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Ancient Fluvial Systems
Cyclicity, Tectonics and Coal: Some Aspects of Fluvial Sedimentology in the Brazeau-Paskapoo Formations, Coal Valley Area, Alberta, Canada
The Upper Cretaceous – lower Tertiary Brazeau-Paskapoo Formations in the central Foothills of Alberta encompass an estimated thickness of 3600 m of nonmarine sediments of alluvial plain origin. They were deposited in a foreland, or molasse, basin accompanying orogeny in the Cordillera to the southwest.
Three orders of cyclicity are observed: (1) first order cycles representing mappable lithostratigraphic units; (2) second order cycles representing successive recurrences of fluvial channels at a particular location; and (3) third order cycles representing high- and low-energy overbank deposits on a flood plain.
The main allocyclic control on sedimentation was tectonics. Progressive, but sporadic, encroachment of the thrust belt of the Rocky Mountains on the foreland basin produced sporadic loading and subsidence by isostatic compensation. Coarser members of first order cycles reflect major thrust loading events that initially produced maximum aggradation on the southwestern flank of the foreland basin and then was transmitted progressively eastward. Continued subsidence due to sediment loading produced further aggradation but at a reduced rate, resulting in the finer members of first order cycles which are characterized by a greater proportion of overbank to channel deposits. Second order cycles are predominantly of autocyclic origin but with an allocyclic override. Third order cycles are entirely autocyclic.
Rapid marine transgressions are related to rapid (104 – 105 years) thrust loading and widespread subsidence of a flexurally rigid lithosphere. Slower transgressions are a function of sediment starving at the coastal plain by preferential sedimentation at the proximal edge of the foreland basin, or by creation of local base levels.
Calculated rates of deposition suggest that long periods of nondeposition must have occurred.
Modern river classifications appear to be inadequate for application to ancient fluvial systems because they lack the perspective of time. Present models for ancient fluvial sequence interpretation are premature and often misleading.
Channel sands, interspersed with various thicknesses of overbank deposits, have characteristics of both braided and meandering rivers, suggesting that both of these morphological types were present. Many channel deposits, composed of horizontally stratified, coarse-grained sandstones, are interpreted to have a flood origin.
The inferred tectonic – sedimentation framework for the Brazeau-Paskapoo sequence is analogous in many respects to the modern Indogangetic Plain of northern India and Bangladesh.
Thick coal deposits are of alluvial plain origin and are associated with a climatic change and widespread floral extinction at the Cretaceous-Tertiary transition. They also coincide with a major orogenic event producing widespread subsidence and marine transgression.
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