BODY SIZE AS AN INDICATOR OF SEQUENCE STRATIGRAPHIC ARCHITECTURE IN THE EARLY MISSISSIPPIAN (KINDERHOOKIAN) LODGEPOLE FORMATION, MONTANA

The Early Mississippian Lodgepole Formation contains cleary recognizable sedimentary sequences and parasequences deposited along an outer carbonate ramp to basin transition. Transgressive packages are generally represented by crinoidal packstones and grainstones, and flooding surfaces are overlain by thin-bedded, laminated mudstones and wackestones. Additionally, fossils are well-preserved throughout the Lodgepole, providing an opportunity to study the biotic responses to bathymetric change. Specifically, we tested the hypothesis that body size varies predictably with depth.

Crinoid columnal diameters were used as a proxy for body size throughout the formation. Over 1400 round columnals were measured from over 49 independently identified stratigraphic units and through nearly 600’ of the Lodgepole, Madison Range, Montana. Bootstrap analyses of the data indicate that temporal changes in mean and maximum columnal diameters correlate significantly with facies and sequence stratigraphic architecture: the largest columnals are associated with relatively shallow grainstones, and the smallest columnals are associated with relatively deep mudstones.

Because the taxonomic composition of crinoid faunas may vary widely over disparate facies, potentially influencing the observed trends in body size, an additional (~1400) crinoid columnal measurements were made from the elliptical columnals of platycrinitids. Columnal diameters from platycrinitids change significantly in concert with bathymetry as do those of round crinoid columnals.

The result that temporal changes in round and elliptical crinoid columnal diameters mirror patterns of sea level change indicate an ecological (vs. taxonomic) driving mechanism for patterns of body size change among crinoids. Although the specific driving mechanism is unknown, increased predation, higher environmental energy, or greater nutrient availability in more proximal facies may explain the observed pattern of larger size in shallower depths. Finally, the significant relationship between body size and sequence stratigraphic architecture suggests that body size data may be used as a useful tool for stratigraphic correlation.