Allostratigraphic relationships of Early Proterozoic iron-formations in the Lake Superior region

Early Proterozoic sequences in the Lake Superior region, which are sedimentologically related and contain major units of iron-formation, are divisible into allostratigraphic sequences-packages of rock bounded by unconformities. Analysis that emphasizes the bounding unconformities therefore emphasizes processes external to the depositional system, which initiate and terminate deposition of the sedimentologically related successions of rock types. An analysis of this type shows that the region's iron-bearing strata were deposited throughout the progressive growth and ultimate destruction of a rifted continental margin. The extensional phase of this continuum is marked by sedimentary units having an Archean provenance to the north and iron-formation having many descriptive attributes associated with Algoma-type iron-formation. Unconformities developed during extension resemble the rift-onset and the breakup unconformities recognized in younger continental margin assemblages. The compressional phase deposited in a northward-migrating foredeep basin is marked by sedimentary units having an Early Proterozoic provenance to the south and iron-formation having Lake Superior-type attributes. The boundary between the two is time-transgressive, migrating to the north as the basin migrated in that direction. This stratigraphic reconstruction departs significantly from those of the last 50 years which were based on the assumption of a single correlatable Early Proterozoic iron-formation in all the Lake Superior iron ranges. Sedimentation, although episodic, started around 2200 Ma and was essentially over by 1850 Ma. Thus units of iron-formation were deposited over some 350 m.y. of geologic time, an interval approximately 60 percent as long as that attributed to all of the Phanerozoic eon. Thus precipitation of iron-formation in the Lake Superior region is not a unique product of a single set of factors restricted to a short interval of geologic time. Our challenge will involve identifying underlying factors on a range-by-range basis.