Iron-rich chemical sedimentary rock, ironstone, occurs as an extensive bed, an iron formation, of Eocene age near Paz de Rio, Colombia. This deposit is fairly typical of iron formations which have formed throughout the past 2,200 m.y. in shallow inland seas rich in sandy, clayey, and oolitic sediment. The iron formation is only a few meters thick and its ironstone is oolitic, chert-poor, aluminous, and phosphatic. Similar Tertiary iron formations occur elsewhere in northwestern South America.

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