RELATING OUTCROP AND SUBSURFACE EXPRESSIONS OF THE MAGALLANES-AUSTRAL BASIN: PROVENANCE OF THE EASTERN MAGALLANES-AUSTRAL BASIN REVISED BY DETRITAL ZIRCON U-PB SEDIMENT CORE AGES, SOUTHERN PATAGONIA

Integrating outcrop and subsurface data is crucial to understanding basin fill history. The Magallanes-Austral Basin of southern Patagonia provides an excellent opportunity to constrain such relationships because the Cretaceous fill crops along a 500+ km outcrop belt adjacent to the subsurface basin. Outcrop data suggest that the basin filled longitudinally from the north with local drainages laterally tapping the Andean fold-thrust belt. How outcrop inferences relate to subsurface stratigraphy, however, remains poorly understood. New detrital zircon U-Pb ages (n=2258) from nine subsurface core samples from the Springhill, Piedra Clavada, and Anita Formations along a 140 km east-west seismic reflection transect between Lago Argentino and the Atlantic Ocean reveal provenance and filling history of the basin subsurface. Maximum depositional ages (MDAs) from the Springhill Formation range from 159.1–155.8 Ma, which are older than correlative outcrop ages. MDAs from the Piedra Clavada and Anita Formations range from 94.7–92.4 Ma and 79.4–78.5 Ma, respectively, and overlap in age or are slightly younger than outcrop ages. The Piedra Clavada Formation samples have a dominant 200–172 Ma Early Jurassic age population similar to outcrop ages from the northern, Austral sector Punta Barrosa Formation (stratigraphically equivalent to the Piedra Clavada Formation), suggesting Magallanes-Austral Basin sediment was sourced from the North Patagonia Massif to the northeast. By the Late Cretaceous, the basin was more locally sourced, as suggested by the muted signal of Early Jurassic ages in Anita Formation samples and a second mode of 157–145 Ma Late Jurassic and 144–137 Ma earliest Cretaceous ages, which reveal local sourcing from recycled El Quemado Formation from the fold-thrust belt and the Patagonian arc. Seismic reflection data reveal onlapping and prograding packages within the Springhill Formation. Overlying foreland basin strata are represented by poorly resolved flat and continuous reflection packages but nevertheless permit location of our subsurface samples within the subsurface basin stratigraphy. We suggest that the longitudinal and local transverse drainages that sourced the Magallanes-Austral Basin outcrop belt also propagated into the subsurface to the east.