5.3 PRIMARY INORGANIC SEDIMENTARY STRUCTURES
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is not always easy to make. In particular, the huge polygons of modern playas, into whose
cracks a camel may fall, may be due to a combination of subaerial and subaqueous dehydration with complex histories related to Quaternary climatic changes.
5.3.5.5 Sand Dikes
Sand dikes are vertical sheets of sand that have been intruded into muds from sand beds
beneath. Though they are sometimes polygonally arranged, they can be distinguished
from desiccation cracks by their tendency to die out upward and by the fact that they
are rooted to the parent sand bed below. Sand dikes are intruded as liquefied quicksand
into water-saturated mud. Like desiccation cracks, they often show ptygmatic compaction effects (Fig. 5.36). A notable example of sand dike intrusion occurs in the Miocene
of the Panoche Hills, California, where more than 350 sand dikes and sills intrude the
Moreno shale (Smyers and Peterson, 1971). They range from 1 dm to 7 m in width. Individual dikes are up to 1 km in length.
A particular variety of sand dike is the sedimentary boudinage structure. This is morphologically similar to tectonic boudinage. Instead of being due to the necking of a competent limestone or sandstone in incompetent shale, the converse is true. Sedimentary
boudinage typically occurs where interbedded, unconsolidated, water-saturated sands
and muds are subjected to tension, for example, adjacent to a slump. Clay beds develop
necks and are sometimes divided up into blocks by sand intrusion from above and below (Fig. 5.37).
5.3.5.6 Sedimentary Volcanoes
Volcanoes of ejected sediment are sometimes found whose gross morphology of vent
and cone are analogous to igneous volcanoes. These sedimentary volcanoes range in
scale from diameters of only a few centimeters or so to several kilometers.
Monroes are small (5-10 cm in diameter) mud volcanoes with a mammillated appearance that have been recorded on recent tidal fiats. They appear to be generated by
the escape of water charged with biogenic gas (Dionne, 1973).
Fig. 5.37. Sedimentary boudinage. Formed by tensional splitting of clay bed accompanied by quicksand injection along the incipient fissures.
165
is not always easy to make. In particular, the huge polygons of modern playas, into whose
cracks a camel may fall, may be due to a combination of subaerial and subaqueous dehydration with complex histories related to Quaternary climatic changes.
5.3.5.5 Sand Dikes
Sand dikes are vertical sheets of sand that have been intruded into muds from sand beds
beneath. Though they are sometimes polygonally arranged, they can be distinguished
from desiccation cracks by their tendency to die out upward and by the fact that they
are rooted to the parent sand bed below. Sand dikes are intruded as liquefied quicksand
into water-saturated mud. Like desiccation cracks, they often show ptygmatic compaction effects (Fig. 5.36). A notable example of sand dike intrusion occurs in the Miocene
of the Panoche Hills, California, where more than 350 sand dikes and sills intrude the
Moreno shale (Smyers and Peterson, 1971). They range from 1 dm to 7 m in width. Individual dikes are up to 1 km in length.
A particular variety of sand dike is the sedimentary boudinage structure. This is morphologically similar to tectonic boudinage. Instead of being due to the necking of a competent limestone or sandstone in incompetent shale, the converse is true. Sedimentary
boudinage typically occurs where interbedded, unconsolidated, water-saturated sands
and muds are subjected to tension, for example, adjacent to a slump. Clay beds develop
necks and are sometimes divided up into blocks by sand intrusion from above and below (Fig. 5.37).
5.3.5.6 Sedimentary Volcanoes
Volcanoes of ejected sediment are sometimes found whose gross morphology of vent
and cone are analogous to igneous volcanoes. These sedimentary volcanoes range in
scale from diameters of only a few centimeters or so to several kilometers.
Monroes are small (5-10 cm in diameter) mud volcanoes with a mammillated appearance that have been recorded on recent tidal fiats. They appear to be generated by
the escape of water charged with biogenic gas (Dionne, 1973).
Fig. 5.37. Sedimentary boudinage. Formed by tensional splitting of clay bed accompanied by quicksand injection along the incipient fissures.