Superposition, the presumption that older strata and fossils underlie younger ones, is a basic tenet of stratigraphy and paleontology. At very fine scales of resolution (e.g., centimeters of vertical accumulation), however, mixing by animal burrowing or reworking by physical events (storm surge, current winnowing) may overprint or obscure the original succession. The physical condition of buried shells may or may not be reliable indicators of the extent to which the original stratigraphic sequence is preserved. Radiocarbon dating of shells in a microstratigraphic sequence is another way to ascertain the degree to which the preserved sequence reflects the original vertical succession. Drs. Judith Parrish and Karl Flessa propose an intensive study of several such vertical sequences from the shallow marine environments of the northern Gulf of California. By analyzing the preservation state of shells relative to their vertical sequence and isotopically-determined ages, paleontologists and paleoecologists will gain new insights into the limits of resolution in the fossil record. The results of this study will be cited widely as a yardstick for "calibrating" the reliability of the fossil record, and it will provide a methodological model for future studies.
