SILICIFIED BONES, INVERTEBRATE BURROWS, AND EOLIAN ADHESION STRUCTURES IN THE PENNSYLVANIAN WESCOGAME FORMATION, SUPAI GROUP, GRAND CANYON, ARIZONA

In exposures along the South Kaibab Trail in Grand Canyon National Park, the Upper Pennsylvanian Wescogame Formation of the Supai Group is exposed as a succession of dark red fluvial mudstones, siltstones, sandstones, and local gravelly sandstones, interbedded with tan eolian sandstones. The Wescogame is unconformably overlain by the Permian Esplanade Sandstone, and it is unconformably underlain by the Manakacha Formation. In addition to conspicuous tetrapod trackways that have long been recognized in some eolian beds, we report the discovery of silicified limb bones in a silty, very fine-grained, lenticular, fluvial sandstone bed with indistinct internal stratification. As far as we are aware, these bones are the first skeletal fossils of terrestrial vertebrates reported from Paleozoic strata in the Grand Canyon.

Eolian sandstone beds in the Wescogame are very fine- to medium-grained and are characterized internally by cyclic, upward-coarsening wind-ripple laminae, and crossbed sets, up to 4 m thick, composed of abundant ripple and grainfall strata and sparse sandflow strata. We observed unusual wavy, intersecting ridges, about 1.0 cm high, oriented parallel to dip on grainfall strata. Our provisional interpretation is that the ridges are shallow tunnels made by invertebrates, probably insects. Local eolian adhesion ripples and warts interrupt sets of wind-ripple laminae at nearly regular intervals. Modern adhesion bedforms develop in sand moistened by rain or tidal flooding. These bedforms migrate and climb upwind and deposit pseudo-crosslaminae that dip downwind. Adhesion bedforms in the Wescogame have steep slopes that face northeast and pseudo-crosslaminae that dip southwest. Together, these traits suggest that the ancient adhesion bedforms migrated and climbed upwind in a northeastward direction. Eolian sand for the adhesion structures was most likely moistened during episodes of rain during the Late Pennsylvanian. The greater cohesion of the moist sand inhibited frequent grainflows on dune slopes and facilitated preservation of the adhesion structures, invertebrate tunnels, and tetrapod tracks.