EO-Ulrichian to Neo-Ulrichian Views: Lessons from an Ordovician “Layer Cake”
March 10, 2014
Classical notions of “layer cake stratigraphy” have been denigrated as representing an antiquated view of the geologic record. The American paleontologist E.O. Ulrich was vilified as the quintessential advocate of “layer cake stratigraphy”, perhaps best exemplified by his views of the Ordovician Cincinnatian rocks in the Ohio-Kentucky area, because he failed to acknowledge the possibility of lateral facies change. Even the most casual observer knows that conditions are far from uniform across virtually any part of the globe. The recognition of facies revolutionized geologists’ view of time-space relationships in stratigraphy; strata came to be seen as parts of diachronous facies mosaics and inferred time-parallel layer cake patterns were viewed with suspicion. However, with the rise of event and sequence stratigraphy, the pendulum has started to swing the other way. Field stratigraphers have long known that some unique beds can be traced for miles, but, ironically, it took seismic stratigraphy to convince most geologists that major through-going surfaces exist within rocks. Careful re-inspection of the geologic record in many regions reveals remarkable a “layer cake” patterns, after all. The geologic record of many basins also contains a far more intricate pattern of layering that cuts across local facies. The marine record is divisible into packages (analogous to cake layers) that are bounded by thin, through-going marker beds (analogous to frosting layers). Both consistency and thickness of the “cake” layers may vary substantially. The fact that certain thin “frosting” beds may show remarkable lateral persistence both along and across facies strike may seem paradoxical. Detailed work on the iconic “layer cake”, the Upper Ordovician Kope Formation in Ohio illustrates many aspects of this issue. Like many Phanerozoic marine successions, the Upper Ordovician (Katian/Cincinnatian; Edenian) Kope Formation in southwestern Ohio and northern Kentucky exhibits distinct, correlatable alternations of thick (meter- to decameter-scale) mudrock-dominated intervals and thinner (decimeter-to meter-scale) shell bed-dominated units; these cycles and their component beds have been correlated across the entire outcrop belt and into the “Utica” facies of the Sebree Trough. We infer that the thinner bioclastic units are condensed, relative to the mudrock-dominated intervals. The former represent the long-term accumulation of thin, mud starved veneers of winnowed/reworked parautochthonous skeletal debris, whereas the latter involved the episodic deposition of mud and silt layers up to several centimeters thick. Certain of these yield distinctive features and can be traced for tens of kilometers, as well. Evidence presented herein indicates that the decimeter-scale shell beds of the Kope Formation formed during millennial-scale periods of widespread siliciclastic sediment starvation, combined with episodes of storm-related reworking and winnowing. This constitutes an alternative to the “tempestite” interpretation of shell bed genesis in the Kope Formation, as well as in many other mixed siliciclastic–carbonate successions, which is more in accord with taphonomic, sedimentologic and paleontologic evidence.The paradox of “frosting continuity” is explicable by processes at two ends of the timespectrum: widespread, instantaneous events and widespread condensation. Theextreme “layer-cake” interpretations of E.O. Ulrich are demonstrably incorrect, but a revised (“Neo-ulrichian”) view is emerging that carries many of the same correct observations of pattern, recast in terms of event and sequence stratigraphy.