“Funnel Grabens” and Early Paleozoic Tectonism in South Sweden
The province of Scania, which is situated on the boundary between the Baltic Shield and the North European Depression, contains Lower Paleozoic and Mesozoic sedimentary rocks that are faulted mainly along northwest-southeast and west-northwest-east-southeast lines. Fairly extensive outcrops of Lower Cambrian sandstones are shown to be thoroughly marked with structures that are tectonically controlled by fractures with the said directions. These structures formed before the consolidation of the sandstones was completed. A few of the described structures existed even before the end of the Early Cambrian. Others formed not too long after the beginning of the Late Cambrian. The described structures are conical depressions, 1 to 200 m across and 1 to over 50 m deep. The depressed sandstone beds were either smoothly bent from the horizontal to their present dips of over 45 ° toward the center, or lowered along straight to curved, concentric fractures. The name “funnel grabens” is suggested for these structures. Associated structures (sandstone plugs and dikes, intra-formational breccias, and lineations) indicate local mobilization of parts of the affected deposits. North-south extension and opening of fractures in the underlying Precambrian crystalline basement is suggested as the direct cause of the funnel grabens. When the largest of these grabens formed, the sandstone was covered by at least 50 m of black, soft shale of mainly Middle and Late Cambrian age, and probably by some depth of sea water as well.