Origin of red beds in a moist tropical climate (Etruria Formation, Upper Carboniferous, UK)

Abstract

Summary The Etruria Formation is a Westphalian mudstone-dominated red bed succession, which is interdigitated with, and overlies coal-bearing paralic sediments in Central England. Sedimentological studies indicate that the formation mainly comprises swamp and muddy alluvial plain deposits. The distribution of red beds shows that the red coloration formed in areas of improved drainage soon after deposition, and was associated with the formation of complex palaeosols. The occurrence of coals and humid climate floras within the red bed succession suggests that the onset of oxidizing conditions was not the result of climatic change. There are no fundamental differences in the mineralogy of associated grey and red beds: both have distinctive residual characteristics, and consist mostly of kaolinite and quartz. The alluvium was derived from intensely weathered source areas, and was reddened during and soon after deposition by the dehydration of detrital ferric hydroxides and the oxidation of ferrous iron associated with organic material. Palaeomagnetic results support the view that reddening took place shortly after deposition, when the area lay in close proximity to the Carboniferous equator. The Etruria Formation thus provides an excellent example of the genesis of red beds as muddy alluvium under moist tropical climatic conditions.