Facies model for fluvial systems in the seasonal tropics and subtropics

Facies models that summarize the deposits of fluvial systems are well established for humid climate settings and for desert environments, but the deposits of rivers in the sub-humid and semiarid seasonal tropics have been largely ignored. Our observations and data from modern streams and recent deposits in northeastern Australia show how such rivers, with extremely variable discharge, have distinctive deposit characteristics that are substantially different from conventional fluvial facies models. These properties include (1) erosionally based channel-fill lithosomes that exhibit complex lateral facies changes, with (2) abundant, pedogenically modified mud partings, (3) complex internal architecture that may lack the macroform elements typical of other fluvial sediment bodies, (4) an abundance of sedimentary structures formed under high flow stage, and (5) an abundance of in situ trees that colonize channel floors and are adapted to inundation by fast-flowing water. We illustrate examples of this fluvial style from the rock record, and set out a new facies model. The recognition of such a distinctive fluvial character, and of changes in character through vertical successions, will aid paleoclimate and reservoir analysis.