Jurassic geologic formation in Europe
The Ciechocinek Formation (also known as the Gryfice Formation at Suliszewo), known in Germany as the Green Series/Grimmen Formation (German: "Grüne Serie") is a Jurassic (lower Toarcian) geologic formation that extends across the Baltic coast, from Grimmen, Germany, to Lithuania, with its major sequence in Poland and a few boreholes in Kaliningrad. It represents the largest continental area defined as deltaic in the fossil record, estimated to cover ~7.1 × only in the Polish realm. It is mostly known for its diverse entomofauna, composed of more than 150 species of different groups of insects, as well as its marine vertebrate fossils, including remains of sharks, actinopterygians and marine reptiles, along with terrestrial remains of dinosaurs, including the early thyreophoran "Emausaurus" and others not yet assigned to a definite genus. Its exposures are mostly derived from active clay mining of a dislocated glacial raft with exposed Upper Pliensbachian to late Toarcian shallow-marine sediments. Starting with coarse and fine sand deposits with concretions, the pure clay of the Ciechocinek Formation, after the falciferum zone, was deposited in a restricted basin south of the Fennoscandian mainland. It hosts a layer full of carbonate concretions, where a great entomofauna is recovered.
The Ciechocinek Formation is the sister unit of the Sorthat Formation of Bornholm, being its frontal brackish system (measured thanks to the presence of phyllopods and absence of Echinoderms and other stenohaline invertebrates), and the Lava Formation of Lithuania (that represents a more brackish setting at the east), a foreshore setting of the deltaic/lagoonar depositions of the Sorthat, located at the south of this last one, and sharing material between both due to the presence of a measured deltaic system that developed between the two units. The Ciechocinek Formation was, in the late Toarcian a depositional area located north-eastern margin of the North German Basin, where the Sorthat Formation (Bornholm high, Fennoscandinavian coast) and the northern part of the island of Rügen (Ringkøbing-Fyn High), both to the north, provided the terrestrial elements of the Ciechocinek Formation taphocoenosis. The Posidonia Shale, deposited mostly on nearby deeper parts of this basin interfinger with the Ciechocinek Fm in the western parts of the states of Mecklenburg-Western Pomerania and Brandenburg. Its main equivalents are the Posidonia Shale, upper part of the Rydeback Member, Rya Formation (Southern Sweden), the Fjerritslev Formation (Danish Basin), the Sorthat Formation (Bornholm) or the Lava Formation (Lithuania). There are also coeval abandoned informal units in Poland: Gryfice Beds (Now fused with the Ciechocinek, Pomerania region), Lower Łysiec beds (Częstochowa region), or the "Estheria series".
History.
The clay pits of Dobbertin have been exploited in the past, being the most famous being the Schwinzer Hellberg clay pit. This clay layer appear on the northeastern slope of Hellberg, and was first found and excavated out in 1879 by the Rostock geologist Eugen Geinitz, and being recognized as such. The sediments of the Ciechocinek Fm in Grimmen where found in 1873. The first described deposit consists on several Grey, Plastic Clay from a 300 m railway cutting near the village of Schönenwalde, at at the north of Grimmen. The mined clay of Klein Lehmhagen and Dobbertin was used as an addictive for concrete production. The first fossils, mostly ammonites (that allowed a more precise datation) and insects where recovered in 1894, where Geinitz related the local finds with the southern liassic shales that he studied at the same time, yet he was surprised about the abundance of phyllopods and coeloptera elytrons in the sediments, suggesting a more freshwater/terrestrial influence. Latter, using the ammonites as a reference, it was established in 1909 that the Grimmen Pits were different, but the regional equivalent of the, present in mostly southern Germany and pelagic, Posidonia Shale (being both liassic in age). Also, some of the sediments were vinculated with finds of the Lias of Bornholm (Moslty the Hasle Formation). In 1954 a first drill process was done in Ciechocinek, where the geology of the zone was related to Jurassic sedimentation, although a concrete datation beyond lias was not possible. That first drilled borehole would become latter the main pit of the Formation. It wasn't until 1958, when the eminent Geologist Stefan Zbigniew Różycki proposed the name Ciechocinek Series, that was left temporally as an informal unit. Różycki was the first to study in-depth the strata recovering the formation, that he called "Seria Ciechocińska" (Ciechocinek Series), and report a structure based on clay rocks, represented by mudstones, claystones, and shales with the find of clays with quite a high kaolin content. It also does the first comparation with the slightly younger Seria borucicka (Borurice Beds), and estimate that the rocks were approximately of Late Liassic in age, with the possibility of finding Dogger Sediments on the uppermost parts. Also, suggests that they were analogue to the Ostrowiec series of Świętokrzyskie Mountains. Studies on the region found that the Clay sediments found in Grimmen had a wider distribution on the surrounding areas, which led to the opening of a Klay Pit near Klein Lehmhagen in between 1959 and 1961. After that year, the pit extension grown, allowing to study it deeper and have detailed insights into its sedimentology, as well in the Dobbertin pit, being both compared in depth. It was found that this strata had depth and clear glacial deformation, with several layers displaced as effect of glacial erratics. Ammonites found on the layers of Dobbertin where identified as Pliensbachian in age, yet it was proven that they were Toarcian ammonites latter. Later works use the name Ciechocinek Series for the Polish Basin sediments, doing studies about sedimentology on the layers already Drilled, or founding new Boreholes with similar composition of Kaolinite and related materials. Was in the 1960s, when the name "Formazaja Ciechocińska" was first suggested, at the same time the stratigraphy of the formation led to exclude Middle Jurassic Strata, being linked now in first instance to the Posidonia Shale of Germany. At this time, the Kaolinitic content was related to a depositional setting based on a large deltaic succession, and compared to large modern rivers, suggesting a tropical climate for the formation. In the late 1970s was recovered as the Toarcian succession of the Polish Basin, linked in age to the Posidonia Shale and to the deposits of Dobbertin and Grimmen of the same age. The presence of the Posidonia Shale in the German realm was quoted until the 90's, yet this deposit is dominated by siderites &amp; mudstone and the true Posidonia Shale is limited to southern pelagic deposits. The clay pits of Grimmen and Dobbertin ended with different destiny: the first one was closed in 1995, and has filled with freshwater since 2002. The Dobbertin pit however, has been under protection as a geological natural monument since 1991, as the exposed layers are considered an exceptional land–sea facies distribution during the Toarcian and also its international fame, due to its wealth of fossils from the northern margin of Fennoscandia. In the 2000s the greatest part of the work on the Formation was done, studying its geology, stratigraphy and sedimentation. The German layers where on several recent works classified as part of the Ciechocinek Formation.
Toarcian material found in glacial Erratics in Ahrensburg and the Hagen Forest has been in controversy due to its dubious origin, being linked with the Rya Formation and Sorthat Formation, as well this unit. They were originally considered or local or Baltic in derivation, but that changued with the recovery of erratic concretions in the Baltic sea cliffs near Lübeck, being found as part of the Weichselian Glacial Maximum. Liassic–Cretaceous sediments in the assemblage are most probably associated with the tectonic Sorgenfrei–Tornquist Zone. The origin of this erratics from southwestern Baltic, Poland or Danish archipelago is unlikely, as those zones are dominated by Late Cretaceous–Paleocene strata, suggesting that this Toarcian assamblages should come from south/SW between STZ–TTZ and the German Baltic coast. The most clear hint link this deposits with the Ciechocinek Fm, as they're identical in fauna and facies composition of Grimmen and Dobbertin, also affected by subglacial erosion and thrusting, suggesting a close stratigraphic and palaeogeographical origin.
Lithology.
The Ciechocinek is composed mostly by muds and silts, along with poorly consolidated Mudstones and Siltstones with lenses and subordinate intercalations of fine-grained sandy lenses, typically from 1 mm to less 20 cm thick, extended a few meters, along also with sandstones. Diagenetic siderite intercalations and concretions, around 20 cm thick, as well as pyrite concretions are also present. The lithology of the Pits consists mostly on gray-yellow and red-brown clay marl follow over pyrite and gypsum-bearing clay stones of the upper part of the formation strata, that have abundant "marl" concretions, called "Amaltheentone". The mineralization of siderite occurs in all types of Ciechocinek Formation deposits. Include fine crystalline variety, coarse crystalline rhombohedra and fine crystalline aggregates impregnated by a mixture of undetermined iron hydroxides. The mineralization of the siderites on certain layers is associated to the abundance of organic matter, with the decay of it ending on a progressive carbonate supersaturation, with an early precipitation on the nucleus of the siderites. They reflect also the conditions of salinity on the water, showing variances in short periods, associated with the abundance of organic matter. Siderite spherulites have also been recorded on heaps near Kuraszków and also single spherulites or groups of several appear in the "Brody-Lubienia borehole" among the light gray mudstone. The strata is filled with mostly marine fauna in the German realm, Belemnites, Bivalves, Gastropods, whose hard parts become lithified and part of the deposited grains. There is silty to fine sand and weakly carbonatic, mica-bearing facies at the southernmost point. The lithology of the layers is dominated by claystone and clay, along very fossiliferous mudstones, brown cemented sands, siltstone, sandstone and coarse to very sorted sands on the lowermost and uppermost levels. Deeper lithological evaluation subdivided lithology into sections of greenish to grey coarse sand, argillaceous silt with dark clay streaks and fusain (charcoal) particles, dark clay layers with lenticular lamination, limestone concretions with calcitic core and sideritic mantle, pyrite concretions and finally the pure greenish clay layers. Muds and mudstones consist mainly of particles with the size of common clay, some with an admixture of silt; sandy muds and mudstones are also encountered in counterpart to the pure clays and claystones, without admixtures of other fractions, that are rare or extremely rare. Mudstones are poorly consolidated, tend to disintegration, swell and become plastic, being the only exceptional ones those the heavy sideritic mudstones, that range from a wide variety of colors, from brown or cherry-red in colour, with an olive shade often appearing. On the other hand, there are abundant poorly consolidated quartz sandstones, that are regularly intercalated with silt and siltstone. Along the sandstones, there are chlorite and kaolinite grains. Plant microremains are common, concentrated with mica flakes on lamination planes, while larger wood fragments, several centimeters long, are also moderately frequent. Heavy minerals are present and include as the most common euhedral forms such as zircon, rutile and tourmaline, along with angular fragments of rutile, garnet and staurolite, with finally well-rounded oval grains of zircon, rutile, garnet and tourmaline. Outside the main heavy minerals, it is possible to find well-rounded grains of green hornblende, apatite, disthene, epidote, and finally on one study one pyroxene grain was found. With the exception of thin limestone layers, mostly are cemented. Other minerals found include illite, quartz, calcite, feldspar and smectite, as well in less extent Charcoal. There is also a high content and proportion of unstable heavy minerals (pyrolusite, manganite, birnessite, todorokite and rhodochrosite) in the Toarcian clays, that indicate basaltic volcanism sediments, translated probably from nearest inland strata, such as the coeval Djupadal Formation. The local vulcanism started in the late Pliensbachian, and extends along the North Sea and mostly from southern Sweden. At this time, the Central Skåne Volcanic Province and the Egersund Basin expulsed most of his strata, with influences on the local tectonics. The Egersund Basin has abundant fresh porphyritic nephelinite lavas and dykes of lower Jurassic Age, with a composition nearly equal to those found on the clay pits. That reveals the translation of strata from the continental margin by large fluvial channels, that ended on the sea deposits of the Ciechocinek Formation as they are present on the Sorthat Formation. The three-dimensional clay of the Ciechocinek