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Discriminant Analysis of XRF Data from Sandstones of Like Facies and Appearance: A Method for Identifying a Regional Unconformity, Paleotopography,and Diagenetic Histories

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The placement of an unconformable surface within a stratal succession affects the interpreted thickness of units and sequences in contact with that surface. Unit thickness influences the interpretation of basin subsidence, paleotopography, diagenesis, and depositional style. Accurate placement of an unconformity results in true formational thicknesses for formations associated with that unconformity. True thicknesses aid in producing more precise surface to subsurface correlations, isopach maps, and paleogeographic maps. An unconformity may be difficult to identify in the stratal succession due to similar rocks above and below the unconformity and the presence of multiple candidate surfaces. Using statistical discriminant analysis of XRF data, formations bounding an unconformity can be discriminated by elemental composition which results in delineation of the associated unconformity. This discrimination is possible even for rocks that do not have significant differences in provenance if they have experienced distinct diagenetic histories. Elemental differences can be explained by quantity and type of cement. Three discriminant models were created. These models were tested with samples from three formations of similar facies, appearance, and provenance that are all associated with the same regional unconformity. All data, regardless of location, facies, or tectonic feature were used to create the first model. This model achieved moderate success by correctly classifying 80% of known samples. In a second model, data were grouped by facies trends. Separating the data by facies resulted in 94% of known samples being correctly classified. This model was most useful for delineation of an unconformity and discrimination of formations. A third model based solely on location or local tectonic feature produced the best results statistically. 96% of known samples were classified correctly. This third model does not compare locations to each other, thus making it less robust. This last model contributes by adding detail to interpretations made with the facies trend model.
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... The geometries and facies transitions of the Middle Jurassic eolian sandstones, as well as the size and trend of the paleotopographic high, suggest the existence of a forebulge associated with the Utah-Idaho trough ( figure 1). We believe this forebulge/paleotopographic high acted as a hydrologic barrier between time-correlative eolian sandstones to the east (Harris Wash Member of the Page Sandstone) and west (White Throne Member of the Temple Cap Formation), dividing the region into two distinct diagenetic zones ( Phillips, 2012). ...
... The White Throne Member has been shown to be geochemically distinct from the Page Sandstone ( Phillips, 2012). The use of discriminant analysis indicates that mobile elements are the main discriminating elements between the White Throne Member and the Page Sandstone, which reflects their distinct diagenetic histories. ...
... The Page Sandstone typically has large amounts of ferroan dolomite, calcite, iron oxide, and rare poikilotopic calcite cements. The White Throne Member, in contrast, typically has kaolinitic cements ( Phillips, 2012). These distinct diagenetic differences may be explained by a paleotopographic high on the Navajo Sandstone that separated these formations into discrete basins. ...
... Ten field sections and one core from the Covenant oil field were measured or described ( Fig. 1; Phillips, 2013). Lithology, color, grain size, bedding thickness, bedding contacts, geomorphic expression (in outcrop), and primary and secondary sedimentary structures were noted as well as any other significant features. ...
... All samples were collected from sandstones. For consistency, samples for XRF analysis were collected from the middle of dune foresets rather than from the dune toes or crests (Phillips, 2013). ...
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