EOLIAN PIN STRIPES IN THE NAVAJO SANDSTONE: ORIGIN, DISRUPTION, AND RECONSTITUTION

Pin stripes are thin (<2 mm-thick) laminae rich in very-fine sand and coarse silt that separate grainflows (dry avalanche deposits) consisting of medium sand. They are dominantly planar, but commonly contain concave-up portions that define down-dip-oriented troughs about 30 cm wide and 5 cm deep. Deposition of pin stripes has been previously attributed to fallout of grains onto the slip face, or to sorting processes that take place during avalanching. In the Navajo Sandstone of southern Utah and northern Arizona, pin stripes were dominantly produced by upslope winds that drove fine grains obliquely across the lower slip face. Although these airflows were generally erosive, pin stripes formed as saltating fines lodged amongst the medium-grained sand on the upper surface of grainflow deposits. When upslope winds moved sufficient sand laterally, the slip face was undermined, and pin stripes were disrupted by small-scale grainflow tongues. If these “secondary” grainflows lasted for more than a few seconds, the fines from the disrupted pin stripe descended to the concave-up shear surface at the base of the grainflow and formed a distinct lamina that reconnected the ruptured edges of the pin stripe. The resulting reconstituted pin stripe is thus polygenetic: the planar portion was formed via saltation, but concave-up portions were produced by secondary grainflows. The obliquely upslope winds that generated pin stripes were likely the lee eddy generated by high velocity winds or a roller flow generated by winds that were not perpendicular to the crestline of the dune.

Modeling studies of eolian grainflows suggest that relatively small volumes of sand can be stored just leeward of the dune crest before avalanching is initiated. Pin stripes in the Navajo Sandstone, however, define the tops and bases of grainflow strata that are up to 17 cm thick. Because of their great thickness and because they form laterally extensive sheets rather than narrow tongues, the thick grainflow strata of the Navajo are best explained as vertico-lateral amalgamations of many individual grainflow tongues. Paucity of fines within distal grainflows probably contributed to amalgamation: on the large Navajo dunes: the lower slip face would have been beyond the reach of grainfall, and kinetic filtering during avalanching would have led to rapid loss of fines.