STRATIGRAPHIC SEQUENCE OF PALEOCENE DACITE INTRUSIONS AND THE FOUNTAIN ARKOSE WITHIN THE AGGREGATE QUARRY SOUTHWEST OF LYONS, COLORADO

The extent of Paleocene volcanic intrusions related to Laramide orogenic activity are difficult to study in the northern Colorado Front Range due to their limited exposure at the surface. Several aggregate quarries between Boulder and Fort Collins may offer insight on the subsurface geology surrounding these intrusions. However, geologic studies conducted in these areas are often proprietary information and are seldom shared with the scientific community at large. Recently, Boulder County Open Space acquired the property at Hall Ranch II southwest of Lyons, which included a large, inactive dacite quarry. While in the planning stages for reclamation, the location presents a unique opportunity to study the exposed local subsurface stratigraphy. The set of geologic cross-sections and stratigraphic columns completed during this study add to the body of work in the area and increase understanding of the relationships between Laramide intrusions and thermally altered sedimentary country rock.