SOIL DEVELOPMENT ON MODERN DISTRIBUTIVE FLUVIAL SYSTEMS: PRELIMINARY OBSERVATIONS WITH IMPLICATIONS FOR INTERPRETATION OF PALEOSOLS IN THE ROCK RECORD

Understanding of controls on the distribution of soils in modem sedimentary basins facilitates interpretation of paleosols in the rock record. Here, we present information on soil distribution from a number of modem distributive fluvial systems (DFSs) in sedimentary basins developed in different climatic and tectonic settings. DFSs form an important part of modem alluvial sedimentary basins, and an understanding of the controls on soil development in these settings should facilitate interpretation of the alluvial rock record. The studied areas include: the Pilcomayo and Bermejo DFSs in the Andean foreland of Argentina, the Tista DFS of the Himalayan foreland basin in northern India, and the Okavango DFS developed in an intracontinental rift basin in Botswana. Soils in each of the examples are relatively immature and weakly developed. Where present, downdip changes (over distances >100 km) from relatively well-drained, relatively dry soils in sandy proximal areas to more poorly drained, relatively wet soils in more distal, clay-rich areas can be recognized. In the Andean example, this change is considered to be related to a downdip increase in precipitation and decreasing depth to water table. In the Himalayan system, this is considered to be due to a combination of decreasing depth to water table and increased surface flooding due to direct, monsoon-driven precipitation on the DFS surface. An increase in poorly drained soil development occurs near the toe of the DFS in Botswana, despite high transmission losses across the system. A key implication from these modem systems is that a change from well-drained to poorly drained soils is controlled by hydrology. This change occurs along a single isochronous surface that may extend for hundreds of kilometers and could be preserved in the rock record. Rock record examples that describe a downdip change from well-drained to poorly drained soils have been documented previously and are attributed to tectonic, climatic, autocyclic, and hydromorphic controls. Our studies from modem DFSs would suggest that a hydrornorphic control is likely to be the most important factor. Criteria derived from modem DFSs for distinguishing between changes in soil type that record climate change include the observation that paleosols developed in the proximal well-drained area are likely to be associated with a sandy parent material and sand-dominated channel fad. In contrast, in the distal DFSs, more poorly drained soils are likely to be developed on a silt- or clay-rich parent material interbedded with a mixture of sandy and muddy meandering channel-fill deposits, crevasse splays, and floodplain sands and muds. Paleosols that record climate change should show no discernible relationship between parent material and soil type. While similar relationships between soil type and parent material have been described previously, their distribution within the context of a DFS has not been widely documented.