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Petrologic Discrimination Between the Neogene Formations in Adams and Wilkinson Counties, Southwestern Mississippi
Kenneth F. Rhinehart, Maurice A. Meylan
The outcropping Neogene sediments of Adams and Wilkinson Counties, Mississippi, include the Miocene(?) Hattiesburg Formation,the Plio-Pleistocene(?) Citronelle Formation, the Pleistocene Natchez Formation, and Quaternary terrace deposits, overlain by a now erosionally dissected loess blanket that thins eastward away from the Mississippi River. These units can be distinguished both on the basis of outcrop characteristics (which were used to identify them in the field) and petrologic parameters (which may be useful for determining lithostratigraphic relationships between units during detailed geologic mapping, as well as for identifying units in the subsurface).
Samples of the pre-loess Neogene section have been studied utilizing grain size (sieve and pipette) analysis, thin-section petrography of sands, and X-ray diffraction analysis of the clay-sized fraction. The Hattiesburg Formation consists primarily of clay and silt; the coarsest bed sampled is a fine-grained sand. Smectite is the dominant clay mineral in the Hattiesburg, but kaolinite is the only clay present in all samples of this formation. The Hattiesburg is increasingly siltier southward, and this increase in grain size coincides with a decline in smectite abundance.
The coarsest unit is the Citronelle, which is commonly graveliferous near its unconformable basal contact with the Hattiesburg Formation, fining upward to sands. This contact is found consistently at an elevation of about 300 feet. Gravel is also present in the Natchez Formation and terrace deposits, and fining upward sequences were observed in both units. The Citronelle, Natchez, and terrace deposits also include beds of fine-grained sediments, the Citronelle even having a relatively well indurated shale. Kaolinite is the dominant clay mineral in the Citronelle, and in most of the samples is the only clay detected.
Whereas Hattiesburg, Citronelle, and terrace deposit sands can be classified as quartz arenites or sublitharenites, the markedly greater feldspar content of the Natchez Formation gives it a subarkose to arkose composition. Additionally, it is the only unit displaying volcanic rock fragments, as well as the only unit with illite identified in all samples; kaolinite is also abundant in the Natchez Formation. Unfortunately, the Natchez is essentially a local unit, the remnant of a Mississippi River terrace constructed of glacial debris from a northern mid-continent provenance area; thus, the diagnostic characteristics of the Natchez Formation do not have regional applicability.
Terrace deposits lie unconformably on the Hattiesburg Formation, and likely have been derived mostly from Citronelle, and to a lesser extent, Hattiesburg sediments. Kaolinite is present in all terrace deposit fine fractions; illite and smectite are important but less frequently occurring.
Miscellaneous sedimentary features that may be useful for distinguishing between units are also present. Lignitic material, calcite nodules, and trace fossils are found within the Hattiesburg section. Large-scale crossbedding, rip-up clasts composed of clay, and manganese oxide-encrusted gravel clasts occur in the Citronelle Formation; and armored clay balls were noted in the Natchez Formation.
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