Early Tertiary Climate Change and its Impact on the Matrix Mineralogy of “auriferous gravels” in the Sierra Nevada Foothills

This is an Power Point presentation delivered at the GSA Cordilleran Conference (Session T5: Critical Zone: Where Rock Meets Water and Life at Earth’s Surface) in Fresno CA on May 20, 2013

Summary: The data from continuing mineralogical studies show that the sedimentary unit known as the Early Tertiary “auriferous gravels” in the Sierra Nevada foothills is much more complex than previously realized.  The popular assumption is that this fluvial package of sediments was deposited during a short window of time solely in the Middle Eocene and contains kaolinitic matrix clay.  X-ray data is presented to show that the fluvial sedimentary unit classified by the 19th Century hydraulic miners as the “lower or channel gravels” and the underlying paleosols are indeed kaolinitic and likely of Middle Eocene in age.  In striking contrast, smectite clay dominates the matrix of the terrace sedimentary unit previously termed the “upper or bench gravels” and their underlying paleosols.  The end of the deposition of the terrace sediments is early Oligocene based on the Ar/Ar dates of stratigraphically interbedded rhyolitic tuff beds at the top of the smectitic section.  These tuffs date to between 30Ma and 33Ma in various localities.

The transition from 1:1 climax clay mineralogy (kaolinite) in regional soils and sediments to a 2:1 climax clay assemblage dominated by smectite in the upper terrace sediments and their underlying soils is indicative of a decrease in the hydrolytic weathering intensity that would have accompanied the “climatic crash” that occurred between the Late Eocene to the Early Oligocene. Thus, two distinct fluvial sedimentary systems comprise the “auriferous gravels” of the Sierra Nevada Foothills and other adjacent areas of the Sacramento/San Joaquin Valley fringe zones.  The kaolinitic lower unit is equivalent to the Ione Formation of Middle Eocene age, while the smectitic upper unit is Late Eocene to Early Oligocene in age and whose matrix mineralogy records the events of the rapidly cooling global climate in the Late Eocene.  A discussion of the micromorphology of clay materials occurring in these respective fluvial systems is presented in the accompanying GSA presentation here.


Slide presentation (pdf)….view here

Slide notes (pdf)….view here