SPELEOGENESIS AND SAFETY ASSESSMENT OF SURPRISE CAVE, SAN JUAN MOUNTAINS, COLORADO

Surprise Cave is a previously unmapped cavern system located at 3,650 meters (10,500 feet) in the southern San Juan Mountains, southwest of Molas Pass. The cave formed in an extensively jointed limestone bed of the Pennsylvanian Hermosa Group. Approximately 321 meters (923 feet) of the estimated 0.8 km (0.5 mile) long cave was recently surveyed. Characteristic features of the narrow, joint-controlled cave include undulatory passageways with scalloped walls, potholes, fluted bedrock, and incised phreatic tubes. These features suggest a polygenetic cave origin (formed or modified under both phreatic and vadose conditions). The cave floor, for the first 60 meters, consists of angular, pebble-sized rock debris genetically related to the cave walls, scattered with larger, well-rounded quartzite boulders (10-15 cm in diameter) derived from the Precambrian Uncompahgre Quartzite. The mapped location of the boulders suggests that the cave was also modified by an influx of glacial backwash. Actively forming speleothems, including popcorn, soda straws, ribbons, incipient flowstone, stalagmites, stalactites, and a few columns, are locally present. Small alluvial fans of soil (<1 meter wide at the toe) occur at the base of a few crosscutting joints, and plant roots penetrate the cave near the terminal breakdown room. The soil fan and plant roots indicate that the cave ceiling is close to surface. Measured carbon dioxide levels of the cave atmosphere were generally less than 500ppm. Water depths exceed 1.2 meters (4 feet) in some keyhole passageways; fluctuations of less than 30 cm were noted during times of heavy precipitation during the Fall 2002. Because of the proximity of the cave to the surface, a sudden increase in water depth may occur during times of heavy precipitation or runoff.