EVIDENCE FOR EOLIAN DEPOSITION AND DUNE EVOLUTION IN TWO UNDESCRIBED SECTIONS OF THE UNKPAPA FORMATION, BLACK HILLS, SOUTH DAKOTA

The Jurassic Unkpapa Formation is reexamined in the Rapid City, South Dakota area to determine its paleoenvironment. Two previously undescribed sections are discussed and interpreted to be dune and interdune deposits. This claim is supported by the presence of pinstriping (wind ripples), fine laminae (adhesion ripples), wedge planar cross bedding, tabular planar cross bedding, and well sorted, subrounded to rounded quartz sands of a uniform composition; all of which are criteria of eolian deposits. This work supports previous interpretations that this unit represents an ancient eolian environment. Further scrutiny of bedding reveals tabular planar crossbeds underlying wedge planar crossbeds. These structures illustrate the process of transverse dunes grading directly into barchanoid ridge dunes during the process of dune evolution.