Abstract

The Littleham Mudstone Formation of southeast Devon consists predominantly of red lutites of Late Permian or Early Triassic age. Present within the lutites are spherical pale patches that have sharp boundaries against the red lutite and often contain dark, organic centers rich in uranium and vanadium. Chemical and Mössbauer analysis of the oxidation state of iron in this formation shows that the red lutite contains an average total iron content of about 5.5%, with an Fe2+/Fe3+ ratio of 0.16, while the pale material has a total iron content of 2.7%, with an Fe2+/Fe3+ ratio of 1.2. In the pale material, iron is present only within silicate (clay material) lattice sites, but the red lutite contains hematite as well. The hematite has an important superparamagnetic component and is considered to have a bimodal grain size distribution with two modes of occurrence: as interlayer crystals within clay minerals and as external coatings or grains. The Fe2+/Fe3+ ratios and total iron content show that the red lutite is not merely the pale material with hematite added, but that about 50% of the Fe2+ in the clay mineral lattice in the pale material is oxidized to Fe3+ in the red lutite. This change in oxidation state is not coincident with the pale-red boundary but begins within the pale patches. The mechanism by which this occurred is considered to be an in situ oxidation that produced the red lutite, accompanied by the precipitation of ferric hydroxide from solution. This process is inhibited in the pale patches where Fe2+ ions would be more soluble and readily removed. Geotechnical tests on the lutites show that the spherical patches must have formed after a thickness of 1,000 to 1,300 m of overlying strata had been deposited.

This thickness encompasses the whole of the Triassic beds of southeast Devon and implies that the reddening occurred at the end of Triassic or in early Jurassic time. It is suggested that the reddening was aided by a change from arid to humid climate that occurred in the area at this time.

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