Abstract

The Eocene(?) El Rito Formation in north-central New Mexico is the basal Tertiary unit of the Chama basin and the northwestern Española basin. Deposition of the El Rito Formation occurred in the final stages of the Laramide orogeny. The formation is predominantly an alluvial-fan deposit, showing two principal facies: quartzite conglomerate and quartzose sandstone. Postdepositional pore filling by calcite, zeolite, and hematite, alteration of feldspars, and removal of heavy minerals are observed in the sediments of the El Rito Formation. The extensive diagenetic effects are interpreted as resulting from diagenesis under semi-arid to arid conditions. Paleocurrent directions determined from imbricated clasts, cross-bedding, and parting lineations indicate that the primary sources of the El Rito Formation were in the north and northeast.

The clasts of the early, conglomeratic facies were derived from the Precambrian crystalline highlands of the Brazos-Sangre de Cristo uplift. The conglomerates represent debris-flow, sieve, and high-energy braided-stream deposits. Point counts of thin sections of four sandstone samples show high QFL% quartz and QpLvmLsm% metamorphic lithic fragments. These results also indicate that the provenance was the Precambrian metamorphic rocks of the Brazos uplift. The sandstone units represent sheet-flow sediments deposited from lower-energy, braided streams with more easterly sources than the sources for the conglomerates. Only sandstones in the western portion of the El Rito depositional basin may have had significant sources in the Nacimiento uplift, the Gallina-Archuleta arch, or the southern San Juan uplift at the northwest.

The El Rito Formation was deposited in a south-southeast-trending, asymmetrical basin with through-flowing drainage in the southern Rocky Mountains east of the Colorado Plateau. The basin formed in response to compression associated with the north-northeast jamming of the Colorado Plateau into the stable craton during the Laramide orogeny. The El Rito depositional basin probably was continuous with the Galisto basin before northeast-trending normal faulting of the Rio Grande rift separated the two basins in the Miocene to Holocene.

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