Tephrostratigraphic and paleomagnetic correlations of the Miocene Peach Spring Tuff (PST) in the Alvord Mountain area, central Mojave Desert, California

Tuesday, 16 December 2014
David Buesch and John Hillhouse, US Geological Survey, Menlo Park, CA, United States
An ignimbrite in the Spanish Canyon Formation of the Alvord Mountain area, southeastern California, is correlated to the 18.78 ± 0.02 Ma PST (age from Ferguson and others, 2013) on the basis of feldspar mineral chemistry and paleomagnetism. Intermittent exposures along ~9 km of exposure reveal a northward decrease in thickness of tuff from 7 to 1 m.

Hillhouse and others (2010) identified a 7.0 m thick, partially welded, and crystallized ignimbrite with distinctive inclination as the PST. They inferred a clockwise vertical-axis rotation of ~56° relative to the Colorado Plateau based on declination of remanent magnetization. About 3.2 km north is a 3.3 m thick, zeolitically altered ignimbrite that when structurally restored has a declination of 33.4°, inclination of 76.8°, and clockwise vertical-axis rotation of ~45°. About 5.8 km northwest is a 1.5 m thick, nonwelded, argillically altered ignimbrite with no measureable paleomagnetic direction.

Petrographic features and electron microprobe analyses of sanidine and plagioclase compare well with 3640 analyses from widespread sites of PST, and differ from three other tuffs (Buesch, 1993, unpublished data). Textural classification includes grains with attached glass, in pumice, or mantled feldspar or crystal clots as “magmatic”, “xenolithic”, or non-diagnostic “grains”. Samples from the three sections are 2 (of 7) m, 2 (of 3.3) m, and 0.7 (of 1.5) m above the base. Variations in feldspar endmenbers (Or, Ab, An) in magmatic and non-diagnostic grains from the two thicker sections are consistent with lower parts of PST measured sections from elsewhere. The 1.5 m-thick section is petrographically similar to other sections, but with more xenolithic grains. Tephrostratigraphic and paleomagnetic similarities support correlation with the PST, and local facies in the tuff (including welding, crystallization, alteration, and incorporation of locally derived clasts) support deposition across a broad area of low-relief, such as medial to distal alluvial fan and lacustrine environments, as described in Buesch (2014).

The PST in the Alvord Mountain area (1) expands distribution of the tuff, (2) enables interpretations of pyroclastic flow processes, (3) constrains stratigraphic facies at 18.78 Ma, and (4) provides a framework for tectonic deformation.