Posidonia Shale

The Posidonia Shale was originally referred to as the Schwarzjura lias was first recovered from the Franconian Jura, that borders to the northeast on the Obermainisches Hügelland and the Oberpfälzisch-Obermainisches Hügelland, tectonically part of the Faulkschollenland. The Franconian Jura rocks recovered are west of the Saxothuringian basement bordering the Franconian Line. It is recovered laterally extensive within Germany belonging to the early Toarcian Central European Epicontinental Basin, that evolved gradually from low tophography tidal flat to floodplains to a shallow shelf sea with intermittent N connection with the Viking Corridor and the Proto-Atlantic Ocean and with the Tethys Ocean towards the S, that filled with seawater the area, subdivided in several subasins with heterogeneous conditions and biotas, from the Netherlands to the Tirol Area. The CEB stablished a relatively shallow transcontinental seaway between the Tethyan and Boreal Arctic Sea biota, thus mixing cold and hot waters. In the Mesozoic was marked by the breakup of Pangea during the Late Triassic, what led to the appearance of the early Atlantic connecting with the Boreal waters of the Panthalassa Ocean, and several marine to continental subasins locally. Developed by the sinking of the southern German Jura Plateau during the Miocene, it separates the higher, older crystallization of the 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 which flooded during the early Toarcian and reemerged during the Bifrons substage with a changing coastline, thanks to rhythmic uplifts and subsidence of older Paleozoic and Triassic siliciclastic deposits from the east. The granites and gneisses resulting from crystallization were eroded from the Paleozoic exposures on the east, and 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 bituminous facies overlie the Pliensbachian, and are mostly clay marl to marl shales with an organic carbon content of over 2%, with some levels referred as "bitumen-free" or "bitumen-poor". The lowermost sequence is known as Seegrasschiefer (seagrass slate), appearing just above the limit, being actively burrowed horizons, approximately 15–20 cm thick with clay marl stone appear in the darker, brownish to gray, bituminous clay marl/marl slate. This initial horizons have abundant foraminifera and ostracods as the medium to light gray color without a brown tint. On the Unken Syncline near Lofen, basinal deposits with abundant Aragonite and Calcite helped to know the major Jurassic basin geometry, where on several layers of the same age was complicated due to the posterior Alpine deformation. Correlated Unken and Diessbach basins developed mostly during the Toarcian, with deposition of abundant material from the near Emerged Landmasses. On the Unken Syncline, the breccias associated with the normal faults were deposited until Oxfordian age. == Lithology ==

The black shales are the main part of the strata present, with a major composition of bacterial origin. The shale is made represent blackish gray to dark brown bituminous, fine-leaved, somewhat sandy marl slate, that lies on the profile of the strata, alternated with storage light brown (max. 4 mm) and darker layers (rarely over 2 mm) characteristic. There is a relatively common presence of blue fittings, as well as Wood and Fish remains (Bones, scales). Dinoflagellates are the major organic component and the most abundant microfossils. Manganese is present, such as in the Toarcian deposits of Hungary. Those are completed by the marl levels, composed by lithoclasts. quartz and smectite are the main minerals, along with illite, chlorite, and plagioclase in minor amounts. Bächental bituminous marls consist mainly of quartz and carbonate minerals. Isorenieratene derivatives are highly abundant on this level, related to several processes such as sedimentary iron, influenced by anoxic conditions. Rhodochrosite and manganese rich calcite are present in the manganese levels, while the Black Shale levels are rich in Pirite. The lower matrix is composed by clay and carbonate minerals, such as muscovite and feldspar. The presence of altered Celadonite, suggest volcanogenic solutions as the most probable source, where the high amounts of dissolved manganese of continental origin was translated to the epicontinental margins of the Tethys. On the Bächental bituminous marls had a bulk mineralogy where the Calcite is the most abundant fraction (49%), followed by Phyllosilicates (35%), Quartz (11%) and Pyrite (5%). While the Clay mineral distribution includes a large amount of Illite (51%), Montmorillonite (40%) and Kaolinite (9%). == Dating ==

According to sedimentologic and palynologic features, a tidally influenced transgressive development within the Lower Toarcian is inferred with increased continental matter being moved to marine areas, causing anoxic conditions, with the Posidonia Shale being the reference formation for this interval. The Posidonia Shale of Dotternhausen and Schesslitz is well dated on the basis of ammonite and microfossil biostratigraphy. The Lower Toarcian sections are subdivided into three ammonite biozones (Dactyloceras tenuicostatum, Harpoceras falciferum, and Hildoceras bifrons) and several subzones. The study of the different layers and strata of the Posidonia Shale has given different data about the chronology of the formation. Dormettingen shales have been calculated biochronologically and with isochron data, giving an approximate age of 183-181 million years, being close to the Pliensbachian boundary based on the recent revisions of the Early Jurassic Subperiods. The Toarcian and the Pliensbachian are considered as strongly constrained in terms of chronology, where the deposition has been estimated to have lasted 3.2 Myr in the South Germany Basin, with the uppermost sequences estimated to be Bifrons in age. The Posidonienschiefer lasts until the Late Toarcian (Variabilis Biozone) in the NW German Basin with the "Dörntener Schiefer", while it mostly disappears in the SW, substituted by the Jurensismergel Fm, with few deposits where it lasts (Wutach area, Nürtingen). == History ==

The Posidonia Shale has been a focus of scientific interest for the last 100 years. The first fossils were recorded in 1598 by the medical doctor Johannes Bauhin, who interpreted the local ammonites as "metallic things" in rocks and as "miraculous tricks" of nature, while the crinoids where interpreted as either huge flowers or heads of medusa, and evidencie of the biblical flood. Many people did important geological and paleontological research on the Swabian Posidonia Shale, including Carl Hartwig von Zieten (1785–1846), Eberhard Fraas (1862–1915), Bernhard Hauff senior (1866–1950) and Adolf Seilacher (1925–2014). The first insight on the flora was done in 1845, with partial leaf fragments. Boué in 1829 did a study of the general geology of the Jurassic along Germany, recovering limestone and shale facies, with a superficial assignation of what he considered most of the main Jurassic Strata, without classifying the layers on a concrete subperiod. Further geological work was then carried out, recovering examples of marine facies representing various biomes, all associated with black shale deposits in other areas, as towards the NW or at Regensburg. While in 1921, the 1st major fossil inventory was done by Hauff, reporting exquisite specimens, most of them from Holzmaden and some of them nearly complete, including Ammonites, Fish and Marine reptiles, such as Plesiosaurs and Icthyosaurs. Hauff described in 1938 "Acidorhynchus" (Saurorhynchus), the latest surviving of the Saurichthyiformes. In 1953, an impressive Insect fauna was revised in the Northern outcrops. On 1978, Wild described the First and only know Dinosaur Fossil from the formation, what he named Ohmdenosaurus, a small sized Sauropod. Latter works revisited the excepcional preservation of the biota, specially the presence od soft parts. The lithology and sedimentology of the formation was revisited, with several suggestions such as stagnant basin models and restricted open marine ones, all suggested to be deposited on a shallow epicontinental sea. With the addition of multiple new references, the expansion of information thanks to the revision of profiles, boreholes and other outcrops, new works on the characteristics of the deposition, the type of environment and the conditions that led to the exquisite preservation were produced, where paleocurrents where found to be nfluenced from the north and the south of the Central European Basin. The Black shale deposition was found to be related with changues in the oxygen levels. Thanks to the renewed information, a new cycle of publications reviewing the microfacies took place between 1980-1990. The most important works of the XX century where done by Riegraf in 1985-86, being a complete review of all aspects of this formation, updating multiple points based on all the information compiled throughout the century: lithology, stratigraphy, biota list and ammonite biozonation, followed by a focused work on a complete mapping of the microfacies composition and extent of the shale deposits. == Paleogeography and paleoenvironment ==

The Posidonia Shale was located in the SW and NW Germanic basins, as part of a shallow epicontinental sea, surrounded and influenced by various highs and emerged lands that provided most of the terrestrial matter found along the Formation. The main outcrops of the formation are disposed along the modern southern Germany, recovering the locations of Holzmaden, Ohmden, as well at Niedersachsen, and others appearing along the east, such as the related to the Banz Abbey strata or Regensburg. Connected to the SW German basin where the Paris Basin, that recovered central France, with correlated sedimentation to the Shale deposition on Germany, also sharing a epicontinental sea, bordered by carbonate facies, specially towards the south. At the north, the Wenzen Well report little deeper basinal settings, heavily influenced by continental matter coming from the main continental land present anywhere nearby the formation, Fennoscandia. In this area, the main emerged units present was the Rhenish High at the west, being a small land of the size of Sicily, and on the east, the N Bohemian Massif. The Bohemian massif with the Southern Vindelician High represent the major emerged units present on the Central European basin on the Toarcian. The Germanic Epicontinental sea is considered to be an analogue, as compares well to the sedimentation rate in deep-water settings, of the Black Sea. Most of the outcrops (Holzmaden, Dotternhausen, Ohmden or Dormettingen) represent low-energy depositional environments, far from deltaic sediment sources. The black shales characteristic of this unit reveal a shallow marine environment, influenced by arctic and Tethyan waters, with marked episodes of disappearance of benthic biota. Also measure a change in carbon-isotope excursion in marine and terrestrial life, and was probably a perturbator of the carbon cycle. Global seawater has been proved to be approximately, for the interval of the negative carbon-isotope excursion, close to 1.45‰, less than modern values, with estimated 2.34‰. Waters interchange were one of the major effects on the palatine de-oxygenation showed on most of the Lower Toarcian Layers around the word, with the connection with the Viking Corridor as one of the main effects, due to the arctic waters freshening and breaking the oceanic circulation. The effect was consequently negative on the German realm, where the environments expose a tropical fluctuation, with conditions similar to the modern Caribbean Sea, which hosted a high variety of sea fauna, except on the bottom layers, where only a few genera were able to survive until oxygen conditions got slightly better. The changes on the benthic oxygen where common, with most of the animals dying without being scavenged by bottom-dweller organisms, and sessile life, with this biota limited to "benthic islands" associated with ammonite shells or vertebrate carcasses (Except some Polychaetans on higher oxygen conditions). The uppermost layers are marked by regressive sea levels, as it is shown on layers across Bavaria where major events set the fate of the nearshore environments. and adjacent units The main terrestrial environments of the Posidonia Shale are the near emerged lands where the Black Forest High/Swell (known thanks to strata containing fine sand in the tenuicostatum Zone, 'Glaukonit und viel Feinsand', at Obereggenen im Breisgau), located at 70 km at the west and the Ries Swell, W of Regensburg, then far towards the W the Vosges Massif is also suggested to be present (known by the abundant detrital quartz from the EST433 borehole located near Bure, Meuse). The Environments of this highs are assumed to have been trought phases os aridity and humidy marked by the Toarcian Oceanic Anoxic Event on the basis of Palynology. The Bohemian Massif was located in a relatively warm, precipitation-rich climate with Bavarian shallow areas receiving freshwater inflows from the east, which temporarily lowered the salinity of the seawater in the whole basin or in parts. In the south-eastern North-German Basin at Hondelange and Schandelah, the Posidonienschiefer was deposited in the "Oberaller Through", a local depression bordered by the emerged "Calvörde Island" and the submerged Altmark Swell to the N, while the shallow submerged Fallstein Swell closed it at the south, and more towards the E the Bohemian Massif hosted a large delta that discharged towards Oberaller. In Microfacies, after the Pliensbachian-Toarcian locally is observed a significant decrease in the Crinoid skeleton elements, also that of the Ophiurida; the Echinoids take their place, where really blossomed at that time, while Pedicellaria are observed very often. The main maceral found is Lamalginite, which may derive from thin-walled planktonic and benthic organisms, including Green Algae, Cyanobacteria, and Bacterial mats. There is a clear low frequency of Vitrinite and Inertinite, what suggests that terrestrial inputs of organic matter to be of less importance, although, the main part of OM contained in the basal mudstone, including charred material, was derived from terrestrial sources. This Mudstone contains charred organic material typically connected to Wildfires along with large amounts of expandable Smectite possibly derived from alteration of volcanic ash, what indicated a clear contribution of volcanic-derived detritus during deposition. There is measurements of reduction of the local salinity on the water where elevated inputs of freshwater due to an accelerated hydrological cycle resulted in a surface-water layer. These levels lack bituminous facies or are interspersed with them in profiles as one moves westward, indicating that they probably belonged to more coastal sectors with better oxygenated waters, with the full transition from shale to sandstone in Regensburg, Bruck and Naab areas considered as caused by a major regression of the sea level, marked at Irlbach by white-yellow levels indicating karst funnels or Cenote-like depostion. Lehmhagen Member . Animals are based on material found on the Lehmhagen Member, the Grimmen Formation and tracks of the older Drzewica Formation The Lehmhagen Member, designated after Klein Lehmhagen village, was previously referred to the Ciechocinek Formation and then to the Grimmen Formation, yet was in 2025 identified as a new subunit within the Posidonia Shale, notoriously the one with Dinosaur remains and continental biota, otherwise mostly unknown from the formation. It comprises organo-detrital shales interspersed with coastal heterolithes and sandstones ranging from fine to coarse grains. The type section is situated in the northern Grimmen clay pit. This sequence overlies an erosive boundary with upper Pliensbachian shoreface sands, transitioning upward from coarse coastal sands to finer organo-detrital clays and heterolithes, culminating with a sharp contact to the organic-rich clay. In the North German Basin axis, the member thins to a few decimeters, as observed in the Grambow 5 well. Organic-rich clays and heterolithes with concretions are also noted in Dobbertin. Biostratigraphically significant concretions, named for their index ammonites, mark this member. Dörnten Member The Dörnten Member was also named in 2025, despite being known before, and reflects a transgressive phase of the upper Posidonia Shale, overlaying greenish clays of the Grimmen Formation and fluvio-deltaic sandstones of the Glashütte Formation. Equivalent to the 'Dörntener Schichten' in the Salzgitter and NW Germany regions, it is characterized by fossiliferous, organic-rich strata. The reference section is in the Grambow 5 well, a 16 m thick sequence of dark grey laminated claystone. Ammonite records indicate a range from the upper bifrons to upper thouarsense Zone. The member thins eastward in the Reinberg 1E well and transitions upward into marine claystones of the Opalinuston Formation or fluvio-deltaic sandstones of the lower Glashütte Formation. == Economical value ==

The posidonia slate has been mined in the Holzmaden area for centuries to make wall, table and window panels. Other uses of the shale included be made into fireplace stones in Gomaringen-Mössingen until was replaced by Eifel pumice stone. At Dotternhausen, the ROHRBACH Zement company uses oil shale in the production of binders, mining in the 80´s up to 1,600 t. Shale oil, specially after World War II, when where burned on coal ovens was temporally obtained from the bituminous slate through smoldering and distillation by oil works near Reutlingen; Frommem or Holzheim near Göppingen, yet this turned out to have low profitability and the fact they produced a lot of slag and sulfur-containing exhaust gases, production lasted not long. Recent studies have shown that the petroleum generation potential of the PS is high in all studied regions due to the high TOC and Hydrogen Index. However, differences exist which can be expressed by SPI values. The latter are highest for northern Germany, where the PS is richest in TOC and has the highest HI values combined with a thickness of 30 to 40 m at most places. Since the first serious evaluations in the 2000s, different organic samples were extracted to revise the changes and potential presence of the Shale Oil on the main quarries of the southern realm. Based on several core samples with abundant organic material (Dinoflagellate cysts and other microorganism fragments, such as microscopic algae) different thermal maturity has been found, especially on the samples from the Hils Syncline strata. The maturation of this strata has implied losing organic carbon and loss of hydrogen index values. Beyond that, the status of the samples has been stable during at least 40 measured years. == Paleontological significance ==

In addition to their Posidonia bronni, the shales contain some spectacularly detailed fossils of other Jurassic sea creatures—ichthyosaurs and plesiosaurs, spiral-shelled ammonites and crinoids, or sea-lilies. Urweltmuseum Hauff The Main Museum with the taxa Found on the Posidonia Shale, the Hauff Museum recovers the best specimens found in the last 150 years, and it is situated on Ohmden. The museum has several halls with different kinds of fauna found on the layers of the formation, where the vertebrate specimens are exposed on the main parts, including on those Icthyosaur remains and several fishes. The Museum has the world's largest colony of sea lilies, measuring an approximate size of 100 square metres. Rolf Bernhard Hauff is the actual director of the museum. ==See also==