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Re-Evaluating A Mid-Cretaceous ‘Delta’ as Depositional Remnants Driven by Crustal Anisotrophy: Frontier Formation, Powder River Basin, Eastern Wyoming


Linking modern sedimentary environments to sedimentary architecture requires robust calibration of depositional process to sedimentary response. The suite of modern surficial processes used to define deltaic systems, however, often cannot explain the sedimentary patterns observed in ancient deltaic successions. We present data from the Middle Cretaceous “delta” of eastern Wyoming's Frontier Formation, which suggests that the poor correlation between ancient and modern deltas reflects variation in preservation. Considering crustal processes acting during the ancient deltaic system adds important context to spatial and temporal variability in preservation. Joseph Barrell (1917) used deltas to illustrate the physical expression of geologic time as both material (rock) and nonmaterial (surface). The conspicuous eastward distribution of sandstones in the Frontier Formation has long been interpreted to represent the paleo-shoreline bulge of an ancient delta. Consideration of the role of preservation provides a more complete explanation for the following sedimentary patterns; these include: 1) the downward trajectory and seaward stacking pattern of shallow-marine episodes; 2) thin, elongate, offset and isolated sandstone thicks; 3) regional correlation of multiple and closely spaced erosional surfaces; 4) thin interval (100 m) spanning a significant period of time (3 m.y.); 5) the absence of delta-plain deposits; 6) missing biozones; 7) well developed ravinement surfaces at the top of upward shoaling shallow-marine successions; and 8) divergent thickness and sediment transport trends, including internal lithologic variation that sums to, but is inconsistent with, the larger composite pattern interpreted to represent the paleo-shoreline bulge of an ancient delta. The poor correlation between sedimentary patterns in the Frontier Fm. and the suite of sedimentary components of modern deltaic environments is better explained as remnants from a shallow-marine system spread across a broad stable tectonic platform and incompletely preserved. We attribute the development of depositional remnants to decreased subsidence in this region of the foreland system. The variable expression of subsidence in this foreland system is correlated to crustal anisotropy generated by the growth and assembly of the Proterozoic craton. Subsidence in the foreland system of Horton and DeCelles (1997) was lower on a tectonic terrane we term the Wyoming Platform, which distributed middle Cretaceous sand the farthest eastward along a 10,000-km segment of the western margin of the Western Interior Cretaceous Seaway. The basinward propagation of strain resulting in the contraction and loading of different thrust segments varied in response to differences in the inherited crustal rheology and fabric of the continental crust. While these regional sedimentary patterns emphasize recognition of different orders of widespread tectonic deformation, these considerations are very relevant to building reservoir models that better portray geologic heterogeneity created by remnant sedimentary patterns. Creating a hierarchy for the range of many different scale processes that operate over geologic time provides a more predictive framework for the explanation of sedimentary patterns. This hierarchical method takes into account variations in preservation explaining the poor correlation between sedimentary attributes from ancient and modern deltaic systems. This is important in addressing concerns raised about the utility of applying smaller sedimentary attributes to the prediction of larger sedimentary patterns. Finally, these results highlight a fundamental problem with the application of facies models to the prediction of reservoir presence and architecture. The reductionism used to create a facies model involves extracting certain attributes from many modern examples. This simplification selectively combines specific attributes and excludes others in the abstraction of a conceptual model that does not actually exist. This simplification is done to remove variation or “noise” attributed to system complexity, when the real problem is the incomplete, abstract and static facies model formulation. Because they do not contain the essential variation required to determine the contribution from deposition and preservation, facies models have limited predictability. The critical attributes required for prediction are removed in formulation of a facies model. Consequently, over simplification is likely the problem, not complexity. The hierarchical methods illustrated here emphasize the stratigraphic correlation of variation resulting from both temporal and spatial changes in sedimentary systems. This avoids the over simplification embedded in facies models that break the predictive link between hierarchical sedimentary attributes and the multiple scales of geologic heterogeneity impacting reservoir presence and quality.