Pot and gutter casts from the Chapel Island Formation, Southeast Newfoundland

Abstract

ABSTRACT Member 2 of the Chapel Island Formation of southeast Newfoundland contains a diverse suite of erosional structures known as pot and gutter casts. These are particularly abundant in a lithofacies deposited in a nearshore zone characterized by erosion and sediment bypass during storm emplacement of sands. The steep-to-overhanging walls of both pot and gutter casts imply rapid filling during the same storm events that created these depressions. Erosion is thought to have taken place under strong offshore-directed unidirectional flow (storm surge or other relaxation flow). As with many sandstone tempestites, the initial stages of the storm events were dominated by strong--and in this case highly erosive--unidirectional flow, and the latter stages by oscillatory flow, as evidenced by wave-diagnostic lamination in the sandy infills. Pot casts have geometries and soles indicating vortex flow similar to that which forms potholes in bedrock in glaciated regions. Gutter casts that lead into potholes do so at their edge, a requirement for generating rotary currents within the depression. Pot casts are commonly tilted relative to paleohorizontal, and their direction of plunge is either upcurrent or downcurrent. Variation in geometry of the erosional structures from the Chapel Island Formation, and from erosional structures in general, can be accounted for by the following: substrate type; diagenetic history of substrate; and flow parameters such as pattern of water motion, velocity, and intermittency of flow.