Chronology of Deposition and Unconformity Development Across the Cretaceous–Paleogene Boundary, Magallanes-Austral Basin, Patagonian Andes

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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

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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 representative 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) accommodation 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 and Graham, 2015; Gutiérrez basin (Fig. 3), including 6 samples from the Dorotea Formation, the et al., 2017; Daniels et al., 2017). Following Cretaceous basin filling, a youngest deposits preserved below the unconformity, and 5 samples significant regional unconformity developed throughout the basin, from the Man Aike Formation, the oldest deposits preserved above the which has been previously interpreted as the product of widespread unconformity. Moreover, previous studies variably propose between erosion due to Paleocene uplift (e.g., Biddle et al., 1986; Malumián 0.7 and 5 km of erosion across the unconformity (Skarmeta and Castelli, et al., 1999; Fosdick et al., 2015b). The unconformity has been re- 1997; Fosdick et al, 2015a,b; Ali, 2016). To constrain the amount of cognized in both surface and subsurface datasets (e.g., Biddle et al., sedimentary burial preceding unconformity development, we present 1986; Fosdick et al., 2011; Sickmann et al., 2018) and likely correlates vitrinite reflectance data from organic materials in the upper Dorotea with a basin wide Paleocene unconformity in the Malvinas Basin to the Formation. east (Baristeas et al., 2013). Seismic data show that the unconformity The results provide a chronologic, stratigraphic, and provenance exhibits subtle angularity (< 5°) in the west, which diminishes to the framework in which to: (1) constrain the timing and duration of un- east (e.g., Fosdick et al., 2011; their Fig. 6). conformity development, (2) consider changes in sediment provenance, The stratigraphic succession of the Magallanes-Austral Basin pri- sediment routing, sediment burial, and arc magmatism before, during, marily reflects deep-water deposits with a transition to shallow-marine and after unconformity development, (3) evaluate the amount of sedi- and nonmarine facies within the study area (e.g., Romans et al., 2011; mentary overburden removed during unconformity development, and Schwartz and Graham, 2015; Gutiérrez et al., 2017; Manríquez et al., (4) demonstrate the presence of the K/Pg boundary at the Río de las 2019). The Río de las Chinas valley (50°40′S, 72°32′W) is composed of Chinas locality. By integrating regional constraints on unconformity three main units: the Tres Pasos Formation (Campanian–Maas- development, arc magmatism, and stratigraphic burial, we consider trichtian), Dorotea Formation (considered Campanian–Maastrichtian, potential mechanisms that may have generated a ∼20 Myr un- revised here to Campanian–Danian), and Man Aike Formation (late conformity from the early Paleocene to middle Eocene. Lutetian–Bartonian) (Manríquez et al., 2015; Daniels et al., 2017; Gutiérrez et al., 2017; Schwartz et al., 2017; Sickmann et al., 2018; 2. Geologic and stratigraphic framework Manríquez et al., 2019; this study). Previous studies along strike to the north in the Lago Viedma-Lago Argentino region (~49–50.5°S) and to The Magallanes-Austral Basin (Fig. 1) is a retroarc foreland basin the south in the Ultima Esperanza region (~50.5–52°S) have inter- associated with Cretaceous–Cenozoic mountain building in the preted sediment routing patterns in the Magallanes-Austral Basin and southern Andes (Natland et al., 1974; Winn and Dott, 1979; Dott et al., suggest a southward prograding system variably sourced by arc and 1982; Biddle et al., 1986; Wilson, 1991; Sachse et al., 2015; Malkowski peripheral sources (e.g., Natland et al., 1974; Winn and Dott, 1979; et al., 2017). Formation of the Magallanes-Austral Basin was preceded Dott et al., 1982; Biddle et al., 1986; Wilson, 1991; Fildani et al., 2003; by the Rocas Verdes Basin, a north-trending Middle to Late Jurassic Fildani and Hessler, 2005; Romans et al., 2010; Bernhardt et al., 2011; back-arc basin formed during crustal attenuation, including generation Varela et al., 2013; Fosdick et al., 2014, 2015b; Malkowski et al., 2015, of oceanic crust, coeval with bimodal volcanism during initial breakup 2017; Schwartz et al., 2017; Sickmann et al., 2018). of Gondwana (Fig. 2; Dalziel et al., 1974; Dalziel, 1981; Stern and de The Dorotea Formation is a ∼600–1000 m thick fossiliferous Wit, 2003; Malkowski et al., 2016). A spatial gradient in crustal ex- shallow marine to nonmarine unit with plant, invertebrate, and verte- tension and generation of oceanic crust led to the development of a brate fossils (Fosdick et al., 2011; Manríquez et al., 2015; González, triangular-shaped basin (in map view) that widens along strike from 2015; Schwartz et al., 2017; Gutiérrez et al., 2017). The Dorotea For- north to south (Malkowski et al., 2016, 2017). A major tectonic shift in mation has been correlated with the Cerro Fortaleza, Cerro Cazador, the late Early Cretaceous was marked by initial shortening in the An- Chorillo, La Irene, Calafate and Cerro Dorotea Formations in corre- dean fold-thrust belt, closure of the Rocas Verdes Basin, and earliest sponding segments of the basin to the north and east in Argentina flexural subsidence in the Magallanes-Austral Basin (Natland et al., (Macellari et al., 1989; Arbe, 2002; Nullo et al., 2006; Leppe et al., 1974; Biddle et al., 1986; Wilson, 1991; Fildani et al., 2003; Varela 2012; Sickmann et al., 2018). The uppermost Dorotea deposits are et al., 2012; Fosdick et al., 2014). The transition in tectonic regime was primarily interpreted as fluvial, shoreface, and deltaic deposits likely the product of either increased absolute motion of the South (Manríquez et al., 2015; Schwartz and Graham, 2015; Daniels et al., American plate westward over subducting oceanic lithosphere of the 2017). Above the Dorotea Formation is a regional Paleogene un- Pacific basin or opening of the South Atlantic Ocean(Maloney et al., conformity capped with little to no (< 2-5°) angular discordance by the 2013; Ghiglione et al., 2016; Horton, 2018b). shallow-marine Man Aike Formation (∼100–250 m thick) and its The inherited architecture of the Rocas Verdes Basin influenced the equivalents (lower Río Turbio Formation), which are interpreted as initial configuration of the successor Magallanes-Austral Basin (Fildani wave-dominated estuary and tidal flat environments (Fig. 4 and Sup- and Hessler, 2005; Hubbard et al., 2008; Romans et al., 2010; plemental Data; Malumián, 1990; Camacho et al., 2000; Marenssi et al., Malkowski et al., 2016). A southward diachroneity in the evolution of 2003; Schwartz and Graham, 2015; Gutiérrez et al., 2017; Manríquez the foreland basin is expressed in the record of initial coarse-grained et al., 2019). sedimentation and inferred onset of flexural subsidence, with estimates Age estimates for the uppermost levels of the Dorotea Formation ranging from about 115 to 89 Ma from 48° to 55°S (Malkowski et al., vary among studies, with the youngest reported zircon U-Pb maximum 2017). The basin was deeply subsided in the south but in the north was depositional ages of 68.9 ± 2.0 Ma (Schwartz et al., 2017). Studies of floored by less attenuated, more rigid lithosphere, resulting in greater similar clastic deposits to the north and south of the study region in- flexural subsidence in the south and an overall south-dipping deposi- terpreted as equivalents of the upper Dorotea Formation contain evi- tional slope parallel to the basin axis (Dott et al., 1982; Wilson, 1991; dence of early Paleocene deposition (Macellari et al., 1989; Fosdick Romans et al., 2010; Ghiglione et al., 2010, 2016; Fosdick et al., 2014). et al., 2015a). Reported depositional ages for unconformably overlying The proximal westernmost basin adjacent to the north-trending Andean units are distinctly younger, ranging from Eocene to Miocene age, with

3 S.W.M. George, et al. Journal of South American Earth Sciences 95 (2019) 102237

2017; Sickmann et al., 2018). Although age constraints are limited, it appears that in the north sedimentary rocks below the unconformity are older and outcrops capping the unconformity are younger, suggesting a greater hiatus in the north relative to the south (e.g., Biddle et al., 1986; Macellari et al., 1989; Sickmann et al., 2018). Deposition of the Man Aike Formation is thought to have occurred during a period of relative sea-level rise, with progressive flooding taking place throughout an estuary system marked by tidal facies and ravinement surfaces (Casadío et al., 2009).

3. Detrital zircon U-Pb geochronology

3.1. Methods

Samples of medium-grained sandstone were collected from the Dorotea Formation (n = 6) and Man Aike Formation (n = 5) and zircons were separated using standard crushing, density, and magnetic separation techniques. Detrital zircon U-Pb geochronological results were obtained at the University of Arizona LaserChron Center following the laboratory's established LA-ICP-MS (laser ablation–inductively coupled plasma–mass spectrometry) practices (e.g., Gehrels, 2000, 2014; Gehrels et al., 2008). For each sample, back scattered electron images enabled selection of n = 120 inclusion-free spots on zircon grains of varying sizes and morphologies. Corrections for fractionation and common lead were made using a Sri Lanka primary age standard that was analyzed after every five unknown grains. In choosing the best U-Pb ages, a cutoff age of 900 Ma was employed for selecting between 206Pb/238U ages (grains < 900 Ma) and 206Pb/207Pb ages (grains > 900 Ma). Ages with > 10% uncertainty, > 5% reverse discordance, or > 20% discordance were omitted from further consideration. Detrital zircon U-Pb maximum depositional ages have proven to be a valuable technique to assess stratigraphic age for sedimentary basins fed by syndepositional volcanic materials (e.g., Fildani et al., 2003; Dickinson and Gehrels, 2009; Horton et al., 2015; Daniels et al., 2017). In the Magallanes-Austral Basin, maximum depositional ages have been shown to approximate true depositional ages through dating of tuffac- eous materials, biostratigraphy, Sr isotope stratigraphy, and the appli- cation of Bayesian statistics (e.g., Fildani et al., 2003; Bernhardt et al., 2011; Schwartz et al., 2017; Sickmann et al., 2018; Johnstone et al., 2019). Here we report maximum depositional ages for individual samples on the basis of the weighted mean age of the youngest grains that overlap at 1σ uncertainty recognizing that the maximum depositional age may be older than true depositional age. If the youngest two grains do not overlap at 1σ uncertainty, then the maximum depositional age of that sample is provided by the weighted mean of the youngest population of grains that overlap at 2σ uncertainty (ranging from 3 to 44 grains). When that is not possible, the maximum depositional age is given by the youngest single grain age. Uncertainties on maximum depositional ages are reported at 2σ uncertainty. When samples were Fig. 2. Generalized stratigraphic column for the Magallanes-Austral foreland collected in close lateral and stratigraphic proximity (within several basin, showing Cretaceous-Cenozoic stratigraphic units and the basin-wide meters of each other), we combine the youngest grains to ensure robust unconformity separating the Dorotea Formation (and equivalent Cerro Dorotea, maximum depositional ages. La Irene, Chorillo, and Calafate Formations) from the disconformably overlying Paleogene Man Aike Formation (and equivalent lower Río Turbio Formation). Modified after Wilson (1991), Fildani and Hessler (2005), Romans et al. (2010), 3.2. Potential sediment source regions Fosdick et al. (2011, 2014), Schwartz and Graham (2015), Schwartz et al. (2017), and Malkowski et al. (2017). Pre-Cenozoic stratigraphic thicknesses are Potential sediment sources for Cretaceous–Paleogene strata of the from Fosdick et al. (2011) and Cenozoic thicknesses are modified from Fosdick Magallanes-Austral Basin include: (1) Paleozoic to Triassic meta- et al. (2014). morphic and igneous complexes exposed along the western flank of the Patagonian Andes and in the distal foreland northeast of the basin; (2) the Man Aike Formation generally regarded as Eocene in age (Camacho Jurassic extension-related volcanic rocks to the west and northeast of et al., 2000; Marenssi et al., 2002, 2003; Ghiglione et al., 2014; Fosdick the study area, (3) Late Jurassic to Paleogene arc-related rocks now et al., 2015b; Sickmann et al., 2018). The duration of the disconformity preserved in coastal regions to the west, and (4) sedimentary rocks capping the Dorotea Formation varies across the basin, with most es- involved in Andean shortening along the western basin margin (Fig. 1; timates ranging from roughly 5–40 Myr (Biddle et al., 1986; Fosdick Fildani et al., 2003; Romans et al., 2010; Malkowski et al., 2017; et al., 2015a,b; Schwartz et al., 2017; Gutiérrez et al., 2017; Rivera, Sickmann et al., 2018).

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Fig. 3. Geologic and stratigraphic framework for the Río de las Chinas valley in the Magallanes-Austral Basin (after Malumián et al., 2000 and Schwartz et al., 2017). (A) Geologic map showing major units and sample locations for the Dorotea and Man Aike Formations. (B) Local stratigraphy of the Dorotea and Man Aike Formations in the Río de las Chinas valley. The regional unconformity is indicated in purple and detrital zircon and vitrinite samples are marked with circles. The upper Dorotea Formation is dominated by medium grained sandstone and mudstone, while the base of the Man Aike is characterized by coarse sandstone and conglomerate. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

(1) Paleozoic–Triassic metamorphic complexes exposed within the centered at 325–200 Ma, 650–350 Ma, and 1200–900 Ma, with Patagonian Andes likely reflect an exhumed late Paleozoic sub- subsidiary intervening age peaks (Hervé et al., 2003; Malkowski duction complex (Hervé, 1988). From north to south, the meta- et al., 2017). The Eastern Andean Metamorphic Complex lies di- morphic complexes include the Main Range (Devonian–Permian), rectly west of the Río de las Chinas area and is the most proximal Chonos (Triassic), Eastern Andean (Devonian–Permian), Duque de source region with significant pre-Jurassic age populations. York (part of the Madre de Dios accretionary complex, Triassic), and Cordillera Darwin (mid–late Paleozoic) metamorphic complexes (Fig. 1; Faúndez et al., 2002; Hervé et al., 2003, 2008, 2010). (1) To the northeast, additional Paleozoic metamorphic rocks in- These massifs are variably characterized by large age peaks clude crystalline basement rocks of Jurassic extensional

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Fig. 4. Overall setting and outcrop expression of the Paleogene unconformity in the Río de las Chinas valley. (A) Satellite image modified from Google Earth of the valley highlighting bedding (in white) of the Dorotea and Man Aike Formations. Note the lack of angular discordance between the Dorotea and Man Aike strata within the valley. Purple line indicates observed extent of the unconformity within the study area, and stars indicate the location of outcrop photos (yellow = B, C; blue = D, E). (B–E) Views of the contact between the Dorotea and Man Aike Formations in the northern portion of the study area, facing northeast. The unconformity is indicated in purple. In the northern (B, C) and central (D, E) regions of the site the top of the Dorotea Formation is characterized by medium grained sandstone, while the Man Aike Formation is composed of either conglomerates rich in bivalve fragments or red medium grained sandstone. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. Probability density plots and histograms of detrital zircon U-Pb geochronological results from the Dorotea and Man Aike Formations from 2000 to 0 Ma. Pie charts show the percentage of grains in each age bin. Upper figure shows a cumulative distribution function for each sample. Adjacent to the samplenames, N = shows the number of grains displayed versus the total number of grains.

7 S.W.M. George, et al. Journal of South American Earth Sciences 95 (2019) 102237

- In the Northern Patagonian Massif, metasedimentary Paleozoic basement yields strong age peaks between 650–430 Ma and ca. 1200–900 Ma (Uriz et al., 2010; Chernicoff et al., 2013), and Or- dovician to Triassic igneous intrusions yield ages ranging from ca. 475–245 Ma (Pankhurst et al., 2006). (2) Jurassic extension associated with the breakup of Gondwana generated voluminous igneous provinces along the eastern (Atlantic) margin of southernmost South America (Fig. 1). The regionally extensive Chon Aike large igneous province contains volcanic rocks that range in age from 188 to 142 Ma (Pankhurst et al., 2000; Calderón et al., 2007; Malkowski et al., 2016). The Chon Aike igneous rocks (including the Tobífera, Marifil, Ibanez, El Quemado Formations) are exposed in both basement massifs of the northern Patagonian foreland region northeast of the study area (Fig. 1). Importantly, the Northern Patagonian Massif includes diagnostic age populations of 188–178 Ma not present in the more proximal Deseado Massif (Marifil Formation; Malkowski et al., 2017). In addition, the Upper Jurassic Tobífera Formation of the Rocas Verdes Basin is currently exposed along the western flank of the Magallanes-Austral foreland basin and is comprised of volcanic and volcaniclastic rocks (Hervé et al., 2007) in a north-trending belt that provides a potential westerly source of Jurassic-age zircons. (3) Subduction-related igneous rocks crop out within Southern Patagonian Batholith in the Patagonian Andes to the west of the Magallanes Basin (Fig. 1). These rocks of the Andean magmatic arc range from Late Jurassic through Cenozoic age (Hervé et al., 2007; Echaurren et al., 2017). Magmatic compositions include an early phase of voluminous bimodal leucogranites and gabbros (ca. 157–145 Ma), followed by dominantly tonalitic-dioritic Cretaceous plutonism, low-volume isotopically juvenile Paleogene granitoids, and finally voluminous Neogene tonalites and granodiorites with minor dacites (Hervé et al., 2007). (4) Finally, shortening and uplift west of the Magallanes-Austral basin may have exhumed sedimentary rocks from older formations within the Andean fold-thrust belt (Fig. 1; Fosdick et al., 2015b; Ghiglione et al., 2016). In this scenario, synorogenic basin fill will contain specific age groups that mirror that of the older (Jurassic–Cretac- eous) clastic strata, with basin fill recording a systematic upsection shift in age spectra characteristic of unroofing (recycling) of progressively older deposits (e.g., Fosdick et al., 2015a).

3.3. Results

Detrital zircon U-Pb geochronological results define the age distributions for 11 samples of the Dorotea and Man Aike Formations. Samples of the Dorotea Formation show prominent age peaks of 1200–900 Ma, 625–350 Ma, 325–225 Ma, 160–130 Ma, 120–80 Ma, and 70–60 Ma (Figs. 5 and 6). Although proportions vary, all but the youngest age mode (70–60 Ma) are present in all Dorotea samples. Dorotea sample 18BKH06 taken 42 m below the unconformity yields a maximum depositional age of 66.7 ± 1.3 Ma (n = 2 grains). Dorotea samples DT1 and DT2 taken directly below the unconformity give maximum depositional ages of 64.0 ± 1.1 Ma (n = 2 grains) and Fig. 6. Probability density plots and histograms of detrital zircon U-Pb geochronological results from the Dorotea and Man Aike Formations from 200 to 62.1 ± 0.7 Ma (n = 1 grain) respectively. Combined Dorotea samples 0 Ma with maximum depositional ages and associated 2σ uncertainties in bold DT1 and DT2 provide a robust Paleogene maximum depositional age (63.6 ± 1.8 Ma; n = 3 grains). Samples of the Man Aike Formation contain similar age peaks as the complexes, including the Deseado Massif and Northern underlying Dorotea (1200–900 Ma, 625–350 Ma, 325–225 Ma, Patagonian Massif (Pankhurst et al., 2006; Chernicoff et al., 160–130 Ma, 120–80 Ma, and 70–60 Ma) with two notable additions: a 2013; Moreira et al., 2013). The Deseado Massif includes the large age peak from 45 to 40 Ma, and an 80–70 Ma age peak most no- Paleozoic La Modesta Formation in the west, which contains age table in sample 18BKH03. Also notable is the absence of 60–45 Ma peaks of 490–440 Ma, 700–510 Ma and 1200–900 Ma, and is populations in all Man Aike samples (Figs. 5 and 6). We consider the devoid of grains younger than 410 Ma (Moreira et al., 2013). In significant age peak from 45 to 40 Ma comprising > 20% of thetotal the east, the Deseado Massif contains igneous and metamorphic zircons analyzed from the Man Aike to be roughly syndepositional as it rocks of the Río Deseado Complex, with ages peaks ranging from (1) generally becomes progressively younger upsection, and (2) is ∼1300 to 345 Ma (Pankhurst et al., 2003; Moreira et al., 2013). consistent with existing U-Pb geochronological and paleontological age

8 S.W.M. George, et al. Journal of South American Earth Sciences 95 (2019) 102237 constraints (Camacho et al., 2000; Marenssi et al., 2002, 2003; Table 1 Ghiglione et al., 2014; Fosdick et al., 2015b; Sickmann et al., 2018). Vitrinite reflectance results. Fossil constraints indicating an Eocene age for the Man Aike and its Sample meters below grains %Ro (mean+/-s.d.) TOC lateral equivalents include microfossil assemblages, Lopha herminii, unconformity (n = ) (wt %) Ostrea groeberi, Bouchardia conspicua, Testudines indet., Mesoeucroco- dylia indet., and Sphenisciforme indet. (Camacho et al., 2000; Marenssi 18BKH01 60 51 0.47 ± 0.06 18.3 18BKH05 45 51 0.47 ± 0.04 7 et al., 2002; Sallaberry et al., 2010; Otero et al., 2012a,b). Previously 18BKH08 48 46 0.46 ± 0.04 9.3 published maximum depositional ages from the Man Aike Formation 18BKH10 40 50 0.46 ± 0.03 9.1 also support an Eocene depositional age (40.30 ± 0.47 Ma, 18BKH11 35 21 0.46 ± 0.07 15.8 40.48 ± 0.37 Ma, 43.2 ± 2.0 Ma; Le Roux, 2012; Gutiérrez et al., 18BKH12 46 51 0.43 ± 0.01 16.3 2017; Sickmann et al., 2018). U-Pb geochronological results from the Man Aike Formation yield suggests that the modern Cenozoic stratigraphic succession represents statistically robust maximum depositional ages (Fig. 6). Three samples the maximum burial history of uppermost Cretaceous basin fill. taken from the lowermost levels, < 5 m above the unconformity, yield maximum depositional ages of 45.7 ± 0.6 Ma (18BKH04; n = 3 grains), 42.5 ± 1.0 Ma (18BKH13; n = 4 grains), and 42.2 ± 0.5 Ma 5. Discussion (18BKH09; n = 4 grains). These results support a middle Eocene (Lu- tetian) age for the basal Man Aike Formation, suggesting that the un- 5.1. Paleontological implications of a K/Pg boundary succession conformity represents a ∼20 Myr hiatus. Sample 18BKH03 from 4 m above the unconformity gives a maximum depositional age that is older Previous paleontological discoveries have suggested a Campanian than underlying samples, and hence is not representative of a true de- through (at least) Maastrichtian age for the Dorotea Formation positional age. A sample from the middle to upper Man Aike Formation, (Manríquez et al., 2015, 2019; González, 2015; Garrido et al., 2016; 18BKH14, ∼100 m above the unconformity, yields a maximum de- Soto-Acuña et al., 2016; Schwartz et al., 2017). Vertebrate fossils of positional age of 40.5 ± 0.3 Ma, which is in agreement with similar U- Late Cretaceous marine reptiles and non-avian dinosaurs, as well as Pb maximum depositional ages from the top of the Man Aike Formation plant and invertebrate remains, have been reported within the Dorotea (40.30 ± 0.47 Ma and 40.48 ± 0.37 Ma; Le Roux, 2012; Otero et al., Formation in particular abundance in the Río de las Chinas valley 2013; Gutiérrez et al., 2017), suggesting a middle Eocene (Lute- (Cortés, 1964; Katz, 1963; Jujihara et al., 2014; Soto-Acuña et al., 2014, tian–Bartonian) age for the entire formation. 2016; Vogt et al., 2014; Manríquez et al., 2015; González, 2015; Garrido et al., 2016; Leppe et al., 2018). The Dorotea Formation yields 4. Vitrinite reflectance maximum depositional ages of 66.7 ± 1.3 Ma (18BKH06) and 63.6 ± 1.8 Ma (combined DT1 and DT2). These maximum deposi- 4.1. Methods tional ages from the top of the Dorotea coupled with fossil occurrences lower in the section suggest the presence of the K/Pg boundary in the To assess burial heating and estimate the thickness of a former se- Río de las Chinas valley. The K/Pg interval is significant due to the dimentary overburden, several coal horizons from the uppermost occurrence of one of Earth's five largest mass extinction events (Raup Dorotea Formation were sampled for vitrinite reflectance analysis. and Sepkiski, 1982). From a paleontological perspective this invites Organic matter was isolated and analyzed for six samples using kerogen future investigation, as it is one of few described transitional marine to separation through acid digestion followed by measurement of vitrinite non-marine K/Pg sections, as well as one of the only definitive outcrops reflectance using reflected light microscopy (Sweeney and Burnham, that preserves a record of continuous deposition across the K/Pg 1990; Barker and Pawlewicz, 1994; Nielsen et al., 2017). Although boundary at a mid-latitude site in the southern hemisphere different sedimentary basins have variable thermal histories andfluid (Zinsmeister, 1982; Butler et al., 1991; Parras et al., 1998; Hay et al., circulation patterns that can affect estimated temperatures, vitrinite 1999; Leanza and Hugo, 2001; Leanza et al., 2004; Scasso et al., 2005; reflectance has proven to be a useful parameter for the evaluation of Torsvik et al., 2012; Gutiérrez et al., 2017). peak temperatures (Morrow and Issler, 1993; Barker and Pawlewicz, Refined age constraints for the Río de las Chinas valley have im- 1994). To avoid complications due to local thermal effects, multiple portant implications for testing the southern mid-latitude (estimated samples from nearby stratigraphic horizons are commonly analyzed between 60° and 78°S; Torsvik et al., 2012; van Hindsbergen et al., with internal comparison to identify potential local variations and 2015) expression of hypothesized global trends of survivorship and stratigraphic trends. extinction at the K/Pg boundary (Schulte et al., 2010). Most described fossiliferous nonmarine K/Pg boundary sections occur in the Northern 4.2. Results Hemisphere, potentially biasing global interpretations of conditions before and after the K/Pg (Schulte et al., 2010; Fastovsky and Bercovici, In this study, vitrinite reflectance analysis involved 45–50 in- 2015). The majority of K/Pg boundary sections described from South dividual measurements per sample, allowing for calculation of a mean America are preserved in marine settings, limiting interpretations of value and standard deviation for each sample. The mean values for the southern hemisphere conditions to the marine realm during the ex- six samples are relatively low, with a narrow range from 0.43 to 0.47% tinction event (e.g., Albertão et al., 1994; Leanza et al., 2004; Bermúdez Ro and a standard deviation of 0.01–0.06% (Table 1). Conversion of a et al., 2016). Deposits in Argentine Patagonia have yielded extensive mean Ro value into an estimate of peak temperature and maximum Late Cretaceous vertebrate discoveries, but none have been identified in burial depth is subject to kinetic uncertainties related to vitrinite close proximity to the K/Pg boundary (e.g., Novas et al., 2008; Egerton composition, kerogen type, and oxidation. Nevertheless, Ro values of et al., 2013; Lacovara et al., 2014; Lamanna et al., 2018). The Jagüel 0.45% are in accord with other estimates for the region (e.g., Skarmeta Formation in the Neuquén basin of Argentina is the nearest locality to and Castelli, 1997; Ali and Fosdick, 2015) and are most consistent with the Magallanes-Austral sections with a similarly fossiliferous K/Pg vitrinite values measured at 1.0–1.2 km depth in onshore exploration section, but occurs in marine deposits (Legarreta et al., 1989; Leanza wells in the Magallanes-Austral basin (Sachse et al., 2016 and refer- and Hugo, 2001; Leanza et al., 2004). Furthermore, in key fossiliferous ences therein). These values are in close agreement with the present- sections from the Argentine San Jorge Basin that have been proposed to day 800–1000 m sedimentary section preserved above the Dorotea support rapid Southern Hemisphere floral recovery in the Paleogene, Formation in the study region (Fig. 2; Gutiérrez et al., 2017); this the K/Pg boundary lies within an unconformity (Clyde et al., 2014).

9 S.W.M. George, et al. Journal of South American Earth Sciences 95 (2019) 102237

Fossils from the Río de las Chinas valley could expand interpretations of samples, while the Punta Barrosa, Cero Toro, and Tres Pasos Formations global conditions around the extinction event. Notable Cretaceous generally exhibit a stronger separation from the overlying Dorotea and fossils reported thus far include Chilean Patagonia's first Upper Cre- Man Aike Formations (Fig. 7b). These results suggest either comparable taceous hadrosaurs (Jujihara et al., 2014; Soto-Acuña et al., 2014) and sediment source regions during Maastrichtian–Danian deposition of the ornithopods (Jujihara et al., 2014) that represent the southernmost Dorotea and middle Eocene deposition of the Man Aike, or possible findings of non-avian dinosaur fossils in Chile, and numerous large recycling of Dorotea deposits during Man Aike deposition. Such re- plant remains (leaves, fruits, and trunks of angiosperms, conifers, and cycling would be coeval with a middle Eocene return to flexural sub- ferns; Yabe et al., 2006; Leppe et al., 2012, Leppe et al., 2018; sidence, broadening of the fold-thrust belt, and rapid exhumation in the Manríquez et al., 2019). The Río de las Chinas locality has also pro- west (Nelson, 1982; Biddle et al., 1986; Skarmeta and Castelli, 1997; duced the oldest South American record of the Nothofagus genus, Kraemer, 1998; Malumián et al., 2000; Malumián, 2002; Ghiglione linking the site to Campanian–Maastrichtian localities in Antarctica et al., 2016). However, the notable increase in 80–70 Ma and (Leppe et al., 2018). Continuing to refine the ages of these remains, 140–115 Ma grains in the Man Aike Formation with respect to the along with new discoveries from the valley, would improve compar- Dorotea Formation (Fig. 8E and F) suggests the addition of new sources, isons of Late Cretaceous to Paleogene terrestrial fossils across South or exhumation in existing source areas, in addition to recycling of older America, studies of biogeographical relationships between South Cretaceous strata. Generally, contributions from Eocene magmatic arc America and Antarctica, and consideration of global scale extinction sources increase upsection within the Man Aike Formation (Fig. 5) such trends across the K/Pg boundary. that coeval magmatic arc materials become the dominant sediment source by ca. 40 Ma. 5.2. Sediment routing systems 5.3. Potential magmatic lulls Extensive outcrop and provenance studies have characterized sediment routing systems within the Magallanes-Austral Basin (e.g., Fildani Detrital zircon U-Pb geochronological results from the Dorotea and and Hessler, 2005; Romans et al., 2010; Varela et al., 2013; Schwartz Man Aike Formations highlight two notable lulls in age populations at et al., 2012, 2017; Schwartz and Graham, 2015; Fosdick et al., 2015a,b; 140–115 Ma (low abundance) and 60–45 Ma (absent) (Fig. 8). These Malkowski et al., 2017; Daniels et al., 2017; Sickmann et al., 2018). Early Cretaceous and late Paleocene–middle Eocene lulls are also pre- Detrital zircon U-Pb geochronological results for the Dorotea Formation sent in regional detrital zircon age spectra (Fig. 8B). The apparent lulls demonstrate sediment derivation from metamorphic complexes may represent waning of arc magmatism related to variations in sub- (1200–225 Ma age peaks), Jurassic volcanic rocks (160–140 Ma age duction dynamics. Alternatively, the data may reflect the relative peak), and Cretaceous to earliest Paleocene arc-related rocks dominance of Late Cretaceous ages (from the Patagonian Batholith) or (140–60 Ma peaks). While Cretaceous–Paleocene populations clearly source-to-sink dynamics where feeder systems did not tap source re- indicate arc-related sources to the west or axially to the northwest, gions of these ages. Lower Cretaceous arc rocks crop out along the coast Jurassic and Paleozoic populations are less diagnostic and could re- of southern Patagonia (Fig. 1; Hervé et al., 2007), suggesting the ab- present sources to the west (various metamorphic massifs and Rocas sence of Early Cretaceous ages may reflect signal dilution rather than Verdes Basin) or northeast (Deseado or Patagonian Massif). However, cessation of magmatism. This is broadly consistent with a relatively due to a lack of Jurassic zircons older than 160 Ma in the Dorotea stable signal in the magmatic bedrock record (Fig. 8A) and increased Formation, we consider the Northern Patagonian Massif an unlikely Early Cretaceous zircons in the middle Eocene Man Aike Formation, sediment source region (Malkowski et al., 2017). Additionally, due to perhaps implying that feeder systems began tapping Early Cretaceous the presence of late Paleozoic–earliest Mesozoic (Fig. 5; ca. igneous units by this time. 325–225 Ma) populations in the Dorotea Formation, metamorphic In contrast, the Paleogene magmatic lull from 60 to 45 Ma corre- basement in the Deseado Massif is insufficient to explain the Paleozoic sponds to a low abundance of arc magmatic rocks in Patagonia (Figs. 1 age distributions and sourcing from younger metamorphic complexes and 8A; Hervé et al., 2007). The late Paleocene–middle Eocene mag- located to the west of the basin system is required. Therefore, we matic lull is roughly coincident with unconformity development, and suggest that the majority of detritus in the Dorotea Formation was may relate to arc cessation due to slab flattening, as proposed to the derived from westerly sources, potentially with limited input from north of the study area (Nale flat slab; Gianni et al., 2018a,b) or genesis proximal northern sources such as the Deseado Massif. of a slab window (during Aluk-Farallon spreading ridge collision; Previous studies have suggested a provenance shift from an axial Ramos, 2005). Interestingly, Paleogene magmatic centers have not been south-flowing Dorotea system to a west-flowing transverse ManAike documented between ∼49°S and 52°S, roughly coincident with the N-S system (Schwartz et al., 2012). However, detrital zircon U-Pb geo- extent of the protracted hiatus (Fig. 9). chronological results indicate that the Man Aike Formation shares similar age populations as the Dorotea Formation, with the addition of a 5.4. Erosional removal of sedimentary overburden during unconformity syndepositional middle Eocene population (45–40 Ma; Sickmann et al., development 2018; this study) and increased abundance of 80–70 Ma and 140–115 Ma grains in the Man Aike Formation. Here we evaluate the Based on (U-Th)/He thermochronometric data, previous authors continuity of sediment sources from the Dorotea Formation to the Man have interpreted pronounced burial of the uppermost Cretaceous Aike Formation with detrital zircon U-Pb geochronological results. Dorotea Formation beneath a proposed 5 km thick sedimentary over-

Multidimensional scaling (MDS) plots using Kuiper Vmax values are burden that was eroded away during Eocene development of the basin- useful for evaluating similarity between samples, where spatial proxi- wide Paleogene unconformity (Fosdick et al., 2015b). In this inter- mity in the MDS plot reflects a greater likelihood of shared provenance pretation, a very thick Paleocene to middle Eocene succession was (Vermeesch, 2013; Saylor and Sundell, 2016; Sharman et al., 2018). deposited quickly and then rapidly removed, in its entirety, prior to late Composite detrital zircon U-Pb results from the Man Aike, Dorotea, Tres middle Eocene deposition of the Man Aike Formation. Vitrinite re- Pasos, Cerro Toro, and Punta Barrosa Formations were also included in flectance results for coal horizons within the uppermost Dorotea For- the MDS plots for comparison (Fig. 7). mation uniformly indicate limited burial beneath a sedimentary over-

When all age populations are considered, MDS plots show moderate burden. Measurements of vitrinite reflectance yieldo R values less than separation between Man Aike and Dorotea samples (Fig. 7a). However, 0.45, suggesting no more than 1000–1500 m of burial. Given that the when post-Dorotea grains (< 60 Ma) are excluded from Man Aike Dorotea Formation is currently overlain by 800–1000 m of preserved samples, there is substantial overlap between Dorotea and Man Aike sedimentary overburden, in the form of middle Eocene through

10 S.W.M. George, et al. Journal of South American Earth Sciences 95 (2019) 102237

Fig. 7. Multidimensional scaling (MDS) plots for all samples from this study with composite samples from the Man Aike, Dorotea, Tres Pasos, Cerro Toro, and Punta Barrosa Formations. Composite age distributions are shown in Fig. 8 (200-0 Ma). (A) MDS using all age bins. (B) MDS using only ages greater than 60 Ma to allow comparison between the Man Aike and Dorotea Formations without nearly syndepositional Eocene arc populations in the Man Aike Formation. Composite age spectra for the Man Aike, Dorotea, Tres Pasos, Cerro Toro, and Punta Barrosa Formations are from Fildani et al. (2003); Romans et al. (2010); Fosdick et al. (2014); Schwartz et al. (2017); Daniels et al. (2017); Malkowski et al. (2017, 2018); Sickmann et al. (2018); and this study.

Miocene–Pliocene sedimentary and volcanic rocks (Fig. 2), significant (Biddle et al., 1986; Ghiglione et al., 2010; Rivera, 2017). Paleogene burial and subsequent removal is unlikely. Despite the considerable spatial and temporal scale of this un- Ali (2016) demonstrates a thermal inversion, whereby vitrinite re- conformity, its origins remain unclear. Here we consider potential flectance samples collected above the unconformity in the ManAike mechanisms for unconformity generation including: (1) local uplift Formation yield higher maximum temperatures than samples in the related to shortening in the fold-thrust belt; (2) eastward migration of a Dorotea Formation. When coupled with detrital zircon (U-Th)/He broad forebulge, (3) fall in sea level, (4) isostatic uplift associated with thermochronology, an analysis of porosity across the unconformity, and tectonic quiescence, (5) development of a slab window linked to sub- thermal modeling, Ali (2016) posits that the inversion suggests ∼717 m duction of a spreading ridge, and (6) regional foreland uplift during of eroded section consistent with an additional heat mechanism such as flat-slab subduction (Fig. 10). regional volcanism or hot fluid flow. Rather than sedimentary burial, the apparent thermal histories (1) Shortening and uplift of the frontal fold-thrust belt can generate implied by the thermochronometric data may reflect localized heating important unconformities in the proximal foreland basin (Fig. 10A; due to magmatic processes or fluid flow within the basin. In any case, DeCelles and Giles, 1996; Fosdick et al., 2014; Ghiglione et al., our results are inconsistent with deposition then erosional removal of a 2016; Echaurren et al., 2016). Proximal deformation in wedge-top ∼5 km thick sedimentary overburden in the Río de las Chinas region. settings is accompanied by flexural loading and foredeep accumu- Instead, we suggest that the major Paleogene unconformity represents a lation adjacent to the deformation front. Although growth strata period of nondeposition and/or sediment bypass, with relatively minor within the Man Aike and age equivalent Río Turbio and Río Ba- erosion that can account for scour relief on the order of a few tens of guales Formations (Malumián et al., 2000; Le Roux et al., 2010; meters, as observed in the study area. Otero et al., 2013; Ghiglione et al., 2016) attest to middle–late Eocene wedge-top interactions, these records postdate the timing of the regional disconformity, suggesting that wedge top interactions 5.5. Unconformity development alone are not a sufficient mechanism. Moreover, the absence of:(a) wedge-top growth strata, (b) an adjacent thick foredeep package, U-Pb geochronological data refine a ∼20 Myr disconformity be- and (c) significant angular discordance are inconsistent with a tween the Campanian–Danian Dorotea Formation and middle Eocene thrust-generated unconformity. Man Aike Formation. In the Río de las Chinas valley, the Dorotea is (2) Forebulges are broad flexural upwarps in the distal foreland that characterized by marine-deltaic deposits including sandstone, siltstone, tend to be expressed in the sedimentary record as condensed sec- and coal seams and the Man Aike is comprised of marine conglomerate tions in overfilled foreland basins or as unconformities in under- and sandstone deposited in tidal settings (Casadío et al., 2009; Schwartz filled basins (Fig. 10B; Horton and DeCelles, 1997; DeCelles, 2012; and Graham, 2015; Manríquez et al., 2019). The unconformity is re- Horton, 2018a). Passage of a forebulge in the Magallanes-Austral gional in extent, exhibits limited (< 2-5°) angular discordance, and is foreland basin (Wilson, 1991) is broadly consistent with the ∼20 diachronous across the basin, with overlying strata becoming progres- Myr hiatus represented by the unconformity, the limited angular sively younger to the north (Fig. 9; Macellari et al., 1989; Malumián discordance, and the diachroneity of the unconformity across the et al., 2000; Marenssi et al., 2002, 2003; Schwartz et al., 2017; basin. However, this scenario would require that the accumulation Sickmann et al., 2018; this study). The longest documented hiatus oc- of the preceding Cretaceous foreland basin succession occurred in a curs ∼150 km north of the Río de Las Chinas valley near Lago Viedma, backbulge setting, which is inconsistent with the rapid rates and where the Cangrejo Sandstone overlying the unconformity yields Mio- high magnitude of Late Cretaceous subsidence and sediment accu- cene maximum depositional ages (18.1 ± 0.32 Ma; Sickmann et al., mulation (Natland et al., 1974; Biddle et al., 1986; Sachse et al., 2018). Farther south, the hiatus diminishes substantially to < 10 Myr

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Fig. 8. (A) Compiled ages from the Southern Patagonian Batholith compared to: (B) a regional compilation of detrital zircon ages from Upper Cretaceous-Cenozoic deposits within the Magallanes-Austral Basin; compilations of detrital zircon ages from this study (C and F); and composite plots of compiled age distributions for Upper Cretaceous to Cenozoic age spectra (D, E, and G-J). We interpret two magmatic lulls from 140 to 115 Ma and from 60 to 45 Ma. Note that panel E excludes grains younger than 60 Ma and panels F to J were deposited in the Cretaceous and therefore would not record the interpreted magmatic lull between 60 and 45 Ma. Regional compilations are from Malkowski et al. (2017) with U-Pb and K-Ar ages from the Southern Patagonian Batholith from Halpern (1973); Hervé et al. (1984); Suarez et al. (1987); Bruce et al. (1991); and Hervé et al. (2007). Compiled detrital zircon U-Pb ages are from Fildani et al. (2003); Romans et al. (2010); McAtamney et al. (2011); Fosdick et al. (2014); Schwartz et al. (2017); Daniels et al. (2017); Malkowski et al. (2017, 2018); Sickmann et al. (2018); and this study. Adjacent to the sample names, N = shows the total number of samples, and n = shows the number of grains displayed versus the total number of grains.

2015; 2016; Horton, 2018a, 2018b). Moreover, flexural modeling (commonly delta plain) conditions. Although sea level fall may demonstrates limited forebulge migration in the Magallanes-Austral have diminished late Maastrichtian–Danian accommodation, eu- Basin (Fosdick et al., 2014), suggesting that the passage of a fore- static sea level rose above late Maastrichtian levels throughout the bulge was not responsible for the unconformity. early Eocene (Miller et al., 2005), suggesting that this was not the (3) A drop in eustatic (global) sea level can result in regional devel- controlling mechanism of unconformity development. opment of unconformity surfaces (Fig. 10C; e.g., Vail et al., 1984). (4) Slab window development associated with collision of the Aluk- In this case, early Paleocene (Danian) deposits of the uppermost Farallon spreading ridge (Cande and Leslie, 1986; Kay et al., 2002, Dorotea Formation below the unconformity correspond with a fall 2004; Ramos, 2005; Aragon et al., 2013) may have generated re- in eustatic sea level (Miller et al., 2005). Locally, relative sea level gional uplift and unconformity development (Fig. 10D). The slab fall is also supported by facies shifts in the Dorotea that suggest window is roughly coeval with the Paleogene lull in arc magmatism upward shallowing from shallow marine to distal nonmarine (Fig. 8) and extensive alkaline basaltic volcanism occurred in

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Fig. 9. Chronostratigraphic framework for the Magallanes-Austral Basin emphasizing the along-strike variability in the duration of the unconformity. Age constraints from maximum depositional ages and U-Pb geochronology on volcanic horizons. Note that the unconformity decreases in duration from north to south. See Biddle et al. (1986) for a chronostratigraphic correlation from subsurface data farther east (Giascosa et al., 2012; Olivero and Malumian, 2008; Quattrocchio and Sarjeant, 2003).

retroarc regions farther north (Posadas Basalt; ∼44–51°S). Re- Austral Basin (Biddle et al., 1986; Alvarez-Marrón et al., 1993), in cognizing the progressive southward growth of the slab window offshore regions of the Argentina and Malvinas basins (Ghiglione during Paleogene time, it is apparent that the processes of waning et al., 2010, 2014; Pinto et al., 2018). Distal offshore accumulation magmatism and unconformity genesis both initiated in the study (locally up to 900 m) in the Malvinas Basin may be related to the area well before ridge collision (Gianni et al., 2018b). Hence ridge interpreted Paleocene–Eocene phase of sediment bypass (Baristeas collision alone is insufficient to account for the Paleogene un- et al., 2013; Sachse et al., 2015). The broad wavelength and dia- conformity. chroneity of the unconformity along with the apparent coeval (5) The cessation of large-scale thrust loading and flexural subsidence magmatic lull may attest to cessation of regional shortening. (with possible minor extension) is likely to induce a regional iso- (6) Slab shallowing and flat-slab subduction are recognized by inboard static response, resulting in basin-wide low-angle unconformity migration of the magmatic arc followed by arc shutoff (Fig. 10F; surface across which clastic sediment is transported into the distal e.g., Coney and Reynolds, 1977; Dickinson and Snyder, 1978; foreland (Fig. 10E; Heller et al., 1988; Horton and Fuentes, 2016). Constenius et al., 2003; Ramos and Folguera, 2009). These changes For Patagonia, this would suggest a ca. 20 Myr period with no can be accompanied by intraplate deformation (e.g., intraforeland shortening and a stalled thrust front while sediment was reworked basement uplifts; Lawton, 2008; Yonkee and Weil, 2015), and/or by and bypassed into distal regions to the east. Such distal records generation of a regional unconformity surface (e.g., Yu and Chou, would likely be beyond available subsurface data of the Magallanes- 2001; Catuneanu, 2004). Recognizing an eastward migration and

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Fig. 10. Possible mechanisms of unconformity development. (A) local uplift related to shortening in the fold-thrust belt; (B) uplift of a broad forebulge, (C) fall in eustatic sea level, (D) uplift due to slab window development after spreading ridge collision, (E) isostatic uplift associated with tectonic quiescence (or potential minor extension), and (F) regional foreland uplift during flat-slab subduction.

waning of arc magmatism, Gianni et al. (2015, 2018a, 2018b) records of erosion and sediment accumulation in the distal foreland suggested a regional Maastrichtian to Eocene slab shallowing in basin are required to test these mechanisms and to rule out the other Patagonia. In this model, development of the Nale flat-slab at ∼48 possible models. to 35°S was followed by a magmatic lull from ∼65 to 50 Ma (Suarez and De la Cruz, 2000). If the shallow slab persisted farther south, 6. Conclusions this process may have helped generate broad regional uplift and unconformity development in the foreland. 1. Maximum depositional ages from the upper Dorotea Formation in the Río de las Chinas valley of the Magallanes-Austral Basin support On the basis of the large regional extent (from 49° to 54°S; Biddle the presence of the K/Pg boundary. The field area presents a unique et al., 1986; Alvarez-Marrón et al., 1993; Wilson, 1991; Sickmann et al., opportunity for paleontological and geological study of a transi- 2018; Pinto et al., 2018), limited angular discordance, temporal dia- tional marine-nonmarine K/Pg boundary section. The abundance of chroneity, and a relatively thin sedimentary overburden in the Río de vertebrate, invertebrate, and plant fossils may elucidate South las Chinas valley, we rule out options 1–3 as exclusive explanations and American ecological conditions before and after the mass extinction tentatively favor options 4–6 as potentially viable mechanisms for event, providing a southern mid-latitude boundary to compare with generating an unconformity at sufficiently large temporal and spatial those from the Northern Hemisphere. scales. Further constraints on Andean arc magmatism and Paleogene 2. Detrital zircon U-Pb geochronology suggests the Dorotea and Man

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Aike Formations were primarily fed by Andean sediment sources to Santa Cruz, Relatorio del 15 Congreso Geológico Argentino, pp. 103–128. the west, perhaps with limited contributions from the Deseado Barker, C.E., Pawlewicz, M.J., 1994. Calculation of vitrinite reflectance from thermal histories and peak temperatures: a comparison of methods. In: In: Mukhopadhyay, Massif. Based on similarity between age spectra, we suggest con- P.K., Dow, W.G. (Eds.), Vitrinite Reflectance as a Maturity Parameter, Applications tinuity of sediment sources between the Dorotea and Man Aike and Limitations: American Chemical Society Symposium Series, vol. 570. pp. Formations, with possible recycling of Upper Cretaceous strata 216–219. Baristeas, N., Anka, Z., Di Primio, R., Rodriguez, J.F., Marchal, D., Dominguez, F., 2013. during deposition of the Man Aike Formation. The appearance of New insights into the tectono-stratigraphic evolution of the Malvinas Basin, offshore additional age peaks in the Man Aike Formation may reflect the of the southernmost South American Argentian continental margin. Tectonophysics introduction of new source regions or unroofing in existing source 604, 280–295. regions. Bermúdez, H.D., García, J., Stinnesbeck, W., Keller, G., Rodríguez, J.V., Hanel, M., Hopp, J., Schwarz, W.H., Trieloff, M., Bolívar, L., Vega, F.J., 2016. The 3. U-Pb age spectra indicate two lulls in magmatic productivity from Cretaceous–Paleogene boundary at Gorgonilla Island, Colombia, south America. ca. 140–115 Ma and ca. 60–45 Ma. While the decrease in zircons Terra. Nova 28, 83–90. from 140 to 115 Ma may reflect sediment routing systems rather Bernhardt, A., Jobe, Z., Grove, M., Lowe, D., 2011. Palaeogeography and diachronous infill of an ancient deep-marine foreland basin, Upper Cretaceous Cerro Toro than a waning of magmatic activity, we suggest that the apparent Formation, Magallanes basin. Basin Res. https://doi.org/10.1111/j.1365-2117.2011. 60–45 Ma gap represents a magmatic lull, perhaps related to a 00528.x. posited contemporaneous flat-slab (Nalé flat-slab) to the north ofthe Biddle, K.T., Uliana, M.A., Mitchum, R.M., Fitzgerald, M.G., Wright, R.C., 1986. The stratigraphic and structural evolution of the central and eastern Magallanes Basin, study area. southern South America. Int. Assoc. Sedimentol. Spec. Publ. 8, 41–61. 4. U-Pb geochronological results constrain the age and duration of a Bruce, R.M., Nelson, E.P., Weaver, S.G., Lux, D.R., 1991. Temporal and spatial variations major unconformity to ∼20 Myr between the early Paleocene and in the southern Patagonian batholith; constraints on magmatic arc development. In: Andean Magmatism and its Tectonic Setting: Geological Society of America Special middle Eocene. Results from vitrinite reflectance data suggest lim- Paper, vol. 265. pp. 1–12. https://doi.org/10.1130/SPE265 -p1. ited removal of sedimentary overburden during unconformity gen- Butler, R.F., Hervé, F., Munizaga, F., Beck, M.E., Burmester, R.F., Oviedo, E.S., 1991. esis. Due to the regional extent, lack of significant angular dis- Paleomagnetism of the patagonian plateau Basalts, southern Chile and Argentina. J. Geophys. Res. 96, 6023–6034. cordance, coeval hiatus in volcanism, and limited erosion across the Calderón, M., Fildani, A., Herve, F., Fanning, C.M., Weislogel, A., Cordani, U., 2007. Late unconformity, we tentatively suggest a regional-scale process linked Jurassic bimodal magmatism in the northern sea-floor remnant of the Rocas Verdes to ridge collision (and slab-window development), cessation of basin, southern Patagonian Andes. J. Geol. Soc. Lond 162, 1011–1022. shortening and foreland isostatic rebound, and slab shallowing as Camacho, H.H., Chiesa, J.O., Parma, S.G., Reichler, V., 2000. Invertebrados marinos de la Formación man Aike (Eoceno medio), provincia de Santa Cruz, Argentina. Boletín de potential mechanisms of unconformity development. la Academia Nacional de Ciencias, Córdoba 64, 187–208. Cande, S.C., Leslie, R.B., 1986. Late Cenozoic tectonics of the southern Chile trench. J. Acknowledgements Geophys. Res. Solid Earth 91 (B1), 471–496. Casadío, S., Griffin, M., Marenssi, S., Net, L., Parras, A., Rodríguez-Raising, M., Santillana, S., 2009. Paleontology and sedimentology of middle Eocene rocks in lago Argentino Funding for this project was provided by the U.S. National Science area, Santa Cruz province, Argentina. Ameghiniana 46, 1–21. Foundation (NSF) Graduate Research Fellowships to S.W.M.G and Catuneanu, O., 2004. Retroarc foreland systems—evolution through time. J. Afr. Earth Sci. 38, 225–242. S.N.D., NSF grants OPP ANT-1141820 to J.A.C. and EAR-1348031 to Chernicoff, C., Zappettini, O., Santos, J., McNaughton, N., Belousova, E., 2013. Combined B.K.H, the Jackson School of Geosciences, University of Texas at Austin, U-Pb SHRIMP and Hf isotope study of the Late Paleozoic Yaminué Complex, Río the Fondecyt Grant 1151389, and the PIA-Conicyt Grant ACT172099. Negro Province, Argentina: implications for the origin and evolution of the Patagonia composite terrane. Geosci. Front 4, 37–56. Special thanks to the Instituto Antártico Chileno and Estancia Cerro Clyde, W.C., Wilf, P., Iglesias, A., Slingerland, R.L., Barnum, T., Bijl, P.K., Bralower, T.J., Guido for their support during field seasons and access to the field site. Brinkhuis, H., Comer, E.E., Huber, B.T., Ibañez-Mejia, M., Jicha, B.R., Krause, M., We gratefully acknowledge Kristina Butler, Hector Garza, and the Schueth, J.D., Singer, B.S., Raigemborn, M.S., Schmitz, M.D., Sluijs, A., Zamaloa, M.C., 2014. 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