New Mexico Geology
2018, Volume 40, Number 2, pp. 45-60.
Sorting clasts across laminated maar dunes, Kilbourne and Hunts Holes, New Mexico: comparisons to sorting across aeolian and fluvial bedforms
Love, Dave W.; Gutjahr, Allan; Lazari, Andreas,
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Unlike aeolian and fluvial grain-size distributions that vary longitudinally across bedforms, unimodal and multimodal grain-size distributions from individual laminae and thin beds across pyroclastic climbing dunes in exposures at Kilbourne and Hunts Hole maars remain similar, regardless of location on the dunes. By analogy with aeolian and fluvial deposits, distributions in the climbing-dune facies of these maar deposits are interpreted to be dominantly a fine-grained (0.15–0.35 mm) saltation mode, with removal of finer particles in suspension and incorporation of lesser creep fractions coarser than the saltation mode. In some laminae, coarser grains up to 10 mm in diameter are present. Like aeolian and some fluvial dune deposits, the saltation distributions may be described by the four parameters of hyperbolic distributions: slopes of asymptotes on fine and coarse sides of the distributions (φ and γ), abscissa of the vertex where the two limiting lines of the hyperbola cross (μ), and peakedness or the shape of the distribution near the mode (δ). Sampled saltation distributions from dunes at Kilbourne and Hunts Hole maars have gentler asymptotic slopes, compared to aeolian and fluvial deposits, and have slightly smaller average μ and δ. Removal of particles in suspension above individual laminae in the dunes appears to result in two kinds of logarithmic decreases in mass amounts on the fine side of the grain-size distribution curves. One type of decrease is very similar to steep decreases seen in aeolian and fluvial ripples and dunes, perhaps related to flow separation and suspension near the crests of the dunes. The other type is more gradual in laminae that have an order of magnitude more ash incorporated in the thin beds. If the saltation fraction is deposited by aeolian processes, impact threshold velocities at these two maars appear to be on the order of only 0.12 to 4 m/sec. Unlike experimental aeolian saltation fractions, which decrease in modal size and amount downwind in waning wind conditions, the pyroclastic saltation populations remain constant over a distance of at least 0.4 km at Kilbourne Hole, and the saltation populations at Hunts Hole are similar. Coarser-grained creep populations in the surge deposits appear to be more complex than their aeolian counterparts, and in some samples contain maximum grain sizes that are up to nine times coarser than the modal saltation values. The fractions coarser than the saltation mode exhibit a range of decreases in mass amounts and some samples appear to exhibit separate coarse modes similar to distributions seen in some aeolian granule megaripples and fluvial gravelly megaripples. In these pyroclastic surge deposits, coarse grains may have been added by fallout and entrained in thin beds mixed with saltating grains. However, if the coarse grains are mixed with a saltation population and emplaced as thin, dense, granular shear layers, the mechanisms responsible for deposition of discrete, continuous layers across large bedforms are unclear, perhaps analogous in some respects to granular shear layers in fluvial gravelly megaripples.