SEQUENCE AND EVENT STRATIGRAPHY OF THE TYPE WENLOCK (SILURIAN), MIDLAND PLATFORM, ENGLAND: COMPARISON WITH EASTERN LAURENTIA

The Wenlock Series of the Midland Platform, England comprise three conformably successive formations, the Buildwas Formation, Coalbrookdale Shale and Much Wenlock Limestone totaling 291-447m of strata. Though well constrained within the type localities, the graptolite biozonal framework does not provide a clear means of correlation elsewhere on the platform or beyond. Based on the geochemical and geophysical correlation of several K-bentonites, time lines within the basal and uppermost formations have now been established which allow direct correlation across the Midland Platform. From the basal Wenlock there is a clear synchronous regression (C. centrifugus-C. murchisoni), represented by the limestones and shales of the Buildwas Formation. The overlying monotonous shales (M. riccartonensis-C. lundgreni) of the Coalbrookdale Formation represent a major transgression. Though within the southern half of the platform (?C .rigidus-C. linnarsoni) regressive sandstones and limestones occur in the middle Coalbrookdale. The successive Much Wenlock Limestone (G. nassa-M. ludensis) consists of two shallow water carbonates separated by a deep water shale interval in the northwest part of the platform and a coarser siliciclastic dominated equivalent to the south. Within the type locality at Wenlock Edge, carbonate deposition does not start until the M. ludensis biozone and is associated with localized uplift along the platform margin and the onset of the upper platform-wide regression. The Much Wenlock Limestone-Elton Shale contact appears both conformable and synchronous across the platform, indicating rapid transgression. Sedimentation throughout the Midland Platform can be regarded as being largely eustatically controlled. Other factors e.g. platform slope, siliciclastic input and variations in subsidence are of lesser importance. The inferred sea level curve can therefore be directly compared with other coeval stable platforms as a potential means of global correlation. Notably the pattern for the northern Appalachian Basin is closely comparable, with lower Wenlock lowstand carbonates (Irondequoit Limestone), centrally deepened Rochester Shale, and two upper Wenlock shallow carbonates, separated and overlain by (Ludlow ?) deeper water shaly carbonates (Lockport Group).