The Ordovician Deicke and Millbrig K-Bentonite Beds of the Cincinnati Arch and the Southern Valley and Ridge Province

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Abstract
In the southeastern United States, identification of the Rocklandian Deicke and Millbrig K-bentonite Beds is based on differences in phenocryst mineralogy in the tuffaceous zones of each bed, and the two beds can be reliably and consistently distinguished on this basis. The common phenocrysts in the Deicke are labradorite and various Fe-Ti minerals, and in the Millbrig they are andesine, quartz, and biotite. Phenocrysts present in trace amounts are biotite and quartz in the Deicke, and apatite and zircon in both beds. The Deicke is altered dacitic or latitic ash, whereas the Millbrig is altered rhyodacitic ash. Both beds are interpreted as airfall deposits produced by huge volcanic eruptions, each of which was much larger and produced far more ash than the 1815 eruption of Tambora and even the great Toba eruption of 75 Ka. The principal authigenic minerals are feldspars (albite and K-feldspar), the clay minerals mixed-layer illite/smectite (I/S) and kaolinite, and ferroan and ferrotitanian minerals (pyrite, hematite, and the TiO2, polymorphs). The current lateral distribution of these authigenic minerals is the result of variations in regional geochemical conditions during burial diagenesis. The distribution of authigenic feldspars, together with regional changes in the percentage of illite in the I/S, indicates that maximum burial temperatures were higher in the Valley and Ridge province than along the Cincinnati Arch, and that the highest temperatures were in the Virginia Valley and Ridge. Regional variations in the distribution of the ferroan and ferrotitanian authigenic minerals in the K-bentonites and adjacent strata reflect variations in pore water redox conditions during diagenesis. Throughout the Cincinnati Arch and the westernmost Valley and Ridge, sediment pore waters were reducing. In the eastern Valley and Ridge of Alabama and Georgia the pore waters were oxidizing, and in the central and eastern Valley and Ridge from south of Roanoke to Knoxville the pore waters were initially reducing and then oxidizing. The stratigraphy of the Deicke and Millbrig is now very well known from their type area in the Upper Mississippi Valley to the easternmost exposures of Rocklandian strata in the southern Valley and Ridge from Alabama to Virginia. Many previous correlations are verified, and correlation of the two beds is extended along and across strike in the Valley and Ridge between Roanoke, Virginia, and Birmingham, Alabama. The Deicke and Millbrig are the thickest and most wide-spread of the several Rocklandian K-bentonites in this region. Of the two, the Deicke is more laterally persistent along the Cincinnati Arch, but in the southern Valley and Ridge the Millbrig is the more widespread and persistent of the two beds. The relationship of the K-bentonites to several regional and local unconformities is also now better understood. With this documentation of the great lateral extent of these two superb marker beds, it will be possible to link the vertical and lateral distribution of stratigraphically important faunal elements, especially conodonts and graptolites, with the stratigraphic position of the Deicke and Millbrig K-bentonite Beds.

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