THE FORMATION OF CAVERNOUS WEATHERING FEATURES AND ARCHES IN RATTLESNAKE CANYON, COLORADO

Natural sandstone arches are well known from the Colorado Plateau region including Arches National Park in Utah, but a poorly studied concentration of sandstone arches exists in the Rattlesnake Canyon area of western Colorado. Here there are 9 significant arches within a 0.5 km radius, and 25 arches within a 5 km radius. The arches are located along the northern margin of the Uncompahgre Plateau, and are developed in gently dipping strata of the Jurassic Entrada Sandstone. Field observations show several stages of arch development that suggest the following sequence of events: 1) cavernous weathering is initiated along the unconformity separating sandstone of the underlying Slick Rock Member of the Entrada Sandstone from sandstone of the overlying Board Beds unit, 2) caverns enlarge both into the sandstone and upward through the Board Beds, eventually breaching the unit to form a hole in the roof of the cavern, and 3) the hole enlarges and the roof collapses to form an arch.

Critical factors influencing arch formation probably include the local thickness of the Board Beds unit of the Entrada Sandstone and the local hydrology. In the Rattlesnake Canyon area, the Board Beds unit is 6-10 meters thick; this thickness allows both cavern enlargement as well as eventual roof collapse. If the Board Beds unit was substantially thicker, the caverns would not breach the surface. Regional ground water flow is northward, and evidence that water influences cavernous weathering includes slightly higher moisture contents within sandstone caverns versus smooth outcrop surfaces. Similarly, measurements of salt contents of sandstone samples taken within 1-2 cm of the surface of cavern interiors range from 1133-2031 µs whereas samples taken from non-cavernous surfaces average 115 ± 15 µs; the salt content of sandstone in the caverns is ~10-19 times that of background. Another factor that could be important is the absence of large-scale jointing near the arches, which could suggest that arch formation is favored by areas where stresses within the Entrada Sandstone have not been reduced by previous episodes of deformation. In summary, Rattlesnake Canyon arches differ in their formation from other classic areas such as Arches National Park due to their lack of association with rock fins and joints, and their dependence on stratigraphic factors.