Abstract
Bornholm is the easternmost island of Denmark and rather far away from the rest of the country (Fig. 1). Situated in the south-eastern part of the Fenno-Scandian Border Zone (Tornquist Line) with the Fenno-Scandian Precambrian Shield towards the north and the Danish-Polish Subbasin towards the south, Bornholm has a complex tectonic and sedimentological history (like Scania). The pre-Quaternary surface consists of Precambrian basement rocks in the northern part of the island, whereas Palaeozoic and Mesozoic sediments, often separated from the basement by faults, are found in the western and southern part (Fig. 2). The pre-Quaternary rocks are all covered by Quaternary deposits, mainly tills and melt water sediments.
Highlights
This report is a review of pre-Quaternary geological and structural evolution of the Danish area as it can be interpreted from well data, seismic data, and outcrops
The Middle Cambrian sequence is terminated by the 2 m thick lowermost part of the "Upper Alum Shale", which passes into the 21 m thick Upper Cambrian alum shale, "Olenid Shale", a homogene black shale with scattered large anthraconite lenses
The Late Berriasian regressive Langbjerg Member (0-28 m thick) consists of medium to coarse~grained faintly horizontally laminated, bioturbated or cross-bedded quartz sand with Ophiomorpha and occasionally Skolithos, deposited on a prograding barrier island complex and on intertidal/subtidal channels and flats (Gravesen 1977, 1982). This unit is conformably overlain by the regressive Late Berriasian - Valanginian Jydegard Formation which is subdivided into the interfingering Tornh0j and R0dbjerg Members
Summary
To the south the Precambrian basement is overlain by the 100 m thick Early Cambrian (Tommotian) subarkosic, fluviatile, mainly red colored Nex0 Sandstone. This sequence is overlain by the 60 m thick marine, occasionally glauconitic, quartzitic Balka Sandstone, deposited in tidal environment, and indicating the Early Cambrian transgression. Middle and Late Cambrian, and Early Ordovician rocks of Bornholm are dominated by anoxic black claystones (alum shales) with subordinate limestone beds or lenses. The phosphorized top is overlain by the 1.5 m thick "Lower Alum Shale", succeeded by a 25 cm anthraconite bed with a phosphorite conglomerate, which initiates the 1.5 m thick bituminous Andrarum Limestone. The Middle Cambrian sequence is terminated by the 2 m thick lowermost part of the "Upper Alum Shale", which passes into the 21 m thick Upper Cambrian alum shale, "Olenid Shale", a homogene black shale with scattered large anthraconite lenses
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