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
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.
Mossler, John H..
RI-19 Results of Subsurface Investigations in Northwestern Minnesota, 1972.
Minnesota Geological Survey.
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