A reassessment of the Brassington Formation (Miocene) of Derbyshire, UK and a review of related hypogene karst suffosion processes

The Brassington Formation of the Pennines is the most extensive onshore Miocene succession in the UK. It is preserved as outliers in Lower Carboniferous Limestone. During the Cenozoic, central England underwent uplift, with erosion of post-Mississippian strata from the Pennine axis in the Peak District. The Brassington Formation is hence significant in reconstructing Cenozoic geological history. It is non-marine, derived from Triassic sandstone and of Mid–Late Miocene age. The c. 60 outliers occur in three clusters over c. 220 km2. They are remnants of a sedimentary prism at least 75 m thick and with a volume of c. 10 km3. Suffosion of bedrock, conditioned by aggressive precursor fluids of hypogenic origin, was the major control on subsidence. The calculated volume of the fills in the Bees Nest and Green Clay outliers is 3.7–5.0 × 106 m3. Referenced to a sub-Miocene surface at c. 450 m OD in pre-subsidence times, the original volume of this subsidence complex was c. 21.8 × 106 m3 and the aggregated volume of the Late Neogene White Peak palaeokarst was at least 0.66 km3. Subsidence was concomitant with Pliocene uplift of the Pennine Axis, suggesting suffosion accommodation of 3 × 106 m3 per km2 or 244 m3 per annum.