The oolitic iron ore in the lower Tertiary Concentracion formation 'records a unique episode of chemical sedimentation' that seems anomalous in this succession of essentially nonmarine detrital sediments. The bed of ore, with its basal sandstone, is 2 to 20 m thick and has been traced for 20 km. Red goethitic ore contains 45 percent Fe, green siderite-chamosite ore 35 to 40 percent Fe.

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