THE STRATIGRAPHIC PALEOBIOLOGY OF CONTINENTAL SYSTEMS: INITIAL CONSIDERATIONS

The stratigraphic architecture of continental systems is controlled by the rates of accommodation and sedimentation. Coupled with ecological gradients, these rates control the expression of the continental fossil record. These processes generate predictable clusters of first and last occurrences, as well as gradual, abrupt, and repeated changes in ecological composition and morphology.

Continental systems are typically characterized by two end-member systems tracts, the high-accommodation and low-accommodation systems tracts (HAST and LAST). HAST have single-story fluvial channels embedded in extensive floodplain deposits, often with thick and numerous coal beds. In contrast, LAST are characterized by multi-story fluvial channels, minor floodplain deposits, and numerous well-developed paleosols. HAST favor the preservation of species in their preferred habitat and a higher-quality taphonomic grade. In contrast, LAST favor a lower-quality taphonomic grade, and species will commonly be found out of their preferred habitat.

In coastal settings, systematic changes in sedimentary environments are superimposed on these systems tracts in response to shoreline transgression and regression. For example, transgression (equivalent to the marine TST, and usually associated with HAST) drives an upward shift from coast-distal to coast-proximal fossil assemblages, with regression (equivalent to the marine HST and LST, usually associated with LAST) displaying the opposite. As a result, taphonomic grade and the composition of fossil assemblages will change systematically through depositional sequences.

Moreover, different parts of sedimentary basins are characterized as low-accommodation and high-accommodation settings, which tend be dominated by one of the two systems tracts. The geographic distribution of these settings changes over the lifetime of a basin, causing a long-term shift towards low-accommodation systems tracts dominated by coast-distal environments. As a result, taphonomic grade and ecological composition will also change predictably through the history of a depositional basin.