New Mexico Geological Society Annual Spring Meeting — Abstracts

Mount Taylor (3445 m) is a relatively prominent stratovolcano that formed along the Jemez volcanic lineament during Plio-Pleistocene time. Six recently completed 7.5-min geologic maps encompassing the greater Mount Taylor area were completed from 2007 to 2014. These, combined with petrography, 216 chemical analyses, 107 ⁴⁰Ar/³⁹Ar dates, 82 paleomagnetic measurements, and final field checking and photogrammetry were complied into a map that will be published as NMBGMR Geologic Map 80. Eruptive products span basanite to alkali rhyolite compositions but trachybasalt, trachyandesite and trachydacite dominate the upper parts of the volcanic complex. Volcanism is divisible into four phases: 1) development of the volcano floor (3.72–2.90 Ma); 2) initial growth of the cone (2.90–2.75 Ma); 3) final growth of the cone (2.75–2.50 Ma); and 4) terminal mafic volcanism (2.50–1.26 Ma). Most of the volcano formed during the Gauss normal magnetic chron (3.58 to 2.58 Ma) but the last eruptions in the edifice and most of the phase 4 mafic eruptions are magnetically reversed (Matayama chron, 2.58 to 0.78 Ma). Present volume of the stratovolcano is about 25-30 km³, surrounded by an equivalent volume of mafic lavas. Pyroclastic rocks constitute <5% of the volcano, erupted primarily during phases 1 and 2. The edifice is constructed of an eroded central stock, a plexus of radiating dikes, and surrounding domes and flows. The edifice contains a large eastward-facing amphitheater created by erosion during and after phase 3. Volcaniclastic deposits form an apron interlayered with lava flows around much of the volcano but are thickest east and southeast of the amphitheater. No glacial deposits were identified in the volcaniclastic package. More than 15 maar volcanoes formed on flanking mesas near the end of the Pliocene (c.a. 2.5 Ma). Their presence probably indicates a higher water table and wetter climate during the Plio-Pleistocene transition than today. Mount Taylor overlies well-exposed, moderately deformed Jurassic to Cretaceous strata that display classic transgressive-regressive depositional sequences and host the Grants uranium district. Significant coal deposits exist in regional Cretaceous rocks and were mined from the Crevasse Canyon Formation around the western volcano. More than 340 million pounds of U₃O₈ were extracted between 1952 and 2002, primarily from the Westwater Canyon Member of the Jurassic Morrison Formation. The Grants uranium district is possibly the 7th largest in total world production and was once the largest producer in the United States. Our map sections show that depth to ore-bearing strata is about 975 m northwest of the volcano and roughly 1400-1600 m beneath the north flank of Mount Taylor. The 2020 NMGS fall field conference will cover several topics. On Day 1 attendees will examine Mesozoic stratigraphy west and northwest of Mount Taylor and discuss human activities associated with localized uranium extraction. On Day 2 the group will cross the northwest sector of Mount Taylor stratovolcano to look at diverse magmatic products. Day 3 will focus east of the volcano near Laguna Pueblo to present recent geomorphic and geochemical research on uranium transport in and around the Jackpile mine.