Huelster Formation. [Unclear if part of Vieja Group (Correlation of Map Units).] Mapped in the Davis Mountains area, Jeff Davis County, West Texas. Mostly tuff, thin layers of sandstone and conglomerate, lenses of fresh-water limestone, and trachydoleritic lava; forms landslide terrain of hummocky, grass-covered hills, includes displaced blocks of overlying flow-rock units. Thickness as much as 490 feet in Madera Canyon. Jeff Conglomerate Member occurs locally at base; not separately mapped. Occurs below Gomez Tuff. Age is Oligocene. [Author states that correlation of volcanic rocks from area to area on Marfa sheet is uncertain; little attempt is made to correlate between areas. The rocks are described by area.]

Source: Modified from GNU records (USGS DDS-6; Denver GNULEX).

Pg. 13. Huelster Formation. Trachyandesite clast from conglomerate [position within formation not stated] in Little Aguja Mountain quadrangle, Jeff Davis County, southwestern Texas, yielded a K-Ar age of 39.3 +/-0.9 Ma (plagioclase). Data from D.F. Parker and F.W. McDowell, 1979 (GSA Bull., v. 90, p. 1100-1110); age recalculated using decay constants of Steiger and Jager, 1977 (Earth Planet. Sci. Letters, v. 36, p. 359-362).
[Age considered Eocene; Eocene-Oligocene boundary 36.6 (38-34) Ma (from Geologic Names Committee, USGS, 1983 ed. geol. time scale, with additions from N.J. Snelling, 1985, The Geol. Soc. Mem., no. 10).]

Source: Publication.

Is the oldest unit of Eocene or Oligocene age present in the Davis and Barilla Mountains, Jeff Davis County, Texas, Permian basin. Overlain by Star Mountain Formation, or where Star Mountain is absent, by the Gomez Tuff. Divisible into [not a sequence] Jeff Conglomerate Member (at base), Hunsaker Member (present in northern Davis Mountains), Reynolds Member (Adobe and Madera Canyons in Davis Mountains), mafic lava at several horizons, and limestone lenses at many horizons. Age is Eocene or Oligocene. Report includes cross section, geologic map, chemical analyses.

Source: Modified from GNU records (USGS DDS-6; Denver GNULEX).

Huelster Formation (Map unit Th, and also included in Ql). Mostly tuff, thin layers of sandstone and conglomerate, lenses of nonmarine limestone, and mafic lava. Includes Jeff Conglomerate Member at base; not separately mapped. Most of Huelster is in landslide terrain included under symbol Ql. Thickness at type locality 400 feet, up to 490 feet in Madera Canyon. K-Ar age, <38.4 +/- 0.7 m.y. (age from cobble in conglomerate near base).

Source: Publication.