Assessing weathering intensity factors derived from sandy debris flow and sheet flood deposits of the Neogene Bear Canyon conglomerate, SE California, USA
Friday, December 14th, 2012
The Neogene Bear Canyon conglomerate, a set of three unconformably bounded units, is composed primarily of sandy debris flows and sheet flood deposits. The lower unit of the Bear Canyon, sequence I, unconformably overlies the 23.2 ± 0.2 Ma ignimbrite of Ferguson Wash while the uppermost unit, sequence III is interstratified with a 9.5 ± 0.5 Ma basalt flow unit. While the general timing of deposition of the Bear Canyon is well accounted for, the exact timing during which sequences I and II were deposited is poorly constrained.
Miocene climate studies have highlighted a major climatic change referred to as the middle Miocene climatic transition, between 17 – 12 Ma, during which the southwestern United States climate became cooler and drier. Geochemical and petrological data derived from lithic arenites collected from the three sequences in the Bear Canyon conglomerate were analyzed using multivariate statistics to evaluate whether or not there is a quantifiable signal related to increasing/and or decreasing weathering intensities. Calculated weathering intensities appear to record changes that are consistent with the lower Lower part of sequence I and the whole of sequence III being deposited in a relatively cooler and drier climate regime, while the upper Lower and Upper parts of sequence I, and the whole of sequence II were deposited during a relatively wetter and warmer climatic period. Hence, data presented herein are consistent with the idea that sequence I was deposited prior to the mid-Miocene climatic transition while sequence III was deposited after or during this shift in climate.