RI-16 Revised Keweenawan Subsurface Stratigraphy, Southeastern Minnesota

Abstract

The Midcontinent Gravity High is the major tectonic feature of the northern midcontinent region. Numerous geophysical surveys have shown that this structure is mainly a sequence of basaltic lava flows which form steep-sided, en echelon blocks, on the average about 40 miles wide and several miles thick. Clastic rocks occur in graben-like flanking basins and in axial basins on top of the blocks. Of particular interest are the sedimentary rocks of Keweenawan age that flank and overlie the St. Croix horst, an uplifted basalt block in east-central and southeastern Minnesota. Because of their red color, these sandstones and shales have been grouped together under the name "Red Clastic Series," a "temporary" term first proposed in 1911. However, a detailed study of approximately 4,000 feet (1,220 meters) of diamond drill core from a number of localities has demonstrated the presence of at least three distinct lithic units which can be traced laterally for some distance. Accordingly, it is recommended that the name "Red Clastic Series" be abandoned and replaced by a more suitable nomenclature. The three Keweenawan formations recognized in the subsurface are: (1) Hinckley Sandstone, a buff to tan rock containing 95 or more percent quartz; (2) Fond du Lac Formation, consisting of intercalated moderate red shale and reddish-brown sandstone containing quartz, orthoclase, microcline, sodic plagioclase and IIgraniticll rock fragments; and (3) Solor Church Formation, a newly named formation consisting of dark reddish-brown mudstone and pale reddish-brown sandstone, containing variable amounts of quartz, plagioclase of intermediate composition (oligoclase-andesine), and aphanitic rock fragments. The first two formations are named from surface exposures; however, the Solor Church Formation, so far as is known, is confined entirely to the subsurface. A stratigraphiC analysis indicates that, in the flanking basins, the Solor Church Formation is overlain unconformably by the Fond du Lac Formation, which in turn is gradationally overlain by the Hinckley Sandstone. In contrast, the Solor Church Formation overlies basaltic rocks on top of the horst and in turn is unconformably overlain by the Hinckley Sandstone; at places a regolith separates the two formations. Either the Fond du Lac Formation was never deposited on top of the horst, or it was removed prior to the time of Hinckley deposition.