Abstract

The Upper Cambrian Franconia formation in southeast Minnesota and west-central Wisconsin consists of glauconitic, quartzose sandstones that average 175 feet in thickness. Previous subdivision of the Franconia resulted in faunal zones to which geographic member names were given. These zones are not rock units and cannot properly be called members.

In this paper, members are based on rock type. They are, in ascending order, the Woodhill member—medium- to coarse-grained sandstone; the Birkmose member—fine-grained, glauconitic sandstone; the Tomah member—sandstone and shale; and the Reno member—glauconitic sandstone. A fifth unit, the Mazomanie member—thin-bedded or cross-bedded sandstone, forms a nonglauconitic facies that interfingers with and replaces the Reno member. Faunal zones are independent of the lithic units.

The Woodhill member was deposited in the transgressing Franconia sea. Birkmose greensands formed in shallow waters far from shore, while Tomah sand and shale and Mazomanie thin-bedded sand were deposited nearer shore. Later, Reno greensands formed offshore, Mazomanie thin-bedded sand was deposited shoreward, and cross-bedded Mazomanie sand accumulated nearest the strand line.

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