H. B. Willman and T. C. Buschbach
The Hennepin Member of the Pecatonica Formation (Templeton and Willman, 1963, p. 75) is named for Hennepin County, eastern Minnesota.
The type section of the Hennepin Member is in a bluff on the west side of the Mississippi River at Lock and Dam No. 1 in Minneapolis (NE SW NW 17, 28N-23W), where the member is 2.3 feet thick.
Extent and Thickness
The Hennepin Member is locally present in extreme northern Illinois, where it is a dolomitic sandstone containing phosphate pebbles and is commonly 2-4 feet thick.
The Hennepin Member varies from a dolomitic sandstone to a siltstone and to very sandy silty dolomite, which differentiates it from the relatively pure, although sandy, Chana Member above. It differs from the sandstone in the Glenwood Formation below in that it contains carbonates and its sands have a texture like that of the St. Peter. It appears in places to be a reworking of Glenwood sediments at the beginning of Platteville carbonate deposition.
TEMPLETON, J. S., and H. B. WILLMAN, 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois State Geological Survey Bulletin 89, 260 p.
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