Precambrian clastic sedimentation systems

Abstract

The unique and evolving nature of the Precambrian geological environment in many ways was responsible for significant differences between Precambrian clastic sedimentary deposits and their Phanerozoic-modern equivalents. Some form of plate tectonics, with rapid microplate collisions and concomitant volcanic activity, is inferred to have led to the formation of greenstone belts. Explosive volcanism promoted common gravity-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 emergence of proto-cratons, where cratonic and rift sedimentary assemblages developed, and these became widespread in the Proterozoic as cratonic plates stabilised. Carbonate deposition was restricted by the paucity of stable Archaean terranes. An Early Precambrian atmosphere characterised by greenhouse gases, including CO 2, in conjunction with a faster rotation of the Earth and reduced albedo, provide a solution to the faint young Sun paradox. As emergent continental crust developed, volcanic additions of CO 2 became balanced by withdrawal due to weathering and a developing Palaeoproterozoic microbial biomass. The reduction in CO 2, and the photosynthetic production of O 2, led to aerobic conditions probably being achieved by about 2 Ga. Oceanic growth was allied to atmospheric development, with approximately 90% of current ocean volume being reached by about 4 Ga. Warm Archaean and warm, moist Palaeoproterozoic palaeoclimates appear to have become more arid after about 2.3 Ga. The 2.4–2.3 Ga Huronian glaciation event was probably related to continental growth, supercontinent assembly and weathering-related CO 2 reduction. Despite many analogous features among both Precambrian and 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 foreshore deposits, particularly those related to barrier island systems. The significance of these differences is hard to evaluate, particularly with the reduced palaeoenvironmental resolution because of the absence of invertebrate and plant fossils within Precambrian successions. The latter factor also poses difficulties for the discrimination of Precambrian lacustrine and shallow marine deposits. The temporal distribution 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 deposits prior to 1.8 Ga may merely reflect non-recognition or non-preservation. Precambrian shallow marine environments may have been subjected 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 shelves mostly observed in present settings. Poorly confined Precambrian tidal channels formed sheet sandstones, easily confused with fluvial or offshore sand sheets. Epeiric seas were possibly more prevalent in the Precambrian, but active tectonism as proto-continents emerged and amalgamated to form early supercontinents, in conjunction with a lack of 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 Precambrian palaeoenvironments are more easily reconstructed. Ancient delta plain channels were probably braided, and much thicker preserved delta 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 changed 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 CO 2-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.