See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/334025185 Chronology of deposition and unconformity development across the Cretaceous–Paleogene boundary, Magallanes-Austral Basin, Patagonian Andes Article in Journal of South American Earth Sciences · January 2020 DOI: 10.1016/j.jsames.2019.102237 CITATIONS READS 12 887 7 authors, including: Sarah George Sarah Nicole Davis The University of Arizona University of Texas at Austin 19 PUBLICATIONS 41 CITATIONS 9 PUBLICATIONS 34 CITATIONS SEE PROFILE SEE PROFILE Roy A. Fernández Leslie Manríquez University of Concepción Universidade do Vale do Rio dos Sinos 17 PUBLICATIONS 32 CITATIONS 20 PUBLICATIONS 38 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: A late Cretaceous pre-accretional forearc basin: Alamor-Lancones Basin, Ecuador View project Influence of Late Cretaceous magmatism on the Sevier orogenic wege, western Montana View project All content following this page was uploaded by Sarah George on 23 August 2019. The user has requested enhancement of the downloaded file. Journal of South American Earth Sciences 95 (2019) 102237 Contents lists available at ScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames Chronology of deposition and unconformity development across the T Cretaceous–Paleogene boundary, Magallanes-Austral Basin, Patagonian Andes ∗ Sarah W.M. Georgea, , Sarah N. Davisa, Roy A. Fernándezb, Leslie M.E. Manríquezc, Marcelo A. Lepped, Brian K. Hortona,e, Julia A. Clarkea a Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, TX, USA b Departamento Ciencias de la Tierra, Universidad de Concepción, Concepción, Chile c Programa de Post-Graduação em Geologia, Universidade do Vale do Rio dos Sinos, Escola Politécnica, São Leopoldo, Rio Grande do Sul, Brazil d Laboratorio de Paleobiología Antártica y Patagonia, Instituto Antártico Chileno, Punta Arenas, Chile e Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, TX, USA ARTICLE INFO ABSTRACT Keywords: The Magallanes-Austral foreland basin preserves an important record of orogenesis and landscape evolution in Andes the Patagonian Andes of Chile and Argentina. Throughout the retroarc foreland basin, a regional disconformity Chile with little to no angular discordance separates Upper Cretaceous–lower Paleocene strata from overlying deposits Patagonia of diachronous Eocene to Miocene age. Here we report detrital zircon U-Pb geochronological results for 11 Foreland basin sandstone samples, and vitrinite reflectance data for 6 samples of organic matter from a fossiliferous dinosaur- Unconformity bearing mixed nonmarine and marine clastic succession in the Río de las Chinas valley (50–51°S) of central- Provenance U-Pb geochronology southern Patagonia to: (1) determine the timing and duration of the unconformity using U-Pb maximum de- K/Pg boundary positional age constraints, (2) reconstruct sediment provenance and dispersal patterns, (3) assess possible temporal variations in arc magmatism, (4) evaluate the amount of sedimentary overburden removed during unconformity development, and (5) confirm the presence of a fossiliferous southern hemisphere Cretaceous–Paleogene (K/Pg) boundary site. Samples from the Dorotea Formation yield maximum depositional ages spanning Maastrichtian to Danian time (with ages as young as ∼65–63 Ma), confirming preservation of the K/Pg boundary in a section with recently discovered fossils of dinosaurs, other terrestrial vertebrates, and plants. Samples from directly above the unconformity in the Man Aike Formation, yield middle Eocene maximum depositional ages (with a prominent 45–40 Ma age cluster), indicating a long-lived ∼20 Myr hiatus re- presentative of nondeposition or erosion. Analyses of organic matter preserved in multiple coal horizons of the uppermost Dorotea Formation show consistently low vitrinite reflectance values, requiring limited sedimentary burial, consistent with nondeposition or sediment bypass rather than deposition and later erosional removal of a previously proposed thick package of Paleocene to middle Eocene clastic material. On the basis of regional trends in the age and geometry of the unconformity, timing of arc magmatism, and temporal variations in sediment provenance, we consider a range of potential mechanisms for unconformity genesis, including (1) shortening-related uplift of the frontal fold-thrust belt, (2) cratonward advance of a flexural forebulge, (3) ac- commodation changes driven by regional or eustatic variations in sea level, (4) ridge collision and slab-window genesis, (5) isostatic rebound during tectonic quiescence (or minor extension), or (6) regional foreland uplift during flat-slab subduction. 1. Introduction growth of the Andean fold-thrust belt (Horton, 2018a and references therein). While studies of the fold-thrust belt and foreland basin in the Mesozoic–Cenozoic subduction along the western margin of South southern Andes often focus on periods of rapid subsidence and accu- America generated a series of flexural foreland basins that preserve mulation associated with shortening and crustal loading (e.g., Manceda important records of retroarc shortening and crustal loading during and Figueroa, 1995; Ghiglione et al., 2010; Giambiagi et al., 2012; ∗ Corresponding author. E-mail address: [email protected] (S.W.M. George). https://doi.org/10.1016/j.jsames.2019.102237 Received 10 January 2019; Received in revised form 2 June 2019; Accepted 14 June 2019 Available online 25 June 2019 0895-9811/ © 2019 Elsevier Ltd. All rights reserved. S.W.M. George, et al. Journal of South American Earth Sciences 95 (2019) 102237 Fig. 1. Simplified regional geologic map of southernmost South America including the Magallanes-Austral foreland basin, Patagonian Andes, Andean magmatic centers (triangles, color coded by crystallization age), and metamorphic complexes (CMC, MRMC, EAMC, MDT, CDMC) after Hervé et al. (2007), Romans et al. (2010), Schwartz and Graham (2015), Schwartz et al. (2017), and Malkowski et al. (2017, 2018) up- dated with new age constraints presented in Pilger (2019). The field site is indicated by the black rectangle. Letters correspond to locations listed in Fig. 9: A Lago Viedma, B Lago Argentino, C Ul- tima Esperanza, D Cordillera Chica, E Seno Skyring and Otway, F North Cordillera Darwin. (For interpretation of the references to color in this figure legend, the reader is referred tothe web version of this article.) Fosdick et al., 2014; Fuentes et al., 2016), recent studies highlight the deformation (Fosdick et al., 2014; Ghiglione et al., 2016) or more importance of enigmatic Paleogene unconformities and condensed generally to a coupled tectonic and erosional event (Biddle et al., 1986; stratigraphic sections within the Andean foreland (e.g., Fosdick et al., Sickmann et al., 2018). While previous studies have demonstrated that 2015a; Ghiglione et al., 2016; Horton and Fuentes, 2016; Horton et al., localized segments of the basin contain a stratigraphic record spanning 2016; Horton, 2018b; Sickmann et al., 2018). A growing appreciation the Cretaceous–Paleogene (K/Pg) boundary (e.g., Fosdick et al., 2015a), of protracted phases of diminished accumulation, bypass, or minor the presence of a regional diachronous unconformity (Fig. 2) suggests erosion underscores the importance of transient processes that may potential local or regional removal of this boundary (Macellari et al., punctuate cycles of orogenic growth along convergent margins. 1989; Sickmann et al., 2018). Southern high and mid-latitude fossili- The Magallanes-Austral Basin (Fig. 1), the southernmost foreland ferous boundary sites with nonmarine flora and fauna enhance under- basin of the Andean chain, records flexural loading associated with standing of the biotic recovery after the end-Cretaceous extinction Cretaceous–Cenozoic shortening and crustal thickening in the Patago- event as they provide meaningful points of comparison to more abun- nian Andes. Development of a basin-wide Paleogene unconformity dant low-latitude sites proximal to areas of potential extinction triggers (Malumián et al., 2000) has been attributed to thrust-related (Zinsmeister, 1982; Parras et al., 1998; Vellekoop et al., 2017). 2 S.W.M. George, et al. Journal of South American Earth Sciences 95 (2019) 102237 Understanding the age, duration, geometry, and lateral extent of fold-thrust belt was markedly affected by thrust loading of oceanic and major disconformities (unconformities that lack major angular dis- attenuated continental crust, inducing a stage of deep-water foredeep cordance) within the Andean foreland basin system is important for sedimentation that resulted in deposition of > 2000 m of mud-rich inferring shifts in basin dynamics and interpreting the driving me- strata. Deep-marine depositional conditions persisted into the Late chanisms of accommodation and surface uplift. We present detrital Cretaceous and are characterized by progradation of the basin slope zircon U-Pb geochronological results for 11 samples associated with the towards the south and subsequent upward shoaling to shallow-marine Paleogene regional unconformity within the Río de las Chinas area and deltaic environments in the uppermost Cretaceous deposits (50–51°S) of the central to northern segment of the Magallanes-Austral (Romans et al., 2010, 2011; Schwartz