Fine-Grained Clastic Fades
Lithofacies Ss: Scour-Fill Sand. Above fluvial erosion
surfaces there commonly are a few decimeters to a
meter of coarse- to very coarse-grained, poorly
sorted sand, which may show poorly defined crossbedding and contain abundant intraclasts and other
lag materials (e.g., waterlogged plant debris) (Fig.
5.29). The bedding may drape over the irregular
shape of the basal bounding surface. Sole markings,
including flute marks and tool marks, are common
on these basal bounding surfaces.
The characteristics of lithofacies Ss indicate rapid
deposition of poorly sorted, coarse bed load. While
the process is clear from the facies characteristics,
the details of the environmental interpretation of the
lithofacies depend on its facies associations. For example, what is the type of bounding surface on which
it rests? Questions of this type relate to the architectural-element level of the analysis. For example,
lithofacies Ss may comprise the basal fill of elements
CH (channels) or HO (scour hollows). The same type
of deposit may also occur at the base of macroforrils,
or draping the third-order surfaces that define their
growth increments. A description of the geometry of
the basal bounding surface should not, therefore,
comprise part of the lithofacies definition, as originally proposal by Miall (1977), but should be part of
the architectural description of elements and
bounding surfaces.
Distinction between Ss and large, solitary St sets
migrating along a channel floor may be difficult and
even, perhaps, arbitrary. However, possible confusion about a lithofacies classification is less important than what the observations tell us about
sedimentary processes during deposition of a unit of
interest.
Lithofacies Sm: Massive Sand. Sandstone beds in
outcrop may appear massive if weathering does not
pick out the lamination. However, true massive
sandstones do exist. They may show grading, or
reveal very faint, patchy lamination. Such beds are
the deposits of sediment gravity flows. A characteristic occurrence of this facies is in small channels
resulting from bank collapse (Sect. 6.2). Massive
texture may also be produced by postdepositional
modification, for example by dewatering and bio�
turbation. Faint residual sedimentary structures
may reveal such origins.
123
Fine�grained clastic sediments are deposited primarily from the suspension load of rivers. This cannot take place within active channels, because the
shear stress and turbulence are such that the load
remains in suspension. Deposits of mud, silt, and
very fine sand therefore indicate deposition in floodplain areas, in abandoned channels, and in abandoned areas of normally active channels, for
example during seasonal low-water stages. The
thickness of depositional units ranges from the few
millimeters of mud drape formed in small abandoned channels, to thicknesses of tens to even hun�
dreds of meters in the floodplains of major,
suspension-load streams.
The original suite of two fine-grained lithofacies erected by Miall (1977) was modified by
Rust (1978b), and McLean and )erzykiewicz (1978)
proposed two additional categories, one of wh i ch
is now considered to be superfluous, as discussed
below.
Lithofacies Fl, Fsm, and Fm represent a gradation
in grain size and bedding characteristics from relatively coarse, proximal floodplain deposits to more
distal deposits.
Lithofa cies Fl: Laminated Sand, Silt and Mud. Interlamination of mud, silt, and very fine-grained sand is
common in overbank areas, and represents deposition from suspension and from weak traction currents (Fig. 5.30). Very small-scale ripples may be
present in the sand and silt beds. Undulating bedding, scattered bioturbation (including animal footprints), desiccation cracks, plant roots, coal streaks,
and scattered pedogenic nodules may be present.
Typical thicknesses of continuous Fl deposits range
from a few centimeters to many meters, depending
on sediment supply, fluvial style, and basin subsidence rates. Individual beds may be as thin as a few
millimeters. Careful observation may indicate that
units are organized into thin fining- or coarseningupward successions.
Discrete sand beds thicker than a few centimeters
may be separately logged as lithofacies Sr, St etc., and
may represent more energetic floodplain traction
currents, such as those that deposit crevasse splays.
Lithofacies Ss: Scour-Fill Sand. Above fluvial erosion
surfaces there commonly are a few decimeters to a
meter of coarse- to very coarse-grained, poorly
sorted sand, which may show poorly defined crossbedding and contain abundant intraclasts and other
lag materials (e.g., waterlogged plant debris) (Fig.
5.29). The bedding may drape over the irregular
shape of the basal bounding surface. Sole markings,
including flute marks and tool marks, are common
on these basal bounding surfaces.
The characteristics of lithofacies Ss indicate rapid
deposition of poorly sorted, coarse bed load. While
the process is clear from the facies characteristics,
the details of the environmental interpretation of the
lithofacies depend on its facies associations. For example, what is the type of bounding surface on which
it rests? Questions of this type relate to the architectural-element level of the analysis. For example,
lithofacies Ss may comprise the basal fill of elements
CH (channels) or HO (scour hollows). The same type
of deposit may also occur at the base of macroforrils,
or draping the third-order surfaces that define their
growth increments. A description of the geometry of
the basal bounding surface should not, therefore,
comprise part of the lithofacies definition, as originally proposal by Miall (1977), but should be part of
the architectural description of elements and
bounding surfaces.
Distinction between Ss and large, solitary St sets
migrating along a channel floor may be difficult and
even, perhaps, arbitrary. However, possible confusion about a lithofacies classification is less important than what the observations tell us about
sedimentary processes during deposition of a unit of
interest.
Lithofacies Sm: Massive Sand. Sandstone beds in
outcrop may appear massive if weathering does not
pick out the lamination. However, true massive
sandstones do exist. They may show grading, or
reveal very faint, patchy lamination. Such beds are
the deposits of sediment gravity flows. A characteristic occurrence of this facies is in small channels
resulting from bank collapse (Sect. 6.2). Massive
texture may also be produced by postdepositional
modification, for example by dewatering and bio�
turbation. Faint residual sedimentary structures
may reveal such origins.
123
Fine�grained clastic sediments are deposited primarily from the suspension load of rivers. This cannot take place within active channels, because the
shear stress and turbulence are such that the load
remains in suspension. Deposits of mud, silt, and
very fine sand therefore indicate deposition in floodplain areas, in abandoned channels, and in abandoned areas of normally active channels, for
example during seasonal low-water stages. The
thickness of depositional units ranges from the few
millimeters of mud drape formed in small abandoned channels, to thicknesses of tens to even hun�
dreds of meters in the floodplains of major,
suspension-load streams.
The original suite of two fine-grained lithofacies erected by Miall (1977) was modified by
Rust (1978b), and McLean and )erzykiewicz (1978)
proposed two additional categories, one of wh i ch
is now considered to be superfluous, as discussed
below.
Lithofacies Fl, Fsm, and Fm represent a gradation
in grain size and bedding characteristics from relatively coarse, proximal floodplain deposits to more
distal deposits.
Lithofa cies Fl: Laminated Sand, Silt and Mud. Interlamination of mud, silt, and very fine-grained sand is
common in overbank areas, and represents deposition from suspension and from weak traction currents (Fig. 5.30). Very small-scale ripples may be
present in the sand and silt beds. Undulating bedding, scattered bioturbation (including animal footprints), desiccation cracks, plant roots, coal streaks,
and scattered pedogenic nodules may be present.
Typical thicknesses of continuous Fl deposits range
from a few centimeters to many meters, depending
on sediment supply, fluvial style, and basin subsidence rates. Individual beds may be as thin as a few
millimeters. Careful observation may indicate that
units are organized into thin fining- or coarseningupward successions.
Discrete sand beds thicker than a few centimeters
may be separately logged as lithofacies Sr, St etc., and
may represent more energetic floodplain traction
currents, such as those that deposit crevasse splays.