ABSTRACT

The major constituents of Upper Devonian sandstones that are exposed around the White River Plateau, the west and north flanks of the Sawatch Range, and the west flank of the Park Range, include quartz, feldspar, and rock fragments. The feldspar is predominantly microcline and the rock fragments are mainly composite grains of quartz. The cementing agent is silica.

Trend surface analysis of the major constituents was used to map areal variation in two stratigraphic units, A and C. The quartz content of unit A sandstone increases from the northeast to the southwest in the eastern part of the area, and increases toward the northwest and southeast in the western part. The rock fragment content decreases in these directions. These compositional trends closely parallel a decrease in mean grain size. The feldspar content decreases from the northeast to the southwest but does not show a close parallelism to grain size or the other compositional trends. A facies map developed from trend surfaces of quartz percent and rock fragment-feldspar ratio for unit A, shows orthoquartzite in the western part of the area, protoquartzite and feldspathic sandstones i the center, and subgraywackes in the eastern segment.

Unit C quartz content increases following the same pattern as that for unit A, however both feldspar and rock fragment content decreases in these directions in unit C. All unit C trends parallel a decrease in mean grain size in the same directions. The facies map shows orthoquartzites to the northwest and southeast, feldspathic sandstones and protoquartzites in the center, and subgraywackes to the northeast.

Longshore drift moving primarily to the southeast and secondarily to the northwest in shallow transgressing sea is suggested as the depositional model for these sediments. Progressive sortings affecting primarily the rock fragments and secondarily the feldspar is probably the primary process controlling these facies.