Print this pageALLUVIAL ENVIRONMENTS
The principal subenvironments of the meandering stream plain are the meander belt and the flood basin or backswamp (Figure 3). Once the stream occupies a course within the valley it meanders within a belt determined by the radius of curvature of the meander. This belt is from 18 to 20 times the width of the stream. Stream meandering results from bank erosion on the outside bank of the loop and deposition on the inside bank (Figure 4). A downstream trading process usually occurs, and much of the material eroded from the outside bank of one meander is deposited on the inside bank of the next downstream meander. Repeated reworking of the deposits within the meander belt winnows the finer grained materials, which are transported downstream or, during overbank stages, to the backswamps and abandoned channels. This results in sand deposition within the meander belt, and silt and clay deposition in the backswamp environments. It also results in a progressive downstream decrease in grain size, with larger grain-size ranges in the alluvial meander belt and smaller grain-size ranges in the deltaic meander belt.
The reader is referred to Bernard and Major (1956c), LeBlanc (1950), and Fisk (1947) for details of meander belt environments and deposits.
The meander belt is subdivided into point bar, natural levee, abandoned channel, and oxbow subenvironments (Figure 5). The point bar, the area within the meander loop, is composed of sand deposits which accreted to the inside bank of the meander. The natural levees flank the main channel and include the areas where fine sand and silt are deposited during overbank stages. Abandoned channels and oxbow lakes receive slack water containing silt and clay size sediments during overbank stages.
Cross sections (Figure 6) of a Brazos point bar illustrate the nature and development of a sequence of point bar deposits. Coarser sediments are deposited near the outer or undercut bank where stronger currents exist. Progressively finer grained sediments are deposited on the upper parts of the depositional bank where currents are weaker. As the meander loop migrates towards the undercut bank, a top-to-bottom sequence of progressively coarser sediments is deposited in the point bar environment. A large range in grain size with depth in point bar deposits does not prove, but strongly suggests, alluvial and not deltaic origin.
As there is a grain-size increase downward (LeBlanc, 1950; Nanz, 1950), the SP character1 for the point bar sequence is more or less bell-like in shape. See Figure 7 and Figure 8, EPR Geol. Misc. 1, revised April and May 1958; Figure 4, EPR Memo 22, September 1956; and Figure 11 and Figure 12, EPR Memo 23, October 1956. An abrupt deflection towards the shale line usually occurs at the base of the section (Figure 6). Four of the electric logs shown in Figure 43 are from the Brazos point bar at Richmond, Texas. Note that all curves are bell-like in shape, and some are somewhat serrate. The serrate character may be caused by blocks of clay derived from the undercut bank by caving during flood stages. The slump blocks not carried away or destroyed are buried in the point bar sands. It is also possible that the finer grained sediments responsible in part of the serrate character of the electric log may have been deposited in chutes during low river stages. The thicker parts of clay drapes deposited in point bar swales should also cause a deflection of the curve towards the shale line.
The sequence of bedding features and grain-size increase in point bar sediments is a most important criterion for recognizing alluvial point bar deposits. The normal sequence results from the offlapping of areas of deposition caused by the migration of the meander loop towards the undercut bank. The sequence consists of four zones, from top to bottom, characterized by small ripple bedded, horizontally bedded, giant ripple bedded, and poorly bedded depositional features (Figure 6).
Small Ripple Bedded Features. Currents directed towards the inside bank and to the area of small ripple bedded features during flood stages become weak and overloaded (Figure 6). Therefore, large quantities of fine sand and silt are transported as bed load material. The sediment moves downcurrent in the form of small migrating asymmetrical ripples. Cross sections (Figure 7) of these deposits show the foreset beds of the small ripples dipping downcurrent. Strike sections show a small-scale festoon character caused by scour and fill. Horizontal sections show a rib-and-furrow pattern (Stokes, 1953) formed by the outcrop of the foreset beds. These outcrops are usually concave downcurrent and are excellent directional indicators of the sand trend. More than 10 feet of sediments characterized by these features have been deposited on the upper part of the point bar during a single flood.
Horizontally Bedded Features. In the area of horizontal bedding, where currents are slightly stronger but still overloaded (Figure 6), fine to medium sand settled from suspension to form horizontally bedded and laminated deposits (Figure 8). Thick sections of these sediments have been deposited during a single flood.
Giant Ripple Bedded Features. In the area of giant rippling, stronger bottom currents exist during floods, and predominantly medium and coarse sand which is carried as bed load material is deposited (Figure 6). The bed load takes the form of giant asymmetrical ripples and migrates downstream (Figure 9). The foreslopes of these ripples always face downcurrent. The resulting deposits take the form of the common type of cross bedding (Gilbert, 1899). It has been observed that larger giant ripples are associated with larger streams and coarser sands. For a very large river like the Mississippi, the average maximum size of these ripples is approximately 100 feet from crest to crest and 26 inches in height. For an intermediate sized river like the Brazos, the average maximum size is 17 feet from crest to crest and 7 to 10 inches in height.
During many small floods giant ripple crests remain more or less parallel as they migrate downstream. This results in the development of continuous incline bedding or planar cross bedding (Figure 9). The cross beds which form on the ripple foreslopes are always directed downcurrent.
It appears that during larger floods giant ripple crests are less parallel and are more lobate in character. Some of the ripple crests (Figure 9) are lobate in ground plan. The resulting bedding features of these deposits take the form of typical festoon type cross bedding which was first described by Knight (1930) (see Figure 10). Downcurrent (dip) sections of these deposits show continuous inclined and tangential cross bedding directed downstream. Strike cross sections of these deposits show typical festoon features which are the result of scour and subsequent fill in the interlobate areas immediately downstream of the ripple crests as the ripple migrates downcurrent. Although not shown here, the outcrop pattern of these ripple foreset beds on a horizontal surface presents a very large scale rib-and-furrow pattern. This feature, which is also common to small ripple bedded deposits, is illustrated in the upper right of Figure 7. The outcrop of these foreset beds is usually concave downcurrent, and this feature is an excellent directional indicator of the sand trend.
Poorly Bedded Features. Adjacent to the undercut bank where the currents are the strongest, poorly bedded gravels are deposited (Figure 11.). Some cross bedding may occur in these deposits.
Gradational Zonation in Sequence of Sedimentary Features. Because the positions of point bar areas of deposition, characterized by the above depositional features, are related to the profile of the inside or depositional bank, and also because the meander migrates towards the undercut bank, a natural top-to-bottom sequence of zones of small-scale ripple bedded, horizontally bedded, giant ripple bedded, and poorly bedded deposits develops (Figure 12). However, it is most important to note that the sequence of bedding features is gradational, since flood stages vary in height. See EPR Geol. Misc. 1, April 1956, and EPR Memo 23, October 1956 for details.
Miscellaneous Sedimentary Features. Common sedimentary features of point bar deposits are steeply dipping bank slope deposits of sand, silt, and clay. The dip direction is usually towards the undercut bank (Figure 13a). Other features are clay drapes, mud cracks, mud balls, and contorted bedding. Clay-silt deposits are draped over the bar during falling flood stages. In Figure 8 the uppermost layer of silt and clay draped over the horizontally laminated sand was deposited during the falling stage of a flood. Most mud balls are derived from the destruction of these silt-clay drapes during low river stages and redeposited together with sand during the next high river stage (Figure 13a). Exceptionally strong localized river currents produce large cross beds in spillover bars (Figure 13b). Frequently, the inclination of the cross bedding reaches the critical angle of repose, and penecontemporaneous contortion of these beds develops (Figure 13c). However, contortion is most common in rapidly deposited small or giant ripple bedded deposits. Single small ripple bedded and horizontally laminated sand or silt units lying in a horizontal position (some less than 3 inches thick) appear to have flowed under their own weight immediately after exposure following a falling flood stage. Only a few contacts per grain may exist in very rapidly deposited sediments, and the enclosing fluids flow laterally as the water table is lowered during a subsiding flood stage. Grains which were almost suspended in the fluids are no longer supported, and under the force of gravity fall ~nd flow laterally in any direction.
Point bar sediments are generally nonfossiliferous but may contain abundant reworked fossils. Many samples of Brazos point bar sediments collected near Richmond, Texas contain a relatively rich Upper Cretaceous foraminiferal fauna. Land snails (pulmonates), fresh water clams, ostracods, crabs, etc. are rare in these deposits. Fossil logs, some in standing position, and wood fragments may be common. Wood fragments are usually aligned parallel to the bank of the stream.
Cross Bedding. Cross bedding directions of giant ripple bedded deposits are downcurrent. Measurement of the dip directions of these cross beds at stations 500 feet apart along the banks of a series of meander loops of the modern and the pre-1800 Brazos courses near Richmond, Texas, gives a rose diagram which points downstream and is parallel to the trend of the meander belt (Figure 14).
Subsurface cross bedding directions observed in cores taken in borings near Richmond agree with the downcurrent direction of the Brazos River as it deposited the sequence of point bar sediments during the past 200 years (Figure 15). The trend of the inside bank and the current direction at various stages are indicated by the accretion scars which are illustrated by the fine lines. The composite cross bedding directions occurring in all borings agree also with the general downcurrent direction during the development of this point bar sequence (Figure 15).
Rose diagrams of expected cross bedding directions for Mississippi point bar deposits, at stations located by a 1 1/2-mile grid, are directed downstream and at right angles to the coast (Figure 16). The average direction is parallel to the regional valley and meander belt trends. Rose diagrams of the cross bedding directions for valley segments of point bar deposits are directed downstream and are parallel to the local trends of the meander belt. See EPR Geol. Misc. 1, revised April 1956 and May 1958, and EPR Memo 23, October 1956.
Grain Orientation. Grain orientation in recently deposited, horizontally bedded, point bar sands is parallel to the trend of the depositional bank and the current direction (Figure 17). See EPR Geol. Misc. 1, revised April 1956 and May 1958. The composite grain orientation for these stations is shown in the rose diagram at the upper left. The average bank and current directions for these stations are shown by the large double-headed arrow (Figure 17). Grain orientation in the subsurface small ripple and horizontally bedded sand zones agrees very closely with the direction of the river current during the development of this point bar (Figure 18). The composite grain orientation for all borings agrees very closely with the average current direction which prevailed during the development of the bar.
Meander belt deposits trend at right angles to the coast and to the regional depositional strike. For intermediate sized streams, like the Brazos, which have meander loops with a radius of curvature of 900 feet, straight-line trends of point bar sands developed within a single meander loop may be predicted for a distance of approximately 600 feet. Straight-line trends for larger streams, such as the Mississippi, may be predicted for several thousand feet (Figure 14, 15, 16, 17, 18, and 19).
Point bar deposits are lenticular. The size varies with the size of the associated stream. Figure 19 compares an immature meander belt of the Brazos with a mature meander belt of the Mississippi. The fine lines conform with cross bedding directions and grain orientation. The point bar sands are indicated by the black dots, and the backswamp clay is shown without dots. The scales for each map are the same, and the dots also represent 40-acre spacings. The thickness of the lower, relatively clean Brazos point bar sands averages approximately 35-40 feet. The width varies from 1.5 to 2.5 miles. Point bar deposits of the Mississippi average approximately 125 feet thick and 11 miles wide.
Long channel segments, and meanders or oxbows abandoned by the stream, are filled gradually with interbedded fine grained sand, silt, and clay (Figure 3). These deposits may be organic-rich. Channel fillings, called plugs, form impermeable barriers to the migration of fluids within the meander belt sands.
Clay and silt deposited in the flood basins during flood stages may or may not be bedded. These sediments are commonly mottled, since they are usually exposed to aeration during dry seasons. Calcareous nodules and caliche are common in these sediments.
1The
recognition and classification of SP log character particularly
as related to the grain size sequence in point bar and barrier
island sands resulted from early work by Fisk (1947), LeBlanc
(1950), and Nanz (1950) and also from discussions between company
and laboratory personnel in the area and on Recent field trips
conducted during the past six years by Bernard and Nanz.
