Abstract

Vertically oriented columns of sandstone, termed clastic pipes, are common in Jurassic deposits of the Colorado Plateau. Exposures in the sabkha to fluvial strata of the Carmel Formation in southern Utah provide an excellent opportunity to understand pipe-formation mechanisms and controls on their expressions. These clastic pipes demonstrate significant variability and fall into two major categories: (1) primary textural pipes and (2) non-textural pipes. Primary textural pipes show significant textural differences relative to the host rock, including an internal outward grain coarsening indicative of traction structures. The pipes contain entrained, brecciated host material, and the surrounding host rock has associated soft-sediment deformation. Primary textural pipes formed through multiple liquefaction and fluidization events as evident from crosscutting pipes, multiple eruption horizons, traction structures, and other evidence of fluid-suspended-sediment transport. Non-textural pipes do not have significant textural differences from the surrounding host rock but have clear diagenetic differences (e.g., mineral coloration and cement differences) relative to the host rock. These non-textural pipes formed through the upward expulsion of fluids similar to primary textural pipes, but the fluid forces were likely insufficient to fully fluidize the sand. Overall, the porous, sandstone host rock plays a key role in explaining pipe characteristics, including their cylindrical geometry (versus tabular geometries in mudstones) and diagenetic expression. Pipe variability typically corresponds to lithologic and stratigraphic changes in the surrounding host rock. These pipe examples are key paleoenvironmental indicators, which give valuable information about subsurface fluids and the presence of a near-surface groundwater system.

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