Coastal-plain origin of trace-fossil bearing red beds in the Early Permian of Southern New Mexico, U.S.A.

The Early Permian Robledo Mountains Formation of the Hueco Group in the Robledo Mountains, south-central New Mexico, U.S.A., is a world-class ichnofossil Lagerstنtte of supposed peritidal origin. Abundant and diverse invertebrate and vertebrate trace fossils occur in several thin horizons of siliciclastic red beds that are intercalated with thick successions of shallow marine limestone and shale. The depositional environment of the red beds has been discussed for decades but without any detailed sedimentary facies analysis. A recently completed systematic excavation shows that the majority of trace fossils is related to mud-draped surfaces within distal crevasse-splay siltstone to fine-grained sandstone. Interbedded mudstone with well-preserved macrofloral remains and a lungfish aestivation burrow represents a hitherto disregarded facies in overbank fines. Both environmental subzones were at least loosely covered with low-growing plants and experienced common periods of subaerial exposure. None of the physical and biological structures recorded in the measured section indicates tidal influence, but instead they support the reconstruction of a freshwater ecosystem dominated by arthropods and tetrapods. Given their proximity to carbonates of unambiguous subtidal origin, we propose that the trace-fossil-bearing red beds of the Robledo Mountains Formation formed in distal parts of an extensive coastal floodplain during alternating wet and dry conditions. If this interpretation is correct, it provides an impetus to continue ichnological and sedimentological field research in the study area, because the true tidal-flat ichnoguilds still await elucidation.