Geochemistry and sedimentology of a facies transition from limestone to iron-formation deposition in the early Proterozoic Transvaal Supergroup, South Africa

This study deals with the geochemistry and sedimentology of a facies transition from interbedded carbonate-shale to banded iron-formation in the Campbellrand carbonate sequence to the overlying Kuruman Iron Formation of the Transvaal Supergroup in South Africa which is approximately 2.3 Ga old. Four major lithologies are: 1) limestone and dolomite, 2) shale and interbedded shale carbonate, 3) siderite-rich banded iron-formation, and 4) iron oxide-rich banded iron-formation. On the basis of the geochemical data and a reconstruction of the depositional basin for the carbonate-shale to iron-formation transition, we conclude that the limestone-dolomite-shale lithologies originated in a water column quite distinct from that in which the iron-formations were precipitated. We propose a model with a stratified water column in which the surface waters were the site of much organic carbon productivity and the locus of cryptalgal limestones and intraclastic limestone deposition; with at somewhat greater depth (below the chemocline) deposition of pyritic carbonaceous shale. Our model depicts the deeper waters as the site for iron-formation deposition. -from Authors