Abstract The Excello Shale is one of the best exposed examples of Pennsylvanian cyclothemic organic-rich shales in the midcontinent region. This study aimed to describe detailed stratigraphy of the study interval along the outcrop belt and to relate the environments of depositon, and paleogeography of the study area during the glacial-related paleoclimatic episodes. Certain present-day shallow silled basins were used as a possible analogue of the Pennsylvanian cyclic epeiric seas. Eustatic changes in sea-level were closely related to cyclic glaciation, global tectonics, basin subsidence and sedimentation pattern. Because of paleobathymetric relief in the cyclic epeiric seas, some units disappear and new ones appear, commonly with a change of facies. In many cases, cyclic coal beds underlie black shales. This suggests that swamp environments were present intermittently due to minor regression preceding episodes of cyclic maximum transgression. The occurrence of the cyclic seas and swamps between 5° and 8° N paleo-latitude all suggest a tropical-wet climate with possible seasonally controlled rainfall. Thin laminae, fine particle size and high TOC content (up to 17 wt%) indicate stagnant conditions with bottom-water anoxia in the Excello Sea. Anaerobic sediments were deposited in more than 100 m depth of water, poor circulation, shallow mixing with atmosphere, and high organic productivity and/or better preservation was present during maximum transgressive episode. The establishment of a density-stratified water column or pycnocline was the most important factor in development of the anaerobic sediments. Perhaps a halocline between bottom normal marine salinnity waters and a surface fresh-water layer probably caused the water stratification and anoxia leading to better preservation of organic matter in the Excello Sea.