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Bio-Stratigraphy Disproves Eustacy as a Primary Factor in the Accumulation of Middle Paleozoic Rocks Around the Transcontinental Arch of North America


In the 70s-80s, Amoco was faced with a compelling commercial need to understand and predict the actual connectivity of certain Mid-Pz rocks across N America, leading to extensive internal works (involving Rich Lane, Jack Babcock and others) that sampled and analysed fossil data, especially conodonts. This information underpinned analyses of rock unit connectivity on the regional scale, which provided rational explanations for facies relationships and thus reservoir property predictability. As more information was assembled by Amoco across a transect of N America, a picture emerged showing that the deposition of rock sequences on one flank of the Transcontinental Arch was not matched by deposition on the opposite side, suggesting a “rocking” motion of the craton, rather than eustacy, as the primary control on depositional patterns. The data also allowed us to identify sediment wedges associated with margins exterior to the NA craton, rather than being the “basin equivalents” of the known NA peri-continental sequences. Less-extensive data was suggestive of similar dis-chroneity of deposition on the eastern strike flank of the TA, implying shorter-scale controls on the creation of accommodation space. In the time since the work was ended at Amoco, other published studies of Mid-Pz rocks provide a more-extensive set of information that reinforces the concept of the primacy of local depositional controls, in opposition to the prevailing notion of eustatic controls. The geodynamic cause that creates such local depo-centers, and that causes them to develop multiple-scale features that include reversals of vertical movements, is not obvious. Existing process models predict vertical displacements that are essentially monotonic, and require that short-scale lateral variations be explained via some ad hoc explanation such as the presence of pre-existing weak faults (whose role may not seem compatible with the overall movement picture). Here we outline some thoughts about the geodynamic model needed to explain the geological record. Regardless of the lack of a suitable geodynamic model, the Mid-Pz record of NA is sufficiently detailed so as to disprove synchroneity of deposition across NA, and thus poses a fundamental challenge to the idea of eustacy as a major factor that controls the deposition of rock sequences.