2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 23
Presentation Time: 1:30 PM-5:30 PM


LOOPE, David B., Geosciences, University of Nebraska-Lincoln, 214 Bessey Hall, University of Nebraska-Lincoln, Lincoln, NE 68588 and ELDER, James F., 101 Smirle Ave, Ottawa, ON KlY0S4, Canada, dloope1@unl.edu

Grainflow strata and the fine-grained laminae that delineate them (pinstripes) are prominently displayed in the Lower Jurassic Navajo Sandstone. Grainflow strata bounded by stratigraphically adjacent pinstripes are commonly thicker (>10 cm) and wider (>10 m) than any observed, modern avalanche tongues. At the toe-of-slope, each grainflow stratum always divides two packages of wind ripple deposits; multiple grainflows never successively downlap onto a single ripple wedge. Although the surfaces of modern slipfaces display cm-scale roughness due to the convexity of avalanche tongues, the upper surfaces of most grainflow strata at Coyote Buttes are planar (suggesting that smoothing of the slipface surface is associated with pinstripe formation).

We recognize two types of pinstripes with distinct origins: Planar pinstripes (type 1) are more extensive than concave-up pinstripes (type 2) and form when strong turbulent air currents move across the slipface, flattening existing topography and emplacing a thin layer of very-fine sand. Concave-up pinstripes (type 2) are associated with individual avalanche tongues and form when sand bearing a type 1 pinstripe is remobilized by slumping; where the slump degenerates into an avalanche, the fine grains of the pre-existing pinstripe sieve through the coarser material and accumulate on a curved, basal shear plane. Concave-up pinstripes are commonly incomplete, with pinstripes that are well defined at the trough sides, but faint or missing on the trough floors. We interpret incomplete type 2 pinstripes as records of slumps that did not fully degenerate into avalanches.

Each grainflow stratum defined by planar (type 1) pinstripes is an amalgamation of tens to hundreds of individual avalanche tongues, each tongue consisting of sand that was depleted of finer grains within a few meters of the dune cornice (long before the tongue reached the lower slope of a long slipface). Each planar pinstripe marks a period of energetic slipface flattening, during which finer sand was deposited onto the lower slipface. Type 1 pinstripe formation may be enhanced when airflow becomes oblique (rather than perpendicular) to the dune crest, and could possibly reflect diurnal wind shifts.