U-PB ages and environmental evolution of the De La Cuesta Formation (late Paleozoic): Record of the Choiyoi Volcanism in the retroarc area of the Paganzo Basin, Western Gondwana

Abstract

This paper examines the relationship between the sedimentation in the retroarc Paganzo Basin and the activity of the magmatic arc developed in the neighboring Andean Cordillera during the Late Pennsylvanian-Cisuralian. The temporal relationships between the Choiyoi Volcanism (Andean region) and the retroarc Paganzo Basin were investigated according to three lines of research: 1. Sedimentology and lithostratigraphy of the De La Cuesta Formation, 2. Geochronology and, 3. Biostratigraphy. Paleoenvironmental studies indicate that small alluvial fans (facies association A) and braided river (facies association B) deposits prevailed in the lower third of the De La Cuesta Formation, which includes a thin intercalation of shallow marine sediments only present in the western part of the area (facies association C). A progressive decrease in the energy of the fluvial systems occurred at the middle part of the unit, where dominant finning-upward successions correspond to meandering rivers (facies association D). In contrast, interbedded sandstones and mudstones were subsequently deposited by low-energy ephemeral rivers (facies association E). Finally, the upper third of the De La Cuesta Formation corresponds to eolian sandstones (facies association F) and shallow water bodies deposits, including evaporites, limestones, and tuffs (facies association G) sedimented under semiarid to arid conditions with periods of explosive volcanism. A U–Pb age of 278.84 Ma was obtained from a tuff level at the uppermost part of the De La Cuesta Formation (facies association G). This age, together with one previously obtained of 311.89 ± 0.21 Ma corresponding to the base of the unit, indicate that the De La Cuesta Formation is Moscovian-Kungurian in age and that it was mostly deposited before the main phase of the Choiyoi magmatism, synchronically with the so-called Pre-Choiyoi magmatism. Furthermore, a rich palynological assemblage belonging to the Lueckisporites/Weylandites Biozone (LW), recovered from deposits of the facies association G, is equivalent to the Lueckisporites virkkiae (Lv) Biozone from the Paraná Basin (Brazil), pointing out a Kungurian age for these biostratigraphic units. The new radiometric age and the palynological taxa recovered from the top of the De La Cuesta Formation (facies association G) confirm a Kungurian age for this interval, establishing a key level for correlation between the sedimentation in the retroarc with the onset of the Choiyoi Volcanism in the magmatic arc.