The stratigraphic position of the Thomson slates [Minnesota]

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ID: 304259
1894
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Abstract
Lo~ation and Extent of the Series.THE Thomson slates (St. Louis slates, Cloquet slates) are schistose or slaty rocks which occupy an extensive area in Eastern Minuesota.They afford continuous exposures along the St. Louis River, not far from Duluth, near Thomson, Oarlton and Oloquet.Here they were first noted and described.They are in this region well cleaved, but possess evident and little-altered sedimentary characters.Further west they are fonnd along and beyond the Mississippi River, in the vicinity of Little Falls.Here they have become schistose; they are often micaceous, staurolitic, hornblendic, or garlletiferous.Frequent outcrops have been found, demonstrating the continuity of the series between the St. Louis and the Mississippi.In a north and south line, the belt has already been shown, by the explorations of O. W. Hall, :Merriam, Upham, and others, to extend, roughly speaking, fl'om near the lower part of Mille Lacs to near the northern boundaries of Aitkin and Oarlton counties.The writer has been able to trace these rocks as far north as T. 51-19, where an exposure is found which will be referred to latm'.This prolongs the known extent toward the north by 10 or 15 miles.The dimensions of the belt are thus about 100 miles east and west by 50 north and south, making an area of not far from 5000 sq uare miles.For the lithological variations from the simple cleaved slates, found within this area~ we may quote from Irving ::j: "They include, among the slates, fine-grained graywackeslates, clay-slates, sericitic quartz-slates, true quartzites, micaslates (often hornblendic), staurolitic mica• slates (often garnetiferous), and hornblende schists; and among the eruptives, diabases, gabbros, and diorites, the latter presumably altered forms of diabase or gabbro."Relations of the Thomson Series to the sW'rounding Rocks.On three sides, south, east, and west, the general houndaries of the Thomson rocks are known.To the south and west lie *To this series the name St. Lou~s has been applied by Irving, Sweet, Van Hise, and others.Inasmnch as this name, however, is already well established for a formation of quite different age, it has been thought best, in accordance with a suggestion of Professor H. S. Wllliams, to designate the Minnesota series in some other way.
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Authors Josiah Edward Spurr
Journal american journal of science
Year 1894
DOI
10.2475/ajs.s3-48.284.159
URL
https://doi.org/10.2475/ajs.s3-48.284.159
Keywords Keywords not found

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