The Lexington Limestone contains unusually high concentrations of phosphate (Cressman, 1973). Phosphate minerals occur in most members of the Lexington Limestone, principally the Tanglewood, Millersburg, and Grier Limestone, and parts of the Lexington were mined in the early 1900's. Although commonly called collophane, the phosphates are actually cryptocrystalline apatite, a calcium phosphate. The phosphates occur within the matrix of the limestones, with concentrations of 10.8 to 2.4 weight percent phosphorus pentoxide (P2O5), usually as coatings and crusts along hardgrounds or as infillings of pore spaces in fossil grains, with only replacement of preexisting calcium carbonate in crinoids (Cressman, 1973).
Phosphate mill near Wallace, Woodford County. Photograph by J.B.Hoeing, 1913.
Many of the phosphate-rich soils in the Bluegrass Region have locally high levels of a radioactive gas, radon, that is known to cause lung cancer. Radon is a daughter product of naturally occurring uranium minerals found in phosphates.
The area of phosphate concentrations is extensive over most of central Kentucky. This region occurs at the apex of the Jessamine Dome, a broad structural upwarping that is part of the Cincinnati Arch. Similar deposits are also known from limestones on the Nashville Dome in Tennessee, which also occurs along the arch (Cressman, 1973).
Phosphatic sands from Woodford County
The fact that phosphates occur on two of the structurally uplifted parts of the Cincinnati Arch suggests that uplift must have played an important role in concentrating the deposits. Because phosphates are most common in deep oceanic waters, we know that the phosphates in the Lexington Limestone formed in deep waters on the flanks of the Jessamine Dome. Upwelling of the deeper water onto the dome caused the phosphate to be transported into the coarser-grained limestones of the Lexington, which are interpreted to have formed as shoals in relatively shallow water (Cressman, 1973).
Phosphates are a natural source of nutrients and fertilizer, which benefits Bluegrass soils and is one of the reasons central Kentucky is a center for the thoroughbred horse industry. These naturally occurring phosphates help build strong bones in mammals.