Dennis I. Netoff

Conical sandstone landforms cored with clastic pipes in Glen Canyon National Recreation Area, southeastern Utah

Abstract Clusters of conical sandstone landforms, many with summit weathering pits, have developed on barren outcrops of the Jurassic Entrada Sandstone in Glen Canyon National Recreation Area, southeastern Utah. The conical landforms have developed on cylindrical bodies of fluidized sandstone (clastic pipes) that typically have near-vertical contacts with the enclosing cross-bedded, eolian sandstone. These landforms vary in size and shape due chiefly to differential erosion of the clastic pipe relative to the enclosing sandstone. The greater resistance to weathering of the clastic pipes is due in part to their higher content of calcite cement. Conical, pipe-cored landforms develop progressively from low domes to cones as high as 70 m. Some of the clastic pipes have relatively soft cores and resistant contacts, leading to the development of conical landforms with summit weathering pits. With time, the size of these pits increases as does the relief of the conical landform. The summit pits are as deep as 16 m and have width–depth ratios as low as 1.5. The resistant rims of these pits are due in part to calcite-enriched pipe contacts. Sandy pit-floor sediment is removed principally by strong wind rotors and vortices. Intense eolian activity in and near the landforms is indicated by abrasional features and pit-floor sand dunes. Factors that promote the development of these conical landforms include (i) the presence of clastic pipes, some with relatively soft cores; (ii) porous, friable, fine-grained pipe and host sandstones; (iii) aridity; (iv) strong winds; and (v) virtually sediment-free, unvegetated bedrock outcrops.

Clastic-injection Pipes and Syndepositional Deformation Structures in Jurassic Eolian Deposits: Examples from the Colorado Plateau

A remarkable range of syndepositional deformation structures are present in eolian and sabkha strata across the Colorado Plateau. In a classification based on scale and style of deformation, these structures include (1) millimeter- to centimeter-scale crinkly and contorted laminae and liquefaction structures; (2) meter-scale folded, contorted cross-strata and liquefied zones; (3) decameter-scale rotated blocks, slumps, and mass-flow deposits; and (4) bed-scale to multiformation-scale clastic pipes. These syndepositional structures record a range of brittle, hydroplastic, liquefaction, and fluidization properties of the sediment at the time of deformation. The pipes are the most enigmatic structures, and these can range from simple forms with structureless fill to complex forms with structureless fill, breccia blocks, and warping or faulting of surrounding and encasing host strata. A disproportionate abundance of syndepositional deformation structures in Jurassic strata of the Colorado Plateau is attributed to (1) the deposition and dissolution of evaporites associated with interdunes, sabkhas, and adjacent shallow seas; (2) a high water table, especially in response to adjacent marine transgressions; (3) dune progradation and loading over saturated, poorly consolidated, marine, and sabkha substrates; and (4) the interbedding of mobile sabkha deposits with bedded and laminated eolian sands capable of recording the overpressurization and deformation of sabkha and eolian deposits. External triggering events may have included catastrophic flooding events, bolide impacts, and seismicity. The range of deformation structures has important implications for understanding syndepositional processes. Furthermore, these studies have applications to interpreting the interconnections of high-permeable injectite conduits across multibed to multiformational scales.