New Mexico Geological Society Annual Spring Meeting — Abstracts

A thick succession of trachyandesite, trachydacite, volcaniclastic sediments, and minor welded ash-flow tuffs is preserved on three N-S striking fault blocks on the west side of Sierra Blanca. The volcanic rocks on the eastern fault block, which includes Sierra Blanca, are composed of a basal thick (>250 m) sequence of pyroxene-phyric trachyandesite breccia and debris flow deposits overlain by a stack of plagioclase-phyric trachyandesite lava flows that are variably fine-grained to crystal-rich. The trachyandesite flows, with published ⁴⁰Ar/³⁹Ar ages of 29.3-37.3 Ma, are among the oldest volcanic rocks in the area. Volcaniclastic sediments are not common in the stacked lava flow section, but are preserved in paleo-channels cut in the upper part of the plagioclase-phyric trachyandesite. The volcanic rocks in middle fault block between Sierra Blanca and the Godfrey Hills are primarily the pyroxene-phyric trachyandesite breccia.

The western fault block, including the Godfrey Hills, exposes the younger part of the volcanic succession of the Sierra Blanca volcanic field. The oldest flows, found at low elevation on the south and west side of the Godfrey Hills, are dark-colored, porphyritic trachyandesite similar to the plagioclase-phyric trachyandesite on Sierra Blanca. Volcaniclastic channel fills are locally present between these flows. The dark-colored porphyritic trachyandesite flows are thinner and the volcaniclastic sedimentary rocks are thicker toward the north end of the hills. The darkcolored flows are overlain by (1) a discontinuous fine-grained, flow-banded, 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± 0.05 Ma), (4) a sparsely porphyritic trachyandesite with zones of breccia (28.59 ± 0.07 Ma), and (5) an upper fine-grained trachyte. The Palisades tuff is thicker at the north end of the Godfrey Hills. In addition, a thin tuff (tuff of Bucky Pasture) is preserved in the volcaniclastic interval in the northern Godfrey Hills. Thickness variations and the degree of welding of the tuffs suggest the presence of an undetected local caldera northeast of the Godfrey Hills.