CORRELATION AND SEQUENCE STRATIGRAPHY OF JURASSIC SAN RAFAEL GROUP, SE UTAH: IMPLICATIONS FOR MODELLING DEPOSITION OF WET EOLIAN SYSTEMS

The dominantly continental San Rafael Group (Middle-?Upper Jurassic) is an unconformity-bounded succession of arid-cycle sediments between the Lower Jurassic Glen Canyon Group and the Upper Jurassic Morrison Formation. Measured sections carefully correlated by physical stratigraphy along an ~ 60 km transect between Bluff and the Abajo Mountains in SE Utah reveal the following: (1) the Carmel Formation is continuous, 8-18 m thick and disconformable on the Navajo Sandstone (J-2 unconformity); (2) the Entrada Sandstone (Slick Rock Member) is continuous, 13-33 m thick and conformable(?) on the Carmel; previously used informal members of the Entrada based largely on color are of no stratigraphic merit; (3) the Summerville Formation is continuous, 27 to 33 m thick south of the Bluff pinchout, and does not intertongue with the Entrada; (4) the Bluff Sandstone grades northward into the upper Summerville, changing from ~ 100 m thick at Bluff to ~ 5 m thick about 15 km south of the Abajo Mountains; (5) the Recapture Member of the Bluff is physically continuous with at least part of the Tidwell Member of the Summerville; and (5) the base of the Salt Wash Member of the Morrison is a pervasive unconformity (J-5) with demonstrable local stratigraphic relief of up to 17 meters. These observations counter previous claims of extensive Entrada-Summerville (or “Wanakah”) intertonguing in SE Utah. Our field data do not support recognition of four depositional sequences in the San Rafael Group. They also do not allow Entrada deposition to be modelled as a wet eolian system responding to several sea level changes. Instead, our data identify the Entrada as representing dunal and interdunal facies deposited during a relative low of regional sea level.