Seismically induced dewatering structures in the oldest (Rhaetian, Triassic) part of the Blue Lias formation at Lyme Regis, UK

Abstract

Soft-sediment deformation structures (SSDS) including slumped beds and dewatering and diapiric structures are relatively common in the youngest part of the Triassic succession exposed on the English south coast between Seaton, Devon and Lyme Regis, Dorset. They have been attributed to fluidisation as a result of slope failure or shockwaves caused by earthquakes, storm waves or bolide impact. Few of these structures can be proved to be the result solely of a seismic event. However, diapiric structures that are restricted to a single bed and over a small area in the oldest (Rhaetian, Triassic) part of the Blue Lias formation at Pinhay Bay, Lyme Regis are morphologically similar to structures developed in present-day sediments in seismically active areas. The Pinhay Bay examples appear to have been initiated by a single event: some of them are associated with minor movements along fractures in the underlying beds. They are interpreted as the result of a small local earthquake, the last in a series of minor tectonic events in the latest Triassic that were related to the opening of the English Channel.