SURFICIAL GEOLOGY AND GEOLOGIC HAZARDS OF ORGAN PIPE CACTUS NATIONAL MONUMENT, SOUTHERN ARIZONA

We are mapping the surficial geology of publicly accessible areas in Organ Pipe Cactus NM (ORPI) in cooperation with the National Park Service to understand the recent geologic history of the area and assess potential geologic hazards and land-use issues. ORPI encompasses a swath of classic Basin-and-Range topography in the Sonoran Desert along the U.S-Mexico border. The spatial distribution of deposits of different ages is complex in detail, but general trends are evident. Late to middle Holocene sediment consisting of channel gravel and much more extensive fine-grained deposits with stage I calcic horizons dominate lower and middle piedmont areas and along axial drainages. Coarser latest Pleistocene to early Holocene deposits with stage II calcic horizons filled or partially filled valleys in middle and upper piedmont areas and locally resulted in substantial changes in loci of deposition. This deposition may be related to the last glacial/interglacial climate transition. Middle and upper piedmont areas are dominated by incised, relatively coarse late to middle Pleistocene alluvial fan deposits with stage II to IV calcic horizons. A few high, deeply dissected alluvial fan remnants with stage IV-V calcic horizons are preserved in upper piedmont areas.

ORPI is lightly developed, but several geomorphic processes present potential geologic hazards. Debris flows are important factors shaping the mountain landscape and transferring coarse sediment from the mountains to upper piedmont areas. Fairly large debris flows occurred locally in the summer of 2008, and Holocene debris flow deposits are common along most sizable washes exiting the mountains. Fortunately, no NPS facilities appear to be at high risk from debris flows. Distributary flow systems common in middle and lower piedmont areas are subject to changing channel patterns and extensive sheetflooding. The principal N-S highway through ORPI has been repeatedly impacted by flood inundation and erosion where it crosses large distributary systems. Extensive incised channel networks (arroyos) have developed historically in broad shallow valleys partially filled with fine-grained Holocene deposits and are continuing to extend upstream. In these areas, border-related off-road vehicle traffic has the potential to exacerbate erosion problems.