THE IMPORTANCE OF NEOGENE STRUCTURAL FEATURES IN DECIPHERING LARAMIDE BASIN AND UPLIFT HISTORY

Detailed geologic mapping in the Medicine Bow Range and Upper Laramie River valley near the Colorado-Wyoming state line has identified numerous faults which offset upper Oligocene or younger sediments. The upper Oligocene to lower Miocene? North Park Formation overlies thin remnants of the upper Eocene to lower Oligocene White River Formation within the study area and contains abundant clasts of volcanic rock derived from Oligocene (~27 Ma) sources to the south and southwest in the Never Summer Mountains and Rabbit Ears Range. The North Park Formation has been broadly correlated with the Miocene Browns Park Formation. These fluvial deposits in several locations lie within large valleys that were later filled by stream gravels most likely representing the upstream sources of parts of the Miocene Ogallala Formation to the east. This entire section is deformed significantly by a series of faults with a strong west-northwest trend (~300°). These near-vertical faults are characterized by a lack of deformation away from the fault plane. In several locations, horizontal slip can be demonstrated. Tight folding along the Bull Mountain fault indicates strong compression between left-stepping strands of the fault, but in most locations, the faulting is best characterized as oblique normal. These younger faults truncate north to north-northwest trending folds and faults that were active during and after deposition of the Paleocene to Eocene Coalmont Formation in the North Park basin. Neogene faults are responsible for the highest topography in the ranges and form the margins of the current structural basins. The northern end of the North Park basin is defined by this younger faulting which continues to the northwest along the Saratoga Valley and through the Sierra Madre Range. The southern end of the Laramie basin and the northern end of the Colorado Front Range are dominated by these Neogene faults. Similar truncating relationships between Neogene faulting and older Laramide structures can be demonstrated throughout northern Colorado and southern and central Wyoming where Neogene deposits are wide spread which has significant implications for reinterpretation of Laramide structures.