ERUPTIVE HISTORY OF THE GRAND MESA BASALT FIELD, WESTERN COLORADO

Grand Mesa (GM) exists today because it is capped by a Miocene basalt sequence that has a present-day footprint of about 155 km². The largest surviving part of the field is to the west (Palisade lobe, Flowing Park lobe, and Crag Crest); additional outliers exist to the east (e.g., Leon Peak, Crater Peak, Mt. Hatten, and Mt. Darline). The flow sequence rests on a west-tilted (average gradient = 10 m/km) paleo-topograhic surface that ranges in elevation from 3,422 m (Crater Peak) to 2,936 m (Palisade lobe). Corehole data (N = 9) for western GM show the sequence to thin significantly from east to west. Total thickness of the flow sequence ranges from 56 to 189 m for western GM and from 15 to 34 m for eastern GM.

Twenty three ⁴⁰Ar/³⁹Ar age dates (USGS and NMT) currently exist for the GM area. Because these two labs use different standards, all values have been normalized relative to FC-Sanidine (28.02 Ma). The normalized values show a range from 9.45-10.99 Ma, a mean of 10.17 Ma and a standard deviation of 0.33 Ma. These values show geographic variability. Eastern GM values (N = 7) have a narrow range (10.10-10.41 Ma), with a mean of 10.25 Ma and a standard deviation of 0.13 Ma. Western GM values (N = 16) have a broader range (9.45-10.99 Ma), with a mean of 10.14 Ma and a standard deviation of 0.39 Ma. Field evidence suggests that emplacement of the basalt occurred in two phases and from two separate vent areas. Initial eruptions possibly came from the "Lombard" vent, which is currently represented by two east-west oriented basalt dikes near Mt. Darline. Flows from this vent followed a series of northwest-trending paleo-valleys positioned on the north flank of the West Elk Mountains. The second eruption sequence occurred at the "Lily Lake" vent, which is documented by a northeast-oriented basalt dike and scattered pyroclastics just north of Crag Crest. The Lily Lake dike clearly crosscuts an older flow sequence (age dates in progress). Eruptions from the Lily Lake vent formed an elongate shield volcano that today forms the surface of western GM, Crag Crest, and possibly Leon Peak. Pleistocene mass wasting has significantly displaced much of the eastern half of the shield and has offset parts of the Lily Lake dike.