New Mexico Geological Society Annual Spring Meeting — Abstracts

This study compares deposits of the Cerro Toledo interval, the 360,000 year interval between the major eruptions of the Bandelier Tuff, from two deeply incised canyons located 12 km apart on the Pajarito Plateau. New ⁴⁰Ar/³⁹Ar ages and geochemical data are used to compare air-fall pumice deposits in Alamo Canyon with previously studied deposits in Pueblo Canyon to the north. Geochemical correlation of major and trace elements is performed using bivariate plots, calculation of a similarity coefficient, statistical distance, and hierarchical cluster diagrams.

The deposit in Pueblo Canyon consists of six tephras intercalated with volcaniclastic sandstone and conglomerate. Previously published ⁴⁰Ar/³⁹Ar ages for the Pueblo Canyon tephras range from 1.65 Ma to 1.25 Ma. The deposit in Alamo Canyon is composed of a basal fluvial sandy conglomerate with several thin tephras near the top of the unit, an ignimbrite containing obsidian breccia derived from collapse of a Rabbit Mountain Rhyolite dome, and an upper fluvial sandy conglomerate with tephra. The tephras from the basal conglomerate and the ignimbrite contain abundant xenocrystic sanidine derived from the underlying Otowi Member of the Bandelier Tuff.

The Rabbit Mountain Rhyolite contains sparse sanidine, so this component is poorly represented in the dated sanidine population. A tephra in the upper conglomerate yielded a 1.42 ± 0.03Ma ⁴⁰Ar/³⁹Ar sanidine age. The lack of abundant primary sanidine in the Alamo Canyon deposit favors the use of geochemical data over geochronological data for discriminating the sources of deposits in these two canyons.