Abstract

The stratigraphy of Middle and lowermost Upper Devonian rocks in Iowa and the central Midcontinent region is revised and refined. These rocks, the Wapsipinicon and Cedar Valley groups, overlap to the northwest across Iowa and southward towards the Ozark Uplift. The Cedar Valley Group reaches its maximum thickness in central Iowa, delineating the central region of the Iowa Basin. The Iowa Basin was the site of shallow marine to supratidal carbonate and evaporite deposition during the Devonian. Sedimentation kept pace with subsidence, and the Iowa Basin did not develop as a bathymetric basin during Devonian time.

Basal Wapsipinicon nonmarine dolomitic strata (Bertram Formation) were laid down in a fault-related topographic depression in east-central Iowa. Subsequent Wapsipinicon deposition expanded during late Eifelian marine transgression, with accumulation of Otis Formation carbonates in east-central Iowa and northern Illinois, containing a restricted marine fauna, and Spillville Formation dolomites in northern Iowa and southern Minnesota, with a diverse marine fauna. The Transcontinental Arch appears to have been breached at that time, establishing open marine connection with the Elk Point Basin of Canada. However, physical barriers restricted marine connections in the area of Otis deposition, isolating it from open marine deposition of the Spillville to the north. Upper Wapsipinicon strata, the Pinicon Ridge Formation, overlap the Otis-Spillville edge and, although the seaway expanded, are characterized by unfossiliferous carbonates, shales, and evaporites. These lower Givetian strata consist of a sequence of carbonate-evaporite cycles that were deposited in restricted environments, probably in a seaway with anti-estuarine circulation patterns.

The Cedar Valley Group overlaps the Wapsipinicon edge to the south and west and includes four major transgressive-regressive (T-R) depositional cycles, each given formational status. Each formation is subdivided into constituent members which are defined by fossiliferous carbonates below and laminated, evaporitic, or restricted marine facies above. Carbonates make up the bulk of these cycles and become increasingly dolomitic to the north. Tidal flat and lagoonal facies prograded toward an intracratonic shelf margin in east-central and southeastern Iowa during the first three T-R cycles; they are represented to the east-southeast by thinner, open marine facies. The first cycle, the Little Cedar Formation (new), was initiated during the Taghanic onlap, and spans the upper Middle varcus Subzone through Lower subterminus Fauna (a probable Lower disparilis Zone equivalent), and contains extensive evaporites in central Iowa. The third cycle, the Lithograph City Formation, spans the insita Fauna (= Lowermost asymmetricus Zone) through the upper Lower asymmetricus Zone, and also includes evaporites in central Iowa. The fourth cycle, the Shell Rock Formation, contains Middle asymmetricus Zone faunas and was removed by pre-Lime Creek erosion in southeastern Iowa and west-central Illinois.

The depositional and faunal sequences in Iowa, central Missouri, and Manitoba are comparable. The Cedar Valley Group correlates with the Callaway and lower Synder Creek formations of central Missouri, while the Wapsipinicon and Cedar Valley groups together correlate with the Elk Point and Manitoba groups, respectively, in Canada.