Sorthat Formation

The Sorthat Formation is a geologic formation on the island on Bornholm, Denmark. It is of latest Pliensbachian-Lower Toarcian age. Plant fossils have been recovered from the formation, along with several traces of invertebrate animals. The "Sorthat Formation" is overlain by fluvial to lacustrine gravels, along with sands and clay along with parts covered by coal beds, that are part of the Aalenian-Bathonian Bagå Formation. In fact the "Sorthat Formation" was included on most of late SXX studies as the lowermost part of the "Bagå Formation", recovering the latest Pliensbachian to lower Aalenian boundary, that was dismissed after comparing the geology of the two. The Sorthat strata recover a mostly marginal deltaic to marine unit, where large streams fluctuated to the east where a large river system was established at the start of the Toarcian. At the northwest the local Vulcanism, that started on the lower Pliensbachian, extended 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 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".
Stratigraphy.
On Bornholm, the lower-middle Jurassic succession is composed of the Rønne (Hettangian-Sinemurian), Hasle (Lower-late Pliensbachian), "Sorthat" and Bagå Formations. The major Pliensbachian–Bathonian coal-bearing Clays and Sands that overlie the Lower Pliensbachian Hasle Formation are distributed between both the "Sorthat Formation" and the overlaying "Bagå Formation". The Sorthat Formation is the regional equivalent of the Ciechocinek Formation on Baltic Germany and Poland, the Fjerritslev Formation on the Danish Basin, the Rya Formation on Scania. The "Sorthat Formation" beds where referred originally to the Levka Beds, Sorthat and Bagå beds. The major section of the Formation is the "Korsodde coastal section" located on the south-west part of the Island. A detailed stratigraphic interpretation of the beds has been difficult to achieve due to the complicated block faulting, but especially due to the absence of marine fossils and distinct marker beds. Using the Megaspore contents, the rocks were dated originally as Middle Jurassic, with the Levka and Sorthat beds being roughly contemporaneous, and the Bagå beds being possibly slightly younger. Latter the "Bagå Formation" was limited, including the Coal Clays of the Levka Beds, along with the also Coal Dominant beds of the Korsodde and Onsbæk sections. With more advanced Palynological studies recovered from locations such as the Levka-1 core-well and the Korsodde section Upper Pliensbachian strata appeared. As at the time several Megaspores collected were found to be common on both the "Bagå Formation" and Sorthat beds, implying at least the presence of Toarcian-Aalenian strata. Although it was difficult to set a concrete age for the Megaspore-bearing strata. With both, the palynological–sedimentological study of all available exposures and cores from the Lower–Middle Jurassic lead to the revelation that the Hasle Formation (Lower-Middle Pliensbachian) are covered by a succession referable to both the Levka and Sorthat Beds, that are composed mostly by bioturbated Sands, Heteroliths and Clays along with abundant Coal seams containing relatively diverse brackish-marine dinoflagellate assemblages that are indicative of the upper Pliensbachian, Toarcian and possibly lower Aalenian strata. While the upper strata is covered by the fluvial gravels and sands, along with lacustrine clays, carbonaceous clays and coals belonging to the "Bagå Formation".
Lithology.
The "Sorthat Formation" has a highly variable lithology. The main core studied from the rocks, the Levka-1 well reveal first a sharp-based, fining-upwards units, 3–14 m thick, consisting of coarse-grained, occasionally pebbly Sand, overlain by muddy, with Coal and Mica, fine- to medium-grained sand, that is laminated to homogeneous Clay and coal seams with roots. On most of the strata there is a common Parallel Lamination with subordinate Cross-bedding, Cross-lamination and Flaser lamination. There are abundant Large plant fragments and small Quartz. Related to this level, there is a clear absence of Marine palynomorphs are not found, as a main probe that this level was deposited on a coastal or Delta plain with fluvial channels, Lakes and Swamps. This is also related to the most recent finds on the German realm of the Ciechocinek Formation, where a large Deltaic System ended. The large Toarcian–Bajocian deltaic systems locally where the shoreline influenced by the vicinity between brackish to freshwater and continental biofacies. 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). 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). Most of the Palynomorphs found on the Toarcian strata are connected with the ones found on the "Sorthat Formation". With nearly 40 m thick, the upper section of the formation is composed mostly by a series of cross-bedded, cross-laminated, wave-rippled and bioturbated sand and heteroliths with sporadic Syneresis cracks, Pyrite nodules, the ichnofossils Planolites isp. and Teichichnus isp. and brackish-marine palynomorphs, mostly dinoflagellates. This upper part has a strata more common to be deposited on nearshore environments with abundant lagoons, coastal lakes and fluvial channels with the clean sand at the top probably representing a marine shoreface. The Korsodde Section, with 93 m thick is composed mostly by coarse-grained sands with cross-bedding and parallel lamination, being mostly of Black due to an abundant organic debris. This Section has been interpreted as part of the large local fluvial system, probably as a series of minor fluvial channels, that were connected with coastal lakes and lagoons, where riparian vegetation was relatively abundant, judging by the presence of megaflora remains and palynomorphs. Small ichnofossil burrows and larger burrows, including Diplocraterion isp. are common, interpreting that there was at least one subunit that was the fill of an estuarine channel. The uppermost part of the formation in the Korsodde section consists of fine-grained, sands with cross stratification and parallel-lamination, yellowish–brown color, along with Sandstones with thin bioturbated and wave-rippled heterolithic beds.
Profile.
At Korsodde, the Evironment Includes:
Palynostratigraphy.
One of the most complete floras found in Europe dating to the Pliensbachian-Toarcian boundary, and among all the Jurassic palynological deposits found worldwide.
Environment.
Due to a Late Pliensbachian marine regression, deposition of coal-bearing strata on the "Sorthat Formation" was resumed on Bornholm until an Early Toarcian transgression terminated peat formation. The two main deposits of the formation, the Levka-1 Well and the lower part of the Korsodde Section where deposited on an environment influenced by the sea, being the Levka location populated by Lagoons, lakes, channels and low fluvial areas. The "Bifrons to Levesquei Zone" in the coeval units at the east and west of Prignitz a sandy coastal-deltaic succession was replaced by laminated shales with pelagic marine fauna, reflected on the shoreline shifts on the Northeast, what contributed to retrogradational stratal pattern architectures. On the "Sorthat Formation" is present a transition from upper to lower shoreface environments, indicating a deepening trend. In the Younger "Levesquei" subzone delta plain environments were replaced by shoreface setting with active bioturbation and hummocky cross-stratification. The Rǿnne Graben shows seismic lines with onlapping patterns that have been correlated to this Lower Toarcian marine shoreface deposits with intense bioturbation.
The depositional environments Include:
Inertinite has been recovered from the Coal-Bearing levels of the formation, where the palynology shows that the mire vegetation may have been dominated by gymnospermous plants and a secondary proportion of ferns characterised by the genera "Dicksonia" or "Coniopteris" and the family Osmundaceae. O several coal seams there where found several Biomolecules, where Euulminite and Attrinite were the most abundant huminite macerals recovered. The Levka-1 well section represents fluvial channels, floodplain areas with shallow lakes and lagoons, and small crevasse deltas, with abundant coalified wood fragments and stems, being most of them found associated to sandy channel fills and on heavily rooted crevasse and lake deposits in shallow inter-fluvial areas. On the Toarcian at Bornholm strata indicates a warm, humid climate suggested by the large number of plant species from interconnected Jameson Land, and thin cutinised leaves of "Podozamites" and Equisetales comparable in size to modern subtropical bamboos are thought to reflect favourable conditions for plant growth. There is abundant Coal, which indicates that wildfires occurred in the bog. Wood particles from this section, both charcoalified and unburned (coalified), with a rounded and worn nature on many particles, that implies the influence of greater transportation energies. The Toarcian Erratics from Northern Germany are assigned to Southern Sweden and also to the "Sorthat Formation", being suggested as the source area of the Ahrensburg erratics. On the Toarcian, the sketchy facies on Sweden suggests a general lateral coarsening of the lithofacies, specially on Helsingborg (Rya Formation) and the "Sorthat Formation", where there is the probe of the presence of a large deltaic system that fluctuated from the SE of Sweden and Bornholm, that ended on a prodeltaic facies belt stretching from Vorpommern to Mecklenburg Bay.
Coal.
Mostly of the coal seams recovered from the formation come from the Levka 1 and the Korsodde section, and are derived on most of the cases from a densely
vegetated, anoxic swamp, which was probably rheotrophic and rich in nutrients. The study of the peat accumulation indicates that peat accumulation occurred in rather short time intervals (around 2.300 years). The deposits have a great amount of thin and clean coal seams, covered by lacustrine-lagoonal flooding peaks, indicating rapid changes on the environment that were controlled by fairly rapid subsidence of the Rönne Graben, that along the coeval eustatic rise in sea level, causing downs and rises in the base level on the coastal plain. The majority of the samples where immature, low rank coals with generally very high content of humified organic matter, what indicates prevailing anoxic and fully saturated conditions during peat formation, with ocassional inundations by freshwater which favoured humification of the plant tissues and also may have increased the gelification processes, raising the ph. Hopanoids are abundant and an indicator of common bacterial activity. The vegetation, both the nearby plants and the peat swamp plants where probably small in stature, and it´s diversity suggests a humid, warm-temperate to subtropical climate which favoured a prolific vegetation. 
Phytoplankton.
On the Lower Jurassic of Bornholm there where several successions of nearshore Peat Formations with Dinoflajellates. Coal-bearing strata were deposited in an overall coastal plain environment during the Hettangian-Sinemurian and then during the Early Pliensbachian was interrupted until the late Pliensbachian-Lowermost Toarcian due to a Sea regression.
Terrestrial palynology.
In Early Toarcian carbonates local bulk organic matter, and wood fragments have been related to carbon cycle perturbations, helping to know the reaction of the Continental Biota to the TAOE that happened along contemporaneous large scale volcanism. There where several changes on the woody vegetation in the wood-derived carbon, with pollen assemblages are dominated by Cupressaceae pollen types such "Cerebropollenites" associated with Cycad pollen types ("Chasmatosporites") and the Hirmerielliaceous "Corollina". The local palynology has allow to state terrestrial changes on the local flora, where before the Pliensbachian-Toarcian boundary the dominant palynofacies where the Cupressaceae such as "Perinopollenites" along with cycads such as "Cycadopites", found related with middle latitude mediterranean climate. Then, at the start of the event the local pollen assamblages suffer a shift to spore rich layers, showing an increase of ferns and lycophytes, indicator of more humid conditions during a long term. Finally, after the Toarcian AOE event, the Sorthat Formation suffered the most abrupt rise of pollen of Hirmeriellaceae such as "Corollina" and specially "Spheripollenites", both indicators of semidesertic to dry mediterranean climates, impliying an abrupt warming event, coeval with the changes happening on the sea. 
Fossil Wood.
Is also common to found wood from the nearshore deposits of Korsodde, with two sets: macroscopic wood, recognizable to the naked eye, up to branch-sized; and microscopic wood (0.25 to 1 mm average dimension). The woods hasn´t been assigned to a concrete genus and include large coalified trunks, isolated coalified logs. This wood shows isotopic patterns similar to those found on the Late Palaeocene thermal maximum, and have allow to measure higher atmospheric CO2 concentrations on this interval. 
Plant macrofossils.
The major deposits of macroflora are the Hasle Clay Pit and the Korsodde section. The flora is was originally stated as coeval with the Rhaetian-Hettangian floras of Sweden, but found latter to be Pliensbachian-Toarcian. Möller did the two major studies on the local flora, with 68 species described, 50% of them ferns. The Late Pliensbachian section is dominated by ferns, that suggest a warm and humid climate, what fits with the paleolatitude of Bornholm, firmly within the Jurassic warm temperature biome. But the presence of Ginkoaleans and absence of large leafed Bennnettites suggest this warm climate was seasonal. Ferns and sphenophytes in the assemblage are interpreted to have occupied the lowermost stratum. Bennettites where mid-storey shrubs and Conifers, such as "Pagiophyllum", together with ginkgoaleans, make the main arboral flora. All the flora developed on a meandering river system with wellvegetated marshy flood plains. The Toarcian section shows a radical change on the local flora, as Hirmeriellaceae Conifers take the role of dominant flora, being the 95% of the pollen recovered on the interval assigned to this family, along with a rise of Seed Ferns, Bennettites and Czekanowskiales. The dominance of "Pagiophyllum" and its related pollen "Corollina torosus" indicate high temperature/aridity with seasonal wildfires (tough, some sections show low coal ratio, derived from slightly more humid environments), with rare occurrences of "Brachyphyllum" and one "Cyparissidium". 