PRESERVATION OF RARE EVENTS: RELATIONSHIPS OF EVENT AND SEQUENCE STRATIGRAPHY

Contrary to expectation, detailed stratigraphic study of many well known successions (e.g. Ordovician to Devonian strata of the Appalachian Basin and platform) commonly reveals highly predictable, "layer cake" stratigraphies, even across facies strike. The phenomenon is accounted for, in part, by the extraordinary persistence of marker beds that represent rare, large magnitude events. Such beds comprise several disparate categories of event deposits, including: K-bentonite beds, seismically deformed horizons (seismites), widespread tempestite, or turbidite beds, and obrution deposits. Detailed local microstratigraphic studies indicate that many intervals of offshore, marine strata consist of stacks of rapidly deposited, thin sediment packages, separated by condensed, debris-rich horizons that record "background" conditions. As with ash layers, beds with unique sedimentological, taphonomic, or faunal signatures (e.g., trilobite molt layers) are "fingerprinted" and provide excellent markers that imply widespread (10s of square km) and uniform sediment blanketing of broadly similar environments on gently dipping ramps. However, over larger areas, across facies strike, subtle facies changes may occur within events beds, indicating that the layers are, indeed, isochronous. Various types of event beds occur non-randomly within depositional sequences. Relatively condensed offshore transgressive marine facies may exhibit a disproportionate preservation of rare events due to "event concentration". This effect is exemplified by a frequent occurrence of K-bentonites, and well preserved hardgrounds at flooding surfaces. Event deposits actually may have higher absolute frequencies in expanded (e.g. highstand ) deposits; however, these horizons are commonly obscured by "dilution". Conversely, seismites appear most frequently in silty to fine sandy ("deformation prone") sediments of late highstands. Regional high resolution event stratigraphy in combination with sequence stratigraphy permits temporal correlations that are orders of magnitude greater than those based on traditional litho- and biostratigraphy.