The sedimentology of Lower Palaeozoic black shales from the shallow wells Skelbro 1 and Billegrav 1, Bomholm, Denmark

Abstract

Previous studies of the Lower Palaeozoic shales on Bornholm have mainly been based on the outcrops along the streams. The outcrops provide data on the lateral continuity of the facies while the selection of (2-3 cm wide) cores for the present study focused the attention on the vertical sequence of facies. Cores of high quality were obtained and have been found to provide an excellent basis for a study of structures, sediment composition and diagenesis. In the outcrops of shale fossils are locally present in high numbers and such levels are also recognized in the cores. Seven sedimentary facies, ranging from black mudshale to greyish mudstone and silt-streaked shale, are distinguished in the Middle Cambrian to lowermost Silurian shales. Gamma-ray logs were run in the two wells and a convincing correlation to the cored sequence can be demonstrated. Gamma-ray logs provide thus a good means of correlation to wells where no cores have been cut. The shales are all interpreted as deposited in an epicontinental sea due to their geological setting i.e. the association with shallow water carbonates (Andrarum Limestone and Komstad Limestone) and their stratigraphical position above the shelf to shoreface sandstones of the Lresa formation. Three facies associations are distinguished: The mudshale association comprises black organic-rich shales which represent a low-energy anoxic depositional environment which prevailed in the Middle Cambrian to Lower Ordovician. The mudstone association is typical of the Middle and Upper Ordovician and represents a continuation of low-energy environments though mottling indicates that ventilation improved in certain periods. The siltshale association represents higher energy environments which were dominant in the lowermost Silurian. A well defined Upper Ordovician CU sequence probably reflects the global eustatic fall in sea level caused by the extensive glaciation in Gondwanaland. In the late Silurian the average rate of deposition increased in response to the approaching Caledonian orogeny.