The Gunflint cherts also have evidence of microscopic unicellular algae , indicating an oxygen-producing biologic system had evolved by this time. Other geologists argue that some of the iron minerals could have formed after deposition and during burial of the sediment. It could be that iron was precipated very rapidly in agitated shallow-water environments around the basin margin, but not in the middle. Interbedded dark-colored sandstones and siltstones are very poorly sorted both from the standpoint of grain size and minerals present. Such poor sorting is often evidence of rapid deposition by turbidity currents in a basin which was rapidly subsiding and therefore structurally unstable.
Graded bedding is also present, making turbidity current deposition a likely candidate.
Elongate markings on the underside of beds, produced by currents dragging some sort of tool along the bottom, indicate current directions toward the old basin center The history of the western part of the basin in which the Lake Superior Iron Formations and overlying dark-colored mudstones, sandstones and siltstones were deposited, is shown in the accompanying diagram . Early Proterozoic rocks to the south of the Superior Basin were deposited on a basement of ancient Archean gneisses Rocks of the same age to the north were deposited on a basement of greenstone and granite. The gneissic basement may have behaved in a more plastic manner and melted more readily than the basement to the north, resulting in more deformation and metamorphism toward the south . The boundary between the two types of basement is proposed to be a fault zone, called the Great Lakes Tectonic zone . This zone is still active, as most of Minnesota's modern earthquakes occur along its trace.