Thin, fine-grained storm layers in a muddy shelf sequence: an example from the Lower Jurassic of the Stenlille 1 well, Denmark

Thin storm-sand layers are a characteristic feature of many sandy shelf sequences, while little evidence of storm layers is preserved in muddy shelf deposits where bioturbation is often total. The present study shows that storm-generated depositional processes governed sedimentation in a muddy shelf environment. The studied sequence is dominated by blackish mudshale and silt-streaked mudshale with thinner intervals of pale, fine-grained heterolithic sandstone. It represents the Lower Jurassic Fjerritslev Formation in the eastern part of the Danish Basin and has been studied in four cores from the Stenlille 1 well. Within the muddy outer shelf association four sedimentary facies and 14 types of laminae, 0.5–10 mm thick, may be distinguished. Most of the laminae share the characteristics of graded bedding, erosive bases and occasional fading ripples with storm layers of sand grade. The structures indicate that the mudshale laminae formed by settling of the suspension load carried by storm-generated currents. This kind of episodic deposition of fine-grained storm layers may be common, though rarely preserved, in outer shelf environments.