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RHAETIAN AND JURASSIC PLANTS OF SCANIA

History of the investigation of Age and Stratigraphy of the Plant Bearing Formation

Emanuel Swedenborg (1772), Carl von Linné (1751) and Samuel Gustaf Hermelin (1773) were the first to describe profiles from the Scanian coal formation (the Höganäs Formation). However, it was not until 1808 that Vilhelm Hisinger attempted to correlate units within the Scanian coal formation. From the area of Helsingborg he described “flötssandsten" (“seam sandstone") overlaying “the transitional sandstone" and breccia. Moreover, he described coal layers intercalated with the sandstone layers (Hisinger, 1808, Lundgren, 1878). In the beginning of the 19th century various theories on the age and chronostratigraphy were put forward by scientists such as Hausmann, Örsted, Esmarch, Forchhammer, Wahlenberg, and Brongniart. Some theories considered the Scanian coal formation coeval to the Carboniferous coal formations of England and France, other theories considered the age to be either Cretaceous, or more precisely Wealden. These early theories were summarized by Bernard Lundgren (1878).
The collections, work and discoveries by Sven Nilsson (e.g. 1819, 1820a, 1823) laid the foundation for understanding the geology of the Scanian coal formation. In 1819 he recognized the Höör Sandstone, containing fossil plants, as a separate stratum (Nilsson, 1819, 1820a, b). Earlier, it had been considered a part of the “transitional sandstones", e.g., the Hardeberga Sandstone. In 1823 Nilsson described plant-bearing layers at Höganäs, Raus, and Bosarp. To determine the fossils he was assisted by Carl Adolf Agardh and Elias Fries. In the same year Agardh (1823) published his own description of Nilsson´s material. Based on studies of the fossil content Nilsson was convinced that the coal formation was different from, and younger than the Carboniferous coal formations of England and France. In the beginning he thought that the coal formation was of marine origin. However, in the course of a re-examination of his collections it was found that a tooth previously interpreted as a fish tooth actually originated from a crocodile. Also, an insect wing and various molluscs, among which was Avicula inaequivalvis Low, were found in the layers of Höganäs deposits. Based on these findings Nilsson concluded that the coal originated from plants growing close to the area of deposition, and under limnic conditions. Because A. inaequivalvis is a key fossil for the Lower Jurassic (Liassic), the age of the Höganäs Formation could be determined more precisely, and was considered older than and clearly separated from the Höör Formation.
In 1859 Nils Peter Angelin published a geological map of Scania, followed by an explanatory text in 1877, in which he compared European strata from Keuper and Lias with the Scanian strata thought to be Liassic. He recognized the strata underlying the coal formation to be of Keuper age and the coal formation at Höganäs to represent the Triassic-Jurassic transitional beds, so-called Infra Liassic (Bonebed zone, Rhätische Stufe, Avicula contorta zone) (Angelin, 1859/1877). Angelin did not define the stratigraphic position of the Höör Sandstone. Alphonse von Dittmar agreed with Angelin in that at least a part of the Höganäs Formation belonged to the Avicula contorta zone and he considered Nilsson´s Avicula inaequivalvis zone to be Liassic. Von Dittmar was the first who mentioned that both Keuper and Rhaetian, as well as the Liassic were present in Scania (Lundgren, 1878, Dittmar, 1864). Edmond Hébert was the first scientist after Nilsson who made new field investigations and collected material in 1865 and 1869. The identification of this material was carried out by Louis G. de Saporta and Wilhelm P. Schimper. Hébert thought that the coal formation belonged to the same geological system and that it was not possible to distinguish a lower stratum with coal and plants from an upper stratum with marine animal remains, but he agreed with Angelin on the Avicula contorta zone (Lundgren, 1878, Hébert 1869).
Few plant fossils were known at the time, but this radically changed with the work of Alfred Gabriel Nathorst. He described new species from Scania from three different plant-bearing strata, which, in his opinion, belonged to the Rhaetian Stage (Nathorst 1876, 1878a and 1878b). At the same time Bernhard Lundgren investigated and described a number of localities and referred to the works of Nilsson, Hébert and Nathorst. He identified the Avicula contorta zone and a zone with Ammonites planorbis (psilonotus) and Ammonites angulatus. Like Angelin and Hébert he assigned them “Infra Liassic". In contrast, he placed the older flora of Bjuv and the older flora of Höganäs into the Rhaetian, and the younger flora of Höganäs and the flora of Pålsjö into the Lias, based on the investigations by Nathorst (Lundgren, 1878). In 1910 Nathorst presented his conclusions on the 11th International Geological Congress in Sweden. He described 16 plant bearing zones based on his own investigations and the works of Lundgren and Erdmann among others (Nathorst, 1910). Nathorst defined the Rhaetian-Liassic boundary as lying between the Mytilus bed and the zone with Nilssonia polymorpha (Table 1).

Table 1. Principal fossil zones of the Rahetian-Liassic strata of Scania (Nathorst, 1910).

Another to investigate the Scanian stratigraphy was Edvard Erdmann (1872, 1887, 1911-1915), who presented a comprehensive description of the geology and mining techniques at the Scanian coal mines (Table 2).

Table 2. Selection of stratigraphic profiles of the Scanian Coal Formation from a shaft at the Bjuv mine (Erdmann, 1911-1915).

He put the shales, clays and sandstones that lie immediately on the Scanian Silurian systems into the Keuper. Based on petrographic evidence these layers most likely belong to Keuper. Above the Keuper strata follows the coal formation of Scania, which is of Rhaetian-Liassic age. The lower part is generally believed to be Rhaetian and the upper part Liassic, but they are not separated by a clear boundary. The Höör Sandstone is considered a part of the Liassic.

In the middle of the 20th century, Anna Birgitta Lundblad wrote that professor Thore G. Halle “repeatedly pointed out" the need of a complete revision of Rhaetian-Liassic floras of Scania (Lundblad, 1950). Previously, Tom M. Harris (1937) had asked for a “critical revision of the stratigraphy and the specific determinations in the flora". The stratigraphy and the many biostratigraphic zones established by Nathorst and others were, until Gustaf Troedssons investgations, not subject to any major investigations, even though Antevs (1919) and Johansson (1922) pointed out the weakness in Nathorst´s zones. The need for revision was partly redressed by Troedsson (1938, 1940, 1943, 1950, 1951). In a number of publications he gave comprehensive, partly new, conclusions on the stratigraphy of the Rhaetian-Liassic succession of Scania, including chronostratigraphy, delimitation of some lithostratigraphical units (e.g., the Vallåkra Member, the strata of Boserup). Furthermore, he described the cyclic depositional environment linked to transgressions and regressions. In his publication of 1950 he presented a summary of the older Mesozoic strata of Scania (Table 3).

Table 3. Older Mesozoic of Scania (Troedsson, 1950).

Along with Troedsson, Lundblad also contributed to the revisions, presenting descriptions of local floras, including taxonomic revisions, and investigations on the stratigraphy (Lundblad, 1950, 1959). She considered the Triassic-Jurassic boundary somewhere between the upper coal seam and the Boserup beds. Harris (1937) also questioned the many biostratigraphic zones established by Nathorst and others. In addition, he identified two zones in the Scanian Rhaetian-Liassic that he believed to be similar to the Scorseby Sound flora of Greenland (Harris, 1937).

The main features of Troedsson´s scheme (Troedsson 1950) are still a valid framework for the Rhaetian-Liassic stratigraphy of Scania. Improved techniques and continued investigations have shed new light on the question of the age of the plant bearing strata and the question of the Triassic-Jurassic boundary. However, it is very difficult to mark the Rhaetian-Liassic boundary with the lithology only. Biostratigraphic data too, is needed, which is the case for many drilling cores (Sivhed, 1980). In more recent time it has mainly been palynological studies that provided new data (e. g., Norling, 1972, Lund, 1977, Guy-Ohlson, 1981, Guy-Ohlson and Norling, 1994). Ulf Sivhed summarized current opinions on the Rhaetian to Middle Jurassic in Scania, with references to the works of Nilsson, Angelin, Lundgren, Nathorst, Erdmann, Troedsson, and Lundblad (commented on above). Sivhed (1984) also referred to more recent works of Edmund Bölau, 1949, Hans Tralau, 1975, Jens Lund, 1977 and Dorothy Guy-Olsson, 1981. Beside the summary of Sivhed there are older summaries by Nathorst (1876), Lundgren (1878), Troedsson (1943, 1950) and Lundblad (1946). Most recent, a comprehensive overview was given by Norling et al. (1993) in the “Guide to the Upper Triassic and Jurassic geology of Sweden". A stratigraphic scheme is presented in Table 3 (Sivhed, 1984). Today, the Triassic-Jurassic boundary seems to be undisputed lying between the Bjuv Member and the Helsingborg Member.

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Text: Ove Johansson

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