RI-19 Results of Subsurface Investigations in Northwestern Minnesota, 1972

Abstract

Two test wells were drilled in northwestern Minnesota during a test drilling program conducted by the Minnesota Geological Survey in 1972. Total footage drilled was 851.5 feet (260 m). Together, the wells represent a relatively complete stratigraphic section of the Paleozoic and Mesozoic rocks found in northwestern Minnesota, which are covered by thick Pleistocene overburden and do not crop out. Four stratigraphic units are present. The basal unit is the Winnipeg Formation, a Middle Ordovician unit approximately 160 feet (49 m) thick composed of quartzose sandstone and shale. It is overlain by the Red River Formation, an Upper Ordovician unit about 200 to 300 feet (61 to 91 m) thick composed of dolomitic limestone and dolomite. The other two units are interpreted to be Mesozoic. The lower unit is a reddish-brown shale with a thin layer of dolomite at the top. It is as much as 105 feet (32 m) thick in Minnesota. It may be equivalent to the red beds in adjoining parts of Manitoba and North Dakota that are assigned a Jurassic age, although no paleontologic evidence was found to indicate its age. The uppermost unit is a gray shale that is provisionally interpreted to be Cretaceous. Two tills can be recognized in the Pleistocene section on the basis of the lithology of the sand-size fraction and size-grade distribution. The surficial Pleistocene unit is clay associated with Glacial Lake Agassiz. The Precambrian basement in the area is principally volcanogenic metasedimentary rock and metamorphosed volcanic rock of Early Precambrian age. It was penetrated in one well where it is represented by actinolitic hornblende-plagioclase schist, a mafic, tuffaceous, sedimentary rock that has been metamorphosed.