PLANKTIC FORAMINIFERAL CYCLES IN THE NIOBRARA FORMATION: PRELIMINARY RESULTS AND INTERPRETATIONS

The cyclic bedding of the Late Cretaceous Niobrara Formation, has long been recognized within the Western Interior of the United States. Joining the many who strive to understand the forcing mechanisms behind the cyclic deposition, we examined the porosity of the planktic foraminifer Hedbergella delrioensis. Drawing from the work of others who demonstrated in the laboratory that many species of modern foraminifera respond to warmer temperatures and lower salinities by making more porous shells, we have investigated the porosity of samples from the Smoky Hill Member of the Niobrara Formation at its type locality in Kansas. Currently, our Kansas data is concentrated in rocks that are the stratigraphic equivalents of the lower shale unit, lower limestone unit, and middle shale unit of Pueblo, Colorado. The data indicate that Hedbergella delrioensis did respond to the forcing mechanism(s) that produced lithologic cyclicity. Mean sample porosity is lower in the lower shale unit equivalent, nearly doubles at the base of the lower limestone unit equivalent, declines upward through the lower limestone unit equivalent, and then in the middle shale unit equivalent returns to mean values slightly higher than those of the lower shale unit equivalent. The porosity distribution indicates that the upper water column was significantly warmer and perhaps somewhat less saline during the deposition of the lower two-thirds of the lower limestone equivalent.

The type section stratigraphic equivalents of the lower shale unit, upper portion of the lower limestone unit, and middle shale unit have Hedbergella delrioensis mean sample porosities similar to those of the Cenomanian-Turonian aged Greenhorn Formation of the Western Interior, with most mean porosities ranging from 11 to 18%. Porosities ranging between 11-18% fall within the Bé (1968) modern foraminiferal porosity based subtropical ocean climate province. Mean sample porosities from the lower two-thirds of the stratigraphic equivalent of the lower limestone unit are distinct from the Greenhorn Formation in that they range from 19-27%, placing them in the Bé (1968) Tropical ocean climate province.