Late Ordovician–Early Silurian δ13C chemostratigraphy in the Upper Mississippi Valley: implications for chronostratigraphy and depositional interpretations

ABSTRACTA pioneer study of the previously unknown δ13C chemostratigraphy in the Ordovician/Silurian boundary interval in eastern Iowa and northeastern Illinois resulted in the discovery of the Hirnantian Isotope Carbon Excursion (HICE). The presence of this major isotope excursion in the Mosalem Formation in Iowa and the Wilhelmi Formation in Illinois, which indicates that the excursion interval in these units is of Hirnantian (latest Ordovician) rather than Early Silurian age, necessitates a revised chronostratigraphic classification of these units. Although the precise level of the Ordovician/Silurian boundary remains somewhat uncertain in the absence of the diagnostic graptolites, it is herein placed in the upper part, but well below the top, of the Mosalem Formation and at the top of the Wilhelmi Formation. During a major regression following the deposition of the Maquoketa Shale, the upper part of the latter clastic unit was in some places deeply eroded, resulting in a topographically dissected landscape with upland areas separated by wide incised valleys. During a subsequent late Hirnantian transgression, these palaeovalleys were gradually filled with marine sediments, but the upland areas were not transgressed until earliest Silurian times. The new chemostratigraphical evidence is in good agreement with the available biostratigraphical data, especially from corals, conodonts, and brachiopods. A preliminary chemostratigraphical study of the presumably coeval Edgewood Group successions in Pike County, northeastern Missouri failed to document any heavy δ13C values characteristic of the HICE and some, or all, of the Hirnantian values obtained there may be diagenetically overprinted.

Stratigraphic and Paleogeographic Significance of An Upper Ordovician Acritarch Flora from the Maquoketa Shale, Northeastern Missouri, U.S.A.

A well-preserved and moderately diverse acritarch assemblage was recovered from the upper Ordovician Maquoketa Shale of northeastern Missouri. Based on graptolite and conodont evidence, the Maquoketa in northeastern Missouri is considered Richmondian (=Ashgill) in age.The acritarch assemblage comprises 28 species, distributed among 17 genera including two new genera,CaelatosphaeraandStictosoma.There are 11 new species,Baltisphaeridium adiastaltum, Caelatosphaera verminosa(type species),Dorsennidium undosum, Elektoriskos aktinotos, Lophosphaeridium acinatum, L. varum, Micrhystridium hirticulum, M. prolixum, Peteinosphaeridium accinctulum, P. septuosum, andStictosoma gemmata(type species); together with two species similar to previously named species and three species left in open nomenclature. Cosmopolitan and stratigraphically important species identified includeBaltisphaeridium perclarumLoeblich and Tappan, 1978;Cheleutochroasp. cf.C. diaphorosaTurner, 1984;Dorsennidium hamii(Loeblich, 1970) Sarjeant and Stancliffe, 1994;Excultibrachium concinnumLoeblich and Tappan, 1978;Orthosphaeridium insculptumLoeblich, 1970;O. rectangulare(Eisenack, 1963) Eisenack, 1968, andVillosacapsula setosapellicula(Loeblich, 1970) Loeblich and Tappan, 1976.Polygonium gracileVavrdová, 1966 emend. Sarjeant and Stancliffe, 1994 andMicrhystridium hirticulumnew species dominate the assemblage, followed byDorsennidium undosumnew species,Baltisphaeridium oligopsakiumLoeblich and Tappan, 1978,Veryhachiumsp. cf.V. oklahomenseLoeblich, 1970,Peteinosphaeridium accinctulumnew species, andLophosphaeridium acinatumnew species. Additionally, the palynoflora contains chitinozoans, scolecodonts, cryptospores, and the enigmatic palynomorphGloeocapsomorpha priscaZalessky, 1917 emend. Foster, Reed, and Wicander, 1989.Paleontologic-palynologic and sedimentologic evidence indicates that the Maquoketa Shale at the two studied localities accumulated under low energy, somewhat offshore, and unrestricted marine conditions. The present Maquoketa Shale acritarch palynoflora shows greatest similarity to those of the Sylvan Shale (Ashgill; Richmondian) of Oklahoma and the Maquoketa Shale (Caradoc and Ashgill) of Kansas. Whereas there are some cosmopolitan acritarch species present in the Maquoketa Shale, its palynoflora shows no pronounced similarity with age-equivalent acritarch suites from outside of North America.