A multidisciplinary study of the Lower Cretaceous marine to continental transition in the northern Austral-Magallanes basin and its geodynamic significance

Tectonic evolution of the northern Austral-Magallanes basin in the Southern Patagonian Andes from provenance analysis

“…The medium-grained sandstones represent the continuity of the marine regression that begins with the upper section of the Río Mayer Formation in the Posadas Lake area. The stratigraphic relation between Río Belgrano and the underlying Río Mayer formation is pointing out the paleoenvironmental shift from an external shelf to a transitional environment close to the coastline (Aramendía et al, 2018′. This unit represents a particular regression of the northern sector of the Austral-Magallanes basin, developed from Pueyrredón Lake to the Arroyo Potranquitas to the south (Figure 3′.…”

“…The sedimentary beds commonly present trough and planar cross-bedding, some siltstones beds are interbedded and contain wood remains. The medium-grained clast-supported conglomerates represent amalgamated channels that reach 25 meters thick (Aramendía et al, 2018′. Amalgamated conglomeradic channels are also interbedded with coarse-to very coarse-grained sandstones.…”

“…This is an important issue, because it means that the northern sector was uplifted and acted as a major sedimentary source for the southern foreland basin depocenter beginning in Cenomanian time (Ronda et al, 2019′. The reasoning to explain this early uplift includes a regional tectonic framework and local-regional stratigraphic-structural considerations: By 115-112 Ma acceleration in the convergence rate between Farallón and South America plates led to subduction-related tectonic crustal shortening and thickening (Suárez et al, 2009;Somoza and Ghidella, 2012′. To the west of the Posadas Lake, contractional deformation is recognized, affecting the upper member of the Río Tarde Formation, with fault-propagation folds. Above, and in angular unconformity the Posadas Basalt and/or the El Chacay Formation are found in a subhorizontal position (Figures 5c and 10h; Suárez et al, 2000;Aramendía et al, 2018′. Given the broad hiatus represented by this unconformity (Cenomanian-Eocene; Figure 5c′, it is difficult to assign an age for the contractional deformation event that took place at some time during that interval. However, recently published anisotropy of magnetic susceptibility data (AMS′ revealed that the deposition of the units of the Aptian-Albian age occurred in a region very close to the orogenic front, possibly in an environment of wedge top depozone (Aramendía et al, 2018′, together The Eocene-Miocene geodynamic scenario is characterized by oblique convergence of the Farallón and Nazca plates (Cande and Leslie, 1986;Pardo Casas and Molnar, 1987;Somoza and Ghidella, 2012′, that would have caused strike-slip deformation in the basement domain, which is comparable to the current dynamics in the Northern Patagonian Andes (Cembrano and Hervé, 1993;Rosenau et al, 2006;Cembrano and Lara, 2009;Georgieva et al, 2016′. In the Miocene a renewed foreland stage began with the Burdigalian marine deposits of El Chacay Formation (Cuitiño et al, 2015′. This unit represents the Neogene Atlantic Ocean ingression (Ramos, 1979;Chiesa and Camacho, 1995′ which advanced from the SE, continued to the W and NW (Cuitiño et al, 2015′.…”

“…The oldest exposed rocks are the metasediments of the Río Lácteo Formation (Bianchi, 1967′ of Late Devonian to early Carboniferous age Hervé et al, 2007;Calderón et al, 2016′. The metamorphic basement lays unconformably under the volcanosedimentary successions of the El Quemado Complex (Riccardi, 1971′. These rocks were dated at 157-153 Ma (Nullo et al, 1978;Ramos, 1981;Pankhurst et al, 2000;Iglesia Llanos et al, 2003′ and are related to the latest phase and the westernmost occurrence of the Jurassic silicic volcanism of Patagonia (stage V3 according to Pankhurst et al, 2000;Figure 5 a-b′ The El Quemado Complex is overlain in erosive unconformity or paraconcordance by continental deposits, then followed by silicoclastic conglomerates and sandstones deposited in a marine shelf environment, both included within the Springhill Formation (Thomas, 1949′, of Berriasian-Valanginian age (Ramos, 1979′. The black marine shales of the Río Mayer Formation (Riccardi, 1971′, of Hauterivian-Barremian age, were deposited in a transitional contact over the Springhill Formation, during a thermal subsidence stage (Arbe, 1986′. In a transitional contact, deltaic deposits composed of green sandstones and shales are found, grouped in the Río Belgrano Formation (Ramos, 1979;Aguirre-Urreta and Ramos, 1981;Aramendía et al, 2018′ of Aptian age according to U-Pb dating on detrital zircons and tuffs (Ghiglione et al, 2015′. The Río Tarde Formation is overlaying, characterized by conglomerates and reddish sandstones in its lower member, interpreted as a high energy fluvial system (Arbe, 1986;Aramendía et al, 2018′. The upper member is dominated by reworked tuffs and sandstones deposited in a floodplain (Arbe, 1986′.…”

“…Formation represents a destructive deltaic environment with dominant wave action(Arbe, 1986′ with the transitional passage from a marine environment (Río Mayer Formation′ to a littoral high energy environment close to the coastAramendía et al, 2018′. The progradation of this system was from E to W and ENE to WSW, considering a paleoshoreline oriented NNW (Figures10d and 10e, Aguirre-Urreta and Ramos, 1981′. The following, lower member of the Río Tarde Formation, on the other hand, has been interpreted as a continental high energy fluvial system with intercalated floodplain deposits(Giacosa and Franchi, 2001;Escosteguy et al, 2003; Figure 10e′.The distribution of the U-Pb detrital zircon ages indicates a main contribution of the Jurassic volcanic stage V1 located in the North Patagonian massif and V2 from the Deseado massif, while there is a lack of ages from the El Quemado complex (V3′ outcropping along the SPA.…”