Santonian-Campanian continentalization in the Austral-Magallanes basin: regional correlation, provenance and geodynamic setting

GHIGLIONE, MATIAS C.; ROCHA, EMILIO; RAGGIO, MARIA FERNANDA; RAMOS, MIGUEL E.; RONDA, GONZALO; MOYANO PAZ, DAMIÁN; VARELA, AUGUSTO N.; VALENCIA, VICTOR

Puerto Madryn

Congreso; XXI Congreso Geológico Argentino; 2022

Asociación Geológica Argentina

The initiation of the foreland stage in the Austral-Magallanes basin, which is indicated by the appearanceof the fi rst sand-dominated and conglomeratic sequences with provenance sources from the SouthPatagonian Andes, presents two relevant characteristics. First, the initiation of the foreland basin stagewas diachronous: Aptian in the northern depocenter (~46-49º), and Albian to Cenomanian in the central andsouthern depocenters (~50-51º and ~51-53º). A second striking fact is that the onset of synorogenic sedimentationwas represented by littoral to fl uvial sequences in a restricted northern continental depocenter(Río Belgrano and Río Tarde Formations), but south of 50º, basal synorogenic sedimentation is representedby marine, sandy turbiditic layers (Lago Viedma, Cerro Toro and Punta Barrosa Formations). The ensuingCretaceous evolution of the Austral-Magallanes foreland basin system also presents contrasting histories.The northern depocenter was inverted and exhumed during basins inversion in a Cenomanian advance of theorogenic front, and constituted an elevated sector as shown by a conspicuous Upper Cretaceous-Paleocenehiatus; a striking diff erence when the hiatus is compared with the ~5000 m thick Upper Cretaceous depositssouth of Lago Viedma.Another key event in the paleogeographic evolution of the Austral-Magallanes foreland basin was theSantonian-Maastrichtian continentalization, marked by the appearance of deltaic, fl uvial and wetland sequencesstacked on top of deep marine deposits. The expanding ecological niche was exploited by theropodsand infl uenced the evolutionary history of dinosaurs. We present new detrital zircon U-Pb geochronologicaldata across this littoral to continental depocenter, in order to provide insights of its age, geographicalextension, source regions and geodynamic setting. Littoral sandstones from La Anita Formation yield MaximumDepositional Ages (MDA) between ~86-80 Ma (weighted mean ages), and ~79-78 Ma youngest zircons.Our results are in accordance with a Santonian-Campanian depositional age. The sequence continues withbadlands and fl uvial systems (Cerro Fortaleza and La Irene Formations), followed by meandering fl uvialand lacustrine deposits (Chorrillo Formation). Transgressive marine facies topping the sequence (CalafateFormation) yield a preferred Maastrichtian 69 ± 2 Ma MDA from the youngest zircon.The Andean sources denote, in the lower samples, exhumation of Mesozoic-shallow structural levels.Towards the younger Campanian-Maastrichtian sequences, a growing relative number of Late Jurassic zirconsfrom the Andean synrift and Permo-Triassic and Paleozoic-Proterozoic zircons which resemble the frequencyhistograms from Andean metamorphic units appears. The co-appearance of 147 Ma peaks from the Andeansyrift (V3) with Paleozoic peaks, suggests that the Paleozoic zircons were derived from Andean metamorphicunits. Together they indicate tectonic exhumation in the Basement domain during this period. The unroofi ngof progressively deeper structural levels is registered in the basin fi ll, which suggests exhumation in theBasement domain during the Santonian-Maastrichtian, in accordance with the coetaneous closure of theRocas Verdes Basin and ophiolite emplacement against the South American continent.We propose that the eastward advance of the orogenic front, and establishment of a wedge-topdepozone, triggered the continentalization phase highlighted in this work. In a context of abundant clasticsupply, shortening-related uplift in the wedge-top competed with fl exural subsidence, causing the uppersurface to rise towards base level.