Jurassic of the Williston Basin and Adjacent Areas

ABSTRACT The Upper and Middle Jurassic of the Williston Basin region comprises nine stratigraphic units or regional correlation intervals, and two regional unconformities. On the basis of faunal, environmental and tectonic evidence these units are grouped into four major depositional sequences or stage-like intervals which Imlay refers to as the Gypsum Spring (or Piper), Sawtooth, Rierdon and Swift formations. Miscorrelation of the type sections of these formations are a result of variations in the stratigraphic succession caused by the sub-Swift and sub-Rierdon unconformities which correspond respectively to Arkell's (1956) Lower Callovian, and uppermost Callovian to Lower Oxfordian marine transgressions. Recognition of the regional extent and significance of these unconformities suggests these are the following stratigraphic variations from those generally accepted:--(a) the Lower Swift (Stockade Beaver/Hulett of the Lower Sundance) is older than the type Swift and younger than the type Rierdon formations; (b) the Sawtooth is a discrete stratigraphic unit which is younger than the Piper or Gypsum Spring.