Foraminifera as Indicators of Slope Environments in the Northern Gulf of Mexico

Paleobathymetric reconstructions of the northern Gulf of Mexico are based mainly on models derived from distribution patterns of modern benthic foraminiferal taxa. This method of paleoenvironmental analysis is commonly supplemented by information derived from species diversity patterns, planktonic/benthonic ratios, and shell-type ratios. Based on these lines of evidence, environment subdivision of the northern Gulf slope in the 1960s recognized two zones, upper bathyal (200 to 500 m) and lower bathyal (500 to 2,000 m). Later studies defined three bathyal depth zones; upper bathyal (200 to 500 m), middle bathyal (500 to 1,000 m), and lower bathyal (1,000 to 2,000 m). Both qualitative and quantitative analyses indicate that this tripartite pattern is the natural subdivision of the slope that can be recognized in foraminiferal assemblages at both the species and generic level. The movement of exploration onto the slope in the 1980s stimulated attempts to further subdivide the bathyal realm. For example, a recent study defined a model consisting of seven a priori bathyal depth zones and statistically investigated zone membership on the basis of generic assemblages. Another approach involved analysis of the depth distribution of assemblages defined by shape of test rather than taxonomic composition. All of these studies utilizemore » modern models of depth relationships but such models become less precise in increasingly older strata. To address this problem, time-specific paleobathymetric models can be derived for fossil foraminiferal assemblages organized along datum planes.« less