Quantifying controls on the occurrence of nonmarine fossils

Although numerous studies have described differential preservation of nonmarine fossils in channel and floodplain facies, quantitative comparisons are lacking. We present measurements of the probability of occurrence of plant, mollusk, and vertebrate fossils in the Campanian Judith River Formation of north-central Montana, USA. The data reveal little difference in the probability of occurrence among higher taxa in channel and floodplain facies, except for plants, which have a higher probability in floodplain deposits. This surprising result supports more recent models in which many organisms are buried initially in floodplain deposits, but those deposits are subsequently exhumed by migrating rivers, and finally buried in those channels. Comparisons across systems tracts revealed intriguing patterns in which plants have a higher probability of occurrence in high-accommodation systems tracts in channel and floodplain deposits, and vertebrates have a higher probability of occurrence in channels of the low-accommodation systems tract. These results confirm that sequence-stratigraphic architecture should be considered in interpretations of the nonmarine fossil record. This probability of occurrence method has promise for many other comparisons of fossilization potential.