DUNBAR, N.W., New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, NM, 87801

The Carrizozo lava flows, in south-central New Mexico, represent one of the youngest-appearing volcanic features in the contiguous United States. The two basaltic lava flows display impressive, well-preserved pahoehoe ropes, pressure ridges, and collapsed lava tubes without much soil development or vegetation. These apparently-young lava flows have proven very difficult to date. Cosmogenic dating techniques have been successfully applied to dating of young, basaltic lava flows that have intact surface features, and hence have undergone little erosion. These techniques rely on measurement of cosmogenic nuclides that begin to build up as soon as a rock is exposed to cosmic rays. In the case of an extrusive volcanic rock, buildup of cosmic rays begins when the rock is erupted, so measurement of the ratio of a radiogenic isotope to a stable isotope can provide an estimate of eruption age.

Three samples from the Carrizozo lava flows, two from the upper flow, and one from the lower flow, yield cosmogenic 36Cl ages of 5.3±0.5, 5.4±0.9, and 6.0±0.9 ka, respectively. These ages are considerably older than some early estimates of <1000 to 1500 yrs for the age of the flow, based on visual observations. However, our ages are in good agreement with the estimates of Salyards (1991) based on secular variation magnetostratigraphy. The magnetostratigraphic age depends on correlation to incomplete magnetic field direction curves, and that the magnetic field represented by the Carrizozo samples may reflect a more recent excursion in field direction, and hence a younger eruptive age. However, the correspondence between the 36Cl and magnetic field direction age suggests that the correct correlation was chosen.

Although the 36Cl and magnetostratigraphy ages are dramatically older than early estimates of flow ages based on geomorphic observations, they appear to be consistent with now-known ages for other recent basaltic lava flows within New Mexico. For instance, the McCarty's flow, in the Zuni-Bandera volcanic field has been dated by 14C at around 3200 yrs, in close correspondence to a 36Cl age of 3.9 ±1.2 ka. Although the McCarty's flow and the Carrizozo lava flows are geomorphically similar, upon closer examination, the McCarty's flow appears distinctly younger. This interpretation is based on a greater degree of glassy pahoehoe rope preservation, larger amount of remaining iridescent glass, and more poorly developed vegetation.

There appears to be no significant time break between the lower and upper Carrizozo flows, at least no time break of greater than 1000 yrs. This interpretation is consistent with the geochemically continuous nature of the two lava flows observed by Faris (1980) as well as field observations of no soil development or significant weathering on the top of the lower flow prior to emplacement of the upper flow.