Ciechocinek Formation

Schwinzer Hellberg clay pit Brickyard (Mecklenburg) on the 1900

The Green Series (German: Grüne Serie) comes from a series of lower Jurassic sediments found in 1873. The first described deposit consist on several Grey, Plastic Clay from a 300 m railway cutting near the village of Schönenwalde, at 4 kilometres (2.5 mi) at the north of Grimmen.[10][11] Using the ammonites as a reference, it was established in 1909 that the Grimmen Pits were the regional equivalent of the mostly southern Germany Posidonia Shale. Also, some of the sediments were vinculated with finds of the Toarcian of Bornholm.[12] During the 1950-60 there was recorded a wider distribution of the Grimmen Clay sediments on the surrounding areas, what ned to opening a Klay Pit near Klein Lehmhagen in between 1959–1961.[10] Since that year the extension of the former pit grown, giving detailed insights of the sedimentology.[13] The liassic strata shows a clear Glacial Deformation, with several layers being deposited as a result of last Glacial period erratics.[10] The mined Clay of Klein Lehmhagen was used as an addictive for concrete production.[14] Was closed in 1995, and become to fill with freshwater since 2002.[10] Later, the Green Series was established as part of the Ciechocinek Formation, and referred to as such in later works.[15] Other of the related strata are the clay pits along Dobbertin, with the most famous as the Schwinzer Hellberg clay pit.[16] The Toarcian sediments & occurrences in the Schwinzer-Hellberg appear on the northeastern slope of Hellberg, and was first found and scavated out in 1879 by the Rostock geologist Eugen Geinitz, being recognized as such.[17][18] After that, Geinitz did various research on the Pit,[19] with studies about the fauna and fossil content, mostly on the Ammonite fauna.[20] He compared it to the Posidonia Shale that he had studied at the same, and stimated a Toarcian age for the Pit, based mostly on the Ammonite taxa.[21] Other studies pointed out that the strata of the Pit contained also Pliensbachian taxa, with incorrectly classified Ammonites, mostly of Lower to Middle Toarcian, with others from the Latest Pliensbachian.[12] Was Oertel on several studies who divided the strata delimitating it to the late liassic (Pliensbachian-Toarcian) based on the Ammonite Fauna.[16] The former clay pit 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.[16]

Brigde over the Schwinzer Hellberg clay pit

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 strata is filled with mostly marine fauna, 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 Posidonia Shale facies at the southernmost point.[22] Along with that, there is abundance of layered blue-gray, lime-free Clays of the "Green Series" with lime concretions that contain a rich fauna, such as Ammonites, Bivalves, Insects,[23][24] Crabs and fishes of several orders, all from the Toarcian, since the Pliensbachian strata is not part of the Green Series.[25] Other pits, like the Grambow well, at 9 km distance from the Dobbertin pit, shows stratification on a 70–120 m thick succession, that is composed by an almost monotonous dome with changing Clay, Silt and fine Sand fractions, while in western and central Mecklenburg closes with a blue-gray, locally also greenish, brown-banded Claystone horizon.[25] It follows more at the south the Toarcian black-brown to dark gray-colored, Posidonia shale.[25] The “Green Series” is presented locally with bold clays that begin in Altmark and W-Mecklenburg with an extension to the east of over 100 m thick. There is also a high content and proportion of unstable heavy minerals in the Toarcian clays, that indicate basaltic volcanism sediments, translated probably from nearest inland strata.[26] The local Vulcanism started on the lower Pliensbachian, and extends along the North Sea and mostly from southern Sweden.[27] At this time, the Central Skåne Volcanic Province and the Egersund Basin expulsed most of his strata, with influences on the local tectonics.[27] The Egersund Basin has abundant fresh prophyritic 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 Green series.[27]

Schwinzer Hellberg clay pit. Ichtyosaur, Insects and several indeterminate vertebrates have been found here.

The Dobbertin & Grimmen Clay Pits are the main ones, where the Liassic rocks are known only from deposits in the Pleistocene regional modifications.[28] Reflection by seismic measurements show that the upper edge of the Mesozoic layers is at a depth of approximately at 500 m.[25] These are highly deformed sediments of the lower Toarcian, that in contrast to the typical Posidonia Shale in Central and Western Europe, is filled by plastic Clay facies, that get the name of "Green Series". This clay is considered in part as the weathering product of an early Jurassic Alkaline-Basalt volcanism that developed in Skåne,[29] that was transported from the Fennoscandian continental mainland more than 100 km away on the sea through large rivers to the adjacent epicontinental marginal area of the North German Basin.[22] The coastline of the main continental series was divided by Flat islands, Bays and Lagoons. Minor changes in salinity led to more brackish conditions, that result on a lack of abundant marine fauna, such as Echinoderms, Belemnites. There is a glacial deformation of the sediments, that showed thanks to the now low erosion conditions, preventing a detailed recording of the entire sequence, which depends essentially to be reconstructed by the fossil content of the concretions.[22] In the western part of the pit, open, fine-grained Quartz sands with traces of charred plant remains have been interpreted as interglacial moved strata of the formation or rearranged Tertiary material.[30] But it could also be higher areas of the "Green Series" and thus represent an important layer in the Toarcian sequence, impliying a descent of the water levels trougth the Middle Toarcian.[22] Several other pits on the north German Basin, make possible to correlate this and other erratic deposits, using the stratigraphic sequence present, common to most of them.[25] On the Grimmen & Dobbertin Clay Pits, Tertiary coverage over the upper lias strata is already lost, while in Goldberg the strata thickness has decreased due do recent erosion.

Professor Stefan Zbigniew Różycki in Hungary, 1966. Różycki named the Ciechocinek formation in 1958.

Several works were done in the 1920s on the Jurassic sediments of Poland, where the main stages, specially Lias and Dogger where stablished. At the same time, facies along the Town of Ciechocinek were classified as possible Lias-Dogger transition, without anything more clear after this.[32] 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.[33] 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.[2] 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.[2] Also, suggests that they were analogue to the Ostrowiec series of Świętokrzyskie Mountains.[2] Later works use the name Ciechocinek Series, doing studies about sedimentology on the layers already Drilled, or founding new Boreholes with similar composition of Kaolinite and related materials.[34] 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.[35] The first Polinic studies where done.[36] And in 1966, the name was already established. 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.[37] The First Floral analysis came from the comparison with the Borurice Formation in 1973, finding pollen of Lycophyta and others assigned to Isoëtaceae.[38] 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.[39] A major floral analysis was done in 1980, where Rogalska described an amplious variety of flora based on microparts of leaves and pollen, but using modern genera to describe its finds, such as Cedrus.[40] Several works were done related to tectonic changes that affected the sediments in 1988, relating for the first time the formation with possible regional vulcanism.[41] In the 2000s the greatest part of the work on the Formation was done, studying its geology, stratigraphy and sedimentation.[5][6] And in the 2010s was related to the Northern German Deposits, that were confirmed as part of the formation in various studies in 2017[42] and 2018.[3]

Wapienno Cement Quarry, where strata of the Formation emerges. Plant matter & Fish Teeth have been found here.

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, ~20 cm thick, as well as Pyrite concretions are also present.[5] 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 Fehydroxides.[6] 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 alternances in short periods, associated with the abundance of organic matter.[6] 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, tren 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.[5] On the other hand, there are abundant poorly consolidated Quartz sandstones, that are regularly intercalated with Silt and Siltstone.[5] 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 cm long, are also moderately frequent.[5] 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.[5] The presence of Kaolinite, related to continental formation in tropical climate environments for the chemical weathering of different igneous, metamorphic and sedimentary rocks, along with Chlorite & Biotite, suggest a deposition linked to fluvial & deltaic forces. The eastern Sudetes and their foreland are the best candidates to be the origin of the transported material, as they have a nearly identical composition, being the Lower Jurassic deposits in the Fore-Sudetic Monocline linked Cracow-Czêstochowa Monocline, with the strata of the Formation.[5]

Czatkowice Cement Quarry, with exposed strata of the Formation

Deposits of the Ciechocinek Formation are related to a brackish-marine origin, being deposited in an epicontinental sedimentary basin on Poland, that was the eastern arm of the Mid-European Toarcian Basin.[5] Sedimentological analyses pointed to a deposition in a shallow basin, prevalently at <20 m deep, with present features of a large, shallow, brackish embayment.[43] The Structure of the main depositional setting has been seen divided in 3 parts: on the center, near Kaszewy Kościelne there was a major restricted brackish-marine basin, with seasonal influxes of marine water.[43] Around it, a series of restricted brackish-marine embayments where developed, leading to a transitional environment, formed by Lagoons, Dune Barriers, Delta Fronts, Delta Plains and Marshes, influenced by brackish-marine water.[43] All of this was influenced by deltaic facies, that has its origin on marginal parts. Well preseved estructural successions along the strata show that the depositional environment of the formation was strongly influenced by storm periods.[44] During that storm periods, paleocurrents transported sand and other components from nearshore to distal settings, being after that reworked by waves and distributed along the sea floor, by several wave related flows.[44] Distal settings show the influence of the storms with due to the presence of several lamintaed deposition of sand-silt streaks, cross-laminated Silt, small Sand lenses and interlaid Mud-Silt-Sand Heteroliths.[44] Near Settings are represented on the strata by several centimetre thick layers and sand packets, with several traces and sedimentary structures, such as wave parallel and crossbed ripplemarks. Along this strata, trace fossils are deposited on settings where geochemical analises show the presence of a brackish water influence on the basin.[44] The main component of the deposits are muds, intercalate sandstones & silts disposed on a high variety of structures, with at least twelve lithofacies that run from fine-grained sedimentations to wave-dominated sandy shoals. It is believed that the dominant sedimentation was by quiet mud and silt sedimentation from suspension, which probably come from near River Mouths. It was a sedimentary basin with a continuous influx of Clay and fine Silt, to which sand was brought by events such as river transportation, storms, eustatic effects, and other related.[45] There is a detailed succession of changes in the strata with time, where the location of the river mouths shifted because of effects such as the progradation of deltas, the stop of the flux of river channels and the long-lasting eustatic sea-level changes, all of that can be confirmed by the presence of microstructures as result of erosion and cross lamination in the mud-silt facies.[45] A Marine transgression correlated to the rise of the sea level on the Lower Toarcian is clearly visible on the Silesian-Cracow area, where the sea encroached the Polish Trough, which resulted in the end of the Pliensbachian alluvial sedimentation, that can be observed on the underlying Blanowice Formation. The presence of alluvial sedimentation still on the nearshore environments, as an effect probably related to floods.[45] On some layers it was observed that the chemistry of the sea water was changing as the deposition continues on the most recent layers of the Ciechocinek Formation, implying the possible formation of a brackish environment, probably due to the sedimentation of fluvial currents. Most of the invertebrate traces reported from the formation, come from these layers.[45] It contains several marine dinoflagellate cysts, Foraminiferal linings and a relatively di verse trace fossil, such as Planolites, Palaeophycus, Helminthopsis, Gyrochorte, Protovirgularia, Spongeliomorpha and Diplocraterion associated with pyrite mineralization.[45][44] On the upper levels of the formation there is a clear change in the sedimentation: a sea regression with a palatine alluvial progradation. The sea left seashore lakes, lagoons, deltas, Mangroves, with the emergence of the land confirmed by the occurrence of plant roots and desiccation cracks.[45] This is also shown on the changes on the fauna present, where the trace fossils get more abundant, specially Planolites and there is appearance of phyllopods of the genus Estheria, reflecting decline in water salinity.[44]

The organic matter found includes the oldest known Biomolecules (Labdanoic Acid, Ferruginol, Sugiol and 7-Oxototarol) from the “Blanowice brown coals”, what probe the presence of abundant Wildfires and/or peat fires on the formation, with the Cupressaceae and/or Podocarpaceae families the main peat-forming plant species.[51] Posterior revision of the Lignites of the Brown Coals had revealed a major distribution of Benzohopane derivatives in these coals and surrounding sandstones, that implicate probable differences in the degree of Biodegradation, and also a low Coalification range, typical of Lignites.[52] Later largers studies cover a really big influence of the fires on the region.[43] After the Toarcian Anoxic Event on the called "Kaszewy-1" (Where the Toarcian makes ~150 m of the strata) the Wildfire activity was widely recorded.[43] The great abundance of Charcoal is the main indicator of the fire activity locally, but also the Polycyclic Aromatic Hydrocarbons, whose abundance reflects an increase in wildfire activity.[43] Coarse Charcoal particles abundance is low, while the fine Charcoal particles are more abundant on nearly all the measured samples, vinculated to small reductions of the Sea Level locally.[43] The most abundant Polycyclic Hydrocarbon found locally is Phenanthrene, and along the Charcoal data shows how the Fires locally increased around the Carbon Isotope Excursion on the Toarcian Anoxic Event Worldwide.[43] Along this period, mostly of the strata of the region shows at least 6 periods of fire intensification, that are coeval to anothers found on Yorkshire, Wales and Peniche.[43]

Sporomorphs have been found, with Minerisporites richardsoni as one of the most abundant, being a genus related to Isoëtaceae. Other examples of flora include Ferns, Bennettitales ands Cycads.[53] On the recent layers there are more flora as a response to the fall of the sea level, with the presence of larger wood fragments (Protocupressinoxylon & Podocarpoxylon), up to 1 m long.[45][47] There is a high dominance of spores in the whole Polish Basin is observed on the Lower Toarcian strata, with average only 20% of bisaccate pollen grains against 80 % of spores, contrasting with the other strata from older intervals of the Early Jurassic.[54] The presence of abundant spores is related to palaeoclimatic factors, as sediments show that the climate at that time was much warmer and much more humid (with an small exception on the Tenuicostatum biochronozone) than in the Hettangian-Pliensbachian interval.[54] The bisaccate pollen grain/spore ratio in the Lower Toarcian deposits in Poland is always strongly biased towards spores, which dominate even in the brackish-offshore settings.[54] The dominance of Spores was probably associated with regional climate fluctuations, associated with proximity of the West European Sea in Pomerania, and more continental climate related to higher altitudes in the East.[54] This finds are related with the Pollen and plant remains found on the Chinese Hsiangchi Formation (Also Toarcian), pointing warm and humid climate, that changed after to a drier conditions, as observed on the Quaidam Basin.[54]

The German realm shows to be a mostly marine unit, composed by Basinal Marine to marginal marine deposits, with washed fauna from the continental deposits, where the anoxic oceanic bottom waters prevented high predatory behavior on the carcasses of most of the deceased animals. Mostly of the deposits around Grimmen, Dobbertin and others where on the Toarcian fully marine environments, that advanced towards the east forming first an enclosed inland sea in the Hettangian-Sinemurian[55] and towards the Pliensbachian-Toarcian transforming to an open shelf sea connected to both the Boreal and the Tethys Oceans.[55] The Clay Pits of Grimmen and Dobbertin tend to occur as concretions within clay to Marlstones, that are related to a pre-diagenetic carbonate migration and concentration, along early diagenetic cementation, often around organic remains.[56] The changes in the mineralogy of the environment is vinculated with the freshwater basins of Fennoscandia, draining the continent to the south. A General Moonsonal events are believed to occur on the two realms, being the main probe of it the washed insects to the sea on the German realm.[57][58] Mostly of the fossils come from benthonic zones, where the whased fossils of where deposited[59] The Marine origin of the Sediments on the German Realm are well known due to the fossil-rich carbonate concretions of the Toarcian.[60] The marine setting was regionally uniform, where sandstones were deposited as carbonate-cemented silts within fine sand lenses, which can be seen as channel fillings within the clay. Along with Ahrensburg layers are likely to show a continuous marine basin setting from Grimmen to the Baltic Sea, where marine marginal marine, to depht basin deposits occur.[60] This is also proved due to the presence of salt structures from Malchin and Dobbertin.[60] There are several strata with marine fauna, with a 15 cm thick conglomerate-like pack of Belemnites and rearranged and unrolled Ammonites along with vertebrate remains, related to an Anoxic Sea Bottom deposition, where there is also the presence of terrigenous material, including Kaolinite and several Volcanic detrits, probably coming from the Scania section of Fennoscandia, that was volcanic active during the time of deposition of the German Margin.[60]

With the presence of diagenetic carbonation that has left to an early deposition is accompanied by the presence of fine-grained sandstones and the presence of terrestrial fragments, such as dinosaur remains and plant fragments, including trunks.[61] There is a clear reduction of salinity, as probes the absence of euryhaline macroinvertebrates.[61] Depending on the interpretation of the sediments of the erratic boulders, the Ringkøbing-Fyn Island may have also played a role as a sediment source. Most of the washed sediments can have come also from the Polish realm and the seashore formations of Bornholm. There is a visible correlation between the sandstones and conglomerates from the Rya Formation. The Sorthat Formation shows clearly prodeltaic facies, part of the large Toarcian–Bajocian deltaic systems, that come from Sweden and the Polish Realm.[61]

The large Toarcian–Bajocian deltaic systems locally where the shoreline influenced by the vicinity between brackish to freshwater and continental biofacies.[62][63] The North German Basin shows that on approximately 14.4 m.a, four third-order relative sea-level fluctuations led the subsequent formation of four individual delta generations in the Bifrons-Thouarsense (Toarcian), Murchisonae-Bradfordensis (Aalenian) and Humpresianum-Garatiana (Bajocian).[62] The Toarcian section was dominated by regressive elongated river-dominated deltas, where due to the fall of the sea level the south to southwest directed delta progradation between the Lower-Upper Toarcian, that was deposited as 40 m of deltaic successions, found on places like Prignitz (East) and Brandenburg (North).[62] On the Bifrons zone (180.36-178.24 m.a) to the Thouarense zone (176.23-174.97 m.a) there was the final outbuilding of the local delta plains, where there was a stretching of about 200 Km from the northern margins of the basin to the center.[62] The Toarcian local deltas are mostly regressive or constructive, with a characterised elongate morphology, covering with its plains approximately 15,000 km² (Bifrons) to 20,000 km² (Thouarsense).[62] The Upper deltaic plains lack any marine influence, with biofacies composed mostly by Palynomorphs, where in the southwest the lower part of the plains shows the influence of temporal marine incursions. The lower plain of the delta covered approx. 10,000 km² (Bifrons).[62] The deltas where connected with several networks of delatic channel belts, where on zones like Usedom (Northeast) there is a clear path with bifurcations and reunification of the channel belts.[62] On the lower delta plain lithofacies plant detritus and wood debris are very common, deposited probably on interdistributary bays formed embayments, thanks to overbank flooding from near distributaries, that covered approx. 2000 km² (Bifrons).[62]