THE MEADE PEAK MEMBER OF THE PHOSPHORIA FORMATION: TEMPORAL AND SPATIAL VARIATIONS IN SEDIMENT GEOCHEMISTRY

The Permian Phosphoria Formation in the northwest United Sates is a marine sedimentary deposit of significant economic and scientific importance because of its widespread phosphorite deposits. Detailed geochemical profiles through the basal Meade Peak Member were examined in six sections with respect to variations in the concentrations of terrigenous- and marine-derived elements. The six sections transect the phosphate depocenter, extending from south-central Idaho to western Wyoming.

Inter-element relationships in the detrital sediment component appear useful for chemo-stratigraphic correlation. A step-like decrease in K:Al ratios, persisting across lithologic changes, occurs in all but the most northeasterly section, wherein the offset is evident only in minimum values. The larger and lower-most offset corresponds closely to the Leonardian – Guadalupian Series boundary coincident with a change from major low-stand to transgressive conditions (Ross and Ross, 1995). The decrease may thus be related to transgression of the Phosphoria Sea upon the Wyoming Shelf. Given that the detrital fraction is mostly wind-transported material, such a reduction in the K:Al values could be related to changes in paleo-atmospheric circulation or simply flooding of source areas and increased transport distances. Intermittent maxima in Fe:Al ratios correlate well between the easternmost sections. These maxima are interpreted as resulting from an increase in the amount of Fe-rich material transported from the Goose Egg Basin in eastern Wyoming. The Fe-rich horizons may represent temporary shifts from northeasterly to more easterly wind patterns at the margins of the Late Permian trade-wind belt.

Inter-element relationships in the marine fraction imply that the bottom waters of the Phosphoria Basin were dominantly denitrifying, although temporary sulfate-reducing conditions may have occurred intermittently. A decrease in Cd, Cu, Mo, Zn and V and an increase in REE concentrations near the center of the most northeasterly and shoreward section, apparently unrelated to lithologic changes, may represent a localized shift to higher redox conditions. Such a change could be the result of shoaling.