The 2.7-2.0 Ga volcano-sedimentary record of Africa, India and Australia: evidence for global and local changes in sea level and continental freeboard

The 2.7-2.0 Ga volcano-sedimentary records of the African, Indian and Austr alian cratons indicate two broadly defined periods of extensive drowning of the emergent continental areas, concomitant with lowered freeboard. Carbon ate-banded iron formation (BIF) platforms characterised the first such even t, at ca 2.6-2.4 Ga (Africa and Australia) to 2.7 Ga (India). These earlier globally enhanced sea levels are ascribed to increased mid-ocean ridge act ivity, possibly related to breakup of a postulated Late Archaean 'southern' supercontinent. Alternatively, a transition from global-scale catastrophic mantle overturn events to the onset of plate tectonics may have occurred i n the Late Archaean (Nelson, 1998. Earth Planet. Sci. Lett. 158, 109-119). Both explanations of increased mid-ocean ridge activity are compatible with significant Early to Middle Archaean crustal growth (Armstrong, 1981. Phil . Trans. R Soc. London A 301, 443-472), with the emergent high freeboard cr atons being subjected to aggressive weathering and erosion. Enhanced contin ental crustal growth near the Archaean-Proterozoic boundary (McLennan and T aylor, 1982. J. Geol. 90, 347-361), related to the development of significa nt island are complexes, would have resulted in common lowered freeboard-en hanced sea level conditions at the passive margins of the 'southern' craton s. The diachronous nature of these earlier transgressions in the Various cr atons may reflect the effect of local tectonic movements and/or the thermal state of the cratons. From ca 2.4-2.2 Ga, cratons that make up the present -day continents of India, Africa and Australia had relatively high continen tal freeboard and lowered sea levels. Glacigenic deposits are preserved on the Kaapvaal (Africa), Singhbhum (India) and Pilbara (Australia) cratons. T he second broadly defined drowning event, at ca <2.2 and >2.15 Ga, was prob ably due to post-glacial climatic amelioration. Freeboard was reduced by th e combination of eustatic rise and the reestablishment of aggressive weathe ring as warmer palaeoclimates returned. In India, carbonates were more prom inent than the siliciclastic sediments (including prominent black shales) s een in Africa and Australia. (C) 1999 Elsevier Science B.V. All rights rese rved.