RARE EVENTS, TIME-AVERAGING, AND THE PARADOX OF “LAYER CAKE STRATIGRAPHY"

Modern shallow marine settings typically display a pronounced patchy distribution of facies and limited lateral continuity of facies. Such spatial facies mosaics undoubtedly existed in ancient marine environments and should be anticipated in the stratigraphic record. However, contrary to expectation, detailed stratigraphic study of many well known successions (e.g., mid Paleozoic; Appalachian basin) reveal highly predictable, “layer cake” stratigraphy, even across facies strike. This phenomenon is due, in part, to the extraordinary persistence of thin (centimeters to a few meters)intervals or beds, despite major changes in thickness (up to 100-fold) and lithofacies of surrounding strata. Marker beds belong to several disparate categories, including both single event deposits (e.g., bentonites, tempestites, seismites) and highly condensed shell/bone beds. Causes of bed persistence are varied, but include: a) gentle sloping ramps and broad facies belts, b) condensation and time-averaging, and c) disproportionate preservation of rare major events. Event deposits, including final reworking of condensed beds, may also preserve lateral gradients of facies that existed synchronously at moments in time. Conversely, time averaging and event condensation in condensed beds may "homogenize" short term spatial mosaics and produce composite beds with rather uniform characteristics over broad areas. Moreover, certain condensed beds relate directly to key sequence stratigraphic surfaces (e.g. transgressive lags/sequence boundaries; condensed phosphatic beds/flooding surfaces) and thus are linked to environmental processes. Finally, some “layer cake" patterns may be present over even broader continental or global scales. These apparently reflect responses to major sea-level and climatic change (e.g. oceanic anoxic events), and bioevents. Such patterns permit temporal correlations that are orders far more precise than those based on traditional litho- and biostratigraphy.