ORIGIN OF BASALTS IN THE MIDDLE-UPPER TRIASSIC STRATA OF THE ISCHIGUALASTO BASIN, NW ARGENTINA

A stratigraphic, structural, and petrologic investigation of Triassic rocks in the eastern part of the Ischigualasto Provincial Park, NW Argentina was conducted in order to determine the origin of igneous rocks intercalated with the fluvial and lacustrine strata of the Chañares, Los Rastros, and Ischigualasto formations. Objectives of the study were to determine: 1) the vertical and lateral relationships between basalts; 2) if basalts were emplaced by lava flows coeval with basin deposition, or were intruded at depth at a later date; and 3) if the basalts have the potential to yield meaningful radiometric ages that can date the timing of deposition within the basin.

Based on stratigraphic relationships and internal morphologies, we have determined that the basalts of the study area were formed by surface lava flows. This interpretation is based on: 1) concordant lower contacts displaying brecciation; 2) upper contacts displaying significant surface relief, pedogenic modification, and lacking baked contacts with overlying lithologies; 3) the presence of elongated amygdules arranged in elliptical patterns that are interpreted as forming in surface-flow lobes; and 4) pilotaxitic crystal texture oriented parallel to stretched vesicles. In addition, petrographic and petrologic analyses indicate that some samples may be good candidates for radiometric dating.