UNUSUAL SECTIONS OF THE READSTOWN MEMBER, ST. PETER FORMATION, AT DECORAH, NORTHEAST IOWA

New data from the Decorah, Iowa area. derived from water well drill-cutting and driller logs, borehole geophysics and outcrop provide the basis for the recognition of an unusually thick St. Peter Formation section composed of a typical Tonti Member and an atypical underlying Readstown Member. In this area the Tonti ranges from 50 to 140 feet thick and the Readstown from zero to 630 feet thick. Thick Readstown sections appear to have accumulated within a paleovalley above a widespread unconformity that separates the Sauk and Tippecanoe cratonic sequences, a topographic setting similar to that envisioned by Mai and Dott (1985) for thick Readstown sections in the subsurface of southern Wisconsin. A post-Prairie du Chien fault, described in a deep well at Decorah, and possibly other faults, may have influenced the development of this erosional surface. Readstown strata at Decorah rest on units as old as Upper Cambrian Wonewoc Formation to as young as Lower Ordovician Shakopee Formation. Facies of the Readstown include fine-to-coarse-grained quartzose sandstone, medium-grained to pebbly sandstone containing rare pink granite pebbles and white quartzite pebbles up to 9 mm in diameter, glauconitic dolostone and sandstone, and laminated, noncalcareous sandy shale displaying reddish-brown, grayish-green and dark grey coloration. Chert is not a major constituent as it is in other Readstown sections.

A distinctive laminated green to dark gray pyritic shale unit, up to 125 feet thick in one well, appears as a consistent uppermost lithic unit underlying the Tonti Member. The upper part of this shale, found in one small outcrop, has yielded Middle Ordovician Chazyan age conodonts (including Archaeognathus, Erismodus, Erraticodon and Phragmodus etc.), at least two conodont assemblages, lingulid brachipods, compressed carbonaceous fossils resembling brown algae, and crustaceans (?Caryocaris). A gravimetric thermal decomposition test indicated fixed carbon in the shales of 5.34-6.44%, and in the algae-like "flakes" of 81.3%; infra-red spectrometry showed an active band indicative of hydrocarbons. Local newspaper accounts from 1922 suggest that a coal-like layer may exist at depth within the shale in the vicinity of the outcrop. We are considering a new name for this local shale: Freeport Bridge Shale.