Heterogeneity, paleohydrology, and 3D facies architecture of ancient point bars, Ferron Sandstone, Notom Delta, South-central Utah



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This study examined ancient exhumed channel belts from the Cretaceous Ferron Sandstone, Notom Delta in south-central Utah. Extensive plan view exposures with local vertical cliff exposures allowed documentation of channel belt dimension, migration pattern (translation versus expansion) and facies architectures. The cliff exposures allow documentation of channel fill thickness and bedding structure that were used in paleogeographic and paleohydraulic reconstructions. Paleocurrent measurements are consistent within one depositional unit (such as a unit bar or channel belt) and can be used to infer channel flow and bar migration patterns. Four hundred and eight paleocurrent directions were integrated with grain-size measurements to reconstruct the 3D facies architecture of the ancient channel belts. Based on the presence of distinct types of inclined strata of large scale foresets and the channel sinuosity (1.01-1.44.) measured from the paleographic map, the formative river of the point bar complex was interpreted to be a low sinuosity river. The point bar complex consists of four successive component point bars and shows a migration pattern that changes from expansion to translation with increasing sinuosity. By using empirical equations, average channel depth and channel width were determined to be 1.7m-3.6m and 23m-89m respectively. Channel width (around 50m) as measured from abandoned channel lags falls into this range. The large variation of channel belt width may be a result of increasing channel dimension due to increasing sinuosity. Therefore, empirical equations for calculating paleohydraulic parameters are generally applicable but need further refinement for specific river types. Bar thickness measured from vertical outcrops range from 5.4m to 6.3m which gives a much narrower range of 47 to 59 m for the channel width estimation. The discharge of the formative river is estimated to be 115 m3/sec to 387 m3/sec. Grain-size variation shows a distinct coarsening trend from inner bar to outer bar at both the scale of individual point bars and the meander loop complex. Because of the absence of extensive shale drapes between each point bar, petrophysical heterogeneity introduced by grain-size variation of sand and directional permeability anisotropy introduced by different types of cross bedding will be the main type of heterogeneity. The grain-size distribution shows a more complicated pattern compared to previous models and the resulting fluid flow pattern is expected to be more complex as well. Finally, the major controls on the formation of low sinuosity river are gradient, sediment supply, and bank erosion, although high frequency climate changes are interpreted to be responsible for the filling of the whole incised valley system. The fluvial style is a bed-load to mixed-load, high net-to-gross river system with limited suspended content in the flow.



Point bars, Facies architecture, Low sinuosity river, Ferron sandstone