Multiphase reddening and weathering events in Upper Carboniferous red beds from the English West Midlands

Abstract

Sediments of the Halesowen and Salop Formations (Westphalian D-Stephanian) 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 sandstones and mudstones in the Salop Formation includes large detrital hematite grains; these yield a primary component of magnetization indicating that sediments were deposited close to the Carboniferous palaeo-equator. Variscan uplift, erosion and subsequent deep oxidation in an arid climate during the Permian resulted in the precipitation of finer 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° and 15°N, the position that Britain occupied during the Permian. The pattern of Permian remagnetization (particularly in borehole samples) suggests that the secondary fine-grained hematite was precipitated as a result of lateral flow of meteoric water through connected, permeable sandstone units, rather than downward percolation through the entire succession. Present-day surface weathering 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 Carboniferous and Permian magnetization events is commonly incomplete in strata close to the present day ground level.