Origin and palaeomagnetism of the Mesoproterozoic Gangau tilloid (basal Vindhyan Supergroup), central India

Abstract

The stratigraphic position and claimed glacial origin of the diamictitic 'Gangau tilloid' in central India have long been controversial. Our study shows that the ∼ 50-m-thick tilloid overlies the Palaeoproterozoic Bijawar Group with angular unconformity and forms the base of the Mesoproterozoic (∼ 1200-1400 Ma) Vindhyan Semri Group. The tilloid contains clasts up to 1 m across, virtually all of stable siliceous lithologies like those of the Bijawar Group, supported by a fine-grained haematitic matrix. Glacially faceted and striated clasts were not observed. The tilloid displays much disruption of beds, and pebble-sized clasts of lamellar and nodular chert-quartz underwent brittle fracture and plastic puckering during deposition. Values are high for SiO2 (68.8-85.5%) and Fe2O3 (7.73-20.1%), and low for Al2O3 (1.03-5.50%) and other elements. The composition of the tilloid is similar to that of laterites formed on sandstone bedrock. Thermal step demagnetisation of 91 core samples from seven sites spanning the tilloid revealed three components: a low-temperature Tertiary component A, an intermediate-temperature steep downward component B and, more rarely, a high-temperature less steep component C. Haematite is the likely carrier of the remanence, interpreted as chemical remanent magnetisation. C may be older than B and date from near the time of deposition. Bedding-corrected C has a direction of D = 161.4°, I = 63.2° (α95 = 12.7°) that gives a pole at 18.2°S, 93.4°E (dp = 15.8°, dm = 20.0°). The pole plots near 1200 Ma on the Australian Precambrian apparent polar wander path using the Veevers et al. (1991) reconstruction for East Gondwanaland. The sedimentology, composition and geochemistry of the Gangau tilloid accord with deposition by continental debris-flows derived mainly from a ferruginous regolith formed on sedimentary rocks. Zoned chert-quartz bodies may have been precipitated in shrinkage cracks and voids in the lower, saturated zone of the regolith, and possibly also as thin beds or crusts of silica within the drainage system. The inferred ferruginous weathering in the source area favours warm and humid conditions with a dry season, and precipitation of silica in the regolith implies seasonal or longer wet and dry intervals. Debris-flow activity indicates brief episodes of abundant runoff. There is no evidence of glaciation. Production of abundant ferric iron in the source regolith implies the presence of appreciable atmospheric oxygen by ∼ 1200-1400 Ma.