Stratigraphic architecture of a paleoproterozoic iron formation depositional system : the Gunflint, Mesabi and Cuyuna iron ranges

Abstract

Understanding iron formation depositional processes has been hindered by the lack of precise modern analogues. However, by combining a regional basin analysis of sedimentary and volcanic rocks surrounding an iron formation with detailed examination of sedimentary structures and lithic associations within an iron formation, the depositional setting and physical processes of sedimentation can be inferred (Fralick and Barrett, 1995). This technique departs significantly from previous methods used for interpreting iron formation sedimentoiogical environments. Instead of focusing on their mineralogy, the description of iron-bearing formations and members, and the erection of stratotypes the basis of this method is the application of Walther's Law and the facies model concept to large scale depositional tracts, up to and including the entire basin (Miall, 1984). Three Paleoprotoerozoic iron formations from the Lake Superior region were investigated using this approach. The Gunflint, Biwabik, and Trommald iron formations are correlative ferruginous units from the Animikie and North Range Groups along the north shore of Lake Superior in Ontario and Minnesota. Two clastic, two volcaniclastic, nine chemical sedimentary, and two cryptalgal facies are present: clast and matrix supported pebble conglomerate, massive quartz sandstone, slate, lapilli tuff, massive and/or cross stratified chert-carbonate grainstone, massive and/or cross stratified hematite-rich grainstone, massive chert-carbonate grainstone with rip up clasts, flaser bedded chert-carbonate grainstone, wavy bedded chert-carbonate grainstone, hummocky cross-stratified chert grainstone, parallel and wavy laminated chemical slate interbedded with chert grainstone, parallel and wavy laminated chemical slate, stromatolites, and oncolites. Chemical sedimentary facies may be grouped into associations comprising seven informal stratigraphic members. Lateral and vertical facies transitions within these members are similar to many Phanerozoic marine shelf systems. Like sediments accumulating along the margins of modern oceans, the chemical sedimentary rocks in Ontario and Minnesota also form a sedimentary wedge which fines and thickens from coarse wave reworked, nearshore deposits of the foreshore and shoreface to parallel laminated mudstones of the outer shelf and slope break. One important difference is the absence of a mud dominated inner shelf. This may be the result of the development of "grainstone factories" below fairweather wave base. Grainstone factories are thought to have formed in proximal offshore areas where iron oxide and silica gel actively precipitated, and where currents could rework these chemical precipitates into rip-up grains. They are broadly similar to modern "subtidal carbonate factories". Stratigraphic data also indicates that iron-bearing members are diachronous and that their deposition was governed by five changes in relative sea level and three pulses of volcanism. The final volcanic episode marks the cessation o f iron formation accumulation as the depositional system was rapidly overwhelmed with volcaniclastics.