104: Wanakah Shale

AN EARLY LATE DEVONIAN BONE BED-PELAGIC LIMESTONE 

SUCCESSION: THE NORTH EVANS-GENUNDEWA 

LIMESTONE STORY 

In memory of Daniel B. Sass (1919-2006) 

Gordon C. Baird, Dept of Geosciences, SUNY-Fredonia, Fredonia, NY 14063; William T. 

Kirchgasser, Dept. of Geology, SUNY-Potsdam, Potsdam, NY 13676; D. Jeffrey Over, 

Dept. of Geological Sciences, SUNY-Geneseo, Geneseo, NY 14454; Carlton E. Brett, Dept. 

of Geology, Univ. of Cincinnati, Cincinnati, OH 45221. 

STOP 1. Taghanic disconformity exposed in small cut (Loc. 2) along Lake Shore Road 

opposite parking area (see Fig. 9b). Exit vehicles and proceed across Lakeshore Road to 

roadcut slightly south of the parking lot entrance. People must watch for traffic coming down 

the hill around the blind comer. 

This roadcut displays, in upward succession, the upper medial part of the Windom Member of 

the Moscow Formation (Fig. 9a), the Taghanic disconformity, a thin, local expression of the 

North Evans lag deposit below the Genundewa Formation (Fig. 9c), a thin unnamed and very 

localized dark gray to black shale unit within the Genundewa Formation, and the resistant ledge 

of styliolinid limestone representing the upper division of the Genundewa Formation (Fig. 9d). 

The disconformity below the North Evans spans possibly 7 conodont chronozones representing 

approximately 2 million years of geologic time (see text); units observed in the Finger Lakes 

region and in eastern New York (topmost parts ofMoscow Formation, Tully Formation, and 

lower half of Genesee Group succession) are missing here such that Late Devonian strata belong 

to the upper part of early Frasnian MN Zone 2 are juxtaposed onto Middle Devonian beds 

belonging to the Middle varcus chronozone (see text). 

The North Evans is here represented by a thin, 1-2 em-thick Jag unit rich in pelmatozoan debris, 

glauconite grains, a conspicuous mixed conodont fauna, and fish teeth, incuding numerous small 

cladodont shark teeth and rare ptychtodont tritors (Fig. 9c). Secondarily oxidized (limonitized) 

detrital pyrite grains are common in this mix, especially at the base. The tubular clasts represent 

pyritized burrows that were exhumed from the underlying Windom Shale (see text). Conodonts 

at the bedding-plane contact with the Windom include Ancyrodella recta that indicates the upper 

part of Frasnian conodont MN Zone 2. Above the North Evans is a 28 em-thick fissile dark 

shale unit that can be accessed below the Genundewa Limestone overhang. As noted in the text, 

this shale is extremely local in extent, being found only in the vicinity of the Lake Erie shore 

(Fig. 9). Because there is no lag deposit and implied discontinuity separating the shale from the 

overlying Genundewa upper division limestone bed at this section and at Pipe Creek, we believe 

that the shale unit is a local basinal subfacies of the upper Genundewa division or a post-North 

Evans Gcnundewa division not normally expressed elsewhere (see text). The presence of the 

dark sha1e unit roofing the North Evans in this section is believed to explain, in part, the thin, pyrite-rich character of the North Evans lag deposit in sections where it is present (Fig. 9). More 

intense dysoxia associated with the basinal subenvironment represented by onset of black shale 

deposition could explain local dissolution of the carbonate fraction normally seen in the North 

Evans (see text). The overlying ledge of the upper Genundewa division (Fig. 9d) displays the 

typical outcrop character of this unit. This bed is composed almost exclusively of uncompressed 

Styliolina fissurella with accessory occurrences of the small bivalve Pterochaenia, wood debris, and rare goniatites, and it probably represents an offshore dysoxic regime characterized by 

prolonged sediment starvation and surface water productivity supporting Sty/iolina as plankton.