EARLY TERTIARY CLIMATE CHANGE AND ITS IMPACT ON THE MATRIX MINERALOGY OF “AURIFEROUS GRAVELS” IN THE SIERRA NEVADA FOOTHILLS

“Early Tertiary auriferous gravels” is a non-descript early mining term that remains in use on modern maps of both the USGS and the California Geological Survey (CGS). Additional terms such as “lower gravel/upper gravel” and “channel gravel/bench gravel” all once held important economic connotations for 19^th century miners but few modern geological studies have correctly characterized the physical and temporal nature of these important sedimentary deposits.

Recent field investigations and analytical data collected from several locations of the “auriferous gravels” throughout the Sierra foothills show this map unit to be a complex sedimentary assemblage comprised of at least two geologic units possessing drastically contrasting mineralogical and age relationships. Kaolinitic quartz-rich fluvial sandstones dominate the “lower or channel gravels” and represent the proximal Ione Formation fluvial system (≈50Ma). In contrast, the “upper or bench gravels” is a smectitic unit with complex sandstone matrix mineralogy and represents a much younger fluvial system (≥ 30 Ma). These smectitic sediments are best represented at Chalk Bluff in Nevada County. In addition, similar kaolinite/smectite mineralogical distinctions are observed in the paleosols buried immediately below these respective fluvial units.

The mineralogical record contained in these sedimentary and pedologic units collectively forms an important addition to other climatic data indicating major global climate change at the end of the Eocene period. Thus, the 19^th century nomenclature spawned during the heyday of gold mining in California should be abandoned in favor of a new classification based upon important differences in matrix mineralogy Such revised classification will help more clearly define the character of these Early Tertiary sediments and their important role in understanding the complex geological evolution of the Sierran region as well as global climate change during this dynamic period.