Abstract

In the Uinta Mountain area, the Gartra Formation (Middle? Triassic) unconformably overlies the Moenkopi Formation on the east and the Ankareh Formation on the west, and interfingers with the overlying purple unit of the Popo Agie Formation (Late Triassic). Based on lithology, sedimentary structures, and weathering characteristics, the Gartra is informally divided into three subunits. The lower subunit is characteristically conglomeratic, poorly sorted, massive, and poorly bedded sandstone. The middle subunit is characterized by finer-grained sandstone and by dominant planar and trough cross-stratification. The upper subunit is finer grained than either of the subjacent subunits, consisting of claystone, siltstone, and very fine to medium sandstone. Horizontal and small-scale cross-stratification are characteristic of the upper subunit. Based on 42 modal analyses, the sandstone of all units is dominantly subarkose. Sandstone composition is apparently not related to grain size, and average constituents of the various grain sizes show no significant trends. The suite of sedimentary structures is small. The percent of each bedding type, based on its total thickness in the Gartra in 209 beds, is: cross-stratification, 50; horizontal stratification, 30; "structureless," 16; and graded stratification, 4. Dominant paleocurrent directions, based on 313 measurements, are to the west and northwest. Fifty-eight percent of all paleocurrent directions are between 241 and 330 degrees. Their distribution is unimodal. Detritus was derived from plutonic, sedimentary, and gneiss-schist terranes on the east and southeast (Ancestral Rockies and Uncompahgre uplift). The Gartra probably was deposited by a series of west-northwest flowing streams on a broad alluvial plain. Gradually decreasing velocity and turbulence of stream currents were responsible for the fining-upward sequences.

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