Depositional Processes and Lithologic Characteristics
Figure 1 illustrates the basic sedimentary model for meandering channels in an alluvial flood plain.
As shown, there are three main depositional subenvironments: the active channel, the abandoned channel, and the flood plain itself. As with braided streams and alluvial fans, flooding is the main process that causes deposition within each.
The creation of sand bodies occurs as a result of two factors: channel migration, with the simultaneous deposition of point bar sand; and, channel abandonment, where point-bar sands are buried beneath finer-grained, backwater material in oxbow lakes. The former is by far the most important to petroleum geology, since the latter results in very thin, isolated sand layers.
Meandering stream channels display an asymmetrical profile which reflects the distribution of current velocity and thus deposition and erosion (Figure 1). Basically, flooding causes erosion of outside meander (“cut”) banks as current strength rises, and subsequent deposition on the inner banks as floodwaters recede. Point bars build out into the channel, and their progradation mostly keeps pace with the erosion of the opposite cut bank. The specific direction of their accretion is largely at right angles to the downstream current at any one point. Point bars are characterized by a fining-upward sequence of grain sizes and sedimentary structures that reflect the decreasing current velocity from the gravelly channel floor to the silts and muds of the flood plain. (Figure 2, Idealized “outcrop” showing upward succession of grain size and sedimentary structures in a preserved point bar.) Most have slopes of 10° to 20°.
They are the site where the majority of sand deposition fakes place in the alluvial systems of meandering streams.
An abandoned channel results when a meander is cut off. This is, to some extent, the inevitable culmination of meander development. Continual cut bank erosion both increases the amplitude of individual loops and “tightens” their necks. (Figure 3, Sequential diagram to show how oxbow lakes, i.e., abandoned meanders, form as a result of continual meander migration.
Erosion of the outside cut bank occurs simultaneously with deposition of sand on the point bar of the inside bank. Abandoned meanders are eventually filled in with a plug of clay and silt. Most meander belts, therefore, are composed of both shoestring point-bar and oxbow deposits.) At some point, flooding will cause currents to overflow the point bar and erode a new, straighter channel across it, thus decapitating the old meander bend. Thereafter, the sinuous oxbow lake which remains will be filled in by finer grained material, mostly introduced by subsequent floodwaters. The point-bar sequence will thus be incomplete, its lower, coarser sand and gravel overlain by silts and clays (Figure 1).
The flood plain itself is composed predominantly of backwater muds (possibly including swamp peat), which are intermittently blanketed by thin layers of fine sand and silt deposited by periods of overbank discharge. Along the banks of the river are two other, localized floodplain subfacies: levees and crevasse splay deposits. Levees form as a result of floodwaters dropping their heavier, coarser load when they first overflow the channel and enter the flood plain. Crevasse splays occur when these same waters break through preexisting levees to spread a cone of sandier material further out from the river banks.
Channel migration is largely unpredictable in its specific evolution, and thus the location and vertical arrangement of point-bar sands within a preserved stratigraphic section is often difficult to determine. Furthermore, as with braided stream deposits, full sequences for individual channels are not always seen in the record. Continual lateral migration of channels from one side of the flood plain to the other means that point-bar deposits will suffer a certain degree of cut and fill. Usually, however, this is far less pronounced than in the case of braided stream alluvium. Individual point-bar or abandoned channel sands form relatively isolated “shoestring” bodies, encased within impermeable flood-plain deposits. (Figure 4, Isopach map and east-west cross section of two stream channels in the Muddy Sandstone of South Glenrock oil field, Wyoming-southwestern Powder River basin.
Note how two channels fill a single meander belt. Oil is stratigraphically trapped in point-bar sandstones within the channel fill.)
These subenvironments all exist within a single meander belt, which may range from tens to thousands of meters in width. In the case of a subsiding basin, where the flood plain is broad yet of low slope, tectonic tilting may force the river to either change course altogether (Figure 5, Idealized block diagram showing a meandering river system over a region of low slope and continual subsidence), or to migrate laterally across the entire basin.
In each case, new bodies of sand will be created by the new meander belts, each composed of stacked channel and point-bar sequences. Thus, meandering alluvial systems do not always result in shoestring reservoirs.
