Print this pageMISCELLANEOUS SEDIMENTARY FEATURES OF POINT BAR DEPOSITS
Contorted Bedding
Contorted
bedding is associated with steeply inclined foreset beds of
spillover bar features occurring in the depositional areas of
both the giant-ripple and "horizontally" bedded sands.
Contortions of foreset beds have been observed a few days
following the flood which deposited the sediment.
A contorted zone of fine-grained sand beds approximately 1 foot
thick (Figures 49, 50, and 51), having a very gentle slope
and lying between undisturbed beds of sand, has been traced the
entire distance of a 50-foot trench cut into the Brazos River
deposits of the 1951 high-water stage. These deposits have not
been flooded since. Right-angle cross sections of the contorted
beds and associated inclined beds suggest that the contorted
nature of the beds resulted from a penecontemporaneous
hydroplastic mass flowage of steeply inclined sediments. The
direction of mass flowage, indicated by most of the contorted beds, conforms with the downstream
dip direction of the inclined beds. However, a component of
movement in the direction of the more gentle depositional slope
of the bar is apparent. Since these contorted beds are overlain
only by a few inches of sediment and have not been flooded since,
it is most likely that contortion was produced by gravity and not
necessarily by turbidity currents (Haaf, 1956) (Mellen, 1956),
air heaving (Stewart, 1956), or unequal loading and dewatering of
sediments (Migliorini, 1950).
Similar contorted beds occur in similar facies of the Eocene
Carrizo formation of east Texas (see Stenzel, 1950).
Slumping
Slumping is also associated with steeply dipping foreset beds
of spillover bars (see Figures 52 and 53).
Initial Dips
Most of the basal sediment layers of a flood area are
conformable with the depositional surfaces of an earlier flood or
erosional surfaces which developed on the point bar during low
river stages (Figure 54). Depositional surfaces outnumber erosional surfaces.
The basal sediments of a rising river stage are literally draped
over older depositional surfaces of the point bar. The
topographically low places are eventually filled with layers of
sediments: the first layers conform to the original slopes,
either depositional or erosional, and the subsequently formed
layers to the previously formed depositional surface. Filling of
the low places may occur during a single flood or a series of
floods. Deposition is most active on those parts of the point bar
which are adjacent to the river and where the depositional slope
is gentle. Most of the small-scale depositional surfaces, such as
ripple fore slopes, slope downstream; some slope towards and some
away from the depositional bank, but upstream slopes are rare.
Most erosional slopes on the point bar slope either upstream or
downstream.
Silt and Clay Drapes
The capacity of the stream to transport sediment is reduced
rapidly as velocities diminish during falling river stages.
Currents are relatively slack near the depositional bank and are
capable only of transporting very fine materials in suspension, such as
silt and clay. During falling river stages, these fine sediments
settle over depositional surfaces which had been adjusted to the
previous higher river stages, and form a silt-clay drape (Figures
55, 56, and 57). The drape is thinner in
the locally high places and thicker in the lower places.
The draped silts and clays are finely laminated. The drape may be
ripple-marked (Figure 40).
The silt and clay drape is in many cases sun-dried' mud-cracked,
and curled (Figures 58 and 59), and is generally destroyed
within a few days or weeks. However, if a subsequent flood
shortly follows the previous flood, the clay drape may be covered
with coarser river deposits and preserved All stages of the
destruction of drapes have been observed on the bar and in cross
sections (Figures 40, 41, 42, 43, 52 and 53).
Sections across steeply inclined drapes will reveal a thin clay
layer cutting diagonally across sand deposits. Some geologists
attribute the origin of sections similar to the above to sediment
flowage.
The maximum stage of a single flood controls the nature and
distribution of the deposits of that flood. The flood may be
recorded only by a silt and clay drape and driftwood near the
high-water line on the point bar, but by several feet of
"horizontally" and cross-bedded sands on the lower
slopes of the bar.
Mud Balls
Most mud
balls observed in point bar deposits are produced by gully
erosion of silt and clay drapes during low-water stages. In many
cases mud balls occur in small alluvial fans (Figures 60 and 61) formed near the low stage
bank during low-water stages. Many mud balls are deposited in the
lower point bar deposits during high-water stages (Figures 54 and 62). Some mud balls are armored
with sand and pea gravel and some develop spindle shapes during
transportation.
Borings
Borings of
land animals, usually insects, and concretion-like features which
are cocoons of insect larvae, are somewhat common in the
"horizontally" bedded and small-ripple bedded deposits
(see Figures 45, 46, 47, and 63).
Other
Miscellaneous Features
Other minor features which are common to the
depositional surfaces but have not yet been observed in cross
sections, are insect trails and bubble pits (see Figures 64, 65, and 66). The latter features are
formed by escaping air which was trapped in sediments during
rapidly rising river stages. Disturbed or contorted bedding
produced by air-heaving as described by Stewart (1956) has not
been recognized in hundreds of trenches cut in these deposits.
Root marks and fragments (Figures 47 and 48) are common in the upper
point bar deposits.
