VOLCANIC STRATIGRAPHY OF THE NORTHWESTERN SIERRA BLANCA VOLCANIC FIELD, SOUTH-CENTRAL NEW MEXICO

A thick (>750 m) succession of 28-38 Ma alkalic basalt, andesite, and dacite flows, volcaniclastic sediments, and minor welded ash-flow tuffs is preserved on three N-S striking fault blocks on the northwest side of Sierra Blanca. The volcanic rocks on the eastern fault block, which includes Sierra Blanca, are composed of two sequences. The lower sequence contains the following relatively laterally continuous units (bottom to top): interbedded pyroxene-phyric trachybasalt flows, flow breccia, and debris flows, plagioclase-phyric basaltic trachyandesite, and fine-grained trachybasalt to trachydacite interbedded with maroon volcaniclastic sediment. A rheomorphic tuff that thickens northward caps the lower sequence. The upper sequence contains laterally discontinuous volcaniclastic sediment, trachyandesite flows, and trachydacite flows of variable thickness. Volcanic rocks in the middle fault block between Sierra Blanca and the Godfrey Hills are composed exclusively of the pyroxene-phyric trachybasalt and debris flows. The western fault block, including the Godfrey Hills, exposes the younger part of the volcanic succession of the Sierra Blanca volcanic field. The oldest units in the Godfrey Hills are distal equivalents of the pyroxene-phyric and plagioclase-phyric alkalic flows on Sierra Blanca. These older flows are overlain by (1) a discontinuous trachyte, (2) a thick volcaniclastic interval containing a few thin, discontinuous trachyandesite and trachydacite flows capped by a continuous trachyandesite flow, (3) a welded ash flow tuff (Palisades tuff; 28.67± 0.07 Ma), (4) a sparsely porphyritic trachyandesite with zones of breccia (28.59 ± 0.05 Ma), and (5) an upper trachyte. The Palisades tuff thickens to the north. In addition, a thin lithic-rich tuff with biotite and mafic clots (tuff of Bucky Pasture) is preserved in the volcaniclastic interval in the northern Godfrey Hills; this tuff may correlate with a similar thin tuff that crops out just above the rheomorphic tuff on Sierra Blanca. Thickness variations and the degree of welding of the tuffs suggest the presence of undetected vents or small calderas located north to northeast of Sierra Blanca.