Aspects of Mesozoic Shelf in Western Europe: ABSTRACT

Abstract

During the Mesozoic, the major paleogeographic units of western Europe were the Tethyan Ocean on the south, where predominantly calcareous marine sediments were deposited, and a shelf zone on the north, where dominantly arenaceous and argillaceous marine sediments and nonmarine intercalations were deposited. Clear-cut distinctions cannot be made between the two units because of gradational changes and fluctuations in space and time. An additional complicating factor is the fact that in some parts of the section, particularly in lower Mesozoic strata, extensive Tethyan deposits on the margins of the present-day Meditteranean apparently were laid down in shallow water. Within the areas of sedimentation there can be distinguished a series of basins, such as the Lower Saxony, Paris, and Wessex basins, characterized by relatively thick, continuous sequences, and swells which are areas of relatively thin, discontinuous sequences which in many places correspond to the margins of Precambrian or Paleozoic massifs, e.g., Scandinavia, Scottish Highlands, Brittany, Massif Central, and Harz Mountains. These massifs were transgressed by the sea only to a limited extent before Late Cretaceous time. The Triassic of the northern shelf zone is composed largely of continental redbeds with subordinate evaporites. However, there is a marine intercalation, the Muschelkalk, between the Bunter and Keuper of Germany and the southern North Sea region. The Muschelkalk consists of limestone and dolomite with a restricted fauna suggesting abnormal salinity. The Triassic deposits of the southern (Tethyan) zone are thick and largely marine; particularly striking are several thousand meters of Carnian, Norian, and Rhaetian shallow-water limestone and dolomite. Just before the Jurassic, the sea began to transgress progressively northward across the shelf zone. Except for some minor regressions, the transgression persisted until late Oxfordian-early Kimeridgian time and was accompanied by the gradual northward spread of shallow-water, calcareous, relatively open-sea deposits at the expense of terrigenous clastic and ferruginous deposits laid down close to river deltas. Salinity probably controlled the regional faunal distribution. The latest Jurassic and Early Cretaceous was a time of widespread regression when nonmarine deposits (Purbeckian-Wealden) were laid down from southern England across northern France to Germany. Renewed transgression in Aptian-Albian time preceded the major Mesozoic transgression in the Late Cretaceous, when great th cknesses of chalk were deposited. During the deposition of the ancient coccolith ooze, most of western Europe was, for the first time, a deep shelf. Mesozoic history ended with a Cretaceous regression. End_of_Article - Last_Page 720------------