Mathematics – Logic
Scientific paper
Aug 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990esrv...28..321c&link_type=abstract
Earth Science Reviews, Volume 28, Issue 4, p. 321-364.
Mathematics
Logic
Scientific paper
The recurrence of major geological events within the 3600 Ma stratigraphic record of southern Africa suggests a periodicity of320 + - 25Ma for at least the last 3200 Ma. This periodicity is reflected in major episodes of continental volcanism, granitoid plutonism, orogeny, reciprocity of terrestrial- and marine-dominated sequences, platform-carbonate sedimentation, glaciation and first-order changes in sea level. These serve to define 11 completed megacycles, each introduced by a major rifting event accompanied by listric faulting, alluvial sedimentation in restricted fault-bound troughs, volcanism, and A-type granitoid plutonism. Prior to 3200 Ma, continental rift magmatism occurred subaqueously, due to very high early Archean sea levels, resulting in pillow lavas and a lack of interbedded terrigenous siliciclastics. The long-term reciprocity of terrestrial- and marine-dominated sequences records eustatic first-order changes in sea level, with highstands characterized by generally humid, warm continental climates, low relief, and tropical afforestation. On the other hand, lowstand climates were hot and arid, with high relief and widespread desertification. Thick marine sequences accumulate in areally extensive epeirogenic basins, without significant volcanism or faulting, and commonly are divided into an early phase of open-shelf sedimentation followed by deposition in a brackish-marine foreland basin. Within openshelf environments, an early phase of biochemical precipitation and/or siliciclastic reworking along linear shorelines is separated from a later phase involving rapid deposition of immature siliciclastics along lobate shorelines. Evolution of the foreland basin is related to orogenic activity and the isolation of an epeiric sea which sometimes developed the characters of a barred basin. Marine-dominated sequences display a deterministic stacking of lithofacies, important glacial epochs commonly occur midway through first-order regression, sometimes with lesser events closely following peak transgression; potentially they display 320 Ma periodicity, but with an unexplained mid-Proterozoic gap. Glaciation is related to the combined effect of climatic cooling associated with first-order regression and depletion of atmospheric CO2 by biological uptake associated with platform-carbonate development. Terrestrial volcanism is believed to be the main contributor of atmospheric CO2. Climatic change is accompanied by cyclical patterns of cratonic weathering and pedogenesis.
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