Middle Mesozoic Sedimentation in Relation to Tectonics in the British Area

Abstract

A new method of graphical analysis shows that lateral and vertical variations in thickness of sediments in Great Britain diminished from the Lower and Middle to the Upper Jurassic. These variations were related to faulting in the Hercynian Basement. An estimation of the mean stratigraphic thickness per ammonite zone for each stage leads to the inference that widespread deepening of the sea recorded by the change from sandstones and limestones to Kimmeridgian shales was marked by substantial increase in rates of subsidence. Mid-Mesozoic sedimentation can be related to a combination of eustatic rise of sea-level coupled with the elevation and subsequent collapse of rifted domal upwarps. Firstly, in the Lower Jurassic, sands were shed eastwards into southern England and Portugal from a land margining the early Atlantic rift. Subsequently, at the close of the Lower Jurassic and early in the Middle Jurassic widespread regression and volcanism related to the domal upwarp of the Central North Sea. This was followed by widespread collapse leading to the deposition of Late Jurassic marine shales. Finally upwarping to the west of Britain from the latest Jurassic through into the early Cretaceous provided westerly sources for the bulk of contemporary clastics. Rifting in the North Sea was associated with volcanism but failed to provide new oceanic crust whereas that in the west developed into the Rockall Trough and its associated structures.