S a nd Facies
109
Fig. 5.13. Example of cross-bedded gravel deposit (lithofacies Gp), with variable textures, controlled by the overpassing
process. Pleistocene, near Mito, Japan
5.3 Sand Fades
5.3.1 Sand Bedform Genesis and Classification
Sand lithofacies in fluvial systems result from the
transport of sand by traction currents, as bed load
and in intermittent suspension ("saltation"). The
principles of fluid turbulence that control bedform
generation are discussed briefly in Sect. 2.4.1. Empirical observation of bedforms in flumes and in a
variety of modern environments, including rivers
and estuaries, have led over the years to a prolifera�
tion of morphological terms for the various types
that have served to confuse rather than elucidate
their origins. However, a recent synthesis and review
of this work have cut through much of the confusion
and resulted in some useful simplifications (Ashley
1990).
The morphology of sand bedforms depends primarily on three parameters, sand grain size, flow
depth, and flow velocity. Other parameters, such as
fluid viscosity and temperature, exert a minor control over bedform morphology, but can be safely
ignored for the study of most natural systems. For
some years it has been known that the main types of
bedform are stable in flowing systems only under
limited ranges of these three main parameters, and
that these stability ranges, for the most part, do not
overlap significantly. Figure 5.14 illustrates the stability fields for the principal bedform types. Each of
these classes gives rise to a characteristic lithofacies,
as described in the next section.
One of the problems with the application of this
type of classification to natural systems is that bed�
forms occurring in the dune field have a wide range
of morphologies and sizes (height, wavelength), resulting in a corresponding variability in sedimentary
structures. There has been considerable debate regarding the genetic significance of these variations,
including whether they indicate a corresponding
range of distinct depositional processes.
The major and most obvious subdivision of
dunes is into those of two-dimensional (2-D) type
and those of three-dimensional (3-D) type. Twodimensonal forms tend to occur at lower flow speeds
for a given grain size. They have simple, prismatic
cross sections and give rise to planar-tabular crossbedding, bounded by simple, planar first-order
109
Fig. 5.13. Example of cross-bedded gravel deposit (lithofacies Gp), with variable textures, controlled by the overpassing
process. Pleistocene, near Mito, Japan
5.3 Sand Fades
5.3.1 Sand Bedform Genesis and Classification
Sand lithofacies in fluvial systems result from the
transport of sand by traction currents, as bed load
and in intermittent suspension ("saltation"). The
principles of fluid turbulence that control bedform
generation are discussed briefly in Sect. 2.4.1. Empirical observation of bedforms in flumes and in a
variety of modern environments, including rivers
and estuaries, have led over the years to a prolifera�
tion of morphological terms for the various types
that have served to confuse rather than elucidate
their origins. However, a recent synthesis and review
of this work have cut through much of the confusion
and resulted in some useful simplifications (Ashley
1990).
The morphology of sand bedforms depends primarily on three parameters, sand grain size, flow
depth, and flow velocity. Other parameters, such as
fluid viscosity and temperature, exert a minor control over bedform morphology, but can be safely
ignored for the study of most natural systems. For
some years it has been known that the main types of
bedform are stable in flowing systems only under
limited ranges of these three main parameters, and
that these stability ranges, for the most part, do not
overlap significantly. Figure 5.14 illustrates the stability fields for the principal bedform types. Each of
these classes gives rise to a characteristic lithofacies,
as described in the next section.
One of the problems with the application of this
type of classification to natural systems is that bed�
forms occurring in the dune field have a wide range
of morphologies and sizes (height, wavelength), resulting in a corresponding variability in sedimentary
structures. There has been considerable debate regarding the genetic significance of these variations,
including whether they indicate a corresponding
range of distinct depositional processes.
The major and most obvious subdivision of
dunes is into those of two-dimensional (2-D) type
and those of three-dimensional (3-D) type. Twodimensonal forms tend to occur at lower flow speeds
for a given grain size. They have simple, prismatic
cross sections and give rise to planar-tabular crossbedding, bounded by simple, planar first-order