Sandstone dikes

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Sandstone dikes form by hydraulic fracturing, either under load of overlying sediments or during shortening in tectonic processes. The latter commonly results in sand squirts along a cleavage.

Dikes related to loading

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84_134.  Sandstone dike. WA. Olympic peninsula, Neah Bay. [48.371891,-124.587893]         84_135.  Sandstone dike. WA. Olympic peninsula, Neah Bay.  [48.371891,-124.587893]


85_160.  Hydraulic fracturing in Lewisian gneiss, filled by mud (dark) from the overlying Stoer Gp. Scotland. NW Highlands, Clachtoll. [NC038271.  58.188953, -5.339307].


85_161.  Breccia filled fracture in Lewisian gneiss. Scotland. NW Highlands, Clachtoll. [NC038271.  58.188953,-5.339307].


85_162.  Hydraulic fractures in Lewisian gneiss. Scotland. NW Highlands, Clachtoll. [NC038271.  58.188953, -5.339307].


72_062.   Sand squirts viewed on a bedding plane. The random orientation of these cross sections suggest the mechanism is not tectonic. MT. Quartz Meadow Gulch. [47.053844,-114.818573]

Dikes related to shortening


69_389. Sandstone dike parallel to cleavage in the Martinsburg formation.  Belvidere, NJ. Stop 12 p125 of GSA guide.


72_081. Sandstone dikes in PreCambrian-Wallace formation. Many of the dikes form from the keels of tight synforms where the strain is highest. MT. First Creek.


72_083. Sandstone dikes in PreCambrian-Wallace formation. In this example the small sand squirts are parallel to a weak cleavage. MT. First Creek.


72_093.  Sandstone dikes in PreCambrian-Wallace formation. In this example the small sand squirts are parallel to a weak cleavage; many originate at synform cores. MT.  Second Creek.

Dikes with complex histories


85_432. Ptygmatic sandstone dikes with 30% vertical shortening. The book lies on a bedding plane. The rocks are Carboniferous sandstone and shale. England. Devon, N. of Bude, Northcott Mouth. [SS202085.  50.848006,-4.555493].


85_433. Folded dike with 30% shortening (close-up of 85_432). England. Devon, N. of Bude, Northcott Mouth. [SS202085.  50.848006,-4.555493].


85_434.  Folded dikes. England. Devon, N. of Bude, Northcott Mouth. [SS202085.  50.848006,-4.555493]. John Cosgrove of Imperial College.


86_135.  Folded clastic dike in the Precambrian Prichard Fm.  Naples, ID. [48.577663,-116.39863]

Enigmatic structures


Metamorphic differentiation during solution cleavage formation leads to structures that superficially resemble dikes. The example below shows dark layers rich in mica where quartz has dissolved under greenschist conditions. The quartz is then re-deposited in quartz extension fractures.

65_008.  Original bedding in a small fold (the cm scale is in the core), and near-vertical cleavage layers of mica and graphite cutting across the quartz-rich sandy layer.  Australia. NSW. Bermagui beach. [-36.42428,150.078535]

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