Hemipelagic shelf sedimentation and climatic cycles: the basal Jurassic (Blue Lias) of South Britain

Abstract

The sedimentology, cyclicity and hiatuses of the Blue Lias Formation (basal Jurassic) were investigated at five localities in South Britain. Most carbonate mud, which is the dominant carbonate component, has been neomorphosed to microspar. However, carbonate mud was apparently supplied by coccoliths in zooplankton faecal pellets, suggesting hemipelagic sedimentation. Clay is assumed to have been supplied by rivers. The three sediment types involved reflect cyclic changes in bottom-water oxygenation levels combined with changes in the clay-to-carbonate mud ratio. Walsh power spectra have been applied for the first time to measured sections in order to investigate the control of cyclicity. This application is analogous to palaeoclimatological studies of Pleistocene oxygen-isotope (ice-volume) records. The spectra show that 1 or 2 sedimentary cycles, of constant thickness, occur in four out of five sections, each of which represents several million years. The maximum possible duration of these cycles is too short for their generation by sea-level changes. Their regularity, stability and order of duration (tens of thousands of years) are consistent with the hypothesis of sedimentary cyclicity controlled by climatic change, itself forced by orbital changes in insolation (Milankovitch theory). The duration of both cycles recognised is less than 93,000 years (93 ka) and may record changes in orbital precession (21 ka) and obliquity (41 ka). Assuming that one ammonite zone represents 1 Ma, comparison of the longest-possible with the implied durations of the cycles, suggests that the sections are at best 20–40% complete. Loss of sediment may have occurred during winter storms or hurricanes. A model relating the sedimentary cycles to changes in the volume of runoff is proposed.