PALEOECOLOGY OF JURASSIC NAVAJO SANDSTONE INTERDUNE ENVIRONMENTS: AN INTEGRATED VIEW BASED ON SEDIMENTOLOGY, GEOCHEMISTRY, AND PALEONTOLOGY

The Navajo Sandstone erg dominated southwest Laurentia during the Early Jurassic, covering 265-660 x 10³ km² (Marzolf, 1988). These widespread eolian deposits include rare interdune deposits composed of limestones, siltstones, and cherts. The present study focuses on a paleoenvironmental reconstruction of interdune deposits around Moab, Utah. We integrate observations from stratigraphy, sedimentology, geochemistry and paleontology at four key sites, to infer primary depositional environments. Sites chosen for comparison include fluvial channel (with Unionoida clam coquinas) and lacustrine carbonate end-members, the latter preserving fossil cycadeoid forests and exhibiting extensive pedogenic overprints. Results show that interdunes were dominated by freshwater groundwater sources, likely derived from the adjacent Uncompahgre Uplift to the east. Flow gradients were parallel to the flow directions of precursor fluvial channels of the underlying Kayenta Formation (Luttrell, 1987).

Evidence suggests at least two periods of increased precipitation during deposition of the Jurassic Navajo erg. During these periods, dry, upland (xeric) habitats were dominated by fossil cycadeoids (preserved as rings of permineralized secondary xylem in large, meter-sized chert disks), while lake margin (mesic) habitats were dominated by Araucarian conifers (preserved as permineralized wood, cones scale and seed casts, and branch casts). Lacustrine carbonate deposits were periodically inundated by migrating dune sands, but were subsequently re-established with elevation of the ground water table. For the largest multi-level interdune deposits, long-term climatic stability is suggested by the presence of up to 250 growth rings preserved in Araucarian conifer wood. The upper levels of these deposits show evidence for pedogenic (calcrete) overprints of primary lacustrine facies.