TIMING AND EVOLUTION OF MIOCENE FORELAND SEDIMENTATION IN THE AUSTRALMAGALLANES BASIN DURING ANDEAN UPLIFT, SOUTHERN

ARAMENDÍA, I.; CUITIÑO, J.; GHIGLIONE, M.; BOUZA, P.J.

Comodoro Rivadavia

Congreso; XXI Congreso Geológico Argentino; 2022

Asociación Geológica Argentina

Foreland basins are elongated regions developed on continental crust near or associated with mountain ranges. Their infill is complexly linked to phases of orogenic uplift, quiescence and decay, which impact in retroarc subsidence dynamics, rates of mountain erosion and transport efficiency of sedimentary systems. The Austral-Magallanes Basin is a well-known example of foreland basin, developed along the retroarc of the Southern Patagonian Andes (SPA). The Neogene foreland deposits of the Austral-Magallanes Basin crop-out along the foothills of the SPA and record the initial marine depositional phase associated to the Andean uplift together with the subsequent continentalization related to the cratonward migration of the deformational front . We studied the Miocene foreland sedimentary record of northwest Austral-Magallanes Basin, including a basal ~165 m thick marine sequence (i.e. El Chacay Formation), followed by ~415 m to ~850 m of continental strata (i.e. Río Zeballos Group and Santa Cruz Formation). New zircon ages of 6 sandstone and tuff samples, combined with a detailed stratigraphic and paleoenvironmental analysis in 3 stratigraphic sections constrain the marine-continental sedimentary cycle between ~20 and ~12 Ma. We define 3 sedimentary units (SU): (i) SU-I: shallow marine deposits (i.e. El Chacay Formation); (ii) SU-II: mixed-load fl uvial deposits (i.e. Río Jeinemeni, Cerro Boleadoras and Santa Cruz formations) and (iii) SU-III: alluvial fan deposits (i.e. Río Correntoso Formation). The paleoenvironmental analysis in a N-S stratigraphic transect shows a correlation of SU-I and SU-II in the 3 stratigraphic sections while SU-III is represented only on the northern area. Sedimentation of the Río Zeballos Group begins at ~19 Ma (i.e. Río Jeinemeni Formation) and culminates close to ~12 Ma (i.e. Río Correntoso Formation). An upsection enrichment of zircons derived from hinterland Jurassic sources between ~16 and 12 Ma documents an unroofing erosional process due to synchronic Andean Miocene uplift. A tuff located between the Santa Cruz Formation and the Belgrano Basalt yielded an age of ~8 Ma representing the youngest acidic volcanic event of the area derived from the orogenic belt or the late-Miocene basalt effusion.