A COMPOUND BRECCIA PIPE IN EVAPORITE KARST; MCCAULEY SINKS, ARIZONA

The McCauley Sinks, in the Holbrook Basin, Arizona, are comprised of 50 individual sinkholes within a 3-km-wide depression. The sinks are grouped semi-concentrically in three nested rings. The outer ring contains ring fractures and is apparent tension zone. The two inner rings are semi-circular chains of large sinkholes, ranging up to 100 m across and 50 m deep. Several sub-basins within the larger depression show local downwarping and possible incipient sinkholes. Limestone of the Permian Kaibab Formation is the principal surface lithology, less than 15 m thick, and near its easternmost outcropping. Although surface rillenkarren are present and the sinks occur within the Kaibab limestone outcrops, the Kaibab is mainly a passive unit that has collapsed into solution cavities developed in underlying salt. Beneath the Kaibab is Coconino Sandstone, which overlies the Permian Schnebly Hill Formation, the unit containing the evaporite rocks (principally halite in the Corduroy Member). Evaporite karst in this part of the Basin is different from the eastern part, probably because of the disappearance of the Holbrook Anticline, a structure that has major joint systems that help channel water to the salt beds below. The McCauley Sinks area also approaches the western edge of the evaporite basin. The structure at McCauley Sinks suggests a compound breccia pipe, with multiple sinks contributing to the inward-dipping major depression. The depression at Richards Lake, 5 km southeast of McCauley Sinks, is similar in form and size but has only a single, central sinkhole. An apparent difference in hydrogeology at McCauley Sinks is their proximity to the adjacent, deeply incised Chevelon Canyon drainage, but the hydrologic connections are unknown. The 3-km-wide karst depression at McCauley Sinks, along with five other depressions nearby, provides substantial hydrologic catchment. Because of widespread piping into karst features and jointed bedrock at shallow depth, runoff seldom ponds at the surface. There appears to be greater recharge efficiency here than in alluvial areas; thus concern exists for groundwater users downgradient from the karst area, specifically by introducing pollutants into the karst openings. Thus, sinkholes and open fissures must not be used for waste disposal. Although a remote area, the likelihood of collapse around engineered structures is also real.