Posidonia Shale

Sachrang Formation
Sachrang Formation
Stratigraphic range: Early-Late Toarcian; ~ H Sin. Plie. Toar. A B B C Ox. Ki. Ti. Possible Latest Pliensbachian records
	Posidonia Shale (Posidonienschiefer) outcrop
Type	Geological formation
Unit of	Altena Group (Netherlands); Black Jurassic (Germany); Northern Calcareous Alps (Austria);
Sub-units	Dactyliocerassandstein; Dörnten Member; Lehmhagen Member; Schistes bitumineux (Luxembourg);
Underlies	Jurensismergel Formation (Germany); Klaus Formation (Austria); Marnes à Bifrons Formation (Luxembourg); Glashütte Formation (Germany); Werkendam Formation (Netherlands);
Overlies	Aalburg Formation (Netherlands); Amaltheenton Formation (Germany); Scheibelberg Formation (Austria); Wolgast Formation (Germany);
Lithology
Primary	Black shale
Other	Lime mudstone, nodular claystone
Location
Region	Western & Central Europe
Country	Germany, Netherlands, Austria, Switzerland, France, Luxembourg
Extent	Northern Limestone Alps; North German basin; South German Scarplands;
Type section
Named for	The village of Sachrang, Bavaria
Named by	Jacobshagen
Location	Border with Tyrol above Sachrang
Year defined	1965
Coordinates	47°41′N 12°14′E / 47.69°N 12.24°E
	Posidonia Shale (Germany); Holzmaden, location of the main outcrop of Posidonia Shale

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.