THE GRAND MESA BASALT FIELD, WESTERN COLORADO

Grand Mesa is a prominent east-west landform capped by basalt (published argon-argon dates of 9.22 to 10.76 Ma; average = 9.92 Ma). The original basalt field may have had a surface area of up to 750 mi2, but due to extensive Neogene erosion, the present-day remnant is about 53 mi2 This remnant has a "Y-shaped" outline, with Crag Crest on the east forming the stem, and the Palisade and Flowing Park lobes forming the branches. Thickness of the in-place basalt ranges from about 600 ft near Crag Crest to less than 200 ft on the western tips of the lobes. Small, isolated remnants (< 200 ft thick) of 10 Ma (?) basalt also occur 15 mi east of Crag Crest in the Crater Peak area.

The basalt sequence rests on a paleo-topographic Miocene surface that ranges in elevation from 11,227 ft on the east (Crater Peak) to 9,632 ft on the west (tip of Palisade lobe); the average east-to-west gradient is about 54 ft/mi. Rocks beneath the basalt sequence include the Uinta, Green River, and Wasatch Formations (Paleocene-Eocene), plus an unnamed Miocene (?) sequence of mudrock, lithic sandstone, and volcaniclastic conglomerate. Vent areas for the basalt were probably in the Electric Mountain area, where several major east-west dikes occur. Stretched vesicles (N = 647) in the western part of the field show flow movement to the southwest (vector mean = 244º). This observation is consistent with the general slope of the sub-basalt surface and changes in thickness.

Details on the basalt stratigraphy are available from nine core holes drilled on the Palisade lobe by the U.S. Bureau of Reclamation in 1985-86. In these cores, thickness of the basalt sequence ranges from 233 to 616 ft, with conspicuous thinning from east to west. Flow thickness ranges from 3.7 to 70.9 ft (average = 23.8 ft). The maximum number of flows observed in a single core was 26. Beds of rust-colored, fine-grained sedimentary rock commonly occur between the flows, but correlate poorly from core to core and in nearby outcrops. These beds probably represent wind-blown detritus, weathering residuum, and incipient soils. Each core hole penetrated into the Miocene (?) unnamed unit. In outcrop, flows have lenticular cross sections and may up to 1,000 ft across, but most are less than 300 ft across.