Petrofacial Analysis of Late Paleozoic Sandstones of Western Argentina: Its Paleotectonic Significance

Oscar R. Lopez Gamundi, Irene S. Espejo

Late Paleozoic sandstones of western Argentina show an evolutionary trend which highlights their paleotectonic settings and the increasing importance of magmatic arc-derived detritus toward the Late Permian. For the Carboniferous-Early Permian interval, sandstones were subdivided into two contrasting petrofacies:

(A) A quartzofeldspathic petrofacies which prevails on the western and central areas, characterized by high percentages of monocrystalline quartz (Qm, 60-90%) and K-feldspar (K, 5-25%) and low percentages of strained polycrystalline quartz (Qp) and plagioclase (P). Unstable polycrystalline lithic fragments (L) include small amounts of metamorphic (Lm) and sedimentary (Ls) lithics. These rocks show a close provenance from plutonic-high metamorphic complexes which constituted the cratonic eastern basin margin.

(B) A lithic petrofacies which comprises the sandstones exposed on the western part and distinguished by high percentages of Ls and low percentages of K, Qm, and Qp. This detrital association was derived from Ordovician up to Devonian sedimentary rocks which were folded, faulted, and uplifted during Late Devonian orogenic movements, having constituted a recycled orogene source area located west of the cratonic flank.

Both petrofacies evolved in time with a convergent pattern toward a lithofeldspathic-feldspatholitic (volcaniclastic) petrofacies with high percentages of P (30-50%) and Lv (30-40%), low percentages of Qm (0-20%) and K (0-10%), and high P/F and Lv/L ratios. This petrofacies suggests that coeval volcanic activity in the arc placed on the western margin of Gondwanaland became an important source area during Late Permian times.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.