Eruptive proceses responsible for fall tephra in the Upper Miocene Peralta Tuff, Jemez Mountains, New Mexico
Sharon Kundel and Gary A. Smith
Upper Miocene pyroclastic-fall deposits in the southeastern Jemez Mountains consist of of a distinct tripartite stratigraphy. A lower white interval is a thick (~0.5-3.0 m), massive, moderately sorted, coarse-grained, pumice-lapilli layer, with abundant, non-glassy lithic fragments. A central pink interval is a much thinner (~8-12 cm), moderately sorted, laminated fine ash with few dispersed coarser fragments. A top gray interval (~15 cm-1.1 m) is stratified and composed of well-sorted, poorly vesicular perlite fragments. At least eight pyroclastic-fall deposits contain this stratigraphy, suggesting a repetitive process of changing eruption dynamics that was common to several different eruptions. Detailed study of two deposits implies a three-stage eruptive history. White intervals were produced by explosive plinian or subplinian eruptions of gas-rich magma. The pink intervals were deposited from coignimbrite ash clouds, and one pink layer is traceable laterally into a pyroclastic-flow deposit. The gray intervals resulted from fragmentation and rapid decompression of highly viscous, partly solidified magma high in the conduit as a consequence of surface dome collapse. This latter stage of activity may have been intermittent and coeval with a long period of dome growth, as suggested by minor erosional disconformities within gray-interval tephra.
- Kundel, Sharon; Smith, Gary A., 2007, Eruptive proceses responsible for fall tephra in the Upper Miocene Peralta Tuff, Jemez Mountains, New Mexico, in: Geology of the Jemez Region II, Kues, Barry S.; Kelley, Shari A.; Lueth, Virgil W., New Mexico Geological Society, Guidebook, 58th Field Conference, pp. 268-274.