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Data from modern deltas have made it possible to interpret the origin of the Pennsylvanian "shoestring sands" of Kansas and Oklahoma. Objective criteria from cores and outcrops are used to define specific areal and vertical depositional patterns. Criteria include: (1) vertical patterns of sedimentary structures, bedding, and grain size, (2) clay mineralogy and detrital clasts, (3) trace fossils, and (4) detailed analysis of textures.
The historic development of a delta provides the insight for interpreting the deposition patterns observed in ancient deltaic sandstone bodies. The processes of progradation and maturation are used to develop a 4-dimensional deltaic model. Six subdivisions are distinguishable: (1) lower alluvial plain, (2) upper deltaic plain, (3) lower deltaic plain, (4) subaqueous sand sheet, (5) marginal basin, and (6) marginal plain.
The lower alluvial plain is characterized by stream meandering, point bars, and unidirectional channel flow. Development of the lower deltaic plain is controlled by crevassing of natural levees, by tides, and by floods. The subaqueous sand sheet is developed by shallow-water currents, modified by progradation and commonly replaced by deltaic-plain environmental units. The marginal basin and depositional plain are produced by longshore drift, and reflect a balance between subsidence, sediment supply, and wave energy. These environmental units may be modified or replaced by other deltaic elements.
The Bluejacket-Bartlesville Sandstone of eastern Oklahoma was selected as a model because of its importance as an oil reservoir, the large amount of available subsurface data, and the simplicity of the stratigraphic and structural framework. The model is used for interpreting many other oil-productive Pennsylvanian sandstones in eastern Oklahoma.
Lower Pennsylvanian strata were deposited during an overall transgression; but, the transgression is marked by extensive regressions. These are represented by widespread sandstone sheets commonly underlain and overlain by marine shale or limestone. Particular sandstone units are distributed across thousands of square miles, but locally the sands are lenticular. Each regression is in response to the outbuilding of sediment under static sea-level conditions. The supply of sediment is related genetically to a river system, and deltaic patterns from the same river can be traced through Morrowan, Atokan, and Desmoinesian strata.
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