New Mexico Geological Society Annual Spring Meeting — Abstracts

Detailed geologic and petrologic studies of extrusive rocks from Tetilla Peak, New Mexico permit evaluation of the influences of geomorphic and pedogenic processes on the chemical and mechanical weathering of parent materials. Basaltic andesites, andesites, and dacites of Tetilla Peak were extruded approximately 2.5 M.Y.A.. The oldest flow in the area is a low silica (54.6%) basaltic andesite with large phenocrysts of hornblende, plagioclase and trace hypersthene and augite. Above this are hornblende andesites (SiO₂=57.2%) grading into a hornblende-poor, pyroxene-rich andesite (SiO₂=61.9%). Capping the volcano is a dacite flow (SiO₂=63.6%) with microcrystalline plagioclase, acicular hornblende and Fe-Ti oxides.

Mechanical weathering resulted in the formation of two pediment surfaces with soil profiles which reveal a continuous coating of dense, laminar secondary carbonate. Andesite clasts within the calcic horizon and fresh andesite parent material are chemically identical. Moreover, weathered material shows no chemical alteration at the anqesite/carbonate interface. This suggests the formation of the carbonate rind produce a barrier which impedes chemical alteration of the andesite. The carbonate is neither a product of chemical alteration nor a result of precipitation from groundwater but a result of eolian influx of dust, which subsequently precipitates to form a pedogenic calcic horizon.