Depositional environments, diagenetic history and source areas of some Bunter Sandstones in northern Jutland

Cores from the wells Mors 1 and Gassum 1 representing the Lower Triassic Bunter Sandstone Formation in the Danish Subbasin have been reexamined with regard to their primary sedimentary structures and their petrology. Four sedimentary facies are recognized within the series of fine- and mediumgrained , reddish sandstones in which fossils and trace fossils have not been found. The facies and the facies sequences suggest deposition of the sandstones by braided rivers.

The petrographical study of thin sections demonstrates two separate phases of diagenetic alterations . The first: infiltration of clay and precipitation of iron oxides comprises features wellknown from recent desert sediments. This phase may or may not be accompanied by growth of authigenic quartz and feldspar. The second phase: growth of authigenic clay and dolomite or ankerite represent deeper burial diagenesis.

The mineralogy of the sandstones indicate a metamorphic-plutonic terrain as the main source area, presumably the Fennoscandian Shield. However, zeolitic aggregates of volcanic origin in the Mors 1 samples and absence of these aggregates in the Gassum 1 samples coupled with the occurrence of sillimanite-bearing grains suggest different sub-source area for the two localities.

It is concluded that the regional pattern of sediment transport into the Danish Subbasin has involved transportation from various parts of the Fennoscandian Shield via a fringe of alluvial fans along the Fennoscandian Border Zone. The present study thus adds new evidence in support of the existing interpretations of early Triassic paleogeography and palaeoclimatology.