Article: Coniferous trees associated with interdune deposits in the Jurassic Navajo Sandstone Formation, Utah, USA
The Lower Jurassic Navajo Sandstone Formation of south-west USA represents one of the largest erg deposits ever to have developed on Earth. Here, we report the widespread occurrence of silicified conifer stumps and trunks within interdune deposits near Moab, south-east Utah. Where present as (par)autochthonous assemblages, trees are associated with the deposits of spring-fed carbonate lakes. A few stumps preserved in growth position are rooted in aeolian sandstone immediately below the lake deposits, and evidently established on interdune soils in response to a rising water table. Following at least several decades of growth, trees were killed as the water table continued to rise forming shallow lakes containing ostracodes. Where present as allochthonous assemblages, randomly orientated tree trunks are associated with massive sandstone beds interpreted as fluidized mass flow deposits. These may have formed when dune slip-faces collapsed during occasional heavy downpours of rain, destroying stands of trees. The occurrence of large conifers over a wide area of the Navajo Sandstone Formation in south-east Utah may record long-lived pluvial episodes during which the dune field stabilized, or reflect the erg-margin position of the localities.