New Mexico Geological Society Annual Spring Meeting — Abstracts

Labrador Oil Company drilled a deep petroleum exploration well, Mescalero No.1, in the Tucamcari Basin, Guadalupe County, east central New Mexico in 1996. This well penetrated of ~2650 m of Precambrian basement consisting of gabbro at the top of the section, followed by a metavolcanic-sedimentary sequence, syenite, granite and another gabbroic body at the bottom of the section. Analysis of these rocks is important for (1) understanding the nature of basement reflectors on seismic lines across the Tucumcari Basin, and (2) constraining the Proterozoic history of east-central New Mexico.

The gabbro is medium to coarse-grained, and consists mainly of plagioclase, pyroxene, and olivine. This rock has a cumulate to sUbophitic texture. Gabbro is the most abundant rock type in the basement section, and characteristically occurs as sill-like bodies. The two sills at depths of 1759-1859 m and 1905-2155 m strong are reflectors on seismic lines in the Tucumcari Basin. A number of small dikes are inferred in the lower part of the well. The gabbro appears to be the youngest rock unit within the basement, since it intrudes the other Precambrian rock units.

The sequence (2155-2664 m) consists of rhyolitic tuff, gneissic metasediment and meta-arkose. Rhyolitic tuff consists of sparsely angular to sub-rounded fragments of quartz, feldspar, and lithic fragments (igneous rocks) supported by a fine-grained groundmass, which is composed by microlites of quartz, carbonate, chlorite, and iron oxide. This unit appears to pass to dark green, fine medium-grained rhyodacite. Gneissic metasediment is characterized by a salt and pepper-sugary appearance with poorly developed bands of light-colored minerals intercalated with bands of medium dark minerals. The meta-arkose is fine to medium grained and pink to orange in color. Quartz, feldspar and fragments of igneous rocks cemented by carbonate and iron oxide characterized this rock. In its lower portion, fragments of underlying are abundant.

The syenite (2664-3740 m) is medium to coarse-grained, pink, light orange to light gray, and is dominated potassium This rock exhibits some variation in composition, passing from syenite to syenite downhole. Near its top, the syenite is strongly altered and exhibits a red-iron coloration, This zone, which shows up as a on lines in the Tucumcari basin, may represent an erosional contact between the arkosic unit and the syenite.

The granite (3740 to 4100 m) has a porphyritic texture comprising large phenocrysts of plagioclase, K-spar and quartz, supported by a matrix composed mostly by quartz, feldspar and mafic minerals, in decreasing order. The age relationship between the granite and the other basement units is not known.