| Field Trip Description: We propose this trip to present a depositional and high-resolution sequence stratigraphic analysis for the Upper Cretaceous Mt Garfield Formation, Grand Junction, Colorado area. The Mt Garfield Formation is interpreted as coastal plain to shoreline/nearshore environments, and represents the final regression of the western interior seaway. Nearshore and tidal environments are well exposed. Following detailed field study, five depositional sequences have been identified, each bracketed by a well-documented sequence boundary (regional erosion surface overlain by a basinward shift of facies). The five sequences will be examined in stratigraphic succession, starting with the most proximal (western) exposures and working progressively eastward (i.e., more distally) with each stop. Highstand deposits will be traced along depositional dip, from proximal to distal exposures. These consist primarily of wave-dominated shoreface successions, and range from distal offshore siltstones to shoreline sandstones. Parasequence architecture of the highstands will be emphasized, including the regional distribution of component subenvironments. Facies of the lowstand and transgressive systems tracts will also be emphasized, and consist primarily of incised valley fill deposits. Incised valley fill strata are complex: some valley fills are characterized by vertically nested fluvial successions, while others contain vertically stacked, tidally influenced sandstones. Many localities contain stacked incised valley fills, which are discernable only through detailed outcrop mapping. A sequence stratigrahic interpretation of the Mt Garfield Formation illustrates the complex stratal geometries associated with higher frequency sequences, as well as the compartimentalization of a formation caused by the depositional response to rapid and successive changes in base level. |