WAS THE DEPOSITION OF LARGE PRECAMBRIAN IRON FORMATIONS LINKED TO MAJOR MARINE TRANSGRESSIONS

Geochemical evidence suggests the iron- and silica-rich precipitates t hat compose large Neoarchean and Paleoproterozoic iron formations form ed along a chemocline between iron-rich deep water and iron-poor surfa ce water. Sedimentological evidence indicates many large iron formatio ns accumulated in marine shelf environments during major transgression s. We suggest this occurred because the chemocline could only impinge on continental shelves during times of sea-level rise or highstand. On the other hand, the stratigraphic record indicates that deposition of iron formations ceased during regressions in some basins and transgre ssions in others. To explain this, we propose that iron concentrations in the Precambrian ocean were greatest at mid-water depths in a zone on the order of 100 m thick. Radiometric age dates from the Hamersley Basin of Western Australia suggest major iron-formation deposition had a periodicity of ca. 20 to 125 m.y., close to second-order cycles in Phanerozoic sea level driven by long-term tectonic processes such as c hanges in the rate of crustal production. While the time scale of the deposition of large iron formations appears to be too long-for glacioe ustatic or Milankovitch forcing, smaller-scale cycles exist within man y iron formations which could reflect the latter.