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The Eocene El Rito Formation of north-central New Mexico is the basal Tertiary unit of the Chama basin and the north-western Espanola basin. Deposition of the El Rito occurred in the final stages of the Laramide orogeny. The formation is an alluvial-fan deposit, showing two principal facies, quartzite conglomerate and quartzose sandstone. Paleocurrent directions determined from imbricated clasts, cross-bedding, and parting lineations indicate that the primary sources for the sediment were in the north and northeast.
The clasts of the early, conglomeratic facies were derived from the reworking of sediments eroded from the Precambrian crystalline highlands of northern New Mexico early in the Laramide orogeny. The conglomerates represent debris-flow, sieve and high-energy, braided-stream gravel deposits. The immature, sand-sized particles were derived from proximal sources in the Brazos uplift during the final stages of the Laramide orogeny. The sandstone units represent sheet-flow sediments deposited from lower energy, braided streams with more easterly sources than the sources for the conglomerates.
Point counts of thin sections of four sandstone samples show high QFL% quartz and QpLvmLsm% metamorphic lithics. These results are consistent with a provenance in the Precambrian metamorphic core of the Brazos uplift. Extensive diagenetic effects typical of Cenozoic desert alluvium include calcite, zeolite, and hematite pore-filling and replacement of grains, alteration of feldspars, and removal of heavy minerals.
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