Geosphere; February 2008; v. 4; no. 1;
p. 36-74; DOI: 10.1130/GES00114.1
© Geological Society of America
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Figure 4. Photographs of middle Miocene sedimentary and volcanic units in the Chimney basin. A: Fine-grained, thin-bedded, ash-rich lacustrine sedimentary strata overlying a thick ash bed at the base of the exposure. Photo was taken in the western part of the basin. B: Phreatic, vitric megabreccia in base of basal flow of the 15.5 Ma Little Humboldt rhyolite above lacustrine sedimentary units (blocky hill), possibly indicating eruption onto wet sediments or water. The overlying, younger flow unit (left background) overlies both the breccia and the sedimentary units but does not have a basal breccia. Photo was taken at the Little Humboldt ranch along the Little Humboldt River (Fig. 3). C: Conformable contact (dashed white line) between the lacustrine and fluvial facies; a tephra sample from 2 m below the contact produced a 14.7 Ma correlation age. Photo taken between Chimney Reservoir and the Snowstorm Mountains (right far background). D: Closer view of contact between lacustrine (below, light colored) and conformably overlying fluvial sediments (tan). The cobbles concentrated along the contact zone were derived from volcanic units exposed in the Snowstorm Mountains to the east (right).
Copyright © 2008 by Geological Society of America
