Was the deposition of large Precambrian iron formations linked to major marine transgressions?

Abstract

Geochemical evidence suggests the iron- and silica-rich precipitates that compose large Neoarchean and Paleoproterozoic iron formations formed along a chemocline between iron-rich deep water and iron-poor surface water. Sedimento-logical evidence indicates many large iron formations accumulated in marine shelf environments during major transgressions. 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 transgressions 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 changes in the rate of crustal production. While the time scale of the deposition of large iron formations appears to be too long-for glacioeustatic or Milankovitch forcing, smaller-scale cycles exist within many iron formations which could reflect the latter.

Publisher

University of Chicago Press

Publication Date

11-1-1996

Publication Title

Journal of Geology

Department

Geology

Document Type

Article

DOI

https://dx.doi.org/10.1086/629861

Language

English

Format

text

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