Sedimentology of Bajocian rocks from the Ravenscar group of Yorkshire
There is a dramatic change in Jurassic sedimentary patterns in Yorkshire from shallow marine Liassic deposits to the mixed coastal plain/marine sequences seen in the Middle Jurassic. The Middle Jurassic Ravenscar Group overlies Aalenian Dogger Formation shallow marine sediments. Facies analysis shows that the lower parts of the Ravenscar Group includes three non-marine horizons (the Saltwick Formation, the Sycarham Member and the Gristhorpe Member) interbedded with two marine sequences (the Eller Beck Formation and the Lebberston Member) and the section studied is terminated by thick offshore marine shales of the Scarborough Formation. Biostratigraphic evidence suggests a continuous period of coastal plain and marine sedimentation lasting approximately five million years for the sequence. Three marine horizons, the upper parts of the Dogger Formation,the Eller Beck Formation and the basal member of the Scarborough Formation show coarsening-upward profiles indicative of coastline progradation following periods of sudden transgression. By comparison the lower part of the marine Lebberston Member resulted from a gradual relative sea level rise which reworked both underlying and exotic sediment into broad sand sheets. As the sea level stabilised carbonate sedimentation occurred as ooids formed in a shallow offshore marine environment, adjacent to a low energy coastline which lay to the north. The marine horizons demonstrate shallow water deposition under variable energy waves and micro- to mesotidal currents. Rapid progradation of fluvial dominated coastal plains occurred at three horizons and the palaeocurrents from these non-marine beds show a strong southerly transport direction. Highly constructive river systems of variable magnitude deposited the majority of the clastic sediment seen in the Ravenscar Group, with large amounts of predominantly fine-grained material laid down in Lacustrine, crevasse splay, levee and alluvial floodplain environments. The rivers included high and ?low sinuosity forms. Some of the very large channel sand bodies were sites of complex deposition and erosion of sediment for long periods of time, and these rivers probably represent major distributary channels which dominated the coastal plains. The upper parts of the coastal sections of the Lebberston Member indicate the rapid progradation of a wave-tide-fluvial delta front over offshore carbonate sediments. From the evidence of wave and tidal activity in the marine horizons this model is tentatively applied to the other non-marine deposits. The overall facies analysis of the sediments allows a description of the history of sedimentation of the Ravenscar Group to be made, including a summary of the depositional palaeogeographies. Palaeocurrent, compositional and grain size evidence suggest that the non-marine horizons represent repeated periods of progradation of the same system, -under the influence of tectonic activity (uplift?) in the hinterland, allied with delta abandonment within the outcrop area. Petrographic analyses combined with a consideration of the Kesosoic tectonic history of the North Sea area indicates a small scale sedimentary system deriving clastic detritus from dominantly Carboniferous outcrops in the region of the Kid-North Sea High. The sandstones are all sub-arkoses or quartz arenites. Diagenetic studies of the sediments indicate widespread kaolinite formation but calcite cementation is restricted to the marine horizons. Clastic diagenesis in the Ravenscar Group shows similarities with other contemporaneous deposits in the North Sea region, with variable quartz redistribution and illite and illite/smectite formation. Illite is the dominant clay mineral present in the shales and mudstones, with kaolinite locally abundant in organic rich sediments. There is little difference between the clay mineral suites of the marine and non-marine muarocks. The lower, carbonate bearing, parts of the Lebberston Member show evidence of syn-sedimentary lithification in submarine and beach environments prior to a sealing of the limestones by a coarse sparry calcite cement. This horizon is analagous in many respects to modern Bahaman environments.