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Magnetic Fabric Investigations of the Sapinero Mesa and Fish Canyon Tuffs, Northern Part of the Southern Rocky Mountain Volcanic Field, Colorado

Michael J. Martin, Christopher N. Harper, and John W. Geissman

Exposures of two laterally extensive ignimbrites in the northern part of the Southern Rocky Mountain Volcanic Field (SRMVF) allow examination of the emplacement mechanisms of these pyroclastic deposits as a function of deposition on irregular preexisting topography using magnetic fabric techniques. The Sapinero Mesa Tuff (28.19 ± 0.03 Ma) was erupted from the San Juan/Uncompahgre Caldera Complex and the Fish Canyon tuff (ca. 28.02 ± 0.16 Ma) was erupted from the La Garita caldera. These ignimbrites are major components of the SMRVF and were emplaced on highly irregular paleotopography. The Sapinero Mesa Tuff was emplaced directly on the late Eocene West Elk Breccia near and west of the Blue Mesa reservoir and directly on Precambrian crystalline rocks south of the Blue Mesa reservoir. The Fish Canyon Tuff was emplaced directly on Precambrian crystalline rocks south and southwest of Gunnison. Our study of these two ignimbrites is concentrated in the northern part of the SMRVF and examines how these deposits were emplaced on different topographic features. To date, a total of 16 sites in the Fish Canyon and Sapinero Mesa tuffs have been collected and analyzed for determination of magnetic fabrics through measurements of anisotropy of magnetic susceptibility (AMS). These samples are currently being analyzed for measurements of anisotropy of anhysteretic remanent magnetization (AARM). Initial analyses indicate transport directions that generally coincide with the inferred regional north--northwest transport direction. Additional sampling includes a greater spatial extent and a more focused method of collection with an emphasis on localities chosen with careful consideration of relationships to paleotopographic features present during emplacement of the ignimbrites. Specifically, we have selected linear features of varying scales with strikes that vary significantly from the regional, inferred transport directions. Our work provides a means to compare regional inferred azimuthal flow directions and source location to localized flow directions. The resulting discrepancies may suggest a transport--driven response to local Precambrian topography attending deposition.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013