Earthquakes as Probable Causes of Chaotic and Deformed Stratigraphy in an Ancient River Meander Deposit, Dinosaur Provincial Park, Alberta

An ancient (Upper Cretaceous, 77-76.5 Ma, Oldman Formation) river meander deposit, exposed in the Steveville Badlands of Dinosaur Provincial Park, AB., exhibits extensive deformed and chaotically bedded strata. The most impressive features are large scale rotations of inclined heterolithic stratified (sandstone and shale) blocks, up to 6 m high and 50 m long, dipping in the opposite direction to that of the lateral accretion trend (Fig. 2). We observe three separate sets of reversely dipping beds along one badland gully, oriented parallel with the direction of lateral accretion. The large reverse cross-stratified structures rest on shale failure planes, suggesting the structures formed as back-rotational slumps of inclined heterolithic strata that slid down an active point bar slope into the channel before it was buried by subsequent lateral accretion sediments. Chaotic and disturbed sandstone and shale blocks, soft sediment deformation, and evidence for sediment foundering in the upper 3 m of point bar stratigraphy are common throughout the ancient meander bend, atypical of meandering river deposits (Fig. 1). Some of the broken and blocky sandstone strata displays a domino-like effect, with all blocks leaning in the same direction. Overturned sandstone beds resting on interpreted failure planes, attributed to slumping, are suggestive of down-slope failures (Fig. 3). Faulting represents the final form of deformation of stratigraphy with displacements of up to 2 m, and the hangingwall always on the channel side of the meander lobe (Fig. 4). We interpret all of these structures as having been caused by large magnitude earthquakes and tremors associated with Laramide thrusting. The three sets of reversed inclined heterolithic strata encased within normal lateral accretion bedding, are interpreted to record three major seismic events, separated by periods of relative