Taphonomic approaches to temporal resolution in stratigraphy: Examples from Paleozoic marine mudrocks

One of the most important and challenging aspects of stratigraphy is the interpretation of the temporal scope of sedimentary units (Schindel, 1980, 1982; Sadler, 1981; Brandt Velbel, 1984). The problem arises at the scale of individual beds and of stratigraphic intervals up to many meters thick. Does a particular bed or interval-represent hours, days, years, centuries, or millennia? Resolution of this question is critical for determination of rates of sedimentation and biotic processes, and in assessing the reliability of the sample for paleoecological or evolutionary analysis. In the absence of a reliable framework of absolute radiometric dates the question can only be answered by indirect inference. Biostratigraphic zonation is a critical first step. But zonation is typically too coarse to resolve temporal scales less than 106years and many zones are not firmly anchored to absolute dates. It is also important to keep separate the issue of temporal duration represented byfossils (as bioclasts)within a given stratum and that of the deposition of thesedimentaryunit itself. There are many instances of thin graded beds full of fossils, which would be characterized unambiguously by sedimentologists as deposits of a single event of sedimentation, but in which the fossils may differ in age by thousands or even millions of years. Examples include many condensed, lag deposits of bones and conodonts (Baird and Brett, 1991), and condensed ammonoid beds containing fossils of several ammonite zones (Fürsich, 1971). Sedimentologic criteria provide one avenue of approach to this issue but commonly fall short of unambiguous answers.