THE GENESIS AND RECOGNITION OF ANASTOMOSED FLUVIAL DEPOSITS - DATA FROM THE ST-MARY RIVER FORMATION, SOUTHWESTERN ALBERTA, CANADA

Data from modern fluvial systems show that the term anastomosed is bes t reserved for suspended-load systems with multiple channels and very low slope that flow in regions where the hydrograph has a seasonal pea k. The interchannel areas are topographic lows relative to the channel banks and are places where crevasse-splay, marsh, and lacustrine sedi mentation occurs. Channel geometry is intrinsically a function of the rate of sediment partitioning; the coarsest sediment is deposited in t he channels, on the levees, and in proximal splay complexes, whereas t he silts and clays are deposited on the rest of the floodplain. Extrin sic controls on the formation of these systems are geological and clim atic. They include a source of suspended sediment and a highly seasona l water budget. Preservation of anastomosed fluvial deposits requires a mechanism for maintaining very low depositional slopes over geologic time scales. Data from the St. Mary River Formation in southwestern A lberta show that the stratigraphic equivalents of these rivers are rib bon sandstone bodies encased in thin sandstones, shales, and siltstone s. The total distribution of facies, rather than proof of simultaneous flow in multiple channels, can be used to interpret correctly a depos it as anastomosed fluvial, Although anastomosed fluvial deposits form in a variety of tectonic settings, they are apparently most common in foreland basins because these basins provide suitable source rocks and thrusting as a mechanism that can maintain the low depositional slope s over millions of years. The mutual interaction among source rock, cl imate, and subsidence, all of which play critical roles in forming and preserving these distinctive fluvial deposits, allows inferences to b e drawn regarding paleoclimate and paleoslope that may help constrain models of basin evolution.