THE STRATIGRAPHIC DISTRIBUTION OF FOSSILS WITHIN GAP-BOUND SEDIMENTARY ROCK PACKAGES

Shifts in the relative magnitudes of sediment supply and the formation/destruction of accommodation space result in up-section changes in rates of sedimentation and environmental characteristics, both of which influence the stratigraphic distribution of fossils. Numerous studies have shown that such factors result in the non-uniform distribution of fossils within sedimentary successions, but most of these have focused on their qualitative distribution within individual stratigraphic sections and basins. Here we combine a comprehensive North American macrostratigraphic database (Macrostrat) with fossil occurrence data from the Paleobiology Database (PaleoDB) to measure quantitatively the large-scale stratigraphic distribution of fossils within gap-bound sedimentary rock packages. Approximately 30,000 fossil collections from the PaleoDB were matched to one of 18,019 Phanerozoic sedimentary lithostratigraphic rock units from 779 geographic locations in North America. The distribution of fossils within gap-bound packages, corresponding roughly to third-order sequences, was then calculated as relative occupancy within each package scaled to unit-duration. The distribution of lithofacies within gap-bound packages was similarly calculated, and the results of each were combined statistically into mean occupancy curves. In general, we find that fossils and, to a lesser extent, lithofacies are more concentrated in the top half of gap-bound packages than expected by chance alone. The implications of these results for paleobiological patterns and for the evolution of sedimentary systems are explored.