ORIGIN AND PALEOMAGNETISM OF THE MESOPROTEROZOIC GANGAU TILLOID (BASAL VINDHYAN SUPERGROUP), CENTRAL INDIA

The stratigraphic position and claimed glacial origin of the diamictit ic ''Gangau tilloid' in central India have long been controversial. Ou r study shows that the similar to 50-m-thick tilloid overlies the Pala eoproterozoic Bijawar Group with angular unconformity and forms the ba se of the Mesoproterozoic (similar to 1200-1400 Ma) Vindhyan Semri Gro up. The tilloid contains clasts up to 1 m across, virtually all of sta ble siliceous lithologies like those of the Bijawar Group, supported b y a fine-grained haematitic matrix. Glacially faceted and striated cla sts were not observed. The tilloid displays much disruption of beds, a nd pebble-sized clasts of lamellar and nodular chert-quartz underwent brittle fracture and plastic puckering during deposition. Values are h igh 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 simil ar to that of laterites formed on sandstone bedrock. Thermal step dema gnetisation of 91 core samples from seven sites spanning the tilloid r evealed three components: a low-temperature Tertiary component A, an i ntermediate-temperature steep downward component B and, more rarely, a high-temperature less steep component C. Haematite is the likely carr ier of the remanence, interpreted as chemical remanent magnetisation. C may be older than B and date from near the time of deposition. Beddi ng-corrected C has a direction of D = 161.4 degrees, I = 63.2 degrees (alpha(95) = 12.7 degrees) that gives a pole at 18.2 degrees S, 93.4 d egrees E (dp = 15.8 degrees, dm = 20.0 degrees). The pole plots near 1 200 Ma on the Australian Precambrian apparent polar wander path using the Veevers et al. (1991) reconstruction for East Gondwanaland.The sed imentology, composition and geochemistry of the Gangau tilloid accord with deposition by continental debris-flows derived mainly from a ferr uginous regolith formed on sedimentary rocks. Zoned chert-quartz bodie s may have been precipitated in shrinkage cracks and voids in the lowe r, saturated zone of the regolith, and possibly also as thin beds or c rusts of silica within the drainage system. The inferred ferruginous w eathering in the source area favours warm and humid conditions with a dry season, and precipitation of silica in the regolith implies season al or longer wet and dry intervals. Debris-flow activity indicates bri ef episodes of abundant runoff. There is no evidence of glaciation. Pr oduction of abundant ferric iron in the source regolith implies the pr esence of appreciable atmospheric oxygen by similar to 1200-1400 Ma.