VISUAL ENHANCEMENT OF TRACE FOSSILS AND OTHER SEDIMENTARY STRUCTURES IN THE ESCALANTE SANDSTONE MEMBER OF THE ENTRADA FORMATION (MIDDLE JURASSIC) BY ENDOLITHIC MICROBES; TWENTY MILE WASH, GRAND STAIRCASE-ESCALANTE NATIONAL MONUMENT, UTAH

When first discovered in 1998, the Twenty Mile Wash Dinosaur Tracksite (TMWDT) was found to be covered with over 1300 distinct dinosaur tracks and an abundance of invertebrate dig traces. High color contrast between individual laminae associated with undertracks and burrows made the features readily visible from great distances. The weathered surface of the outcrop was a medium gray, which contrasted with the yellowish color of freshly exposed sandstone. Subsequently the TMWDT surface has started, perhaps because of extended drought conditions, to spall, exposing fresh sandstone of low contrast in which individual tracks are much more difficult to see. This lead us to the conclusion that something other than inherent physical properties of the rock substrate was responsible for long term stability and visual enhancement of tracks at the site. Thin sections of the un-spalled (or intact) TMWDT surface reveal coccoid and filamentous endolithic microbes. Other authors have described “armoring” effects that endoliths can have on friable sandstone rock outcrops in arid environments. We now believe that endolithic bacteria, in addition to producing previously documented armoring affects which create long term stability of the surface, also dramatically enhance the visibility of trace fossils and sedimentary structures including dinosaur tracks, animal burrows, and cross-bedding by their selective colonization (equals greater abundance) of more porous lamina. Extended drought appears to accelerate the loss of these features by stressing the endolithic flora and increasing the track surface’s vulnerability to physical weathering and erosion.