Elemental geochemistry of shales in Pennsylvanian cyclothems, midcontinent North America

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Title: Elemental geochemistry of shales in Pennsylvanian cyclothems, midcontinent North America
Author: Teo, Wee Seng
Abstract: 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 .
URI: http : / /hdl .handle .net /2346 /18458
Date: 1991-08

Citation

Elemental geochemistry of shales in Pennsylvanian cyclothems, midcontinent North America. Doctoral dissertation, Texas Tech University. Available electronically from http : / /hdl .handle .net /2346 /18458 .

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