The Gogebic Iron Range — A sample of the northern margin of the Penokean fold and thrust belt
The Gogebic iron range is an elongate belt of Paleoproterozoic strata extending from the west shore of Lake Gogebic in the upper peninsula of Michigan for about 125 km westward into northern Wisconsin. It is one of six major informally named iron ranges in the Lake Superior region and produced about 325 million tons of direct-shipping ore between 1887 and 1967. A significant resource of concentrating-grade ore remains in the western and eastern parts of the range. The iron range forms a broad, gently southward-opening arc where the central part of the range exposes rocks that were deposited somewhat north of the eastern and western parts. A fundamental boundary marking both the tectonic setting of deposition and the later deformation within the Penokean orogen lies fortuitously in an east-west direction along the range so that the central part of the range preserves sediments deposited north of that boundary, whereas the eastern and western parts of the range were deposited south of the boundary. Thus, the central part of the range provides a record of sedimentation and very mild deformation in a part of the Penokean orogen farthest from the interior of the orogen to the south. The eastern and western parts of the range, in contrast, exhibit a depositional and deformational style typical of parts closer to the interior of the orogen. A second fortuitous feature of the iron range is that the entire area was tilted from 40° to 90° northward by Mesoproterozoic deformation so that the map view offers an oblique cross section of the Paleoproterozoic sedimentary sequence and structures. Together, these features make the Gogebic iron range a unique area in which to observe (1) the lateral transition from deposition on a stable platform to deposition in a tectonically and volcanically active region, and (2) the transition from essentially undeformed Paleoproterozoic strata to their folded and faulted equivalents. Paleoproterozoic strata in the Gogebic iron range are part of the Marquette Range Supergroup. They were deposited unconformably on Neoarchean rocks consisting of a diverse volcanic suite (the Ramsay Formation) which was intruded by granitic rocks of the Puritan Quartz Monzonite. The Marquette Range Supergroup in this region consists of a basal sequence of orthoquartzite (Sunday Quartzite) and dolomite (Bad River Dolomite), both of which are part of the Chocolay Group. The group is preserved only in the eastern and western parts of the range but was probably present throughout before the erosion interval that separated it from the overlying Menominee Group. The Menominee Group consists of basal clastic rocks (Palms Formation) that grade upward into the Ironwood Iron-Formation, which is the principal iron-bearing unit of the range. The Ironwood interfingers with the Emperor Volcanic Complex in the eastern part of the range and with volcanic rocks and gabbro in the western part of the range. The Ironwood is overlain unconformably by the Tyler Formation in the central and western parts of the range and by the Tyler’s equivalent, the Copps Formation, in the eastern part of the range. Strata in the central part of the iron range are entirely sedimentary. Deposition occurred in a relatively stable tectonic setting, at least until the deposition of the Tyler Formation. The Tyler consists largely of turbidites deposited in a foreland basin in advance of accreting volcanic arcs to the south. Penokean deformation in the central part of the range was very minor; the evidence of deformation consists of steep faults with small offsets and a few bedding-parallel faults that also have small offsets and that are recognized only in mine workings. In both the eastern and western parts of the iron range, abrupt facies changes mark a passage into a more tectonically and volcanically active belt. These relationships are especially well displayed in the east where a graben, the Presque Isle trough, began to subside during deposition of the Ironwood Iron-Formation. The thickness of the Ironwood increases into the graben and its internal stratigraphy also changes. The most prominent changes in the graben are the presence of a thick volcanic unit, the Emperor Volcanic Complex of the Menominee Group, and comagmatic gabbro sills that interfinger with the Ironwood. In the western part of the range, volcanic rocks and comagmatic gabbro sills are also present in the Ironwood, but a graben that is equivalent to the Presque Isle trough is not evident. Penokean structures are well developed in both the eastern and western parts of the iron range. They consist of folds ranging from outcrop to regional scale and thrust faults which, in places, either repeated the section or detached it from Neoarchean basement. The sharp transition from the little-deformed central part of the range to the more intensely deformed eastern and western parts coincides closely with the earlier developed transition from the stable sedimentary setting in the central part to the tectonically active sedimentation in the east and west parts. The extensional structures that formed during sedimentation may have helped to control the extent of later Penokean compressional structures.