Onset of foreland basin sedimentation in the northernmost Magallanes-Austral basin

GHIGLIONE, MATÍAS; NAIPAUER, MAXIMILIANO; SUE, CHRISTIAN; BARBERON, VANESA; BLAMPIED, JORGE; RONDA, GONZALO; RAMOS, VICTOR; AGUIRRE-URRETA, MARÍA BEATRIZ; VALENCIA, VICTOR

Congreso; Geosur; 2013

The southern Patagonian Andes exposes sediments of the Austral-Magallanes foreland basin system along 800 km, characterized by a southward axial sediment dispersal pattern, and a younging of basal orogenic sequences on the same direction (Aguirre-Urreta and Ramos, 1981; McAtamney et al., 2011; Bernhardt et al., 2012). Detrital and volcanic zircon U-Pb geochronology was undertaken on the first coarse-grained regressive units represented by the Río Belgrano and Río Tarde Formations at the northern end of the basin in Lago Pueyrredón, revealing the maximum age of the foreland stage. The maximum depositional age of the basal green sandstones of Río Belgrano Formation is 115.1 ± 2.1 Ma based on the youngest single zircon age, although a more conservative estimate would be represented by a ~122 Ma age-peak, including 36% of the 98 dated zircons. The youngest single zircon of the overlaying red coarse sandstones of Río Tarde Formation yields an age of 118.4 ± 1.5 Ma, however the ~ 122 Ma peak (31% of dated zircons) is also present. Our data indicates that first coarse-grained regressive sequences indicating the initiation of thrust-belt deformation and uplift around this area, are ca. 17-18 Myr older that those located 400 km on the southern end of this axial depocenter at Última Esperanza region. At that southern point, basal foreland sequences are the latest Aptian (~U/Pb zircon age of 101 ± 1.1 Ma, Fosdick et al., 2011) Punta Barrosa and the Coniacian-Santonian (~89-84 Ma) Cerro Toro Formations (review by Bernhardt et al., 2012). This pronounced southward younging trend continues at least another 250 km to the south up to Peninsula Brunswick where foreland basin sedimentation initiated at 88.49 ± 0.61 Ma (McAtamney et al., 2011) around 11 Myr later than Punta Barrosa and 26 Myr after Río Belgrano. An abundant Mesozoic detrital zircon population characterize both the Rio Tarde (23%) and Rio Belgrano (32%) Formations, including a 179-188 Ma peak clearly associated with the V1 volcanic stage associated to the break up of Gondwana, developed to the northeast in the North Patagonian Massif (178-188 Ma Pankhurst et al., 2000; Ferraud, 1999; Cuneo et al., 2013). Subordinate population with zircon ages between ~165 and 173 Ma are also present in both units; these ages are comparable with the V2 volcanic stage (162-172 Ma, Pankhurst et al., 2000). However, zircons of 153-157 Ma representing the age of El Quemado Complex underlying the sag units, and exposed along the Patagonian thrust front to the west and the Deseado Massif to the east (Pankhurst et al., 2000) are under-represented. The data suggest that contemporanous uplift during deposition of Río tarde and Río Belgrano Formation was not concentrated to the west, where El Quemado Complex is yourger, but rather to the north and northeast. This may be associated with with exhumation concentrated on the North Patagonian Massif, and can be associated to deformational events recorded on the eastern rim of the Neuquina basin, as recorder in the detritical content of the Cenomanian basal Neuquen Group (cita Italianos y Ramos). This peak is not represented on detrital zircon populations from Punta Barrosa and younger foreland sequences from Última Esperanza and Peninsula Brunswick, showing that the northern sector of the basin did not aport sediment to the south. The next main peak at 122 Ma seems to represent a contemporaneous volcanic activity and is present in both units. The roots of this volcanism are clearly identified on the western basement domain, were the North Patagonian Batholith in Aysenpresent U-Pb ages of 120-130 Ma (Pankhurst and Herve 1994, Pankhurst et al., 1999). This 122 Ma peak is also a pervasive characteristic of detrital zircon populations from Punta Barrosa and younger foreland sequences from Última Esperanza and Peninsula Brunswick, and could correspond to the southern continuation of the Aysen volcanic arc, altought its roots are not registered on the dating of the Patagonian batolith (i.e. Hervé et al., 2007). In Rio Tarde Formation a well represent group of ages appear from the Paleozoic to the Neoproterozoic (27%) with maximum peaks at ~354 Ma, 456-470 Ma, 514 Ma, and 680 Ma. A smaller group with Grenville ages also appear. These old age peaks indicate that there are exposed basement sources, which are comparable with ages in the North Patagonian Massif to the north and in the Deseado Massif to the east.