Abstract

The Solor Church Formation of Late Keweenawan (Late Precambrian) age occurs in the subsurface of southeastern Minnesota where it comprises an integral part of the St. Croix horst, the major structure that underlies the northern part of the Midcontinent Gravity High. The formation consists of intercalated red intraformational conglomerate, sandstone, siltstone, mudstone/shale, and lesser amounts of limestone. The clastic rocks were derived from a dominantly basaltic terrane much like that which now flanks and underlies the formation. The limestones have a limited distribution and consist of admixed oolites and various kinds of intraclasts indurated by a sparry cement or a micritic groundmass. Two kinds of sedimentary cycles, each characterized by "fining-upward" attributes, are recognized in the Solor Church Formation. (1) Minor cycles, 3-50 ft thick, which consist of a sandstone/conglomerate facies overlain by a siltstone/mudstone facies. Each minor cycle contains a vertical succession of primary structures and textures indicative of alluvial sedimentation. (2) Major cycles, which may be as great as 600 ft thick, are characterized by a systematic increase, stratigraphically upward, in the amount of mudstone relative to sandstone. This fining upward trend reflects a more or less systematic increase in the thickness of the fine-grained facies, whereas the thickness of the coarse-grained facies in each minor cycle remains more or less constant. As the proportion of siltstone/mudstone increases, there is a corresponding decrease in the apparent grain size of the sand- and silt-size detritus. Each major cycle is terminated by an abrupt decrease, over a short vertical distance, in the relative amount of fine-grained detritus and a corresponding increase in the amount of sandstone. The Solor Church Formation most likely was deposited by rivers flowing on an alluvial plain; in each minor cycle the conglomerate and sandstone were deposited in a migrating channel, whereas the mudstone and limestone were deposited on inter-channel flood plains and in shallow lakes. The general increase upward in the amount of silt and clay implies that the upper part of each major cycle was dominated by overbank or inter-channel deposition rather than channel deposition Thus, each major cycle records a progressive change toward a quasi-equilibrium balance between rate of subsidence and rate of infilling. However, the sudden reappearance of abundant conglomerate and sandstone, at the base of the succeeding major cycle, indicates an abrupt increase in the rate at which materials were supplied to the system. It is inferred that these abrupt changes record periods of tectonism during the evolution of the St. Croix horst.

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