DIFFERENTIATING CLASTIC PIPES FROM STEM CASTS: PAGE SANDSTONE (MIDDLE JURASSIC), SOUTHERN UTAH, USA

Many weathered surfaces of flat-bedded fluvial sandstones of the Page Sandstone (Middle Jurassic) display three-dimensional patterns we interpret as bark casts of large, fallen trees. Prostrate casts are closely intermingled with upright sandstone cylinders previously interpreted as eruptive pipes linked to subsurface liquefaction. Near the UT/AZ boundary and Paria River, these sandstones infill a paleovalley incised into eolian strata of the Leche-e Member of the Page Sandstone by a NE-flowing stream. The casts (which also trend NE) are defined by 10’s-cm-thick, weathering resistant, pale sheets of sandstone cemented by syn-sedimentary, sparry calcite. Distinctive geometric patterns are formed by elongate (up to several meters), curving, cm-wide ridges that join and separate at ~20-50 cm intervals. These patterns are similar to those of thick-barked living gymnosperms (Sequoia, Metasequoia, Taxodium, Tsuga, Pseudotsuga, Gingko, Cupressus). We hypothesize that trees were felled by floodwaters and then rapidly buried and flattened by floodplain sediments. Anoxia due to rotting wood elevated bicarbonate, causing calcite to precipitate along the sand-bark interface. Most casts represent “face-down” bark on the underside of the trunks.

We propose that many of the cylinders are better interpreted as upright casts because of their close association with bark casts and because they show evidence of spiral growth, branch scars, buttressed trunks, lean in a downflow (NE) direction, and occur in close proximity to vegetation-induced sedimentary structures. Ring faults and downward sagging strata mark the tops of many large, upright casts. At several sites, sagging strata contain casts of fallen trees, indicating that faulting and sagging post-dated burial of a large, upright tree. This delay shows that deformation was not caused by de-watering of erupted sand, but by collapse of overburden into a large void generated by the decay of wood in the subsurface. The interpretation of these sandstone pipes as trunk casts adds to a growing body of evidence that despite the arid paleoclimate and proximity to large eolian dunes, that vegetation was locally abundant in riparian settings and/or areas where flow from valley walls created a relatively high water table.