Abstract

Evidence from stratigraphy, carbonate mineralogy, and primary sedimentary structures indicates that the Rakes Creek Shale, Ozawkie Limestone, and Oskaloosa Shale Members in eastern Nebraska and western Iowa were deposited along the shore of a fluctuating sea Adjacent positive land areas were located to the west (Nehawka Arch) and locally to the south (Redfield Anticline); the subsiding Forest City basin was located to the southeast.

Stratigraphic evidence supporting these conclusions includes the progressively decreased thickness, increased insoluble content, and increased amount of red shale in these members toward the west. There is also a general increase in the dolomite content of these rocks from the ancient intertidal to the supratidal zone. Primary sedimentary structures suggestive of supratidal and intertidal origin such as birdseyes, desiccation structures, “worm” trails, and thin, wavy laminations are also more abundant toward the west. However, the most diagnostic shoreline features are the well-developed and varied cryptalgal structures of the lower Ozawkie Member which are arranged in subparallel belts analogous to those in the modern intertidal and supratidal zones of Shark Bay, Western Australia. The cryptalgal structures further indicate that the tidal range was at least 8 in., and the paleoslope of the intertidal-supratidal zone was only about 2 in./mi.

The interval from the Rakes Creek to the Oskaloosa Shale is part of a typical midcontinent Pennsylvanian megacyclic sequence, the Deer Creek Megacyclothem (Moore, 1936). The sequence began with the subaerial accumulation of clastic sediments of the Rakes Creek Shale Member followed by marine invasion and deposition of the Ozawkie Limestone Member. The overlying Oskaloosa Shale Member represents a minor regressive phase which ended with a second marine transgression during deposition of the Rock Bluff Limestone Member. Another minor regression with restricted circulation is represented by the lower Larsh Shale Member, which in turn was succeeded by a third transgression during deposition of the upper Larsh and lower Ervine Creek Members. The megacyclic sequence ended with a final major regression at the end of Ervine Creek time.

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