The Excello Shale covers an area of almost 200,000 sq mi in the Mid-Continent and the Illinois basin, and is the most widespread lithologic unit of the Summum cyclothem. It overlies both marine carbonates and coals, and is overlain by marine carbonates and deltaic clastics. The typically carbonaceous Excello grades laterally into more oxidized greenish-gray facies as it onlaps and crosses structural highs.

It is difficult to explain the great lateral extent of the thinly-laminated, fine-grained, organic-rich Excello Shale by a nearshore or lagoonal origin. The stratigraphic relations and lithologic characteristics of the Excello Shale are indicative of deeper water sedimentation in a stratified anoxic water system.

The accumulation of organic-rich bottom sediments resulted from the growth of a widespread density gradient (thermocline) which inhibited circulation of bottom waters of the epicontinental Excello sea to such an extent that anoxic water conditions arose as a result of the subsequent processes of deoxygenation, denitrification, and sulphate reduction. The distribution of the sulfide-bearing bottom waters, and hence the distribution of the organic-rich bottom sediments, was controlled by submarine topographic highs and basinal depressions. Sedimentation was restricted to the slow influx of clay minerals (probably clad with organic films) and detrital plant and animal remains. Return to the normal marine environment and clastic deposition was brought about as the Excello sea became shall w, thus breaking up the thermocline and destroying the anoxic bottom waters.

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