Jökull


Jökull - 01.12.1980, Page 6

Jökull - 01.12.1980, Page 6
erosion, during which an extensive shelf was carved into the Icelandic plateau. The “Red Crag” deposits of Tjörnes were thought to have been deposited during the highest level of this period of abrasion, resting unconform- ably on the tilted and subsided basalts. Thor- oddsen described the eastern part of Tjörnes as a basaltic horst, where the old basalts dipped 4—5° towards northwest, compared to a dip of 4° towards north within the “Red Crag” deposits. In Thoroddsen’s scheme the Pala- gonite Formation and the Dolerite Formation were considered to have been formed simult- aneously during the time from early Pliocene up into the Glacial Period, which Thoroddsen considered to have been uninterrupted by in- terglacials, and to have begun with the for- mation of mountain glaciers in late Pliocene time. The youngest rocks of Iceland belonged to a formation consisting of moraines and other sediments and lava flows from the Glacial Period and Postglacial time. Thoroddsen’s “classic” division was chal- lenged by Pjetursson in a number of papers during the first decade of the twentieth cen- tury. Tjörnes played an important part in Pjetursson’s new contribution (Pjetursson 1901, 1905a, 1905b, 1906a, 1906b, Pjeturss 1908, 1910, 1924). Fle divided the Icelandic sequence into three major formations (Table 1). A Regional Basalt Formation was built up during middle Eocene to early Miocene time as a part of the North Atlantic basalt region. In the uppermost part of the Regional Basalt Formation Pjetursson (1905b) found evidence of repeated Tertiary glaciations, and originally defined a “Grey Stage” at that level. This conclusion was partly based on the assumption that intercalated tillites west of Tjörnes belonged to a stratigraphical level below the Pliocene deposits on Tjörnes. Later, Pjeturss (1939) abandoned the idea of Miocene glaciations in Iceland. According to Pjetursson (1905b) the piling up of the Regional Basalt Formation was followed by a reduction in volcanic activity, and the thick Pliocene Crag deposits on Tjörnes were deposited in a littoral environment during subsidence, disturbed towards the end of the Pliocene by local uplifts. During the Pliocene volcanic lull Ice- land had become separated from the rest of the North Atlantic basalt region, after which extensive eruptions set in again and the Insu- lar Basalt Formation was now built up during the Pleistocene. On Tjörnes Peninsula Pjeturs- son found dolerites and glacial sediments, in- cluding an interglacial marine sequence bet- ween two lithified “moraines” in Breiðavík above the Crag Formation, but near the base of the Insular Basalt Formation. He did not exclude the possibility that the uppermost part of the Crag deposits reached up into the Pleistocene (Pjeturss 1908). Pjetursson found evidence of at least four or five glaciations separated by interglacials in various sections through the Insular Basalt Formation, and suggested that marine fossils found at several localities might yield information about climatic conditions during the Pleistocene. Clarification of the stratigraphy and palaeontology Schlesch visited Tjörnes in 1921 and studied the palaeontology of the Tjörnes sequence (Schlesch 1924, 1925a, 1925b, 1931). He presented an outline .of the geological relationships, largely based on the work of Pjetursson. Schlesch (1924) identified 146 species of molluscs out of which 39 were extinct or unknown at that time. He concluded that the fauna showed similarities with the English and Belgian Crags although the Tjörnes Crag did have a more northern character. Schlesch suggested that the very mixed faunistic cha- racter indicated that the climate had under- gone some changes during the time of deposition. The first detailed investigation of the stratigraphy of the coastal sections between Héðinshöfði and Voladalstorfa (Fig. 2) was carried out by Bárðarson (1925), who mapped individual beds and analyzed the fossil material. Several workers have referred to Bárðarson’s work as a classic one for the sequence on Tjörnes. Earlier, Bárðarson (1918, 1922) had published general descriptions of 4 JÖKULL 30. ÁR

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Jökull

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