Triassic outliers in the Peak District, central England, and a stratigraphical revision of the Miocene Brassington Formation

The Miocene Brassington Formation of Derbyshire and Staffordshire in central England, as originally conceived, is preserved in sinkholes in the Lower Carboniferous/Mississippian Peak Limestone Group. However, contrary to the original Neogene age interpretation, most of the Brassington Formation is demonstrated herein to be of Triassic age, based upon gross lithofacies, material properties, palaeontology, palaeoclimatic considerations and palaeomagnetism. Specifically, the supposedly Miocene Kirkham and Bees Nest members are reinterpreted as the Triassic Sherwood Sandstone and Mercia Mudstone groups respectively. The Brassington Formation is therefore emended to exclude the Kirkham and Bees Nest members, and to include the original Kenslow Member and the new Friden Member. The latter unit is a clay-dominated succession exposed at Kenslow Top Pit, near Friden, Derbyshire. Both the Kenslow and Friden members have yielded Middle–Late Miocene pollen and spores. Evidentially, both the Lower–Middle Triassic Sherwood Sandstone and Mercia Mudstone groups were originally deposited in the White Peak area of Derbyshire and Staffordshire before subsiding into karstic voids. Geological implications of this revised interpretation include that the Sherwood Sandstone Group formerly extended further northwards in the Peak District than was previously supposed, and was laid down in a more complex and extensive depositional setting. The newly recognized Sherwood Sandstone Group and Mercia Mudstone Group successions in the Peak District have implications for the interpretation of hydrological evolution and karstification in the subjacent Peak Limestone Group. Furthermore, the palynology of the emended Brassington Formation may help to determine altitude of deposition, and hence provide improved understanding of Cenozoic uplift rates and interpretations of the drainage and landscape evolution of upland Britain.