Phosphatic pebbles are widespread in basal units of the Marquette Range Supergroup (Precambrian X) in the central part of the Upper Peninsula of Michigan. At one locality, a conglomeratic bed about 15 m thick averages about 15 percent P2O5, and many thinner beds are of comparable grade. This occurrence is believed to be the richest sedimentary phosphate deposit known in the Precambrian of the United States, as well as one of the oldest. Although outcrops are generally scarce, similar material has been found at four other localities in the Upper Peninsula of Michigan.
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USGS Numbered Series
Phosphorite and other apatite-bearing sedimentary rocks in the Precambrian of northern Michigan
Dept. of the Interior, Geological Survey : distributed by Branch of Distribution, U.S. Geological Survey,