Sedimentation of the Middle Precambrian Tyler Formation of Northcentral Wisconsin and Northwestern Michigan

Abstract

The Tyler Formation crops out in a northeasterly-trending belt in Northcentral Wisconsin and Northwestern Michigan along the Gogebic Iron Range. Good exposures are found along several of the major streams draining the area, road cuts and railroad right-of-ways. The Tyler is considered Late Middle Precambrian in age, the Wisconsin equivalent of the Baraga Group of Michigan's Marquette Range Supergroup. The formation was warped and slightly metamorphosed during the Penokean Orogeny (1.7 b. y.). Primary sedimentary structures have been generally preserved. The sedimentologic aspects of seven outcrops in the Hurley, Wisconsin area were studied in detail and a measured stratigraphic section was established. Three distinct lithologies are present, sandstone (graywacke), siltstone and shale in varying degrees of metamorphism. Forty-one percent of the beds measured are argillites or slates and fifty-nine percent are sandstones or siltstones. Volumetrically, the sandstones and siltstones are more abundant than the shale units. A thinning-upward trend in the shale beds suggests a decreasing rate of sedimentation up section which in turn suggests increasing tectonic stability in the source area relative to the depocenter. Alternatively, the thinning-upward trend in the shales may reflect increased periodicity of the events which deposited the coarser-grained elastic beds. A corresponding volumetric increase in sandstones and siltstones up section is explained by the peculiarities of local basin bottom topography. Primary sedimentary structures including alternation of sand-mud units, laterally extensive bedding, graded bedding, rnicrocross-bedding, sole marks, rip-up clasts and Bouma sequences suggest a turbidity current mechanism for sediment transport and deposition in a realtively deep water environment. Twenty-four percent of the beds studied in detail are recognizable turbidites while thirty-five percent do not contain specific telltale sedimentary structures. A grain-flow origin for at least some of these latter beds is suspected. Because of the similarity between the lithologies and facies of the Tyler Formation and the ideal facies sequences of submarine fans, it is suggested that that part of the Tyler Formation which was studied in detail was deposited as part of a submarine fan complex. Indicators of current movement found in the rocks of the Tyler are of several types including both interstratal and intrastratal. Sole marks and cross-bedding are most useful. The currents which deposited the Tyler sediments moved from the east-southeast toward the west-northwest. A new type of sole mark, called a ridge mold or negative groove, is described and a possible method of formation is suggested. Two different methods for estimating current velocity were employed, one based on maximum clast size and one based on spacing of ridge molds. Both methods yielded velocities of a few tens of centimeters per second and primary slope is estimated at 0°10'. Petrology reveals that the major framework constituents of the Tyler graywackes are quartz, plagioclase and rock fragments set in a chlorite- and mica-rich matrix. The "average" graywacke is a lithic graywacke with 28% matrix. Quartz and chert comprise 73% of the framework grains, rock fragments 17% and feldspar 10%. The source terrain was probably mostly granitic with some contribution from older sedimentary, metamorphic and volcanic rocks. The Lower Precambrian rocks to the south and southeast of the Tyler outcrop belt were the probable source. Paleoslope was probably at right angles to current flow. Thus a southern limit to the extent of the Middle Precambrian depositional basin is defined. Similarity of the Tyler and other Middle Precambrian sedimentary rocks in the Lake Superior region suggests a common depocenter in a cratonic basin but at different depositional loci. Reconstruction of current movement and probable source area for the Tyler, Rove and Virginia (Thomson) Formations suggest that the depocenter was landlocked on three sides but may have been open to the northeast.