RECONNAISSANCE MAPPING OF HIGH-POTASSIUM MAFIC FLOWS IN THE WESTERN GRAND MESA VOLCANIC FIELD (WESTERN COLORADO) USING SPECTRAL GAMMA-RAY DATA

Mafic flows of the western Grand Mesa volcanic field range in age (⁴⁰Ar/³⁹Ar) from 9.45 to 10.99 Ma (mean = 10.26; N = 19) and have a large (138 km²) "Y-shaped" outline. Crag Crest forms the stem of the "Y" on the east, with the Palisade and Flowing Park lobes forming the branches on the west. Maximum observed thicknesses of the sequence is 187 m (26 flow units), as penetrated in the USBR D-9 corehole, which is about 8 km southwest of Crag Crest. The Palisade and Flowing Park lobes have limited topographic relief; however, where they join, a noticeable low-relief dome exists. This dome and Crag Crest represent the eroded remains of a local shield volcano associated with the “Lily Lake dike.” This north-south trending dike is 8-9 m wide, and is dated at 10.49 to 10.51 Ma (N = 2). It cross-cuts a flow sequence dated at 10.82 Ma.

Geochemical analyses (ICP-MS) of 64 outcrop and core samples across the Grand Mesa field show a surprising diversity of rock types (TAS classification), including basalt (N = 20), trachy-basalt (N = 7), shoshonite (N = 14), basaltic andesite (N = 18), and latite (N = 5). Geographically, nine of the shoshonite and all of the latite samples are associated with Crag Crest shield volcano and Lily Lake dike. Shoshonite (2 samples) also occurs in the D-9 core between depths of 52 and 134 m. This interval is bracketed between 10.08 and 10.49 Ma (N=3).

Because the shosonites and latites have elevated potassium values, spectral radiometric (K, U, Th) analyses (N = 295) were completed on outcrops, roadcuts, and core (D-9) using a Radiation Solutions RS-125 Super-Spec gamma-ray spectrometer (assay time = 300 sec.). Radiometric potassium values (N = 24) for the Crag Crest flows and Lily Lake dike ranged from 2.1 to 4.4 wt%, with an mean of 3.0 wt%. By comparison, surface and core samples (N = 271) to the west had radiometric potassium values ranging from 0.9 to 3.8 wt%, with a mean of 1.4 wt%. Based on these analyses, plus other field observations, the minimum, present-day footprint of Crag Crest volcano appears to be about 9.1 km², which is about 6.6 % of the western field. The high-K flows observed in the D-9 core were tentatively correlated to roadcuts at Skyway, which are on the north edge of the Palisade lobe, about 2 km (1.3 mi) distant.