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The Baca Formation, which crops out discontinuously in east-west-trending exposures from Socorro, New Mexico, to the New Mexico-Arizona border, is composed of fluvial, alluvial-fan, and lacustrine deposits. It was deposited in a basin which formed during the Laramide orogeny in New Mexico and Arizona. Uplifts bordering the basin include the Defiance, Zuni, Lucero, and Sierra-Sandia, and the Mogollon highland. In the late Eocene, volcanism began in the Datil-Mogollon volcanic field in southwestern New Mexico and adjacent Arizona, south of the study area. Sediments were derived from adjacent uplifts and from the volcanic rocks. The Mogollon highland and the Zuni uplift supplied metamorphic, sedimentary, and plutonic detritus to the Baca Formation.
There is a westward increase in volcanic detritus in the Baca Formation, and faunal evidence shows an early Oligocene, Chadronian, age for western Baca outcrops north of Quemado, younger than outcrops further east which are Eocene in age. With the beginning of Eocene volcanism, volcanic detritus was supplied to the Oligocene Baca sediments which were deposited in a coarse-grained meander belt. Other depositional environments identified include fine-grained meander belt, alluvial fan, and distal, braided alluvial plain. Flow direction was dominantly eastward.
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