WHATEVER HAPPENED TO UNCONFORMOLOGY? (AND WHY SHOULD WE CARE?)

Although contentious, sequence stratigraphy has had many positive impacts on sedimentary geology and paleontology over the last 20 years. One disappointment, however, is that it did not spawn a subdiscipline of unconformology to counter the Tyranny of Depositional Facies. As a result, despite the widespread use of discontinuity surfaces as a basis for genetic stratigraphic subdivision and as a set of hypotheses for correlation and baselevel analysis, we have only the most rudimentary phenomenology of discontinuity surfaces at the outcrop and basin scales relevant to most paleontologic-geohistory analysis.

In marine siliciclastic systems, nondepositional facies (primary physical surfaces generated by erosion, starvation, and/or sedimentary bypass, including their associated mantling materials and altered underbeds) are integral, dynamic counterparts to depositional facies. We know that these complicate the paleontologic record, e.g. erosional truncation of ranges, potential for faunal condensation, taphonomic feedback, and diagenetic extinction during omission. However, case studies are too few to test for geographic and secular trends in the paleontologic expression and consequences of non-deposition. For example, in shallow-marine Neogene records, mollusk-dominated skeletal concentrations are common along all ranks of discontinuity surfaces, even in coastal fanglomerates, and are especially diverse and complex along higher order transgressive ravinements. Are there analogs in Mesozoic and Paleozoic records, and at similar stratigraphic frequencies? Do they have the same environmental deployment, taphonomic/ichnologic features, and implications for bias, e.g. degree of faunal condensation? What are the variants in tropical carbonates, where early cementation and pressure solution are additional factors? Pilot work on secular trends in palimpsest and lag deposits indicates that differences need to be conceived in terms of frequency distributions rather than first/last occurrences or extreme-values, using stratigraphic sections as sampling transects and exploiting lateral changes in hiatal duration to test for time-sensitive effects. Fundamentally, however, the search is for data that are rarely published or gathered: non-deposition is indeed the waif of stratigraphy.