CONODONT DISTRIBUTION, PALEOECOLOGY, AND PRELIMINARY BIOSTRATIGRAPHY OF THE UPPER CHEROKEE AND MARMATON GROUPS (UPPER DES MOINESIAN, MIDDLE PENNSYLVANIAN) FROM TWO CORES IN SOUTH-CENTRAL IOWA
Iowa Department of Natural Resources, Geological Survey Bureau,
Technical Information Series 14, 1985, 71 p.
The conodont faunas of six successive upper Desmoinesian cyclothems, studied sequentially in two overlapping cores from south-central Iowa, show a cyclic succession similar to that described by Heckel and Baesemann (1975) from the higher Missourian sequence cropping out in eastern Kansas. Offshore phosphatic shales ranging from black fissile to green clayey facies, show extremely high conodont, abundance (typically thousands/kilogram) and carry mainly Idiognathodus, Neognathodus, Idioprioniodus, and Gondolella (in certain cycles). Nearshore sandy shales are characterized by a variety of sparse faunas (no more than tens/kilogram) usually dominated by Adetognathus or Idiognathodus. Regressive (upper) limestones generally show diminishing-upward conodont abundances (hundreds to tens/kilogram), generally dominated by Idiognathodus, with Neognathodus, Anchignathodus and sometimes Aethotaxis at horizons throughout; Idioprioniodus generally occurs only toward the base, and Adetognathus and Stepanovites occur only toward the top; Diplognathodus dominates the youngest regressive limestone studied (Cooper Creek). Transgressive deposits consist mainly of thin shales and limestones in the cycles studied, and, where thick enough for vertical differentiation of faunas, show upward-increasing conodont abundance with Adetognathus dominance toward the base, Idiognathodus dominance upward, Idioprioniodus mainly toward the top, and Neognathodus, Anchignathodus and Diplognathodus at horizons throughout. "Super" limestones,, which lie above upper limestones and apparently represent minor transgressive-regressive cycles of deposition, show a variety of conodont faunas like those of the upper limestone; these also generally diminish in abundance upward with mixed dominance of Idiognathodus and toward the top, Adetognathus.
The onshore-offshore trends and lithologic preferences exhibit by the conodont genera in these cores allow construction of a model for upper Desmoinesian conodont paleoecology based on principles reviewed by Klapper and Barrick (1978). The four genera found abundantly in the anoxic black shales were pelagic and probably inhabited different parts of the overlying stratified water column. Idiognathodus and Neognathodus, which are also common in most other facies of the sequence, probably inhabited the warm well-oxygenated, surface water layer. Idiopriniodus, which is confined to off-shore parts of all cycles, probably inhabitated slightly cooler, less oxygenated water near the top of the thermocline. Gondolella, which is further confined to the middle parts of certain phosphatic offshore shales, probably inhabited an even deeper, and probably colder and less oxygenated, water layer that did not become involved in the upwelling in every cycle in this area. Adetognathus, which dominates many nearshore deposits, probably inhabited the shoreline water mass of fluctuating conditions brought about by fresh-water influx and attendant turbidity. The rare but similar occurrences of Stepanovites suggest a similar though perhaps more restricted habitat. Near confinement of Anchignathodus and Aethotaxis to carbonate lithotopes indicates a warm clear-water habitat, which was possibly benthic in view of their apparent absence in similar water at the top of the stratified water column over the black shales. The scattered distribution of Diplognathodus makes it the most difficult genus to interpret, but an intermediate position between the nearshore fluctuating water mass and the offshore stratified succession of water masses is suggested.
Biostratigraphic trends are apparent in the abundant offshore phosphatic shale faunas upward through the six cyclothems. The Oakley Shale of the Verdigris cycle is characterized by subequal dominance of Idiognathodus spp. 1 and 2, and occurrence of Gondolella sp. 1. The Excello Shale of the lower Fort Scott cycle is characterized by dominance of I. sp. 1, and G. sp. 2 (in the middle) with occurrence of I. spp. 2, 3. The Little Osage Shale of the upper Fort Scott cycle is characterized by dominance of I. sp. 1, with occurrence of I. spp. 2, 3, and 4, and lack of Gondolella. The fauna of the Anna Shale of the Pawnee cycle closely resembles that of the Little Osage. The Lake Neosho Shale of the Altamont cycle is the only shale in which Neognathodus predominates over Idiognathodus; furthermore, only I. sp. 5 occurs in the lower and middle part, whereas only I. sp. 1 occurs in the upper part, and Gondolella is absent throughout. The thin shale below the Cooper Creek Limestone (Lost Branch Formation) is characterized by dominance of I. sp. 6 over I. sp. 1, occurrence of G. sp. 3 and G.denuda, and the youngest abundant occurrence of Neognathodus. Diplognathodus coloradoensis was found only in the Verigris cycle but has been reported elsewhere from the lower Fort Scott cycle. D. iowensis n. sp. ranges from the lower Fort Scott to the Cooper Creek. D. n. sp. 2 ranges from the Pawnee through the Cooper Creek, and D. illinoisensis was found only in the Cooper Creek. The possibility of correlating cycles throughout the Midcontinent by means of these distinctions in conodont faunas of the offshore shales is strongly suggested by the discovery of similar distinctive faunas in previously correlated horizons in Kansas and Illinois.