Deciphering Tectonic Influence on Sediment Routing System Configuration via Large-n U-Pb Detrital Zircon Geochronology, Cretaceous Magallanes-Austral Basin, Chile and Argentina

Abstract

Foreland basin fill is widely used to help inform environmental controls on sediment routing system evolution (e.g., uplift). U-Pb ages from detrital zircons, for example, inform sediment provenance and paleodrainage evolution in many ancient basins. However, many studies focus on discrete intrabasinal units (1-5 m.y. depositional duration), and attempts to constrain paleodrainage over a basin’s entire history (>20 m.y.) are few. We analyze >6200 new U-Pb detrital zircon ages (13 samples) from Cenomanian-Maastrichtian units (Punta Barrosa, Cerro Toro, Tres Pasos, Dorotea Formations) of the Magallanes-Austral Basin in order to inform sediment routing history over ~32 m.y. of compressional tectonism. We use these data along with >9500 U-Pb zircon ages (50 samples) reported from equivalent units across the basin to: (1) deduce spatial/temporal sediment provenance trends; (2) hone depositional timing for basin units; and (3) examine the impact of tectonism on sediment routing.

U-Pb age and paleocurrent data suggests that basin units were mostly sourced from the north and west. Sources include: (1) Devonian-Triassic metamorphic rocks; (2) Jurassic volcanic units; and (3) Cretaceous Andean arc rocks. Punta Barrosa Formation (100-86 Ma) submarine fan units show a decline in Jurassic ages and an increase in Cretaceous ages distally (north-to-south). We link these trends to tributaries that routed arc material to the basin in the south, likely related to enhanced uplift. Fine-grained lower Cerro Toro Formation units, which record coarse-grained sediment starvation, contain large amounts of Jurassic and Cretaceous zircons; conglomeratic channel-levee strata in the upper Cerro Toro Formation (88-76 Ma) feature abundant Paleozoic zircons. We link this shift to exhumation of Paleozoic units in the Andean hinterland via thrusting, coupled with efficient delivery of detritus to the basin across a narrow shelf. Tres Pasos and Dorotea Formation units (80-68 Ma) record basin infill via high-relief clinoforms related to decreased tectonic activity. Strata mostly contain Paleozoic zircons, though Jurassic zircons are abundant in northern locales. The paucity of Jurassic ages in the south implies that Jurassic sources were diluted over space/time, perhaps as more Paleozoic material became exposed due to erosion of Jurassic cover. These results provide key insight into deep-time tectonosedimentary evolution that can help constrain sediment routing in other basins globally.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018