PRELIMINARY REPORT ON DEWATERING PIPES IN THE LOWER PART OF THE LOWER JURASSIC NAVAJO SANDSTONE, CANYONLANDS NATIONAL PARK, SOUTHEASTERN UTAH: IMPLICATIONS FOR PLUVIAL EPISODES AND THE OCCURRENCE OF LAKES, TREES, AND MAMMAL BURROWS

Numerous dewatering pipes crosscut eolian dune-interdune deposits in the lower part of the Lower Jurassic Navajo Sandstone in and around Canyonlands National Park, near Moab, Utah. The Navajo Sandstone was deposited as a vast erg system over northeastern Arizona, southwestern Colorado, and most of Utah, and unconformably overlies the Upper Triassic Chinle Formation in most of the four corners area.

The dewatering pipes range in size and distribution greatly. In some places they are so abundant that they form fields of dewatering pipes. The smallest pipes are 4-200 mm in diameter. The largest pipes are 1-4+ m in diameter. In some cases pipes are so large and oddly shaped that the exact size is indeterminate, even in the best exposed outcrops. In many places, smaller diameter pipes originate from larger diameter pipes, and are often connected by finger-like projections. The internal morphology of the pipes includes cone-shaped, concentric sediment layers, microfaults, and wallet- to box-sized rotated blocks of cohesive laminated sediments.

The fields of de-watering pipes appear to terminate at several levels associated with vast interdune deposits traceable for several kilometers. Interdune deposits in the Moab area contain abundant evidence of pedogenic modification and wet ecosystems. Pedogenesis is indicated by massive fine-grained quartz sandstone and pale-reddish brown siltstone and mudstone with 3- to 100-mm-wide rhizoliths, bleached rhizohalos, calcareous rhizocretions, insect-bored steinkerns and permineralized tree trunks and roots, 3- to 5-mm-wide adhesive meniscate burrows, 5-mm-wide subvertical burrows, and large-diameter subhorizontal burrows with T- and Y-shaped intersections and switchback tunnels that form pseudo-spiraled ramps. Also present are intertwined and interpenetrated, high density occurrences of 2- to 4-mm-wide burrows with and without linings associated mostly with rhizoliths. The traces likely represent activity of soil bugs, beetles, social termites, social therapsids or mammals, and vegetation. The traces bear striking resemblance to traces in other Mesozoic and Cenozoic continental deposits. The association of pipes, interdune deposits, and pedogenesis indicate that local-regional groundwater recharge supported this ecosystem during pluvial intervals.