Possible evidence for episodic epeiric marine and fluvial sedimentation (and implications for palaeoclimatic conditions), c. 2.3–1.8 Ga, Kaapvaal craton, South Africa

Abstract

The debate on Palaeoproterozoic atmospheric composition, climate, and the timing and nature of the postulated “great oxidation event” (possibly at c. 2.3 Ga) remain matters of debate within Precambrian geology. This paper contributes to the debate through examination of physical sedimentary structures preserved within two separate groups of sedimentary deposits, from the central and northern portions of the Kaapvaal craton, South Africa. Within the c. 2.3–2.1 Ga Pretoria Group, study of epeiric marine sandstones and mudrocks (Magaliesberg and Silverton Formations, c. 2.1 Ga) suggests that fluvial input was predominant over low energy reworking by waves, winds and tides. This together with extensive evidence for microbial mat growth within the littoral sandstones of the Magaliesberg Formation, points to strongly episodic sedimentation by both marine and fluvial processes. Within two younger basins, belonging to the c. 2.05–1.8 Ga Waterberg Group, anomalously high palaeoslopes (calculated according to standard palaeohydrological methods), combined with direct field evidence for sediment gravity-flows and sheetflood deposition within an overall braided fluvial setting, again suggest episodic sedimentation. Strongly episodic sedimentation can most easily be reconciled with a greenhouse palaeoclimate, at least for that part of Kaapvaal at that time. Taken together, these data sets point to the postulate that the generally accepted change from a Palaeoproterozoic greenhouse through a “great oxidation event” (c. 2.3 Ga?) to less extreme climatic conditions, may have been gradational and even “diachronous” across the Earth's cratons at that time.