Stratigraphy and tectosedimentary evolution of the late Paleozoic Ancestral Andes between 33° and 25° SL

Abstract

The presence and evolution of a late Paleozoic mountain chain with a paleogeographic disposition similar to the present-day Andean Cordillera is analyzed in this paper. This mountain range, here named Ancestral Andes, was formed during the Gondwanan orogeny (Permian) and separated the Paleopacific Sea from the eastern retroarc basins in this part of South America. Three basement types are involved in the roots of the Ancestral Andes: 1. poorly exposed Mesoproterozoic-Neoproterozoic high-grade metamorphic rocks probably remains of the Chi-Cu terrane, 2. Cambrian-Silurian sedimentary and metamorphic rocks formed during the Famatinian orogeny, and 3. Neoproterozoic to early Carboniferous sedimentary and metamorphic rocks originated during the Chanic orogenic cycle. According to tectonic activity, sedimentary facies, and magmatic record, the Gondwanan cycle comprises seven tectosedimentary stages: 1) Late Mississippian-Early Pennsylvanian (post-Chanic orogeny marine sedimentation), 2) late Early Pennsylvanian (sea-level fall and progradation of sandy wedges in an extensional pre-orogenic period), 3) Middle-Late Pennsylvanian (sea level fluctuations at the end of the pre-orogenic phase), 4) latest Pennsylvanian-early Cisuralian (active tectonism during the beginning of the uplift of the Ancestral Andes, 5) middle Cisuralian (continental and shallow marine sedimentation during the main phase of the Ancestral Andes uplift), 6) late Cisuralian-Guadalupian (continental sedimentation and beginning of the post-orogenic volcanism), 7) late Guadalupian-Lopingian (most active post-orogenic volcanism). The stages 1, 2, and 3 correspond to the pre-orogenic phase of the Gondwanan cycle recorded in the Agua Negra Formation, which is mainly marine with thin continental intercalations. Stages 4 and 5 represent the orogenic conditions that promoted the uplift of the Ancestral Andes and a sudden change from mainly marine (Agua Negra Formation) to continental (San Ignacio Formation) facies. Finally, stages 6 and 7 show the intense magmatic activity developed during the Guadalupian-Lopingian and probably early Triassic times (Choiyoi Group, anaorogenic conditions). The late Paleozoic paleofloras recovered from different stratigraphic intervals into the Ancestral Andes allow establishing that stages 1, and 2, bear remains of the Nothorhacopteris-Botrychiopsis-Ginkgophyllum (NBG) Biozone (latest Mississippian-Early Pennsylvanian). Stage 3 yielded Late Pennsylvanian plant remains and invertebrates while the Stage 5 provided silicified trunks of Cuyoxylon (San Ignacio Formation) and palynofloras characterized by Lueckisporites (La Puerta Formation) of Cisuralian age.