RE-EVALUATING THE MORRISON FORMATION–LYTLE FORMATION CONTACT PLACEMENT ON SKYLINE DRIVE, CAÑON CITY, COLORADO

Previous research on the Jurassic Morrison Formation (MF) and Cretaceous Dakota Group (DG) has placed their contact in multiple stratigraphic positions over the last 50 years. In the Cañon City, Colorado, area, Skyline Drive provides an ideal place to re-evaluate previous contact placements using petrographic techniques. At Skyline Drive the MF-DG contact is unconformable with the basal DG Lytle Formation (LF) eroding into the uppermost Brushy Basin Member of the MF. The MF-LF contact is bound between four lithologically similar appearing sandstones, listed in ascending order; the Big Red (BR), the White Sandstone (WS), the Buff Sandstone (BS), and the Lytle Conglomerate (LC). The BR is a known part of the upper MF, whereas the LC is the known basal portion of the LF. The purpose of this study was to constrain the MF–DG contact along Skyline Drive. The objectives were to: 1) Petrographically characterize samples from the BR, WS, BS, and LC; and 2) Statistically test similarities between mineral compositions of these samples. The contact will be identified based on mineralogical differences, as the MF is derived from sediment shedding off the Ancestral Rockies and the LF consists of sedimentary fill shedding off the Sevier orogenic belt. The three MF-LF contact hypotheses tested were: 1) The contact was at the top of the BR; 2) The contact occurs between the WS and the BS; or 3) The contact was at the base of the LC. A thin section of each sample was made and the percentage of Quartz, lithics, Feldspar and matrix from each thin section was point counted using the program JMicroVision. 250 points were completed per image, 20 images were taken per sample, giving an overall total of 25,000 points for this study. The mineralogical compositions of BR, WS and BS were not significantly different from each other; however, LC was significantly different from the other three. Therefore, this study concludes that the MF-LF contact at Skyline Drive is indeed located at the base of the LC.