Four solitary rugosan assemblages are recognized within the uppermost Ordovician-lowermost Silurian sequence in the east-central United States: (1) Late Ordovician “epicontinental” assemblage (Richmondian); (2) Late Ordovician “continental margin” assemblage (Gamachian); (3) Edgewood assemblage (Gamachian-early Early Llandovery); and (4) silurian assemblage (post- Edgewood Llandovery). A Late Ordovician ‘epicontinental” Assemblage is present in the upper Maquoketa Group (Richmondian). Salvadorea randi (Elias, 1981) occurs in Southern Illionois, northwestern Illinois, and eastern Iowa. Grewingkia canadensis (Billings, 1862) has been identified in eastern Wisconsin. These species represent the Red River-Stony Mountain and Richmond solitary rugose coral provinces, respectively. They became extinct when the epicontinental sea withdrew at the end of Richmondian time, due to a major glacio-eustatic sea-level drop. A late Ordovician “continental margin” assemblage is represented by Rhegmaphyllum sp. In the Cason oolite (Gamachian) of eastern north-central Arkansas.

This study is focused on the Edgewood assemblage, situated stratigraphically above the Late Ordovician “epicontinental” assemblage and geographically lateral to the “continental margin” assemblage. The Keel Formation of south-central Oklahoma contains Streptelasma subregulare (Savage, 1913b), Streptelasma amsdeni, n. sp., Streptelasma leemonense Elias, 1982a, Streptelasma sp. cf. S. leemonense Elias, 1982a, Greengkia sp. A, and Keelophyllum oklahomense, n. gen., n. sp. Streptelasma sp. cf. S. subregulare (Savage, 1913b) and S. leemonense occur in the Cason shale of wester north-central Arkansas. In southernIllionois and southeastern Missouri, species within the Leemon Formation are S. subregulare, S. leemonense, and Bodophyllum shorti Elias, 1982a, Streptelasma sp. A is present in the Noix Limestone of northeastern Missouri. The overlying Bryant Knob Formation yields S. subregulare from the unnamed member, and S. subregulare, S. leemanense, and Grewingkia sp. A form the Kissenger Limestone Member. The Cyrene Formation, which is the lateral equivalent of the Noix and Bryant Knob, contains S. subregulare in the middle part. Strptelasma subregular is present in the Schweizer and Birds members of the Wilhelmi Formation in northeastern Illinois, and has been identified from the middle portion of the Mosalem Formations in northwestern Illinois. These taxa comprise the Edgewood Solitary Rugose Coral Province. The overall assemblage, in which 97.8 percent of specimens belong to S. subregulare, S amsdeni, and S. leemonense, seems to be most similar to that in the Dalmanitina Beds (Hirnantian) or possibly earliest Llandovery beds of Östergotland, Sweden, and the Guanyinqiao Beds (Dalmanitina Beds; Hirnantian) of Guizhou Province, China.

The Keel Formation, Cason shale, Leemon Formation, Noix Limestone, and lower to middle Cyrene Formation are considered to be late Ordovician (Gamachian) in age. The lower Schweizer Member of the wilhelmi Formation and lower Mosalem Formation may also be Gamachian. The upper Schweizer Member and the Birds Member of the Wilhelmi Formation, and the middle Mosalem Formation are Early Silurian (early Early Llandovery). The Bryant Knob Formation and upper Cyrene Formation may also be early Early Llandovery. Thus, the Edgewood assemblage spans the time interval from Gamachian to early Early Llandovery, and solitary Rugosa cannot be used to delineate the Ordovician-Silurian boundary in the east-central United States. Gamachian units in the southern portion of the Edgewood Province mark the regressive phase corresponding to the glacial maximum, but could have been deposited during minor transgressions if sea level regressive phase corresponding to the glacial maximum, but could have been deposited during minor transgressions if sea level fluctuated during that time interval. The Gamachian(?) to Early Llandovery sediments of northern Illinois were deposited during the major latest Gamachian-Early Llandovery transgression associates with deglaciation. The Edgewood solitary Rugosa were not derived from corals of the late Ordovician “epicontinental” assemblage. Their resemblance to some taxa previously restricted to the continental margin suggests that they originate from such forms.

Stratigraphically above the Edgewood is an assemblage characterized by genera typical of the Early to Middle Silurian. Dinophyllum sp., Dalmanophyllum sp., Cyathactis? Sp., and Rhegmaphyllum sp. are found in the Bowling Green Dolomite (late Early Llandover) of northeastern Missouri, the Elwood Formation (late Early to Middle Llandovery) of northeastern Illinois, and the upper Mosalem Formation (late Early Llandovery) in northwestern Illinois. Cyathactis? sp. is present in the Cochrane Formation (Llandovery) of south- central Oklahoma, and Dalmanophyllum sp. occurs in the Sexton Creek Limestone (Llandovery) of southeastern Missouri. These corals were not derived from Edgewood taxa, and must have been introduced from elsewhere. Rhegmaphyllum Wedekind, 1927, was confined to areas near the North American continental margin in the Richmondian and Gamachian. As water depth and temperature increased during the Early Llandovery transgression related to deglaciation, the Silurian Assemblage succeeded the Edgewood assemblage, possible after a minors regressive event.  

R.J. McAuley, R.J. Elias

Pages: 82, 14 pls.

Year published: 1990