Named as basal member of Escalante Desert Formation of Needles Range Group for occurrence at Marsden Spring, Iron Co, UT in the Great Basin province. Type section designated in sec 25, T31S, R15W, Lund 7 1/2 min quad. Exposed in Wah Wah Mountains and Needles Range. Areal extent map. Consists of white to pale-gray, green, orange, and pink densely welded ash-flow tuff which has less than 6 percent phenocrysts of sodic plagioclase, quartz, and trace of biotite, hornblende, and iron-titanium oxides. Is easily eroded and poorly exposed because of fractures. Originally covered an area of 1,000 sq km. Had an estimated volume of 300 cubic km. Is 110 m thick. Reference section designated at SW1/4 secs 1 and 2, T29S, R19W. Pine Valley caldera was the source area. Rests on a local unnamed conglomerate at type; overlain by Lamerdorf Tuff Member (new) of Escalante Desert. Assigned to the Oligocene. Stratigraphic chart.

Source: GNU records (USGS DDS-6; Denver GNULEX).