5.3 PRIMARY INORGANIC SEDIMENTARY STRUCTURES
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tory experiments have shown that this process can indeed generate convolute bedding
(Selley, 1969). These experiments showed that the sands could fall into a tighter packing both by vibration and by turbulent eddies in the overlying water.
Convolute bedding has been recorded in modern sediments both as a result of earthquakes and without them (e.g., Barratt, 1966, and McKee et al., 1967, respectively). Allen
(1986b) has even established an empirical relationship between the frequency of earthquake-induced deformation, distance from the epicenter, and quake magnitude on the
Richter scale. The downcurrent overturning of convolute folds and their association
with downcurrent deformed foresets strongly suggest that powerful currents play a significant part in their genesis. This structure is not restricted to aqueously deposited sediment, however, but also occurs in eolian ones (Doe and Dott, 1980).
On a smaller scale, laminated fine sands and silts also show penecontemporaneous
vertical deformation structures termed convolute lamination. This is similar in geometry to convolute bedding, but occurs in finer grained sediment on a much smaller scale;
generally in beds only a decimeter or so high. Convolute lamination is especially characteristic of turbidites, involving deformation of both the laminated and cross-laminated
Bouma units. Correlation of fold axes with ripple crests, and the presence of deformed
intrabed scour surfaces, suggests that movement was virtually synchronous with deposition. Convolute lamination probably originates, therefore, by the dewatering of the
sediment aided by the shear stresses set up by the turbidity flow itself (see also Davies,
1965; Anketell et aL, 1970; Visher and Cunningham, 1981).
Convolute bedding, recumbent foresets, and convolute lamination are the three main
types of intrabed vertical deformational structures. Dish structure is a particular variant of intrasand deformation. This is seen where laminae or bedding planes are intermittently disrupted and upturned like the rim of a dish. Dish structure is a type of
dewatering phenomenon that is particularly characteristic of fluidized sand beds (see
Section 4.5.3). It testifies to the loose and unstable packing of the sand when first deposited (Lowe and Lopiccolo, 1974; Lowe, 1975). Dish structure is often associated with
vertical pipes or pillars that look like organic burrows. The association with dish deformation suggests, however, that the pipes are water escape conduits.
A variety of structures develop where sands overlie muds. The mud:sand interface is
often deformed in various ways. Most typically irregular-rounded balls of sand depend
from the parent sand bed into the mud beneath. These structures are variously termed
loadeasts, ball and pillow structures, etc. They are a variety of the broad group of structures termed sole markings or bottom structures. It is important, however, to distinguish deformational bottom structures, like loadcasts, from erosional markings such
as grooves and flutes. Sometimes erosional bottom structures become deformed. In
extreme cases the sand lobes may become completely detached from their parent bed
above. Similarly, thin sand beds may split along their length to form isolated cakes of
sand in mud (Fig. 5.30). These discrete bodies of sand in mud are termed pseudonodules to distinguish them from normal diagenetic nodules (Macar and Antun, 1949).
Loadcasts and pseudonodules occur at sand:mud interfaces in various environments,
both modern and ancient. They are a common feature of turbidite deposits, yet they
also occur in deltaic and fluvial sediments. There is general agreement that these structures are generated by the differential loading of a waterlogged sand on an unconsolidated mud. They are easy to make in the laboratory (e.g., Kuenen, 1958, Owen, 1996).
159
tory experiments have shown that this process can indeed generate convolute bedding
(Selley, 1969). These experiments showed that the sands could fall into a tighter packing both by vibration and by turbulent eddies in the overlying water.
Convolute bedding has been recorded in modern sediments both as a result of earthquakes and without them (e.g., Barratt, 1966, and McKee et al., 1967, respectively). Allen
(1986b) has even established an empirical relationship between the frequency of earthquake-induced deformation, distance from the epicenter, and quake magnitude on the
Richter scale. The downcurrent overturning of convolute folds and their association
with downcurrent deformed foresets strongly suggest that powerful currents play a significant part in their genesis. This structure is not restricted to aqueously deposited sediment, however, but also occurs in eolian ones (Doe and Dott, 1980).
On a smaller scale, laminated fine sands and silts also show penecontemporaneous
vertical deformation structures termed convolute lamination. This is similar in geometry to convolute bedding, but occurs in finer grained sediment on a much smaller scale;
generally in beds only a decimeter or so high. Convolute lamination is especially characteristic of turbidites, involving deformation of both the laminated and cross-laminated
Bouma units. Correlation of fold axes with ripple crests, and the presence of deformed
intrabed scour surfaces, suggests that movement was virtually synchronous with deposition. Convolute lamination probably originates, therefore, by the dewatering of the
sediment aided by the shear stresses set up by the turbidity flow itself (see also Davies,
1965; Anketell et aL, 1970; Visher and Cunningham, 1981).
Convolute bedding, recumbent foresets, and convolute lamination are the three main
types of intrabed vertical deformational structures. Dish structure is a particular variant of intrasand deformation. This is seen where laminae or bedding planes are intermittently disrupted and upturned like the rim of a dish. Dish structure is a type of
dewatering phenomenon that is particularly characteristic of fluidized sand beds (see
Section 4.5.3). It testifies to the loose and unstable packing of the sand when first deposited (Lowe and Lopiccolo, 1974; Lowe, 1975). Dish structure is often associated with
vertical pipes or pillars that look like organic burrows. The association with dish deformation suggests, however, that the pipes are water escape conduits.
A variety of structures develop where sands overlie muds. The mud:sand interface is
often deformed in various ways. Most typically irregular-rounded balls of sand depend
from the parent sand bed into the mud beneath. These structures are variously termed
loadeasts, ball and pillow structures, etc. They are a variety of the broad group of structures termed sole markings or bottom structures. It is important, however, to distinguish deformational bottom structures, like loadcasts, from erosional markings such
as grooves and flutes. Sometimes erosional bottom structures become deformed. In
extreme cases the sand lobes may become completely detached from their parent bed
above. Similarly, thin sand beds may split along their length to form isolated cakes of
sand in mud (Fig. 5.30). These discrete bodies of sand in mud are termed pseudonodules to distinguish them from normal diagenetic nodules (Macar and Antun, 1949).
Loadcasts and pseudonodules occur at sand:mud interfaces in various environments,
both modern and ancient. They are a common feature of turbidite deposits, yet they
also occur in deltaic and fluvial sediments. There is general agreement that these structures are generated by the differential loading of a waterlogged sand on an unconsolidated mud. They are easy to make in the laboratory (e.g., Kuenen, 1958, Owen, 1996).