Evolving Flexural Depocentres in the Middle Cretaceous of the Western Canada Foreland Basin
University of Western Ontario, London, Ontario, Canada
Regional stratigraphic analysis of Late Albian (~104 M.a.) to Middle Turonian (~92 M.a.) marine and marginal marine successions shows that discrete, unconformity-bounded units (composed of numerous high-frequency sequences), fill a succession of arcuate, flexural depocentres, typically of a few hundred kilometers dip and strike extent. In dip view, the component high-frequency sequences tend to show a predictable pattern in which lower sequences are strongly wedge-shaped whereas upper sequences are very gently tapered, or even tabular. Wedge-shaped sequences tend to be characterized by finer-grained facies whereas tabular sequences are commonly coarser-grained, and may include evidence for major coastal progradation.
These patterns of geometric and facies variability can be interpreted in terms of initially high-rates of flexural subsidence close to the orogen that, over time, gave way to diminished, but more spatially-uniform subsidence. Sand and gravel tended to be trapped close to the orogen during times of high subsidence rate, but were able to prograde distally during times of low, or negligible subsidence, as predicted by numerical models (e.g. Jordan & Flemings, 1991).
Changing rates of subsidence may be interpreted as a response to initially rapid advance and thickening of the adjacent fold and thrust belt. Diminished rates of thickening of the orogenic wedge may have been accompanied by erosional degradation and basinward redistribution of mass, leading to isostatic subsidence of the basin beyond the deformation front, producing more tabular sequences. In some instances, it is evident the high-frequency sequences show systematic onlap and offlap against the flank of the forebulge.
Onlap onto the forebulge could be interpreted as a response to eustatic rise, advance of the thrust tip, and /or sediment transport further into the basin, causing isostatic subsidence and eastward forebulge migration. Offlap away from the forebulge could be interpreted in terms of eustatic fall, or out-of- sequence thrusting that shifted mass towards the west, causing forebulge uplift and migration to the west.
Successive flexural depocenters show lateral offset from each other along the strike of the Rocky Mountain Orogen, on length scales of ~200-400 km and timescales of <0.5 to ~1 m.y. This pattern suggests that the locus of thrust activity in the orogen was spatially and temporally variable on a relatively short timescale. However, this tectonic complexity is only evident if rock units are isopached on timescales of << 1 M.a. Isopach maps of units spanning several M.a. show rather simple wedge-shaped patterns that mask the variability discernable at shorter timescales. Patterns of subsidence in the foreland basin imply major thrust sheets were bounded by lateral transfer zones that allowed discrete portions of the orogenic wedge to be active at any given time.
AAPG Search and Discovery Article #90101 © 2010 AAPG Foundation Distinguished Lecturer Series 2009-2010