Abstract

Along the west flank of the Sawatch and Park Ranges the Parting Formation is composed of quartzose sandstone and some conglomerate; on the White River Plateau the formation consists of interbedded quartzose sandstone, sandy shale, and thin dolomite beds. Dispersal patterns in sandstones of units A and C, lowest and uppermost of the three stratigraphic units that compose the formation, are indicated by, 1) crossbedding, 2) variations in grain size, and 3) variations in sorting values.

The crossbedding azimuths for both stratigraphic units are oriented predominantly in two directions. One group of crossbeds dips to the northwest; the other dips to the southeast. This northwest and southeast trend parallels the zero isopach for both units.

Linear trend surface analysis indicates a decrease in mean grain size in both units in a general south-southwesterly direction. The quadratic trend surface for the lower stratigraphic unit shows a marked decrease in mean grain size from east to west for the eastern half of the area, and a gradual decrease in mean grain size to the northwest and southeast. The quadratic trend surface for the upper stratigraphic unit shows decrease in mean grain size from the north to the south, southeast and to the west. These trend surfaces show a northwestward shift in the area of influx of coarser material, from early to late Parting time. Also shown is a prominent southeasterly trend in the decrease in mean grain size. Longshore currents are suggested as the main dispersal agent, with southeasterly-moving currents the most prominent.

Linear trend surfaces demonstrate that the sorting, as measured by standard deviation of grain size, becomes better to the southwest in the lower stratigraphic unit, and decreases slightly in the same direction in the upper unit. Deviations from the linear trends are used to indicate sorting patterns. Prominent elongate areas of better sorting oriented northwest-southeast are indicated by the deviations. Longshore currents are suggested to explain theses sorting patterns.