Environmental Interpretation of Conodont Distribution in Upper Pennsylvanian (Missourian) Megacyclothems in Eastern Kansas

Abstract

The cyclic pattern of conodont distribution in the lower Upper Pennsylvanian (Missourian) sequence of eastern Kansas reflects with only minor adjustments the pattern of megacyclothems recognized by Moore and supplemented by Weller. Each megacyclothem consists basically of an ascending sequence of: thick, locally nonmarine (outside) shale--thin middle limestone member--thin black shale member--thick upper limestone member--outside shale. Conodont distribution (chiefly multielement) ranges from (1) low abundance and diversity, commonly with Adetognathus dominance, in the outside shales and tops of upper limestone members, through (2) increasing Ozarkodina minuta and Idiognathodus dominance in the adjacent parts of both limestone members, to (3) maximum abundance and diver ity with Idiognathodus dominance and exclusive Idioprioniodus lexingtonensis, Gondolella spp., and an unassigned O1 element in the core of the megacyclothem, which consists of the black shale member and commonly includes the adjacent parts of the two limestone members. Interpretation of depositional environments indicates that a transgressive-regressive sequence with deepest water during deposition of the black shale member was responsible for each megacyclothem. This interpretation is compatible with the Seddon and Sweet pelagic depth-zone model of conodont distribution in which diversity in the sediment increases in an additive fashion with increasing depth. In this model, the deeper water conodonts are found only in the deep-water facies but may be associated there with all the shallower water conodonts, whereas the shallow-water conodonts alone characterize the shallow-water nearshore facies. The progression of conodont depth-zone tops for the Missourian sequence in eastern Kansas is, from shallowest to deepest water: Adetognathus-Ozarkodina min ta (multielement, with a spathognathodontan element)-Aethotaxis-Idiognathodus-Idioprioniodus-Gondolella- unassigned O1 element. This specific paleoecologic model may be applied with certain cautions to environmental interpretation and perhaps to correlation of other Upper Pennsylvanian sequences in the Mid-Continent and probably elsewhere as well.