A REINTERPRETATION OF THE STRUCTURAL DEVELOPMENT OF THE OUTCROPPING GEOLOGICAL SECTION EAST OF BOULDER, COLORADO

The upper Cretaceous section in the northwest portion of the Denver Basin is structurally located within the Laramide Orogeny Boulder-Weld Fault Zone. Geological outcrops east of Boulder, Colorado at White Rocks and both the Marshall and nearby Davidson Mesas were investigated utilizing core data and high quality resistivity geophysical well logs to map the area geological fabric and formulate a model for area structural development. These three outcrops have been extensively investigated since the area discovery of economic coal deposits just southeast of Boulder in 1859. Numerous excellent publications identified listric growth faults, thrust faults, high-angle reverse faults, normal faults and near-vertical strike-slip faults without an identification of a coherent model explaining the development of these divergent fault styles. The author's presentation proposes a structural model explaining their genesis involving an area gravity detachment of the upper Cretaceous bedrock above a singular unfaulted bentonite layer upturned nearly 20 degrees during the Laramide Orogeny uplift of the nearby Front Range Mountain Range to the west. Following their updip detachment, the rocks forming the present-day Marshall and Davidson Mesas moved nearly 2 miles from their original location, while the White Rocks outcrop moved from 2.5 to 3 miles.