Funnel structures and clastic dykes in Cambrian sandstones of southern Sweden - indications for tensional tectonics and seismic events in a shallow marine environment

In the study area, the coast of the Baltic Sea south of Vik (Scania) is made up of Lower Cambrian sandstones belonging to the Vik Member. Several enigmatic tub- and funnel-shaped structures occur within this more or less flat-lying shallow water- and tide-influenced sedimentary succession. Some of these centroclinal structures attain diameters of up to 36 in characterized by minor normal faults sub-concentrically arranged. The bedding planes of the down-faulted sediments are funnel-like bent. Close to the centers of these funnel structures the layers dip with partly more than 50 degrees. Besides these circular structures, the sandstones are cut by several generations of upwards-filled elastic dykes. Both the funnel structures and the elastic dykes are of synsedimentary origin but of different ages. The formation of the first one is regarded to be associated with suddenly opening tensional fissures in the crystalline basement below. Such downwards-filled sugtrusive elastic dykes are known to occur near-by on Bornholm. The generation of the upwards-filled extrusive elastic dykes within the Cambrian sandstones themselves was triggered by strong seismic events prior to the formation of the funnel structures.