VARIATION IN THE PROBABILITY OF OCCURRENCE OF NONMARINE FOSSILS IN THE CAMPANIAN JUDITH RIVER FORMATION OF NORTH-CENTRAL MONTANA

Quantifying the occurrence of nonmarine fossils in depositional environments and systems tracts provides the data necessary for the development of numerical models of the nonmarine fossil record that aid in the interpretation of paleobiologic patterns. These estimates can also be used to understand the general controls on preservation potential. Many studies have described differences in the abundance and preservation of nonmarine fossils (and concentrations of fossils) in channel and floodplain facies. Here, we present quantitative estimates of the probability of occurrence of plant, mollusk, and vertebrate fossils in channel and floodplain facies as well as in nonmarine systems tracts. This data was collected by measuring stratigraphic columns and counting fossil occurrences in the fluvial and paralic McClelland Ferry and Coal Ridge Members of the Campanian Judith River Formation. The architecture of these two units corresponds respectively to the low-accommodation and high-accommodation systems tracts (LAST and HAST), allowing for comparison between systems tracts. The results show few differences in the probability of occurrence of fossils among facies, and the probability of occurrence of vertebrate and mollusk fossils is indistinguishable between channel and floodplain facies. Plant fossils, however, have a higher probability of occurrence in floodplain facies. When probabilities are limited to higher abundance thresholds (i.e., concentrations vs. individual fossils), patterns of occurrence among taxa change. At low thresholds (>10 specimens), mollusks have the highest probability of occurrence, followed by vertebrates and plants. At high thresholds (>100 specimens), only mollusks have a non-zero probability of occurrence. Probabilities of occurrence may differ among nonmarine systems tracts, although confidence intervals overlap. Plant fossils occur more commonly in the floodplain and channels of the HAST (Coal Ridge) than the LAST (McClelland Ferry), and vertebrates are more common in the channels of the LAST. Although a larger data set is needed for a more robust comparison, the current data support that sequence stratigraphic architecture influences patterns of nonmarine fossil occurrences and should be taken into consideration when interpreting paleobiologic events.