Upper Eocene and Oligocene volcaniclastic sedimentary stratigraphy of the Quemado-Escondido Mountain area, Catron County, New Mexico

RICHARD M. CHAMBERLIN and JAMES S. HARRIS

New Mexico Geological Society, Guidebook 45, p. 269-275, 1994

Abstract - An upper Eocene to Oligocene (ca. 40-26 Ma) volcaniclastic sedimentary apron derived from the Mogollon-Datil volcanic field can be divided into three mappable lithostratigraphic units in the Quemado-Escondido Mountain area. From oldest to youngest we propose the new names volcaniclastic unit of Largo Creek (VLC), volcaniclastic unit of Cañon del Leon (VCDL), and sandstone of Escondido Mountain (SEM) as informal divisions of the gently south-southeast-dipping Spears Group. A geologic cross section of the area indicates that VLC, VCDL and SEM are approximately 600, 180 and 240 m thick, respectively. Gray, quartz-poor andesitic sandstones and interbedded red mudstones of the main body of the VLC (lower 540m) conformably overlie the middle to upper Eocene Baca Formation. The main body of the VLC represents the distal fluvial equivalent of andesitic debris-flow deposits of the Dog Springs Formation (ca. 39.6-36.9 Ma), which is well exposed in the Datil region southeast of Quemado. An upward coarsening interval (~60 m) of gray to light brown, quartz-poor to moderately quartz-rich, andesitic conglomeratic sandstones forms a local (unmapped) transition zone in the upper VLC. A unique quartz-microcline-rich basement-derived sandstone near the top of the VLC may be correlative with mixed-clast conglomerates (basement and volcanic derived) recently observed in the middle Spears Group in the Alpine No. 1 Federal Well near Alpine, Arizona. A cliff-forming marker bed composed of light gray tuffaceous sandstones and thin pumiceous mudstones as much as 18 m thick defines the base of the VCDL and conformably overlies the upper VLC. Numerous gray tuffaceous sandstone beds intercalated with brownish gray to bluish-green andesitic conglomeratic sandstones comprise the VCDL at its type section in upper Cañon del Leon. Sanidines from pumice in syneruptive tuffaceous sandstones and plagioclase from a rhyolitic ash bed in the VCDL yield ⁴⁰Ar/ ³⁹Ar ages that imply aggradation on a distal alluvial apron from approximately 35.3 to 34.2 Ma. Tuffaceous sandstones of the VCDL thicken to the southwest and appear to form part of the andesitic volcaniclastic apron of the Pueblo Creek Formation, which was shed northward from a late Eocene eruptive center near Reserve. Pale, yellowish-brown, medium-grained, planar to high-angle cross-bedded, volcaniclastic sandstones of the SEM are locally well exposed where they paraconformably overlie VCDL in uppermost Cañon del Leon. These eolian sandstones of the SEM are discontinously exposed on the flanks of several ranges capped by Oligocene basaltic andesites, from Alegres Mountain westward to Escudilla Mountain in eastern Arizona. The SEM appears to represent a southeasterly tapering erg that was blown over the north-facing epiclastic apron of the middle Spears Group (e.g. VCDL) by prevailing westerly winds off the Colorado Plateau in Oligocene time (ca. 32-26 Ma).