Bimodality of Pure Compaction Bands, Buckskin Gulch, Utah

Abstract:

The exposures of the aeolian Navajo Sandstone at Buckskin Gulch, Utah, have received much attention for their spectacular displays of compaction bands. These bands are described as tabular zones of reduced porosity, and are generally categorized into shear-enhanced compaction bands (SECBs), which accommodate equal amounts of compaction and shear strain, and pure compaction bands (PCBs) that have experienced no shearing. PCBs display a wavy or crooked geometry with vertically dipping bands ~1 mm thick. Conversely, SECBs include planar bands up to 1 cm thick with moderate dips defining conjugate geometries. The Buckskin Gulch field site was surveyed, and compaction bands systematically mapped in a sub-area representative of the heaviest deformation. Geologic and structural map units include dune boundaries within the Navajo Sandstone, areas of soft-sediment deformation, and PCBs and SECBs. Mapping shows that SECBs have outcrop characteristics consistent with those reported in previous studies, and are found with varying degrees of deformation in almost all dune units. However, the mapping and compass measurements also reveal a previously unrecognized bimodal pattern of PBC orientation with bands occurring in strands of up to four individual PCBs. One set of orientations only includes single-strand PCBs, whereas the other set is composed of multi-strand PCBs. Both sets of PCBs have nearly vertical dips but their strikes clearly differ from another by up to 25°. Further, PCBs and SECBs show distinct patterns in areas of soft-sediment deformation. Although individual PCB orientations are consistent with the overall trend of PCBs in the area, SECB orientations are chaotic, their growth was clearly affected by the soft-sediment deformation. These field observations are inconsistent with the current understanding of pure compaction band mechanics and so indicate that their growth is more complex than previously thought.