WILLIAMS FORK VALLEY GRABEN, COLORADO: THE NORTHERNMOST DOCUMENTED LATE QUATERNARY STRUCTURE IN THE RIO GRANDE RIFT SYSTEM

The Williams Fork Valley graben is a late Cenozoic structure in north-central Colorado. It formed in the overthrust block of the Laramide-age Williams Range thrust. The graben is one of several extensional structures in the northern part of the Rio Grande rift, all of which were filled with Neogene sediments eroded from adjacent uplifts. Strong evidence of repeated late Quaternary movement exists along the northern and central sections of the Williams Fork Mountains fault, a northwest-trending, east-dipping, normal fault along the western margin of the graben. Discontinuous scarps cut alluvial deposits in narrow, densely forested tributary valleys that cross the Williams Fork Mountains fault. Some of these scarps may be a result of fluvial erosion, landslides, or debris flows, but scarps that align with Neogene bedrock faults found on drainage interfluves likely have a tectonic origin. Middle Pleistocene gravels cap several mesas within the center of the graben. We attribute tilting of these gravel-capped mesas and elevation differences between adjacent mesas to tectonism. Anomalous parallel and linear drainages within the graben also likely relate to late Quaternary structural deformation. Conclusive evidence of late Quaternary activity on the southern end of the Williams Fork Mountains fault and on the Neogene fault that forms the eastern margin of the graben was not discovered during our reconnaissance. Detailed studies are needed to confirm this interpretation. Preliminary analysis of late Quaternary scarps along the Williams Fork Mountain fault involved measuring numerous profiles across the scarps and evaluation of pedogenic soils exposed in pits excavated into faulted deposits in the footwall of the structure. Scarp heights increase in older alluvial deposits, documenting recurrent late Quaternary activity. The most recent rupture cuts alluvium that is late Pleistocene or younger in age. These scarps mark the northernmost, well documented, late Quaternary fault within the Rio Grande rift system. The presence of these scarps in densely forested areas where aerial photography is of limited use suggests that other late Quaternary structures may exist in similar environments.