Clastic dike

A clastic dike is a seam of sedimentary material that fills an open fracture in and cuts across sedimentary rock strata or layering in other rock types. Clastic dikes form rapidly by fluidized injection (mobilization of pressurized pore fluids) or passively by water, wind, and gravity (sediment swept into open cracks). Diagenesis may play a role in the formation of some dikes. Clastic dikes are commonly vertical or near-vertical. Centimeter-scale widths are common, but thicknesses range from millimetres to metres. Length is usually many times width.

Clastic dikes are found in sedimentary basin deposits worldwide. Formal geologic reports of clastic dikes began to emerge in the early 19th century.

Terms synonymous with clastic dike include: clastic intrusion, sandstone dike, fissure fill, soft-sediment deformation, fluid escape structure, seismite, injectite, liquefaction feature, neptunian dike (passive fissure fills), paleoseismic indicator, pseudo ice wedge cast, sedimentary insertion, sheeted clastic dike, synsedimentary filling, tension fracture, hydraulic injection dike, and tempestite.

Clastic dike environments include:

Tens of thousands of unusual clastic dikes (1 mm—350 cm wide, up to 50 m deep) penetrate sedimentary and bedrock units in the Columbia Basin of Washington, Oregon and Idaho. Their origin remains in question. The dikes may be related to loading by outburst floods. Other evidence suggests they are sediment-filled desiccation cracks (mudcracks). Some have suggested the dikes are ice wedge casts or features related to the melting of buried ice. Earthquake shaking and liquefaction is invoked by others to explain the dikes (i.e., sand blows).

The silt-, sand-, and gravel-filled dikes in the Columbia Basin are primarily sourced in the Touchet Formation (or the Touchet-equivalent Willamette Silt) and intrude downward into older geologic units, including:

With phrasing typical of an early-century American geologist, Olaf P. Jenkins provides one of the first descriptions of the features,

It appears, then, that in every case fissures formed and then fragmental materials are dropped, washed, or pressed into them, from above, below, or from the sides. This action has taken place in open fissures; under water in fissures on the bed of the sea or other bodies of water; and also far below the surface of the earth in consolidated rocks. The filling from below has come about by pressure of some sort, in some cases undoubtedly hydrostatic.

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