Pb ages to calculate Late Cretaceous ... - Wiley Online Library

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 retroarc foreland basin of Patagonia. Progressive younging of maximum depositional ages (MDAs) calculated 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 volcanogenic 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 aggradation 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 Campanian-Maastrichtian time, and demonstrates the influence of predecessor basin history on foreland basin dynamics.

INTRODUCTION In addition to providing information about sediment source regions, DZ ages can help constrain depositional age. The youngest DZ age population present in a sample puts an upper constraint on its age, defining its maximum depositional age (MDA; after Fedo et al., 2003; Dickinson & Gehrels, 2009). Many studies have utilized DZ for this purpose (e.g. Nelson, 2001; Stewart et al., 2001; Surpless et al., 2006; Dickinson & Gehrels, 2009; and many others), as it can be especially informative for sedimentary successions which lack biostratigraphic data and/or volcanic rocks (ashes, tuffs and lavas). In many cases, however, a MDA interpreted from DZ may not reflect the true depositional age (TDA) of the sample. For example, Correspondence: Theresa M. Schwartz, Department of Geology, Allegheny College, 520 North Main Street, Meadville, PA 16335, USA. E-mail: [email protected]

Sircombe (1999) demonstrated that Holocene beach sands from the passive margin of eastern Australia yield a Permian MDA, approximately 250 Myr older than their true age of deposition. Thus, interpreting a MDA to reflect a TDA relies on a thorough knowledge of sediment source areas, sediment routing systems and the adequate representation of source areas within a sample. In basins associated with protracted volcanism, such as forearc and some foreland basins, connectivity between the volcanic arc and sedimentary depocenter allows delivery of volcanogenic sediment to the basin as wind-blown ash or by fluvial systems that drain the arc. Although there can be exceptions and complications (e.g. McKay et al., 2015), ashes and tuffs are most desirable for radiometrically dating sedimentary successions because they generally display geologically insignificant lag times between phenocryst crystallization, extrusion and deposition (Fedo et al., 2003; Painter et al., 2014). However, 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 of erosion, reworking, bioturbation and pedogenesis. In such cases, DZ may provide supplemental age data. Upper Cretaceous strata of the Magallanes-Austral retroarc foreland basin, exposed in the south Andean foldthrust belt, record an approximately 34 Myr history of arc development, foreland subsidence and basin filling. The ca. 6 km-thick siliciclastic package that fills the Late Cretaceous Magallanes foredeep is a conformable succession of deep-marine turbidites capped by a prograding slope and shelf sequence (e.g. Romans et al., 2011 and references within). Throughout Late Cretaceous time, the foredeep trough was fluvially connected to the westward-adjacent arc (Romans et al., 2011). Previous provenance studies of the deep-marine members of the basin fill have compared DZ MDAs to fossil data and ashes preserved in time-equivalent fine-grained units and have demonstrated that DZ MDAs can be reliable measures of TDAs in the Magallanes-Austral basin. This has provided crucial information regarding the tectonic and sedimentary history of the foreland basin system including the age of onset of sedimentation in the basin (Fildani & Hessler, 2005; Malkowski et al., 2015), spatial variability in sedimentation patterns (Romans et al., 2010; Bernhardt et al., 2011; Malkowski et al., 2015) and constraints on episodic fold-thrust belt activity (Fosdick et al., 2011). Although there is some biostratigraphic control in overlying shallow-marine units, similar age data from ashes are not currently available. For this reason, there are relatively few absolute age constraints for the final filling of the Late Cretaceous phase of the Magallanes-Austral basin. This study (i) explores the feasibility of using DZ U-Pb ages, in the absence of ashes, to determine the absolute depositional age range of the Upper Cretaceous Dorotea Formation; (ii) uses new DZ age controls to calculate temporal trends in sedimentation rates, with emphasis on the final phase of Cretaceous basin filling; and (iii) compares these rates to those of other advancing margins to assess the validity of the rates obtained using DZ U-Pb ages. Collectively, this information provides important age control on latest Cretaceous filling of the basin and helps to constrain the timing of tectonic events leading up to Cenozoic inversion of the Late Cretaceous foredeep.

GEOLOGIC SETTING The Magallanes-Austral basin is a retroarc foreland basin associated with Late Cretaceous to Neogene uplift of the southern Andean orogen (Fig. 1) (Wilson, 1991). Upper Jurassic to Miocene rocks that are exposed in the Patagonian sector of the basin, between 48°S and 53°S, record a dynamic basin history including: (i) the development of an extensional back-arc basin associated with the breakup of Gondwana; (ii) the initiation of subduction and partial inversion of the back-arc basin, resulting in the growth of

2

the southern Andes and an eastward-adjacent, deepmarine retroarc foreland basin; (iii) filling of the Late Cretaceous foredeep trough by a thick assemblage of basinal, slope and shelf deposits; and (iv) PaleoceneEocene inversion of the Late Cretaceous foredeep and eastward migration of the foreland depocenter (Fig. 2).

Preforeland basin history The predecessor Rocas Verdes back-arc basin developed during latest Jurassic and Early Cretaceous time in association with the break-up of Gondwana (Dalziel et al., 1974; Dalziel, 1981; Biddle et al., 1986; Wilson, 1991). The Rocas Verdes basin widened southward, with a higher degree of extension in the south than in the north (De Wit & Stern, 1981; Stern et al., 1992; Mukasa & Dalziel, 1996; Stern & De Wit, 2003). Extension was sufficient to facilitate the development of quasi-oceanic crust in the central part of the basin (Allen, 1982; Stern et al., 1992), and was accompanied by deposition of bimodal volcanic/volcaniclastic strata represented by the Jurassic Quemado Formations (ca. 188–153 Ma; Tobıfera and El Fig. 2) (Katz, 1963; Wilson, 1991). Subsequent thermal subsidence was accompanied by deposition of belemniteand ammonite-bearing shale and marl of the Lower Cretaceous Zapata and Rıo Mayer Formations (Fig. 2) (Biddle et al., 1986; Fildani & Hessler, 2005; Malkowski et al., 2015). Closure of the Patagonian sector of the Rocas Verdes basin began in Early Cretaceous time (Fildani & Hessler, 2005; Malkowski et al., 2012; Betka et al., 2015) as subduction began along the western margin of South America. This caused obduction of the Sarmiento ophiolite complex and growth of the Andean volcanic arc and foldthrust belt (Wilson, 1991; Fildani & Hessler, 2005; Fildani et al., 2008; Calderon et al., 2012; Torres-Carbonell & Dimieri, 2013).

Foreland basin history East of the developing arc, the Late Cretaceous Magallanes foredeep underwent a protracted phase of deepmarine deposition. The under-filled foredeep hosted a long-lived (ca. 20 Myr), orogen-parallel and basin-axial, southward-directed sediment dispersal system (Katz, 1963; Scott, 1966; Winn & Dott, 1979; Fildani & Hessler, 2005; Crane & Lowe, 2008; Hubbard et al., 2008; Armitage et al., 2009; Covault et al., 2009; Schwartz & Graham, 2015). As a consequence, Upper Cretaceous Magallanes-Austral stratigraphy provides a classic example of highly diachronous basin fill in which deposits in the northern part of the basin are substantially older than their mapped equivalents to the south (e.g. Fig. 3) (Hubbard et al., 2010; Romans et al., 2010, 2011; Bernhardt et al., 2011; Malkowski et al., 2015). Thus, formation boundaries are commonly lithostratigraphic, rather than chronostratigraphic, in nature (e.g. Fig. 3).


U-Pb ages in the Magallanes-Austral basin –75°0’00”

–70°0’00”

Chile Argentina

(a)

–72°30’00”

(b) Cerro Escondido

Sierra Baguales

Cerro Cagual

–51°0’00”

–51°0’00”

Cerro Guido

iento

rm L. Sa

Cerro Cazador

–50°0’00”

–50°0’00”

El Chaltén

El Calafate

Argentina Chile

Cerro Castillo

–51°30’00”

–51°30’00”

Puerto Natales

l Toro

L. de

Punta Arenas

Cenozoic foreland basin fill Mesozoic foreland basin fill South Patagonia batholith U. Jurassic bimodal volcanics Paleozoic metasediments

Chile Argentina

–75°0’00” TECTONOSTRATIGRAPHIC UNITS

–70°0’00” OTHER

Neogene (25-15 Ma) Paleogene (67-40 Ma) L. Cretaceous (126-70 Ma) M. Cretaceous (136-127 Ma) E. Cretaceous (144-137 Ma) Jurassic (157-145 Ma)

Patagonia ice sheet major lakes

0

60

–72°30’00” STRATIGRAPHIC UNITS

BATHOLITH INTRUSIVE AGES

120 km

OTHER major lakes

Cenozoic (undivided) Dorotea Fm (Kd) Tres Pasos Fm (Ktp) Punta Barrosa & Cerro Toro Fms (Kpb/Kct; undivided)

tina Argen Chile

Sierra Dorotea

Cerros & Sierras detrital zircon samples

0

7.5

15 km

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 Herve et al. (2007) and are adapted here from Fosdick 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 represents a succession of Neogene basalts that intrude the foreland basin succession. White circles show the lateral distribution of detrital zircon samples used in this study, with the majority of samples in the Rıo 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 paleodrainage 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 Formation record the onset of sedimentation in the flexural foredeep (Fig. 2) (Wilson, 1991; Fildani & Hessler, 2005). The Punta Barrosa Formation yields decreasing depositional ages from north to south, from ca. 99–94 Ma in the Argentinian sector of the basin

(Malkowski et al., 2015) to ca. 92–85 Ma in the Ultima Esperanza district of Chile (Fildani & Hessler, 2005), reflecting a diachronous onset of coarse clastic deposition (Malkowski et al., 2015). The Punta Barrosa Formation is conformably overlain by the Cerro Toro Formation (ca. 90–76 Ma; Fig. 2) (Bernhardt et al.,


3

T. M. Schwartz et al. 1000

RB-3B JCF 09-208 RB-1A

900

FA6

Quaternary gravels

M Cenozoic retroforeland basin strata (undivided), shallow-marine to nonmarine

O

800

E P

Panopaea (Maast.)

700

upper delta plain:

supratidal +/- intertidal

LCH-2A Hadrosaur (Maast.)

FA5

TERTIARY

Quat.

RT-2A shark teeth (Maast.)

mouth bar complex (shelf ); subtidal

Dorotea Fm

600

Cerro Toro Fm

Magallanes-Austral retroforeland basin: deep-marine to marginal marine

lower delta plain:

FA4

Thickness (m)

Upper

CRETACEOUS

Tres Pasos Fm

500

Titanosaur (Campanian) 400

LCH-7.1A

Punta Barrosa Fm

lower delta plain:

Paleozoic metasedimentary basement

FA2

Tobifera Fm Rocas Verdes extensional back-arc basin: Sarmiento shallow-marine to Ophiolite deep-marine

tidal channels, interdistributary bays; subtidal

200 LCH-1C LCH-1B LCH-1A

upper delta front:

FA3

Zapata Fm

100

ammonites (Campanian)

mouth bars, distributary channels, and tidal channels

BWR CM-1 lithostratigraphic base

FA1

L

Upper

JURASSIC

300

Pz

and paleosols; intertidal to supratidal

upper slope: slumped, 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-marine, conglomeratic, axial channel complexes encased in overbank mudstone and sandstone. Shoaling of the Magallanes foredeep is recorded by the Tres Pasos and Dorotea Formations. Together, they represent a genetically linked shelf–slope system that

4

prograded southward along the axis of the foredeep (Macellari et al., 1989; Shultz et al., 2005; Covault et al., 2009; Hubbard et al., 2010; Schwartz & Graham, 2015). The Tres Pasos Formation (ca. 83–70 Ma; Fig. 2) (Romans et al., 2010) is composed of continental marginscale slope clinoforms that are mudstone-dominated but


U-Pb ages in the Magallanes-Austral basin Río de las Chinas

~40 km

~60 km

Cerro Cazador

Paleocene-Eocene unconformity

Sierra Dorotea

?? Kd: heterolithic delta plain facies

TS13-RB-3B JCF 09-208 TS13-RB-1A TS12-LCH-2A TS11-RT-2A TS13-LCH-7.1A TS12-LCH-1C TS12-LCH-1B TS12-LCH-1A

JCF 09-226

Kd: sandy delta-front clinoforms BWR CM-1 Pos sibl e t im e line

Ktp: shelf-edge slumps

BWR CCS-1

BWR SD-06

Ktp: muddy to heterolithic slope clinoforms

N

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 Magallanes-Austral foredeep during latest Cretaceous time. Clinoform surfaces represent approximate ‘time lines,’ or paleodepositional surfaces, in the stratigraphy. Stars indicate the positions of detrital zircon samples.

contain sandy channel networks (Armitage et al., 2009; Romans et al., 2009; Hubbard et al., 2010). Topsets of the slope clinoforms interfinger with shelf-edge deltaic deposits of the Dorotea Formation (ca. 80–68 Ma; Figs 2 and 3), which is characterized by a thick set of sandstonedominated, delta-front clinoforms overlain by heterolithic delta-plain facies (Schwartz & Graham, 2015). The Dorotea Formation is separated from Cenozoic foreland basin sediments by a regional, Paleocene to midEocene unconformity that represents an episode of crustal shortening and eastward fold-thrust belt migration (Malumian et al., 2000; Fosdick et al., 2015). This event caused partial cannibalization of Upper Cretaceous to Paleocene foredeep deposits as they were incorporated into the fold-thrust belt (Fosdick et al., 2015). Cenozoic foreland basin deposits that overlie the unconformity constitute a relatively thin succession of sandstone-dominated, shallow-marine to terrestrial deposits that are midEocene to Miocene in age (Malumian et al., 2000).

STUDY AREA: ULTIMA ESPERANZA DISTRICT, CHILE

Potential sediment source regions

Detrital zircon samples encompass the entire exposed thickness of Dorotea Formation strata in the vicinity of the Sierra Baguales, Chile. Medium-grained sandstone was preferentially collected from varying sedimentary facies within the Dorotea Formation (Figs 2 and 3) (after Schwartz & Graham, 2015). Sample locations and descriptions are listed in Table 2.

Previous provenance studies based on thin-section petrography, mudstone geochemistry and DZ analysis indicate that the bulk of siliciclastic sediment delivered to the Magallanes-Austral foredeep was derived from terranes in the westward- to northwestward-located arc and foldthrust belt, primarily from Late Jurassic to Neogene plutons of the Patagonia batholith (Forsythe & Allen, 1980; Macellari et al., 1989; Wilson, 1991; Fildani et al., 2003; Herve et al., 2003; Pankhurst et al., 2003; Herve et al., 2004; Fildani & Hessler, 2005; Romans et al., 2011; Bernhardt et al., 2011; Fosdick et al., 2015; Schwartz & Graham, 2015; and references within). Other sources were located to the east (the ‘forebulge’ region) and northeast (intraforeland uplifts) (Fosdick et al., 2015; Schwartz & Graham, 2015). The known DZ signatures of these source terranes are summarized in Table 1.

The Upper Cretaceous Dorotea Formation and its equivalents crop out intermittently for >150 km between the cities of Puerto Natales, Chile, and El Calafate, Argentina (Fig. 1a). The primary focus of this study is in the northeast corner of the Ultima Esperanza District of Chile (Fig. 1b). Here, the Dorotea Formation is exposed in the east-dipping frontal monocline of the fold-thrust belt. Samples are from the Dorotea Formation stratigraphy described in Schwartz & Graham (2015), in the Rıo de las Chinas area (Fig. 1b). Additional samples from adjacent (Cerro Cagual) and southern (Cerro Cazador, Sierra Dorotea) outcrop areas are published by Romans et al. (2010) and Fosdick et al. (2015) (locations in Fig. 1b).

DETRITAL ZIRCON GEOCHRONOLOGY Sampling strategy

Detrital zircon U-Pb analytical techniques Analytical methods Detrital zircon grains were isolated at Stanford University following standard density and magnetic separation techniques, summarized in Appendix S1 (e.g. DeGraaffSurpless et al., 2003; Romans et al., 2010). U-Pb geochronology of zircons was conducted by laser ablation multicollector-inductively coupled plasma-mass


5

6

Local name

*Depositional age of protolith.

Andean volcanic arc

Jurassic rift-related volcanic rocks

Southern Patagonia batholith

Granodiorite and tonalite, gabbro

Mafic intrusive and extrusive igneous rocks Patagonian Andes, 40°–56°S

Western flank of Patagonian Andes, 51°–53°S

Gust et al. (1985), Wilson (1991), Pankhurst et al. (2000), Calderon et al. (2007), Herve et al., 2007 Allen (1982), Stern et al. (1992)

Herve et al. (2003), Lacassie et al. (2006)

Herve et al. (2003)

Ramos (1989), Herve et al. (2003)

Reference(s)

Stern & Stroup (1982), Late Jurassic – Miocene K1: 144–137 Ma; Herve et al. (2007) K2: 136–127 Ma; K3: 126–75 Ma; 67–40 Ma

Early Cretaceous

184–183 Ma

Devonian – earliest ca. 310–260 Ma Triassic* Post-Early Permian to 290–270 Ma pre-Early Cretaceous*

ca. 310–260 Ma

Dominant zircon age range

Sarmiento ophiolite complex

Eastern flank of Patagonian Andes south of 51°S Western flank of Patagonian Andes, 49°–52°S

Meta-sandstone and -mudstone; marble Meta-sandstone, -siltstone, -shale, -conglomerate

Devonian – early Carboniferous*

Depositional age

V1: 188–178 Ma; V2: 172–167 Ma; V3: 162–153 Ma

Eastern flank of Patagonian Andes, 48°–51°S

Modern location

Meta-sandstone and -mudstone; marble

Lithology

Jurassic Tobıfera (Chile) and El Ryholitic-andesitic ignimbrites, Thrust sheets within fold-thrust belt south lavas, tuffs, and Quemado Formations of ca. 49°S; Deseado Massif volcaniclastic rocks (Argentina) intraforeland uplift

Duque de York Complex (DYC)

Paleozoic metasedimentary Eastern Andean basement complexes Metamorphic Complex (EAMC) Staines Complex

Source terrane

Table 1. Potential sources of siliciclastic sediment for the Late Cretaceous Magallanes-Austral basin

T. M. Schwartz et al.


Formation

Dorotea Fm

Dorotea Fm

Dorotea Fm

Dorotea Fm

Dorotea Fm

Dorotea Fm

Dorotea Fm

Dorotea Fm

Dorotea Fm

Dorotea Fm

Dorotea Fm

TS13-RB-1A

TS11-RT-2A

TS12-LCH-2A

TS13-LCH-7.1A

TS12-LCH-1C

TS12-LCH-1B

TS12-LCH-1A

BWR CM-1*

Cerro Cazador area BWR CCS-1*

Sierra Dorotea area JCF 09-226‡

BWR SD-06*

*Romans et al. (2010). †Macellari et al. (1989). ‡Fosdick et al. (2015). §H€ unicken (1955). Elev., Elevation.

Dorotea Fm

JCF 09-208‡

Rıo de las Chinas area TS13-RB-3B Dorotea Fm

Sample name

72.449910 72.452810 72.537010 72.566040 72.511512 72.512053 72.512788 72.661492

50.754340

50.754530

50.713410

50.704530

50.820329

50.820454

50.820557

50.789957

72.438361 72.460430

51.680844

51.608225

72.348411

72.395144

50.831544

51.224200

72.385870

Longitude (°W)

50.838270

Latitude (°S)

670

574

293

1029

837

867

889

918

1125

964

944

422

546

Elev. (m)

Maastrichtian†,§

Maastrichtian†,§

Campanian†

Campanian

Campanian

Campanian

Campanian

Campanian

Maastrichtian

Maastrichtian

Maastrichtian

Maastrichtian

Maastrichtian

Stratigraphic age

Prodelta/shoreface

Delta-front clinoforms

Prodelta/shoreface

Prodelta/shoreface

Lower distributary mouth bar complex

Fluvial distributary channel

Upper distributary mouth bar complex




Upper tidal flat

Delta plain (poorly exposed)

Lower tidal flat

Depositional environment

Greenish, medium- to fine-grained sandstone; pervasively bioturbated Greenish, medium- to fine-grained sandstone; pervasively bioturbated

Tan, medium- to fine-grained sandstone; pervasively bioturbated

Tan, coarse- to medium-grained sandstone; large-scale tangential foresets lined by ripple bedforms and mud drapes; minor bioturbation Greenish, medium-grained sandstone; massive; abundant bioturbation Tan, medium- to fine-grained sandstone; mud-lined flaser and wavy bedding; abundant bioturbation Tan, medium- to coarse-grained sandstone; medium- to large-scale foresets and trough cross-bedding lined by mud drapes; minor bioturbation Tan, medium- to coarse-grained sandstone; trough cross-stratified; abundant plant debris White, ashy, medium- to fine-grained sandstone; trough cross-stratified; abundant detrital biotite Tan, very coarse- to medium-grained sandstone; amalgamated cut-and-fill units with abundant mud rip-up clasts and trough cross-stratification; rare bioturbation Greenish pebble conglomerate with lenses of medium- to coarse-grained sandstone; planar-tabular cross-stratification Tan, fine- to medium-grained sandstone; amalgamated units of planar, trough, and ripple cross-stratification; minor bioturbation Tan, medium- to fine-grained sandstone; hummocky to swaley cross-stratification

Sandstone description

Table 2. Descriptions of sandstone samples used for detrital zircon geochronology, listed in stratigraphic order by geographic location

U-Pb ages in the Magallanes-Austral basin


7

T. M. Schwartz et al. programs available from the University of Arizona LaserChron Center. Normal and cumulative probability plots in Figure 4 display data between 0 and 500 Ma to highlight primary DZ populations. To determine the MDA of each sample, we compared three different DZ-based measures of MDA (after Dickinson & Gehrels, 2009) and compared zircon ages with the stratigraphic (fossil) age of each sample. Table 3 is a compilation of DZ age measures for each sample including the age of the youngest grain analyzed, the youngest peak age present in the probability curve and the calculated weighted mean age of that youngest peak. For this study, the ‘youngest peak’ is defined as the youngest population of grains with overlapping ages (within error) that constitute a peak, with n ≥ 2 (after Dickinson & Gehrels, 2009). For samples with indiscriminate young peaks, the

spectrometry (LA-MC-ICPMS) at the University of Arizona LaserChron Center using standard analytical techniques, also summarized in Appendix S1 (after Gehrels et al., 2006, 2008). Analytical data are reported in Appendix S2. Statistical methods Probability density plots were created using ISOPLOT 3.7 in Microsoft Excel (Ludwig, 2008). Data were plotted as histograms with superimposed probability curves to display both age measurements and associated uncertainties, thereby highlighting the presence and/or absence of key DZ populations (raw histograms and Concordia plots are summarized in Appendix S3). Composite age-probability plots (Fig. 4) were constructed using open-source Excel

Age (Ma) 0

50

100

150

200

250

300

350

400

450

500

Cumulative probability

0.8

0.6

TS13-RB-3B JCF 09-208 TS13-RB-1A TS11-RT-2A TS12-LCH-2A TS13-LCH-7.1A TS12-LCH-1C TS12-LCH-1B TS12-LCH-1A BWR CM-1

0.4

0.2

SPB

tr. RVB

EAMC

TS13-RB-3B (n = 43/64)

JCF 09-208 (n = 87/93)

Normalized probability

TS13-RB-1A (n = 74/87)

TS11-RT-2A (n = 77/91) TS12-LCH-2A (n = 74/92)

TS13-LCH-7.1A (n = 85/99) TS12-LCH-1C (n = 73/95) TS12-LCH-1B (n = 75/99)

TS12-LCH-1A (n = 82/94) BWR CM-1 (n = 46/60) 0

50

100

150

200

250

Age (Ma)

8

300

350

400

450

500

Fig. 4. Normal and cumulative probability plots for all samples in the Rıo de las Chinas study area. Normal probability plots are stacked in stratigraphic order, and emphasize key grain populations between 0 and 500 Ma. Coloured fill represents primary source regions in the Andean orogen and fold-thrust belt (SPB: South Patagonia batholith; RVB: Rocas Verdes basin volcanic rocks; tr: transitional volcanic rocks; EAMC: East Andean Metamorphic Complex; see Table 1). Grey box represents the fossil age of the Upper Cretaceous Dorotea Formation.


91.0 69.3 78.1 69.0 71.3 79.2 78.4 94.0 90.2 95.4 81.9 65.8 71.3

3.8 0.9 8.3 9.0 3.7 3.6 1.2 0.7 3.0 2.6

82.0 1.4

65.6 3.3 70.4 3.5

Campanian‡

Maastrichtian‡,¶ Maastrichtian‡,¶

Youngest peak age* (Ma)

90.5 69.7 69.8 64.6 68.2 73.6 78.1 82.3 85.7 93.9

Age of youngest grain (Ma 1r)

Maastrichtian Maastrichtian Maastrichtian Maastrichtian Maastrichtian Campanian Campanian Campanian Campanian Campanian

*Excludes youngest single grain. †Romans et al. (2010). ‡Macellari et al. (1989). §Fosdick et al. (2015). ¶H€ unicken (1955). TDA, true depositional age.

Rıo de las Chinas area TS13-RB-3B JCF 09-208§ TS13-RB-1A TS11-RT-2A TS12-LCH-2A TS13-LCH-7.1A TS12-LCH-1C TS12-LCH-1B TS12-LCH-1A BWR CM-1 Cerro Cazador area BWR CCS-1† Sierra Dorotea area JCF 09-226§ BWR SD-06†

Sample name

Stratigraphic (fossil) age

5 4

2

2 2 2 2 2 21 8 10 3 3

Number of grains in youngest peak (n)

Table 3. Range of maximum depositional ages (MDAs) of detrital zircon samples

3.2 1.7 5.7 2.0 5.0 1.8 2.0 1.5* 3.4 4.0

68.6 0.5 72.4 3.9

83.2 2.4

96.3 69.7 76.8 68.9 70.5 78.0 79.3 95.5 88.2 95.1

Weighted mean age (Ma 1r)

2.70 0.50

2.40

3.00 0.02 0.80 0.23 0.71 0.20 0.63 0.68 0.50 0.23

Mean square weighted deviation (MSWD)

0.121–2.775 0.072–3.117

0.001–5.020

0.001–5.020 0.001–5.020 0.001–5.020 0.001–5.020 0.001–5.020 0.480–1.710 0.241–2.286 0.300–2.111 0.025–3.690 0.025–3.690

Acceptable MSWD (Mahon, 1996)

3.2 1.7 5.7 2.0 5.0 1.8 2.0 0.7 3.4 4.0

68.6 0.5 72.4 3.9

83.2 2.4

96.3 69.7 76.8 68.9 70.5 78.0 79.3 82.3 88.2 95.1

Interpreted MDA (Ma 1r)

2.0 5.0 1.8 2.0 0.7

1.7

68.6 0.5 72.4 3.9

83.2 2.4

– 69.7 – 68.9 70.5 78.0 79.3 82.3 – –

Possible TDA (Ma 1r)

U-Pb ages in the Magallanes-Austral basin


9

T. M. Schwartz et al. Unmix function in Isoplot was used to elucidate the youngest peak prior to calculating its weighted mean age. The weighted mean age weights each measurement within the cluster by its uncertainty, such that measurements with the smallest uncertainty have the greatest impact on the final calculated age. For this calculation to be meaningful, we assume that all zircon grains within the cluster are of the same true age and that scatter around that age can be explained by analytical uncertainty. A mean square weighted deviation (MSWD) was calculated for each weighted mean age to evaluate the likelihood that all analyses within the cluster are coeval. This study presents a range of acceptable MSWDs for each weighted mean age calculation, based on the number of analyses contributing to each calculation (Table 3; after Mahon, 1996). Weighted mean ages for samples first published in Romans et al. (2010) and Fosdick et al. (2015) were recalculated using the same parameters as all new data, and therefore, vary slightly from their original publications. Figure 5 displays the statistical results for all weighted mean age calculations.

Results of DZ U-Pb geochronology Sandstone provenance Similar to previous studies, Dorotea Formation sandstones yield DZ populations consistent with sources in the arc, including the Southern Patagonia batholith (ca. 144–65 Ma) and its metasedimentary host rocks (ca. 310–270 Ma), as well as in the interior fold-thrust belt, including Rocas Verdes basin volcanic rocks (ca. 188–153 Ma) (Fig. 4). Thin-section petrography supports derivation from these sources, with primary grain types being quartz, feldspar (plagioclase potassium feldspar), intermediate volcanic rock fragments and minor (meta-) sedimentary rock fragments. In addition, many DZ samples yield a small population of relatively young zircon grains that range in age from ca. 80–68 Ma, which overlap with the mid-Campanian to Maastrichtian fossil age of the Dorotea Formation (Fig. 4). Maximum depositional ages from detrital zircon The age of the youngest single-grain analyzed in a sample was not considered to be a robust measure of MDA due to the possibility of sample contamination, zircon lead loss, etc. (after Dickinson & Gehrels, 2009). An exception was made for sample TS12- LCH-1B, which yielded a youngest single-grain age of 82.3 0.7 Ma (Table 3). For this sample, the single-grain age is considered to be possible (although not robust) because (i) there is good correlation between the single-grain age and fossil data; (ii) there is a discrete age difference between the youngest single grain and the next-youngest group of ages in the sample (ca. 12 Myr; Fig. 4); and (iii) the analysis has high precision (standard error