Elemental geochemistry of shales in Pennsylvanian cyclothems, midcontinent North America
Teo, Wee Seng
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Pennsylvanian cyclothemic marine shales present a wide range of depositional environments that allow the study of depositional controls on distribution of certain elements in shales. Samples were collected from upper Desmoinesian to lower Virgilian units in north-central Texas, Kansas, and Oklahoma, The samples were analyzed for Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, Sc, V, Cr, Co, Ni, Cu, Zn, Be, Sr, Ba, Zr, Y, Rb, S, and total organic carbon (TOC). X-ray diffraction showed that illite, kaolinite, and quartz were the predominant minerals. The weathering index and the chemical index of alteration both indicate that the source minerals of the shales were highly weathered. Thin sections reveal the presence of red brown aggregates of clay, organics, and oxides, gray clay aggregates, and quartz grains. Abundances of Mn and Fe are quite variable (except for Mn in calcareous shales, and Fe in pyritic shales). Core shales, deposited during maximum transgression, may be high or low TOC shales depending on the original sedimentary redox conditions. In high TOC core shales (TOC/Al ratio above 1.2), abundances of V, Zn, and Cr correlate strongly with TOC. Sulfur correlates strongly with Fe. In low TOC core shales (TOC/Al ratio below 1.2), abundances of V, Zn, and Cr do not correlate with TOC. In some low TOC core shales, Zn, Cr, Ni, and Cu increase in maximum transgressive intervals and decrease stratigraphically upwards due to dilution by deltaic clays. Outside shales, deposited during regression, are normal to marginal marine shales with low TOC. Carbonaterelated elements (Ca, Sr, Zn, Mn, P, Y, Ni) are more abundant where the shale contains more calcareous skeletal material. Marginal marine shales show widely variable TOC and elemental composition. This study indicates that the main factors controlling the distribution of elements in cyclothemic shales are (1) the degree of weathering before deposition, (2) redox condition in the depositional environment, (3) settling time of the clay and organic matter through the water colunm, (4) conditions conducive to the formation and deposition of carbonates, (5) the composition of the organic matter, and (6) dilution by fine-grained terrestrial sediments.