The Genesis and Recognition of Anastomosed Fluvial Deposits: Data from the St. Mary River Formation, Southwestern Alberta, Canada

Abstract

ABSTRACT Data from modern fluvial systems show that the term anastomosed is best reserved for suspended-load systems with multiple channels and very low slope that flow in regions where the hydrograph has a seasonal peak. The interchannel areas are topographic lows relative to the channel hanks and are places where crevasse-splay, marsh, and lacustrine sedimentation occurs. Channel geometry is intrinsically a function of the rate of sediment partitioning; the coarsest sediment is deposited in the channels, on the levees, and in proximal splay complexes, whereas the silts and clays are deposited on the rest of the floodplain. Extrinsic controls on the formation of these systems are geological and climatic. They include a source of suspended sediment and a highly seasonal water budget. Preservat on 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 Alberta show that the stratigraphic equivalents of these rivers are ribbon sandstone bodies encased in thin sandstones, shales, and siltstones. The total distribution of facies, rather than proof of simultaneous flow in multiple channels, can be used to interpret correctly a deposit 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 slopes over millions of years. The mutual interaction among source rock, climate, and subsidence, all of which play critical roles in forming and preserving these distinctive fluvial deposits, allows inferences to be drawn regarding paleoclimate and paleoslope that may help constrain models of basin evolution.