Abstract

By analyzing Mid-Continent Pennsylvanian cyclothems, a model for the succession of depth communities is proposed. The basic composite cyclothem contains the following community succession: shallow-water molluscan communities; moderate-depth brachiopod, crinoid, coral and fusulinid communities; and "deeper" water molluscan communities, of which the latter are formally defined herein.

The "deeper" water sediments and communities are found in black, fissile, phosphatic shales, black, clay-rich shales, and dark gray shales. The deepest community is named the Caneyella, Dunbarella-Ammonoid-Radiolarian Community, and is characterized by Caneyella and Dunbarella bivalves, ammonoid and nautiloid cephalopods, sharks, conodonts, radiolarians and conulariids. The taxa present in this community were pelagic, epipelagic, nektonic and nektobenthic organisms. Benthic organisms are absent from this community due to an anoxic bottom resulting from a widespread O₂ minimum zone. The Caneyella, Dunbarella-Ammonoid-Radiolarian Community is preserved in black, fissile, phosphatic shales, which were deposited at maximum transgression. The next deepest community is the Trepospira, Sinuitina-Ammonoid-Anthraconeilo Community. This community is subdivided into the Sinuitina-Juvenile Ammonoid-Anthraconeilo Subcommunity and the Trepospira-Mature Ammonoid-Anthraconeilo Subcommunity. The Sinuitina-Juvenile Ammonoid-Anthraconeilo Subcommunity is typically found in the black, clay-rich, nonfissile shales, which represent dysaerobic bottom-water conditions. This subcommunity is characterized by fossils in the juvenile to early mature stages of development, and is dominated by ammonoid cephalopods, bactritoid cephalopods, and Sinuitina (bellerophontid) gastropods. The lack of mature forms is thought to be due to fluctuating low-oxygen, bottom-water conditions. The second subcommunity, the Trepospira-Mature Ammonoid-Anthraconeilo Subcommunity, is found in dark-to-medium gray clay-rich, shales and represents a more fully oxygenated bottom-water environment. Mature ammonoids, nautiloids, Trepospira gastropods and Anthraconeilo bivalves dominate this subcommunity, and the majority of the invertebrates is mature or nearly mature. In addition to the size range of the organisms (relatively juvenile vs more mature, larger forms), this subcommunity differs from the Sinuitina-Juvenile Ammonoid-Anthraconeilo Subcommunity in that nautiloid cephalopods are more abundant in the Trepospira-Mature Ammonoid-Anthraconeilo Subcommunity. Also, Sinuitina, which dominates the Sinuitina-Juvenile Ammonoid-Anthraconeilo Subcommunity, is very seldom found in the Trepospira-Mature Ammonoid-Anthraconeilo Subcommunity.

The maximum diversity of benthic and nektobenthic invertebrates at higher taxonomic levels occurs in the Trepospira-Mature Ammonoid, Anthraconeilo Subcommunity. This high diversity diminishes gradually into the black, fissile phosphatic shales containing the Caneyella, Dunbarella-Ammonoid-Radiolarian Community, due to decreasing oxygen levels. This high diversity of the Trepospira-Mature Ammonoid-Anthraconeilo Subcommunity diminishes also into the shales and carbonates containing the moderate-depth stenohaline communities and shallow-water mollusc-dominated communities. In the entire cyclothem, the diversity of pelagic taxa (such as radiolarians) is highest in the dark, phosphatic, organic-rich shales, indicating open marine conditions as opposed to marginal marine environments.

This report represents the first documentation in Pre-Permian strata for the autochthonous occurrence of ammonoid cephalopods in "deeper" water deposits in cyclothemic units in North America. These ammonoid cephalopods occur consistently with the above-mentioned communities (Caneyella, Dunbarella-Ammonoid-Radiolarian Community and Trepospira, Sinuitina-Ammonoid-Anthraconeilo Community) in the dark organic-rich shales. These shales are interpreted as having been deposited in "deeper" water (100 to 200 meters) which was present only at or near maximum transgression. Other ammonoid occurrences in shallower communities are limited to allochthonous, nekroplanktic situations, which are exceedingly rare in Mid-Continent Pennsylvanian cyclothems.

This report supports glacial-eustatic, sea-level control as the primary mechanism for cyclothem formation. It is also suggested that subsidence, tectonics, and delta progradation and abandonment played important secondary roles and in some cases overprinted the glacial-eustatic sea-level fluctuations.