New Mexico Geological Society Annual Spring Meeting — Abstracts

Enigmatic structures in the Bearwallow Mountain Andesite, or Bearwallow Mountain Formation, are here interpreted to be flow features. The Bearwallow Mountain Andesite is a voluminous assemblage of andesitic to basaltic lava flows that covers large areas of the Mogollon-Datil volcanic field of southwestern New Mexico. Flows were erupted from numerous poIygenetic, and perhaps some monogenetic, centers best described as shield volcanoes. Many literature references to them as stratovolcanoes seem inappropriate because oftheir composition, low, domal profiles, and lack ofappreciable interlayered pyroclastic material. The Bearwallow volcanoes erupted about 25 million years ago, and many are aligned along predominant northeast-southwest and westnorthwest-eastsoutheast trends believed to reflect early crustal extension that preceded Basin and Range deformation in this area.

The spheroidal and ellipsoidal structures are somewhat uncommon, but have been seen at widely separated localities, in lavas from different eruptive centers. They may be most easily observed in road cuts along the National Forest road leading south and east from Reserve to Negrito Creek, and in cliffs along the Tularosa River about two miles above its confluence with the San Francisco River, in Catron County.

The structures are most conspicuous where they are spheroidal or elliptical in form, but at some places, they appear to grade into ramp structures, as commonly found at flow fronts, or where flows traverse uneven topography. Where spheroidal in form, they are commonly 1-5 meters in diameter, but the long axes of more ellipsoidal structures may be 10 meters long.

Initially, these spheroidal and elliptical features might bring to mind filled lava tubes, but the presence of circumferential, stretched vesicles would seem to rule out that idea. Alternative explanations, gleaned from a summary search ofthe literature, include: accretionary lava balls; lava channel fillings,; lava squeeze-ups; lava coils; and structures formed by interaction oflava and water. However, none ofthese seem to quite fit the ellipsoidal and spheroidal structures in the Bearwallow Mountain Andesite. Then, I came across Kenneth Hamblin's excellent descriptions of lava flows and lava dams in the Grand Canyon in GSA Memoir 183. Dr. Hamblin describes ellipsoidal structures in Flows E and F of the Toroweap lava dam, and his sketch of these features, in his figure 29, closely resembles those in the Bearwallow Mountain Andesite at the Tularosa River locality. However, the features in the Toroweap basalt flows are associated with other structures that Hamblin identified as basalt pillows, some ofwhich are 10-20 meters across, and this association led him to attribute the ellipsoidal structures, as well as the giant pillows, to interaction with water, which is supported by the position of the Toroweap Flows relative to the Colorado River, past and present. Where the similar structures in the Bearwallow Mountain Andesite are found, there are no obvious relationships with major pre-existing water courses, but further consideration ofthis possibility may be warranted.

I conclude that the spheroidal and ellipsoidal structures in the Bearwallow Mountain Andesite probably are flow structures because of the encircling, stretched vesicles, and because they appear to grade into more common flow structures such as ramped flow layers. In keeping with the interpretation as flow features, I offer the perhaps outrageous hypothesis that they might be akin to standing waves in water, but because of the much greater viscosity ofandesitic and basaltic lava, they are essentially standing waves frozen in space.

Finally, I hope others will find an opportunity to see these intresting flow(?) structures in the Bearwallow Mountain Andesite, and look for new evidence to confirm their origin.