EO-ULRICHIAN TO NEO-ULRICHIAN VIEWS: THE RENAISSANCE OF “LAYER CAKE STRATIGRAPHY”

Classical notions of "layer cake stratigraphy” have been denigrated as representing an antiquated “Neptunian” view of the geologic record. The American paleontologist E.O. Ulrich was vilified as the quintessential advocate of "layer cake stratigraphy”, 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 something of a "layer cake" pattern, after all. Possible causes of "layer cake" patterns are numerous and varied. Time-averaging homogenizes local facies patches. Moreover, the parallelism of sedimentary strike and outcrop belts ensures that many long stretches of outcrop run along originally elongate environmental belts of gently dipping ramps. But 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 seems truly paradoxical. The paradox of the “frosting continuity” is explicable by processes at two ends of the time-spectrum: widespread, instantaneous events and sequence-related condensation. The extreme “layer-cake” interpretations of E.O. Ulrich are demonstrably incorrect, but a revised (“Neo-ulrichian”) view carries many of the same correct observations of pattern, recast in terms of event and sequence stratigraphy.