SUBSURFACE VOLCANIC GEOLOGY OF THE HOT CREEK CALDERA, NYE COUNTY, NEVADA

In the 1960’s numerous boreholes were drilled, and geologic mapping completed in the Hot Creek Valley and surrounding mountains in support of underground testing at the Central Nevada Test Area (CNTA). In 2005 and 2009 additional boreholes were drilled in support of Department of Energy (DOE) remediation activities. Analysis of the data from both the legacy and recent studies at CNTA, coupled with recently published age data, provides a more detailed understanding of the Hot Creek caldera. Initial caldera collapse resulted from the Oligocene age eruption of the tuff of Six Mile Canyon and the tuff of Hot Creek Canyon. Caldera wall collapse resulted in megabreccias interbedded with these tuffs. Additional disruption of the caldera resulted from subsequent eruptions of local intracaldera tuffs accompanied by local caldera wall collapse and localized faulting that created depressions within this segmented caldera. During periods of volcanic quiescence tuffaceaous sedimentary and lacustrine deposits filled in these local depressions. Lava domes formed along the southern margins of the caldera ring fracture system.

The Hot Creek Valley graben formed during Basin and Range faulting that post-dated Oligocene volcanism of the Great Basin ignimbrite flare-up. As erosion wore down the surrounding mountains, progressively older strata were exposed to progressively younger erosion. Basin and Range style normal faulting that created the Hot Creek graben preserved younger volcanic rocks in the valley center even as it eroded these same rocks from the adjacent mountain ranges. Thus, valley-fill deposits preserve an inverted stratigraphy with younger alluvium composed of progressively older rock fragments. The youngest volcanic strata associated with the Hot Creek caldera are preserved beneath these valley-fill deposits, with tuffs encountered at shallow depths missing from the volcanic strata exposed in the adjacent uplands. The lateral and vertical variability in the alluvium resulted from late Tertiary and Quaternary faulting within the Hot Creek Valley, as exhibited in the boreholes at CNTA where adjacent boreholes manifest markedly different geophysical log characteristics and lithologic details.