The Permian Phosphate Deposits of Western United States Page: 8
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8
the boundary between the miogeosyncline and craton as defined by other criteria.
These areal facies relationships may be illustrated by analysis of strati-
graphic sections at a number of points from west to east across the field. As
shown in the sections in figure 2 and table 1, the Phosphoria formation in
south-central Idaho is about 680 feet thick and consists of about 3 percent
carbonate-fluorapatite, 59 percent chert, and 38 percent carbonaceous mudstone.
This is the typical geosynclinal facies. Calcareous rocks, as well as sandy
layers, are absent altogether. Westward, in eastern Nevada, about. 1,000 feet
of cherty carbonate rocks appear, overlying about 160 feet of mudstone, chert,
minor thickness of rather weakly phosphatic rocks (a few beds of which display
disconformable contacts), and a thin redbed. Eastward from south-central Idaho,
the Phosphoria formation thins to a thickness of aboJ. 300 feet in the vicinity
of the Wyoming-Idaho border as it passes into the platform faciee Phosphatic
rocks increase in thickness to a maximum of the equivalent of 73 feet of
carbonate-fluorapatite in southeastern Idaho, then decrease sharply within
a span of a few miles in western Wyoming. The phosphate also dwindles rather
steadily eastward, and in central Wyoming it disappears altogether (fig. 3).
Cherty mudstones in south-central Idaho give way to massive cherts that make
up about half of the section in southeastern Idaho; these in turn become nodular
and calcareous in western Wyoming, then pass to nodular, cherty limestones,
some of which persist to east-central Wyoming. Carboniecu mudstone, which
makes up about a third of the formation in southeastern Idaho, likewise thins
progressively eastward. Carbonate rocks first appear as discontinuous lenses
or concretions in southeastern Idaho, are freely interbedded with the phos-
phatic beds in western Wyoming, and increase in thickness as phosphatic rocks
decrease to form nearly 50 percent of the section in west-central Wyoming,
then tongue out into redbeds in central and east-central Wyoming. Sandstone,
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McKelvey, V. E.; Swanson, Rowena W. & Sheldon, Richard Porter. The Permian Phosphate Deposits of Western United States, report, October 1952; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc304530/m1/10/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.