The Permo-Triassic Gondwana Sequence, Central Transantarctic Mountains, Antarctica: Zircon Geochronology, Provenance, and GEOSPHERE; V

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GEOSPHERE The Permo-Triassic Gondwana sequence, central Transantarctic Mountains, Antarctica: Zircon geochronology, provenance, and GEOSPHERE; v. 13, no. 1 basin evolution doi:10.1130/GES01345.1 David H. Elliot1, C. Mark Fanning2, John L. Isbell3, and Samuel R.W. Hulett4 20 figures; 1 table; 3 supplemental files 1School of Earth Sciences and Byrd Polar and Climate Research Center, Ohio State University, Columbus, Ohio 43210, USA 2Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia 3Department of Geosciences, University of Wisconsin–Milwaukee, Wisconsin 53201, USA CORRESPONDENCE: elliot.1@osu.edu 4Department of Earth, Atmospheric and Planetary Sciences, Notre Dame University, Notre Dame, Indiana 46556, USA

CITATION: Elliot, D.H., Fanning, C.M., Isbell, J.L., and Hulett, S.R.W., 2017, The Permo-Triassic Gond- wana sequence, central Transantarctic Mountains, ABSTRACT (McLoughlin and Drinnan, 1997a, 1997b) are part of the East Gondwana interior Antarctica: Zircon geochronology, provenance, and rift system (Harrowfield et al., 2005) and are linked byGlossopteris and palyno basin evolution: Geosphere, v. 13, no. 1, p. 155– The most complete Permian–Triassic Gondwana succession in Antarctica morphs to the Transant arc tic Mountains and other Gondwana successions. 178, doi:10.1130/GES01345.1. crops out in the central Transantarc tic Mountains. The lower Permian strata Vertebrate remains and plant assemblages (Glossopteris and Dicroidium were deposited in an intracratonic basin that evolved into a foreland basin in floras) provide paleontological ages for the Beacon strata, giving a Devonian Received 15 April 2016 Revision received 24 August 2016 late Permian time. Sedimentary petrology and paleocurrent data have been age for the Taylor Group and a Permian through Triassic age for the Victoria Accepted 24 October 2016 interpreted as indicating a granitic (craton) provenance and a West Antarctic Group (Truswell, 1991; Young, 1991). Pollen and spore analysis has established Final version published online 20 January 2017 volcanic provenance. To address the West Antarctic provenance and evolution a palynostratigraphy for the Victoria Group (Schopf and Askin, 1980; Kyle and of the detrital input, detrital-zircon grains from nine sandstones (plus three Schopf, 1982; Farabee et al., 1990) and enabled correlation with the Austral- sandstones reported previously) representing the full extent of the Permo-Tri- asian palynostratigraphic zonation (Mantle et al., 2010). Strata in eastern Ells- assic succession exposed on that flank of the basin, have been analyzed by worth Land and the Ellsworth Mountains have yielded Glossopteris and thus sensitive high-resolution ion microprobe. Results define three provenances: have Permian ages (Gee, 1989; Taylor and Taylor, 1992). Vertebrate faunas have an early (early to middle Permian) provenance that is dominated by zircons been recovered from Lower and Middle Triassic strata (Smith et al., 2011; Sidor having Ross orogen ages (600–480 Ma); a middle (late Permian) provenance et al., 2014). dominated by Permian zircons with subordinate older magmatic arc grains Sandstone petrology and paleocurrent data for the Victoria Group in plus lesser Ross orogen–age grains; and a late (Triassic) provenance, which is the central Transantarc tic Mountains (CTM) established a granitic source quite variable and in which the predominant Triassic arc component ranges interpreted to be the East Antarctic craton and a volcanic source located in from very minor to significant and again with a component of Ross orogen– West Antarctica (Barrett et al., 1986). Outcrops that might be representative age grains. Detrital zircons imply a more extensive Permo-Triassic arc, in time of the potential sources in East Antarctic are confined in CTM to the Miller and space, on the Gondwana margin than is evident from outcrop data. The Range (Fig. 3). The volcanic source has been correlated with the few isolated zircon data are integrated into a model for basin evolution and infilling, pro- Permian and Triassic granitoids and gneisses along the West Antarctic margin viding broad constraints on the timing of tectonic events and the provenance. (Pankhurst et al., 1993; Collinson et al., 1994; Pankhurst et al., 1998; Mukasa and Dalziel, 2000; Elliot and Fanning, 2008). A volcanic provenance in Permian time was also recorded from Mount Weaver in the Scott Glacier region (Min- INTRODUCTION shew, 1967), and although such a source was not noted by Long (1965) for the Permian sandstones in the Ohio Range, petrological reexamination of the The Permo-Triassic Gondwana stratigraphy is best developed in the Trans sandstones has revealed a volcanic component in the upper part of the section. antarc tic Mountains (Figs. 1 and 2), where it is divided into a lower Taylor No unequivocal tuffs have been identified in the upper part of thePermian Group (Devonian) and an upper Victoria Group (Permian to Triassic), the two Buckley Formation in the CTM despite the presence of much volcaniclastic together often referred to as the Beacon Supergroup (Barrett, 1991). In West material derived from an active magmatic arc (Collinson et al., 1994). Glass Antarctica, Permian strata crop out in the Ellsworth Mountains (Collinson et al., shards have been identified in a number of Permian and Triassic sandstones For permission to copy, contact Copyright 1992) and in small isolated outcrops in eastern Ellsworth Land (Laudon, 1987). (Collinson et al., 1994), but attempts to date the rocks have been unsuccessful. Permissions, GSA, or [email protected]. Permian and Lower Triassic strata in the northern Prince Charles Mountains Tuff beds in the lower part of the middle informal member of the siliciclastic

GEOSPHERE | Volume 13 | Number 1 Elliot et al. | The Permo-Triassic Gondwana sequence Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/1/155/1000843/155.pdf 155 by guest on 28 September 2021 Research Paper

CTM Central Transantarctic Mtns. 0° Central South MR Miller Range Prince Age Ohio Range Transantarctic Charles Victoria Land NVL North Victoria Land Mountains SVL South Victoria Land Mtns. ANTARCTICA Theron 90°E Weddell Sea Mtns. 90°W Falla 80°S 160–282m

VVVV VVVV 180° VVV Lashly VVV VVVV 520+m Filchner- Triassic VVVV VVV Ronne 0° VVV Fremouw VVVV Ice Shelf VVVV 700m VVV VVVV East VVV Feather Haag N. Pensacola Mtns. Erehwon N. 250m T Antarctica VVV r a VVVV VVVV 90°W Ellsworth n 90°E Mount VVV 70°S Mtns 80°S s ctoria Group Ohio Ra. a Glossopteris Vi VVVV Buckley n t 690+m VVV 750m West CT a r Thurston I. Mt. Weaver M c Weller t i Antarctica c Permian 250m Scott Gl. MR M Fairchild Marie Shackleton Gl. 130–220m Byrd o Discovery Beardmore Gl. Fig. 3 u Mackellar Land n Ridge 60–140m Ross t 165m Byrd Gl. a

SVL Ice 80°S i n Buckeye Pagoda Metschel Shelf s Triassic gneisses 275m 0–175m 0–70m Late Triassic granitoids P erm Carboniferous Permian-Triassic strata o-T rias Permian gneisses N s ic Ross V Permian granitoids ar L Aztec c Beacon Hts Generalized areas of Sea bedrock outcrop Devonian Horlick Alexandra Arena Edge of shelf ice 0–150m 0–300m Altar 500 km New Mtn 2000 m bathymetric contour 180° Taylor Gp . 70°S 300–1500m

Figure 1. Location map for Antarctica. Stretched continental crust underlies the Ross Sea, Ross Sandstone (volcaniclastic: Coal Ice Shelf, and interior West Antarctica through to Ellsworth Land (the Ross embayment) and VVVV was generated during Late Cretaceous to Late Cenozoic time. Stretched continental crust also arc derived) Carbonaceous shale underlies the Filchner-Ronne Ice Shelf region and the adjacent Weddell Sea (the Weddell em- Sandstone (quartzose) bayment) but was extended during the Early–Middle Jurassic breakup of Gondwana. I.—Island; Shale, siltstone N.—Nunatak; Gl.—Glacier. Diamictite Hiatus

Permian Polarstar Formation of the Ellsworth Mountains (Fig. 1) have yielded Figure 2. Simplified geologic columns for Devonian to Lower Jurassic strata in the zircons with Late Permian U-Pb sensitive high-resolution ion microprobe Transantarctic Mountains (Ohio Range, central Transant arctic Mountains, south (SHRIMP) ages of 258.0 ± 2.1 Ma and 262.5 ± 2.2 Ma (Elliot et al., 2016a). Victoria Land). Sources: Ohio Range—Long (1964, 1965); central Transant arctic The aim of this paper is to evaluate the evolution of the Permo-Triassic ba- Mountains and south Victoria Land—Elliot (2013). Note that the marine Horlick Formation, although a siliciclastic succession of similar Early Devonian age, is not sin in the light of new and existing detrital-zircon geochronologic data. Specifi- part of the Taylor Group. Gp.—Group. cally, this paper records new detrital-zircon data for nine sandstones and inter- prets the results in terms of the evolution of the basin and the source regions. An initial program of detrital-zircon geochronology (Elliot and Fanning, 2008) REGIONAL GEOLOGY showed that the Permian upper Buckley Formation in the Shackleton Glacier region includes near contemporaneous igneous zircon grains. The possibil- The pre-Devonian basement in the CTM (Fig. 3) comprises Neoproterozoic ity existed, therefore, that reanalysis of the young zircons in these and other and Cambrian siliciclastic and volcaniclastic strata and limestones (Stump, samples using SHRIMP II in the higher resolution geochronology mode would 1995) intruded by granitoids emplaced during the Cambrian to Early Ordovi- provide refined and improved age data for those formations. cian Ross orogeny (Goodge et al., 2012). A regional erosion surface, the Kukri

Mt. Weeks 175°E Surficial deposits 83°30′S Clarkson Pk. v v L. Jurassic basaltic rocks (11-4) v v Mt. Miller 83°30′S L. Jurassic Hanson Fm. Tillite Gl. Triassic strata

Permian strata Fremouw Peak Devonian strata Mt. Ropar Pre-Devonian igneous 84°S and metamorphic rocks n Alexandra Ra. Layman Peak Quee (96-35) l. Figure 3. Geologic sketch map of the cen- G y tral Transantarctic Mountains. Numbers e s r refer to locations of analyzed samples. m e i Mt. Kirkpatrick a c Ra.—Range; Gl.—Glacier; I.—Island. R a Mt. Falla (90-2) l

84°30′S G

n N to

e l

a (MBB 73-3) h Buckley I. S D Mt. Butters (11-5) Mt. Bowers om Mt. Rosenwald inion Range (07-2) 85°S (96-36) Collinson Ridge Cumulu (07-6) Halfmoon Bluff s Hills 0 km 40 Misery Peak 175°E 160°E 165°E 170°E 180° 175°W 85°30′S

Erosion Surface (summarized by Isbell, 1999), was developed on those rocks was uplift and erosion prior to glacigenic deposition in latest Carboniferous (?) and is overlain by quartzose sandstones of the Alexandra Formation of inferred to early Permian time (Kyle and Schopf, 1982). Devonian age. A younger erosion surface (Maya Erosion Surface) truncates Glacigenic facies and ice flow indicators in the lower Permian Pagoda the Alexandra Formation and older strata, and on that surface, the Victoria Formation (Victoria Group) suggest deposition occurred in a high-relief, Group (Fig. 2) was deposited. The Victoria Group is overlain by Lower Jurassic topographically expressed, narrow and elongate glaciomarine and/or glacio sandstones and tuffaceous beds of the Hanson Formation (Elliot et al., 2016b) lacustrine basin oriented subparallel to the present trend of the range (Isbell and then the basaltic pyroclastic rocks and flood lavas of the Ferrar Large Igne- et al., 1997). Ice flow was transverse and convergent across basin margins, ous Province (Elliot and Fleming, 2008), which also includes massive dolerite with glaciers only extending short distances into the basin, and was accom- intrusions distributed throughout the Beacon Supergroup. panied by axial mass-transport (slides and slumps) and turbidite flow (Isbell The Alexandra Formation and the much thicker Devonian quartzose suc- et al., 2008; Isbell, 2015). These characteristics are similar to those of fault- cession in south Victoria Land (Bradshaw, 2013) are interpreted as shallow bounded rift basins (Isbell, 2015). Paleocurrent orientations and paleoenviron- marine deposits that grade up into alluvial plain strata and were developed on ments interpreted from turbidites and deltaic facies within the overlying post- a slowly subsiding passive margin (Collinson et al., 1994; Bradshaw, 2013) or glacial Mackellar Formation indicate that sediment was fed into the narrow in a successor basin (Isbell, 1999). The surface truncating those strata reflects basin, across high-relief basin margins, from both the East Antarctic craton a period of tectonism, possibly linked either to the Devonian–Carboniferous and from the direction of West Antarctica. Axial drainage within the basin was active plate margin or to accretion of an allochthonous and/or para-autoch to the southeast (present coordinates) toward the Ohio Range. Basement rocks thonous Late Devonian–Carboniferous arc (Elliot, 2013), during which there along the basin margins were buried and overtopped by prograding deltas and

Table S1. Locations of analyzed samples braided stream deposits of the Fairchild Formation. Uniform paleocurrents ori- results. The geographic locations of samples are given on Figure 3, except ented toward the southeast together with highly interconnected channel stack- for the sandstone from the Ohio Range (see Fig. 1), and in Supplemental File ing patterns suggest slow basin subsidence during braided stream deposition S11. The samples are also projected onto a composite stratigraphic column Sample no. Long. Lat. (Isbell et al., 1997). The sedimentology and sandstone petrology of the overly- for the Shackleton-Beardmore region on Fig. 4A. Petrographic descriptions of 07-2-4 178° 14.2' W 85° 04.9' S ing fluvial and coal-bearing middle and upper Permian Buckley Formation and the analyzed sandstones are given in Supplemental File S22. Analytical proce- 07-2-12 178° 14.2' W 85° 05.0' S MBB 73-3 177° 26.6' W 84° 52.8' S Triassic strata suggest input of detritus from both the East Antarctica craton dures for the SHRIMP U-Pb zircon analyses are given in the Appendix, along 11-4-3 164° 30.6' E 83° 21.1' S and from the Gondwana margin in West Antarctica. With the incoming of vol- with probability density plots, weighted mean age plots, and cathodolumines- H3-384b 113° 25.0' W 84° 48.0' S caniclastic debris in upper Buckley time, the succession is interpreted to be cence (CL) images for each sample. These are accompanied by discussion of 11-4-10 164° 34.2' E 83° 19.4' S the filling of a foreland basin related to the Gondwana plate margin (Collinson each sample. 96-35-2 179° 51.6' W 84° 48.4' S et al., 1994). This basin was initially under filled during the Permian as ex- Ross orogen–age zircons predominate in the lower part of the succession 96-36-1 175° 15.4' W 85° 13.0' S 07-6-2 175° 19.6' W 85° 13.7' S pressed by convergent flow across basin margins and longitudinal flow down (Fairchild and lower Buckley formations), and, with one exception, Permian or 07-6-3 175° 20.3' W 85° 13.8' S the basin axis, whereas predominant uniform flow out of West Antarctica and Triassic grains predominate in the upper Buckley and younger strata. With the 90-2-52 164° 40.9' E 84° 21.2' S oblique across the location of the upper Buckley basin axis indicates evolution exception of sample 07-2-4 from the Fairchild Formation, the age distribution into an overfilled basin during Triassic time. of zircon grains older than 600 Ma is sufficiently scattered that no probability 1Supplemental File S1. Sample locations. Please visit The Permian section in the Ohio Range (Long, 1965; Bradshaw et al., 1984) peaks exist, even though cumulatively they form a not insignificant propor- http://dx.doi.org/10.1130/GES01245.S1 or the full- text article on www.gsapubs.org to view Supplemen- comprises the glacigenic Buckeye Tillite overlain by beds equivalent to the tion. A similar situation exists for the 480–385 Ma interval, except for 07-6-2, tal File S1. Mackellar and Fairchild formations (Discovery Ridge Formation and the lower which has a cluster but no probability peak of Silurian zircons. Zircons in the part of the Mount Glossopteris Formation, which is marine to marginal ma- Late Devonian to early Carboniferous interval are to be expected given the rine). The upper part of the Mount Glossopteris Formation (700 m thick) is a granitoids of that age in Marie Byrd Land. The abundance of Triassic grains in Petrographic descriptions for sandstone samples for which detrital zircon U-Pb geochronological data are provided in Tables S3-S13. fluvial succession, which consists of sandstones, finer grained beds, and coal. Permian beds is most probably the result of radiogenic Pb loss and the uncer- Twelve samples from the Victoria Group reported here come from the Shackleton Glacier and Queen Alexandra Range regions, and one sample from the Ohio Range. These are listed in approximate Paleocurrent data indicate easterly flow (present coordinates); but high in the tainties arising from detrital mode analyses; this is supported by the weighted stratigraphic order (see also Fig. 4a). Geographic locations are shown in Fig. 3, and full analytical data for new detrital zircon U-Pb analyses are presented in Supplementary Tables S3-S13. section, one thick sandstone bed shows major flow to the south (Long, 1965), mean ages derived from the geochronology mode (six-scan) analyses. 07-2-4. Fairchild Formation, Mt. Rosenwald. One hundred and thirty six meters above the base of the formation. Fine-grained sandstone, with minor plagioclase (sodic andesine) and lesser K-feldspar and a sample from that bed contains a high proportion of volcanic detritus. The calculated weighted mean ages are interpreted as maximum ages of (orthoclase and microcline). Accessory micas (muscovite and lesser biotite). Heavy minerals include zircon and garnet, lesser opaques, and rare tourmaline. Carbonaceous shreds are scattered. Matrix of phyllosilicate shreds and some secondary silica.. A Permo-Triassic arc formed the active margin of Gondwana from Papua deposition of the host sandstones. Because these are detrital-zircon grains, no 07-2-12. Lower Buckley Formation, Mt. Rosenwald. Sixty four meters above the base of the formation. New Guinea to Patagonia and in Antarctica is represented by granitoids and assumption can be made about the coherence of the population such as can Medium-grained quartzo-feldspathic sandstone. Much of the quartz is strongly corroded; quartz includes polycrystalline grains. Minor plagioclase (oligoclase-andesine) and traces of K-feldspar. Micas include muscovite and lesser biotite. Heavy minerals are mainly garnet and zircon, with a trace of apatite and gneisses in Marie Byrd Land, Thurston Island, and the Antarctic Peninsula be made for zircons in a tuff bed. Although grains were selected after examina- tourmaline. Fine-grained white mica forms the abundant matrix; replaces detrital quartz grains and such replacement is probably widespread and accounts for the abundance of matrix phyllosilicate. (Pankhurst et al., 1993, 1998; Mukasa and Dalziel, 2000; Millar et al., 2002; Riley tion by transmitted and reflected light and CL imaging, the grains of any one MBB 73-3. Pagoda Formation, Mt. Butters. Seventy three meters above the base of the formation. Medium-grained well-sorted quartz-rich sandstone. Grain margins of quartz are commonly sutured. et al., 2012). By inference, that Permo-Triassic magmatic arc was the source age grouping are likely to have come from many rocks and not necessarily all Plagioclase (oligoclase-andesine) and lesser K-feldspar are minor. Accessory muscovite and biotite. Heavy minerals include garnet, epidote, opaques and zircon. Matrix includes patchily distributed for the volcaniclastic detritus in Victoria Group strata and in the Permian beds of precisely the same age. There can be little doubt that some of the analyzed secondary fine-grained brown phyllosilicate, and fine-grained recrystallized quartz.

11-4-3. Lower Buckley Formation, south spur of Clarkson Peak. One hundred and twenty two meters in eastern Ellsworth Land and the Ellsworth Mountains. The eastern Ellsworth zircons have been affected by secondary processes such as radiogenic lead above the base of the formation and two meters above the upper contact of a major sill. Medium-grained quartz-rich sandstone, moderately well sorted, angualr to sub-round grains. Sparse highly altered Land Permian beds accumulated adjacent to or within the axial part of the loss. This is demonstrated in two samples (96-35-2 and 96-36-1; see Fig. 4A for feldspar. Heavy minerals include garnet and zircon. Abundant siliceous matrix along with colorless and pale brown phyllosilicate flakes; largely resulting from the replacement of detrital feldspar; minor silica overgrowths on quartz grains. Scattered carbonaceous matter. deposi tional basin, whereas the Permian strata of the Ellsworth Mountains stratigraphic positions) by grains younger than the Permo-Triassic boundary 11-5-22. Upper Buckley Formation, Mt. Bowers. Thirty three meters above the lower Buckley/upper were deposited on the flank of the basin in the backarc region of the Antarc- but in Glossopteris-bearing strata and thus almost unequivocally of Permian Buckley contact and 425 m above the base of the Buckley Formation. Described in Elliot et al. (2015). 11-4-10. Upper Buckley Formation, south spur of Clarkson Peak. Three hundred and eighty four meters tic Peninsula sector of the plate margin (Elliot et al., 2016a). In Permo-Triassic age (see next section). in section (the second sandstone in the upper white cliff). Fine-grained volcanic sandstone, moderately sorted, sub-angular to sub-round. Quartz with lesser feldspar (andesine) which is highly altered and time, prior to Jurassic breakup of Gondwana, the Ellsworth Mountains were partially replaced by zeolite. Muscovite and biotite are accessory. Heavy minerals include zircon, garnet and sphene. Abundant volcanic rock fragments: grains are flow banded, microcrystalline and siliceous, fine to very fine-grained and feldspathic with or without flow texture, and microporphyritic. Sparse located along strike eastward from the Cape Fold Belt of South Africa in the quartz-mica schist rock fragments. The matrix includes siliceous matter and phyllosilicate flakes and shreds. Carbonaceous matter is common. proto–Weddell Sea region (e.g., Grunow et al., 1987; Elliot et al., 2016a). Stratigraphic Ages

2 Supplemental File S2. Petrographic descriptions. No complete section of the Buckley Formation has been measured in the Please visit http://dx.doi.org/10.1130/GES01245.S2 or the full-text article on www.gsapubs.org to view Sup- DISCUSSION Shackleton Glacier region. Hence, the position of sample 96-35-2 with respect plemental File S2. to the non-volcanic lower Buckley is uncertain; this sample is 70 m below the Analytical Results Buckley Formation upper contact and ~10 m above the base of the exposed section northeast of Layman Peak (Fig. 2). Sample 96-36-1 is well located strati- Detrital zircons from nine sandstones have been analyzed by SHRIMP; graphically, coming from the top meter of the Buckley Formation at Collinson in addition, three of those plus two sandstones analyzed previously (Elliot Ridge. Although the Permian/Triassic boundary had long been considered to and Fanning, 2008) have been reanalyzed in the higher resolution geochro- lie at the Buckley Formation upper contact, at Collinson Ridge the Era bound- nology mode (see the Appendix). Table 1 summarizes the geochronological ary lies in a 10 m interval that begins at ~36 m above the Buckley/Fremouw

TABLE 1. SUMMARY OF DETRITAL-ZIRCON AGE PROBABILITY AND WEIGHTED MEAN AGE PLOTS Ordovician– Lower Devonian– Lower Carboniferous– Archean Proterozoic Grenville Neoproterozoic Ross Devonian Early Carboniferous Permian Triassic Weighted Sample no. >2.5 Ga 2.5–1.1 Ga 1.1–0.9 Ga 0.9–0.6 Ga 600–480 Ma 480–385 Ma 385–325 Ma 325–252 Ma 252–200 Ma mean age 90-2-52 ca. 515 Ma ca. 205 Ma 206.6± 2.2Ma. (1) (2) (9) (6) (18)(2) (2)(30) MSWD= 1.08 (15) 07-6-3 ca. 570, ca. 490 Ma ca. 365 Ma ca. 255 Ma ca. 240 Ma 242.3± 2.3Ma. (8) (4) (19)(3) (14) (7)(15) MSWD= 0.78 (14) 07-6-2 ca. 330 Ma ca. 310 Ma ca. 245 Ma 245.9± 2.9Ma. (3) (4) (3) (3) (5) (9)(12)(16)(15) MSWD= 1.2 (14) 309.8± 3.2Ma. MSWD= 0.28 (8) 96-36-4 ca. 580, ca. 545, ca. 510 Ma (4) (2) (3) (11) (28)(3) (4)(3) (2) 96-36-1 ca. 260 Ma.250.3 ± 2.2Ma. (1) (9) (13)(5) (5)(20)(7) MSWD= 1.5 (16) 96-35-2 ca. 265 Ma.253.5 ± 2.0Ma. (1) (1) (1)(41)(16) MSWD= 1.5 (20) H3-384b ca. 260 Ma. (1) (1) (1) (1) (5)(47)(13) 11-4-10 ca. 555 Ma ca. 260 Ma (3) (5) (5) (18)(1) (4)(27)(5) 11-5-22 ca. 260 Ma 258.1± 1.9Ma. (1) (4) (3) (2) (11) (3)(42)(6) MSWD= 1.2 (32) 11-4-3 ca. 570 Ma (9) (7) (6) (8) (37)(2) MBB 73-3 ca. 585, ca. 545 Ma (1) (14) (5) (4) (41)(5) 07-2-12 ca. 575, ca. 560, ca. 540 Ma (4) (7) (2) (10) (36)(1) 07-2-4 ca. 2250 Ma ca. 1050 Ma ca. 580 Ma (2) (17) (10) (10) (17)(2) Notes: Ages in bold are predominant probability peaks; those underlined are minor peaks. Total numbers of grains of any particular age range are given in parentheses (excluding repeat six-scan analyses). Probability peaks do not exist in many age intervals because the distribution of ages is too scattered. See the Appendix for probability plots, cathodoluminescence images of zircon grains, and discussion of the probability plots. Data for sample 96-36-4 and additional data for samples 96-35-2 and 96-36-1 from Elliot and Fanning (2008). MSWD—mean square of weighted deviates.

contact and ends immediately below the lowest quartzose sandstone of the probably diachronous due to progradation of facies across the Shackleton- Fremouw Formation; that 10 m interval lies between the last occurrence of Beardmore area. the Glossopteris flora and the first occurrence of fossils of the Lystrosaurus The interpreted depositional age for sample 96-35-2 (253.5 ± 2 Ma) is no fauna (Collinson et al., 2006). The Buckley/Fremouw contact is interpreted by older than the Changhsingian Stage (Fig. 4B) of the late Permian (International Collinson et al. (2006) as a disconformity at many other localities, although de- Chronostratigraphic Chart, 2016), whereas sample 96-36-1 has a maximum tailed reinvestigation of the boundary at those and other localities may demon- depositional age of 250.3 ± 2.2 Ma, which is within uncertainty of the age strate continuity of some sections. Furthermore, the formational contact is for sample 96-35-2. Nevertheless, given the 70 m of strata between the two

Age Formation Period Stage Age A 90-2-52 (206.6 ± 2.2 Ma). B Jurassic Hettangian Diamictite 201.3 ± 0.2 Ma Rhaetian Falla Coal ca. 208.5 Ma U Norian Shale ca. 227 Ma Carnian Fine sandstone, Triassic ca. 237 Ma siltstone Ladinian VVVVV Sandstone M ca. 242 Ma VVV Anisian upper VVVVV (volcaniclastic: arc derived) 247.2 Ma VVVVV Olenekian Sandstone L 251.2 Ma

iassi c (quartzose) VVVVV Induan Tr Fremouw 07-6-3 (242.3 ± 2.3 Ma) 252.17 ± 0.06 Ma Changsingian VVVVV Permian 254.14 ± 0.07 Ma Wuchiapingian middle VVVVV

07-6-2 (245.9 ± 2.9 Ma) VVVVV lower 96-36-4 (ca. 335 Ma)

VVVVV 96-36-1 (250.3 ± 2.2 Ma) VVVVV 96-35-2 (253.5 ± 2.0 Ma) VVVVV

ctoria Group VVVVV

Vi VVVVV VVVVV VVVVV 11-4-10 (ca. 260 Ma) Buckley VVVVV Figure 4. (A) Composite stratigraphic column for the Shackleton-Beardmore 11-5-22 (258.1 ± 1.9 Ma) VVVVV region showing the projected and approximate positions of the analyzed sandstone samples. H3-384b, the Ohio Range sandstone, comes from low in the volcaniclastic upper part of the Mount Glossopteris Formation, and 11-4-3 (ca. 570 Ma) its stratigraphic position on this column is not well constrained. (B) Permian and Triassic chronostratigraphic ages. Reproduced from the International Chronostratigraphic Chart, International Commission on Stratigraphy (2016). Note that Burgess et al. (2014) proposed an age of 251.9 Ma for the Permian/ MBB 73-3 (ca. 470 Ma) Triassic boundary. 07-2-12 (ca. 540 Ma) m 500 Permian 07-2-4 (ca. 525 Ma)

Fairchild

250 Mackellar

Pagoda 0

samples, this suggests a real age difference and erosion of an actively growing ject to local conditions, and diachroneity is to be expected. The Cynognathus magmatic arc. The age for sample 96-36-1 is younger than, but within uncer- fauna comes from close to the base of the upper member of the Fremouw tainty of, the age for the Permian–Triassic boundary (ca. 251.9 Ma; Burgess Formation near the type section in the Queen Alexandra Range, over 200 km et al., 2014), yet comes from beds 37 m below the last occurrence of the Glos- from Halfmoon Bluff, and therefore chronostratigraphic correlation with upper sopteris flora (Collinson et al., 2006). However,Glossopteris has been reported Fremouw beds in the Halfmoon Bluff section is subject to considerable uncer- to occur above the boundary in India (Pant and Pant, 1987; Bose et al., 1990) tainty. The geologic age assignment for the vertebrate fauna is unlikely to be and Lystrosaurus below it in South Africa (Neveling, 2004; Botha and Smith, in significant error. 2007). As previously noted, and setting aside the possibility of the Glossopteris Sample 90-2-52 comes from 43 m below the upper contact of the Falla For- flora continuing into the Triassic, in particular sample 96-36-1 (from ~37+ m be- mation, which at the type section is marked by a lag gravel that is interpreted low the paleontologically defined Permian–Triassic boundary interval) points as a disconformity where there is also a significant change in sandstone petrol up the uncertainties in using detrital-zircon geochronology to establish precise ogy and provenance (Elliot et al., 2016b). The numerical age (207 ± 2 Ma) is stratigraphic ages. The original detrital analyses (four-scan or detrital mode) early in the Rhaetian Stage of Late Triassic time. This is consistent with the for the two Buckley Formation sandstones (Elliot and Fanning, 2008) recorded palynological age assignment of Carnian to Norian for two samples collected a range of zircon ages that spans the Permian and indicated that source region from 150 and 220 m, respectively, below the top of the Falla Formation (Fara magmatism probably occurred throughout late Permian time and probably be- bee et al., 1990). The numerical age also suggests that the Falla Formation gan much earlier, which is supported by an early Permian U-Pb age for a Marie probably does not extend across the Triassic/Jurassic boundary. Byrd Land granitoid (Mukasa and Dalziel, 2000). The middle member of the Fremouw Formation is recognized by the lack of quartzose channel sandstones and the dominance of green fine-grained Provenance floodplain deposits, whereas the upper member consists of volcanic sandstone with subordinate intervals of fine-grained beds (Barrett et al., 1986). Sample The detrital-zircon results suggest three provenance scenarios that match 07‑6‑2 lies at 46 m in the middle member at Halfmoon Bluff (Collinson and those evident in the sandstone petrology: an early provenance recorded in the Elliot, 1984); the middle to upper boundary was placed at 100 m above the sandstones from the Fairchild and lower Buckley formations (samples 07-2-4, base of the middle member, but reexamination suggested that it should be 07-2-12, MBB 73-3, and 11-4-3); a middle provenance recorded in the upper at 170 m. Sample 07‑6‑3 lies at 80 m (above the revised member boundary) in Buckley Formation (samples 96-35-2, 96-36-1,11-4-10, 11-5-22, and H3‑384b); the Fremouw upper member, which now has a minimum thickness of 300 m. and a late provenance documented in the Triassic sandstones (samples 96‑36‑4 Sample 07‑6‑2 has a maximum depositional age (246 ± 3 Ma) that is lower [Elliot and Fanning, 2008], 07-6-2, 07-6-3, and 90-2-52). Anisian (Fig. 4B) and close to the Lower to Middle Triassic boundary (ca. 247.2 The early provenance is dominated by zircons falling in the age span of Ma; International Chronostratigraphic Chart, 2016), and sample 07‑6‑3 (203.5 m the Ross orogen (Goodge et al., 2012; see also Adams et al., 2014). Although stratigraphically above sample 07-6-2) has a depositional age no older than the analyses for two samples have been deconvolved and yield a range of 242 ± 2 Ma, which lies at the Anisian–Ladinian boundary (within the Middle probability peaks, there is no consistency, except for a prominent late Neo- Triassic). The Cynognathus vertebrate fauna recovered from the upper member proterozoic peak at ca. 575–585 Ma and a lesser peak at ca. 560 Ma. Younger of the Fremouw Formation in the Queen Alexandra Range has been assigned grains, consistent with the dated Ross granitoids (ca. 545–480 Ma), are less a late Scythian (late Olenekian) or early Anisian age (ca. 247 Ma) that is late abundant. One of the Buckley Formation samples (MBB 73-3) has an enigmatic Early or early Middle Triassic (Hammer, 1995), whereas Sidor et al. (2014) sug- Middle Ordovician minor peak (ca. 470 Ma) that may reflect radiogenic Pb loss gested correlation with the Cynognathus C subzone of the Karoo in South Africa or document a very late magmatic episode that is not represented in outcrop. (Abdala et al., 2005) and therefore a slightly younger age of late Anisian (ca. There is a spread of Grenvillian-age grains (ca. 900–1100 Ma), but only the 244 Ma). Both of these age assignments are older than the maximum deposi- Fairchild sample (07-2-4) has a significant probability peak, which lies at ca. tional age for the Fremouw upper member at Halfmoon Bluff. The maximum 1050 Ma. This sample also has a minor early Mesoproterozoic cluster at ca. depositional age for sample 07-6-3 would suggest that the vertebrate fauna has 2250 Ma. These sandstones were derived from the West Antarctic flank of the a late Anisian or younger age. Nevertheless, some uncertainties must be con- depositional basin, but the age spectra are similar to those of samples from sidered. Radiogenic lead loss from the zircon grains (areas analyzed) would Mount Bowers, which were clearly derived from the craton (Elliot et al., 2015). bias any weighted mean to younger numerical ages; considerable diachroneity The paleocurrent data for the lower Buckley Formation sample MBB 73-3 are in the middle to upper Fremouw Formation boundary may exist; and the nu- clear and indicate derivation from the West Antarctic flank (used here only in merical age assigned to the base of the Anisian is not particularly precise. a geographic sense); for the other three, there is some uncertainty, and flow The informal members of the Fremouw Formation represent depositional may have been less directly off West Antarctica and more nearly down the facies and therefore, in a terrestrial environment, the boundaries will be sub- basin axis (NW to SE in present coordinates). The lower Buckley sample MBB

73-3 suggests derivation from unexposed Upper Proterozoic to lower Paleo- strata and/or recycling out of lower Paleozoic strata rather than directly from zoic granitoids in West Antarctica or from Paleozoic sedimentary successions granitoids and gneisses. (Ordovician Swanson Formation and Devonian Taylor Group), which were The third (youngest) provenance was more diverse, reflecting a variety themselves probably derived to a significant extent from Ross orogen gran of sources. Principal flow directions were the reverse of those of the lower itoids. Extension of the Ross orogen into West Antarctica is probable, but how Buckley Formation (Barrett et al., 1986; Isbell, 1991; Collinson et al., 1994). The far is unknown. The Swanson Formation (Bradshaw et al., 1983), a turbidite lower Fremouw sample (96-36-4) has a scattering of grains younger than ca. succession in western Marie Byrd Land, may extend eastward (subglacially), 380 Ma but too few for any significance to be attached. The case is different and the Devonian Taylor Group (Bradshaw, 2013) extended out into West Ant- for the remaining samples that have yielded a significant number of Triassic arctica; zircons in both sets of strata have the Gondwana margin detrital-zircon grains and weighted mean ages have been calculated. They show that Trias- signature (Ireland et al., 1998; Pankhurst et al., 1998; Wysoczanski et al., 2003; sic magmatic arc rocks were being eroded, as well as the Permian arc rocks Elliot et al., 2015; Yakymchuk et al., 2015) of numerous upper Neoproterozoic to that dominated the upper Buckley zircons. Carboniferous (sample 07-6-2; lower Paleozoic (Ross orogen–age) grains plus lesser numbers of Grenvillian- sample 96-36-4 [Elliot and Fanning, 2008]) and Devonian (sample 07-6-3) arc age grains and are therefore appropriate sources. The persistent occurrence rocks formed a minor component of the source region during Fremouw time. of small numbers of Neoproterozoic (0.9–0.6 Ga), Paleo- and Mesoproterozoic The middle Fremouw sample (07-6-2) also has a string of Upper Ordovician (2.5–1.1 Ga), and Archean grains is, similarly, most probably the result of recy- to lowest Devonian grains, which have no known potential source in Antarc- cling through Swanson and Taylor Group strata. tica. Ross orogen–age grains are present in samples 96-36-4 (lower Fremouw The middle provenance, associated with the switch in paleocurrent direc- sandstone), 07-6-3 (upper Fremouw sandstone), and 90-2-52 (upper Falla sand- tions following the transition from the lower to the upper Buckley Formation stone). The Fremouw sandstones, for which paleoflow indicates derivation (Barrett et al., 1986; Isbell, 1991; Collinson et al., 1994), is dominated by zircon from West Antarctica, have both arc and Ross orogen–age grains, the latter grains with Permian ages. The two samples remeasured in geochronology probably derived by recycling from underlying strata and/or lower Paleozoic (six-scan) mode are within uncertainty of each other and therefore indistin- strata. In the case of the upper Falla sandstone (90-2-52), which comes from guishable in age although separated stratigraphically. In sample 96-35-2, from the axial part of the basin, the Ross-age and older zircons were not necessarily 70 m below the Buckley Formation upper contact near Layman Peak, 55 of derived by recycling but might have been eroded directly from the craton or 62 grains analyzed (excluding those reanalyzed in geochronology mode) are from Ross orogen foreland basin strata (Elliot et al., 2015). It should be noted Permian (and younger) in age and indicate a flood of grains from a magmatic that the known Ross orogen rocks were covered by Victoria Group strata in arc. Sample 96-36-1, from 1 m below the upper Buckley contact, has 27 of Permian and Triassic time and could not have formed the immediate source 60 analyzed grains (excluding six-scan results) being Permian or younger in for zircons giving a “Ross orogen age.” age. In sample 11-4-10, from Clarkson Peak and relatively low in the upper Buckley strata, 32 of 70 analyzed grains gave Permian and younger ages, and sample 11-5-22 from 40 m above the base of the upper Buckley strata at Mount Regional Context—Gondwana Plate Margin Bowers gave Permian or younger ages for 48 out of 72 zircons analyzed. The one sandstone from the Ohio Range (H3-384b) comes from high in the Paleocurrent data and sandstone petrology of the Victoria Group indicate stratigraphic section but is overlain by carbonaceous beds, which implies a that a major source of volcaniclastic detrital material lay in what is now West Permian age because, regionally, Lower Triassic strata are non-carbonaceous Antarctica (Barrett et al., 1986; Barrett, 1991; Collinson et al., 1994). Except for (see Collinson et al., 2006). As noted earlier, the few zircon grains giving Tri- the Pacific rim (Marie Byrd Land and Thurston Island) and the anomalous assic dates in all these samples are considered to be analyses of areas that Ellsworth-Whitmore Mountains block (Fig. 1), West Antarctica is an ice-cov- have lost radiogenic Pb. The abrupt influx of volcaniclastic detritus argues ered Cretaceous and Cenozoic rift system (Behrendt, 1999). However, the geo- for a significant shift in the provenance and the tapping of a magmatic arc, graphic relationships of the components of West Antarctica are the result of which had been active from at least early Permian time onward; the arc only Gondwana fragmentation and subsequent rotations and translations of the sparsely documented in West Antarctica (Pankhurst et al., 1993, 1998; Mukasa various crustal blocks, plus the later extension between Marie Byrd Land and and Dalziel, 2000) is clear for Zealandia and the Antarctic Peninsula (Wandres the Transant arc tic Mountains. Thus discussion of the provenance of the zir- and Bradshaw, 2005; Barbeau et al., 2010). One sandstone has a small cluster cons has to be in the context of the reconstruction of Gondwana for pre-Juras- of Carboniferous grains and three of the sandstones have minor clusters of sic time, specifically Permo-Triassic time. Reconstruction is not straightforward Devonian igneous zircons, demonstrating that older magmatic arcs were a because of the lack of geologic pinning points, uncertainties in crustal block component of this new source region. However, Ross orogen–age grains are dimensions given probable collapse of free edges, the uncertain nature of the present in three of the sandstones, and still older grains are present in one subglacial regions of West Antarctica, unknown basement lithologies beneath of them; these grains were more likely derived by erosion of lower Permian the sediment cover in the Ross and Weddell embayments, let alone the geol-

ogy of other submerged continental blocks generated by breakup. Some of these uncertainties in reconstructions have been discussed previously (Elliot, Cape 2013; Elliot et al., 2016a). Africa

F The reconstruction in Figure 5 assumes continuity of geologic provinces, ol d B Gondwana recognizing that there are significant uncertainties in the relationships in the el Patagonia t deformation eastern Marie Byrd Land–Thurston Island–southern Antarctic Peninsula re- Tierra del front gion. Correlation of West Antarctic geology with the South America–south- Fuego ern Africa sector of the Gondwana margin is particularly complicated because FI of the rotations and displacements of various continental fragments, but the Lachlan orogen of southeastern Australasia (Foster and Gray, 2000; Glen, 2005) provides a framework for Marie Byrd Land and the Ross embayment, EM albeit with the probability of major margin-parallel displacements of terrains Permian- Fore-arc Triassic AP (DiVenere et al., 1996; Adams et al., 1998; Bradshaw, 2007; Bradshaw et al., strata EL East 2009). The Lachlan orogen has a complicated history of subduction, rifting, Volcaniclastic extension, slab rollback, marginal sea formation, and magmatism (episodic V TI Pensacola strata (Per-Tri.) Back-ar Mtns from Ordovician to Carboniferous time). The Ordovician turbidites alone indi- Orthogneisses (Tri.) cate a close and direct connection with the Marie Byrd Land–Ross embayment Granitoids (Tri.) region of West Antarctica, the region of particular interest for this study. The Orthogneisses (Per.) Antarctica New England orogen (Glen, 2005) is marginal to and outboard of the Lachlan c Beardmore Gl. orogen but continues the record of magmatism into the Triassic and clearly Granitoids (Per.) links through Zealandia to eastern Marie Byrd Land. C Beacon R Fore-arc The probable existence of a belt of Permian intrusive and metamorphic AP Antarctic Peninsula MBL strata rocks on the Panthalassic margin of Antarctica was first recognized by Crad- CP Campbell Plateau dock et al. (1964), Halpern (1972), and Wade (1972), and was subsequently con- ChP Challenger Plateau, V firmed by Pankhurst et al. (1998) and Mukasa and Dalziel (2000), the latter two W. New Zealand V CP papers also documenting Triassic granitoids and orthogneisses (Fig. 5). The CR Chatham Rise V Permo-Triassic arc rocks together with a few Silurian granitoids constitute the EL Ellsworth Land E Amundsen Province of Pankhurst et al. (1998). Investigations on the Antarctic EM Ellsworth Mtns block FI Falkland Is. NZ Peninsula have also documented granitoids and orthogneisses of Permian and ChP LHR Lord Howe Rise V Triassic age (Storey et al., 1996; Vaughan and Storey, 2000; Millar et al., 2002; MBLMarie Byrd Land V Flowerdew, 2008; Riley et al., 2012). In contrast, in the New Zealand sector of NZE Eastern New LHR the margin, the Permian and Triassic record is dominated by stratified succes- Zealand. Permo- Australia sions, both volcanic arc-derived (Murihiku, Maitai, and Caples terranes) and TI Thurston I. Triassic 1000 km of continental derivation (Rakaia terrane) (Mortimer, 2004; Wandres and Brad- arc shaw, 2005). Associated igneous rocks are confined to the oceanic volcanic Figure 5. Gondwana reconstruction to illustrate the distribution of Upper Carboniferous, rocks of the early Permian Brook Street terrane and an upper Permian–Lower Permian, and Triassic granitoids and orthogneisses (modified from a reconstruction pro- Triassic intraoceanic I-type batholith (Price et al., 2011; McCoy-West et al., 2014). vided by the PLATES Project at the Institute of Geophysics at the University of Texas at Older magmatic belts are recognized along the Gondwana margin (Fig. 6). Austin). Note the right lateral translation of eastern Marie Byrd Land with respect to western Marie Byrd Land (DiVenere et al., 1996). X marks present-day South Pole in Antarctica. A belt of Upper Devonian and Carboniferous granitoids crops out in the West- I.—Island; Is.—Islands; Gl.—Glacier ern Province of New Zealand (Mortimer, 2004; Wandres and Bradshaw, 2005; Allibone et al., 2009; Tulloch et al., 2009), in Tasmania (Gray and Foster, 2004; Glen, 2005), southeastern Australia (Foster and Gray, 2000), and in the prox- very sparse Cambrian orthogneisses, define the Ross Province of Pankhurst imal north Victoria Land (Henjes-Kunz and Kreuzer, 2003) in a reconstructed et al. (1998), which lies inboard of the Amundsen Province. Possible extension Gondwana. This belt is also developed in Marie Byrd Land (Weaver et al., 1991; of the Ross Province into the Antarctic Peninsula is not well documented, con- Pankhurst et al., 1998; Mukasa and Dalziel, 2000; Siddoway and Fanning, 2009; sisting of Devonian orthogneisses in two isolated basement complexes and a Yakymchuk et al., 2013, 2015), where Upper Devonian and Lower Carbonifer- single Carboniferous granitoid (Millar et al., 2002). In the Australasian context, ous granitoids and sparse orthogneisses crop out. These rocks, together with the Silurian–lower Carboniferous and Permo-Triassic magmatic belts are spa-

extend to the Permo-Triassic plate margin, it implies that the younger magmatic belts were built on the outer margin of a very wide arc region of the Africa Ross orogen or that an allochthonous ribbon of Ross orogen rocks had been detached and moved outboard. The former is regarded as improbable because the deep-water Ordovician (?) Swanson turbidites were deposited on the oce- Patagonia anic flank of the Ross orogen and lie inboard of the Silurian and younger magmatic arcs. The latter interpretation is advocated. The Ross orogen, including FI granitoids and both siliciclastic and carbonate shallow water successions of Cambrian age, may have extended for some distance outboard of its known extent but would have been mainly covered by the Ordovician and Devonian EM strata. The regional context of the Ross orogen is complicated by the contrast AP between the Cambrian strata of the CTM, north Victoria Land, and Tasmania, East and the possible allochthonous nature of Tasmania (Bradshaw et al., 2009; EL Cayley, 2011). Devonian strata TI The Ross orogen–age and older zircons of the early provenance are there- (Transantarctic fore attributed mainly to recycling through sedimentary strata (Swanson For- Mountains, EM, EL) mation and inferred equivalents and/or Devonian quartzose strata) and/or Orthogneisses Antarctica Marie Byrd Land paragneisses (Yakymchuk et al., 2015) that were located out- Granitoids (Carb.) Beardmore Gl. board of the Transant arc tic Mountains during Permo-Triassic time, with per- Granitoids (U. Dev.) haps a small component derived by direct erosion of Ross orogen rocks out- C board of its known exposures. During Triassic time (late provenance time), R MBL Taylor Gp. AP Antarctic Peninsula and in particular late Falla time, the Ross orogen–age and older zircons could CP Campbell Plateau equally well have been derived from the craton flank of the basin. ChP Challenger Plateau, W. New Zealand CP Detrital Zircons and Basin Filling CR Chatham Rise EL Ellsworth Land E Detrital zircons clearly indicate a major source region along the contemporaneous active Gondwana margin during late Permian through Triassic time. EM Ellsworth Mtns block NZ FI Falkland Is. ChP Studies at Mount Bowers and in the Queen Elizabeth Range demonstrated sig- LHR Lord Howe Rise nificant detrital input from the craton (Isbell, 1991; Elliot et al., 2015) on that MBLMarie Byrd Land flank of the basin, but major flow directions recorded for Permian strata (Barrett LHR NZE Eastern New Devonian-Early Australia et al., 1986; Isbell, 1991; Isbell et al., 1997) suggest that the present outcrop belt 1000 km Zealand. Carboniferous of the Permian strata in CTM aligns approximately with the basin axis, with an arc TI Thurston I. overall sense of flow to the southeast (present coordinates) toward the Ohio Range; this regional flow pattern was gradually reversed in late Permian time. Figure 6. Gondwana reconstruction to illustrate the distribution of Upper Devonian and Lower Carboniferous granitoids and orthogneisses. I.—Island; Is.—Islands; Gl.—Glacier; The Pagoda Formation basin received detrital sediment from both flanks Gp.—Group. (Fig. 7), although the principal paleoflow was along the axis of an elongate basin (Isbell et al., 2008). The postglacial Mackellar Formation, deposited in tially separated; it is possible that the Ross and Amundsen province spatial a brackish to marine environment (Miller and Collinson, 1994; Jackson et al., distinctions result from inadequate exposure, and there could be overlap of all 2012), also shows sparse evidence for derivation from the basin flanks. The the magmatic belts in Antarctica. overlying Fairchild Formation represents the deltaic infilling and fluvial cover- Although isolated Cambrian orthogneiss localities are known from eastern ing of the shallow Mackellar Sea and, like the underlying Permian formations, Marie Byrd Land (Pankhurst et al., 1998; Mukasa and Dalziel, 2000), there is records minor input from the basin flanks and predominantly southeasterly little indication that the Ross orogen extended to the Permo-Triassic Gond- paleoflow (Isbell, 1991; Isbell et al., 1997). Together, the Mackellar and Fair- wana plate margin; however, the occurrence of Cambrian basement in Tierra child formations reflect deposition in a 100–200-km-wide, elongate, slowly del Fuego (Pankhurst et al., 2003) suggests that a Cambrian arc might have subsiding basin. The quartz-pebble conglomerate lenses marking the base of extended along the Gondwana margin to link up with the Ross orogen. If it did the Buckley Formation (Barrett et al., 1986) suggest a hiatus and a tectonically

Magmatic arcs W. Ant SHG MB Craton Falla

Fremouw iassi

Buckley n

Fairchild ermi Figure 7. Schematic space-time illustration

P Mackellar of the Victoria Group in the central Trans Pagoda antarctic Mountains to show the various

Alexandra ev detrital inputs from the basin flanks and D principal paleoflow directions. The depositional basin was narrow in early Permian time, and broadened in Buckley time, as it evolved into a foreland basin. Ra.—Range. Swanson Fm. Ross Orogen

Regional paleoflow to the NW (SVL) Mixed detritus in axial part of basin Transitional switching paleoflow Arc-derived volcaniclastic detritus (P,Tr) Regional paleoflow to the SE (Ohio Ra.) Paleocurrent directions West Antarctic-derived granitic detritus (P) Inferred paleocurrent directions Craton-derived granitic detritus (Tr) MB Mt. Bowers SHG Shackleton Glacier Craton-derived granitic detritus (P) SVL South Victoria Land W. Ant West Antarctica Craton-derived granitic detritus (D)

driven change in environment. This is not observed in the sandstone petrology reflect the migration of principal river systems across the flood plain and the but is in the detrital-zircon patterns that now have a greatly diminished com- depositional sites of volcaniclastic sediment, shifting entry points of prograd- ponent of pre–Ross orogen–age grains. Furthermore, dispersal patterns were ing alluvial systems into the basin, relative uplift and relief of the basin flanks reorganized at this time, with a change from uniform southeasterly flow in the (Conaghan et al., 1982), and tectonism and sediment loading resulting in an Fairchild Formation to transverse flow into the basin along the flanks of the expanding and migrating basin system (Flemings and Jordan, 1990). outcrop belt and axial flow along the basin, suggesting the lower Buckley beds The advent of volcaniclastic detritus and contemporaneous magmatic arc were deposited in a trough-shaped basin (Isbell et al., 1997). The lower Buck- zircon grains, together with the associated change in paleoflow, marks the ley depositional system was, at least locally, a low-lying alluvial plain (Isbell most significant event in the history of basin filling (Figs. 7 and 8). The input and Flaig, 2005) with coal deposition recorded in sections near Mount Rosen of volcanic detritus into the upper Buckley depositional system in late Permian wald (below and above sample 07-2-14 in a 64-m-thick succession capped by a time resulted in major alluvial fans prograding into and across the basin and dolerite sill) and in the region of Mount Ropar and Mount Weeks (Barrett et al., reflected tectonism and magmatism along the plate margin. The timing of on- 1986) (Fig. 3). set of volcaniclastic input to the basin is not well constrained but has to be late The transition from the quartzose lower Buckley beds to the volcaniclastic Permian in age, and the detrital-zircon age data suggest an age no older than upper Buckley strata must be diachronous, depending on the source regions ca. 260 Ma for the CTM and the Ohio Range. Zircon grains in Buckley Formation being tapped and the locations of alluvial systems and depositional centers. sample 90-35-2 are almost exclusively Permian in age, but other samples with At Mount Miller on the arc flank, the first occurrence of volcaniclastic detritus significant numbers of Permian grains also have a small and older Devonian( is ~150 m above the base of the formation and at Mount Ropar (Fig. 3) on the and Carboniferous) Pacific margin component as well as Ross orogen–age cratonic flank at ~200 m (Barrett et al., 1986). However, at Mount Bowers, also clusters and older Precambrian grains. The Cambrian and older zircons, except on the cratonic flank, it occurs at 392 m (Elliot et al., 2015). Diachroneity must for the Mount Bowers sample (11-5-22), could have been derived by reworking

C 0 km ~100 Upper Buckley strata Victoria Gp. basin Lower Buckley strata Figure 8. Schematic cross sections of the thrust belt Pagoda-Fairchild strata Fremouw strata Devonian strata Gondwana margin and Victoria Group depositional basin in the central Transant arctic Mountains. (A) Early Permian time at the end of Fairchild deposition. (B) Lat- est Permian time at the end of upper Buck- ley deposition. (C) Middle Triassic time at the end of Fremouw deposition. These schematic cross sections illustrate the km Upper Buckley strata B 0 ~100 Victoria Gp. basin pre-Devonian crustal provinces and the Lower Buckley strata Gondwana margin arc terrains related to Permian beds Pagoda-Fairchild strata Victoria Group deposition. They also illus- Devonian strata trate the proposed backarc basin in which lower Permian arc-derived sediment was trapped, the release of a flood of Permian zircons from that basin and the arc in late Permian time, and the progressive on lap and expanding basin boundaries through time. The location of the inferred thrust 0 km ~100 A Victoria Gp. basin Pagoda-Fairchild strata belt is illustrative only; it could have in- Permian beds Devonian strata volved strata on the basin margin. The Ross orogen Paleozoic strata contact must be a significant unconformity, but other province boundaries undoubtedly are diffuse and merge from one into another. The horizontal scale is approximate. Gp.— Group.

Fore-arc P-TR arc Dev. arc Paleozoic strata Ross Orogen Proterozoic rocks

of the Swanson Formation, Marie Byrd Land paragneisses, and Devonian (sample 96-36-4; Elliot and Fanning, 2008) predominantly have Ross orogen quartzose sandstones, or from unconsolidated lower Buckley beds. The Cam- ages but with scattered older Precambrian grains and a smattering of Devo- brian grains in the Mount Bowers sample (11‑5‑22) were most probably de- nian, Carboniferous, and Permian zircons. The latter were ultimately derived rived directly from the craton, as inferred by the paleocurrent data suggesting from the Gondwana margin but probably were recycled from upper Buckley flow off East Antarctica (Elliot et al., 2015). The initial flood of detrital sediment beds. The paucity of volcaniclastic detritus in the lower Fremouw member im- with Permian zircon grains, associated with a significant tectonic event that plies that the principal source for the Ross-age grains was the craton. Input led to the gradual reversal in regional paleoflow along the basin axis, must from the margin must have been largely blocked by rivers carrying the prod- have spread across the basin and beyond its current extent, forming a series ucts of craton erosion and/or a hiatus in sediment supply from the margin of low-gradient alluvial fans (Isbell, 1991). The distal margins of the fans on resulting from tectonism intermittently blocking sediment dispersal from the the cratonic flank fed into the dispersal of volcaniclastic sediment back toward arc. The sample from the Fremouw middle member (07-6-2) marks a return to the main basin axis (see Elliot et al., 2015), possibly resulting from continuing significant Gondwana margin input with abundant grains having Early–Middle development of a forebulge. Triassic ages and a small but important set of upper Carboniferous grains. The upper provenance, documented by the Triassic detrital zircons, is asso- In addition, a cluster of Upper Ordovician to Silurian grains must also have ciated with strong southwesterly (present coordinates) paleoflow off West Ant- come from the margin where a Silurian granitoid and an orthogneiss have arctica (Vavra et al., 1981; Collinson and Elliot, 1984; Collinson et al., 2006) in the been recorded (Pankhurst et al., 1998). The more quartzose sample (07‑6‑3) region of the Shackleton Glacier and with paleoflow veering northwesterly in from the Fremouw upper member has a major group of Middle Triassic grains the axial part of the basin in the Queen Alexandra Range region (Barrett et al., and minor upper Permian and Upper Devonian clusters. The cluster of Ross 1986; Isbell, 1991). In a general sense, this documents fans prograding off West orogen–age and older grains could have come directly from the craton or by Antarctica and a change in direction of the paleoflow on entering the axial part recycling through the inferred lower Paleozoic strata in West Antarctica. Zir- of the basin. However, the zircon data suggest this is oversimplified. Zircons cons from the Falla Formation sample (90-2-52) exhibit a major group of near in the quartzose sandstone of the lower member of the Fremouw Formation contemporaneous Late Triassic age and a minor group of Early Triassic age,

which might have been reworked from older Triassic beds or eroded directly substantial magmatic activity on the Gondwana plate margin from Silurian to from magmatic arc rocks. The significant cluster of Ross orogen–age grains Late Triassic time, with major episodes in the Devonian, Permian, and Trias- was probably derived from the craton. Apart from the Fremouw lower mem- sic, and lesser episodes during the Carboniferous. The widespread occurrence ber, the variability of the zircon spectra in the three remaining samples sug- from Zealandia to the Antarctic Peninsula of Permian volcaniclastic strata, gests that detrital inputs from the plate margin and craton sources were being igneous rocks, and detrital zircons suggests a major tectonic adjustment, at- mixed through reworking on a sandy braided plain (Collinson et al., 1994) and tributable to plate reorganization or increasing closure rates. Granitoids and that interfingering of stream deposits with different proportions of plate mar- orthogneisses along the Pacific margin must be more prevalent in time and gin and craton detritus was widespread. space than is apparent from limited outcrop data in that now ice-covered The abrupt switch from coal-bearing Buckley strata to non-carbonaceous region. Further, the zircon data suggest that Permian magmatism possibly sedimentation in Fremouw time marks a significant environmental change. In started earlier than is evident in the currently exposed stratigraphic and mag- addition to a change in climate (Collinson et al., 2006), it is probable that tec matic record and was episodic to continuous through to Late Triassic time. tonism increased the rates of erosion in the source region and also caused ero- The Gondwana plate margin in the Antarctic sector was a complex and sion of older strata on the basin margin, resulting in the Victoria Group depo varied active plate boundary with at least episodic magmatism from Silurian sitional system passing from an under filled to an overfilled basin (Catuneanu through Triassic time, yet the lower part of the Victoria Group (Pagoda to lower et al., 1998; Isbell and Flaig, 2005). Nevertheless, the fluvial environment in- Buckley formations), which was derived in part from the West Antarctic region, cluded coal formation in late Fremouw and early Falla time. lacks detrital material derived from those Paleozoic magmatic arcs. The ero- sional products of those magmatic arcs must have been trapped between the arc and an elevated source region during early Permian time, in some type of Magmatism and Tectonism on the Gondwana Margin backarc basin (Fig. 8). The flood of volcaniclastic detritus and Permian zircons indicates breaching of that barrier in late Permian time. This probably resulted Detrital zircons in the upper Permian to Upper Triassic sandstones provide from thickening of the crust, uplift of the arc, and development of a foreland additional information on the overall makeup of the Gondwana margin. Except fold and thrust belt, all attributable to increasing closure rates on the plate mar- for the McQuarrie arc in far-distant New South Wales, Australia (Glen, 2005), gin. Although the late Permian–Triassic Victoria Group setting is interpreted as sources for Upper Ordovician zircon grains are unknown, which implies un- a foreland basin, thickening of foredeep strata toward the plate margin cannot exposed rocks of that age in the Antarctic sector. The few detrital zircons with be documented because of the ice cover. The uncertainties are compounded Silurian ages are consistent with granitoid and orthogneiss of that age on the by the fact that the intensity of deformation affecting the lower Permian fore- plate margin (Pankhurst et al., 1998). Silurian magmatic rocks are very sparsely deep strata in the Karoo of South Africa and the Falkland Islands diminished distributed in Marie Byrd Land but, in contrast, are a major component of the toward West Antarctica in reconstructed Gondwana (Tankard et al., 2009; Elliot Lachlan orogen. The Upper Devonian Ford granitoids (Pankhurst et al., 1998; et al., 2016a), suggesting that the foredeep may have shallowed in that direc- Mukasa and Dalziel, 2000; Yakymchuk et al., 2015) were clearly the source for tion. In Zealandia, only the Triassic Topfer Sandstone and Jurassic Kirwans detrital zircons of that age in the Permian strata of the CTM. Lower Carbon- Dolerite west of the Alpine fault (Mortimer et al., 1995) are comparable to the iferous granitoids were reported by Pankhurst et al. (1998) and Mukasa and Victoria Group. It is not clear if and how any of the New Zealand terranes are Dalziel (2000) and formed a minor contributor to the Victoria Group. Upper related, particularly given the argument that they may have been translated Carboniferous granitoids and orthogneisses, known from eastern Marie Byrd long distances (Adams et al., 2007). The Permo-Triassic arc and associated Land and Thurston Island (Pankhurst et al., 1993, 1998), constitute a source foreland basin emerge again in the New England orogen and Sydney Basin of for grains of that age found in the middle Fremouw sample (07-6-2) and in the eastern Australia (Veevers et al., 1994). Permian sandstone from Erehwon Nunatak in eastern Ellsworth Land (Elliot et al., 2016a). The abundance of detrital zircons with Permian ages demon- strates that the late Permian was a time of major magmatic activity, and the CONCLUSIONS spread of detrital-zircon ages suggests magmatism may have been active throughout Permian time. The presence of Triassic zircons implies that gran- 1. Three distinct provenances, defined by detrital-zircon age data, are rec- itoids and orthogneisses of that age are more widespread than might be ap- ognized and are consistent with changes in sandstone petrology. parent from the dated granitoids at Kinsey Ridge, western Marie Byrd Land, at 2. The Permian magmatic arc on the Gondwana plate margin was initiated Mount Murphy, eastern Marie Byrd Land (Pankhurst et al., 1998), and at Mount early in the Permian, reached peak activity in late Permian time, and was prob- Brammall, Thurston Island (Pankhurst et al., 1993). ably a more significant magmatic episode than that during Triassic time. Overall, the detrital-zircon data together with the previous results from dat- 3. The detrital-zircon data show that a barrier (elevated terrain) separated ing granitoids and gneisses in Marie Byrd Land and Thurston Island suggest the Victoria Group basin from the Gondwana margin until late Permian time.

Table S3. Summary of SHRIMP U-Pb zircon results for sample 90-2-52.

Total Ratios Radiogenic Ratios Age (Ma) deposition, selected young grains were reanalyzed in geochronology mode using SHRIMP II to 206 204 238 207 206 207 207 206 207 Grain. UTh Th/U Pb* Pb/ f206 U/ Pb/ Pb/ Pb/ Pb/ Pb/ Pb/ % 4. A basin existed adjacent to the arc, in which arc-derived sediment was spot (ppm)(ppm) (ppm) 206Pb % 206Pb ± 206Pb ± 238U ± 235U ± 206Pb ± ρ 238U ± 206Pb ±Disc

SHRIMP RG: detrital analyses (4 scan data) gain better precision on the youngest detrital-zircon age grouping. 2.1303 2130.71220.0000960.0411.994 0.1430.05800.00080.0833 0.0010 516 6 3.1299 82 0.27 39 0.0000990.176.659 0.0790.07230.00090.1499 0.0018 1.4660.028 0.0709 0.0010 0.629 901 10 955306 4.12373178 0.08 1900.0000040.0010.7350.111 0.0592 0.0003 0.0932 0.0010 574 6 trapped, probably from late Carboniferous time until breaching in late Permian 5.1134 1310.9840.0004230.3125.7740.438 0.0535 0.0017 0.0387 0.0007 245 4 6.1943 7070.75750.0001060.3710.7900.137 0.0621 0.0004 0.0923 0.0012 569 7 Overall, the data have been reduced using the SQUID Excel Macro of Ludwig (2001). The Pb/U 7.1284 77 0.27 21 0.0001360.0111.656 0.1420.05810.00080.0858 0.0011 531 6 8.1258 1050.4180.000210<0.01 27.673 0.3740.05040.00120.0362 0.0005 229 3 9.1129 76 0.59 40.0005110.3726.5810.436 0.0539 0.0027 0.0375 0.0006 237 4 10.1 332526 1.58 10 0.0006340.0628.8920.418 0.0510 0.0011 0.0346 0.0005 219 3 11.1 498449 0.90 13 0.0000800.1131.6840.383 0.0510 0.0009 0.0315 0.0004 200 2 time, as a result of uplift in the arc and backarc region. 12.1 541473 0.87 15 0.0002570.2631.7270.377 0.0522 0.0009 0.0314 0.0004 200 2 ratios have been normalized relative to a value of 0.0668 for the TEMORA reference zircon, equiv- 13.1 270730.27200.0000860.1911.331 0.1360.05990.00080.0881 0.0011 544 6 14.1 91032183.54270.0001600.1128.7810.320 0.0514 0.0006 0.0347 0.0004 220 2 15.1 193920.4813 -0.2512.3150.157 0.0593 0.0010 0.0810 0.0011 502 6 16.1 490360.0743 -<0.01 9.7300.109 0.0607 0.0006 0.1028 0.0012 631 7 17.1 134900.6710 -<0.01 11.255 0.1540.05700.00110.0890 0.0012 550 7 alent to an age of 417 Ma (see Black et al., 2003). Uncertainty in the reference zircon calibration 18.1 586483 0.82 19 0.000098<0.01 26.158 0.3060.05040.00080.0383 0.0005 242 3 5. Detrital zircons suggest that the Triassic arc documented in the Antarctic 19.1 223160 0.72 6 -<0.01 30.992 0.4540.04920.00140.0323 0.0005 205 3 20.1 422122 0.29 63 0.0000200.035.788 0.0640.07410.00050.1727 0.0019 1.7590.024 0.0738 0.0006 0.822 102711 1037161 3 21.1 262117 0.44 19 0.0000190.1112.0990.157 0.0584 0.0009 0.0826 0.0011 5117 22.1 239790.3334 -<0.01 6.0990.076 0.0742 0.0024 0.1640 0.0020 1.6800.057 0.0743 0.0024 0.364 979 11 1049647 for each analytical session is given in Supplemental Files S3–S13 for each sample. Uncertainties 23.1 852410.05610.000022<0.01 12.005 0.1290.05710.00050.0834 0.0009 516 5 24.1 1373 2480.1870 -0.0116.8690.179 0.0541 0.0006 0.0593 0.0006 371 4 25.1 794247 0.31 3340.0000090.012.043 0.0210.17960.00040.4893 0.0051 12.1100.130 0.1795 0.0004 0.979 256822 264843 Peninsula was more extensive in West Antarctica than is apparent from very 26.1 200150 0.75 15 0.0000660.4011.755 0.1500.06110.00100.0847 0.0011 524 7 27.1 528760.14750.0001520.266.059 0.0670.07410.00050.1646 0.0018 1.6320.025 0.0719 0.0008 0.717 982 10 983220 given for individual analyses (ratios and ages) are at the one sigma level (Supplemental Files 28.1 1413 1095 0.77 40 0.000027<0.01 30.668 0.3340.04990.00060.0326 0.0004 207 2 29.1 401251 0.63 29 0.000068<0.01 12.077 0.1380.05760.00070.0828 0.0010 513 6 30.1 24 30.1120.0008621.2810.9950.266 0.0691 0.0029 0.0898 0.0022 554 13 31.1 664241 0.36 67 0.0000320.758.477 0.1090.06930.0005 0.11710.0015 714 9 204 206 32.1 206530.26280.0001120.196.311 0.2250.07120.00080.1449 0.0018 1.4040.041 0.0729 0.0018 0.797 950 33 1012516 limited exposure. 33.1 337141 0.42 42 0.0005060.886.854 0.0780.07580.00060.1446 0.0017 1.3660.030 0.0685 0.0013 0.521 871 9884 39 1 S3–13 [see footnote 3]). Correction for common Pb was either made using the measured Pb/ Pb 34.1 1082 6440.60159 0.0000090.015.851 0.0610.07360.00030.1709 0.0018 1.7320.020 0.0735 0.0003 0.913 101710 1028101 35.1 333235 0.70 10 0.0000600.1529.2980.375 0.0516 0.0011 0.0341 0.0004 216 3 36.1 464230.05330.0000670.0112.1270.138 0.0576 0.0007 0.0825 0.0010 5116 37.1 92 50 0.55 15 0.0002710.465.197 0.0780.07520.00110.1830 0.0022 1.8440.045 0.0755 0.0025 0.824 1135 17 108165-5 38.1 352133 0.38 55 0.0000390.075.459 0.0610.07400.00060.1830 0.0021 1.8520.027 0.0734 0.0007 0.776 108411 102518-6 6. The Gondwana margin detrital-zircon signature of late Neoproterozoic– ratio in the normal manner or for grains younger than say 800 Ma (or those low in U and radio- 39.1 81 49 0.60 70.0005690.6810.0920.153 0.0656 0.0015 0.0984 0.0015 605 9 40.1 150151 1.00 40.0005720.6531.4290.496 0.0553 0.0017 0.0316 0.0005 201 3 41.1 190970.51140.0000090.1911.866 0.1540.05930.00100.0841 0.0011 521 7 42.1 476251 0.53 16 0.000117 0.12 26.104 0.3090.05200.00080.0383 0.0005 242 3 207 207 43.1 728590 0.81 20 0.000110 <0.0131.6470.362 0.0488 0.0007 0.0317 0.0004 201 2 genic Pb), the Pb correction method has been used (see Williams, 1998). When the Pb correc- 44.1 223142 0.64 21 -0.089.337 0.1150.06210.00080.1070 0.0013 655 8 45.1 97 52 0.53 60.0002500.4712.9750.196 0.0604 0.0015 0.0767 0.0012 476 7 Early Ordovician grains and lesser Grenville-age grains persisted on a dimin- 46.1 258840.32180.0001070.3712.2490.149 0.0604 0.0009 0.0813 0.0010 504 6 47.1 340215 0.63 10 0.000017<0.01 29.465 0.3790.05030.00110.0339 0.0004 215 3 207 206 48.1 347530.15270.000133<0.01 11.115 0.1290.05810.00070.0900 0.0011 556 6 49.1 593417 0.70 16 -0.2032.0420.380 0.0517 0.0009 0.0311 0.0004 198 2 tion is applied, it is not possible to determine radiogenic Pb/ Pb ratios or ages. In general, for 50.1 531533 1.00 14 0.0001570.1831.8480.379 0.0515 0.0009 0.0313 0.0004 199 2 51.1 118175 1.48 30.0005000.6031.8940.553 0.0549 0.0023 0.0312 0.0006 198 3 52.1 369326 0.89 10 0.000024<0.01 31.165 0.4010.04870.00110.0321 0.0004 204 3 ishing scale into the Mesozoic. 206 238 53.1 355204 0.57 32 0.0000160.079.423 0.1100.06190.00070.1060 0.0013 650 7 54.1 359194 0.54 17 -<0.01 18.669 0.2230.05270.00080.0536 0.0006 337 4 grains younger than ca. 800 Ma, the radiogenic Pb/ U age has been used for the probability 55.1 1047 1610 1.54 28 -<0.01 32.636 0.3620.04940.00060.0307 0.0003 195 2 56.1 139136 0.98 4 -0.2131.0610.497 0.0519 0.0017 0.0321 0.0005 204 3 57.1 105157 1.50 7 -<0.01 12.546 0.1860.05700.00140.0797 0.0012 494 7 58.1 113330.30150.000002<0.01 6.6440.098 0.0719 0.0011 0.1505 0.0022 1.4920.032 0.0719 0.0011 0.693 904 12 983318 207 206 59.1 138740.5425 -<0.01 4.7220.061 0.0805 0.0009 0.2119 0.0027 2.3630.041 0.0809 0.0010 0.737 123915 121923-2 7. The Ordovician Swanson Formation and Devonian Taylor Group must density plots (and for areas that are low in U and therefore radiogenic Pb). The Pb/ Pb age is 60.1 756292 0.39 49 0.000010<0.01 13.373 0.1500.05620.00050.0748 0.0009 465 5 be more extensive in the Ross embayment of West Antarctica in order to form used for grains older than ca. 800 Ma or for those enriched in U. Tera and Wasserburg (1972) con- 3Supplemental Files S3–S13. Analytical results for cordia plots, probability density plots with stacked histograms, and weighted mean 206Pb/238U age detrital zircons in 11 sandstones. Please visit http:// the source, together with Marie Byrd Land paragneisses, for the Gondwana calculations were carried out using Isoplot/Ex (Ludwig, 2003). dx.doi .org /10 .1130/GES01245 .S3 or the full-text arti- margin detrital-zircon signature documented in lower Permian Fairchild sand- As a general comment about the methods used, for detrital-zircon age spectra, we have ana cle on www.gsapubs.org to view Supplemental Files stones and for Ross orogen–age zircons in all early provenance sandstones. lyzed grains at random within the total zircon aliquot that was poured (not hand selected) onto S3–S13. double-sided tape and cast into an epoxy disk. From paired reflected and transmitted light images 8. On the assumption that Glossopteris does not range into the Triassic, of the zircon population, a photographic pair was selected, and where possible, every zircon grain results for a sandstone from tens of meters below the Permo-Triassic bound- within those photographs was analyzed. Initially the transmitted light image was used to locate ary illustrate some of the uncertainties in using detrital-zircon data to establish an area within any one grain that was free of cracks and inclusions and so of good-quality zircon. precise depositional ages. Having chosen that area, the CL image was then used to ensure that the area analyzed was of a single zircon component, i.e., there was no overlap of two CL-structured areas. Further, at this stage, the area selected for analysis of any one grain was, on the basis of the CL image, the youngest component in that grain. ACKNOWLEDGMENTS Following this detrital age spectrum procedure, it is often apparent that there are some analy ses that are either younger than the inferred stratigraphic age or very close to that age. In this This research was supported by National Science Foundation (NSF) grants ANT 0944662 to DHE situation, it is then important to revisit those grains and determine in geochronology mode a more and ANT 0838851 to JLI. The Ohio Range sandstone was collected by W.E. Long and provided by precise and accurate date on the specific zircon area and/or grain. In this manner, reanalysis of the U.S. Polar Rock Repository at Ohio State University. DHE wishes to acknowledge significant those grains in higher precision mode yields a better defined estimate for the time of deposition. support over many years from the Office of Polar Programs at NSF. Review by John Bradshaw and In the case of the samples analyzed in this study, a few of the younger analyses were subsequently comments by the editor were particularly helpful in improving the manuscript. This paper is Byrd determined to be of areas that were clearly altered and had lost radiogenic Pb. Such analyses have Polar Research Center contribution no. 1550. not been included in the final weighted mean age calculation for the youngest age grouping for that sample. APPENDIX. ANALYTICAL PROCEDURES AND RESULTS Results

SHRIMP U-Pb Geochronology The geographic locations of samples are given on Figure 3, except for the sandstone from the Ohio Range (Fig. 1), and in Supplemental File S1 (see footnote 1). The samples are also projected Zircon grains were separated from total-rock samples using standard crushing, washing, onto a composite stratigraphic column for the Shackleton-Beardmore region on Figure 4A. heavy liquid (specific gravities 2.96 and 3.3), and paramagnetic procedures. The zircon-rich 07-2-4; Fairchild Formation, Mount Rosenwald; 136 m above the base of the formation. Detrital- heavy-mineral concentrates were poured onto double-sided tape, mounted in epoxy together with zircon grains are principally broken and/or round to subround crystals, although a few euhedral chips of the TEMORA reference zircon, sectioned approximately in half, and polished. Reflected grains are present. The CL images show that a number are oscillatory-zoned igneous crystals, and transmitted light photomicrographs were prepared for all zircons, as were cathodolumines- amongst which a few exhibit sector zoning, although many exhibit only poorly defined to indistinct cence (CL) scanning electron microscope (SEM) images. These CL images were used to decipher zoning. A number are homogeneous under CL, suggesting a metamorphic origin. The age spectrum the internal structures of the sectioned grains and to ensure that the ~20mm SHRIMP spot was (Fig. 9) shows a principal peak at ca. 580 Ma, but deconvolution using the mixture modeling algo- wholly within a single age component within the sectioned grains. rithm (Isoplot) yields a prominent peak at ca. 580 Ma and three lesser groupings at ca. 525 Ma, ca. The U-Th-Pb analyses reported herein were made using sensitive high-resolution ion micro- 565 Ma, and ca. 610 Ma, all within the age span of the Ross orogen (Goodge et al., 2012). There are probe (SHRIMP) II or SHRIMP reverse geometry (RG) at the Research School of Earth Sciences, minor Grenville-age and early Mesoproterozoic peaks at ca. 1050 Ma and ca. 2250 Ma. the Australian National University (ANU), Canberra, Australia, following procedures given in Wil- 07-2-12; lower Buckley Formation, Mount Rosenwald; 64 m above the base of the formation. liams (1998, and references therein). For the detrital age spectra, between ~60 and ~70 grains Zircon grains are mainly round to subround, with some broken fragments of presumed euhedral were chosen in a random manner from the total zircon population, and each analysis consisted crystals. Internal CL characteristics range from strong oscillatory zoning to homogeneous un- of four scans through the mass range, with a TEMORA reference zircon grain analyzed for every zoned grains of possible metamorphic origin; some grains have distinct cores. The age spectrum five unknown analyses. Previously published data for samples 96-65-2 and 96-36-1 (see Elliot and is dominated by grains in the range 535–610 Ma (Fig. 10). This main cluster can be deconvolved Fanning, 2008) were analyzed using SHRIMP I (at ANU), with the U/Pb calibration relative to the into three more prominent subgroupings at ca. 540 Ma, ca. 560 Ma, and ca. 575 Ma, with other Duluth gabbro reference zircon FC1. The youngest grains in these two samples have now been minor subgroups at ca. 515 Ma and ca. 605 Ma. These correspond with the upper Neoproterozoic reanalyzed in geochronology mode using SHRIMP II, each analysis consisting of six scans through to Lower Ordovician Ross orogen; many of these grains are homogeneous, suggesting a meta- the mass range, with a TEMORA reference zircon grain analyzed for every three unknown analy morphic origin. There is a scattering of older ages back to ca. 3 Ga. ses. Samples H3-384b 07-6-2, 07-6-3, and 90-2-52 contain igneous grains with apparent young MBB 73-3; lower Buckley Formation, Mount Butters; 73 m above the base of the formation. 206Pb/238U detrital ages. Because such grains have high significance with respect to the time of Zircon grains from this sandstone are mainly broken and round to subround crystals. A wide

7 16 7 ca. 580 Ma ca. 560 Ma 9 A ca. 525 Ma ca. 565 Ma ca. 580 Ma ca. 610 Ma ca. 515 Ma ca. 560 Ma ca. 575 Ma ca. 605 Ma 6 14 8 ca. 540 Ma 6 A 07-2-12 5 07-2-4 7 Relative probabilit

12 Relative probability 5 4 6 5 ca. 525 Ma 3 10 4 r 4 2 8 3 1 3 2

Number

Numbe 0 6 1 500 520 540560 580600 620640

ca. 1050 Ma y 2 0 4 490510 530550 570590 610 630

1 2

0 0 200 600 1000 1400 18002200260033000 400 400800 1200 1600 2000 2400 2600 3200 Age (Ma) Age (Ma)

B B

500mμ 500 μm

Figure 9. (A) U-Pb zircon age-probability density plot and (B) cathodoluminescence Figure 10. (A) U-Pb zircon age-probability density plot and (B) cathodoluminescence (CL) image for Fairchild Formation sandstone sample 07-2-4. (CL) image for lower Buckley Formation sandstone sample 07-2-12.

variety of CL characteristics are present, including oscillatory and rare sector zoning, as well as internal structure. Grains without internal zoning are also present, and these are probably of meta- complex interiors indicative of more than one period of zircon crystallization. Homogeneous, pos- morphic origin. The zircon age spectrum (Fig. 12) is dominated by grains in the range 520–600 Ma, sibly metamorphic, zircon occurs sparsely. The broad and irregular pattern of the predominant with a dominant peak at ca. 570 Ma, again corresponding with the Ross orogen. Older grains are component of the age spectrum lies in the range 530–600 Ma, with prominent peaks at ca. 545 and scattered through the Precambrian and include eight grains of Archean age. ca. 585 Ma (Fig. 11). These ages fall in the range of the upper Neoproterozoic to Lower Ordovician 11-5-22; upper Buckley Formation, Mount Bowers; 33 m above the contact between the lower Ross orogen. Minor clusters have Early to Middle Ordovician (ca. 470 Ma) and late Grenvillian (ca. Buckley and upper Buckley members, and 425 m above the base of the formation. The sample is 930 Ma) ages. A scattering of older ages is also present. described in Elliot et al. (2015). The zircon population (48 of 72) is dominated by grains of Permian 11-4-3; lower Buckley Formation, south spur of Clarkson Peak; 120 m above the base of the for- age with a probability peak at ca. 260 Ma. The weighted mean 206Pb/238U age for 32 analyses is mation (immediately above the 250-m-thick sill marking the top of section Z2 in Barrett et al., 1986). 258.1 ± 1.9 Ma (mean square of weighted deviates [MSWD] = 1.2) (Fig. 13). There is a minor Ross- Zircon grains form a heterogeneous population with round to subround grain shapes the more age peak (Elliot et al., 2015). The Permian grains are predominantly euhedral (or broken) crystals dominant; many of these are broken fragments of prismatic crystals. Well-defined oscillatory zon- with oscillatory zoning and often with terminations. Ross-age grains are mainly broken, subround, ing is not common, with many of those grains exhibiting weak zoning and some having complex and with oscillatory zoning. Older grains are round to subround and have weak or complex zoning.

16 18 A ca. 585 Ma ca. 545 Ma 10 10 MBB 73-3 A ca. 570 Ma 14 ca. 545 Ma ca. 585 Ma 16 ca. 570 Ma 9 11-4-3 8 8 14 12 7 Relative probabilit

6 12 6 Relative probability 10 5 4 ca. 470 Ma 10 4 8 2 3 8

Number 2 6

0 Number 6 1 440480 520 560 600 620 680 ca. 470 Ma y 4 0 4 420 460 500 540 580 620 660 700

2 2

0 0 0 500 1000 1500 2000 2500 3000 3500 0 500 1000 1500 2000 2500 3000 3500 Age (Ma) Age (Ma)

B B

100 μm 100 μm Figure 12. (A) U-Pb zircon age-probability density plot and (B) cathodoluminescence (CL) image for lower Buckley Formation sandstone sample 11-4-3. Figure 11. (A) U-Pb zircon age-probability density plot and (B) cathodoluminescence (CL) image for lower Buckley Formation sandstone sample MBB 73-3.

H3-384b; Mount Glossopteris Formation, Terrace Ridge, Mount Schopf, Ohio Range; 390 m age spectrum (four-scan data) showed a dominant peak at ca. 270 Ma with a minor but signifi- above the base of the section (Long, 1964, 1965). The zircon grains are predominantly elongate cant grouping around 245 Ma. Many of the analyses yielding young ages plotted above the Tera- to stumpy, euhedral (or broken) crystals, including many with pyramidal terminations. The CL Wasserburg concordia due to enrichment in common Pb. It was suspected that the areas analyzed images show that fine oscillatory zoning is very common, with a few grains having sector zon- had also lost radiogenic Pb, and so, a number of the youngest Permian-age grains were reanalyzed ing. Carboniferous grains are similar to those of Permian age. Grains older than Carboniferous in geochronology mode. Unfortunately, not all the Triassic grains could be reanalyzed. In the com- are round or subround, broken, and with weak to strong oscillatory zoning. The initial detrital posite plot of the original detrital data, together with the updated geochronology analyses of 15

Ma. 30 230 A ca. 270 Ma 7 11-5-22 ca. 245 Ma ca. 260 Ma ca. 270 Maca. 280 Ma 25 6 H3-384b Relative probabilit 240 5

20 4

3 250 15 2

Number

10 1 260 y 0 230240 250260 270 280 290 300 5 270 Weighted mean = 258.1±1.9 Ma. 0 MSWD 1.2 for 32 analyses 0500 1000 1500 2000 2500 3000 3500 280 Age (Ma)

Figure 13. U-Pb zircon weighted mean 206Pb/238U age plot for upper Buckley Forma- tion volcanic sandstone sample 11-5-22. B

grains, there remains a subordinate grouping at ca. 245 Ma that is not considered to be geologi- cally meaningful due to inferred radiogenic Pb loss. The composite probability density plots show the dominance of Permian grains with the main peak at ca. 270 Ma and slightly lesser peak at ca. 260 Ma (Fig. 14). This suggests that an age of ca. 260 Ma may represent the best estimate of the maximum age of deposition. 11-4-10; upper Buckley Formation, south spur of Clarkson Peak; 330 m above the base of the formation. The age spectrum is dominated by Permian grains with the principal probability peak at ca. 260 Ma, a minor but prominent peak at ca. 555 Ma, and small clusters of grains at ca. 360 Ma and ca. 1100 Ma (Fig. 15). The Permian grains have 206Pb/238U ages that range from ca. 250 to ca. 275 Ma, and a depositional age no older than ca. 260 Ma is implied. Most of the Permian (32 grains) and Devonian (four grains) zircons are euhedral, equant to prismatic, crystals of which some are broken; pyramidal terminations are common; all have strong oscillatory zoning, indicating igneous crystallization. The minor but prominent grouping at ca. 555 Ma has, in detail, a range in ages from ca. 500 Ma to ca. 600 Ma, again corresponding with Ross orogen ages, and the 1100 Ma peak lies in the Grenville-age range. Most grains older than Devonian are round to subround, commonly broken, and with less well defined CL zoning. 96-35-2; upper Buckley Formation, near Layman Peak; 70 m below the top of the formation. 100mμ Analytical data for the detrital age spectrum of 60 grains have been published in Elliot and Fanning (2008). The youngest 23 grains, out of the 58 Permian grains originally analyzed, have been rean- Figure 14. (A) U-Pb zircon age-probability density plot and (B) cathodoluminescence alyzed in six-scan geochronology mode in order to provide a better estimate of the depositional (CL) image for Mount Glossopteris Formation sandstone sample H3-384b. Note that age. The zircons are dominated by stumpy euhedral grains, with many retaining crystal termi- only the geochronology data for the Permian grains have been included in the plots. nations, and showing well-developed oscillatory zoning (note that in Elliot and Fanning, 2008, figure 5 is mislabeled: fig. 5a should be 96-36-4, and fig. 5c should be 96-35-2). The analyses form a single tight grouping close to or within uncertainty of the Tera-Wasserburg concordia, indicating that the areas analyzed are dominated by radiogenic Pb and essentially unaffected by radiogenic Pb loss. Twenty of the 23 reanalyzed zircon grains give a weighted mean 206Pb/238U age of 253.5 ± some Devonian grains at ca. 360 Ma and a Cambrian cluster with weak probability peaks at ca. 2.0 Ma (MSWD = 1.5) (Fig. 16), which is interpreted as the maximum age of deposition. 520 Ma and ca. 540 Ma. For the current study, the 22 grains reanalyzed in six-scan mode form a 96-36-1; upper Buckley Formation, Collinson Ridge; 1 m below the top of the formation. Ana slightly dispersed cluster, close to or within uncertainty of the Tera-Wasserburg plot; some taper lytical data for the detrital age spectrum of 60 grains have been published in Elliot and Fanning ing off on the younger age side suggests some areas have lost radiogenic Pb. Sixteen of the (2008). Zircons in this sample include more broken and abraded grains than in 96-35-2, suggesting reanalyzed 22 grains have a weighted mean 206Pb/238U age of 250.3 ± 2.2 Ma (MSWD = 1.5) (Fig. 17), a higher degree of surface transport. Nevertheless, there are whole and broken euhedral crystals, which is interpreted as the maximum age of deposition. some of which are elongate, suggesting volcanic or subvolcanic crystallization. Zoning includes 96-36-4; Fremouw Formation, lower member, Collinson Ridge; 46.5 m above the base of the fine concentric growth patterns, broad and ill-defined zones, as well as occasional sector zoning. formation. Analytical data were published in Elliot and Fanning (2008), and no further analyses From Elliot and Fanning (2008, fig. 7), apart from the dominant Permian grouping, there are also have been made. The published age spectrum for this sandstone is dominated by grains in the

Ma. A 16 ca. 260 Ma 240 96-35-2 ca. 260 Ma 14 11-4-10 20 12 244

10 Relative probability

15 8 248

6 252 4 10 Number 2 256 0 230 240 250260 270280 290 300 260 5 ca. 555 Ma

264

0 Weighted mean = 253.5±2.0 Ma. 0500 1000 1500 2000 2500 268 MSWD 1.5 for 20 analyses. Age (Ma) Figure 16. U-Pb zircon weighted mean 206Pb/238U age plot for upper Buckley Forma- tion volcanic sandstone sample 96-35-2. External uncertainties are included in the B weighted mean age calculation.

Ma. 238 96-36-1

242

246

250

254

258

100 μm 262

Figure 15. (A) U-Pb zircon age-probability density plot and (B) cathodoluminescence Weighted mean = 250.3±2.2 Ma. 266 (CL) image for upper Buckley Formation volcanic sandstone sample 11-4-10. MSWD 1.5 for 16 analyses.

Figure 17. U-Pb zircon weighted mean 206Pb/238U age plot for upper Buckley Forma- tion volcanic sandstone sample 96-36-1. External uncertainties are included in the range ca. 470–600 Ma, with a subsidiary cluster at ca. 630–660 Ma. The principal set, correspond- weighted mean age calculation. ing to the Ross orogen, shows probability peaks at ca. 545 Ma and ca. 580 Ma and a decreasing tail of younger grains. Insignificant clusters have Devonian and Permian ages, and there are scattered grains older than 660 Ma. 07-6-2; Fremouw Formation, middle member, Halfmoon Bluff; 46 m above the base of the of Triassic analyses around 230 Ma. It was therefore decided to reanalyze these youngest grains member (which is 170 m thick where measured). Zircons in this sandstone include euhedral crys- and a number of new grains in geochronology mode (six-scan data). The second analysis on these tals, often broken, and round to subround grains. CL images show that oscillatory zoning is ubiqui grains did not reproduce the younger dates, and so, it is concluded that the areas analyzed had lost tous, but weak non-oscillatory zoning and sector zoning are also present. Most grains are of igne- radiogenic Pb; all analyses are of igneous zircon grains. The composite detrital and geochronology ous origin. The initial detrital analyses recorded a main age peak at ca. 245 Ma with a secondary data set reaffirms the principal probability peak (Fig. 18A) at ca. 245 Ma and a secondary grouping prominent grouping at ca. 310 Ma and then scattered older ages. There was also a small grouping at ca. 310 Ma, with dispersed ages in the range 400–500 Ma (Early Devonian to Late Cambrian).

18 10 Ma. A ca. 245 Ma 230 ca. 245 Ma 16 07-6-2 07-6-2 8 B 14 234 6 Relative probabilit ca. 310 Ma 12 4 238

r 10 ca. 310 Ma 2 242 8

Numbe 0 6 220 240 260 280 300 320 340 360 y 246

4 250 2

254 0 Weighted mean = 245.9±2.9 Ma. 0500 1000 1500 2000 2500 3000 3500 MSWD = 1.2 for 14 analyses Age (Ma) 258

D 296 C 07-6-2 300

304

308

312

316

320 Weighted mean = 309.8±3.2 Ma. 500 μm MSWD = 0.28 for 8 analyses. 324

Figure 18. (A) U-Pb zircon age-probability density plot; (B) U-Pb zircon weighted mean 206Pb/238U age plot for Permian zircons; (C) U-Pb zircon weighted mean 206Pb/238U age plot for Carboniferous zircons; and (D) cathodoluminescence (CL) image for Fremouw Formation sandstone sample 07-6-2. Note that for the Permian grains only the geochronology data have been included in the plots. External uncertainties are included in the weighted mean age calculation.

A weighted mean for the 14 analyses in the dominant grouping gives a 206Pb/238U age of 245.9 ± age spectrum, combining original detrital data with selected grains reanalyzed in geochronology 2.9 Ma (MSWD = 1.2). The older group of eight Carboniferous analyses has a weighted mean age mode (Fig. 19A) has two major peaks, one at ca. 240 Ma (Early Triassic) and the other at ca. 365 Ma of 310 ± 3.2 Ma (MSWD = 0.28) (Fig. 18B). (Late Devonian). There are minor older peaks at ca. 490 Ma and ca. 570 Ma, both falling in the age 07-6-3; Fremouw Formation, upper member; Halfmoon Bluff; 75 m above the base of the range of the Ross orogen. In detail (see inset Fig. 19A), the ca. 365 Ma cluster is rather diffuse, but member. The zircon grains are principally euhedral crystals with pyramidal terminations and in nevertheless the ages correspond with the Upper Devonian Ford granitoids of Marie Byrd Land CL images exhibiting strong oscillatory zoning and scattered sector zoning. However, some, par- (Pankhurst et al., 1998; Mukasa and Dalziel, 2000; Yakymchuk et al., 2015). In terms of the dominant ticularly the older grains, are broken fragments of euhedral crystals, and a few show weak non- Early Triassic analyses, a weighted mean for 14 analyses gives a 206Pb/238U age of 242.3 ± 2.3 Ma oscillatory zoning or no zoning, the latter suggesting a metamorphic origin. The composite zircon (MSWD = 0.78) (Fig. 19B).

16 (A) 14 A ca. 240 Ma 07-6-3 14 ca. 240 Ma 12 10 12 8 Relative probabilit

6 ca. 365 Ma 10 ca. 255 Ma 4 C(C) 8 2

Number 0 6 220 240 260 280 300 320 340 360380 ca. 365 Ma

ca. 490 Ma y 4 ca. 570 Ma 2

0 200 400 600 800 1000 1200 Age (Ma) Ma. (B) 07-6-3 232 B

236

240 500 μm

244

248

252 Weighted mean = 242.3±2.3 Ma. MSWD 0.78 for 14 analyses

Figure 19. (A) U-Pb zircon age-probability density plot; (B) U-Pb zircon weighted mean 206Pb/238U age plot; and (C) cathodoluminescence (CL) image for Fremouw Formation sandstone sample 07-6-3. Note that for the Permian grains only the geochronology data have been included in the plots. External uncertainties are included in the weighted mean age calculation.

90-2-52; Falla Formation, Mount Falla; 252 m in the type section of the formation. The com- ous and Devonian grains, far fewer in number, are predominantly similar. These Devonian posite zircon age spectrum, combining original detrital data with selected grains reanalyzed to Triassic zircons are mainly of igneous origin and are most probably first-cycle detrital in geochronology mode (Fig. 20A), has a dominant age peak at ca. 205 Ma with a clustering grains. Older grains (mainly of Ross orogen age together with some Proterozoic and minor at ca. 245 Ma, a prominent cluster at ca. 515 Ma, corresponding with Ross orogen ages, and Archean grains) are commonly broken and often round to subround, which suggests recy- scattered older grains including some between ca. 900–1100 Ma. In general, the Triassic grains cling through sedimentary successions. Th/U ratios in igneous grains with oscillatory zoning are euhedral prisms, or occasionally fragments of prisms, with pyramidal terminations and range down to 0.02; low Th/U arises from U enrichment as part of the igneous process exhibiting oscillatory CL zoning. Older zircons are more varied; many are fragments; round to rather than Th depletion as would be expected in metamorphic zircon. Oscillatory zoning is subround grains are common; cores are present in many grains; and zoning may be weak and relatively common, but many grains have complex zoning or have cores, reflecting incorpo- non-oscillatory. A weighted mean for 15 analyses forming the dominant age grouping gives a ration of xenocrystic zircon or various stages of resorption and growth, and others display 206Pb/238U age of 206.6 ± 2.2 Ma (MSWD = 1.08) (Fig. 20B). weak to very weak non-oscillatory zoning or essentially no zoning, being homogeneous. The Permian and Triassic zircon grains are euhedral, sometimes broken and forming The latter are probably of metamorphic origin; however, none of those have very low Th/U fragments, often with terminations and exhibiting strong oscillatory zoning; the Carbonifer- ratios (<0.05).

A ca. 205 Ma 10 ca. 205 Ma 90-2-52 12 8

10 6 Relative probabilit

4 8 2

Number 6 0 190 200 210 220230 240250 260

4 ca. 515 Ma y C 2

51 0 100300 500700 9001100 1300 Age (Ma) Ma. 90-2-52 196 B

200

200 μm 21 204

208

212

Weighted mean = 206.6±2.2 Ma. 216 MSWD 1.08 for 15 analyses

Figure 20. (A) U-Pb zircon age-probability density plot; (B) U-Pb zircon weighted mean 206Pb/238U age plot; and (C) cathodoluminescence (CL) image for Falla Formation sandstone sample 90-2-52. Note that for the Triassic grains only the geochronology data have been included in the plots. External uncertainties are included in the weighted mean age calculation.

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