COALESCING CALDERAS AND VOLCANIC DEBRIS AVALANCHE DEPOSITS IN THE NORTHERN KAWICH RANGE, CENTRAL NEVADA

Five mid-Miocene calderas were discovered in the northern Kawich Range between U.S. Highway 6 and the Bellehelen townsite. The calderas are filled with intracaldera rhyolite crystal to lithic tuff and debris avalanche deposits. ⁴⁰Ar/³⁹Ar dating and geochemical studies of the intracaldera tuffs are currently in progress. Based on overlapping relationships determined by detailed field mapping, the calderas are from oldest to youngest the Clifford Spring, Tobe Spring, Cow Canyon, and Bellehelen. No preferred direction of migration was noted. The Bellehelen caldera is elongated east west and is approximately 9 km long and 4.5 km wide. It is filled with a mineralogically distinct intracaldera tuff with 20-30% phenocrysts including 40% sanidine and 60% quartz. The tuff lacks biotite and hornblende phenocrysts but locally contains large pumice fiamme. The long axis of the caldera is parallel to what was previously mapped as the Bellehelen lineament. This lineament separated a topographic high area to the south from a lower area to the north and was thought to control mineralization in the Bellehelen area. Our new mapping indicates that most of the numerous faults that were previously mapped as the Bellehelen lineament by Gardner et al.,(1980) do not exist. We suggest instead that the mineralization and linear features are associated with the Bellehelen caldera. The topographic change reflects the southern wall of the Bellehelen caldera. A large volume volcanic dry avalanche deposit was identified within the Bellehelen Caldera. Evidence for this deposit includes fractured megablocks in a clastic matrix, hummocky terrain, and ~0.7 km long ridges parallel to direction of flow. The debris flow deposit is roughly 4 km long and 1 km wide. Lithic fragments within the debris flow range in size from less than 1 mm to a block 15 m by 28 m by 26 m. Lithic fragments are composed of tuff from the Bellehelen, Tobe Spring, and Cow Canyon calderas as well as meso-breccia from the Bellehelen caldera wall. Lithic fragments decrease in size and abundance away from the Bellehelen Caldera wall. The volcanic dry avalanche, therefore, appears to have originated by the collapse of the caldera wall and flowed from the caldera wall into the interior of the caldera.