152
5 SEDIMENTARY STRUCTURES
....
~
L
Preserved
facies
Shallow marine
shelf sands
Storm deposits
Turbidites
J,DePi hs of activity~
Turbidity currents
Storms
Tidal currents
Fig. 5.23. Illustration of the operation, depths, and preserved intervals of turbidite and storm deposits. This
shows that hummocky cross-stratification indicates deposition between the fair weather and stormy weather
wave bases.
and redeposited by normal traction currents down to a fair weather wave base. Storm
deposits can occur from sea level down to a storm weather wave base (Aigner, 1985).
They too will be reworked by traction currents above the fair weather wave base, but
may in turn rework turbidites (Fig. 5.23).
These descriptions of the various types of cross-bedding show that it is a very complex sedimentary Structure. More properly it is a group of structures of diverse morphology and genesis. Particular attention has been paid by geologists to determine
depositional environment from the type of cross-bedding. This has not been notably
successful because, though the structural morphology is closely related to hydrodynamic
conditions, the same set of hydrodynamic parameters can occur in various environments. Hummocky cross-bedding is perhaps the only exception.
Another line of approach has been to try to determine water depth from set height.
This has not been very successful either, for several reasons, not the least of which is
that the preserved set height is controlled by the degree of erosion that occurred after a
set was laid down. Nevertheless, in underwater cross-bedding, water depth cannot have
been less than the preserved set height. Set height has also been used to try to distinguish
eolian from subaqueous dunes. The folklore holds that eolian dunes deposit higher set
heights than subaqueous ones. This is not universally true as the studies cited in Section 4.3.1 show. No satisfactory height limit for subaqueous cross-bedding can be fixed
because the internal morphology of submarine dunes is so little known (see Section
6.3.2.7.2). One of the most important things that can be learned from cross-bedding is
the flow direction of the currents which deposited them. This can give important clues
to the environment, paleogeography, and structural setting of the beds in which they
occur. This important topic of paleocurrent analysis is discussed later in the chapter.
5.3.3.5 Ripples and Cross-Lamination
Ripples are a wave-like bed form that occurs in fine sands subjected to gentle traction
currents (Fig. 5.24). Migrating ripples deposit cross-laminated sediment. Individual
5 SEDIMENTARY STRUCTURES
....
~
L
Preserved
facies
Shallow marine
shelf sands
Storm deposits
Turbidites
J,DePi hs of activity~
Turbidity currents
Storms
Tidal currents
Fig. 5.23. Illustration of the operation, depths, and preserved intervals of turbidite and storm deposits. This
shows that hummocky cross-stratification indicates deposition between the fair weather and stormy weather
wave bases.
and redeposited by normal traction currents down to a fair weather wave base. Storm
deposits can occur from sea level down to a storm weather wave base (Aigner, 1985).
They too will be reworked by traction currents above the fair weather wave base, but
may in turn rework turbidites (Fig. 5.23).
These descriptions of the various types of cross-bedding show that it is a very complex sedimentary Structure. More properly it is a group of structures of diverse morphology and genesis. Particular attention has been paid by geologists to determine
depositional environment from the type of cross-bedding. This has not been notably
successful because, though the structural morphology is closely related to hydrodynamic
conditions, the same set of hydrodynamic parameters can occur in various environments. Hummocky cross-bedding is perhaps the only exception.
Another line of approach has been to try to determine water depth from set height.
This has not been very successful either, for several reasons, not the least of which is
that the preserved set height is controlled by the degree of erosion that occurred after a
set was laid down. Nevertheless, in underwater cross-bedding, water depth cannot have
been less than the preserved set height. Set height has also been used to try to distinguish
eolian from subaqueous dunes. The folklore holds that eolian dunes deposit higher set
heights than subaqueous ones. This is not universally true as the studies cited in Section 4.3.1 show. No satisfactory height limit for subaqueous cross-bedding can be fixed
because the internal morphology of submarine dunes is so little known (see Section
6.3.2.7.2). One of the most important things that can be learned from cross-bedding is
the flow direction of the currents which deposited them. This can give important clues
to the environment, paleogeography, and structural setting of the beds in which they
occur. This important topic of paleocurrent analysis is discussed later in the chapter.
5.3.3.5 Ripples and Cross-Lamination
Ripples are a wave-like bed form that occurs in fine sands subjected to gentle traction
currents (Fig. 5.24). Migrating ripples deposit cross-laminated sediment. Individual