| dc.description.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. | |
