Abstract

Specific causes of unusual events recorded in the geological record are commonly difficult to distinguish and isolate; in some instances, event strata contain features that cannot be explained by a single causal mechanism. Unicausal hypotheses, when applied to complex problems, can lead to the misidentification, misinterpretation, and force-fitting of observations (“great expectations syndrome”). The close timing or temporal overlap of significant events, although statistically improbable on short time scales, becomes possible on long time scales. Event coincidence may occur on a wide range of scales, from local to global. On the local scale, a multiple-event interpretation is offered for both the concentration and clustering of bivalves at specific levels within the Upper Cretaceous Bearpaw Formation of southern Alberta. For this example, the relative timing of fluctuations in benthic substrate texture, oxygen concentration, abundance of planktotrophic larvae, and degree of sea-floor scouring was crucial to the formation and preservation of shell concentrations. On the sharply contrasted global scale, the implications of multiple events warrant much closer consideration than they have received hitherto in terms of major proposed causes for the Cretaceous–Tertiary (K–T) mass-extinction event: bolide impact, sea-level change, climatic change, and flood-basalt volcanism. By considering the predictable effects of these synchronous factors, both individually and in combination, a multiple-cause explanation of the K–T mass extinction emerges as entirely plausible. Certainly it needs to be considered in all future investigations of this important issue.

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