PRECAMBRIAN ELASTIC SEDIMENTATION SYSTEMS

The unique and evolving nature of the Precambrian geological environme nt in many ways was responsible for significant differences between Pr ecambrian elastic sedimentary deposits and their Phanerozoic-modem equ ivalents. Some form of plate tectonics, with rapid microplate collisio ns and concomitant volcanic activity, is inferred to have led to the f ormation of greenstone belts. Explosive volcanism promoted common grav ity-flow deposits within terrestrial greenstone settings, with braided alluvial, wave/storm-related and tidal coastline sediments also being preserved. Late Archaean accretion of greenstone terranes led to emer gence of proto-cratons, where cratonic and rift sedimentary assemblage s developed, and these became widespread in the Proterozoic as cratoni c plates stabilised. Carbonate deposition was restricted by the paucit y of stable Archaean terranes. An Early Precambrian atmosphere charact erised by greenhouse gases, including CO2, in conjunction with a faste r rotation of the Earth and reduced albedo, provide a solution to the faint young Sun paradox. As emergent continental crust developed, volc anic additions of CO2 became balanced by withdrawal due to weathering and a developing Palaeoproterozoic microbial biomass. The reduction in CO2 and the photosynthetic production of O-2, led to aerobic conditio ns probably being achieved by about 2 Ga. Oceanic growth was allied to atmospheric development, with approximately 90% of current ocean volu me being reached by about 4 Ga. Warm Archaean and warm, moist Palaeopr oterozoic palaeoclimates appear to have become more arid after about 2 .3 Ga. The 2.4-2.3 Ga Huronian glaciation event was probably related t o continental growth, supercontinent assembly and weathering-related C O2 reduction. Despite many analogous features among both Precambrian a nd younger sedimentary deposits, there appear to be major differences as well. Two pertinent examples are rare unequivocal aeolian deposits prior to about 1.8 Ga and an apparent scarcity of Precambrian foreshor e deposits, particularly those related to barrier island systems. The significance of these differences is hard to evaluate, particularly wi th the reduced palaeoenvironmental resolution because of the absence o f invertebrate acid plant fossils within Precambrian successions. The latter factor also poses difficulties for the discrimination of Precam brian lacustrine and shallow marine deposits. The temporal distributio n of aeolian deposits probably reflects a number of possible factors, including few exposed late Archaean-Palaeoproterozoic cratonic areas, extensive pre-vegetative fluvial systems, Precambrian supercontinents and a different atmosphere. Alternatively, the scarcity of aeolian dep osits prior to 1.8 Ga may merely reflect non-recognition or non-preser vation. Precambrian shallow marine environments may have been subjecte d to more uniform circulation systems than those interpreted from the Phanerozoic-modern rock record, and Precambrian shelves probably were broad with gentle seaward slopes, in contrast to the narrow, steep she lves mostly observed in present settings. Poorly confined Precambrian tidal channels formed sheet sandstones, easily confused with fluvial o r offshore sand sheets. Epeiric seas were possibly more prevalent in t he Precambrian, but active tectonism as proto-continents emerged and a malgamated to form early supercontinents, in conjunction with a lack o f sufficient chronological data in the rock record, make it difficult to resolve the relative importance of eustatic and tectonic influences in forming epeiric embayments and seaways. Other differences in Preca mbrian palaeoenvironments are more easily reconstructed. Ancient delta plain channels were probably braided, and much thicker preserved delt a successions in the Precambrian are compatible with the inferred more active tectonic conditions. Pre-vegetational alluvial channel systems were almost certainly braided as well. Common fluvial quartz arenites are ascribed to differences in weathering processes, which probably c hanged significantly through the Precambrian, or to sediment recycling . Although Precambrian glacigenic environments were probably the least different from younger equivalents, their genesis appears to reflect a complex interplay of factors unique to the Precambrian Earth. These include emergence and amalgamation of proto-continents, the early CO2- rich atmosphere, the development of stromatolitic carbonate platforms, early weathering, faster rotation of the Earth and the possible role of changes in the inclination of the Earth's axis. (C) 1998 Elsevier S cience B.V. All rights reserved.