Abstract
The Cretaceous Rollins Sandstone Member (Mount Garfield Formation) is the youngest marine sandstone deposited within the Sevier foreland basin in Colorado. The Rollins Sandstone Member is a complicated stratigraphic unit that consists of four progradationally stacked sequences. These sequences were deposited as a result of high-frequency changes in sea level. Each sequence initiates with an incised valley fill and contains a single parasequence within the highstand systems tract. Parasequences within highstand systems tracts contain offshore to marine-shoreface deposits. The top of the Rollins Sandstone Member is a surface that results from the progradation of a single strandline. This surface can be used as a regional datum. This new datum indicates there is no upward-climbing geometry at the top of the Mount Garfield Formation, and the Rollins Sandstone Member and the Cameo Wheeler coal zone (of the Williams Fork Formation) are not time-equivalent units. The marine- shoreface deposits within the Rollins Sandstone Member represent high-energy shorefaces. These shorefaces had daily wave heights of 1-2 m and Nor'easter-scale storms occurring several times a year. These high-energy conditions produced a straight coastline along the western edge of the Cretaceous Western Interior seaway.