APPLICATION OF FLOODPLAIN PALEOSOLS TO MODELS OF ALLUVIAL ARCHITECTURE

In alluvial architecture models, the floodplain deposits are merely background for the channel sandstones; no lithologic or pedologic details are provided. Because the paleosols contain important information about the fluvial system, a simple model has been generated that shows how paleosols change in response to changes in basin subsidence and for different relationships between avulsion frequency and sediment accumulation rate. The model mirrors that of Heller and Paola (1996); however, the fine-grained deposits, rather than the sand bodies, are emphasized. The underlying principle of the paleosol model is that, in avulsion-dominated alluvial successions, the degree of soil development is controlled by both the rate at which the parent material accumulated and the frequency of channel avulsion.

If avulsion frequency is not related to accumulation rate, as early alluvial architecture models assumed, an increase in subsidence rate produces a stratigraphic section in which strongly developed paleosols are overlain by paleosols that are thicker, less well developed, and less densely spaced. The paleosols are thicker because more fine-grained sediment, the parent material for the soil, was deposited between avulsions. If avulsion frequency increases more rapidly than sediment accumulation rate, an increase in subsidence rate causes strongly developed paleosols to be overlain by a stratigraphic interval with paleosols that are thinner, less well developed, and more densely spaced. The paleosols are thinner because, with more frequent avulsion, less fine-grained sediment is deposited on the floodplain between avulsions.

The significance of the paleosol models is that, where paleosols are present, they should help constrain field-testing of models of alluvial architecture. Careful analysis of the paleosols in different parts of a basin or in different parts of the stratigraphic column should provide clues as to the local relationship between avulsion frequency and sediment accumulation rates. Study of the thickness and degree of development of the paleosols together with analysis of the sandstone bodies should provide a clearer understanding of what factors influenced the stratigraphic architecture in a particular alluvial basin.