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The Characteristics and Origins of Dish and Pillar Structures
Donald R. Lowe, Robert D. LoPiccolo
Dish structure is defined by the presence of thin, subhorizontal, flat to concave-upward, argillaceous laminations in siltstone and sandstone units. It is commonly associated with vertical or nearly vertical cross-cutting columns and sheets of massive sand termed pillars. Both form commonly in sediment ranging in grain size from coarse-grained siltstone to coarse-grained, conglomeratic sandstone. In sedimentation units greater than about 0.5 m thick, dish structure is faint and neither cuts across nor is cross-cut by other sedimentary structures. In thinner units dish structures commonly cut across primary flat laminations, climbing-ripple cross-laminations, and convolute laminations.
Dish and pillar structures form during the consolidation of rapidly deposited, underconsolidated or quick beds. During gradual compaction and dewatering, semi-permeable laminations act as partial barriers to upward-moving fluidized sediment-water slurries, forcing horizontal flow beneath the laminations to points where continued vertical escape is possible. As water seeps upward through the confining laminations, fine sediment, planar, and low-density grains are filtered out and concentrated in the sediment pore spaces. The resulting clay- and organic-enriched laminations are flat dishes that may be later deformed by the upward pressure of flow around their margins and central subsidence as underlying sediment and water escape. Pillars form during forceful, explosive water escape. It s suggested that the shapes of dishes and pillars within an individual bed can be related to its original water content, thickness, and grain size; to the rate and magnitude of dewatering including consideration of water entering the bed from underlying consolidating sediments; and to the types and distribution of earlier-formed sedimentary structures.
Dish structures cannot be used directly to infer transport or depositional processes. Where dishes are associated with or cut across primary sedimentary structures, the latter indicate deposition from currents.
The study, indicates that coarse-grained terrigenous sediments often have pronounced and complex consolidation histories. Many rapidly deposited beds undergo partial liquefaction and fluidization during consolidation but retain sufficient strength to resist wholesale downslope flowage in response to gravity.
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