Testing the Utility of Vertebrate Remains in Recognizing Patterns in Fluvial Deposits: An Example from the Lower Horseshoe Canyon Formation, Alberta

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Abstract
Depositional cycles in fluvial successions are described here as chronostratigraphic packages of strata founded on a laterally extensive, scour-based, amalgamated channel-sand body, overlain by mudrocks, isolated channel fills, avulsion and splay complexes, and paleosols. Ten packages are described from the lower Horseshoe Canyon Formation (Campanian-Maastrichtian), one of a succession of clastic wedges filling the Alberta foreland basin in south-central Alberta. The structure of these packages is consistent with the fall-rise-fall cycle of base-level described in other studies, but the package-bounding scours and internal surfaces are discontinuous and difficult to trace in the mudrock-dominated strata. Terrestrial vertebrate fossils are preserved in relatively fossiliferous, facies-independent horizons 1 to 3 m thick that statistically correlate with the stratigraphic position of package scours and surfaces. Fossiliferous horizons formed as a result of attritional accumulation under an optimum, relatively low, regional deposition rate. Not only do these horizons aid in locating package surfaces, but they also provide insight to the interaction of the package-scale, base-level oscillation with the larger-scale fluctuation in accommodation associated with the formation of the clastic wedge. As such, fossiliferous horizons in the Horseshoe Canyon Formation make better boundary markers than do paleosols, splays, coal seams, or even the surfaces associated with package structure. Therefore, the vertebrate fossil record may supply a means of stratigraphically evaluating sections in other locations in which typical sedimentological and architectural cues for surfaces are absent.