New Mexico Geological Society Annual Spring Meeting — Abstracts

Exposed sections in erosional scarps northwest and southwest of Los Lunas volcano and a reflection seismic profile reveal a complex history of sedimentation, deformation, and erosion related to aggradation of the Albuquerque Basin and local tectonics adjacent to the emplacement of two volcanoes within the central basin. Low within the exposed southwest section are andesitic flows with an age of 3.88 ± 0.04 Ma. Northwest exposures reveal an angular unconformity within the basin fill with lowest fluvial beds dipping steeply or vertical. This deformation appears to correlate with emplacement of the volcano to the south. The andesitic flows and beveled dipping beds are overlain by a thick (20 to 40 m) eolian interval, followed by 150 m of fluvial and eolian deposits and pedogenic intervals typical of upper basin fill. Within the fluvial units are glassy obsidian pebbles and pumice debris interpreted to be from the Jemez Mountains. These units are overlain by tephra and basaltic-andesite flows of the 1.219 ± 0.01 Ma Los Lunas volcano. The basin-fill units and tephra are domed around the volcano over several square miles; uplift of the basin fill directly beneath the trachytic dome and flows at the top of the edifice is about 166 m. Dips of the upper deformed basin fill on the flanks of the structural dome range from 5 to 20 degrees. Several faults cut the basin fill and older volcanic units. Other faults cut the younger volcano as well. The tephra and underlying units are deformed into sharp synclines and anticlines and cut by reverse faults in northwestern exposures. Deformation took place after initial tephra deposition, but before tephra was reworked to fill the synclines. More than one tephra layer has draped over eolian dunes farther northwest, indicating intervals of time between tephra falls.

After the eruptions, the domed basin fill was eroded near the volcano and redeposited in a prograding alluvial apron around the flanks of the volcano. The volcano/uplift-derived alluvium is as much as 48 m thick and consists of reworked tephra, blocks of basaltic andesite, alluvial sands and silts, eolian sands, and pedogenic horizons. A buried paleovalley cut in the domed basin fill contains as much as 0.6 m of ash from the 1:1 Ma Bandelier eruption from the Jemez Caldera.

A shallow seismic reflection line running northwestward from near the volcano seems to show the same structural features as those observed at the surface and appears to show the two unconformities and lower steep dip away from the volcano to a depth of about 1,000 m.

Two crude rates of sedimentation may be inferred from the dates. Between 4.88 and 1.22 Ma, 148 m of sediment accumulated, suggesting a rate of 55 m/Ma. Between 1.22 Ma and present, the alluvial apron aggraded as much as 48 m, yielding a rate of 39 m/Ma. Undoubtedly, the eolian sediment took longer to accumulate than the fluvial sands and clays, and the pedogenic intervals indicate episodes of stability in the range of thousands to tens of thousands of years.