New Mexico Geological Society Annual Spring Meeting — Abstracts

The late Miocene Peralta Tuff Member of the Bearhead Rhyolite, in the southeastern Jemez Volcanic Field, contains crossbedded tuffs of both pyroclastic surge and eolian origins. Study of textural characteristics, stratification, and flow directions was undertaken to disciminate deposits produced by these very different processes. Surge deposits examined in this study are part of a phreatomagmatic tuff-ring exposed at the Tent Rocks; their characteristics were also compared to others previously studied by the senior author and to published textural data. Other crossbedded tuffs are commonly found overlying pyroclastic-fall tuffs in the Peralta Tuff. Strongest evidence for eolian origin is measured paleocurrent directions indicating flow from the south and southwest. All known Bearhead Rhyolite vents are located to the north and the fluvial paleoslope was inclined to the southeast. Although there is some overlap in textural parameters, the eolianites are generally better sorted than the surge deposits, lack fine ash, and lack grains larger than 0.5 cm, which are abundant in the surge deposits. The textural characteristics of the eolianites are slightly coarser and more poorly sorted than published values for quartz-arenite eolianites; the probably reflects the abundance of low-density grains in the reworked tuffs. Grains in eolianites appear slightly better rounded than those surge deposits. The eolianites consist of two types. The most common are units 0.2 to 2.5 m thick composed of low, undulating cross-stratification typical of sand-sheet deposits. Laminae are less than 1 cm thick where not disturbed by abundant burrows. The second type are dune facies composed of wind-ripple and grain-flow strata, 0.2 to 1.2 cm thick. These latter deposits were formed by migrating dunes that were 1 to >4 m high. Surge deposits form low-angle, climbing cross-strata, generally >1 cm thick, in sets averaging 0.5 m thick, with common preservation of stoss sides of the migrating bedforms. These features are typical of high accretion rates and fast moving currents and are rarely observed in the eolianites. Fine-grained surge deposits and sand-sheet eolianites appear very similar in the field and great care must be taken to distinguish them. The ubiquitous occurrence of eolianites with fall deposits of similar grain size suggest that the falls were extensively reworked by wind to produce facies that could be easily mistaken as the product of surges.