Sedimentology and sequence stratigraphy of the paleoproterozoic Rove and Virginia formations, southwest Superior Province

Abstract

The Paleoproterozoic Rove and Virginia Formations are lithostratigraphically and chronostratigraphically correlative units which comprise the upper sedimentary strata in the Animikie basin. They sharply overlie an intensely altered zone within the upper Gunflint and Biwabik iron formations which was subaerially exposed by compressional forces during the Penokean Orogeny. Dating of volcaniclastic zircons from the upper Gunflint yielded a pre-Penokean age of 1878 Ma. Tuffaceous layers very near the base of the Rove and Virginia Formations provided U-Pb zircon ages of approximately 1835 Ma placing commencement of sedimentation into the newly resubmerged basin during the final stages o f Penokean igneous activity. This study involved examination of 3200 m of drill core from eleven continuously drilled holes and one twiced drilled hole extending over 424 km from south of Duluth to south of Thunder Bay. Observation of the lithofacies present and their stratigraphie relationships provided insight into the depositional environment as well as the tectonic regime operating at the time. The basal Rove and Virginia Formations were deposited as transgression progressed across the depressed basinal area. They consist of black, carbonaceous shale with thin interbeds of siltstone, very-fine grained sandstone and friable green tuffaceous layers, possibly contributed by volcanic activity within the Penokean terrain. From approximately 5 m above the base, siltstone and sandstone layers gradually diminish in frequency upward, until the succession is almost completely dominated by approximately 100 to 150 m of fissile black shale. Microscopic examination of thin sections of this unit revealed the presence of very thin shale laminae and other laminae composed of angular silt grains or microlayers consisting of carbon. This sediment-starved, condensed sequence developed with increasing water depth, and with anoxic conditions probably caused by high organic loading in the bottom sediments. A siltstone and very-fine grained sandstone-rich unit traceable across the basin occurs midway through the shale-dominated succession. This coarser unit thickens near both the northern and southern margins of the basin. Above it another coarser-grained interval within the shale-dominated succession is observed in the southern third of the basin, probably representing sediment contributed by Penokean sources to the south. A gradational transition between the shale and an overlying sandstone-shale unit occurs over 80 m in the north, thinning to the south. The upper sandstone-shale unit varies in thickness, with a maximum of 350 m, and consists of over one hundred individual coarsening upwards parasequences. The individual packages are composed of graded, commonly massive, fine-grained sandstones separated by shale layers millimetres to centimetres thick. Shale units separating the parasequences are decimeters to one or two meters in thickness. The sandstone-shale assemblage fines to the south. Approximately 500 m above the base of the section the uppermost unit is dominated by lenticular bedding of fine-grained sandstones in the black shale, with both current and wave ripples present. The entire succession represents the transition from a sediment-starved basin, with exceedingly slow deposition rates, to active deltaic progradation with sediment probably derived from the Trans-Hudson orogenic zone to the north.