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
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
SP-07 The Geology of the Middle Precambrian Rove Formation in northeastern Minnesota.
Minnesota Geological Survey.
Retrieved from the University of Minnesota Digital Conservancy,