DIASTEMS AND TIME-AVERAGING: THE LIMITS OF RESOLUTION IN STRATIGRAPHY AND PALEONTOLOGY

Two concepts of resolution can be applied to the stratigraphic record: stratigraphic resolution determined by the duration of interstratal diastems and unconformities (temporal spacing between preserved depositional events) and depositional resolution determined by the temporal span of physical records (e.g., fossils) preserved within strata. Studies of modern and ancient depositional systems indicate that whereas both types of resolution vary widely, their typical temporal range is strikingly comparable: hundreds to thousands of years. Despite those similarities, the two types of resolution are only partly correlated. Depositional resolution is controlled primarily by temporal mixing, which is a function of types of fossils, taphonomic processes, depths and rates of bioturbation, depositional/tectonic settings, and sea level history. Stratigraphic resolution is controlled by the amount of accommodation space, latitude, bathymetry, and Milankovitch-style climatic cycles. Depositional and stratigraphic resolutions are partly independent parameters and both may place limits on the resolution of stratigraphic and paleontological data. Because bioclasts often predate depositional events by hundreds or thousands years, paleontological data may often provide complete, albeit mixed, documentation for time intervals missing from the geologic record (beds record depositional events, fossils record the preceding diastems). Thus, paleontological completeness may be often higher than stratigraphic completeness, and the average estimates of parameters obtained from fossils may be more representative of their true long-term, average values than is suggested by the sporadic nature of depositional events. Despite differences in underlying secular factors, both depositional and stratigraphic resolution appear to have decreased through the Phaneorozoic. When considered together, the two measures of resolution serve as a useful conceptual tool for evaluating the limits of resolution inherent to geoscientific analyses and offer a more realistic picture of the temporal nature of paleontological data.