Historical:Spechts Ferry Formation
H. B. Willman and T. C. Buschbach
The Spechts Ferry Formation is named for Spechts Ferry, Dubuque County, Iowa (SW NW 4, 90N-2E).
The type section for the Spechts Ferry Formation is in a ravine at Spechts Ferry, Dubuque County, Iowa (SW NW 4, 90N-2E), where it is 7.8 feet thick (Agnew et al., 1956, p. 307; Templeton and Willman, 1963, p. 106).
The Spechts Ferry Formation is correlated with the Selby Member of the Rockland Formation in New York and the Curdsville Limestone in Kentucky and Tennessee.
Extent and Thickness
The Spechts Ferry Formation is exposed in northwestern Illinois along the Galena River in Jo Daviess County (cen. 34,29N-1E) and in small areas near West Point Landing in Calhoun County, and Valmeyer, Monroe County. It is commonly 5-10 feet thick in wells in western Illinois but is absent in the central and eastern parts of the state.
The Spechts Ferry Formation is the basal formation of the Galena Group, underlying the Kings Lake or Guttenberg Formations and unconformably overlying various Platteville formations. It rests on a distinctive, pitted, ferruginous, and phosphatic surface on the top of the Quirnbys Mill Formation, and the basal beds locally contain fragments of that formation. In northwestern Illinois, where the Kings Lake is absent, the Spechts Ferry is overlain by the Guttenberg Formation, the basal beds of which contain phosphate nodules.
The Spechts Ferry Formation (Kay, 1928, p. 16; 1935b, p. 287) consists of as much as 15 feet of interbedded shale and limestone. Limestone is dominant in the south and shale in the north. Much of the shale is bright green, but it is locally greenish gray and the basal few inches is dark brown. The formation is characterized by two bentonites and by the persistence of distinctive thin beds of dense, fine-grained to earthy limestone, coquinite, and dark purplish gray, coarse-grained calcarenite. It is subdivided into the Castlewood Member (at the base), which is dominantly limestone, and the Glencoe Member (at the top), which is limestone and shale-- in places dominantly shale. In the mining district in northwestern Illinois, it is called the "Clay bed."
Pionodema subaequata (fig. O-5) is abundant in the Spechts Ferry. Other Trentonian fossils are common, particularly bryozoans (Perry, 1962). The Spechts Ferry Formation is the Stictoporella bed of early reports.
AGNEW, A. F., A. V. HEYL, Jr., C. H. BEHRE, Jr., and E. J. Lyons, 1956, Stratigraphy of Middle Ordovician rocks in the zinc-lead district of Wisconsin, Illinois and Iowa: USGS Professional Paper 274-K, p. 251-312.
KAY, MARSHALL, 1928, Divisions of the Decorah Formation: Science, v. 67, p. 16.
KAY, MARSHALL, 1935b, Ordovician System in the Upper Mississippi Valley: Kansas Geological Society Guidebook, 9th Annual Field Conference, p. 281-295.
PERRY, T. G., 1962, Spechts Ferry (Middle Ordovician) bryozoan fauna from Illinois, Wisconsin, and Iowa: Illinois State Geological Survey Circular 326, 36 p.
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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