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Using detrital zircon U-Pb ages to calculate Late Cretaceous sedimentation rates in the Magallanes-Austral basin, Patagonia
Article in Basin Research · April 2016 DOI: 10.1111/bre.12198
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The user has requested enhancement of the downloaded file. EAGE Basin Research (2016) 1–22, doi: 10.1111/bre.12198 Using detrital zircon U-Pb ages to calculate Late Cretaceous sedimentation rates in the Magallanes-Austral basin, Patagonia Theresa M. Schwartz,* Julie C. Fosdick† and Stephan A. Graham‡ *Department of Geology, Allegheny College, Meadville, PA, USA †Department of Geological Sciences, Indiana University, Bloomington, IN, USA ‡Department of Geological Sciences, Stanford University, Stanford, CA, USA
ABSTRACT Determining both short- and long-term sedimentation rates is becoming increasingly important in geomorphic (exhumation and sediment flux), structural (subsidence/flexure) and natural resource (predictive modelling) studies. Determining sedimentation rates for ancient sedimentary sequences is often hampered by poor understanding of stratigraphic architecture, long-term variability in large-scale sediment dispersal patterns and inconsistent availability of absolute age data. Uranium– Lead (U-Pb) detrital zircon (DZ) geochronology is not only a popular method to determine the provenance of siliciclastic sedimentary rocks but also helps delimit the age of sedimentary sequences, especially in basins associated with protracted volcanism. This study assesses the reliability of U-Pb DZ ages as proxies for depositional ages of Upper Cretaceous strata in the Magallanes-Austral retro- arc foreland basin of Patagonia. Progressive younging of maximum depositional ages (MDAs) calcu- lated from young zircon populations in the Upper Cretaceous Dorotea Formation suggests that the MDAs are potential proxies for absolute age, and constrain the age of the Dorotea Formation to be ca. 82–69 Ma. Even if the MDAs do not truly represent ages of contemporaneous volcanic eruptions in the arc, they may still indicate progressive-but-lagged delivery of increasingly younger vol- canogenic zircon to the basin. In this case, MDAs may still be a means to determine long-term (≥1– 2 Myr) average sedimentation rates. Burial history models built using the MDAs reveal high aggra- dation rates during an initial, deep-marine phase of the basin. As the basin shoaled to shelfal depths, aggradation rates decreased significantly and were outpaced by progradation of the deposystem. This transition is likely linked to eastward propagation of the Magallanes fold-thrust belt during Campa- nian-Maastrichtian time, and demonstrates the influence of predecessor basin history on foreland basin dynamics.
INTRODUCTION Sircombe (1999) demonstrated that Holocene beach sands from the passive margin of eastern Australia In addition to providing information about sediment yield a Permian MDA, approximately 250 Myr older source regions, DZ ages can help constrain depositional than their true age of deposition. Thus, interpreting a age. The youngest DZ age population present in a MDA to reflect a TDA relies on a thorough knowl- sample puts an upper constraint on its age, defining its edge of sediment source areas, sediment routing sys- maximum depositional age (MDA; after Fedo et al., tems and the adequate representation of source areas 2003; Dickinson & Gehrels, 2009). Many studies have within a sample. utilized DZ for this purpose (e.g. Nelson, 2001; Ste- In basins associated with protracted volcanism, such as wart et al., 2001; Surpless et al., 2006; Dickinson & forearc and some foreland basins, connectivity between Gehrels, 2009; and many others), as it can be espe- the volcanic arc and sedimentary depocenter allows deliv- cially informative for sedimentary successions which ery of volcanogenic sediment to the basin as wind-blown lack biostratigraphic data and/or volcanic rocks (ashes, ash or by fluvial systems that drain the arc. Although tuffs and lavas). In many cases, however, a MDA there can be exceptions and complications (e.g. McKay interpreted from DZ may not reflect the true deposi- et al., 2015), ashes and tuffs are most desirable for radio- tional age (TDA) of the sample. For example, metrically dating sedimentary successions because they generally display geologically insignificant lag times Correspondence: Theresa M. Schwartz, Department of Geol- between phenocryst crystallization, extrusion and depo- ogy, Allegheny College, 520 North Main Street, Meadville, PA sition (Fedo et al., 2003; Painter et al., 2014). How- 16335, USA. E-mail: [email protected] ever, they are difficult to preserve in terrestrial and
© 2016 The Authors Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 1 T. M. Schwartz et al. shallow-marine environments due to higher probabilities the southern Andes and an eastward-adjacent, deep- of erosion, reworking, bioturbation and pedogenesis. In marine retroarc foreland basin; (iii) filling of the Late such cases, DZ may provide supplemental age data. Cretaceous foredeep trough by a thick assemblage of Upper Cretaceous strata of the Magallanes-Austral ret- basinal, slope and shelf deposits; and (iv) Paleocene- roarc foreland basin, exposed in the south Andean fold- Eocene inversion of the Late Cretaceous foredeep and thrust belt, record an approximately 34 Myr history of eastward migration of the foreland depocenter (Fig. 2). arc development, foreland subsidence and basin filling. The ca. 6 km-thick siliciclastic package that fills the Late Preforeland basin history Cretaceous Magallanes foredeep is a conformable succes- sion of deep-marine turbidites capped by a prograding The predecessor Rocas Verdes back-arc basin developed slope and shelf sequence (e.g. Romans et al., 2011 and during latest Jurassic and Early Cretaceous time in associ- references within). Throughout Late Cretaceous time, ation with the break-up of Gondwana (Dalziel et al., the foredeep trough was fluvially connected to the west- 1974; Dalziel, 1981; Biddle et al., 1986; Wilson, 1991). ward-adjacent arc (Romans et al., 2011). Previous prove- The Rocas Verdes basin widened southward, with a nance studies of the deep-marine members of the basin higher degree of extension in the south than in the north fill have compared DZ MDAs to fossil data and ashes pre- (De Wit & Stern, 1981; Stern et al., 1992; Mukasa & Dal- served in time-equivalent fine-grained units and have ziel, 1996; Stern & De Wit, 2003). Extension was suffi- demonstrated that DZ MDAs can be reliable measures of cient to facilitate the development of quasi-oceanic crust TDAs in the Magallanes-Austral basin. This has pro- in the central part of the basin (Allen, 1982; Stern et al., vided crucial information regarding the tectonic and sedi- 1992), and was accompanied by deposition of bimodal mentary history of the foreland basin system including volcanic/volcaniclastic strata represented by the Jurassic the age of onset of sedimentation in the basin (Fildani & Tob ıfera and El Quemado Formations (ca. 188–153 Ma; Hessler, 2005; Malkowski et al., 2015), spatial variability Fig. 2) (Katz, 1963; Wilson, 1991). Subsequent thermal in sedimentation patterns (Romans et al., 2010; Bern- subsidence was accompanied by deposition of belemnite- hardt et al., 2011; Malkowski et al., 2015) and constraints and ammonite-bearing shale and marl of the Lower on episodic fold-thrust belt activity (Fosdick et al., 2011). Cretaceous Zapata and R ıo Mayer Formations (Fig. 2) Although there is some biostratigraphic control in (Biddle et al., 1986; Fildani & Hessler, 2005; Malkowski overlying shallow-marine units, similar age data from et al., 2015). ashes are not currently available. For this reason, there Closure of the Patagonian sector of the Rocas Verdes are relatively few absolute age constraints for the final fill- basin began in Early Cretaceous time (Fildani & Hessler, ing of the Late Cretaceous phase of the Magallanes-Aus- 2005; Malkowski et al., 2012; Betka et al., 2015) as sub- tral basin. This study (i) explores the feasibility of using duction began along the western margin of South Amer- DZ U-Pb ages, in the absence of ashes, to determine the ica. This caused obduction of the Sarmiento ophiolite absolute depositional age range of the Upper Cretaceous complex and growth of the Andean volcanic arc and fold- Dorotea Formation; (ii) uses new DZ age controls to cal- thrust belt (Wilson, 1991; Fildani & Hessler, 2005; Fil- culate temporal trends in sedimentation rates, with dani et al., 2008; Calder on et al., 2012; Torres-Carbonell emphasis on the final phase of Cretaceous basin filling; & Dimieri, 2013). and (iii) compares these rates to those of other advancing margins to assess the validity of the rates obtained using Foreland basin history DZ U-Pb ages. Collectively, this information provides important age control on latest Cretaceous filling of the East of the developing arc, the Late Cretaceous Magal- basin and helps to constrain the timing of tectonic events lanes foredeep underwent a protracted phase of deep- leading up to Cenozoic inversion of the Late Cretaceous marine deposition. The under-filled foredeep hosted a foredeep. long-lived (ca. 20 Myr), orogen-parallel and basin-axial, southward-directed sediment dispersal system (Katz, 1963; Scott, 1966; Winn & Dott, 1979; Fildani & Hessler, GEOLOGIC SETTING 2005; Crane & Lowe, 2008; Hubbard et al., 2008; Armi- tage et al., 2009; Covault et al., 2009; Schwartz & Gra- The Magallanes-Austral basin is a retroarc foreland basin ham, 2015). As a consequence, Upper Cretaceous associated with Late Cretaceous to Neogene uplift of the Magallanes-Austral stratigraphy provides a classic exam- southern Andean orogen (Fig. 1) (Wilson, 1991). Upper ple of highly diachronous basin fill in which deposits in Jurassic to Miocene rocks that are exposed in the Patago- the northern part of the basin are substantially older than nian sector of the basin, between 48°S and 53°S, record a their mapped equivalents to the south (e.g. Fig. 3) (Hub- dynamic basin history including: (i) the development of bard et al., 2010; Romans et al., 2010, 2011; Bernhardt an extensional back-arc basin associated with the breakup et al., 2011; Malkowski et al., 2015). Thus, formation of Gondwana; (ii) the initiation of subduction and partial boundaries are commonly lithostratigraphic, rather than inversion of the back-arc basin, resulting in the growth of chronostratigraphic, in nature (e.g. Fig. 3).
© 2016 The Authors 2 Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists U-Pb ages in the Magallanes-Austral basin
–75°0’00” –70°0’00” –72°30’00” (a) (b)
Chile Cerro Argentina Escondido Sierra Cerro Baguales Cagual
Cerro Guido –51°0’00”
–51°0’00” o L. Sarmient
El Chaltén Cerro Cazador –50°0’00” –50°0’00” ro L. del To El Calafate Cerro Castillo
Puerto Natales Argentina
Chile –51°30’00” –51°30’00”
Punta Arenas Sierra A Dorotea r ge Chile
n
t i na
–75°0’00” –70°0’00” Chile rgentina –72°30’00”
A TECTONOSTRATIGRAPHIC UNITS BATHOLITH INTRUSIVE AGES OTHER STRATIGRAPHIC UNITS OTHER Cenozoic foreland basin fill Neogene (25-15 Ma) Patagonia ice sheet Cenozoic (undivided) major lakes Mesozoic foreland basin fill Paleogene (67-40 Ma) major lakes Dorotea Fm (Kd) Cerros & Sierras L. Cretaceous (126-70 Ma) Tres Pasos Fm (Ktp) South Patagonia batholith detrital zircon samples M. Cretaceous (136-127 Ma) 060120 km Punta Barrosa & Cerro Toro U. Jurassic bimodal volcanics E. Cretaceous (144-137 Ma) Fms (Kpb/Kct; undivided) 07.515 km Paleozoic metasediments Jurassic (157-145 Ma) Zapata Fm (Kz)
Fig. 1. (a) Simplified geologic map of the Patagonian region of South America depicting the primary tectonostratigraphic units that make up the arc complex. Batholith intrusive age intervals are defined in Herv e et al. (2007) and are adapted here from Fos- dick et al. (2015). (b) Geologic map of the Ultima Esperanza study area depicting the distribution of stratigraphic units used in this study for basic basin modelling (modified from Fosdick et al., 2011). The red unit in the northeast corner of the study area repre- sents a succession of Neogene basalts that intrude the foreland basin succession. White circles show the lateral distribution of detri- tal zircon samples used in this study, with the majority of samples in the R ıo de las Chinas area (dashed box). White arrows depict average paleoflow directions interpreted for Cretaceous units of different ages, and illustrate the long-term history of axial pale- odrainage in the Magallanes foredeep (compiled from Romans et al., 2011; Bauer, 2012; Schwartz et al., 2012 and Schwartz & Graham, 2015).
Coarse clastic turbidites of the Punta Barrosa Forma- (Malkowski et al., 2015) to ca. 92–85 Ma in the Ultima tion record the onset of sedimentation in the flexural Esperanza district of Chile (Fildani & Hessler, 2005), foredeep (Fig. 2) (Wilson, 1991; Fildani & Hessler, reflecting a diachronous onset of coarse clastic deposi- 2005). The Punta Barrosa Formation yields decreasing tion (Malkowski et al., 2015). The Punta Barrosa For- depositional ages from north to south, from ca. mation is conformably overlain by the Cerro Toro 99–94 Ma in the Argentinian sector of the basin Formation (ca. 90–76 Ma; Fig. 2) (Bernhardt et al.,
© 2016 The Authors Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 3 T. M. Schwartz et al.
1000 RB-3B
JCF 09-208 RB-1A RT-2A shark teeth 900 (Maast.) Quat. Quaternary gravels upper delta plain: FA6 M supratidal +/- intertidal Cenozoic LCH-2A Hadrosaur (Maast.) retroforeland basin 800 strata (undivided), shallow-marine to O nonmarine E P 700 Panopaea mouth bar complex (Maast.) (shelf); subtidal FA5 Dorotea Fm
600 Tres Pasos Fm lower delta plain:
and paleosols; FA4 Magallanes-Austral 500 intertidal to supratidal retroforeland basin: Upper Cerro deep-marine to Toro Fm marginal marine Thickness (m)
CRETACEOUS Titanosaur (Campanian) 400 LCH-7.1A
Punta Barrosa Fm lower delta plain: 300 tidal channels,
L Zapata Fm FA2 interdistributary bays; subtidal
Tobifera Fm Rocas Verdes extensional back-arc basin: 200
Upper Sarmiento shallow-marine to Ophiolite deep-marine LCH-1C upper delta front: JURASSIC LCH-1B LCH-1A mouth bars,
Paleozoic FA3 distributary channels, metasedimentary 100 and tidal channels Pzbasement TERTIARY BWR CM-1 lithostratigraphic base ammonites (Campanian) upper slope: slumped,
FA1 shelf-derived material 0 ms ss & cgl
Fig. 2. Composite stratigraphic column depicting the sedimentary fill of the Magallanes-Austral basin foredeep, with a more detailed emphasis on the Upper Cretaceous Dorotea Formation. The Dorotea Formation is split into six facies associations, FA1 through FA6 (after Schwartz & Graham, 2015). Fossil locations and ages are listed in italics (Maast: Maastrichtian). Detrital zircon sample locations are denoted by stars.
2011), which is characterized by a network of deep-mar- prograded southward along the axis of the foredeep ine, conglomeratic, axial channel complexes encased in (Macellari et al., 1989; Shultz et al., 2005; Covault et al., overbank mudstone and sandstone. 2009; Hubbard et al., 2010; Schwartz & Graham, 2015). Shoaling of the Magallanes foredeep is recorded by the The Tres Pasos Formation (ca. 83–70 Ma; Fig. 2) Tres Pasos and Dorotea Formations. Together, they rep- (Romans et al., 2010) is composed of continental margin- resent a genetically linked shelf–slope system that scale slope clinoforms that are mudstone-dominated but
© 2016 The Authors 4 Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists U-Pb ages in the Magallanes-Austral basin
Río de las Chinas ~40 kmCerro Cazador ~60 km Sierra Dorotea Paleocene-Eocene unconformity TS13-RB-3B ?? Kd: heterolithic delta plain facies JCF 09-208 TS13-RB-1A JCF 09-226 TS12-LCH-2A TS11-RT-2A TS13-LCH-7.1A TS12-LCH-1C TS12-LCH-1B TS12-LCH-1A
Kd: sandy delta-front clinoforms BWR CM-1
P os sibl e tim e line BWR CCS-1 Ktp: shelf-edge slumps BWR SD-06
Ktp: muddy to heterolithic N slope clinoforms
Fig. 3. Schematic dip-oriented cross-section through the Dorotea (Kd) and uppermost Tres Pasos (Ktp) Formations in the Ultima Esperanza district of Chile. Southward-prograding delta and slope clinoforms illustrate a highly diachronous in-filling of the Magal- lanes-Austral foredeep during latest Cretaceous time. Clinoform surfaces represent approximate ‘time lines,’ or paleodepositional sur- faces, in the stratigraphy. Stars indicate the positions of detrital zircon samples. contain sandy channel networks (Armitage et al., 2009; STUDYAREA: ULTIMA ESPERANZA Romans et al., 2009; Hubbard et al., 2010). Topsets of DISTRICT, CHILE the slope clinoforms interfinger with shelf-edge deltaic deposits of the Dorotea Formation (ca. 80–68 Ma; Figs 2 The Upper Cretaceous Dorotea Formation and its equiv- > and 3), which is characterized by a thick set of sandstone- alents crop out intermittently for 150 km between the dominated, delta-front clinoforms overlain by heterolithic cities of Puerto Natales, Chile, and El Calafate, Argentina delta-plain facies (Schwartz & Graham, 2015). (Fig. 1a). The primary focus of this study is in the north- The Dorotea Formation is separated from Cenozoic east corner of the Ultima Esperanza District of Chile foreland basin sediments by a regional, Paleocene to mid- (Fig. 1b). Here, the Dorotea Formation is exposed in the Eocene unconformity that represents an episode of crustal east-dipping frontal monocline of the fold-thrust belt. shortening and eastward fold-thrust belt migration (Mal- Samples are from the Dorotea Formation stratigraphy umi an et al., 2000; Fosdick et al., 2015). This event described in Schwartz & Graham (2015), in the Rıo de las caused partial cannibalization of Upper Cretaceous to Chinas area (Fig. 1b). Additional samples from adjacent Paleocene foredeep deposits as they were incorporated (Cerro Cagual) and southern (Cerro Cazador, Sierra Dor- into the fold-thrust belt (Fosdick et al., 2015). Cenozoic otea) outcrop areas are published by Romans et al. (2010) foreland basin deposits that overlie the unconformity con- and Fosdick et al. (2015) (locations in Fig. 1b). stitute a relatively thin succession of sandstone-domi- nated, shallow-marine to terrestrial deposits that are mid- Eocene to Miocene in age (Malumi an et al., 2000). DETRITAL ZIRCON GEOCHRONOLOGY Sampling strategy Potential sediment source regions Detrital zircon samples encompass the entire exposed thickness of Dorotea Formation strata in the vicinity of Previous provenance studies based on thin-section pet- the Sierra Baguales, Chile. Medium-grained sandstone rography, mudstone geochemistry and DZ analysis indi- was preferentially collected from varying sedimentary cate that the bulk of siliciclastic sediment delivered to the facies within the Dorotea Formation (Figs 2 and 3) (after Magallanes-Austral foredeep was derived from terranes Schwartz & Graham, 2015). Sample locations and in the westward- to northwestward-located arc and fold- descriptions are listed in Table 2. thrust belt, primarily from Late Jurassic to Neogene plu- tons of the Patagonia batholith (Forsythe & Allen, 1980; Macellari et al., 1989; Wilson, 1991; Fildani et al., 2003; Detrital zircon U-Pb analytical techniques et al. et al. et al. Herve , 2003; Pankhurst , 2003; Herve , Analytical methods 2004; Fildani & Hessler, 2005; Romans et al., 2011; Bern- hardt et al., 2011; Fosdick et al., 2015; Schwartz & Gra- Detrital zircon grains were isolated at Stanford University ham, 2015; and references within). Other sources were following standard density and magnetic separation tech- located to the east (the ‘forebulge’ region) and northeast niques, summarized in Appendix S1 (e.g. DeGraaff- (intraforeland uplifts) (Fosdick et al., 2015; Schwartz & Surpless et al., 2003; Romans et al., 2010). U-Pb Graham, 2015). The known DZ signatures of these source geochronology of zircons was conducted by laser abla- terranes are summarized in Table 1. tion multicollector-inductively coupled plasma-mass
© 2016 The Authors Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 5 6 al. et Schwartz M. T. ai Research Basin ©
06Jh ie osLd uoenAscaino Geoscientists of Association European Ltd, Sons & Wiley John 2016 Table 1. Potential sources of siliciclastic sediment for the Late Cretaceous Magallanes-Austral basin
Dominant zircon Source terrane Local name Lithology Modern location Depositional age age range Reference(s)
Paleozoic metasedimentary Eastern Andean Meta-sandstone Eastern flank of Patagonian Devonian – early ca. 310–260 Ma Ramos (1989), basement complexes Metamorphic and -mudstone; marble Andes, 48°–51°S Carboniferous* Herv e et al. (2003) Complex (EAMC) Staines Complex Meta-sandstone Eastern flank of Patagonian Devonian – earliest ca. 310–260 Ma Herv e et al. (2003) and -mudstone; marble Andes south of 51°S Triassic* Duque de York Meta-sandstone, -siltstone, Western flank of Patagonian Post-Early Permian to 290–270 Ma Herv e et al. (2003), Complex (DYC) -shale, -conglomerate Andes, 49°–52°S pre-Early Cretaceous* Lacassie et al. (2006)
Jurassic rift-related Tob ıfera (Chile) and El Ryholitic-andesitic ignimbrites, Thrust sheets within Jurassic V1: 188–178 Ma; Gust et al. (1985), volcanic rocks Quemado Formations lavas, tuffs, and fold-thrust belt south V2: 172–167 Ma; Wilson (1991), (Argentina) volcaniclastic rocks of ca. 49°S; Deseado Massif V3: 162–153 Ma Pankhurst et al. (2000), intraforeland uplift Calder on et al. (2007), Herv e et al., 2007 Sarmiento ophiolite Mafic intrusive and Western flank of Patagonian Early Cretaceous 184–183 Ma Allen (1982), nier n nentoa soito fSedimentologists of Association International and Engineers & complex extrusive igneous rocks Andes, 51°–53°S Stern et al. (1992)
Andean volcanic arc Southern Patagonia Granodiorite and Patagonian Andes, 40°–56°S Late Jurassic – Miocene K1: 144–137 Ma; Stern & Stroup (1982), batholith tonalite, gabbro K2: 136–127 Ma; Herv e et al. (2007) K3: 126–75 Ma; 67–40 Ma
*Depositional age of protolith. © 06TeAuthors The 2016 ai Research Basin © Table 2. Descriptions of sandstone samples used for detrital zircon geochronology, listed in stratigraphic order by geographic location 06TeAuthors The 2016
Elev. Sample name Formation Latitude (°S) Longitude (°W) (m) Stratigraphic age Depositional environment Sandstone description ©
06Jh ie osLd uoenAscaino esinit nier n nentoa soito fSedimentologists of Association International and Engineers & Geoscientists of Association European Ltd, Sons & Wiley John 2016 R ıo de las Chinas area TS13-RB-3B Dorotea Fm 50.838270 72.385870 546 Maastrichtian Lower tidal flat Tan, coarse- to medium-grained sandstone; large-scale tangential foresets lined by ripple bedforms and mud drapes; minor bioturbation JCF 09-208‡ Dorotea Fm 50.831544 72.395144 422 Maastrichtian Delta plain (poorly exposed) Greenish, medium-grained sandstone; massive; abundant bioturbation TS13-RB-1A Dorotea Fm 50.754340 72.449910 944 Maastrichtian Upper tidal flat Tan, medium- to fine-grained sandstone; mud-lined flaser and wavy bedding; abundant bioturbation TS11-RT-2A Dorotea Fm 50.754530 72.452810 964 Maastrichtian Fluvial distributary channel Tan, medium- to coarse-grained sandstone; medium- to large-scale foresets and trough cross-bedding lined by mud drapes; minor bioturbation TS12-LCH-2A Dorotea Fm 50.713410 72.537010 1125 Maastrichtian Fluvial distributary channel Tan, medium- to coarse-grained sandstone; trough cross-stratified; abundant plant debris TS13-LCH-7.1A Dorotea Fm 50.704530 72.566040 918 Campanian Fluvial distributary channel White, ashy, medium- to fine-grained sandstone; trough cross-stratified; abundant detrital biotite TS12-LCH-1C Dorotea Fm 50.820329 72.511512 889 Campanian Upper distributary mouth Tan, very coarse- to medium-grained sandstone; amalgamated bar complex cut-and-fill units with abundant mud rip-up clasts and trough cross-stratification; rare bioturbation TS12-LCH-1B Dorotea Fm 50.820454 72.512053 867 Campanian Fluvial distributary channel Greenish pebble conglomerate with lenses of medium- to coarse-grained sandstone; planar-tabular cross-stratification TS12-LCH-1A Dorotea Fm 50.820557 72.512788 837 Campanian Lower distributary mouth Tan, fine- to medium-grained sandstone; amalgamated units of -bae nteMglae-uta basin Magallanes-Austral the in ages U-Pb bar complex planar, trough, and ripple cross-stratification; minor bioturbation BWR CM-1* Dorotea Fm 50.789957 72.661492 1029 Campanian Prodelta/shoreface Tan, medium- to fine-grained sandstone; hummocky to swaley cross-stratification Cerro Cazador area BWR CCS-1* Dorotea Fm 51.224200 72.348411 293 Campanian† Prodelta/shoreface Tan, medium- to fine-grained sandstone; pervasively bioturbated Sierra Dorotea area JCF 09-226‡ Dorotea Fm 51.680844 72.438361 574 Maastrichtian†,§ Delta-front clinoforms Greenish, medium- to fine-grained sandstone; pervasively bioturbated BWR SD-06* Dorotea Fm 51.608225 72.460430 670 Maastrichtian†,§ Prodelta/shoreface Greenish, medium- to fine-grained sandstone; pervasively bioturbated
*Romans et al. (2010). †Macellari et al. (1989). ‡Fosdick et al. (2015). §Hunicken€ (1955). Elev., Elevation. 7 T. M. Schwartz et al. spectrometry (LA-MC-ICPMS) at the University of programs available from the University of Arizona Laser- Arizona LaserChron Center using standard analytical Chron Center. Normal and cumulative probability plots techniques, also summarized in Appendix S1 (after in Figure 4 display data between 0 and 500 Ma to high- Gehrels et al., 2006, 2008). Analytical data are reported in light primary DZ populations. Appendix S2. To determine the MDA of each sample, we compared three different DZ-based measures of MDA (after Dick- inson & Gehrels, 2009) and compared zircon ages with Statistical methods the stratigraphic (fossil) age of each sample. Table 3 is a Probability density plots were created using ISOPLOT 3.7 compilation of DZ age measures for each sample includ- in Microsoft Excel (Ludwig, 2008). Data were plotted as ing the age of the youngest grain analyzed, the youngest histograms with superimposed probability curves to dis- peak age present in the probability curve and the calcu- play both age measurements and associated uncertainties, lated weighted mean age of that youngest peak. For this thereby highlighting the presence and/or absence of key study, the ‘youngest peak’ is defined as the youngest pop- DZ populations (raw histograms and Concordia plots are ulation of grains with overlapping ages (within error) that summarized in Appendix S3). Composite age-probability constitute a peak, with n ≥ 2 (after Dickinson & Gehrels, plots (Fig. 4) were constructed using open-source Excel 2009). For samples with indiscriminate young peaks, the
Age (Ma) 0 50 100 150 200 250 300 350 400 450 500
0.8
0.6
TS13-RB-3B 0.4 JCF 09-208 TS13-RB-1A Cumulative probability TS11-RT-2A TS12-LCH-2A TS13-LCH-7.1A 0.2 TS12-LCH-1C TS12-LCH-1B TS12-LCH-1A BWR CM-1
SPBtr. RVB EAMC
TS13-RB-3B (n = 43/64)
JCF 09-208 (n = 87/93)
TS13-RB-1A (n = 74/87)
TS11-RT-2A (n = 77/91) Fig. 4. Normal and cumulative proba- bility plots for all samples in the R ıo de TS12-LCH-2A (n = 74/92) las Chinas study area. Normal probability plots are stacked in stratigraphic order,
Normalized probability TS13-LCH-7.1A (n = 85/99) and emphasize key grain populations between 0 and 500 Ma. Coloured fill rep- TS12-LCH-1C (n = 73/95) resents primary source regions in the Andean orogen and fold-thrust belt TS12-LCH-1B (n = 75/99) (SPB: South Patagonia batholith; RVB: Rocas Verdes basin volcanic rocks; tr: TS12-LCH-1A (n = 82/94) transitional volcanic rocks; EAMC: East Andean Metamorphic Complex; see BWR CM-1 (n = 46/60) Table 1). Grey box represents the fossil 0 50 100 150 200 250 300 350 400 450 500 age of the Upper Cretaceous Dorotea Age (Ma) Formation.
© 2016 The Authors 8 Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists ai Research Basin © 06TeAuthors The 2016 © 06Jh ie osLd uoenAscaino esinit nier n nentoa soito fSedimentologists of Association International and Engineers & Geoscientists of Association European Ltd, Sons & Wiley John 2016
Table 3. Range of maximum depositional ages (MDAs) of detrital zircon samples
Mean square Interpreted Possible Stratigraphic Age of youngest Youngest peak Number of grains Weighted mean weighted Acceptable MSWD MDA TDA Sample name (fossil) age grain (Ma 1r) age* (Ma) in youngest peak (n) age (Ma 1r) deviation (MSWD) (Mahon, 1996) (Ma 1r) (Ma 1r)
R ıo de las Chinas area TS13-RB-3B Maastrichtian 90.5 3.8 91.0 2 96.3 3.2 3.00 0.001–5.020 96.3 3.2 – JCF 09-208§ Maastrichtian 69.7 0.9 69.3 2 69.7 1.7 0.02 0.001–5.020 69.7 1.7 69.7 1.7 TS13-RB-1A Maastrichtian 69.8 8.3 78.1 2 76.8 5.7 0.80 0.001–5.020 76.8 5.7 – TS11-RT-2A Maastrichtian 64.6 9.0 69.0 2 68.9 2.0 0.23 0.001–5.020 68.9 2.0 68.9 2.0 TS12-LCH-2A Maastrichtian 68.2 3.7 71.3 2 70.5 5.0 0.71 0.001–5.020 70.5 5.0 70.5 5.0 TS13-LCH-7.1A Campanian 73.6 3.6 79.2 21 78.0 1.8 0.20 0.480–1.710 78.0 1.8 78.0 1.8 TS12-LCH-1C Campanian 78.1 1.2 78.4 8 79.3 2.0 0.63 0.241–2.286 79.3 2.0 79.3 2.0 TS12-LCH-1B Campanian 82.3 0.7 94.0 10 95.5 1.5* 0.68 0.300–2.111 82.3 0.7 82.3 0.7 TS12-LCH-1A Campanian 85.7 3.0 90.2 3 88.2 3.4 0.50 0.025–3.690 88.2 3.4 – BWR CM-1 Campanian 93.9 2.6 95.4 3 95.1 4.0 0.23 0.025–3.690 95.1 4.0 – Cerro Cazador area BWR CCS-1† Campanian‡ 82.0 1.4 81.9 2 83.2 2.4 2.40 0.001–5.020 83.2 2.4 83.2 2.4 basin Magallanes-Austral the in ages U-Pb Sierra Dorotea area JCF 09-226§ Maastrichtian‡,¶ 65.6 3.3 65.8 5 68.6 0.5 2.70 0.121–2.775 68.6 0.5 68.6 0.5 BWR SD-06† Maastrichtian‡,¶ 70.4 3.5 71.3 4 72.4 3.9 0.50 0.072–3.117 72.4 3.9 72.4 3.9
*Excludes youngest single grain. †Romans et al. (2010). ‡Macellari et al. (1989). §Fosdick et al. (2015). ¶Hunicken€ (1955). TDA, true depositional age. 9 T. M. Schwartz et al.
Unmix function in Isoplot was used to elucidate the the youngest peak age within a sample is expected to be youngest peak prior to calculating its weighted mean age. slightly more robust than any youngest single-grain age The weighted mean age weights each measurement because there is minimal reproducibility of young ages within the cluster by its uncertainty, such that measure- (Dickinson & Gehrels, 2009), peak ages do not provide an ments with the smallest uncertainty have the greatest absolute measure of error calculated from that of con- impact on the final calculated age. For this calculation to be stituent analyses. meaningful, we assume that all zircon grains within the We consider the weighted mean age of the youngest cluster are of the same true age and that scatter around that peak within each sample to be the most conservative mea- age can be explained by analytical uncertainty. A mean sure of MDA (Table 3; Figs 5 and 6). Although no square weighted deviation (MSWD) was calculated for MSWDs are ‘good’ (equal to 1.0), all but one calculation each weighted mean age to evaluate the likelihood that all yield low-but-acceptable MSWDs at 95% confidence analyses within the cluster are coeval. This study presents a (after Mahon, 1996). The consistently low MSWDs for range of acceptable MSWDs for each weighted mean age new data indicate that uncertainty has been systematically calculation, based on the number of analyses contributing overestimated for all analyses. There is no evidence that to each calculation (Table 3; after Mahon, 1996). Weighted the youngest grain clusters in these samples contain mul- mean ages for samples first published in Romans et al. tiple age groups, supporting the assumption that all grains (2010) and Fosdick et al. (2015) were recalculated using the within the cluster are of the same true age, and that varia- same parameters as all new data, and therefore, vary tion in individual analyses can be explained through ana- slightly from their original publications. Figure 5 displays lytical uncertainty. the statistical results for all weightedmeanagecalculations. Sample TS13-LCH-7.1A, a reworked ash, is the only sample that yields a weighted mean age with an unaccept- able MSWD at 95% confidence (Table 3; after Mahon, Results of DZ U-Pb geochronology 1996). Similar to other samples, the MSWD calculated for this sample is very low (MSWD = 0.20) in spite of the Sandstone provenance number of grains contributing to the calculation (n = 21), Similar to previous studies, Dorotea Formation sand- further indicating that uncertainty has likely been overesti- stones yield DZ populations consistent with sources in mated. We maintain that the weighted mean age of the the arc, including the Southern Patagonia batholith sample is robust because (i) the sample is a reworked ash (ca. 144–65 Ma) and its metasedimentary host rocks that is lithologically distinct from other samples (Table 2) (ca. 310–270 Ma), as well as in the interior fold-thrust with abundant euhedral quartz and biotite grains, as well belt, including Rocas Verdes basin volcanic rocks as relatively abundant (by volume) Campanian-Maastrich- (ca. 188–153 Ma) (Fig. 4). Thin-section petrography tian DZ grains; (ii) the weighted mean age of the sample is supports derivation from these sources, with primary the most precise calculated age (2.3% error); (iii) system- grain types being quartz, feldspar (plagioclase potas- atic overestimation of uncertainty has produced consis- sium feldspar), intermediate volcanic rock fragments and tently low MSWDs for all samples, suggesting that it may minor (meta-) sedimentary rock fragments. In addition, not be statistically appropriate to eliminate the age based many DZ samples yield a small population of relatively solely on its MSWD; and (iv) as an ash, the sample should young zircon grains that range in age from ca. 80–68 Ma, provide reliable insight into coeval arc activity. which overlap with the mid-Campanian to Maastrichtian fossil age of the Dorotea Formation (Fig. 4). Maximum vs. true depositional age Although DZ data always provide a MDA, the MDA may Maximum depositional ages from detrital zircon not always represent a TDA. Fundamentally, a potential The age of the youngest single-grain analyzed in a sample TDA cannot be older than the MDAs calculated for was not considered to be a robust measure of MDA due to underlying samples, within the margins of error, and/or the possibility of sample contamination, zircon lead loss, should not be older than fossil ages of that horizon. etc. (after Dickinson & Gehrels, 2009). An exception was The MDAs calculated for samples in the R ıo de las made for sample TS12- LCH-1B, which yielded a young- Chinas area reveal a net upward-younging trend through est single-grain age of 82.3 0.7 Ma (Table 3). For this the Dorotea Formation stratigraphy with ages that are sample, the single-grain age is considered to be possible mostly consistent with mid-Campanian to Maastrichtian (although not robust) because (i) there is good correlation fossil ages (Fig. 6). We eliminate MDAs of samples between the single-grain age and fossil data; (ii) there is a TS13-RB-3B and TS13-RB-1A as potential TDAs discrete age difference between the youngest single grain because they are significantly older than those of underly- and the next-youngest group of ages in the sample ing samples (Fig. 6). In addition, although they fit the (ca. 12 Myr; Fig. 4); and (iii) the analysis has high preci- upward-younging trend, we eliminate the MDAs of the sion (standard error <1%). two lowest samples in the stratigraphy because they are Similarly, youngest peak ages (n ≥ 2) were not consid- significantly older than their fossil ages by up to ered to be the most robust measures of MDA. Although ca. 10 Myr (Fig. 6). The six remaining MDAs are
© 2016 The Authors 10 Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists U-Pb ages in the Magallanes-Austral basin
Río de las Chinas area Cerro Cazador area
100 100 100 95 95 95 90 90 90 85 85 85 80 80 80 75 75 75 70 70 70 Best Age (Ma) 65 TS13-RB-3B 65 TS13-LCH-7.1A 65 BWR CCS-1 60 WMA = 96.3±3.2 [3.4%] 60 WMA = 78.0±1.8 [2.3%] 60 WMA = 83.2±2.4 [2.8%] includes SE [1.1], 0 of 2 rejected 55 55 includes SE [1.3], 0 of 21 rejected 55 includes SE [1.0], 0 of 2 rejected MSWD = 3.0, error bars are 1σ MSWD = 0.2, error bars are 1σ MSWD = 2.4, error bars are 1σ 50 50 50
100 100 Sierra Dorotea area 95 95 90 90 100 85 85 95 80 80 JCF 09-226 90 75 75 WMA = 68.6±0.5 [0.8%] 85 70 70 includes SE [1.0], 0 of 5 rejected 80 MSWD = 2.7, error bars are 1σ 65 65 TS12-LCH-1C JCF 09-208 75 60 WMA = 69.7±1.7 [2.4%] 60 WMA = 79.3±2.0 [2.6%] includes SE [1.4], 0 of 8 rejected 70 55 includes SE [1.0], 0 of 2 rejected 55 MSWD = 0.02, error bars are 1σ MSWD = 0.6, error bars are 1σ 65 50 50 60
100 100 55 95 TS13-RB-1A 95 50 WMA = 76.8±5.7 [7.4%] 90 90 includes SE [0.8], 0 of 2 rejected 85 MSWD = 0.8, error bars are 1σ 85 Best Age (Ma) 100 80 80 95 75 75 90 70 70
Best Age (Ma) 85 Best Age (Ma) 65 65 TS12-LCH-1B 80 60 60 WMA = 95.5±1.5 [1.6%] 75 55 55 includes SE [1.1], 1 of 11 rejected MSWD = 0.7, error bars are 1σ 70 50 50 65 BWR SD-06 60 WMA = 72.4±3.9 [5.4%] 100 100 55 includes SE [1.0], 0 of 4 rejected 95 TS11-RT-2A 95 MSWD = 0.5, error bars are 1σ 50 90 WMA = 68.9±2.0 [2.9%] 90 includes SE [1.1], 0 of 2 rejected 85 85 MSWD = 0.2, error bars are 1σ 80 80 75 75 70 70 65 65 TS12-LCH-1A 60 60 WMA = 88.2±3.4 [3.9%] includes SE [0.9], 0 of 3 rejected 55 55 MSWD = 0.5, error bars are 1σ 50 50
100 100 TS12-LCH-2A 95 95 WMA = 70.5±4.9 [6.9%] 90 includes SE [1.0], 0 of 2 rejected 90 85 MSWD = 0.7, error bars are 1σ 85 80 80 75 75 70 70 65 65 BWR CM-1 60 60 WMA = 95.1±4.0 [4.2%] 55 55 includes SE [1.0], 0 of 3 rejected MSWD = 0.2, error bars are 1σ 50 50
Fig. 5. U-Pb detrital zircon weighted mean ages of the youngest zircon population present in each sample, including systematic error (SE) of the standard. Error bars (1r) are reported for individual zircon analyses and do not account for systematic error. Red bars indi- cate grains that were not included in the weighted mean age calculation. potential TDAs, and constrain the age of the Dorotea LCH-2A as a TDA because it has relatively high uncertainty Formation to be between ca. 82 Ma and 68 Ma (Fig. 6). that significantly overlaps the ages of adjacent samples Within this suite of potential TDAs, overlap in error (Fig. 6). In contrast, we consider the weighted mean age makes assigning absolute ages to some horizons problematic. from sample TS13-LCH-7.1A to be the most believable We also disregard the weighted mean age of sample TS12- TDA because the sample is a reworked ash with many young
© 2016 The Authors Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 11 T. M. Schwartz et al.
TS13-RB-3B 96.3 69.7 JCF 09-208
76.8 Fig. 6. Weighted mean ages from detri- TS13-RB-1A tal zircon samples placed in stratigraphic 68.9 context (composite stratigraphy dis- TS11-RT-2A played in Fig. 2), illustrating a net 70.5 upward younging in maximum deposi- TS12-LCH-2A
Sample tional age (MDA). Black MDAs are 78.0 potential true depositional ages (TDAs), TS13-LCH-7.1A whereas red MDAs are unreasonable 79.3 TDAs based on corroborating strati- TS12-LCH-1C graphic data. Error bars are 1r. Vertical 82.3 dashed lines illustrate Late Cretaceous TS12-LCH-1B age divisions (Con: Coniacian; Sant: San- 88.2 tonian; Maast: Maastrichtian) (after TS12-LCH-1A 95.1 Walker et al., 2012). Grey box depicts BWR CM-1 approx. fossil age the fossil age of the Dorotea Formation Cenomanian Turonian Con. Sant. Campanian Maast. (after Hunicken,€ 1955; Macellari et al., 100 95 90 85 80 75 70 65 1989; E. Bostelmann, pers. comm., 2014; Weighted mean age (Ma, ±1σ) M. Leppe, pers. comm., 2014) zircons, and because it has relatively low uncertainty (Fig. 6). model, we assume a basement thickness of 26 km beneath Using that horizon as an age and lithologic datum, we con- the study area (after Robertson Maurice et al., 2003; strain the first phase of deltaic sedimentation in the R ıo de Lawrence & Wiens, 2004). las Chinas area (delta-front progradation, FA3; Fig. 2) to be ca. between 82 and 79 Ma, and the second phase of sedi- Jurassic syn-rift volcanic and volcaniclastic rocks mentation (delta-plain aggradation, FA4 to FA6; Fig. 2) to be between ca. 78 and 69 Ma. Paleozoic metasedimentary basement is unconformably overlain by syn-rift volcanic and volcaniclastic rocks of the Jurassic Tob ıfera and El Quemado Formations (Katz, SEDIMENTATION RATES FROM DZ 1963; Wilson, 1991; Fildani & Hessler, 2005). This suc- GEOCHRONOLOGY cession fills extensional topography, thickening into gra- Calculation of decompacted sedimentation bens and thinning over basement highs, with an average rates thickness of 800 m (Biddle et al., 1986; their fig. 10; Wil- son, 1991). Exposures of the Tob ıfera and El Quemado Decompacted sedimentation rates for the Dorotea For- Formations near the field area indicate an intermediate to mation were calculated using a one-dimensional (1D) felsic composition. Shelfal depositional facies observed in â burial history model in Petromod . The resolution of U- volcaniclastic units indicate deposition in shallow-marine Pb ages reported in this study is insufficient to address environments (Biddle et al., 1986). sedimentary processes that occur on sub-Myr timescales (e.g. delta lobe switching), and as such, all calculations in Lower Cretaceous syn-rift shale: Zapata and Rıo Mayer this study are intended to be long-term (multi-Myr) aver- Formations age sedimentation rates. The model incorporates litho- logic data for the entire history of the Magallanes-Austral Jurassic volcanic and volcaniclastic rocks are overlain by basin area at the study area latitude (ca. 50° to 52°S) the shale-dominated Zapata and R ıo Mayer Formations (Table 4). (Katz, 1963). Seismic-reflection profiles of the Magallanes Basin from east of the fold-thrust belt suggest a thickness of ca. 400 m (Fosdick et al., 2011). Microfaunal assem- Basement blages from the lower Zapata Formation indicate deposi- Basement of the Magallanes-Austral basin is variably tion in water depths ranging from ca. 100 to 500 m attenuated from Jurassic to Early Cretaceous extension (Natland et al., 1974). associated with tectonic separation of the Gondwana supercontinent (Pankhurst et al., 2000). Rifting generated Upper Cretaceous foreland basin fill: Punta Barrosa, oceanic crust in some areas, preserved today by the Sar- Cerro Toro and Tres Pasos Formations miento ophiolite complex in the interior fold-thrust belt. East of the ophiolite complex, the Cretaceous-to-Miocene Although the deep-marine Upper Cretaceous formations basin is floored by attenuated Paleozoic metasedimentary are heterolithic, we use a bulk shale lithology in the model and meta-igneous rocks (Faundez et al., 2002). For this because sandstone and conglomerate units are commonly
© 2016 The Authors 12 Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists U-Pb ages in the Magallanes-Austral basin )
2 laterally discontinuous. Cumulative thicknesses of the deep-marine succession reach ca. 4 km within the study area (Biddle et al., 1986; Romans et al., 2010; Fosdick
Heat flow (mW m et al., 2011). Paleobathymetric estimates from microfau- nal assemblages suggest water depths of 1000–2000 m in the foredeep (Natland et al., 1974). Bulk lithology Shale 75 ShaleRhyoliteSchist 80 90 60 Shale 75 Sandstone 75 Upper Cretaceous foreland basin fill: Dorotea Formation Despite lithologic variability in the Dorotea Formation Eroded to (Ma) (Fig. 2), we model the Dorotea Formation as one unit to address long-term average sedimentation rates. We assign a bulk mudstone composition to the unit because, similar to its deep-marine counterparts, coarse clastic units are Eroded from (Ma) –– –– –– –– –– –– laterally discontinuous and encased in thick mudstone intervals. Paleo-water depths range from 100 m at the delta front to ≤10 m on the delta plain (after Schwartz & 500 10 – Graham, 2015). – Paleo-water depth (PWD) (m) Paleocene to mid-Eocene unconformity Cretaceous and Cenozoic units are separated by a regional Paleocene to mid-Eocene unconformity that represents Deposited to (Ma) ca. 20 Myr of missing time, from ca. 65 to 45 Ma (Mal- umi an et al., 2000; Schwartz et al., 2012; Fosdick et al., 2015; this study). We model the Paleocene unconformity 82 68 100 99 82 1500 43 0 10 Deposited from (Ma) 188310 153 260 100 100 153 99 500 in two ways to address multiple hypotheses regarding the thickness of missing section and the effect of these thick- nesses on burial history and decompaction. In the first model scenario, we use recent results that suggest that a substantial thickness of Paleocene overbur- Eroded thickness (m) – – – – – – den originally blanketed the region. Recent insight from paired DZ U-Pb geochronology and (U-Th)/He thermochronology indicates that Dorotea Formation sediments were buried to peak temperatures of Depth to base (m) ca. 164–180°C with peak burial between 54–46 Ma (Fos- dick et al., 2015). Assuming an average geothermal gradi- ent of 30°Ckm 1 (after Hamza & Munoz,~ 1996; Ramos
Depth to top (m) & Kay, 2002; Breitsprecher & Thorkelson, 2009), this indicates burial depths of 4.8–5.3 km if heating is solely due to post-depositional burial (Fosdick et al., 2015). To
4000 1900 5900 achieve this burial depth, decompacted sediment accu- Thickness (m) mulation rates of ca. 0.3–0.5 mm yr 1 would have had to persist throughout Paleocene time (Fosdick et al., 2015). Assuming a constant rate of deposition, these rates would result in 5 km of sedimentary overburden between ca. 65 and 53 Ma. Following deposition and burial, erosion , 2014) 0 1000 1000 5000 68 53 150 53 43 Shale 65 rates of ca. 0.5–2.7 mm yr 1 (Fosdick et al., 2015)
et al. would be needed to remove the overburden by mid- Eocene time, at ca. 43 Ma. A paleo-water depth of
1-D burial history model inputs ≥150 m and bulk shale lithology is assumed for this sce-
© nario because these high accumulation rates would neces-
1 (c.f. Fosdick 2 (c.f. Skarmeta & Castelli, 1997) 0 1000 1000 2000sitate rapid 68 subsidence 53 of 10 the basin floor 53 and a 43 rise in Shale sea 65 – – level.
PetroMod In a second model scenario, we use results that indi- cate a more modest overburden and burial depth. Coaly debris from outcrops of the Tres Pasos Formation col- (undifferentiated) Table 4. Layer Upper Cretaceous deep-marine foreland basin fill Jurassic syn-rift volcanic rocksPaleozoic metasedimentary basement 26 000 7100 800 6300 33 100 7100 Lower Cretaceous syn-rift shale 400 5900 6300 Unconformity Unconformity Upper Cretaceous Dorotea Formation 900 1000 1900 Cenozoic foreland basin fill (undifferentiated) 1000 0 1000 lected near Cerro Castillo yield vitrinite reflectance
© 2016 The Authors Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 13 T. M. Schwartz et al. values of ca. 0.5 %Ro which were modelled to infer as the foredeep shoaled to sea level, with sedimentation only ca. 2 km of stratigraphic overburden at this loca- rates up to four times lower (190–200 m Myr 1) during tion (Skarmeta & Castelli, 1997). Preliminary vitrinite deposition of the Dorotea Formation (Table 5; Fig. 7). reflectance data from Maastrichtian-Eocene units at Interestingly, recent interpretation of seismic-reflec- Cerro Castillo support this, yielding values between 0.4– tion data from northeast of the study area yields decom- 0.6 %Ro (Ali & Fosdick, 2015). We use the 2 km burial pacted, bulk sedimentation rates of 205 m Myr 1 for depth as an alternative end-member scenario. A paleo- the Upper Cretaceous section (Sachse et al., 2015). water depth of 10 m and bulk shale lithology is Although these rates compare well with our results for assumed because lower accumulation rates may reflect the Dorotea Formation, they are significantly lower than steadier subsidence rates, possibly maintaining coastal our results for the deep-marine members of the Magal- plain to shallow-marine environments similar to those lanes foredeep (Table 5). The difference in sedimenta- represented by the upper Dorotea Formation (e.g. tion rates likely reflects west-to-east lateral variation in Schwartz & Graham, 2015). accommodation and sediment supply during Late Creta- ceous time. The long-term history of basin-axial (south- ward) sediment transport that is recorded by the Punta Mid-Eocene to Miocene foreland basin fill Barrosa, Cerro Toro, Tres Pasos and Dorotea Forma- The Cenozoic foreland succession has a bulk sandstone tions is supported by seismic data that show eastward composition with a composite thickness of ca. 1 km adja- thinning and onlap of Upper Cretaceous strata onto cent to the study area (Malumi an et al., 2000; Fosdick attenuated basement rocks (Biddle et al., 1986; Fosdick et al., 2011, 2015). Depositional facies range from shallow et al., 2011; Sachse et al., 2015). Lithologic data from to nonmarine (Fosdick et al., 2015) with water depths of cores in the eastern Magallanes-Austral basin also reveal ≤10 m. that arc-derived sediment did not reach the eastern basin until mid-Eocene time, and that sediment derived Results of 1-D burial history model: long- from the arc was sequestered adjacent to the fold-thrust term average sedimentation rates belt prior to then (Biddle et al., 1986). Thus, it is rea- sonable that sedimentation rates, sustained by pro- Burial history plots reveal remarkably similar long-term nounced flexural subsidence (Fosdick et al., 2014) and average sedimentation rates in the Magallanes basin for voluminous input of arc- and fold-thrust belt-derived the two unconformity scenarios (Table 5; Fig. 7). Sedi- detritus, were much higher in the Magallanes foredeep mentation rates were highest during the deep-marine than in the eastern sector of the basin. phase of the basin, and decreased significantly as the basin filled. Calculation of deltaic progradation rates During the earliest period of deep-marine sedimenta- tion, represented by the Punta Barrosa and Cerro Toro Detrital zircon thermochronology reveals a remarkably Formations, sedimentation rates were approximately consistent thermal history along the length of the Dorotea 710 m Myr 1 (Table 5; Fig. 7). Aggradation rates Formation outcrop belt, indicating the same magnitude peaked and approached 715–745 m Myr 1 as the Tres and duration of heating of strata near R ıo de las Chinas Pasos shelf margin began to prograde from north to south and Sierra Dorotea (Fosdick et al., 2015). For this reason, (Table 5; Fig. 7). Aggradation rates decreased abruptly we assume that these areas have similar subsidence
Table 5. PetroMod © 1-D burial history model calculations for long-term average sedimentation rates
Case 1*: Case 2†: Sachse et al. Depositional sedimentation sedimentation (2015) Unit Age (Ma) Thickness (m) depth (m) rate (m Ma 1) rate (m Ma 1) (m Ma 1)
Cenozoic foreland basin fill 43–0 1000 ≤10 25.1 23.6 101–197 Paleocene 68–53 2000–5000 10–150 359.4 160 173 (represented by unconformity in Magallanes province) Camp.-Maast. Dorotea Formation 80–68 900 ≤100 201.7 189.8 205‡ Campanian Tres Pasos Formation 83–70 ca. 1000 ≥100 743.6 716.1 205‡ Cenomanian-Campanian Cerro Toro 99–76 ca. 3000 up to 1500 712.4 708.6 205‡ and Punta Barrosa Formations Lower Cretaceous syn-rift shale 153–99 400 500 14.1 14.1 116 Jurassic syn-rift volcanic rocks 188–153 800 100 22.9 22.9 168
*After Fosdick et al. (2015). †After Skarmeta & Castelli (1997). ‡Upper Cretaceous strata are grouped as one unit in Sachse et al. (2015).
© 2016 The Authors 14 Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists U-Pb ages in the Magallanes-Austral basin
Tri. Jurassic Cretaceous Paleogene Neogene Lower Middle Upper Lower Upper Pal. Eocene Oligo. Miocene Kd missing section 0 foreland (a) Ktp Cenozoic, basin undiv. initiation Dorotea Formation
Punta Barrosa, Cerro Toro, & Tres Pasos Fms, undiv. 5000
syn-rift shale
Depth [m] syn-rift volcanics
Paleozoic metasedimentary Sedimentation Rate basement 10000 0 200 400 600 800 [m/Myr]
Tri. Jurassic Cretaceous Paleogene Neogene Lower Middle Upper Lower Upper Pal. Eocene Oligo. Miocene Kd missing section 0 (b) foreland Ktp basin Cenozoic, initiation undiv. Dorotea Formation
Punta Barrosa, Cerro Toro, & Tres Pasos Fms, undiv. 5000
syn-rift shale
Depth [m] syn-rift volcanics
Paleozoic metasedimentary Sedimentation Rate basement 10000 0 200 400 600 800 [m/Myr]
200 150 100 50 5 Time [Ma]
Fig. 7. Burial history plots produced using unconformity scenarios proposed by (a) Fosdick et al. (2015) and (b) Skarmeta & Castelli (1997). Hot colours indicate high sedimentation rates, and cool colours indicate lower rates. Note that both (a) and (b) predict similar rates, which peak during deposition of the Tres Pasos Formation (Ktp) and decrease abruptly during deposition of the Dorotea For- mation (Kd). Models do not account for Neogene (post-18 Ma) uplift. histories and accommodation histories when calculating justified by paleo-transport directions, there is an inher- shelf-margin progradation rates. ent southward-younging of delta-front clinoforms in the To calculate progradation rates for the Dorotea Forma- Dorotea Formation such that: tion delta system, we use the rate calculation: t ¼ t1 t2 R ¼ D=t where t1 is the TDA at R ıo de las Chinas and t2 is the where R = rate (km Myr 1), D = distance (km) and TDA at Sierra Dorotea. t = time (Myr). In this calculation, D is the straight-line Poor exposure of delta-front facies in the R ıo de las distance between DZ samples from the R ıo de las Chinas Chinas area and discontinuities in the Dorotea outcrop area and the Sierra Dorotea area, which is approximately belt (e.g. Fig. 1b) make it difficult to trace deltaic clino- 100 km (Figs 1b and 3). For parameter t, we use TDAs form surfaces and make exact correlations between out- from DZ samples in the north and south to represent the crops along the length of the outcrop belt. However, diachronous nature of the basin fill (e.g. Fig. 3). As stratigraphic positioning of delta-front sandstone samples
© 2016 The Authors Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 15 T. M. Schwartz et al. in both areas allows for a reasonable comparison of ages: Romans et al., 2011; Schwartz & Graham, 2015). Fur- Samples TS12-LCH-1B and TS12-LCH-1C in the north thermore, many provenance studies have illustrated volu- and sample JCF 09-226 in the south are all from the tops minous sediment derivation from the arc and fold-thrust of sandy delta-front clinoforms, close to the contact belt (Macellari et al., 1989; Wilson, 1991; Fildani et al., between delta-front and delta-plain facies (Fig. 3). Using 2003; Fildani & Hessler, 2005; Bernhardt et al., 2011; paired TDAs from these samples, progradation rates cal- Romans et al., 2011), with Late Jurassic to Neogene plu- culated using R = D/t range from 7.3 to 9.3 km Myr 1, tons and volcanic rocks of the Southern Patagonia batho- nearly two orders of magnitude higher than aggradation lith being primary sources (e.g. Forsythe & Allen, 1980; rates calculated for the delta system. Herv e et al., 2003; Pankhurst et al., 2003; Herv e et al., 2004; Fosdick et al., 2015; this study). This provenance signature in fluvio-deltaic deposits of the Dorotea Forma- DISCUSSION tion (Fig. 4) demonstrates fluvial connectivity between Maximum depositional age as a proxy for the arc and the northern part of the Magallanes embay- depositional age: some assumptions ment. Similar to the Upper Cretaceous volcanic rocks that Using DZ to assess the depositional age of sandstone mantled the batholith, deep exhumation of the arc has requires knowledge of source-to-sink pathways in the eradicated any evidence for intra-arc or piggyback basins Magallanes-Austral basin system. We acknowledge the that may have coexisted with the Dorotea delta system. following assumptions: (i) the south Andean arc was Paleobotanical and paleontological data from this sector of active (i.e. extruding volcanic rock and sediment), albeit the basin indicate a warm and humid paleoclimate during episodically, throughout Late Cretaceous time; (ii) there Late Cretaceous time (Iglesias et al., 2011). This would was fluvial connectivity between the arc and the northern promote erosion and sediment bypass, rather than long- Magallanes embayment, thus enabling young zircon term accumulation, in wedge-top basins (e.g. Tucker & grains to reach the Dorotea Formation depocenter; (iii) Slingerland, 1997; Bookhagen & Strecker, 2012). Fur- volcanic detritus was not sequestered long term within thermore, previous age and provenance studies indicate the arc or fold-thrust belt, and the lag time between extru- that lag times between extrusion of volcanogenic zircon sion and deposition of volcanogenic zircon in the Magal- and sedimentary delivery to the foredeep were geologi- lanes embayment was geologically insignificant; and (iv) cally insignificant. For example, comparisons of sand- fluvial facies within the Dorotea Formation should best stone DZ ages to ash ages in the Punta Barrosa Formation represent the youngest available DZ populations from the (Malkowski et al., 2015) and Cerro Toro Formation arc. (Bernhardt et al., 2011) show that DZ MDAs closely overlap with ash ages, indicating relatively rapid transfer of volcanogenic detritus from the arc to the deep-marine South Andean arc activity foredeep. Although similar couplets of samples are not Since Paleocene time, the South Patagonia batholith has available for the Tres Pasos and Dorotea Formations, we been deeply exhumed (Thomson et al., 2001, 2010; Fos- assume the trend to be true through time because (i) the dick et al., 2013). The volcanic carapace to the Late Cre- arc and fold-thrust belt were the primary sources of sedi- taceous Andean arc no longer exists, and provides no ment for the foredeep throughout its history and (ii) insight into the distribution of volcanic rocks that would increasingly younger MDAs indicate progressive unroof- have fed the Magallanes embayment. However, the distri- ing of the arc. bution of Jurassic to Neogene plutons has been mapped across exposed parts of the South Patagonia batholith Depositional environment and detrital zircon heterogeneity between ca. 48°S and 55°S (Herv e et al., 2007). Late Cre- taceous plutons range in age from 126 to 65 Ma and are Even though sediment source areas were consistent scattered along the length of the batholith at this latitude through time, DZ samples from the Dorotea Formation (Fig. 1a). Although the plutonic age data and compiled inconsistently yield a MDA that is as a potential TDA. DZ data from foreland basin sediments support episodic The absence of young zircon populations in some samples arc activity (Fosdick et al., 2015), it is reasonable that does not necessarily indicate temporal changes in sediment young volcanogenic material was continuously eroded source areas, but instead may be a product of insufficient from the arc throughout Late Cretaceous time. DZ analyses per sample. The necessity of a larger sample size, in turn, may be influenced by depositional facies. Provenance studies commonly assume that sandstones Late Cretaceous source-to-sink pathways are homogenized, and that fractionation, segregation or Many studies document the existence of a long-lived, oro- dilution of zircon populations is unlikely to occur once gen-parallel and basin-axial, southward-directed pale- the sediment reaches the basin. This is particularly true odispersal system in the Late Cretaceous Magallanes for deep-marine sequences, where deposition in basin- foredeep (e.g. Katz, 1963; Scott, 1966; Winn & Dott, floor fans is often the final phase of sedimentation prior to 1979; Fildani & Hessler, 2005; Hubbard et al., 2010; lithification. However, these assumptions become
© 2016 The Authors 16 Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists U-Pb ages in the Magallanes-Austral basin increasingly unrealistic with depositional environments of sedimentation history increases the likelihood of suc- higher in the source-to-sink pathway, closer to sediment cess in identifying key grain populations. source areas, where competing depositional and erosional processes are active. Several studies illustrate that hetero- Deltaic sedimentation rates geneity in DZ populations may be profound when parent sands are poorly mixed (e.g. DeGraaff-Surpless et al., Aggradation vs. progradation 2003) and/or when there is secondary dilution or segrega- Long-term average sedimentation rates changed signifi- tion of grain populations (e.g. Williams et al., 2011). cantly during Late Cretaceous time. Adjacent to the fold- Dynamic shorelines, such as the one recorded by the thrust belt, the Magallanes foredeep supported relatively Dorotea Formation (after Schwartz & Graham, 2015), are high sedimentation rates during the deep-marine phase of likely places for dilution and/or segregation of grain pop- the basin. Aggradation rates peaked as the Tres Pasos ulations to occur. Because most sediment was fed to the slope began to prograde southward, approaching Magallanes embayment by large fluvial systems that 745 m Myr 1 (Table 5; Fig. 7). This was followed by a drained the arc, the freshest (i.e. youngest) arc-derived dramatic decrease in aggradation rate as the basin shoaled DZ populations should be located in fluvially dominated to sea level (ca. 200 m Myr 1; Table 5; Fig. 7). Concur- depositional facies. In this study, all DZ samples that rent progradation rates for the Dorotea delta system were yielded reasonable TDAs are from fluvially dominated nearly two orders of magnitude higher (up to depositional facies, including fluvial distributary channels 9.3 km Myr 1), illustrating the strongly progradational and distributary mouth bars proximal to distributary nature of the shelf margin. channels (Table 2; Figs 2 and 3). In contrast, all DZ sam- ples that yielded unreasonable TDAs are from deposi- tional facies that were significantly influenced by tidal Comparison of long-term average rates to analogous shelf currents, such as prodelta, delta-front and tidal flat depos- margins its (Table 2; Figs 2 and 3). Although this observation is not a comprehensive For this comparison, we assume that because the Dorotea assessment of zircon fractionation by depositional envi- delta system was a shelf-edge delta for much of its exis- ronment, it alludes to the effects that secondary sedimen- tence (e.g. Porebski & Steel, 2003; Covault et al., 2009; tary processes may have on provenance and age Schwartz & Graham, 2015), the progradation rate calcu- signatures. In this case, the continual redistribution of lated for the delta also approximates the progradation rate sediment by tides likely resulted in dilution of young of the entire shelf margin. Figure 8 is a compilation of grain populations, leading to the potential for skewing paired aggradation and progradation rates for shelf mar- interpretations towards older, more voluminous grain gins of different ages and tectonic settings. The paired populations. This is undoubtedly a question of DZ sam- rates are long-term (multi-Myr) average rates calculated ple size, but also indicates that a detailed understanding from biostratigraphic and/or magnetostratigraphic
100 Convergent margins 90 Passive margins Rifted margins
) 80 Magallanes basin
–1 modeled after Carvajal et al., 2009 70
60 WSk
50 LFHk 40 GOMm ORm 30 ORpl GOMp GOMo NJm NSAk Progradation rate (km Myr 20 Kd ORm Ktp GOMe 10 NSk SIWm 0 NJo GOMe 0 100 200 300 400 500 600 700 800 900 1000 Aggradation rate (m Myr–1)
Fig. 8. A comparison of long-term average aggradation and progradation rates calculated for the Dorotea (Kd) and Tres Pasos (Ktp) Formations, denoted by red stars, with those reported for other ancient shelf margins from around the world. Points lying directly on an axis do not have a corresponding value of ‘0’, but have only one of the two values reported in the literature. Data are compiled from Carvajal et al. (2009) and references within; Harris et al. (1993), Rao (1993) and Worm et al. (1998), and are tabulated in Appendix S4. Select abbreviations are as follows: GOM, Gulf of Mexico; LFH, Lewis-Fox Hills (Wyoming); NJ, New Jersey; NS, Nova Scotia; NSA, North Slope of Alaska; OR, Orinoco; SIW, Siwaliks (India); WS, West Siberia; pl, Pliocene; m, Miocene; o, Oligo- cene; e, Eocene; p, Paleocene; k, Cretaceous.
© 2016 The Authors Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists 17 T. M. Schwartz et al. records (Carvajal et al., 2009; and references within; Har- environments to highly progradational shelf-slope envi- ris et al., 1993; Rao, 1993; Worm et al., 1998; data tabu- ronments in the Magallanes-Austral basin, and subse- lated in Appendix S4). All data are from Early Cretaceous quent basin filling, is likely a response to arc and/or fold- to Pliocene shelf margins where progradation was driven thrust belt activity. by fluvio-deltaic sediment input (after Carvajal et al., We postulate that southward progradation and shoaling 2009). We exclude Quaternary examples because: (i) of the foredeep deposystem was directly linked to Late many Quaternary systems with documented rates, such as Cretaceous activity in the fold-thrust belt. A significant the Holocene Mekong (Ta et al., 2002) and Yangtze (Hori thrust event, ultimately incorporating Upper Cretaceous et al., 2001) deltas, display excessive reported rates (up to foredeep deposits into the fold-thrust belt, has been three orders of magnitude higher than those reported for loosely constrained between early Campanian and early older systems) that are likely linked to growth and erosion Oligocene time (74–27 Ma Tenerife thrust period; Fos- of the Tibetan Plateau; and (ii) rates for Quaternary sys- dick et al., 2011). This is contemporaneous with a major tems are often calculated over much shorter timescales period of tectonic loading and crustal shortening in the (e.g. kyr instead of Myr), and therefore may represent Fuegian Andes, located immediately to the south (Klepeis individual periods of rapid, focused progradation, rather et al., 2010). Early stages of this thrust event progressively than multi-Myr averages. Only two datasets provide embayed the Magallanes foredeep, effectively ‘closing’ the decompacted sedimentation rates, and as such, most rates basin from north to south as the foredeep narrowed and are considered minimum estimates. accommodation was physically reduced (after Schwartz & The long-term average sedimentation rates calculated Graham, 2015). Thrust propagation also generated sedi- from DZ ages are similar to those reported for other ment source terranes more proximal to the basin, increas- ancient analogues (Fig. 8), indicating that the DZ- ing sediment supply and caliber (after Schwartz & derived rates are reasonable estimates for an advancing Graham, 2015). Conglomerate in the upper Dorotea For- shelf margin. The calculated aggradation and prograda- mation is dominated by felsic to intermediate (meta-) vol- tion rates compare well with other convergent margin canic lithologies (Schwartz & Graham, 2015), indicating deposystems (Fig. 8), but most of these examples are unroofing of thrust sheets containing Upper Jurassic vol- from relatively broad, shallow basins with ramp-like floors canic rocks. Detrital zircon data support this, and display a that lack a discrete shelf-slope-basin floor profile. For this subtle upward increase in ages corresponding to late-stage reason, these systems are not necessarily appropriate ana- Rocas Verdes volcanism (162–153 Ma; V3 of Pankhurst logues for the Late Cretaceous Magallanes-Austral basin et al., 2000) (Fig. 4). The South Patagonia batholith also deposystem. However, the rates are also very similar to remained a primary source of sediment throughout this decompacted rates of high-relief passive margins, such as time (144–65 Ma; Fig. 4) indicating continued connectiv- the Paleogene Gulf of Mexico (Fig. 8). In spite of their ity between the arc and foredeep. Ultimately, subsidence contrasting tectonic settings, the Paleogene Gulf of Mex- in the narrowing basin was overwhelmed by the influx of ico margin and the Late Cretaceous Magallanes-Austral sediment from the arc complex, and the shelf-slope system margin had shelf-to-basin floor profiles of similar propor- prograded southward in response. tions (after Hubbard et al., 2010) and were both charac- terized by high sediment influx. For these reasons, progradation and aggradation rates from passive margin CONCLUSIONS analogues may be a better comparison for the Dorotea- Tres Pasos system than documented convergent margin Detrital zircon U-Pb geochronology is a powerful tool to analogues (e.g. Covault et al., 2009). In all cases, progra- assess the provenance of clastic sedimentary rocks, and in dation outpaces aggradation by at least one order of mag- some scenarios, it is a means to assess depositional age. nitude (Fig. 8). This indicates that sediment supply Using DZ data for this purpose relies on fundamental exceeded accommodation, and that the supply-dominated knowledge of sediment source areas, source-to-sink path- margins (Carvajal et al., 2009) prograded in response to ways and the adequate representation of source terranes sediment influx. within a sample. We argue that MDAs in the Upper Cre- taceous Dorotea Formation can be used as proxies for TDA, and they constrain the age of the Dorotea Forma- Implications for basin evolution tion to be between ca. 82–69 Ma (Fig. 6). Spatio-temporal fluctuations in accommodation and sedi- Even if MDAs do not truly represent the ages of coeval mentation in foreland basins have been linked to episodic volcanic eruptions in the arc, their progressive younging uplift and unroofing of thrust sheets, structural partition- may still indicate a progressive-but-lagged delivery of vol- ing of basins, magmatic fluxes in the arc and even shifts in canogenic sediment to the basin. Assuming consistent lag climatic regime (e.g. Posamentier & Allen, 1993; Kamola rates through time, MDAs can be used to calculate sedi- & Huntoon, 1995; Coney et al., 1996; DeCelles & Currie, mentation rates. One-dimensional burial history models 1996; Horton & DeCelles, 1997; Horton et al., 2004; that incorporate DZ MDAs reveal significant changes in DeCelles et al., 2009; and many others). Similarly, the sedimentation rates during the Late Cretaceous history of transition from rapidly aggrading deep-marine the Magallanes-Austral foreland basin. The long-lived,
© 2016 The Authors 18 Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists U-Pb ages in the Magallanes-Austral basin deep-marine Magallanes foredeep hosted high aggrada- SUPPORTING INFORMATION tion rates that peaked during deposition of the Tres Pasos Formation, and then decreased dramatically as the fore- Additional Supporting Information may be found in the deep shoaled to sea level during deposition of the Dorotea online version of this article: Formation (Table 5; Fig. 7). As aggradation rates slowed, the foredeep deposystem became highly progradational, Appendix S1. Separation, analytical and statistical with progradation rates that outpaced aggradation by techniques for detrital zircon analysis and interpretation. nearly two orders of magnitude. These rates are compara- Appendix S2. U-Pb detrital zircon geochronologic ble to those determined for other ancient advancing mar- analyses by LA-ICP-MS. gins (Fig. 8). Appendix S3. Raw histograms and Concordia plots for The transition from aggradational, deep-marine envi- U-Pb detrital zircon analyses. ronments to a highly progradational slope and shelf Appendix S4. Compilation of reported aggradation reflects activity in the Patagonian arc and fold-thrust belt, and progradation rates for ancient shelf margins. and illustrates the influence of the predecessor Rocas Verdes basin on Magallanes-Austral foreland basin evolu- tion. Southward progradation of the Tres Pasos-Dorotea deposystem indicates that accommodation in the northern Magallanes embayment was insufficient for incoming sed- REFERENCES iment loads. This lack of accommodation was likely due to ALI,R.&FOSDICK, J.C. 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© 2016 The Authors 20 Basin Research © 2016 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists U-Pb ages in the Magallanes-Austral basin
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