Soft-sediment deformation structures in the Late Silurian Stubdal Formation: the result of seismic triggering

The Ringerike Group is a lower Old Red Sandstone fluvial sequence that forms part of the Late Silurian foreland basin succession of the Ringerike Group, Oslo Region, southern Norway. The Ringerike Group exhibits a number of soft-sediment deformation structures in all its constituent formations, but in the type area of the Stubdal Formation many of these can be demonstrated to show preferential distribution along the same stratigraphic horizon. It is suggested that the laterally extensive soft-sediment deformation and mass sediment flows within the braided fluvial environment are the sedimentary expressions of a single large seismic event. The soft-sediment deformation structures include abundant ball-and-pillow structures, convolute cross-stratification, mud-crack diapirism, and spectacular channelised "pillow beds" up to two metres in thickness. These are catalogued and their variety is attributed to lateral differences in sedimentary facies at the time of deposition. The seismic activity responsible for the deformation was possibly due to an event in the Late Silurian tectonic evolution of the Norwegian part of the Caledonide mountain chain and its foreland basin.