VOLCANIC DAM HYPOTHESIS AND POSSIBLE SHERWIN GLACIATION AGE FOR THE MCGEE TILL, LONG VALLEY CALDERA, SIERRA NEVADA, CALIFORNIA

The logical foundation of the Volcanic Dam Hypothesis for the McGee Till rests upon a mistaken interpretation of landscape evolution. The presence of Aggie-McGee Ridge does not argue against capture of a paleo McGee-to-Convict drainage by the present McGee Creek Canyon outlet after deposition of the till. This stream capture could have occurred at any time without beheading the Aggie-Morrison drainage. Rectifying this error in interpretation removes a major objection to Eliot Blackwelder's hypothesis of deposition of the till within a broad upland paleo-valley.

Another objection to Blackwelder's model, put forth within the Volcanic Dam thesis, is that the McGee Mountain north bench quartzite boulder field lays glacier-right to, and seems to have been partly overrun by, the granodiorite component of the till. The known quartzite sources within the McGee Creek drainage are left lateral to a McGee Creek Canyon glacier. One possible explanation for this anomalous distribution of till would involve an initial glacial advance in which quartzite laden ice from the nearby Aggie-Baldwin cirques was spread across the floor of a broad paleo-valley, including the present site of the McGee Mt plateau. Continued glaciation, possibly a later advance, would result in granodiorite laden ice from further up canyon eventually reaching this site and overrunning the original quartzite deposits.

For the North Bench quartzite boulders, Rinehart and Ross proposed a western origin. This would be the adjacent Aggie-Morrison Canyon. Since this hanging valley shows evidence of having once been filled with the 2.7 ma basalt, it is not out of the question that quartzite laden ice originating from between Mt Aggie and Mt Morrison was elevated enough to spread across the McGee Mountain plateau before being overrun by granodiorite bearing ice originating in the cirques from the head of the McGee Creek drainage. This and the above explanation are not exclusive, and both fit within the framework of Blackwelder's model without requiring an overflowing ice pool as proposed in the Volcanic Dam Hypothesis.

The adjacent Long Valley Caldera event would have hastened landscape changes, allowing a younger age for the till than previously supposed, possibly Sherwin Glaciation. But this accelerated landscape evolution with its associated compression of time frames entails a modification of Blackwelder's original thesis, not its rejection. There is no indication of pre-Bishop Tuff silicic or andesitic volcanics near the Plinian vent site. The Volcanic Dam Hypothesis lacks supporting evidence, and is based upon an erroneous assumption of the falsification of Blackwelder's model. Although the Volcanic Dam Hypothesis is not falsified, Occam's Razor clearly favors Blackwelder's non-catastrophic explanation. There is no need to invoke a volcanic dam in accounting for the presence of the McGee Till.

David R Stockton, March 12, 2007

Thanks to Malcolm Clark and Michael Ort for their helpful criticism.

Thanks to the GSA for allowing this correction to be added to the 2006 abstract page.