Proposed members of the Chamita Formation, north-central New Mexico
— Daniel J. Koning and Scott B. Aby
New geologic mapping and stratigraphic considerations lead us to favor retaining the term Chamita Formation, albeit with a geographic limitation for two of its five proposed members. West of the Rio Grande in the north-central Española basin, the predominately fluvial strata of the Chamita Formation (upper middle to upper Miocene) overlie cross-stratified, eolian Ojo Caliente Sandstone of the Tesuque Formation (middle Miocene). Here, it is straightforward to map the lower contact of the Chamita Formation. However, the Ojo Caliente Sandstone only extends about 6 km east of the Rio Grande, where it interfingers with Miocene alluvium derived from the Sangre de Cristo Mountains. The coarser, upper part of this alluvium has been subdivided into two interfingering fluvial units named the Cejita and Cuarteles (new name) Members. These two members prograded west of the modern Rio Grande in the late Miocene and comprise most of the type section of the Chamita Formation. However, progressively east of the Rio Grande, towards the Sangre de Cristo Mountains, the base of these two members becomes increasingly more difficult to recognize. Consequently, we include the Cejita and Cuarteles Members in the Chamita Formation west of the modern Rio Grande and in the Tesuque Formation east of the Rio Grande, as is allowed by the Stratigraphic Code.
We also propose the Pilar Mesa, Vallito, and Hernandez Members for the Chamita Formation, including where these units are locally present east of the Rio Grande. The Pilar Mesa Member represents distal to medial alluvial fan deposits derived from the Picuris Mountains, and generally consists of sand with various proportions of gravel dominated by quartzite and Pilar phyllite. Brownish, sandy basin floor strata deposited by a river draining the southern San Luis basin are assigned to the Vallito Member. The Vallito Member is generally composed of very fine to medium sand and silty sand, with minor pebbles whose abundance increases to the north. The Vallito Member has greater than 20% Paleozoic sandstone + granite, whereas the Pilar Mesa Member has less than 20%. The Hernandez Member is a largely volcaniclastic, fluvial deposit that coarsens up-section. We interpret that it was deposited by a river draining the Tusas Mountains and Abiquiu embayment because of its diverse volcanic clast composition (rhyolite, dacite, andesite, and basalt), the presence of subordinate quartzite, and south-southeast-directed paleoflow data. The proportion of quartzite clasts in the Hernandez Member increases upwards, which probably reflects unroofing of older Tertiary volcaniclastic deposits and volcanic flows from the Proterozoic-cored Tusas Mountains. Clast sizes in the Hernandez Member also increase up-section, as does the proportion of coarse channel deposits. As defined above, the Chamita Formation is lithologically distinct from underlying, middle Miocene strata, and serves as a useful lithostratigraphic unit in the north-central Española basin.
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- Koning, Daniel J.; Aby, Scott B., 2005, Proposed members of the Chamita Formation, north-central New Mexico, in: Geology of the Chama Basin, Lucas, Spencer G.; Zeigler, Kate E.; Lueth, Virgil W.; Owen, Donald E., New Mexico Geological Society, Guidebook, 56th Field Conference, pp. 258-278. https://doi.org/10.56577/FFC-56.258