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.

The Trout Valley Formation of Emsian–Eifelian age in Baxter State Park, Maine, consists of fluvial and coastal deposits that preserve early land plants (embryophytes). Seven facies are recognized and represent deposits of main river channels (Facies 1, 2), flood basin (Facies 4), storm-influenced nearshore shelf bars (Facies 3), a paleosol (Facies 5), and tidal flats and channels (Facies 6, 7). The majority of plant assemblages are preserved in siltstones and are allochthonous and parautochthonous, with only one autochthonous assemblage identified in the sequence above an apparent paleosol horizon. Taphonomic analysis reveals that plant material within allochthonous assemblages is highly fragmented, poorly preserved, and decayed. Plant material within parautochthonous assemblages shows evidence of minimal transport, is well preserved, and shows signs of biologic response after burial. The one autochthonous assemblage contains small root traces. Trimerophytes ( Psilophyton and Pertica quadrifaria ), rhyniophytes (cf. Taeniocrada ), and lycopods ( Drepanophycus and Kaulangiophyton ) are the most common taxa in estuarine environments. Psilophyton taxa, Pertica , cf. Taeniocrada , and Drepanophycus are found also in fluvial settings. The presence of tidal influence in deposits where parautochthonous and autochthonous assemblages occur shows that these plants occupied coastal-estuarine areas. However, the effects on the growth and colonization of plants of the physical conditions (e.g., salinity) that exist in these settings in the Early to Middle Devonian are unknown.