No agreement exists on this question. It could be that iron was precipated very rapidly in agitated shallow-water environments around the basin margin, but not in the middle. The Thompson Formation is well exposed near Cloquet, Thompson, and Jay Cooke State Park. 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 basin in which the Lake Superior Iron Formations and overlying dark-colored mudstones, sandstones and siltstones were deposited, is shown in the accompanying diagram . Note that this diagram is for the western part of the basin, in Minnesota. Farther to the east and south, in Wisconsin and Michigan, the basin was more complicated and tectonically unstable. And perhaps this same setting continued all the way northeast to Labrador, where similar iron formations are present. But this is conjecture at this point.