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 compare 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 stablished 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 succesion is composed of the Rønne, Hasle, Sorthat and Bagå Formations. The major Pliensbachian–Bathonian coal-bearing Clays and Sands that overlie the Lower Pliensbachian Hasle Formation are distributed between the to the new Sorthat Formation and the 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 specially due to the absence of marine fossils and distinct marker beds. Using the Megaspore contents, the rocks where 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 Dominat 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 where 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 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, Murchisonae-Bradfordensis and Humpresianum-Garatiana. 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 and Brandenburg. 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-rippledand 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 where 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.
Flora
One of the Most Complete Floras found on All Europe on the Pliensbachian-Toarcian Boundary, and among all the Jurassic Palynologycal 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. 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, 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 sugested as the source area of the Ahrensburg erratics. On the Toarcian, the facies on Sweden the sketchy facies information suggests a general lateral coarsening of the lithofacies on Helsingborg 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
The Levka- 1 well has a core of approx. 150 m on the Sorthat Formation, covering the underliying marine strata of the Hasle Formation. The lower part includes 112 m with the Coal along sand and clay. There if an abundant presence of large, coalified, wood fragments and stems. The Coal bearing strata of the Levka-1 are interpreted as fluvial channel fills, with active channel deposition followed by abandonment and a passive phase of clay deposition, gradual overgrowth and change into a peat-forming environment. Clay and coal seams present along this strata have abundant rootlets and a non-marine palynomorph assemblage dominated by spores and pollen, interpreted as representing flood plain areas with shallow Lakes, small crevasse Deltas and Swamps. Some section have wave ripples, wavy and flaser bedding, bioturbation and transported Equisetites stems, that are interpreted as the sediment fill of a local lagoon, deposited on a transgressive shoreline with a series of lagoon succesions. Levka-1 coal contain hard, black coal and are very similar petrographically, with Huminite on the major part of the seams, with some seams having up to 90% of Huminite. There is a dominance of macerals from detrital organic matter over macerals derived from more woody material. Gelinite appears as the most common component of the samples, followed by Huminite.The Korsodde section is interpreted as representing an small coastal series of lakes and protected lagoons, where are recovered at least 6 coal seams occur. Above the marine strata of the Toarcian Tranggression strata with abundant Clay fine-grained Sand, Silt, that contains transported, coalified pieces of wood wood, pyrite nodules, rootlets and a diverse miospore assemblage, where there is abundance of the marine dinoflagellate Mendicodinium reticulaturn. On this coal Seams Huminite maceral group comprises the majority of the organic matter, where humotelinite dominates over humodetrinite maceral. Eu-ulminite and Densinite are the most prominent macerals.
Marine Palynology
On the Lower Jurassic of Bornholm there where several succesions 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 has 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 assem-blages are dominated by Taxodiaceae pollen types such Cerebropollenites associated with Cycad pollen types and the Cheirolepidiaceous Corollina, reflecting a warming transition from a wet fern dominated landscape to seasonally dry vegetation.Is also common to found wood from a near-shore setting on the eastern Danish Basin, with two sets: macroscopic wood, recognizable to the naked eye, up to branch-sized; and microscopic wood. Apart from wood and leaf cuticles there is little organic matter present in the section—the sediments comprise sand, silt and mud—and thus there is no possibility of contamination by marine carbon.