Depositional facies of two Canadian modern anastomosed river systems, the upper Columbia River and lower Saskatchewan River, occur in intermontane and plains settings, respectively. Both systems contain low-gradient, multiple, interconnected, laterally stable sand-bed channels, with adjacent splay, levee, and shallow wetland deposits, all aggrading in accordance with channel sedimentation. While aggrading cross-valley alluvial fans or subsidence tend to control sedimentation rates in intermontane valleys, basin subsidence and/or regional tilting controls deposition rates in plains settings.
Deposits in the upper Columbia River valley (120 × 1.5 km) consist of low-sinuosity multistoried stringers (textural cycles) with planar tabular cross-bed sets of channel sands and numerous sandy crevasse-splay deposits. Channel deposits are laterally contained by deposits of levee silt and lacustrine mud, and when buried are vertically mud encased. Aggrading at an average rate of 60 cm/100 years over the past 2,500 years, the anastomosed system is very dynamic, exhibiting many avulsions and channel fills.
Deposits in the lower Saskatchewan River valley (120 × 80 km), a much wider basin with much slower aggradation rates (± 15 cm/100 years), occur as channel sands flanked by laterally extensive (1 km) sheets of overbank levee deposits of fine sandy silt, which grade into even more laterally extensive thick deposits of mud or peat. With time, dominant channels become sinuous, thus causing increased flow resistance, major avulsions upriver, and eventual channel filling and abandonment. Many channel sand-fill deposits form cross-section geometries, ranging up to 15 cm thick by 120 m wide.
Facies differences of the two anastomosed river systems are believed to be caused by both the rate of sedimentation and width of the sedimentary basin. Other than size differences of similar sedimentary environments, Columbia River channels are less sinuous, avulse more frequently, and contain coarser grained sand. In the Saskatchewan, some crevasse-splay (sheet sands) and associated avulsions are laterally extensive (10 × 30 km) and very complex. Wetlands in the Columbia are dominated by marsh (organic-rich mud) and lacustrine silt, whereas thick (up to 3 m) laterally extensive peat bogs dominate in the Saskatchewan system.
Within the upper Mannville Group of the Lloydminster area there exists a large-scale mappable complex of fluvial channel-fill sandstones that exhibit an anastomosed pattern. The complex has areal dimensions of 250 km (width) by 700 km (length).
Channel sandstones are thick (up to 35 km), narrow (300 m), can be traced for several kilometers, and are stratigraphically variable. The channel fills are multistoried, with the predominant sedimentary structures consisting of plane beds, cross-beds, and climbing current ripples. Interchannel sediments consist of interbedded sheet sandstones, siltstones, mudstones, and coals. The predominant sedimentary structures of the interchannel sandstones are the same as those found within channel sandstones.
From a compare-and-contrast approach, it is concluded that meandering, sandy braided, valley-fill, deltaic, or tidal origins cannot account for the observed sand-body geometries and facies distribution.
The modern model that best explains the sediment and facies distributions within the upper Mannville is the anastomosed fluvial model in which narrow, vertically accreting channels are bordered by extensive aggrading interchannel wetland deposits with interbedded crevasse-splay sands.
Hydrocarbon distributions within the upper Mannville are stratigraphically controlled and oil quality can be directly related to depositional facies. Common trapping mechanisms consist of updip shale-filled channels, structural closure formed by differential compaction, and lateral sandstone pinchouts.