Abstract
Based on geologic mapping, measured sections, and lithologic correlations, the local features of the upper and lower type areas of the Early Arikareean (30.8–20.6 million years ago) Sharps Formation are revised and correlated. The Sharps Formation above the basal Rockyford Member is divided into two members of distinct lithotypes. The upper 233 feet of massive siltstones and sandy siltstones is named the Gooseneck Road Member. The middle member, 161 feet of eolian volcaniclastic siltstones with fluvially reworked volcaniclastic lenses and sandy siltstone sheets, is named the Wolff Camp Member. An ashey zone at the base of the Sharps Formation is described and defined as the Rockyford Ash Zone (RAZ) in the same stratigraphic position as the Nonpareil Ash Zone (NPAZ) in Nebraska. Widespread marker beds of fresh water limestones at 130 feet above the base of the Sharps Formation and a widespread reddish-brown clayey siltstone at 165 feet above the base of the Sharps Formation are described. The Brown Siltstone Beds of Nebraska are shown to be a southern correlative of the Wolff Camp Member and the Rockyford Member of the Sharps Formation. Early attempts to correlate strata in the Great Plains were slow in developing. Recognition of the implications of the paleomagnetic and lithologic correlations of this paper will provide an added datum assisting researchers in future biostratigraphic studies. Based on similar lithologies, the Sharps Formation, currently assigned to the Arikaree Group, should be reassigned to the White River Group.
Highlights
The Arikareean North American Land Mammal Age (30.8– 20.6 million years ago) was defined by the Wood committee in 1941, but due to the noncontinuous nature of the stratigraphic column as known at that time, the temporal sequence of a part of the paleofauna was unrecognized
The Wood committee defined the upper boundary of the Whitneyan Land Mammal Age as those taxa representative of the top of the Whitney Member of the Brule Formation in Nebraska
There is no surface lateral continuity of the Gering Formation or Brown Siltstone Beds of Nebraska with the Sharps Formation of South Dakota as the regions were separated by the Chadron Arch, a regional structural feature trending NWSE and oblique to the Nebraska-South Dakota state line [12]
Summary
The Arikareean North American Land Mammal Age (30.8– 20.6 million years ago) was defined by the Wood committee in 1941, but due to the noncontinuous nature of the stratigraphic column as known at that time, the temporal sequence of a part of the paleofauna was unrecognized. Though the lithologies did not correlate, a large overlap of the upper and lower type section sediments was assumed, and the thickness of the Sharps Formation above the Rockyford Member was reported to be 349.3 feet.
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