SP-07 The Geology of the Middle Precambrian Rove Formation in northeastern Minnesota

Abstract

The Middle Precambrian Rove Formation, the upper part of the Animikie Group, is estimated to be at least 3,200 feet thick and is exposed between northwestern Cook County, Minnesota and the Thunder Bay district, Ontario. It is a sequence of graywacke, argillite, locally abundant intraformational conglomerate, quartzite, and carbonate rocks. The formation was deposited some time between 2.0 b.y. and 1.7 b.y. ago in a northeast-trending basin, the configuration of which probably was controlled by a pre-existing structural grain. Detailed mapping in the 7 1/2-minute South Lake quadrangle combined with a field and laboratory study of approximately 150 other scattered stratigraphic sections provide a basis for the recognition of three informal lithic units. From oldest to youngest these are: (1) lower argillite, 400 feet thick; (2) transitional beds' of interbedded argillite and graywacke, 70 to 100 feet thick; and (3) thinbedded graywacke, as much as 2,700 feet thick. It is concluded that the argillite and associated graywackesandstone and graywacke-siltstone units were deposited in moderately deep, quiet water. Repeated graywacke sedimentation units indicate sediment transport and deposition by turbidity currents. A sedimentation unit reconstructed from composite sections consists of (1) a basal conglomeratic graywacke, (2) a structure less unit that grades indistinctly into (3) a graded graywacke that is overlain by (4) a laminated graywacke, which may be modified by (5) small-scale cross-bedding, or (6) contorted bedding. Anyone or several of these may be absent, but the unit is always overlain by (7) an argillite. Post-depositional soft-sediment structures such as load casts, flame structures, clastic dikes, bed pull-aparts, overfolds, and microfaults indicate rapid deposition of Rove sediments, active bottom currents, and post-depositional deformation, implying a significant paleoslope. A detailed analysis of paleocurrent directional indicators such as groove casts, flute casts, dendritic ridges, and cross-bedding shows that the turbidity currents had a southerly trend about perpendicular to the axis of the Rove basin. However, ripple marks, winnowed lag deposits at the tops of many graywacke beds, and possibly some festoon-type cross-bedding show that the turbidities were later modified by bottom currents that trended southwesterly or parallel to the axis of the basin. The heavy minerals of the Rove are characterized by epidotegroup minerals, apatite, sphene, and tourmaline, and are typical of older Precambrian igneous rocks now exposed north of the present Rove outcrop area. Thin-section and X-ray analyses of 200 samples show that the graywackes consist of angular, poorly sorted grains of clastic quartz and plagioclase (An10-An25) embedded in an argillaceous matrix that now consists of quartz, chlorite, and muscovite. The fine-grained, fissile argillite and mudstone have the same mineralogy and microtextures as the graywacke. Erosion subsequent to pre-Keweenawan tilting removed an unknown amount of the formation prior to the deposition of Lower Keweenawan sedimentary rocks. The intrusion of Middle Keweenawan mafic igneous rocks caused local metamorphism of the Rove Formation to a variety of mineral assemblages now assigned to the pyroxeneand hornblende-hornfels facies, but the remainder of the formation is essentially unmetamorphosed.