are limited to Dotternhausen and Holzmaden. The Unteren Schiefern layers (Exaratum) appear next, marked by the highest bitumen content and characterized by very fine light/dark stratification, abundance of pyrite, fine-grained weathering, and the largely absent bioturbation derived from anoxic conditions, while the accumulation of exceptionally preserved fossils indicates very weak water movement. The next, Untere Stein, is the most important level of the formation, especially in southwest Germany, southern France, and Alsace-Lorraine. It appears across the area either as a concretionary "laibstein" horizon in the Aalen region, or as a uniformly layered limestone bank in the Wutach area. Leptolepis coryphaenoides is the characteristic fossil of this limestone bank.
The Mittleren Schiefer/Schieferklotz (upper exaratum to lower elegans subzones) becomes increasingly poorly layered until it becomes a small brittle limestone bank, the "Stinkkalkbank" (Dotternhausen to Gomaringen and Nürtingen) with Coelodiscus shells, low on bitumen and with biota that marks better oxygenated conditions, such as foraminifera and occasional ostracods. The Obere and Wilder Stein (upper elegans) medium to brown-grey always remain formed as a regular limestone bank of approximately uniform thickness, rarely laminated (Dotternhausen) and often shows traces of minor bioturbation (Dotternhausen, Mössingen, Gomaringen; Aalen-Wasseralfingen), becoming calcified towards the upper limit between Nürtingen and Holzmaden, marked with increased presence of Cucullaea muensteri as well foraminifera and ostracods, as well scoria horizons with fish, cephalopod and larger vertebrate remains, as well as belemnite rostrums. The last level of the formation is the Wilden Schiefer (probably reaching Bifrons zone) with the presence of "Monotis bank", from Altdorf to Dotternhausen and Göppingen area, with slates getting poorer in bitumen and less layered. The top section is known as "bollensis camp", marked by the mass deposition of Bositra buchi, closed at the top by a new seagrass slate. The limit with the Jurensismergel Formation is mostly eroded in profiles, evolving into marlstones. Several outcrops, mostly in the north-west (ex. Harz hills), show that relict levels of the Posidonia Shale lasted until the Upper Toarcian, contemporary with the Jurensismergel Fm., known as "Dörntener Schiefer".
Lithology The black shales are the main component of the strata, with a major bacterial component. The shale is represented by blackish-grey to dark brown bituminous, fine-leaved, somewhat sandy marl slate, that lies on the profile of the strata, alternating with storage light brown (max. 4 mm) and darker layers (rarely over 2 mm). The lighter layers get darker while keeping their fine-plate character. The shale has a dark-grey to brownish tone, alternating more rarely with light grey shades. Blue fittings are relatively common, as are wood and fish remains (bones and scales). The younger strata with the fresh outcrops develop on a series of several meters thick wall, that splits into fine paper slates when weathered. The shale is the most abundant rock fraction in these strata, while their carbonate content ranges from 26%-58%, averaging 40.2%. Bituminous claystones with green clay march engagements are present in the edge facies of the Sachrang shale, called the Unken shale. There is no clear separation between "manganese shale" and "bituminous shale" in the main localities of the formation, because the bituminous content fluctuates with the high manganese content. The Unken shales on the Bächental locality is layered on