Origin of Phosphatic Nodules and Cements in the Upper Cretaceous Frontier Formation, Natrona County, Wyoming

Phosphatic pebbles (carbonate-fluorapatite) are common constituents of lag gravels found along Turonian (Upper Cretaceous) disconformities within the Frontier Formation in Natrona County, Wyoming. These pebbles are of two types: 1) phosphate nodules, and 2) phosphate-cemented sandstone clasts. The phosphate nodules contain organic matter and pyrite and formed in fine-grained sediments in a reducing environment. Organic matter was probably the major source of phosphate, although distributaries may also have supplied some phosphorus. The phosphate-cemented sand­stone clasts contain fine- to coarse-grained sand, fish tooth fragments, and previously formed phosphate nodules. The isopachous rims of phosphate cement do not contain pyrite and are considered to have precipitated in an oxidizing environment very near the water-sediment interface. The lowest stratigraphic occurrence of phosphate-cemented clasts is in the upper Turonian which suggests formation during a transgression. Slow sedimentation associated with the transgression may have caused a relative increase in organic matter within the sediment and allowed phosphate-rich in­terstitial water to diffuse into the water column, precipitating carbonate-fluorapatite in the oxidized sands near the water ­sediment interface. The association of phosphate-cemented clasts with the upper Turonian disconformity suggests that they could serve as a useful stratigraphic marker to help identify the disconformity throughout Natrona County.