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''.
Environments of formation
Clastic dike environments include:
*Clastic dikes associated with earthquake
:A large variety of dikes are found in the geologic record. However, clastic dikes are typically produced by seismic disturbance and liquefaction of high water content sediments. Examples of this type are many. Clastic dikes are paleoseismic indicators in certain geologic settings. Several qualitative, field-based systems have been developed to help distinguish seismites
from soft sediment deformation features formed by non-seismic processes.
:Results from analytical modeling of clastic dike injection in soft rocks indicate propagation occurred at a rate of approximately 4 to 65 m/s at driving pressures of 1–2 MPa. Emplacement duration (<2 s) is similar to the speed with which acoustic energy (pressure waves) moves through partially lithified sedimentary rock.
*Clastic dikes associated with debris flow
:Sandstone dikes formed by downward injection are found along Black Dragon wash upstream of the famous petroglyphs area, San Rafael Swell
*Clastic dikes associated with impact crater
during the earliest part of the impact crater excavation stage. The dike is made of cataclastically broken sand grains derived from the White Rim Sandstone. The slightly overturned Organ Rock beds dip
steeply to the left and their tops face toward the right. The White Rim Sandstone, folded to vertical, lies just off the photo to the right. View is to the north. P.W. Huntoon Collection.
:Sandstone dikes with cataclastically deformed sand grains, sourced in the Permian
White Rim Sandstone, are found within Upheaval Dome
, Canyonlands National Park
, at Roberts Rift, and elsewhere. Commonly, the fill is composed of angular grains, evidence that the injected material was lithified prior to impact and was crushed during injection into fractures (preexisting or impact-formed).
*Clastic dikes associated with salt dome
:Clastic dike swarms associated with salt dome diapir
ism are reported from the Dead Sea
*Clastic dikes associated with glacier
:Sand injection features are reported to have formed under heavy loads and confining pressures beneath grounding glacial ice.
*Clastic dikes in resistant bedrock
:Though unusual, a significant number of reports describe sedimentary material intruding fractured crystalline bedrock, usually within fault zones. Some of the articles referenced here describe lithified clastic dikes.
*Clastic dikes in storm deposit
:Cyclic stresses from large waves can cause wet sediments to fluidize, forming various types of soft sediment deformation features including clastic dikes.
Clastic dikes in the Columbia Basin
[[Image:Clastic dike at cecil swc.jpg|Vertically sheeted clastic dike typical of those found in rhythmically bedded Missoula flood slackwater deposits of the Columbia Basin. Yellow field book for scale. Willow Creek Valley at Cecil, OR.
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 River Drainage Basin|Columbia Basin of [[Washington (state)|Washington, Oregon
. Their origin remains in question. The dikes may be related to loading by outburst floods. Other evidence suggests they are sediment-filled desiccation cracks (mudcrack
s). 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:
hillslope colluvium with developed caliche
horizons in the Umatilla Basin near Alderdale, Washington
landslide deposits along Hwy 14 in the Columbia Gorge (Umatilla Basin)
* Pleistocene Clearwater Gravels in the Lewiston Basin
* "Pre-late Wisconsin
" outburst flood and flood-related deposits (includes the "ancient cataclysmic flood deposits" of Allen et al., 2009) in the Walla Walla Valley/Pasco Basin
and Columbia Gorge
* Post-basalt basin fill sediments (Dalles Group, etc.) exposed in tributary stream valleys downstream of Wallula Gap
Snipes Mountain Conglomerate in the Yakima Valley at Granger, Washington
* Miocene-Pliocene Ringold Formation
in the Pasco Basin
* Miocene-Pliocene Ellensburg Formation at Craig's Hill near Ellensburg, Washington
(outside Missoula flood track)
* Miocene-Pliocene Ellensburg Formation at Foster Coulee (WDFW Foster Ck Wildlife Area) near Bridgeport, Washington
* Miocene Columbia River Basalt
(CRB) in the Walla Walla Valley
and Pasco Basin at Gable Mountain
* Latah Formation (interbeds in CRBs) west of Finley, Washington
along Hwy 397 and elsewhere
In 1925, Olaf P. Jenkins described the clastic dikes of eastern Washington state as follows:
[Jenkins, O.P., 1925, Clastic dikes of Eastern Washington and their geologic significance, American Journal of Science, 5th series, v. X, No. 57, p. 234-246]
Sandstone dikes photo gallery