Topics

South American n General Triassic Paleogeography Triassic Events (245- n Overview of events that occurred in SA in the Triassic and early Jurassic 200 mya) ¨ General geological layout Kristin Sturtevant n Main volcanic component of the Triassic n Late Triassic sediment deposition n Contrast to NA during this time period

Triassic Paleogeography Cont’d

n Pangea assembled, stable through most of time period, slowly “cracking” apart by late Triassic n Cracking creates a gap between Laurasia and Gondwana n Climate in this time is warm, and semi-arid to arid

Overview Cont’d n Late Paleozoic subduction caused unstable crustal thickening along western coast n Strike-slip faulting along coast causing n Triassic volcanism a result of ending detachment, rifts fill with marine and fluvial convergence and intercontinental extension sediments ¨ Choiyoi group ¨ Ischigualasto Formation n Some subduction farther south along the coast, n During passive margin, sediment deposition in but most of continent in a passive margin by basins adjacent to margin mid -late Triassic n Subduction begins again in the mid Jurassic, forming a new magmatic arc

1 n Terranes from Paleozoic Choiyoi Group ¨ Pampean n Lower and Upper Members ¨ Lower-andesite, dacite ¨ Precordillera ¨ Upper-rhyolite ¨ Primary and frontal cordilleras n Characteristics n Accrentionary wedge from ¨ Pre-rift volcanics Paleozoic subduction ¨ Volcanic plutons and magmatic arcs n Volcanism from back arc ¨ Form igneous provinces basins and beginning ¨ Calc-alkaline extension n Crustal source (accretionary prism from past orogenies) n Sediment deposition in n Partial melting basins from erosion ¨ Alkaline n Extensional mantle source (back- Example of a section of the arc basin) Choiyoi Group from Llambias et n Higher heat al, 2003 From Franzese and Spalletti, 2001

Formation of the Choiyoi Group Late Triassic Sediment Deposition

Tectonic Environment for the formation of the Choiyoi Group from Franzese and Spalletti, 2001

Cont’d Cont’d Ischigualasto Formation n Ischigualasto Formation Chinle Formation ¨ Fluvial environment Members ¨ Sandstone and mudstone ¨ Preserved dinosaur fossils and ferns n Chinle Formation ¨ Fluvial-floodplain environment ¨ Sandstone, mudstone and a silty layer ¨ Mostly plant fossils preserved n Most known for petrified wood, some dinosaurs and non-marine animals n Most fossils found in the Petrified Forest Section of the Formation

2 Triassic Ischigualasto Formation, Argentina In Contrast to North America

n Triassic, the beginning of a period of multiple orgonies ¨ Sonoma Orogeny in early Triassic n Several island arc terranes accreting to the continent ¨ Known as a docking event n 1st of several events that would produce the Cordilleran Triassic Chinle ranges Formation, Utah n Shifting of NA into an active period, while SA is still in a passive period until near the end of the Jurassic

Nearing the end… References n Franzese, J., Spalletti, L., Perez, I.G., Macdonald, D., 2003. Tectonic and paleoenvironmental evolution of Mesozoic sedimentary basins along the Andean foothills of Argentina (32°-54ºS). Journal of South n All these events are considered “Pre-Rifting” American Earth Sciences 16, 81-90. and “Pre-Andean” n Franzese, J., Spalletti, L., 2001. Late Triassic-early Jurassic continental extension in southwestern Gondwana: tectonic segmentation and pre- break-up rifting. Journal of South American Earth Sciences 14, 257- n Very late Triassic and Early Jurassic 270. continental extension and rifting began… n Jenks, W.F., 1956. Handbook of South American Geology: An explanation of the geologic map of South America. The Geological Society of America Memoir 65, 140-201. n Llambias, E.J., Quenardelle, S., Montenegro, T., 2003. The Choiyoi Group from Central Argentina: a subalkaline transitional to alkaline association in the craton adjacent to the active margin of the Gondwana continent. Journal of South American Earth Sciences 16, 243-257. n Ramos, V.A., Cegarra, M., Cristallini, E., 1996. Cenozoic tectonics of the High Andes of west-central Argentina (30°-36°S latitude). Tectonophysics 259, 185-200.

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