EVIDENCE OF A PROGRADATIONAL SYSTEM IN FLUVIAL SHEET SANDS, MORRISON FORMATION, OJITO WILDERNESS, NM

Sedimentary features (i.e. grain size, rounding, sorting, composition, bedding) observed in a large (up to 15-m thick) sheet sandstone of the Westwater Canyon Member of the Morrison Formation, located in the Ojito Wilderness, New Mexico, show evidence of a progradational fluvial system. Field work conducted at the site includes measurements of paleocurrent direction from cross-bedding and stratigraphic section measurement along talus slopes and from interpretation of terrestrial lidar scans of the cliff face exposures. Preliminary observations show a general upward coarsening grain size trend in the sandstone, with primarily fine sand near the base of the section grading up to medium to coarse sand near the top of the section. Related to this change in grain size, a majority of trough cross-bedding used to obtain paleocurrent measurements are found towards the top of the observed section, with upper-stage plane bed deposits dominating sedimentary structures near the base of the unit. A greater amount of clay rip up clasts occur in channel lag deposits near the bottom of the section with very little found toward the top of the section, indicating less surrounding fine-grained floodplain deposits are available for reworking during later stages of deposition on this system. Finally, clastic sediments are less mature, more felsic and less rounded, at the top of the unit. The increasing grain size, observed sedimentary structures, and clast maturity in this sandstone unit are consistent with more proximal deposits overlying more distal deposits.