The Copper Basin Formation in the eastern Whipple Mountains provides a synorogenic stratigraphic record of the mid-Miocene tectonic development of that region. Genesis of the Whipple Mountain antiforms and normal faults associated with a system of regional, low-angle detachment faults probably reflect the passage of the mid-Tertiary magmatic arc through this region. This deformational episode resulted in the establishment of relatively high sediment source areas and numerous linear depositional basins during late Oligocene through mid-Miocene time. The Copper Basin Formation at its type locality is composed of interbedded arkosic sandstone, siltstone-mudstone, and fanglomerate that exhibit distal alluvial fan facies development. Paleocurrent analyses indicate sediment transport was generally to the south, off the developing structural/topographic highs. Petrographic evidence indicates a local, mixed plutonic-metamorphic-volcanic source terrane for the Copper Basin Formation sedimentary rocks. Faulting occurring simultaneously with sedimentation has resulted in a distinctive, fan-like stratigraphic geometry for the unit. This feature indicates tectonic transport of the upper-plate occurred synchronously with uplift and sedimentation. Stratigraphic and structural features developed in the underlying Gene Canyon Formation support this interpretation, as well. Radiometrically dated volcanic flows, interbedded with both formations, provide temporal constraints that facilitate paleogeographic reconstruction of the region during Oligo-Miocene time.
