EOLIAN PIN STRIPES IN THE NAVAJO SANDSTONE: TWO DISTINCT MODES OF ORIGIN

In the Navajo Sandstone of southern Utah and northern Arizona, pin stripes-- thin (less than 2 mm-thick) laminae composed of fine sand and coarse silt-- separate medium-grained grainflow (avalanche) cross-strata up to 17 cm thick. Compared to other types of eolian strata, pin stripes have very low permeability and commonly are sites of early cementation.

Our field observations and laboratory experiments using a Hele-Shaw cell indicate that pin stripes in the Navajo formed in two distinct ways. Type 1 pin stripes formed by distal grain fallout onto the upper surface of grainflow strata near the toes of long slip faces. Type 2 pin stripes formed by kinetic sieving of the fine grains composing Type 1 pin stripes when the underlying grainflows were remobilized. The fine grains accumulated along the basal shear zone of such a secondary (remobilized) grainflows and typically formed concave-up laminae that extend several meters parallel to dip, but are only a few tens of cm wide. In strike-parallel sections, the Type 2 laminae delineate erosive scours, and are laterally contiguous with Type 1 pin stripes. In simulations with the Hele-Shaw cell, we observed that the fine grains initially composing a grainfall lamina can descend through flowing coarser grains in only a few seconds and that they generate a Type 2 lamina that, in dip-parallel section, is very similar to the initial (Type 1) pin stripe. The abundance of Type 2 pin stripes in the Navajo indicates that remobilization of previously deposited grainflows happened relatively often. Intertonguing of wind-ripple deposits with grainflows at the toe of slope indicates slope-parallel traction transport of sand by lee-side eddies. Undermining of the lee slope by such winds was probably the most important process that remobilized grainflows.

Vertebrate tracks are abundant in some sets of Navajo cross-strata. Grainflows in these sets are typically thin and the laminae that separate them have erosive bases-- a hallmark of Type 2 pin stripes. Animals moving over the lee slopes were the agents of remobilization for the thin grainflows that bear the tracks. Frequent disturbance of the lee slope by animals triggered numerous thin avalanches and prevented preservation of thick grainflows.