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ABSTRACT: Tertiary rocks more than 5,000 feet thick outcrop in central South Park, Colorado; they include continental sediments and volcanics. These rocks have been previously divided into several formations, from youngest to oldest: Denver Formation (Eocene), and lower andesite of the Thirtynine Mile volcanic field, Agate Creek Formation (ash flow-4), and Antero Formation (all of Oligocene age). A newly-recognized unit, the Hatsel Formation, is probably of Pliocene and Pleistocene age. Major unconformities separate Oligocene rocks from older and younger beds. The formations of Oligocene age are separated from each other by minor unconformities. The Denver Formation and older rocks were deformed during the Laramide orogeny. The Oligocene rocks were mildly folded and faulted by later Tertiary deformation in late Oligocene to Miocene time. The main structural features of the area are the Antero and High Creek synclines, and the Santa Maria fault (new name).
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