Revised Timing of Cenozoic Atlantic Incursions and Changing Hinterland Sediment Sources during Southern Patagonian Orogenesis

Abstract

New detrital zircon U-Pb geochronology data from the Cenozoic Magallanes-Austral Basin in Argentina and Chile ~51° S establish a revised chronostratigraphy of Paleocene-Miocene foreland synorogenic strata and document the rise and subsequent isolation of hinterland sources in the Patagonian Andes from the continental margin. The upsection loss of zircons derived from the hinterland Paleozoic and Late Jurassic sources between ca. 60 and 44 Ma documents a major shift in sediment routing due to Paleogene orogenesis in the greater Patagonian-Fuegian Andes. Changes in the proportion of grains from hinterland thrust sheets, comprised of Jurassic volcanics and Paleozoic metasedimentary rocks, provide a trackable signal of long-term shifts in orogenic drainage divide and topographic isolation due to widening of the retroarc fold-thrust belt. The youngest detrital zircon U-Pb ages confirm timing of Maastrichtian-Eocene strata but require substantial age revisions for part of the overlying Cenozoic basinfill during the late Eocene and Oligocene. The upper Río Turbio Formation, previously mapped as middle to late Eocene in the published literature, records a newly recognized latest Eocene-Oligocene (37-27 Ma) marine incursion along the basin margin. We suggest that these deposits could be genetically linked to the distally placed units along the Atlantic coast, including the El Huemul Formation and the younger San Julián Formation, via an eastward deepening within the foreland basin system that culminated in a basin-wide Oligocene marine incursion in the Southern Andes. The overlying Río Guillermo Formation records onset of tectonically generated coarse-grained detritus ca. 24.3 Ma and a transition to the first fully nonmarine conditions on the proximal Patagonian platform since Late Cretaceous time, perhaps signaling a Cordilleran-scale upper plate response to increased plate convergence and tectonic plate reorganization.

The closure of the Rocas Verdes Basin and early tectono-metamorphic evolution of the Magallanes Fold-and-Thrust Belt, southern Patagonian Andes (52–54°S)

“…83 Ma age is coeval with a pronounced zircon U-Pb age peak (ca. 80-66) present in detrital zircon age spectra from the Cenozoic Magallanes-Austral Basin successions (Fosdick et al, 2020), thereby providing additional indirect evidence for this phase of Late Cretaceous arc magmatism.…”

Eocene palaeoenvironments and palaeoceanography of areas adjacent to the Drake Passage: insights from dinoflagellate cyst analysis

“…This scheme is updated here considering the recently published radiometric ages (Fosdick et al . 2020; Bijl et al . 2021) and the International Chronostratigraphic Chart (Cohen et al .…”

“…From the Upper Member Fosdick et al . (2020) dated exclusively the middle part of Sequence VIII and the top of the Sequence IX, representing only the uppermost 90 m of the formation (Leonard et al . 2020).…”

“…2B, zircon U–Pb 1 and 2) gave detrital maximum depositional ages (MDAs) of 47.1 ± 2.7 and 46.3 ± 1.3 Ma (Fosdick et al . 2020). According to our field information, we interpret these levels as corresponding to the Sequence I of the Lower Member of the Río Turbio Fm.…”

“…According to Fosdick et al . (2020) a sample from the contact between the Río Turbio and Río Guillermo formations (Fig. 2B, zircon U–Pb 6) that yields an age of 26.6 ± 0.2 Ma, represents the top of the Río Turbio Fm, extending the age of the continental deposits of the uppermost part of Sequence IX into the early Chattian.…”