Abstract Observations and data from modern streams and recent deposits demonstrate that river systems in subhumid and semiarid seasonal settings in tropical and subtropical latitudes can have deposit characteristics very different from those predicted from previously published facies models based on other climatic settings. The differences result primarily from the extremely variable discharges that are typical of these climatic settings and contribute to a distinct fluvial style. Five critically important characteristics of the deposits are illustrated by data from NE Australia. The deposits with these characteristics occur in streams with variable discharge and are independent of channel size. (1) The channel-fill lithosomes are erosionally based and exhibit complex lateral facies changes. (2) Within the channel-fill lithosomes there are abundant mud partings, some of which are pedogenically modified. (3) In most cases the complex internal architecture lacks the macroform elements, such as lateral-accretion cross bedding, typical of other fluvial sediment bodies. This is the case even within point bars. (4) Sedimentary structures formed under high flow stage are abundant and frequently preserved in the lithosomes. (5) Trees and other vegetation adapted to occasional inundation by fast-flowing water may colonize channel floors (not only on banks), and these both generate organic sediment and influence the flow and resulting sediment deposition patterns. In addition to the characteristics of the channel-fill deposits, the overbank deposits may have diagnostic character and the sand petrography may differ from that in other settings. The same features are identified in the rock record and a detailed case study from the Pennsylvanian of the Maritimes Basin Complex of Atlantic Canada illustrates the distinction between the seasonal tropical and other fluvial styles. The upper Namurian (Yeadonian) Boss Point Formation shows a transition from perennial to subhumid to semiarid, seasonal tropical fluvial styles within a succession of sheet-like braidplain channel bodies. The overlying, basal Westphalian (Langsettian), Little River Formation is composed entirely of more lensoid channel bodies displaying the subhumid to semiarid, seasonal tropical fluvial style, and associated overbank deposits. The overlying coal-bearing Joggins Formation shows a gradual return to a perennial fluvial style with channel body geometries similar to the underlying unit. A clear signal of paleoclimate change can be deconvolved from variations in accommodation regime, providing a hitherto unavailable discriminant for interpreting fluvial successions. Recognition of the strongly seasonal, tropical to subtropical fluvial style in the rock record, and of changes in character through vertical successions, will aid paleoclimate interpretation and subsurface reservoir analysis in fluvial successions.

Abstract The Burdekin River Delta of northeastern Australia has been described previously as a type example of a wave-dominated delta, or as a “mixed-energy” delta. A reexamination of the geomorphology, sedimentology, and Quaternary stratigraphy herein leads, however, to the conclusion that the Holocene delta has been constructed primarily by flood-related processes. The surface of the upper delta plain is composed dominantly of sand and gravelly sand deposited in channel, paleochannel, and floodplain environments. The lower delta plain, by contrast, comprises a lithologically more diverse array of sands, muds, and chemical and organic sediments deposited in a variety of coastal environments. The lower-delta-plain channel and paleochannel sands are similar in texture to their upper-delta-plain counterparts, whereas deposits of smaller tidal creeks are finer-grained, in many cases muddy sands. Beaches, beach ridges, and spits are composed of well sorted, medium-grained sand, and eolian dune facies are somewhat finer-grained but equally well-sorted sands. Three varieties of coastal flats are recognized (mangrove, salt, and other types), the sediments of which are dominated by mud, typically rich in organic debris, and in some cases microbial mats, with minor sand laminae. Mouth bars are well developed at the present river mouth and throughout the lower delta plain. These bodies are composed of medium to very coarse-grained sand, texturally similar to channel sands, though slightly finer-grained. The narrow delta front comprises a sandy-mouth-bar zone passing distally into a mud-dominated clinoform set extending to -10 m (and only 10 km wide in total). Sharp-based coarse sand bodies of interpreted

Abstract Tectonic control of alluvial architecture is commonplace in extensional, transcurrent and compressional tectonic terrains. The primary influence of tectonic activity on floodplain behavior is tilting, which varies areally and temporally and is superimposed on the pre-existing channel gradient. These effects are considered theoretically and illustrated by Pleistocene and Holocene examples from southwest Montana and the Mississippi Valley. A transverse slope will cause the movement of channel belts toward the area of maximum subsidence either 1) by processes of downslope cutoff and preferential erosion, producing an abnormally wide and asymmetric sandstone body, or 2) in steps, by avulsion. Avulsion may be triggered by tilting or may occur subsequently through preferential flooding of the down-tilted side of the floodplain. Although the channel will move toward the position of maximum subsidence, it will often be offset by fans constructed from the footwall or it may flow into a lake occupying the floodplain low. Differential subsidence strongly affects deposition rates, groundwater behavior, pedogenesis and flood distribution, in addition to channel migration.