--> Miscellaneous Sedimentary Features of Point Bar Deposits

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MISCELLANEOUS 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.


Point Bar Depositional Areas
Related to Sedimentary Structures
 
  Organic Remains of Point Bar
Deposits
  Table of Contents
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