Current
.....
I!I
,
5.3 PRIMARY INORGANIC SEDIMENTARY STRUCTURES
149
Fig. 5.17. Formation of tabular planar cross-bedding occurs where dunes migrate downcurrent, and where
stoss side erosion is less than sedimentation of the foreset. Note that the preserved thickness of each set is
less than the height of the dune from which it was deposited.
completely infill the pool with a single set of cross-strata (Jopling, 1965) (e.g., Fig. 5.18;
see also Fig. 6.6).
A second important way in which cross-bedding forms is seen in channels. A channel
may be infilled by cross-bedding paralleling the channel margin. Alternatively cross-bed
deposition occurs on the inner curves of meandering channels synchronous with erosion
on the outer curve (see Section 6.3.2.2.3). By this means, a tabular set of cross-strata
may be deposited in which the foresets strike parallel to the flow direction (Lyell, 1865,
p. 17). This type of lateral cross-bedding is rather larger than most types (sets may be
several meters high) and the base is marked by a conglomerate. Close examination of
the foresets often shows that they are composed of second-order cross-beds or crosslaminae which do in fact reflect the true current direction (Fig. 5.19).
A third important variety of cross-bedding is that formed by antidunes in upper flow
regime conditions. It has been pointed out that at very high current velocities sand dunes
develop that migrate upcurrent (Section 4.2.1.1). These deposit upcurrent dipping foresets. The foresets of these antidunes, as they are called, are seldom preserved. As the
current wanes prior to net sedimentation, antidunes tend to be obliterated as the bed
form changes to a plane bed or dunes. A few instances have been noted. Some have been
described from the A unit of turbidites (Skipper, 1971; Hand et al., 1972) as mentioned
in Section 4.2.1.1. Alexander and Fielding (1997) have described gravel antidunes in the
-
lOOm
2ml
Fig. 5.18. Single large foreset deposited in braided channel chute pool. Cambro-Ordovician, Wadi Rum,
Jordan.
.....
I!I
,
5.3 PRIMARY INORGANIC SEDIMENTARY STRUCTURES
149
Fig. 5.17. Formation of tabular planar cross-bedding occurs where dunes migrate downcurrent, and where
stoss side erosion is less than sedimentation of the foreset. Note that the preserved thickness of each set is
less than the height of the dune from which it was deposited.
completely infill the pool with a single set of cross-strata (Jopling, 1965) (e.g., Fig. 5.18;
see also Fig. 6.6).
A second important way in which cross-bedding forms is seen in channels. A channel
may be infilled by cross-bedding paralleling the channel margin. Alternatively cross-bed
deposition occurs on the inner curves of meandering channels synchronous with erosion
on the outer curve (see Section 6.3.2.2.3). By this means, a tabular set of cross-strata
may be deposited in which the foresets strike parallel to the flow direction (Lyell, 1865,
p. 17). This type of lateral cross-bedding is rather larger than most types (sets may be
several meters high) and the base is marked by a conglomerate. Close examination of
the foresets often shows that they are composed of second-order cross-beds or crosslaminae which do in fact reflect the true current direction (Fig. 5.19).
A third important variety of cross-bedding is that formed by antidunes in upper flow
regime conditions. It has been pointed out that at very high current velocities sand dunes
develop that migrate upcurrent (Section 4.2.1.1). These deposit upcurrent dipping foresets. The foresets of these antidunes, as they are called, are seldom preserved. As the
current wanes prior to net sedimentation, antidunes tend to be obliterated as the bed
form changes to a plane bed or dunes. A few instances have been noted. Some have been
described from the A unit of turbidites (Skipper, 1971; Hand et al., 1972) as mentioned
in Section 4.2.1.1. Alexander and Fielding (1997) have described gravel antidunes in the
-
lOOm
2ml
Fig. 5.18. Single large foreset deposited in braided channel chute pool. Cambro-Ordovician, Wadi Rum,
Jordan.