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Facies and Phases of the Altamont Limestone and Megacyclothem (Pennsylvanian), Iowa to Oklahoma

Dalhousie University, Halifax, Nova Scotia

The Altamont Limestone, Marmaton Group of the Desmoinesian Series, is a thin, widespread stratigraphic unit which crops out for more than 460miles along the eastern flank of the Western Interior basin. The formation consists of two carbonate members (older Amoret and younger Worland Limestones) and an intervening shale (Lake Neosho Shale)—the last member characterized by a median black phosphorite. The formation is the marine portion of the Altamont megacyclothem.

The depositional environment has been interpreted after detailed field work, insoluble-residue analysis, shale disaggregation and sieving, and carbonate petrography of thin-sections and acetate peels. Phosphatic nodules were studied in thin sections and slabs and by qualitative chemical tests. Microscopic point-counts of 23 categories of fauna and flora in limestones and shales yielded 7 biofacies.

Tectonic features affecting sedimentation were the Cherokee, Tri-State, and East-Central negative elements, and the Bourbon, West-Central, Lincoln, and Redfield positive elements. Greater turbulence on the south flanks of arches suggests a prevailing south wind. Composition, habit, association, and occurrence of phosphatic nodules conform to present concepts of phosphatic deposition. Maximum marine transgression occurred during phosphate precipitation. Carbonates are shallow-water fades; shales are deep-water facies. Maximum sea-level fluctuation in the area was between 50 and 200 m. A single transgressive-regressive cycle of the strandline produced the Altamont megacyclothem.

Water depth was the controlling factor in vertical and lateral distribution of lithosomes and biosomes. Tectonic features modify only the carbonates. Eustatic change rather than diastrophism appears best to explain the megacyclothem.

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