A vertic paleosol at the Archean-Proterozoic contact from the Singhbhum-Orissa craton, eastern India

Abstract

We report on an inferred Paleoproterozoic soil horizon from the Singhbhum-Orissa craton near Keonjhargarh, Orissa, eastern India, which preserves vertic (Vertisol-like) features. The ∼2–10 m thick, white shale consists of quartz–muscovite–pyrophyllite interpreted as a paleosol, overlies ∼3.32 Ga granite and is covered by pre-2.0 Ga sandstones (Kolhan Group) deposited in marginal marine environments. The paleosol passes downwards through a partly altered granite (saprolith) zone to fresh granite. Abundant pedogenic slickensides, ped structures, gilgai topography, reddish stain on ped surfaces, and sepic-plasmic microfabric in the paleosol were probably related to shrink and swell processes driven by marked seasonal precipitation in a well-drained landscape. An observed decrease of alkali elements and an increase of silica, alumina and CIA values from granite to paleosol, together with relatively constant Ti/Al ratios suggest in situ weathering of granite protolith. However, a paleo-Vertisol necessitates a mafic parent rock, either directly (igneous) or reworked through alluvium. The c. 2.25 Ga Malangtoli-Jagannathpur basaltic rocks are relatively widespread and close to the inferred paleosol occurrences, and provide a logical solution. Geochemistry of the highly depleted paleosol, in contrast, particularly when applying the isochon method, strongly supports a granitic parent. The apparent conundrum can possibly be solved through repeated alluvial reworking of both granitic and mafic detritus and accumulation of weathered material with varying proportions of these two components, yet enough smectitic clays (∼30%) to form a paleo-Vertisol. This pre-2.0 Ga weathering event in the Singhbhum craton is consistent with a global scenario of Paleoproterozoic weathering which followed an extended global glaciation, and thus has the potential to contribute to understanding of pre-2.0 Ga amalgamation of regional and global cratonic blocks.