Australia: The Land Where Time Began

A biography of the Australian continent 

Volcano-Sedimentary Record of Africa, India and Australia - Evidence of Global and Local Sea Level and Continental Freeboard Changes

There are 2 very extensive periods, that are broadly defined, of drowning of emergent continental areas recorded in the volcano-sedimentary record of the African, Indian and Australian Cratons, concomitant with lowered freeboard, between <2.7->2.0 Ga. The first of these events was characterised by platforms of carbonate-banded iron formation (BIF) at about 2.6-2.4 Ga in Africa and Australia, and about 2.7 Ga in India. According to the authors1 these earlier periods of sea levels that were globally enhanced have been ascribed to increased activity of mid-ocean ridges that were possibly related to the breakup of a postulated 'southern' supercontinent from the Late Archaean. An alternative suggestion is that transition from catastrophic mantle overturn events, that occurred on a global scale, to the onset of plate tectonics may have taken place in the Late Archaean (Nelson, 1998. Earth and Planet. Sci. Lett. 158, 109-119). The authors1 say that both explanations for the increased activity at mid-ocean ridges are compatible with significant crustal growth in the Early to Middle Archaean (Armstrong, 1981, Phil. Trans. R Soc. London A301, 443-472), during which the emergent high freeboard cratons were subjected to aggressive weathering and erosion. Conditions of common lowered freeboard at the passive margins of the 'southern' cratons would have resulted from enhanced continental growth near the boundary between the Archaean and Proterozoic (McLennan & Taylor, 1982. J. Geol. 90, 347-361), which was related to the development of significant island arc complexes. The authors1 suggest the effect of local tectonic movements and/or the thermal state of the cratons may reflect the diachronous nature of these earlier transgressions in the various cratons. Cratons comprising the continents of India, Africa and Australia of the present had relatively high freeboard and lowered sea levels from about 2.4-2.2 Ga. On the Kaapvaal Craton of Africa, Singhbhum Craton of India and the Pilbara Craton of Australia glaciogenic deposits have been preserved. The second drowning event that was broadly defined, that occurred less than 2.2 and greater than 2.15 Ga, is suggested by the authors1 to probably be due to post-glacial climatic amelioration. A combination of static sea level rise and the reestablishment of aggressive weathering  that occurred with the return of warmer palaeoclimates. Carbonates were more prominent in India than the siliciclastic sediments, that included black shales, that are present in Africa and Australia.

Sources & Further reading

  1. Eriksson, P. G., R. Mazumder, S. Sarkar, P. K. Bose, W. Altermann, and R. van der Merwe. "The 2.72.0 Ga Volcano-Sedimentary Record of Africa, India and Australia: Evidence for Global and Local Changes in Sea Level and Continental Freeboard." Precambrian Research 97, no. 34 (9// 1999): 269-302.
Author: M. H. Monroe
Last updated 18/10/2013
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