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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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The Genundewa Member (medial Genesee Formation: Frasnian) is a thin limestone, traceable from Canandaigua Lake westward to Lake Erie. This unit, bounded both above and below by black, fissile shales, is composed mainly of shells of the pelagic, conoidal organism Styliolina fissurella, and lesser amounts of pelmatozoan debris, cephalopod conchs, wood fragments, and conodonts. The Genundewa displays prominent westward sedimentary condensation; eastern sections, up to 13 ft (4 m) thick, are mudstone-rich. West of central Genesee County, the Genundewa is typically a thin, 2 to 14 in. (5 to 36 cm) thick, compact bed composed of Styliolina biosparite with localized lentils of cephalopod coquinite.
The Genundewa is underlain by a prominent discontinuity traceable from central Genesee County westward; this hiatus is coextensive with remanie deposits of the North Evans Member which overlie the Windom-Genesee unconformity in western Erie County. The sub-Genundewa unconformity displays westward erosional overstep of underlying beds across Erie County with consequent increase in hiatal interval. East of Cazenovia Creek, the disconformity is marked by exhumed, glauconite-coated Penn Yan concretions and a thin blanket of remanie sediment. From Cazenovia Creek westward, the remanie blanket thickens with appearance of reworked Windom-derived brachiopods, and concretions, numerous fish bones, and increased pelmatozoan debris.
Pre-Genundewa erosion occurred in an oxygen-deficient outer shelf setting, as indicated by absence of associated benthos and by local occurrence of the hiatus within black shale. Genundewa Styliolina ooze accumulated following the erosion event under conditions of prolonged sediment starvation on a dysaerobic sea floor.
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