A FACIES MODEL FOR RIVERS IN THE SEASONAL TROPICS AND SUBTROPICS, AND APPLICATION TO PALEOCLIMATIC INVESTIGATIONS OF ANCIENT SUCCESSIONS (Invited Presentation)

The regime of extremely variable discharge that is typical of rivers in the seasonal tropics and subtropics leads to a distinctive fluvial style. The sediment bodies constructed by such rivers differ in some important respects from those of both humid climate and arid-zone settings. Sediment bodies are erosionally-based intervals of texturally mature but mineralogically immature sand with local, commonly pedogenically-modified mud partings and subordinate gravel. These bodies exhibit complex lateral facies changes with common internal erosion surfaces. Macroform elements are lacking in many such bodies, and sedimentary structures are dominated by those formed under Froude-transcritical conditions. Furthermore, such bodies preserve the in situ remnants of trees that live(d) in the bed of the river, adapted to frequent inundation by fast-flowing water, and sedimentary features that formed around them. Since this distinctive fluvial style is a response to a strongly peaked (seasonal) precipitation and runoff regime, it follows that it may be used in investigations of the rock record to identify former times of strongly seasonal climate in the paleo-tropics and –subtropics. Further, vertical stratigraphic changes into and out of this style may be useful in developing interpretations of ancient paleoclimatic change. Several such stratigraphic changes have been documented in the Pennsylvanian succession of the Maritimes Basin Complex of Atlantic Canada, and used to infer repeated paleoclimatic fluctuations between humid and subhumid conditions. The ultimate drivers for these changes are complex but likely include cycles of growth and decay of glacial ice during the late Paleozoic Ice Age. Similar fluvial styles are also evident in time-equivalent strata in western Ireland, and may correlate with the Atlantic Canada record.