MULTIPHASE REDDENING AND WEATHERING EVENTS IN UPPER CARBONIFEROUS REDBEDS FROM THE ENGLISH WEST MIDLANDS

Sediments of the Halesowen and Salop Formations (Westphalian D-Stephan ian) in the West Midlands were deposited on the southern margin of the Pennine Basin during a period of relative tectonic quiescence during the Variscan Orogeny. The clastic framework of the predominantly red s andstones and mudstones in the Salop Formation includes large detrital hematite grains; these yield a primary component of magnetization ind icating that sediments were deposited close to the Carboniferous palae o-equator. Variscan uplift, erosion and subsequent deep oxidation in a n arid climate during the Permian resulted in the precipitation of fin er hematite in rocks of both formations (which are now found at depths of over 150 m below the present-day ground level). These grains carry secondary magnetizations which indicate formation at palaeolatitudes of between 5 degrees and 15 degrees N, the position that Britain occup ied during the Permian. The pattern of Permian remagnetization (partic ularly in borehole samples) suggests that the secondary fine-grained h ematite was precipitated as a result of lateral flow of meteoric water through connected, permeable sandstone units, rather than downward pe rcolation through the entire succession. Present-day surface weatherin g has resulted in the dissolution of the finer grains of hematite and, in some cases, where permeabilities are relatively high, it has also affected the coarser fraction. Consequently, the record of late Carbon iferous and Permian magnetization events is commonly incomplete in str ata close to the present day ground level.