Abstract: The Excello Shale is one of the best exposed and most laterally continuous of the Pennsylvanian cyclothemic black shales in the midcontinent region of the United States. Its petrology and paleoenvironmental significance were studied to understand the nature of cyclic black shales in general and how they relate to the habitat of hydrocarbon source beds. The Excello is thinly laminated, fissile where weathered, and rich in organic matter and phosphate nodules; it is 90–120 cm thick. Its thin laminations, fine particle size, and high total organic carbon (TOC) content suggest that it was deposited in a quiet water environment of an epeiric sea having anaerobic bottom water. The Excello Shale consists of two lithofacies: non-bioturbated black shale and bioturbated yellow-brown shale. Petrographic studies show that the black shale contains wavy to straight laminations and that the yellow-brown shale contains discontinuous and random laminae and mottled stratification. The close association of organic matter and phosphate-controlled nodule morphology (spherical, elongated, bladed, and platy) appear to be related to progressive decreases in nutrient supply of the sea water from the ancient ocean (Panthalassa).
Clay-mineral assemblages consist mainly of detrital illite, kaolinite, chlorite, and illite/smectite (I/S). Grain size and amount of these clays increase and TOC decreases shoreward towards the probable source areas of the clay minerals in the northern Ouachita region and northern Iowa. Quartz, carbonate-fluorapatite, carbonate fossil fragments, and minor feldspar and pyrite are the principal minerals in the Excello Shale. Minor amounts of fine-silt-size carbonate minerals are present in some samples. Limestone concretions, 90–150 cm in diameter and 30 cm thick, were found at only one locality in Missouri where the Excello is a maximum of about 3 m thick. Fissility increases with weathering along the bedding plane, and laminae are separated by a limonite film. Four petrographically distinct microfabric variations of the Excello Shale appear to be related to the TOC content and bioturbation. Textural and structural properties of the shale are more developed with increasing organic matter content.