Patterns of sedimentation in the Precambrian

Abstract

The principle of uniformitarianism may be applied to Precambrian basin evolution and to the sedimentary record as a whole. The major difference in the Precambrian Eon lay in variability of rates and intensities of processes controlling weathering, erosion, transport, deposition, lithification, and diagenesis. This paper examines Precambrian sedimentation patterns within the larger framework of Earth evolution. Pre-rock record sedimentation probably comprised deep water oceanic realms within which meteoritic and cometary impact events generated very large tsunamis, resulting in very coarse volcaniclastic detritus combined with fine dust settling out of suspension, all reworked by marine current systems and localised turbidites. From c. 4 to 3.2 Ga, greenstone belts provided the predominant settings for the thin passive margin carbonates, BIF, stromatolitic evaporites, pelites and quartzites, and lesser synorogenic turbidites, conglomerates, and sandstones that accompanied the volcanic and volcaniclastic rocks typical of these settings. Common palaeoenvironments were high gradient alluvial fans, low sinuosity braided rivers, and relatively shallow marine settings, subject to wave and tidal action, and turbidity currents. Although continental crustal growth continued largely through greenstone belts until c. 2.7 Ga, the Witwatersrand basin (c. 3.0–2.7 Ga; Kaapvaal craton, South Africa) reflects initial stabilisation of the oldest craton, with an epeiric sea accumulating largely fluvial detritus subject to tidal (inland) and storm-wave (craton-marginal) reworking within a retroarc foreland basin setting. Neoarchaean–Palaeoproterozoic sedimentation is discussed within a framework of two global “superevents”, at c. 2.7 Ga and 2.2–1.8 Ga, each encompassing major changes in Earth's evolution related to the supercontinent cycle, mantle superplumes, peaks in crustal growth rates, and significant biochemical changes within the atmosphere–hydrosphere system. Concomitant globally raised sea levels led to chemical and clastic epeiric seas within which the first giant carbonate platforms developed, and deposition of iron-formation peaked globally at c. 2.5 Ga. The first global glaciation at c. 2.4–2.2 Ga provides little support for the “Snowball Earth” theory, but does suggest a negative feedback loop model whereby intraglacial CO 2-related warming and synglacial decreases in weathering alternated up to three times. Models for palaeo-atmospheric and -oceanic evolution are mutually exclusive, but by c.1.8 Ga, the existence of large landmasses and free oxygen in Earth's atmosphere enabled erg development and red bed sedimentation globally; the full spectrum of Phanerozoic–Modern sedimentary environments was thus present on Earth. A third postulated “superevent” at c. 0.8–0.6 Ga essentially recreated conditions experienced at c. 2.2–1.8 Ga, with, additionally, at least three global refrigeration events.