Fluvial Sand-Body Dimensions and Architecture, Neslen and Lower Farrer Formations (Campanian), Lower Sego Canyon, Utah

Cole, Rex D.¹ (1) Mesa State College, Grand Junction, CO

The objectives of this study were to create a statistical database for lenticular and channel-form sand bodies in the Neslen and lower Farrer Formations. Three measured sections (total of 1,622 ft) were established and 55 sand bodies mapped (GPS) within an area of 1.4 mi2. Paleocurrent measurements (N = 374) were also collected. The Neslen Formation (260 ft thick), which consists of carbonaceous mudrock, fine- to medium-grained sandstone, and coal, was deposited in a coastal-plain setting, whereas the lower Farrer Formation (290 ft thick), which contains more sandstone and very little carbonaceous mudrock and no coal, was deposited in an alluvial-plain setting. The average sandstone-mudrock ratio is 22:78 for the Neslen and 38:62 for the lower Farrer. Sand-body connectivity is very low (<5%) in both units. For the Neslen sand bodies (N = 18), the thickness range is from 1.4 to 12.9 ft and the apparent-width range is from 52 to 1,222 ft; for the lower Farrer (N = 37), these ranges are 1.5 to 27.0 ft and 92 to 2,556 ft, respectively. The apparent-width parameter is the linear distance between pinchouts of a given sand body. In the Neslen Formation, 11% of the sand bodies consist of single channel fills, 39% are compound channel fills, and 50% are splays; whereas in the lower Farrer Formation, these proportions are 65%, 22%, and 13%, respectively. Paleocurrent data have a vector mean of 40.8º and suggest that the Neslen channel-form sand bodies have greater sinuosities that those in the lower Farrer.