The paleoecology of a “middle limestone member” (Leavenworth) of an upper Carboniferous (Stephanian) cyclothem midcontinent, U.S.A.

Abstract

This paper is devoted to the documentation of the lithological and paleontological parameters relative to deposition of the very thin (usually less than. 1 m thickness) Late Carboniferous limestone unit, the Leavenworth Limestone, extensively exposed in the American midcontinent. The Leavenworth Limestone has long been thought of as a classic example of pronounced lateral sediment homogeneity, and this study corroborates this claim. The Leavenworth outcrop belt is exposed from northern Oklahoma, across eastern Kansas, northwestern Missouri, and into southeastern Nebraska. No outcrops of Leavenworth Limestone are present in southwestern Iowa, but the results from two shallow core bores from this area have been incorporated in this study and complete a rock traverse of approximately 500 kms. Field relationships indicate that the Leavenworth Limestone exposures were essentially deposited parallel to what must have been the eastern shoreline of a Late Carboniferous epicontinental sea. Accordingly, it is shown that the Leavenworth was laid down within intermediate water depths on a slowly subsiding carbonate platform. The facies distribution indicates that the majority of Leavenworth locations (25) can be designated as a skeletal wackestone facies, with only four other locations, all restricted to the southern end of the outcrop belt, assigned to an oncolithic facies. This facies distribution pattern reflects increasing southward proximity to a paleo-highland source area. Still farther southward, beyond the basin margin, the Leavenworth Limestone, and all other thin Late Carboniferous limestones, lens-out and merge into the clastic wedge of the Vamoosa Formation of central and northern Oklahoma. Although the Leavenworth Limestone is unrecognizable per se within this clastic wedge, it does extend farther southward than any other Late Carboniferous unit. The Leavenworth Limestone contains remains of all major Late Paleozoic biotic groups, although none of the represented groups can be considered as occurring in great abundance. The foraminifers (agglutinated, silicified, smaller calcareous, and fusulinids) are perhaps the best represented faunal groups and show extensive lateral distribution patterns thought to reflect the overall Leavenworth lateral homogeneity. The algal assemblage is sparse, although it is characterized by consistent fragments of dasyclads and distinct types of algally coated-grains. Miscellaneous microfossils are represented by sponge spicules, holothurian sclerites, scolecodonts, “fish” remains, and conodonts, all of which occur in rather limited abundances and within spotty distribution patterns. Trace fossils occur mainly on the upper and lower bedding plane surfaces of the Leavenworth Limestone, and these consist ofChondrites, Asteriacites, and plowing trails attributed to gastropods. Throughout its thickness the Leavenworth Limestone shows abundant evidence of extensive organism burrowing and homogenization of the sediment. Many burrows are filled with distinctive fecal pellets associated with dispersed scolecodont remains. This association suggests an abundant original Leavenworth infauna composed primarily of marine worms. The megafauna retrieved from the Leavenworth is sparse and is dominated by a few groups of brachiopods and gastropods. It is thought that the delimiting factor responsible for the rather poor megafossil exploitation can be attributed to the dominantly muddy substrate, which most organisms find as a stress environment. The Leavenworth Limestone has always been thought of as the “middle limestone” member of MOORE's megacyclothem and as such was thought to represent deposition in waters that were “just awash”. Recent work by HECKEL, and his re-interpretation of cyclothem deposition in the midcontinental United States, suggests that the middle limestone member (Leavenworth) is the transgressive carbonate in a cyclothem overlain by fissile black shale now regarded as an offshore core shale that was deposited within the deepest waters of the cyclothem. This present interpretation of cyclothem deposition in the Late Carboniferous rocks of the midcontinantal United States seems best to explain the current observations.