Models of fine-grained meander-belt systems generally emphasize the coarser sandstone facies of channel origin and neglect the relatively fine-grained overbank sediments of the flood plain. This emphasis is unfortunate because of the volumetric importance of the flood-plain sequences in many ancient stratigraphic successions such as the Tertiary coal and uranium-bearing rocks of the northern and central Rocky Mountain and Great Plains provinces. An appreciation of the processes of formation, and lateral and vertical successions of these deposits can provide valuable information for mining and reclamation activities.
Macroscopic and microscopic studies of 18 continuous cores through the lower Wasatch and upper Fort Union Formations in a 4-sq mi (10 sq km) area of southeastern Campbell County, Wyoming, allow recognition of six flood-plain environments associated with point-bar sequences. These flood-plain sequences, recognized on the basis of primary and secondary sedimentary structures, presence and type of bioturbation, organic content, and presence or lack of preferred vertical and lateral successions, include lacustrine, lacustrine delta fill, well-drained and poorly drained swamps, crevasse splay, levee, and abandoned channel. Recognition is based in part on comparison with criteria developed for Holocene sequences of the Atchafalaya flood plain.
Lacustrine deposits consist of highly organic, parallel-laminated clay with some silty laminations. Burrow structures are quite common and nodules of pyrite are present. Calcium carbonate is common along bedding planes. Lacustrine delta fills form coarsening-upward sequences which grade upward from lake deposits into interlaminated silts, clays, and sands with abundant ripple laminations, steeply dipping spill-over foresets, and loading features. Burrowing and pyrite nodules are present in the lower part and coarse organic debris and roots are common in the uppermost part. Poorly drained swamps are best recognized by their high content of organic fragments and root bioturbation that destroys most depositional structures. Pyrite and siderite are common as nodules. Well-drained swamps, in contrast, are relatively high in silt content and contain few organic fragments. Roots and root bioturbation structures are well preserved and stratification is only vaguely discernible. Iron oxide nodules are common, especially as crusts surrounding roots. Calcium carbonate nodules also are present in these deposits. Crevasse-splay deposits are similar to those of well-drained swamps except that stratification is better preserved and thin (less than 1 m) coarsening-upward sequences are common. These sequences contain ripple laminations, shallow scour-and-fill structures, and may be capped by rooted beds of sand. Natural levees are recognizable as fining-upward sequences with a well-stratified, burrowed, sandy lower part grading upward into a finer grained, root-bioturbated upper part with iron oxide and calcium carbonate nodules. Finally, abandoned channel-fill sequences consist of a complex interlayering of swamp, lacustrine, lacustrine-fill, and crevasse-splay deposits lying on, and often below, point-bar sequences.