CHRONOLOGY OF LAVA AND ASSOCIATED SOILS NEAR SAN-MIGUEL, NEW-MEXICO

A lava flow in southern New Mexico extends into the Rio Grande Valley from the lower La Mesa geomorphic surface, a relict basin floor of middle Pleistocene age. Three buried soils occur beneath the lava at its east end. In places, the lava itself is buried by a sediment in which a soil has formed. Pedogenic carbonate was used to calculate the age of this soil, using a soil of lower La Mesa as a chronological guide. Both the latter soil and the Rio Grande Valley must be less than about 620,000 yr old according to dated volcanic ash, and are estimated to be about 500,000 yr old. Based upon this age and a total of 1200 kg/m2 pedogenic carbonate in the lower La Mesa soil, the carbonate accumulated at an average rate of about 2.4 kg/m2/1000 yr. Using the same rate of carbonate accumulation, the postlava soil took about 180,000 yr to form. However, not quite all of the pedogenic carbonate in this soil has been accounted for, and a general figure of 200,000 yr is suggested for its approximate age. The geological setting indicates that the age of the lava should be very close to the age of the postlava soil, and a general figure of 200,000 yr is also suggested for approximate age of the lava. This contrasts with a new K Ar age of 490,000 ± 30,000 yr, which is considered to be too old. However, two previously obtained K Ar ages of 232,000 ± 80,000 and 154,000 ± 119,000 yr are much closer to the estimate based on pedogenic carbonate. The landscape of the soil that buries the lava differs from the landscape of the lower La Mesa soil that was used as a chronological guide. The rate of carbonate movement into the soils could also differ, and for this reason the age obtained by pedogenic carbonate is considered tentative. © 1990.