POINT BAR DEPOSITIONAL AREAS

RELATED TO SEDIMENTARY STRUCTURES

Topographically, the point bar environment may be subdivided into four arcuate-shaped depositional areas (Figure 8) of different, but relative elevations related to river stages (Figure 9) and also of different, but relative, positions within the meander loop related to distances from the undercut bank. Sedimentary structures and grain-size characters diagnostic of each depositional area are, from highest to lowest.

1) Area of small-ripple bedded, mainly poorly sorted, very fine- to fine-grained sands interbedded with thinner "horizontally" bedded, fine-grained sands and thin clay layers.

2) Area of "horizontally" bedded, mainly well-sorted, fine- to medium grained sands interbedded with thinner small-ripple bedded, well- to poorly sorted, very fine- to medium-grained sands

3) Area of giant-ripple bedded, mainly well-sorted, fine- to coarse grained,occasionally graveliferous sands

4) Area of poorly bedded and poorly sorted, coarse- to very coarse grained sands and gravel deposits.

Note The above sedimentary features, grain size, and sorting variations are principally those of the point bar deposits within the first Brazos River meander loop southeast of Richmond, Texas. Grain sizes vary for point bar deposits up and downstream and also for different southeast Texas streams; however, the relative grain size and sorting differences of each point bar area are similar to the above.

Individual depositional features are of diagnostic value, but the sequence and association of deposits and features are probably the most important criteria for recognizing point bar deposits. Vertical sections in borings (Figure 10) near Richmond consist of offlap sequences (Figures 11 and 12) repeating the grain size and sorting variations and sedimentary features which are diagnostic of the four depositional areas. Migration of the meander loop and these areas of deposition towards the undercut bank permits the natural development of a point bar stratigraphic sequence consisting, from the top downward, of progressively coarser deposits (1) small ripple bedded, (2) "horizontally" bedded, (3) giant-ripple bedded, and (4) poorly bedded.

Point bar sediments comprise the coarser alluvial sediments occur ring within stream meander belts which trend usually at right angles to the regional depositional strike. The majority of directional features of point bar sediments are directed towards and aligned at right angles to the con temporaneous regional shorelines of gradually subsiding basins

Area of Poorly Bedded and Poorly Sorted Graveliferous Sands

Poorly bedded and poorly sorted, coarse-grained deposits are typical of the depositional area comprising only the crossover channels and the basal part of the depositional slope which occurs mainly within the low-stage channel (see Figures 8, 9, and 10) A poor form of graded bedding is not uncommon in these deposits. In the downstream sections of the alluvial plains of coastal southeast Texas streams, these graveliferous sediments form the basal sections of point bar sequences. The lack of bedding might be attributed to the high degree of turbulence, large velocity variations of river currents, and also to the coarse-grained nature of the sediments of this part of the channel.

Area of Giant-Ripple Bedded Sands

The area of giant-ripple bedded (Figure 13), mainly well-sorted, fine- to coarse-grained, occasionally graveliferous sands, includes that part of the depositional slope immediately above the area of poorly bedded deposits. Giant asymmetrical ripples (Figures 13, 14, 15, 16, 17, & 18), the typical depositional forms on this part of the slope, occur in most cases between the level of normal high-water stages and a level a few feet below low-water stage. The ripples are aligned nearly normal to the river trend and each ripple migrates downstream over the underlying ripple and associated deposits. Sedimentary features associated with these ripples are typical of the crossbedding described as torrential bedding, continuous-incline bedding, diagonal-incline bedding, etc. (see Figures 19, 20, 21, & 22). The association between foreset bedding of "mammoth" ripples and the familiar cross-bedding so common in many coarse sandstone beds has been recognized by Kindle (1917). Because this zone of paint bar deposition also migrates in the direction of the undercut bank, a vertical section (approximately 10 feet thick for the Brazos River near Richmond) of giant-rippled deposits is laid down on top of the basal, poorly bedded point bar sediments.

Numerous giant ripples on the first four Brazos River point bars southeast of Richmond, Texas, have been measured (see Table 1). The heights of 150 ripples averaged 7 inches and the distances between ripple crests averaged 17 feet. Ripple foreslopes are steep and face downstream; backslopes are very gentle and slope upstream. Foreset dips of 230 ripples ranged from eight to forty-two degrees and averaged twenty-eight degrees. Only a few low-angle dips were measured; approximately 82 percent of the dips varied between twenty-two and thirty-six degrees. Usually the foreset beds which form during the final falling river stages have smaller dips and are tangential to the associated bottomset beds and also to the topset beds of the underlying ripple (see Figures 19 and 21). The upper parts of the foreset beds of many giant ripples are destroyed. The top and bottomset beds are very thin and have an average upstream dip of two degrees

Crests of many giant ripples may be traced for distances of over 150 feet from the normal flood level bank to the low-stage bank and into the river to depths of over 4 feet. Crests of 150 ripples form an average downstream angle of sixty-eight degrees with the bank. Thus, the direction of maximum dip of the foreset beds of most ripples is directed downstream towards the depositional bank at an angle of twenty-two degrees from the river trend (Figure 23). The vertical thickness of any one set of ripple foreset beds is approximately 7 inches.

The predominant dip directions of giant-ripple foreset beds along the inner banks of twelve meander loops of the modern course, and a reconstructed pre-1800 course of the Brazos River near Richmond conform with the downstream direction of the meander belt trend (Figure 24a). These directions should also be related to marine shoreline directions, since meander belt trends parallel valley trends which, in most cases, are approximately at right angles to the regional depositional strike of gradually subsiding depositional basins (see Figures 1, 3, and 24b).

Giant ripples and related cross beds of similar proportions have been recognized by Kindle (1917) in tidal channel deposits of eastern Canada, but the directions of the foreset dips at any one locality change with the tides. Perhaps the cross-bedded nature of Kindel's tidal channel deposits is typical of the herringbone structure of Shrock (1948).

Spillover bars (Figures 25, 26, & 27) deposited by strong local currents transferring flood waters which were trapped in swales, across lower elevations of giant-ripple bar accretions, are common in the upstream and down stream ends of the point bar. Although much larger, spillover bars are somewhat similar to the associated giant ripples. Heights of the foreslopes of Brazos River spillover bars are usually over 3 feet. Fore set beds (Figures 28 and 29) conform to the bar foreslopes and have an average dip of twenty-eight degrees. Most dip directions are downstream; however, directions towards the undercut bank are common. The topset and bottomset beds are thicker than those of the giant ripples, and may be either "horizontally" bedded or small-ripple bedded.

Area of 'Horizontally' Bedded Deposits

The point bar area (Figures 8, 13, and 30) of "horizontally" bedded, usually well-sorted, fine- to medium-grained sands, interbedded with thinner small-ripple bedded, mainly well-sorted, very fine- to medium-grained sands,occurs above the area of giant-ripple bedded sands (see Figures 31 and 32). The area is subject to flooding during high-water and flood stages, but the area does not include the uppermost part of the point bar which is only subject to flooding during maximum flood stages

Most of the deposits are horizontally laminated in contrast to the sedimentary structures of the other point bar deposits, but low initial dips and extremely low-angle cross--laminations are the more common depositional attitudes and features of the "horizontal" beds (Figures 33 and 34).

Numerous horizontal laminae have been traced continuously in cuts (Figure 31) over 100 feet long, parallel and at right angles to the river channel. These laminated sediments settled from suspension over large areas or were bed-load materials deposited by laminar currents. Laminae composed of dark heavy mineral grains, approximately equal in size to quartz grains of associated laminae, are common and suggest a very delicate adjustment of the stream's capacity to winnow and transport quartz grains and deposit heavier minerals. Laminae of heavy minerals could be described as "lag" concentrates. Numerous laminae are composed of finely divided plant fragments and mica; some laminae are rich in shell fragments of land snails and reworked but well-preserved Upper Cretaceous Foraminifera.

Wind-blown sands (Figure 35) are in a few places interbedded with water-laid, horizontally laminated sands (Figure 36).

The small-ripple bedded (cross-laminated) deposits (Figures 37, 38 and 39) which are seldom over 24 inches thick, comprise the foreset, topset, and bottomset beds of migrating, small-scale, asymmetrical ripples; Small-ripple sediments are deposited during rising and falling stages in relatively slack water near the inside bank where currents are usually loaded with sediment. Although these deposits are interbedded with the horizontally bedded sediments they are more characteristic of the uppermost paint bar sediments

The average dip of the small-ripple foreset beds is twenty-seven degrees Most dip directions are downstream; directions towards and away from the undercut bank are not uncommon, but upstream dips are rare. Steeply climbing small-ripple structures (Figures 40 and 41) develop when currents are overloaded These structures have been described as unilateral rolling strata, rolling inclined bedding, and change rolls (see Pettijohn, 1949). The top- and bottomset beds of such ripples may give a false foreset dip direction upstream if the bottom- and topset beds are interpreted as foreset beds.

Cross-bedded spillover bar features frequently develop in the zone of "horizontally" bedded sands.

Thin beds of ripple-marked, finely laminated silt-clay layers, in many cases mud-cracked, are interbedded in many areas with the "horizontally" and small-ripple bedded sands (see Figures 42 and 43).

Area of Small-Ripple Bedded Fine Sands

The area of small-ripple bedded, usually poorly sorted, very fine to fine-grained sands interbedded with thinner beds of "horizontally' bedded fine-grained sand occupies the uppermost part of the point bar area which is subject to flooding only during extreme high-water and flood stages (Figures 8 and 44). The ripple bedding is similar to the small-ripple bedding of the area of "horizontally" bedded sands; however, the scale is usually smaller (Figures 45, 46, 47, and 48). "Horizontally" bedded sands are present and are more common on the ridges and less common in the swales. Silt and clay beds are more common in the swales.

The small-ripple bedding is the predominant feature of this area inasmuch as this part of the point bar is flooded usually by relatively slack flood water which is overloaded with finer-grained sediments.