BASEMENT AND COVER STRUCTURAL ANALYSIS ALONG THE EAGLE RIVER GORGE, SAWATCH RANGE, COLORADO

The Sawatch Range within the Rocky Mountains of central Colorado exposes Paleoproterozoic basement and overlying Paleozoic sedimentary rocks with a complex history of brittle deformation. Ductile deformation in the northeast Sawatch Range created the Homestake shear zone, a NE-striking belt of subvertical tectonites and transposed foliation. The goal of this study is to examine the influence of pre-existing basement structures on brittle structures in the cover strata along the basement-cover contact in the northeastern Sawatch Range along the Eagle River canyon and U.S. Highway 24 from Red Cliff to Gilman, Colorado. Paleoproterozoic basement rocks include the Cross Creek granite, diorites, and gneisses; the contacts between these are gradational and concealed in locations. The basement is nonconformably overlain by the Cambrian Sawatch Formation.

Reverse, normal and strike-slip faults were found with minor offset. SE-dipping dip-slip (normal or reverse offset) faults and south and SW-dipping joints dominate the structures in the basement rocks. Faults in the Sawatch Formation are SE-dipping with left-lateral strike-slip or right-lateral strike-slip displacement. The majority of the joints in the Sawatch formation dip to the SW. 

Strike-slip faults occur in the basement rocks and the Sawatch Formation, while dip-slip faults are abundant throughout the Paleoproterozoic units. Joint orientations in the basement rocks are south to SW-dipping, while joints in the Sawatch Formation dip primarily to the SW. In some locations faults are continuous from the Paleoproterozoic rocks into the Sawatch Formation, suggesting that they are Paleozoic or younger. Joints and faults are more abundant in the Paleoproterozoic units, and the orientations of these preexisting structures could have influenced the orientations of the structures within the Sawatch Formation. Further analysis of this structural data will yield a greater understanding of the tectonic history of the area.