REVISION OF STRATIGRAPHY ACROSS THE TRIASSIC-JURASSIC BOUNDARY, FOUR CORNERS REGION, SOUTHWESTERN USA

Stratigraphy in the Four Corners region in use since the 1970s assigns the Moenave Formation, comprising the Dinosaur Canyon, Whitmore Point, and Springdale Sandstone members, and the partly laterally equivalent Wingate Sandstone to the Glen Canyon Group, which overlies unconformably the Chinle Group (“J-0 unconformity”). The Kayenta Formation overlies these two formations unconformably.

Detailed measurement and correlation of stratigraphic sections from across this region indicates significant errors in this well accepted stratigraphic hierarchy. The Rock Point Formation, previously the uppermost Chinle Group, and supposedly truncated by the J-0 unconformity, is laterally equivalent to the lowest beds of the Dinosaur Canyon Member, and is vertically gradational with the Wingate Sandstone. Furthermore, fossil bones and trackways of Late Triassic age in the Wingate beds, and diagnostic Lower Jurassic bones and trackways in the upper part of the Dinosaur Canyon beds demonstrate that these formations span the Triassic-Jurassic boundary (which appears to be in the middle part of the Dinosaur Canyon Member), and thus that there is no unconformity at the Triassic-Jurassic boundary. The Springdale Sandstone overlies these formations nconformably and has an aspect much like the overlying Kayenta Formation.

We suggest reassignment of the Rock Point, Moenave, and Wingate formations to the new Moenave Group, comprising the Dinosaur Canyon, Whitmore Point, and laterally equivalent Wingate formations. The former Rock Point Formation, which rests unconformably on the Tr-5 unconformity that separating the Moenave Group from the underlying Chinle Group, is equivalent to the basal beds of the Dinosaur Canyon Formation and is assigned member status within this formation. The Springdale Sandstone is excluded from the group and considered a member of the overlying Kayenta Formation. The basal Kayenta unconformity thus is the base of the redefined Glen Canyon Group.