T. C. Buschbach
The Derby and the overlying Doerun were named separately for exposures in the vicinity of mines operated by the Derby Lead Company and the Doerun Lead Company near Elvins, St. Francois County, Missouri. Their similar lithology and a conformable relationship resulted in their being combined as a formation with a hyphened name (Hayes and Knight, 1961).
Extent and Thickness
The Derby-Doerun Member of the Franconia Formation underlies all of Illinois and is about 20 feet thick in the north and it thickens to probably more than 500 feet in the southeast.
The Derby-Doerun Member of the Franconia Formation (Buckley, 1907; 1909, p. 44) overlies the Davis Member in central and southern Illinois. It is included in the Knox Dolomite Megagroup.
In north-central Illinois the Derby-Doerun is silty argillaceous dolomite-- it is the upper part of the Franconia where the lower part of the Franconia is fine-grained glauconitic sandstone. Farther south it becomes quite pure, except for a thin zone at the top that is slightly sandy, argillaceous, and glauconitic.
BUCKLEY, E. R., 1907, Missouri Bureau of Geology and Mines, 2nd Series, v. 10, separate.
BUCKLEY, E. R., 1909, Geology of the disseminated lead deposits of St. Francois and Washington Counties: Missouri Bureau of Geology and Mines, 2nd Series, v. 9, part 1, 259 p.
HAYES, W. C., and R. D. KNIGHT, 1961, Cambrian System, in Stratigraphic succession in Missouri: Missouri Geological Survey, v. 40, p. 14-20.
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