Conodont fauna of the Excello Shale (middle Pennsylvanian), midcontinent North America

Date

2017-08

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Abstract

The Excello Shale member is the laterally extensive core shale of the Lower Fort Scott major cyclothem of the Desmoinesian Marmaton Group in the Midcontinent Basin of the central United States. It is the first major cyclothem of the group and lies above the Mulky Coal, which marks the transition between the Cherokee and Marmaton groups. The Excello Shale member can generally be divided into a lower black shale facies and an upper gray shale facies. Phosphate laminations as well as phosphate nodules occur through the lower black shale, as well as rare acrotretid brachiopods. Three sections of the Excello Shale member in a south to north transect were sampled in small stratigraphic intervals (5-10 cm) and small samples (200 g) of the shale were completely processed in bleach to obtain an accurate census of conodont genera and species. Rare abundance data was converted to approximate numbers of conodont individuals. Idiognathodus elements were divided into three species, I. acutus, I. species 2 and I. sp. 3. Neognathodus comprises four species, N. dilatus, N. roundyi, N. sp. 3 and N. sp. 4. Gondolella was separated into two species, G. wardlawi and the much rarer G. sp. 2.The Excello conodont fauna is significantly different than the more diverse conodont fauna of the highest major Cherokee cyclothem, the Verdigris. The Excello fauna resembles more closely the younger Marmaton conodont faunas, such as the Lost Branch fauna at the top of the Marmaton Group. The distribution of conodont genera in the Excello Shale member is compatible with the Midcontinent conodont biofacies model. The species provide some additional biofacies information, with differences in proportions of Idiognathodus and Neognathodus species occurring across the offshore to onshore transect. However, these species-level biofacies changes are not seen in the vertical succession of lithofacies in any one section. This may because the geographic spacing of sections provides greater ecological separation than can be resolved in the condensed vertical lithofacies section of the cyclothem.

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Keywords

Conodonts, Pennsylvanian, Midcontinent Basin, Black shale, The Excello Shale, Cyclothem

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