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Regional Geology

The Danish Basin & Vomb trough with focus on the Mesozoic

In Sweden, Jurassic sedimentary rocks are restricted to the southernmost province, Skåne. Exposures of Lower Jurassic strata are found mainly in the north-western part of the province and at the eastern margin of the Vomb trough in the central parts, where they rest on Lower Palaeozoic deposits. Exposures of Middle and Upper Jurassic sediments are known, as a result of mining activities, from one locality only, Eriksdal, located within the Vomb Basin. The Vomb Basin is a narrow, elongated graben with a length of c. 80 km and a maximum width of 11 km. During the Mesozoic, this area was tectonically active and the basin was severely affected by faulting leading to the formation of minor troughs within the basin.  The eastern margin of the Vomb Basin is defined by a  major fault forming the western boundary of an early Paleozoic plateu. The sedimentary deposits in the Vomb basin are mainly constituted by Late Cretaceous marine sediments; however a narrow rim of Upper Triassic and Jurassic strata are exposed, mainly composed of argillaceous sandstones and clays. The sediments were formed in deltaic, freshwater environment during warm and humid climatic conditions but also some marine deposits prevail. 
Geological map of Skåne, with Jurassic exposures marked with green. Eriksdal marked with red star.

Bornholm is a Danish island situated almost in the centre of the Baltic Sea at 15 degrees east and 55 degrees north. The island is 40 km across and has a 141 kilometre-long coastline.

Bornholm is located within the Fenno Scandian Border Zone and displays a block-faulted mosaic exposing a wide variety of rocks ranging from Precambrian gneisses and granites to Lower Palaeozoic and Mesozoic strata.

The Mesozoic sequence occurs in narrow fault blocks  along the south and west coasts and offshore in the Rønne Graben west of Bornholm covering around 2500 metres of sediments spanning a period from Late Triassic to Late Cretaceous. The Lower and Middle Jurassic sediments are particularly well exposed and form a general transgressive or deepening sequence. This sequence initiates with deposition of Hettangian lacustrine and fluviatile clay, and coal of the Munkerup and Sose Bugt Members of the Rønne Formation, overlain by Sinemurian tidal flat and channel mud and sand deposits of the Galgelokken Member of the Rønne Formation.

The Lower Cretaceous, Berriasian - Valanginian sand and clay sediments of the Rabekke, Robbedale and Jydegård Formations are mainly deposited in the shallow marine are as, on back-barriers and in lagoons, channels and lakes.

The Upper Cretaceous deposits consist of the Cenomanian Arnager Greensand Formation, the Coniacian Arnager Limestone Formation and the Cenomanian Bavnodde Greensand Formations. The three marine formations consist of marine glauconitic sands, marls, clays and limestones and conglomerates with a lot of marine fossils and trace fossils.

Geological map of Bornholm, with Mesozoic
exposures color-coded

Stratigraphic log over the Mesozoic of Bornholm (Denmark)
and SE Skåne (Sweden). Numbers are indicating the s
tratigraphic positions of the different excursion localities (stops).