AN INVESTIGATION OF THE GEOLOGY AND EMPLACEMENT HISTORY OF THE WETHERILL MESA DIATREME, SOUTHWESTERN COLORADO

The Wetherill Mesa diatreme is located at the northeast edge of the Oligocene Navajo volcanic field (NVF), and is one of several diatreme-dike complexes that are exposed in southwestern Colorado. The Wetherill Mesa complex contains a series of northeast-trending potassic minette dikes that form a linear core to an elliptical apron of minette breccia that was emplaced in Late Cretaceous sedimentary rocks about 2000 feet below the surface. A detailed investigation of this complex has established that the breccia apron dips 40° to 60° away from the central dikes to the east and west, forming a dome-shaped deposit. Bedding within the breccia deposits is thin to medium and contains well-preserved sedimentary structures such as graded bedding, cross lamination, and scour surfaces. We propose that this complex formed by subsurface lateral blasts that were emplaced along areas of structural weakness in the country rock, driven by decompressive gas release as the magma reached depths where gas pressures exceeded lithostatic lodes. Explosive emplacement caused fragmentation of the minette dikes feeding the system, and brecciation of the country rock surrounding the dikes. Our interpetation is not consistent with previous models for diatreme emplacement in the NVF that rely on phreatomagmatic eruptions at depth where magma would interact with groundwater systems. Convincing evidence for phreatomagmatic interaction between minette dikes at Wetherill Mesa was not found.