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 Australian cratons indicate two broadly defined periods of extensive drowning of the emergent continental areas, concomitant with lowered freeboard. Carbonate-banded iron formation (BIF) platforms characterised the first such event, 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 activity, 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 in 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 cratons being subjected to aggressive weathering and erosion. Enhanced continental crustal growth near the Archaean-Proterozoic boundary (McLennan and Taylor, 1982. J. Geol. 90, 347-361), related to the development of significant island arc complexes, would have resulted in common lowered freeboard-enhanced sea level conditions at the passive margins of the 'southern' cratons. The diachronous nature of these earlier transgressions in the various cratons 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 continental freeboard and lowered sea levels. Glacigenic deposits are preserved on the Kaapvaal (Africa), Singhbhum (India) and Pilbara (Australia) cratons. The second broadly defined drowning event, at ca <2.2 and >2.15 Ga, was probably due to post-glacial climatic amelioration. Freeboard was reduced by the combination of eustatic rise and the reestablishment of aggressive weathering as warmer palaeoclimates returned. In India, carbonates were more prominent than the siliciclastic sediments (including prominent black shales) seen in Africa and Australia.