The Brattefors plugs: Collapse structures initiated during a Tremadocian regression

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Abstract
At Brattefors on the southern slope of Kinnekulle, Västergötland, Sweden, Upper Cambrian bituminous shale is cut by plugs consisting of a thick, relatively undisturbed succession of Tremadocian Ceratopyge Shale otherwise not represented in the vicinity. Drillcores show the plugs to reach down into Lower Cambrian sandstone. Along with structures formed by collapse and subrosion of Lower Cambrian sandstone in other parts of Sweden the Brattefors plugs are thought to be due to a drastic lowering of the sea-level in the earliest Ordovician. As a consequence of the groundwater head developed in the emergent sedimentary succession, percolation of groundwater led to subrosion of Lower Cambrian sandstone and maybe leaching and subrosion of the underlying gneiss surface. Middle and Upper Cambrian deposits thereupon collapsed into the voids formed by subrosion. The event is dated by Cordylodus angulatus occurring at the base of the Tremadocian filling of the pipes.