An introduction to Precambrian basins: their characteristics and genesis

Precambrian and younger basins reflect the interaction of sediment supply a nd subsidence; the latter is generally ascribed to tectonic, magmatic and r elated thermal processes. The interplay of supply and subsidence is further modified by eustasy and palaeoclimate, Problems and enigmas inherent in an alysis of Precambrian basin-fills include: a spectrum of ideas on the maxim um age of Phanerozoic-style plate tectonics in the rock record; Archaean he at flow up to two to three times present values; changes in magmatism over time (including global magmatic events); the evolution of atmospheric compo sition and of life and their influence on weathering, erosion and sediment supply rates; degree of preservation, deformation and metamorphism, and pre servational bias (especially of intracratonic basins which would lack evide nce for early plate tectonics); a limited rock record; poor age constraints , inherent errors in geochronological techniques and difficulty in dating t he time of deposition of sedimentary rocks. Major influences on Precambrian basin formation are assumed to include magm atism, plate tectonics, eustasy and palaeoclimate, all of which interacted. Models for greenstone belt evolution include plate tectonic intra-oceanic generation, plume-generated oceanic plateau, and global catastrophic magmat ic events that may have been transitional to a plate tectonic regime over s everal hundred million years. The latter transition may have included the o nset of the supercontinent cycle. Insignificant preservation of Precambrian ocean floor makes evaluation of these models problematic. Eustasy was intr insically related to continental crustal growth rates, continental freeboar d and the hypsometric curves of emerging cratons. Possible maximum crustal growth rates near the Archaean-Proterozoic boundary led to globally elevate d sea levels, and the formation of enormous carbonate-banded iron formation platforms where cyanobacterial mats, which produced oxygen, flourished. Th e combination of changes in cratonic growth rates, thermal elevation of cra tons, eustasy, weathering and palaeo-atmosphere composition may have combin ed to produce the first global glaciation at ca. 2.4-2.2 Ga. Examples of basins discussed here emphasise the interaction of tectonism, m agmatism, eustasy and palaeoclimate in their evolution. For the Neoarchaean Witwatersrand basin (Kaapvaal craton, South Africa), evidence for all thes e factors is preserved in the basin-fill, whereas for the Neoproterozoic Ma caubas basin (Sao Francisco craton, Brazil), clear evidence for eustasy is more Limited. The ca. <2.45- < 1.9 Ga preserved Hurwitz basin (Hearne domai n, Canada) suggests a predominant tectonic control, but with significant in fluences from magmatic processes, eustasy and palaeoclimate. For the ca. 2. 7 Ga Ventersdorp Supergroup, which succeeded the Witwatersrand Supergroup, a strong case can be made for magmatism as a prime influence, with an infer red mantle plume having caused lithospheric stretching and thermal subsiden ce. The Ventersdorp formed part of an inferred global magmatic event, succe eded on the Pilbara and Kaapvaal cratons by the NeoArchaean-Palaeoproterozo ic Hamersley and Lower Transvaal carbonate-banded iron formation platform s uccessions, ascribed largely to globally high sea levers, allied to an aggr essive weathering regime. Evidence for both eustasy and weathering are Limi ted in the preserved basin-fill of the Palaeoproterozoic Timeball Hill (upp er Transvaal Supergroup, Kaapvaal) depository, formed during the ca. 7.4-2. 2 Ga global glaciation, probably due to tectonic subsidence. For the ca. 1. 7-1.5 Ga Espinhaco basin (Sao Francisco craton, Brazil) evidence supports l ithospheric stretching and thermal subsidence as prime influences. The origin of greenstone basins remains contentious. That magmatism was a m ajor factor in their evolution is accepted by most, but whether this was pl ate-independent or plate-driven is less certain; the role of mantle plumes and the possibility of greenstones having been ridge-generated are also dis cussed by some workers. Episodic magmatism on a global scale may have playe d a role in the evolution of early basins such as the greenstones, Witwater srand and Ventersdorp, and with a possible transition to plate tectonics in to the Palaeoproterozoic, mid-ocean ridge growth related to either supercon tinent break-up or to continental custal growth rates probably influenced t he eustatically controlled Hamersley and Lower Transvaal basin sedimentatio n. The possibility that early plate tectonics was characterised by variable spreading and subduction rates is discussed in the light of evidence from the Witwatersrand basin, the North American, Baltic and Siberian cratons, a nd the Transvaal Supergroup. Ln conclusion, Precambrian basin evolution pro bably reflects the variable interaction of tectonism, magmatism, eustasy an d palaeoclimate las also found for Phanerozoic basins), with the most signi ficant difference compared to younger basins lying in the relative rates of processes such as ridge-spreading, subduction, crustal growth, weathering and atmospheric compositional change. (C) 2001 Elsevier Science B.V. All ri ghts reserved.