To Synglacial Rift-Related Volcanism in the Neoproterozoic (Cryogenian) Pocatello Formation, SE Idaho: New SHRIMP and CA-ID-TIMS Constraints

Pre- to synglacial rift-related volcanism in the Neoproterozoic (Cryogenian) Pocatello Formation, SE Idaho: New SHRIMP and CA-ID-TIMS constraints

Joshua A. Keeley1, Paul K. Link1, C. Mark Fanning2, and Mark D. Schmitz3 1DEPARTMENT OF GEOSCIENCES, IDAHO STATE UNIVERSITY, POCATELLO, IDAHO 83209, USA 2RESEARCH SCHOOL OF EARTH SCIENCES, AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, ACT 0200, AUSTRALIA 3DEPARTMENT OF GEOSCIENCES, BOISE STATE UNIVERSITY, BOISE, IDAHO 83725, USA

ABSTRACT

Volcanic and diamictite-bearing strata of the Neoproterozoic Pocatello Formation record middle Cryogenian glaciation and alkaline to subalkaline within-plate magmatism during Rodinia rifting. New mapping along the Oxford Ridge segment of the southern Bannock Range in SE Idaho has resolved stratigraphic relationships between the Scout Mountain and underlying Bannock volcanic members of the Pocatello For- mation. Bannock Volcanic Member metabasalt has an upper gradational contact with over 250 m of Scout Mountain Member that includes extrabasinal and volcaniclastic diamictite, in turn overlain by a volcaniclastic unit (the Oxford Mountain tuffi te). Previous attempts to date the tuffi te include three sets of analyses of the original sample (06PL00) and one resample (04JK09) that yielded sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon concordia ages of ca. 709, 702, and 686 Ma and one isotope dilution–thermal ionization mass spectrometry (ID-TIMS) age of 687.4 ± 1.3 Ma. Several new samples of plagioclase-phyric volcanic sandstone and the tuffi te, dated via high-precision (~0.1%) chemical abrasion (CA) ID-TIMS, have multimodal zircon populations with single-crystal ages ranging from as old as 709 Ma to as young as 685 Ma, confi rming the epiclastic nature of the deposit. The majority of grains in one sample yielded a 206Pb/238U weighted mean age of 685.5 ± 0.4 Ma, which provides a robust maximum age of deposition. From the type section of the lower Scout Mountain Member, Pocatello Formation at Portneuf Narrows, we report four new SHRIMP maximum depositional ages between 705 ± 5 Ma and 682 ± 6 Ma. A 691 ± 4 Ma (SHRIMP) volcanic clast from the cobble conglomerate member provides a maximum depositional age, and provides a geochronologic correlation with the Oxford Mountain tuffi te. The data are interpreted to support a lithostratigraphic correlation between the diamictite on Oxford Mountain and the lower diamictite at Portneuf Narrows and to show that the upper glaciogenic diamictite in the Portneuf Narrows section is younger than 685 Ma. This 685 Ma age from rift-related rocks that underlie the Brigham Group passive-margin succession provides a maximum age for onset of rift subsidence. ε − Lu-Hf analyses of 685–730 Ma igneous zircons yield enriched initial Hf values in the range +2 to 17, indicating that they crystallized from magma that incorporated depleted Paleoproterozoic to Archean crustal components of the underlying Farmington Canyon Complex and Wyoming craton.

LITHOSPHERE; v. 5; no. 1; p. 128–150; GSA Data Repository Item 2013034 | Published online 14 November 2012 doi: 10.1130/L226.1

INTRODUCTION zoic glacial phases are the Middle Cryogenian siderably younger bound is the U-Pb sensitive “Sturtian,” the Late Cryogenian “Marinoan,” high-resolution ion microprobe (SHRIMP) age Volcanic, siliciclastic, and minor carbonate and the Ediacaran “Gaskiers” (Hoffman and Li, of 660 ± 6 Ma from a tuff in a dropstone-bearing strata of the Neoproterozoic Pocatello Formation 2009). The Sturtian, Middle Cryogenian gla- sandstone within the Sturtian glacial succes- in southeast Idaho record rifting of the supercon- ciations derive their name from exposures of sion near Copley in South Australia (Fanning tinent Rodinia during widespread regional gla- diamictite in the “gorge of the River Sturt,” south and Link, 2006, 2008). In South Australia, the ciation (Fig. 1) (Ludlum, 1942; Crittenden et al., of Adelaide, South Australia (Howchin, 1901; postglacial Tapley Hill Formation is composed 1983; Link, 1983; Link et al., 1994; Smith et al., Preiss, 2000; Preiss et al., 2011). This name has of thick transgressive shale that yielded a Re-Os 1994; Lorentz et al., 2004; Corsetti et al., 2007). been applied worldwide, and current age con- age of 643 ± 2.4 Ma from the basal Tindelpina These strata bear on paleogeographic, Snow- straints on the glaciation suggest it was diachro- Shale Member (Kendall et al., 2006). ball Earth, and Rodinia rift models that seek to nous between ca. 716 Ma and ca. 660 Ma. The The chronostratigraphic term “Marinoan” explain drastic tectonic and climatic ﬂ uctuations older bound is the chemical abrasion–isotope comes from the Marino Arkose Member of the near the onset of complex life (Hoffman, 1991; dilution–thermal ionization mass spectrometry Wilmington Formation, which underlies the Hoffman et al., 1998; Li et al., 2008; Hoffman (CA-ID-TIMS) U-Pb zircon age of 716.47 ± glaciogenic Elatina Formation (Williams et al., and Li, 2009). 0.24 Ma from diamictites of the Mount Harper 2008). The Elatina glaciation of Marinoan age in Neoproterozoic glacial successions are rec- Group in the Yukon Territory representing the Australia has been interpreted to have occurred ognized worldwide, but the number, timing, and Rapitan glaciation (Macdonald et al., 2010). The at 635 Ma on the basis of chemostratigraphic duration of glacial episodes remain controver- Ghubrah Formation in Oman is slightly younger, correlation with dated glacials in South China sial. Three generally agreed upon Neoprotero- at 711.5 ± 0.3 Ma (Bowring et al., 2007). A con- and Namibia (Hoffman and Schrag, 2002). The

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W112°30’ W112°00’ W111°30’ and ice-distal debris-fl ow tillite. Rare striated quartzite clasts are present. AB Regional equivalents of the Pocatello For- N 43°00’ N 43°00’ mation include the informal formation of China SNAKE RIVERPeak PLAIN Perry Canyon in the northern Wasatch Range, the Mineral Fork Formation in the central to Pocatello NPR southern Wasatch Range, and the Otts Canyon OR MT and overlying Dutch Peak Formations in the Portneuf Edwardsburg Bannock R Sheeprock Mountains, Utah (Fig. 3; Crittenden Fm. Narrows samples BRR et al., 1971, 1983; Link et al., 1993; Link and Pocatello Fm. - 64JK09 Christie-Blick, 2011). Glacial dropstones that - 15PL08 ID pierce underlying laminations are common in - 63JK09 the Mineral Fork Formation in northern Utah NV UT - 62JK09 (Christie-Blick, 1983). Dropstones have also ID - Idaho O SPR MT - Montana Oxford Peak been found in the upper Otts Canyon Forma- ange NV - Nevada Area of B tion immediately underlying the ~1.5-km-thick OR - Oregon N 42°00’ IDAHO stratifi ed and massive diamictite of the Dutch UT - Utah UTAH CV PT Neoproterozoic exposures Peak Formation (Christie-Blick, 1983; Link and Idaho Neoproterozoic exposures Christie-Blick, 2011). The formation of Perry WF L Abbreviations Wasatch Range Canyon displays two horizons of thick diamic- BRR - Bear River Range tite separated by several hundred meters of gla- CV - Cache Valley ciomarine strata. The lower of these diamictites L - Logan Perry LM - Little Mountain Canyon has been interpreted to bear dropstones. The NPR - Northern Portneuf Range Great two diamictite units here have been interpreted O - Oxford Salt to represent two stades in one glaciation (Crit- PT - Paris Thrust Lake tenden et al., 1983). Overlying the Pocatello SLC - Salt Lake City LM SPR - Southern Portneuf Range WT Formation, in the Brigham Group, incised val- WF - Wasatch Fault leys between the Caddy Canyon Quartzite and WT - Willard Thrust Fremont Is. the Inkom Formation (Fig. 3) are interpreted Thrust fault, dotted where N 41°00’ as indirect evidence for eustatic changes due to concealed late Cryogenian glaciations (Link and Christie- Active normal fault, dashed where concealed Blick, 2011). Town/City SLC The timing and duration of rifting of the Neo- Lower Cambrian exposure proterozoic to Paleozoic western North American N passive margin also remain poorly constrained Neoproterozoic exposure Cottonwood area (Moores, 1991; Karlstrom et al., 1999). How- 0 10 20 mi ever, recent age constraints have signifi cantly 0 25 50 improved our understanding. Radiometric ages km along the western Laurentian Cordillera begin with the ca. 780 Ma Gunbarrel mafi c dike swarm, Figure 1. Regional maps. (A) Map showing the location of Neoproterozoic diamictite-bearing successions along the Cordillera, modifi ed from Fanning and Link (2004). (B) Location map showing interpreted to suggest crustal extension induced the study areas of the Portneuf Narrows and Oxford Mountain in the Bannock Range, SE Idaho. by a mantle plume beneath Rodinia (Harlan et al., Also shown are locations of correlative Neoproterozoic sections: formation of Perry Canyon in Perry 2003). Mechanical rifting along the Cordillera is Canyon and the Mineral Fork Formation in the Cottonwood area east of Salt Lake City. then recorded by thick volcanic- and diamictite- bearing successions from which volcanic rocks provide ages of 716–711 Ma (Macdonald et al., Marinoan, late Cryogenian glaciation is brack- ada (Bowring et al., 2003). It was suggested by 2010), ca. 688 Ma (Ferri et al., 1999), and ca. eted between 655 Ma (Zhang et al., 2008) and Hoffman and Li (2009) that the time constraints 685 Ma (Lund et al., 2003). Thin volcanic rocks 632 Ma (Condon et al., 2005), with equiva- for the Gaskiers glaciation are too brief for a in the overlying Brigham Group and correla- lent 635 Ma U-Pb zircon ages recorded from global glaciation. tives yield ages of 580–570 Ma (Fig. 3; Christie- Namibia (Hoffmann et al., 2004), Oman (Bow- The diamictites that crop out throughout Blick and Levy, 1989; Colpron et al., 2002). It is ring et al., 2007), and South China (Condon et southeast Idaho and northern Utah (Fig. 1) have unknown whether the volcanics represent a pro- al., 2005). long been interpreted as having been deposited tracted rift history or separate rift episodes. Whereas the Sturtian and Marinoan glacia- in a glaciomarine setting (Calkins and Butler, The mafi c volcanic rocks, diamictite, and tions have been considered global glaciations, 1943; Crittenden et al., 1983; Link, 1983). Lines abundant feldspathic sandstones that occur in or “snowball Earth” periods (Kirschvink, 1992), of evidence for regional glaciation are seen in the lower two members of the Pocatello Forma- the Gaskiers glaciation is generally limited to the type area of the Scout Mountain Member, tion (and correlative strata in Utah) are inter- high latitudes (>45°) (Hoffman and Li, 2009) Pocatello Formation (Fig. 2), where massive preted to record the initial stages of rift basin and is directly dated by the Gaskiers Formation and stratifi ed diamictite facies associations development in response to fi rst-order conti- on the Avalon Peninsula of Newfoundland, Can- have been interpreted as ice-proximal lodgment nental rifting during regional glaciation (Link,

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A General Neoproterozoic B Pocatello, Idaho section, Pocatello area 800 m Portneuf Narrows shale, siltstone area limestone 4500 m pillow basalt C diamictite conglomerate 4000 sandstone < 667 ± 5 Ma (tuff -145PL02) mafic volcanics 580 Ma volcaniclastics 600 (Rb-Sr) Fe iron-rich layer* Figure 2. Generalized stratigraphic sec- < 717 ± 4 Ma sequence tions. (A) Entire Neoproterozoic section 3000 (clast - 148PL02) boundary in Pocatello area. (B) Pocatello Formation < 701 ± 4 Ma stratigraphy at Portneuf Narrows. *Black (clast - 24PL05) C Oxford Mountain sandstone of Link (1987). Sensitive high- resolution ion microprobe (SHRIMP) Brigham Group maximum age constrains adjacent to Fault 2000 column are U-Pb zircon concordia ages 400 < 685.52 ± 0.40 Ma (Fig. DR2). (C) Revised stratigraphy of < 689 ±4 Ma Oxford Mtn. Pocatello Formation at Oxford Mountain. Blackrock (clast - 64JK09) tuffite Canyon Chemical abrasion–isotope dilution– Limestone extrabasinal thermal ionization mass spectrometry Fe diamictite (CA-ID-TIMS) age on Oxford Mountain is 1000 a U-Pb zircon weighted mean. Modiﬁ ed < 682 ±6 Ma transitional from Fanning and Link (2004) based on (15PL08) unit new mapping. Sturtian Scout Mountain Scout Mountain Member, Pocatello Fm. Scout Mountain Member, Member 200 Pocatello Fm. < 704 ±5 Ma Bannock (63JK09) Volcanic Member < 705 ±5 Ma (62JK09) Fault Bannock Volcanic Member

Allochthon (UT-ID) Parautochthon (UT-ID) Coal Ck. Inlier, Mackenzie N. British SE Canadian Central Sheeprock Central Uinta Mtns. Grand Cn. Death Valley Age (Ma) Yukon (1,2) Mnts. (3,4) Columbia (5) Cordillera (6,7) Idaho (8,9) SE Idaho N. Wasatch Mtns. Wasatch (10) (10,11,12) (11) Backbone Cls Cls Cls Cls Bouvette Fm. Ranges Fm. Gog Group Gog Group Tintic Qzt. Tonto Group Wood Cn. Fm. C Umbrella Tintic Qzt. Ingta Fm. Butte Fm. Camelback Geertsen Prospect Mtn. Qtz. Cn. Qtz. Mtn. Qtz. Stirling Qzt. Risky Fm. v 570 ‘upper’ grp Blueflower Fm. BHF (13)580 v Misinchinka Hamill Missouri Rdg. Fm Johnnie Fm. Gametrail Fm. Group Mutual Fm. ‘Mutual’ Fm.

Ediacaran 600 Sheepbed Fm. ? Moores Lk. Fm. Hay Ck. Grp. dIce Brook Keele Vreeland Inkom Fm. 635 d Fm. Fm. Fm. r608 Sixtymile Fm. Noonday Dol. Group Horse- Goldman Cut Fm. Caddy Canyon Qzt. ? Thief Ck. Monk Moores Stn. Fm. Papoose Ck. Fm./Kelley Cn. Fm. ? d Rapitan d Rapitan not exposed Grp. Fm. v 664 DGD v667 Poc- Perry Sheep- Mid-Upper Windermere SG Group Group d d v 688 Gataga Irene Volc. d d d 671 d Mineral Neoproterozoic 700 Edwards- v686 atello v rock Fork Fm. v Kingston v 716.5 qzt. Toby Fm. v 685 burg Fm. v Cn. d Peak Fm. v 717.4 d d Fm. d Group MHVC v 700-<717 d708 Fm.* CLG Coates Lake not exposed ? ? LMHG Group not exposed thrust Big v L. Kingston

Cryogenian 742 Upper Cottonwood Uinta Mtn. Pk. Fm. Fm. Chuar Grp. Fifteenmile Little Dal Grp. Group Beck Spr. Dol. MMS 800 v 811.5 Group Little Willow < 766 Nankoweap upper Crystal Late Cryogenian 580 radiometric age (Ma) Fm. Fm. diamictite-bearing interval v volcanic bed Spring Fm.† Middle Cryogenian d glacial diamictite Paleoproterozoic to diamictite-bearing interval r Re-Os date (7) Archean basement Uinta Mountain Group unconformity and correlatives

Figure 3. Composite correlation chart of Neoproterozoic and Lower Cambrian rocks of the Cordillera. The allochthonous and parautochthonous rocks in Utah-Idaho refer to rocks in the hanging wall and footwall, respectively, of the Paris-Willard thrust sheet in, respectively. BHF—Brown’s Hole Forma- tion; CLG—Coates Lake Group; Cls—Cambrian limestone and shale undifferentiated; DGD—Daugherty Gulch diamictite; LMHG—Lower Mount Harper Group; MHVC—Mount Harper Volcanic Complex; MMS—Mackenzie Mountains Supergroup; qtzt—quartzite undifferentiated. (1) Macdonald et al. (2010); (2) Macdonald et al. (2011); (3) Ross et al. (1995); (4) Aitken (1991); (5) Ferri et. al (1999); (6) Colpron et al. (2002); (7) Kendal et al. (2004); (8) Lund et al. (2003); (9) Lund et al. (2010); (10) Dehler et al. (2010); (11) Mahon (2012); (12) Dehler et al. (2012); (13) Christie-Blick and Levy, (1989). *After Balgord (2011); †Proposed as Horse Thief Springs Formation by Mahon (2012). Adapted from Link and Christie-Blick (2011).

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1987; Link et al., 1994; Link and Christie-Blick, Mountain (Fig. 1). The lower contact is not volcanic clasts, including glacially striated 2011). The Paris-Willard thrust sheet of south- exposed (Fig. 2A). The correlative forma- quartzite clasts (Link, 1982). Overlying the upper east Idaho contains the 1500-m-think Pocatello tion of Perry Canyon in northern Utah (Crit- diamictite is a bedded, locally cyclic, pink dolo- Formation overlain by a 4000-m-thick package tenden et al., 1983; Balgord et al., 2011) rests mite and reworked dolomite chip breccia with of westward-thickening quartzose strata, the on Paleoproterozoic Facer Formation above interstratifi ed sandstone and argillite (Meyer et Brigham Group (Crittenden et al., 1971; Link the Willard thrust fault, in the footwall of the al., 2012). This “cap carbonate” (Dehler et al., et al., 1987). Overlying the Brigham Group, active Wasatch fault. Exposed below the Wil- 2011) exhibits a negative δ13C excursion (−6 to there are ~2000 m of dominantly carbonate and lard thrust fault, there is the Paleoproterozoic −3 ‰, Lorentz et al., 2004), similar to the Neo- subordinate siliciclastic Cambrian strata. This Farmington Canyon Complex, which is pro- proterozoic Noonday Dolomite of the Pahrump package thins to the east and gives way to Lower jected northward and thought to underlie the Group in Death Valley (Petterson et al., 2011). Paleozoic to Mesozoic rocks. This sedimentary Bannock and Pocatello Ranges in SE Idaho A porcellanous 10-cm-thick reworked package thickens west of and subparallel to (Foster et al., 2006). green tuff (667 ± 5 Ma, SHRIMP concordia ~112°W longitude, known as the Cordilleran Part of the type section of the Pocatello For- age; Fanning and Link, 2004) overlies the pink hingeline, which is interpreted by many work- mation is located in an overturned limb of an dolomite–bearing unit and is overlain by lime- ers to be the W-facing, trailing edge of Lauren- E-vergent fold, south of China Peak and north stone and argillite of the upper member of the tia after Rodinia rifting. Abundant W-directed of the Portneuf Narrows near Pocatello (Lud- Pocatello Formation (Link, 1983, 1987; Lorentz paleocurrents (Link et al., 1987; Keeley et al., lum, 1942; Trimble, 1976; Link, 1983). Here, et al., 2004). Carbonates from this limestone 2009) and numerous detrital zircon studies link- the ~1500 m Pocatello Formation is divided into interval at the top of the Scout Mountain Mem- ing miogeoclinal sedimentary provenance to the lower basaltic Bannock Volcanic Member, ber record a transition to a positive δ13C excur- basement provinces east of the hingeline (Rain- the middle Scout Mountain Member, containing sion (−5 to +5 ‰) and have been correlated with bird et al., 1992; Stewart et al., 2001; Schoen- diamictite, siliciclastic rocks, and minor carbon- carbonates of the Johnnie Formation in Death born et al., 2012; Rainbird et al., 2012) support ate, and an informal upper member of phyllitic Valley (Corsetti et al., 2007). this interpretation. West of the hingeline, near shale (Crittenden et al., 1971, 1983; Link, 1983) the Idaho-Oregon border, the 0.706 87Sr/86Sr line (Fig. 2B). Pocatello Formation on Oxford Ridge, is interpreted to mark the paleogeographic bor- Southern Bannock Range der of western Laurentia. Bannock Volcanic Member The purpose of this study is to better resolve Harper and Link (1986) measured trace and The east face of Oxford Ridge exposes the age inconsistencies from the Pocatello Forma- rare earth elements of basalt samples from the deepest structural and lowest stratigraphic lev- tion so as to better place it in a global frame- Bannock Volcanic Member and determined a els in the Paris-Willard thrust sheet in south- work for the Neoproterozoic. The work pre- tholeiitic-alkaline to subalkaline composition, east Idaho (Figs. 1 and 4), exposed in a modern sented in this paper fi rst describes new results comparable to other within-plate basalts. Felsic Basin and Range extensional horst. Rocks of of fi eld mapping followed by a detrital zir- porphyritic volcanic clasts within Scout Moun- the Pocatello Formation on Oxford Ridge are con provenance analysis carried out by laser tain Member diamictite have been interpreted to metamorphosed to lower-middle greenschist ablation–multicollector–inductively coupled originate from felsic lava fl ows in the Bannock facies, as evidenced by pervasive chlorite, and plasma–mass spectrometry (LA-MC-ICP-MS). Volcanic Member (Link, 1983). However, the abundant albite and epidote in the mafi c rocks. The latter was also used as a screening process lack of rhyolite in the Bannock Volcanic Mem- Outcrops of the Pocatello Formation contain at to locate Neoproterozoic zircons. Maximum ber led Harper and Link (1986) to conclude the base at least 200 m of pillow basalt (Fig. 5A) age constraints were then obtained by methods volcanism was not bimodal. Keeley (2011) and hyaloclastite (Fig. 5B) of the Bannock Vol- of increasing precision, fi rst SHRIMP, followed reported major- and trace-element geochemistry canic Member. This is overlain by up to 250 m by CA-ID-TIMS. The zircons from the former that suggested a within-plate continental rift set- of Scout Mountain Member (Figs. 5C, 5D, and analyses were then analyzed for lutetium-haf- ting for both Bannock Volcanic Member basalts 5E), with four stratigraphic units (Fig. 2C): a nium isotopes in an effort to begin develop- and the trachytic to rhyolitic suite of felsic vol- lower transitional unit of mainly diamictite with ing a Neoproterozoic Hf database and to make canic clasts from the lower Scout Mountain interbedded mafi c volcanic rock (greenstone); some inferences about the petrogenesis of the Member. Assuming that the analyzed volcanic a middle extrabasinal diamictite containing zircons. Finally, we used the new ages to place clasts (Keeley, 2011) are representative of the diverse quartzite, plutonic, and metamorphic the Pocatello Formation in context with Neopro- full compositional range, the clast chemistries clasts (Fig. 5E); a volcaniclastic interval (the terozoic glaciations and Rodinia rifting. indicate a span of intermediate to felsic volca- Oxford Mountain tuffi te) containing tuffaceous nism interpreted to be associated with the basal- sandstone (Fig. 5D); and an upper sandstone. GEOLOGIC BACKGROUND tic Bannock Volcanic Member. Since the base is not exposed, it is possible that the Bannock Volcanic Member is signifi cantly Geologic Setting and Regional Scout Mountain Member thicker (Fig. 6). Stratigraphy of the Pocatello Formation In more detail, the Scout Mountain Member The complex structure shown in Figure 6 at Portneuf Narrows (Fig. 2B) includes a lower is the result of two or more episodes of normal The Pocatello Formation is exposed in diamictite interval, which contains locally strati- faulting involving the ca. 10–4 Ma Bannock the Bannock and Pocatello Ranges (Ludlum, fi ed, green to brown, matrix-supported diamic- detachment system (Janecke and Evans, 1999; 1942; Link et al., 1993; Link and Christie- tite with mafi c volcanic, argillite, and rare fel- Long et al., 2006) and modern Basin and Range Blick, 2011). The outcrops in Idaho are sic porphyritic volcanic clasts. Above this lies extension. Several WSW-trending and gently aligned N-S and extend ~120 km from east of locally ferruginous sandstone and conglomerate W-dipping low-angle faults are involved in the Pocatello in the north, to the southern end of and an upper massive diamictite unit that con- large-offset and regionally extensive Bannock the Bannock Range, 25 km south of Oxford tains quartzose, gneissic, granitic, and silicic detachment system (Carney and Janecke, 2005).

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W 112°15’ W 111°45’

Bannock Downey Portneuf Zpb Marsh Valley A N B Q 91 N 42°27’30” W 112°5’ Dayton- Wakely I-15 Valley Fault OCu Peak Oxford CZu fault Range RRP CZu Zps Range Zpb A’ Oxford Ridge Zps Zpb DCF CZu Zps N 42°17’30” Swan Lake WF CZu Q OP A Goose- N Q Zps berry Ck. fault Oxford CZu Q B 91 Zpb Oxford Basin Malad Range EDOF Zpb fault CZu Q 0 4,000’ DCHGDCF Zpb Malad ECF Zpb B’ 0 1000m DOF ? City 14

WF W 112°2’30” Preston B CZu N 42°15’ OCu C’ N 42°15’ 11-12 Q Zps I-15 Zps Weston Zpb OCu Idaho C Q

DOF Utah Ti N 42°00’ Q Malad Valley 91 9-10 14 WCF New Canyon 4 fault 1,2 3 D’ Zps ORA Bear River Range D Clifton Canyon fault N 42°

ECF CZu W 112°5’ 8 12’ West Hills Zps 30” Clifton Q Q Quaternary fault

WCF deposits Ti Zpb Tertiary intrusion Q Q T Tertiary strata Zps CZu Cache Valley OCu Cambrian - Ordovician T CZu Neoprot. - Cambrian

Pocatello Fm. Q Mt. Mb. Scout ECF Q T Logan T WF T Zps Q Bear River 91- N 41°44’ T Valley 89 Zpb Bannock Vol. Mb. W 112°15’ W 111°45’ ORA - Oxford Ridge Anticline Zps N 42° 0 10 10’ 5 town CZu mi Key - - 1 LA-MC- 9 CA-ID-TIMS DOF 0 5 10 91 road/highway km ICP-MS Quaternary-Tertiary 118° 109° 1 - 67JK09 9 - 04JK09 deposits CZu 47° 47° 2 - 68JK09 10 - 23JK10 WA Tertiary Salt Lake Fm. 4 - 03JK09 11 - 15JK10 Q MT T T 5 - 73JK09 12 - 13JK10 pre-Tertiary bedrock 13 - 16JK10 Area of 6 - 74JK09 CZu ID Figure 4A Neoproterozoic Pocatello 7 - 75JK09 14 - 06PL00 (ID-TIMS) Formation Q 8 - 24JK09 3 - 34PL05 5 active normal fault OR Abbreviations on map and inset: Zps low angle normal fault DCF = Deep Creek fault T N 42° WY older inactive faults DCHG = Deep Creek Half Graben ORA 07’ DOF = Dayton-Oxford fault 30” SRP attitude of bedding EDOF = East Dayton Oxford fault Q ECF = East Cache fault stratigraphic contact Q OP = Oxford Peak rollover anticline folds RRP = Red Rock Pass NV 6-7 SRP = Snake River Plain T WF UT anticline 5 Mile 40° WCF = West Cache fault T 40° attitude of foliation Canyon 118° 109° WF = Wasatch fault

Figure 4. (A) Simplifi ed geologic map of the Cache Valley, Malad Valley, and Marsh Valley areas. New mapping of Oxford Ridge is bordered in black. Modifi ed from Janecke et al. (2003). Inset is modifi ed from Janecke and Evans (1999). (B) New geologic map of Oxford Ridge showing locations of geochronologic samples. CA-ID-TIMS—chemical abrasion–isotope dilution–thermal ionization mass spectrometry; LA-ICP-MS—laser ablation–inductively coupled plasma–mass spectrometry. Cross sections to section lines A–D are shown in Figure 6.

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D E

B C

Figure 5. Lithologies of the Pocatello Formation on Oxford Mountain in ascending order. (A) Pillow basalt, Bannock Volcanic Member. Dashed white line marks contact between pillow basalt (below) and lobate ﬂ ow (above). (B) Hyaloclastite and volcaniclastic rocks of the Bannock Volcanic Member. (C) Scoured unconformity (arrow) between extrabasinal conglomerate at base of diamictite on the Bannock Volcanic Member. (D) Oxford Mountain graded volcanic sandstone (sample 68JK09). (E) Stratiﬁ ed extrabasinal diamictite of the Scout Mountain Member (scale on left is 6.5 cm).

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A A’ WSW CF ENE CZu 8000 CZu 2438 Zps

CZu Elevation (m) ? Zps CF 7000 Zpb CCF 2134 CCF Zps Zpb Zpb Zpb 6000 Zpb ? DOF 1829 Zpb ? OBF ? Zpb ? T Q

5000 1524 B B’ WSW CF ENE Ti? CZu D 9000 2743 CZu NCF Zps Zpso OCu OCu CF 8000 2438 Zps CCF Zps Zpb Ti? CCF Zpso Zpb Q Quaternary deposits 7000 Zpb 2134 ? ? Ti Tertiary intrusion DOF Q 6000 OBF- 1829 T Tertiary strata ? Zpb

T Elevation (m) OCu Cambrian to Ordovician C NCF CZu CF Ti? OCu C Ti C’ CZu Neoproterozoic to Cambrian Elevation (ft) Elevation SW OCu Zps

NE Mt. Mb Scout Pocatello Fm 8000 Zps Zpso 2438 Oxford tuffite (Zpso) CZu Zpb CCF Zps Diamictite undiff. CCF Zpb 7000 2134 ? Q Zpb Bannock Volcanic Mb. ? ? DOF OBF Basement? 6000 Zpb 1829 ? Bedding orientation T Foliation orientation 5000 1524 D Location of sections D D’ in Figure 7 WSW - ORA B ENE CCF Clifton Canyon fault 8000 2438 CF - Clifton fault Zps Zps Zpso NCF - New Canyon fault 7000 2134 Ti Ti DOF - Dayton-Oxford fault CCF Zpb CCF OBF - Oxford Basin fault 6000 1829 Zpb DOF ORA - Oxford Ridge anticline OBF ? ? Q T No vertical exaggeration 5000 1524

Figure 6. Cross sections A–D along Oxford Ridge from north to south. Locations are shown in Figure 4. (A) Near the northernmost extent of the Clifton fault with the Camelback Mountain Quartzite (CZu) in its hanging wall and the underlying and folded Clifton Canyon fault. (B) Near the northernmost extent of the Oxford Ridge anticline and subsequently folded and back-tilted New Canyon, Clifton, and Clifton Canyon low-angle normal faults. (C) Back-tilted, east-dipping Clifton fault cut by the structurally higher New Canyon fault. (D) WSW-dipping and back-tilted Clifton Canyon fault intruded by a sheet-like Tertiary(?) intrusion.

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They are, from bottom to top, the Clifton Can- Creek fault (Fig. 4). More detailed descriptions soft-sediment deformation. Siltstone and sand- yon, Clifton, and New Canyon faults (Figs. 4 of the regional geology and structural history of stone injections, and basalt inclusions in soft and 6). A sheet-like Tertiary mafi c sill intrudes Oxford Ridge were presented by Carney and sediment are evidence of peperite deposition the Clifton Canyon fault, and its sheared lower Janecke (2005), Keeley and Link (2011), and and/or hypabyssal intrusion of basalt. contact suggests it was emplaced during fault- Keeley (2011). Just below the Clifton Canyon fault north of ing. Carney and Janecke (2005) interpreted Davis Basin in section 18, T14S, R38E, extra- that, during WSW-directed slip on the Bannock Bannock Volcanic Member on Oxford Ridge basinal conglomerate and diamictite of the detachment system, removal of lithostatic over- On the east face of Oxford Ridge, new Scout Mountain Member lie above a locally burden caused isostatic folding, leading to the mapping found two roughly 30-m-thick pil- scoured unconformity on volcaniclastic rocks present Oxford Ridge anticline (ORA; Figs. 4 low basalt fl ows (Fig. 5A), separated by mas- of the Bannock Volcanic Member (agglom- and 6). High- to moderate-angle faults associ- sive basalt fl ows and hyaloclastite (Fig. 5B) in erate facies of Link, 1982) (Fig. 5C). Further ated with Basin and Range extension include the Bannock Volcanic Member. No porphyritic south, along Clifton Road (Fig. 7A), diamic- the E-dipping Oxford Basin fault and the range- felsic volcanic fl ows (Link, 1983) or intru- tite is interbedded with hyaloclastite and basalt bounding Dayton-Oxford fault (Figs. 4 and 6). sions for direct radiometric dating were found. fl ows in the transitional unit at the base of the An intervening extensional episode resulted in Locally, sedimentary textures on the margins Scout Mountain Member (Keeley and Link, E-W-striking cross faults such as the Gooseberry of pillow basalt fl ows and hyaloclastite display 2011).

S N 4.27 km 3.35 km 2.3 km

A: Clifton B: North of C: Clifton Quad D: Oxford Basin Basin Road Clifton Basin North Cliffs Clifton 400 m Clifton Canyon Clifton Fault Canyon Fault 60 m ? 40m Fault 20m 67JK09, 4JK09, 23JK10, 34PL05 68JK09, 06PL00 15JK10 16JK10 50 13JK10 300 30 Oxford 10 Mtn. tuffite Shear 40 20 zone SMM 200 3JK09 BVM

no section 30 10 Tertiary(?) intrusion

100 Sandstone

20 Fault? Felsic volcaniclastic Fault? rocks Fault 24JK09 m s fs cs g p c b SMM pebble - boulder BVM conglomerate Diamictite: extrabasinal/ 10 m s fs cs g p c b intrabasinal

SMM Hyaloclastite/agglomerate BVM Pillow basalt

m s fs cs g p c b Massive metabasalt

Figure 7. Stratigraphic sections of the Pocatello Formation on Oxford Ridge. (A) Section exposed at Clifton Basin Road composed of the lower transitional unit with intercalated metabasalt and diamictite. (B) Section north of Clifton Basin showing diamictite and Oxford Mountain tufﬁ te. (C) Section in the northern portion of the Clifton Quadrangle. (D) Section at the southern end of the cliffs above Oxford Basin. BVM—Bannock Volcanic Member; SMM—Scout Mountain Member.

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Scout Mountain Member on Oxford Ridge (up to 1 m thick) with soft-sediment deforma- Several workers (Hoffman and Li, 2009; tion (Fig. 8C, 13JK10); (4) thin- to medium- Macdonald et al., 2010; Petterson et al., 2011) New mappable, intertonguing stratigraphic bedded, graded volcanic sandstone (68JK09) have suggested that the volcaniclastic unit is units have been identifi ed in the Scout Moun- intercalated with nonstratifi ed volcanic diamic- not in contact with glacial strata and have ques- tain Member on Oxford Mountain (Fig. 7). tite (67JK09) (Fig. 5D); and (5) crudely strati- tioned the reliability of the age constraints. The The lower “transitional unit” of Link (1982) fi ed volcanic diamictite bearing trachyandesite present paper presents the results of two seasons consists of 70 m of green-gray, massive to and rhyolite clasts (Fig. 8D), including angu- of geologic mapping aimed at resolving the age crudely-bedded quartzose diamictite interca- lar to subrounded, white to dark-gray apha- and stratigraphic relations between the Bannock lated with metabasalt fl ows and hyaloclastite nitic clasts. The individual tuffi te beds contain Volcanic and Scout Mountain Members of the units up to 10 m thick, with reworked basalt angular to round felsic volcanic clasts, some Pocatello Formation both on Oxford Ridge and pillows (Fig. 7A). Vesicular and nonvesicular of which have undergone negligible transport at the type section south of Portneuf Narrows basaltic clasts up to cobble size make up 80% from the volcanic vent. The stratifi ed volca- near Pocatello, Idaho. We address the age of the of the clasts in the diamictite (Link, 1982). The niclastic diamictite is interbedded with mul- Neoproterozoic Pocatello Formation with new remaining clasts are dominantly argillite with tiple beds having thin to thick, fi ne-grained SHRIMP and CA-ID-TIMS U-Pb zircon ages. quartzite and sparse basement lithologies. Fine green wavy laminations (04JK09, 23JK10, and grained, light-colored, aphanitic clasts may 16JK10, Fig. 8E). Overlying the tuffi te, there Analytical Methods represent a subordinate felsic volcanic or vol- is normally graded breccia and conglomerate, caniclastic component. The diversity of clasts containing argillite and sandstone clasts, and Samples from the Pocatello Formation both from the Scout Mountain Member diamictite on green feldspathic sandstone (Fig. 8F). Oxford at Oxford Mountain and at the type area south Oxford Mountain suggests a mixture of intraba- Ridge does not expose Neoproterozoic carbon- of Portneuf Narrows, were selected for U-Pb sinal and extrabasinal components. Locally, a ate or dark phyllitic strata as seen in the upper geochronology to (1) determine detrital zircon gray quartzose granule conglomerate is discon- Scout Mountain Member at Portneuf Narrows. provenance ages (data presented in Keeley and tinuously interbedded in this unit. Previous to the mapping in this study, the Link, 2011; GSA Data Repository Table DR11) A 150–190-m-thick, brown-green to purple volcaniclastic unit (now the Oxford Mountain and (2) place constraints on the depositional and locally sandy, massive to stratifi ed, inferred tuffi te) on Oxford Ridge (Figs. 2 and 4) was ages of the succession. Zircon crystals were glaciogenic diamictite, with a variety of extraba- reported by Fanning and Link (2004) to have a extracted from samples by traditional methods sinal clasts up to boulder size, overlies the tran- SHRIMP weighted mean 206Pb/238U zircon age of crushing and grinding at Idaho State Univer- sitional unit. Clast lithologies in the diamictite of 708 ± 5 Ma from original sample 06PL00. sity, followed by separation with a Wilfl ey table, include porphyritic felsic volcanic rocks, basalt, In total, 18 zircon grains were used in the cal- heavy liquids, and a Frantz magnetic separator chloritic argillite, plus granite and gneiss. This culation (n = 18) with a mean square weighted at Boise State University. Samples were pro- heterolithologic unit makes up the dominant deviation (MSWD) of 1.7. Condon and Bow- cessed such that all zircons were retained in the lithology on Oxford Ridge and is thickest (200 ring (2011) suggested that given the sample size fi nal heavy mineral fraction. m) at Fivemile Canyon. Up to 25 m of medium- (average relative to SHRIMP standards) and the Initial detrital zircon provenance analysis to thick-bedded trough cross-bedded sandstones relatively high MSWD of 1.7, the scatter in the and screening were done at the Arizona Laser- are locally present within this diamictite. Simi- data set was possibly due to real age variation in Chron Center following Gehrels et al. (2006, lar sandstone up to 100 m thick forms an upper the sample rather than analytical error. Fanning 2008). Here, the grain separates were incorpo- unit that lies above the diamictite at the head of and Link (2004) noted that there were both clear rated into a 1″ (2.54 cm) epoxy mount together Fivemile Creek. euhedral and possibly reworked zircons. with fragments of the Sri Lanka standard zir- Above and in gradational contact with Further analysis on samples from the same con. The mounts were sanded down to a depth the diamictite on Oxford Ridge is the Oxford stratigraphic level using CA-ID-TIMS was of ~20 µm, polished, imaged, and cleaned prior Mountain tuffi te (tuffi te is a general term for conducted by Condon and Bowring (2011). to isotopic analysis. U-Pb geochronologic a rock unit that contains a mixture of pyro- Condon and Bowring (2011) analyzed three analysis of zircons was conducted by LA-MC- clastic and epiclastic material; Schmid, 1981). zircon grains each from samples 06PL00 and ICP-MS. The analyses involved ablation of The Oxford Mountain tuffi te consists of 40–60 34PL05 (Fig. 4). Of the six total grains ana- zircon with a New Wave UP193HE Excimer m of debris-fl ow diamictite and feldspathic lyzed, four grains were equivalent at ca. 687 laser using a spot diameter of 30 µm. The volcaniclastic sandstone (Figs. 7B–7D). It Ma, one was slightly younger at ca. 684 Ma, ablated material was carried in helium into the is exposed for 5.5 km along strike parallel to and one was older at ca. 705 Ma. Condon and plasma source of a Nu HR ICP-MS, which was and in the upper plate of the low-angle Clif- Bowring (2011) reported a weighted mean ID- equipped with a fl ight tube of suffi cient width ton Canyon fault, which cuts out the top of the TIMS 206Pb/238U age of 687.4 ± 1.3 Ma (n = 4). that U, Th, and Pb isotopes could be measured unit progressively to the east (Fig. 6). Facies They concluded that the lower precision of simultaneously. The ablation pit was ~15 µm in within the Oxford Mountain tuffi te include: (1) SHRIMP U-Pb dates does not offer the ana- mafi c volcaniclastic diamictite, with abundant lytical precision needed to discern subpopula- 1 pebble- to cobble-sized vesicular and non- tions at the <3% level, and confi dently assign GSA Data Repository Item 2013034, laser ablation– multicollector–inductively coupled plasma–mass spec- vesicular basalt clasts, generally found at the depositional ages. These results prompted a trometry (LA-MC-ICP-MS) data table, sensitive high- base (Fig. 8A); (2) nonstratifi ed volcaniclas- second SHRIMP analysis of the same sample resolution ion microprobe (SHRIMP) and chemical abra- tic diamictite that includes rounded pebbles (06PL00), which yielded a revised zircon age sion isotope dilution–thermal ionization mass spectrom- to rare boulders of gray to purple porphyritic of 686 ± 4 Ma (Fanning and Link, 2008). At etry (CA-ID-TIMS) cath o do luminescence (CL) images, and SHRIMP Wetherill concordia plots, is available at trachyandesite, trachyte, dacite, and rhyolite the time, structural complexities on Oxford www.geosociety.org/pubs/ft2013.htm, or on request (Fig. 8B, 15JK10); (3) one or more reworked Ridge made the exact stratigraphic position of from [email protected], Documents Secretary, plagioclase-phyric volcanic lithic wacke beds the unit uncertain. GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.

136 www.gsapubs.org | Volume 5 | Number 1 | LITHOSPHERE

A B

C D

E F

Figure 8. Lithologies of the Oxford Mountain tuffi te. (A) Mafi c volcaniclastic diamictite. (B) Nonstratifi ed volcaniclastic diamictite with trachyte epi- clast (15JK10). (C) Stratifi ed plagioclase-phyric volcanic sandstone and diamictite exhibiting slump folds (13JK10). Dashed white lines mark folded planar beds. (D) Crudely stratifi ed and tectonically stretched volcaniclastic diamictite bearing light-colored, aphanitic, volcanic clasts. (E) Volcaniclastic diamictite with fi ne-grained green wavy laminations (04JK09, 23JK10, and 16JK10). (F) Above the tuffi te lies normally graded breccia, conglomerate, and feldspathic sandstone.

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depth. Detrital zircon spectra were created with Fifty zircons with determined ca. 700 Ma plucked from the epoxy, and then chemically Isoplot (Ludwig, 2008). U-Pb ages from the four samples were ana- abraded following a modifi ed version of Mattin- Further analysis of tuffaceous units was con- lyzed for lutetium and hafnium isotopes using son (2005). The residual crystals were washed, ducted using SHRIMP. U-Pb zircon analyses MC-ICP-MS (multicollector–inductively cou- spiked with an EARTHTIME mixed 202Pb-205Pb- were carried out at Research School of Earth pled plasma–mass spectrometer) at the Austra- 233U-235U tracer solution (ET2535) and dis- Sciences, Australian National University, Can- lian National University using procedures as solved. U and Pb were separated from zircon berra, Australia, using SHRIMP II following described in Munizaga et al. (2008). Two-stage solutions following Krogh (1973), loaded onto procedures described in Williams (1998) and crustal model ages were calculated using the a Re fi lament after Gerstenberger and Haase references therein. Zircon grains from volca- 176Lu decay constant of Söderlund et al. (2004), (1997), and measured using a GVI (IsotopX) niclastic diamictite samples were handpicked model chondritic values of Bouvier et al. (2008), Isoprobe-T MC-TIMS. from heavy mineral concentrates and placed present-day depleted mantle values of Vervoort onto double-sided tape, together with Temora and Blichert-Toft (1999), and average crustal GEOCHRONOLOGY RESULTS reference zircon grains. Epoxy disks were cast, values of Goodge and Vervoort (2006). and the zircon grains were sectioned approxi- Finally, fi ve samples along depositional Detrital Zircon Provenance on Oxford mately in half, and polished. Transmitted and strike were analyzed using high-precision CA- Mountain refl ected light photomicrographs and cathodo- ID-TIMS. CA-ID-TIMS U-Pb analyses of luminescence (CL) images were made for all selected zircon grains from sample 4JK09 (pre- U-Pb ages of ~330 detrital zircon grains grains (Fig. DR1 [see footnote 1]). Four sam- viously analyzed by SHRIMP) and new samples from seven samples of the Scout Mountain ples (4JK09, 62JK09, 63JK09, and 64JK09) of the Oxford Mountain tuffi te were completed Member along Oxford Ridge were analyzed by were analyzed in a single extended probe ses- at Boise State University following procedures LA-MC-ICP-MS. Data are listed in GSA Data sion. Uncertainty in Temora reference zircon U/ described in Schmitz and Davydov (2012). Zir- Repository Table DR1 (see footnote 1). The Pb ratio calibration was 0.39%. Sample 15PL08 con grains from heavy mineral separates were detrital zircon spectra (Fig. 9) exhibit a dominant was run in a separate session, where uncertainty annealed at 900 °C for 60 h in quartz beakers 1.6–1.7 Ga Paleoproterozoic-aged peak with in Temora reference zircon U/Pb ratio calibra- and then handpicked, placed onto double-sided locally important Archean, Mesoproterozoic, tion was 0.58%. Data were processed using the tape, mounted in epoxy, polished, and imaged and Neoproterozoic peaks. Populations include SQUID Excel macro of Ludwig (2000), and via cathodoluminescence (CL) (Fig. DR2 [see a Paleoproterozoic to Neoarchean grouping at plots and age calculations were carried out using footnote 1]). Single grains were selected using 2450–2700 Ma, Mesoproterozoic 1400–1500 Isoplot (Ludwig, 1999, 2008). CL to avoid inherited cores and fractures, Ma grains, and locally common Neoproterozoic

Figure 9. Normalized detrital zircon probability density plots, in stratigraphic order from bottom to 5 top. Frequency on right axis, histo- gram bins are 20 m.y. wide. These Volcaniclastic are laser ablation–multicollector– diamictite inductively coupled plasma–mass 67JK09 n = 11 25 spectrometry (LA-MC-ICP-MS) data G analyzed at the Arizona LaserChron 15 Center. (A) Granitic clast from F Quartzite clast in diamictite cobble- to boulder-conglomerate 03JK09 n = 84 5 ﬁ lling scoured unconformity above the Bannock Volcanic Member 5 Volcanic sandstone 68JK09 n = 39 (24JK09). (B) Lowest exposed E diamictite at Fivemile Canyon 20 (75JK09). (C) Quartzite clast

Diamictite, upper Fivemile Canyon Frequency D 10 from Fivemile Canyon diamictite 73JK09 n = 80 (74JK09). (D) Upper portion of diamictite on Oxford Mountain at Fivemile Canyon (73JK09). (E) Vol- Quartzite clast in diamictite, Fivemile Canyon C 5 canic sandstone of Oxford Moun-

Age Probability 74JK09 n = 22 tain tufﬁ te (68JK09). (F) Quartzite clast in diamictite (3JK09) with a B Diamictite, lower Fivemile Canyon 1.7 Ga provenance. (G) Volcanicla- 10 stic diamictite of Oxford Mountain 75JK09 n = 87 5 tufﬁ te (67JK09). All grains from 68JK09 belong to ca. 700 Ma popu- A Granitic clast from Oxford diamictite 3 lation. See Figure 4 and Table DR2 24JK09 n = 9 (see text footnote 1) for sample locations. Data are listed in Table 500 1000 1500 2000 2500 3000 DR1 (see text footnote 1). Age (Ma)

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ca. 650–720 Ma grains. Grains of Grenvillean gest minor transport. Despite our best efforts at Maximum Age Constraints: Portneuf age (950–1300 Ma) are rare and do not form a picking fi rst-cycle magmatic grains, one equant Narrows Area signifi cant detrital zircon age grouping. Posi- grain was Archean and one was Mesoprotero- tive Kolmogorov-Smirnov (K-S) tests (Press et zoic. Individual crystal age uncertainties are SHRIMP al., 1986) on two diamictite samples (75JK09 generally between 0.5 and 1 Ma. We obtained SHRIMP 206Pb-238U zircon and 73JK09), one volcanic sandstone sample A resample of 04JK09 (Fig. 8E; resample of concordia ages from several diamictite and (68JK09), and one quartzite clast (74JK09) 06PL00 of Fanning and Link, 2004) yielded three sandstone samples from the type section of the between the Fivemile Canyon area and Oxford age groups at 703, 699, and 694 Ma, confi rming Scout Mountain Member south of Portneuf Nar- Ridge suggest all four samples are not statisti- the multimodal, detrital character of the Neopro- rows (Table 1; Fig. 2; Figs. DR1 and DR3 [see cally different. terozoic crystal load of the volcaniclastic diamic- footnote 1]). The diamictite samples are all from tite. A similar conclusion was made by Condon the lower diamictite interval, separated from the Maximum Age Constraints: Oxford and Bowring (2011) and alluded to in Fanning upper diamictite interval by siltstone, sandstone, Mountain and Link (2004). Sample 23JK10 of volcanicla- and conglomerate (Fig. 2). Sample 62JK09, stic diamictite, 20 m to the north along strike of mafi c volcaniclastic diamictite, is from the low- SHRIMP sample 04JK09, has subrounded basalt and felsic est exposed portion of the lower diamictite and Sample 04JK09 is from the Oxford Moun- volcanic clasts with several closely spaced fi ne- has an almost exclusively intraclastic component tain tuffi te unit resampled from the same strati- grained wavy laminations (Fig. 8E). Analysis of of fl attened metabasalt and siltstone clasts, likely graphic unit as 06PL00 of Fanning and Link this sample yielded ages of 697 and 700 Ma, as originating from the underlying Bannock Vol- (2004) and 34PL05 of Condon and Bowring well as a Mesoproterozoic grain. canic Member (Fig. 11). All 18 grains analyzed (2011). The rock is a medium-bedded plagio- Sample 13JK10, located ~3 km further north, in 62JK09 were Neoproterozoic, ranging in age clase-rich volcaniclastic diamictite with gray is composed of plagioclase-phyric, laminated, from 723 to 689 Ma, excluding two discordant and green, 2-cm-thick, thinly laminated siltstone volcanic sandstone with fl oating subangular analyses as old as 774 Ma. The probability den- beds (Fig. 8E). Subangular clasts include basalt to subrounded pebbles (Fig. 5C). The poorly sity plot shows that zircon ages form a simple cobbles, aphanitic gray, trachytic volcanics, and sorted rock contains abundant subangular, bell-shaped distribution (Fig. DR3 [see footnote quartzite. Zircons in sample 04JK09 are gener- light-colored, aphanitic clasts with carbonate- 1]). However, in light of the multimodal age pop- ally small and fragmented. Twenty grains were chlorite alteration on rims. Rounded epiclastic ulations shown by CA-ID-TIMS data on other analyzed using SHRIMP II (Table 1); one grain trachyte and basalt lithic clasts are also present. such zircon populations from Oxford Mountain, recorded a 207Pb/206Pb age of ca. 2445 Ma, while Laminations exhibit soft sediment deformation it is likely that any weighted mean calculation another high-U zircon is discordant with a Ter- and are depressed and punctuated by basaltic from SHRIMP data would encompass a range of tiary 206Pb/238U age. The remaining 18 areas ana- and trachytic epiclasts. This sample yielded actual zircon crystallization events. lyzed are predominantly within uncertainty of crystal populations with diverse ages of 707, Sample 63JK09 is a similarly intraclastic and the concordia curve. A probability density plot 703, 699, 696, and 688 Ma (Table 2; Fig. 11); mixed volcaniclastic diamictite 100 m strati- of 206Pb/238U ages shows a prominent peak at ca. and one Grenville-age grain. graphically higher than sample 62JK09. The 680–710 Ma with scattered older ages extend- Sample 15JK10, further north, is composed probability density plot shows an even distri- ing to 820 Ma (Fig. 10). of volcaniclastic diamictite (Fig. 8B). Clasts bution of zircon ages between 728 Ma and 684 within this diamictite are trachyte, basalt, feld- Ma, with one outlying grain dated at ca. 786 Ma CA-ID-TIMS spathic lithic wacke and quartz arenite. This (Fig. DR3 [see footnote 1]). The SHRIMP con- The range of concordant SHRIMP 206Pb/238U sample yielded zircons ranging from 709 to cordia age, and 206Pb/238U age, of the youngest ages for zircons from the Oxford Mountain 696 Ma (Fig. 11). grain is younger than, but within the SHRIMP tuffi te suggests complex, multi-age compo- The northernmost sample, 16JK10, lies just analytical uncertainty, of the youngest grain in nents. Thus, CA-ID-TIMS analyses (approach- below the Clifton fault and is lithologically the underlying sample (Table 1; Fig. DR3 [see ing 0.1% precision on these samples) were made similar to sample 15JK10 but contains fi ne- footnote 1]). on fi ve new samples along 5.5 km of strike (Fig. grained wavy laminations similar to sample Immediately above the lower diamictite, a 4B) in order to isolate precise single grain ages 23JK10 (Fig. 8E) with cobble-sized volcanic plagioclase-arkose (15PL08) yields Paleopro- and so determine the age of the youngest zircon clasts. Ten of 12 grains yielded a weighted terozoic and Neoproterozoic detrital zircon pop- age grouping. Five to 12 euhedral grains were mean age of 685.5 ± 0.4 Ma, with two older ca. ulations. The Neoproterozoic population ranges selected on the basis of CL images from grain 709 Ma grains (Fig. 11). The CL images for the in age from 700 to 662 Ma (Fig. DR3 [see foot- mounts of samples 04JK09, 23JK10, 13JK10, younger 685 Ma grains show two distinct grain note 1]). As above, the concordia age, and age 15JK10, and 16JK10. Data are presented in Fig- types characterized by oscillatory and weakly of the youngest analysis, is younger than that of ure 11 and in Table 2. zoned internal structure versus sector zoning underlying sample. The zircon grains from these samples are (Fig. DR2 [see footnote 1]). Moving up section, a prominent cobble con- 50–200 µm in length, equant to subprismatic, Zircons from two felsic volcanic clasts glomerate lies below the upper diamictite south and euhedral to subrounded. Cathodolumines- were dated to determine the age of the coarse of Portneuf Narrows. Sample 64JK09 is a one cence (CL) images from the samples show sec- epiclastic portion of the Oxford Mountain of several felsic volcanic clasts from this con- tor zoning and rare narrow to broad oscillatory tuffi te (Table 2). Sample 17JK10 yielded one glomerate that exhibits fl ow-banding and pos- zoning patterns, suggesting that all grains are 700.6 ± 0.5 Ma grain. Sample 19JK10 yielded sible fi amme. Most of the grains are euhedral magmatic. Sector zoning is the dominant pat- two ca. 699 Ma grains and one 712.7 ± 1.4 Ma and exhibit rare resorbtion in CL (Fig. DR1 [see tern for samples 04JK09 and 23JK10, whereas grain. The likely ages of these clasts overlap footnote 1]). Several subrounded grains and one samples 13JK10, 15JK10, and 16JK10 exhibit the 699–700 Ma zircon grain populations in Paleoproterozoic grain suggest some reworking more variation. Rare subrounded grains sug- most of the tuffi te samples. during eruption and/or deposition. The volcanic

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Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/5/1/128/3723042/128.pdf by guest on 25 September 2021 KEELEY ET AL. ) † disc. ±% ±% Continued ( ) Pb Pb a 207 206 M (

e g 1 ± A 2 U 6 Pb 238 206 ±r Pb Pb 207 206 s o i t ± a r

c i n e g 2 o 5 U i Pb 0 d . 235 a 207 0 R 1 0 0 ± 0 . 0 6 9 U Pb 0 0 238 . 206 0 5 0 0 ± 0 . 0 9 8 s Pb Pb 4 o 0 i . t 207 206 0 a r

l a t o 8 T ± 0 . 1 TABLE 1. SUMMARY OF SHRIMP U-Pb ZIRCON RESULTS OF SHRIMP 1. SUMMARY TABLE 8 U 4 Pb . 2 238 206 0 1 8 * 1 . % 206 1 f 5 3 6 Pb Pb 0 0 204 206 0 . 0 3 Pb 4 206 (ppm) 6 0 . 0 Th/U 6 9 Th 2 (ppm) 4 2 U 1 (ppm) 5 1 . 1.15.1 193 44 130 0.67 33 0.75 21 0.000229 5 0.40 0.003960 7.746 6.89 0.091 0.0633 6.994 0.0007 0.120 0.1259 0.1286 0.0090 0.0015 1.063 0.1331 0.0026 0.027 1.269 0.0600 0.0013 0.234 0.471 0.0691 0.0127 780 0.106 9 806 603 15 903 48 –29 378 11 7.1 146 66 0.45 14 – <0.01 8.674 0.102 0.0620 0.0008 0.1153 0.0014 0.989 0.017 0.0622 0.0008 0.673 704 8 681 27 –3 2.1 46 27 0.59 4.6 0.000242 0.42 8.515 0.123 0.0678 0.0012 0.1169 0.0017 1.037 0.043 0.0643 0.0025 0.349 713 10 752 83 5 3.1 155 125 0.80 16 – <0.01 8.589 0.099 0.0623 0.0006 0.1165 0.0013 1.005 0.015 0.0626 0.0006 0.762 710 8 693 21 –2 3 9.14.1 31 1206.1 13 60 178 0.40 0.502.1 75 3 12 130 0.42 0.000536 0.000094 72 18 0.94 0.16 0.56 0.001245 8.858 8.826 13 2.19 0.147 0.106 0.0654 0.000101 0.0632 0.0014 8.563 0.0007 0.18 0.097 0.1118 0.1131 0.0792 0.0019 0.0014 8.674 0.0007 0.888 0.103 0.965 0.1142 0.0643 0.065 0.023 0.0013 0.0007 0.0576 0.0619 0.963 0.0041 0.0013 0.1151 0.034 0.236 0.0014 0.508 0.0612 0.997 0.0021 683 691 0.020 0.326 11 0.0628 8 0.0010 515 697 670 0.586 155 8 44 –33 702 645 –3 8 72 702 –8 35 0 4.1 56 42 0.75 5.7 0.000159 0.28 8.512 0.118 0.0643 0.0010 0.1172 0.0016 1.002 0.030 0.0620 0.0017 0.463 714 9 675 57 –6 8.1 745 1001 1.34 73 0.000208 0.37 8.711 0.090 0.0659 0.0003 0.1144 0.0012 0.992 0.013 0.0629 0.0005 0.780 698 7 705 18 1 7.1 618.1 35 105 0.58 81 6.1 0.77 10.8 – 0.000245 0.43 <0.01 8.389 8.599 0.104 0.117 0.0650 0.0621 0.0008 0.0010 0.1187 0.1167 0.0015 0.0016 1.006 1.051 0.024 0.029 0.0614 0.0653 0.0013 0.0016 0.513 0.488 723 712 9 9 655 784 45 51 –10 9 6.1 61 35 0.57 6.1 0.000120 0.21 8.650 0.118 0.0626 0.0010 0.1154 0.0016 0.969 0.029 0.0609 0.0016 0.464 704 9 636 56 –11 1.1 41 24 0.58 4.5 –7.1 38 <0.01 22 7.854 0.58 0.115 0.0731 0.0012 4 0.1275 0.000221 0.0019 0.39 1.308 0.031 8.678 0.0744 0.132 0.0014 0.0678 0.625 0.0013 0.1148 774 0.0018 11 1.022 1052 0.047 0.0646 37 0.0028 26 0.339 700 10 761 90 8 5.19.1 46 349 25 259 0.55 0.743.1 34.3 51 4.6 0.000017 0.000275 43 0.03 0.48 0.85 8.728 8.689 0.093 0.126 5.0 0.0627 0.0624 0.0004 0.0011 – 0.1145 0.1145 0.0012 0.0017 <0.01 0.986 0.922 0.013 8.710 0.039 0.124 0.0624 0.0584 0.0612 0.0005 0.0023 0.0011 0.818 0.346 0.1155 699 0.0017 699 1.050 10 7 0.039 545 688 0.0659 0.0023 87 16 0.387 –28 –2 704 10 804 72 12 3.1 92 79 0.86 9 0.000422 0.74 8.598 0.112 0.0646 0.0010 0.1154 0.0015 0.931 0.045 0.0585 0.0027 0.278 704 9 549 101 –28 6.1 85 52 0.61 8 0.000112 0.20 8.921 0.114 0.0647 0.0009 0.1119 0.0014 0.973 0.025 0.0631 0.0014 0.492 684 8 711 48 4 11.1 92 59 0.65 16 0.021393 37.31 5.068 0.068 0.3730 0.0038 0.1237 0.0029 1.100 0.305 0.0645 0.0178 0.085 752 17 758 583 1 11.1 50 33 0.67 5.0 0.000021 0.04 8.652 0.122 0.0646 0.0011 0.1155 0.0016 1.024 0.024 0.0643 0.0012 0.613 705 9 750 38 6 11.1 162 88 0.54 16 0.000142 0.25 8.858 0.102 0.0638 0.0006 0.1126 0.0013 0.959 0.019 0.0618 0.0010 0.574 688 8 666 35 –3 19.1 152 108 0.71 18 0.002562 4.45 7.046 0.085 0.0943 0.0029 0.1356 0.0018 1.066 0.099 0.0570 0.0052 0.140 820 10 492 202 –67 10.1 314 158 0.50 126 0.000023 0.03 2.135 0.024 0.1592 0.0009 0.4682 0.0052 10.257 0.127 0.1589 0.0009 0.888 2476 23 2444 10 –1 14.1 253 184 0.73 26 0.000096 0.17 8.313 0.093 0.0634 0.0005 0.1201 0.0013 1.028 0.016 0.0621 0.0007 0.705 731 8 676 24 –8 12.1 47 40 0.85 4.9 0.000345 0.61 8.356 0.123 0.0639 0.0012 0.1190 0.0018 0.966 0.049 0.0589 0.0028 0.297 725 10 564 105 –29 13.1 61 36 0.60 6.0 0.000202 0.36 8.634 0.117 0.0631 0.0010 0.1154 0.0016 0.957 0.035 0.0601 0.0020 0.376 704 9 608 73 –16 16.1 207 13021.0 0.63 77 21 47 – 0.62 14 <0.01 0.023152 40.30 8.661 0.097 0.0629 4.723 0.0005 0.078 0.3937 0.1155 0.0048 0.0013 0.1264 1.012 0.0040 0.015 0.988 0.0635 0.424 0.0006 0.758 0.0567 0.0243 0.073 705 7 767 727 23 478 20 947 3 –60 23.1 66 40 0.60 7 0.000236 0.41 8.628 0.119 0.0643 0.0010 0.1154 0.0016 0.969 0.032 0.0609 0.0018 0.427 704 9 635 64 –11 1 12.117.1 105 562 6218.1 371 0.59 102 0.66 10 56 55 0.55 0.000008 – 0.01 1015.1 0.000076 <0.01 8.794 85 0.092 0.13 8.818 0.0631 0.108 44 0.0003 8.706 0.0622 0.52 0.106 0.0007 0.1137 0.0628 0.0012 18 0.0007 0.1135 0.987 0.0014 0.026845 0.1147 0.012 0.979 46.62 0.0014 0.0630 0.017 0.975 0.0003 4.155 0.0626 0.893 0.020 0.053 0.0008 0.0617 0.4553 0.708 694 0.0010 0.0112 0.589 693 7 0.1285 0.0029 700 707 8 1.267 693 8 11 0.439 663 0.0715 26 2 0.0247 36 0.065 0 –6 779 16 972 706 20 17.1 199 131 0.66 20 – <0.01 8.646 0.098 0.0640 0.0006 0.1157 0.0013 1.027 0.015 0.0644 0.0006 0.773 706 8 754 20 6 14.1 54 53 0.98 5.4 – <0.0113.1 8.614 25 0.120 0.0630 17 0.0010 0.69 0.1167 0.0016 2 1.078 0.000968 0.036 1.70 0.0670 0.0020 0.426 8.746 0.156 0.0657 711 0.0016 9 0.1181 838 0.0019 1.037 62 0.063 15 0.0639 0.0042 0.746 696 13 737 141 6 18.1 44 26 0.60 4.4 0.000038 0.07 8.687 0.130 0.0639 0.0012 0.1150 0.0017 1.004 0.026 0.0633 0.0013 0.590 702 10 719 44 2 12.1 51 39 0.77 5 – <0.01 8.720 0.124 0.0634 0.0013 0.1147 0.0016 1.003 0.025 0.0634 0.0013 0.574 700 9 723 43 3 17.115.1 46 127 29 59 0.64 0.47 4.5 12.5 0.000096 0.000152 0.17 0.27 8.760 8.708 0.128 0.104 0.0624 0.0612 0.0011 0.0007 0.1140 0.1145 0.0017 0.0014 0.958 0.932 0.029 0.022 0.0610 0.0590 0.0016 0.0012 0.476 0.503 696 699 10 8 638 567 58 45 –9 –23 19.1 146 87 0.59 14 0.000125 0.22 8.699 0.102 0.0641 0.0007 0.1147 0.0014 0.985 0.020 0.0623 0.0011 0.572 700 8 685 36 –2 16.1 117 50 0.43 11.3 0.000026 0.05 8.857 0.106 0.0637 0.0007 0.1129 0.0014 0.985 0.017 0.063318.1 0.0008 0.686 116 63 689 0.54 8 11 718 27 – 4 <0.01 8.842 0.107 0.0642 0.0010 0.1131 0.0014 1.007 0.020 0.0645 0.0010 0.614 691 8 759 33 9 16.1 22 13 0.60 2 0.000092 0.16 8.814 0.173 0.0629 0.0016 0.1133 0.0022 0.962 0.038 0.0616 0.0021 0.500 692 13 659 73 –5 10.1 193 126 0.65 19.0 0.000050 0.09 8.696 0.098 0.0633 0.0005 0.1149 0.0013 0.992 0.016 0.0626 0.0007 0.717 701 7 695 23 –1 21.1 34 25 0.73 3 0.000318 0.56 8.856 0.144 0.0669 0.0014 0.1123 0.0018 0.964 0.040 0.0623 0.0024 0.397 686 11 684 81 0 62JK09 63JK09 Grain spot 04JK09

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e g ± A Pb ratio. 206 Pb/ U Pb 204 238 206 ±r Pb Pb 207 206 ) s o i t ± a r

c i n e g Continued o U i Pb d 235 a 207 R ± U Pb 238 206 Error in Temora reference zircon calibration was 0.39% for all analytical sessions, except sample 15PL08, Temora Error in

± level. σ s Pb Pb U data from different mounts). Correction for common Pb was made using the measured U data from different o i t 207 206 238 a r

l Pb/ a t o 206 T ± U Pb TABLE 1. SUMMARY OF SHRIMP U-Pb ZIRCON RESULTS ( U-Pb ZIRCON RESULTS OF SHRIMP 1. SUMMARY TABLE 238 206 * % 206 f Pb Pb 204 206 Pb that is common Pb. Pb 206 206 (ppm) Th/U ) Th (ppm) U (ppm) Continued SHRIMP—sensitive high-resolution ion microprobe. Uncertainties are given at the 1 % denotes the percentage of 206 1.1 167 639.1 0.38 272 16 155 0.000097 0.573.1 165 0.17 27 0.000037 74 8.851 0.102 0.06 0.45 0.0622 0.0006 16 8.797 0.098 0.1128 0.0609 0.0013 – 0.0005 0.945 0.1136 0.020 <0.01 0.0013 0.0608 0.946 0.0011 8.730 0.538 0.014 0.101 0.0629 0.0604 689 0.0006 0.0006 0.738 0.1146 8 0.0013 631 694 0.996 7 39 0.015 0.0630 618 –9 0.0006 0.753 22 –12 699 8 709 22 1 6.17.1 1452.1 177 136 61 81 0.42 54 0.46 14 0.40 17 0.000087 13 0.000107 0.15 0.000006 0.19 0.01 8.841 8.805 0.105 8.802 0.102 0.0630 0.104 0.0633 0.0007 0.0623 0.0006 0.1129 0.0007 0.0013 0.1134 0.0013 0.1136 0.961 0.0013 0.965 0.018 0.974 0.017 0.0617 0.016 0.0617 0.0009 0.0622 0.0009 0.637 0.0007 0.642 0.734 690 692 8 694 8 664 8 665 682 31 30 –4 23 –4 –2 9.1 121 835.1 0.68 115 32 52 0.0000758.1 0.45 0.12 285 11 3.292 156 0.000230 0.049 0.55 0.1056 0.41 0.0012 28 9.049 0.3034 0.000055 0.112 0.0046 0.0643 0.10 4.373 0.0008 0.085 8.868 0.1101 0.1045 0.098 0.0014 0.0013 0.0628 0.924 0.771 0.0005 0.024 1708 0.1127 0.0609 0.0012 23 0.0014 0.963 1706 0.472 0.014 23 673 0.0620 0.0006 0 8 0.741 636 688 50 7 –6 675 21 –2 8.11.1 147 271 102 259 0.69 0.95 15 27 0.000118 0.0000204.1 0.212.1 0.04 347 8.607 98 282 8.619 0.101 0.094 0.0644 0.81 50 0.0636 0.0006 0.0007 0.51 36 0.1159 0.0014 0.1160 11 0.0013 1.001 – 0.000361 1.013 0.020 0.016 0.63 0.0626 <0.01 0.0633 0.0010 0.0007 0.600 7.753 8.364 0.692 0.101 0.090 0.0634 707 0.0626 0.0009 707 0.0004 8 0.1296 0.1196 7 0.0018 696 0.0013 720 1.201 1.033 33 0.049 24 0.013 –2 0.0672 0.0627 0.0022 2 0.0004 0.732 0.859 786 728 11 7 845 697 67 14 7 –4 4.1 2747.1 182 67 0.66 30 26 0.45 0.000047 0.08 7 0.0006078.1 9.040 0.119 1.09 113 0.0620 0.0009 42 8.791 0.179 0.1105 0.37 0.0644 0.0015 0.0019 31 0.934 0.1129 0.000095 0.019 0.0025 0.0613 0.15 0.915 0.0010 0.639 0.073 3.182 0.0587 0.048 0.0039 676 0.1097 0.668 0.0012 8 0.3138 690 650 0.0048 15 4.690 344.1 557 0.092 –4 125 0.1084 146 0.0013 –24 45 0.775 0.36 1759 12 23 1773 0.000077 0.14 23 1 8.652 0.105 0.0625 0.0007 0.1154 0.0014 0.977 0.023 0.0614 0.0013 0.510 704 8 654 44 –8 9.1 2175.1 151 0.70 39 22 23 0.000024 0.59 0.04 4 8.535 0.000279 0.097 0.0626 0.49 0.0005 8.398 0.1171 0.129 0.0013 0.0643 1.005 0.0012 0.015 0.1185 0.0622 0.0018 0.0006 0.985 0.748 0.050 0.0603 714 0.0029 8 0.307 682 722 22 11 –5 614 104 –18 3.1 72 48 0.66 7 0.000190 0.34 8.690 0.166 0.0641 0.0017 0.1147 0.0022 0.970 0.039 0.0613 0.0022 0.470 700 13 651 77 –8 1.1 123 715.1 0.57 267 198 11 0.74 – 266.1 0.000188 107 <0.012.1 0.33 53 9.250 128 0.148 0.49 8.800 0.0631 0.115 28 0.0013 27 0.0638 0.22 0.0009 0.000069 0.1081 0.0017 0.1133 35 0.11 0.0015 0.941 0.000061 0.953 0.025 3.434 0.10 0.054 0.026 0.0631 0.1018 0.0013 0.0610 3.172 0.0013 0.603 0.0014 0.045 0.486 0.2909 0.1066 662 0.0046 0.0011 692 10 4.045 0.3150 0.085 713 9 0.0045 0.1009 4.595 640 45 0.0014 0.081 0.743 51 7 0.1058 1646 0.0011 –8 0.798 23 1640 1765 22 26 1728 0 20 –2 For % disc, 0% denotes a concordant analysis. *f Note: † 11.1 202 100 0.50 20 0.000131 0.23 8.847 0.101 0.0639 0.0006 0.1128 0.0013 0.963 0.018 0.0620 0.0009 0.612 689 7 672 32 –2 11.1 62 23 0.38 6 0.000883 1.61 8.721 0.183 0.0658 0.0020 0.1138 0.0026 0.937 0.070 0.0597 0.0038 0.653 695 15 593 136 –17 16.1 351 247 0.70 34 0.000052 0.09 8.736 0.095 0.0624 0.0005 0.1144 0.0012 0.973 0.015 0.0617 0.0007 0.717 698 7 663 23 –5 15.113.1 290 271 18118.1 160 0.62 161 0.59 28 46 27 0.29 0.000086 – 0.15 44 <0.01 8.717 – 0.097 8.737 0.0631 0.109 0.0005 0.0628 <0.01 0.0005 0.1145 3.137 0.0013 0.1145 0.039 0.978 0.0014 0.1087 0.995 0.018 0.0007 0.0619 0.015 0.3189 0.0009 0.0630 0.0040 0.605 0.0005 4.795 0.843 699 0.069 0.1090 699 7 0.0008 8 0.864 671 709 1784 31 19 17 –4 1783 1 13 0 10.1 228 112 0.49 22 0.000029 0.05 8.939 0.101 0.0621 0.0005 0.1118 0.0013 0.951 0.015 0.0617 0.0007 0.714 683 7 663 24 –3 10.1 138 67 0.49 34 0.00018019.1 0.2817.1 369 259 3.451 285 150 0.051 0.77 0.1086 0.58 0.0011 36 25 0.2890 0.000124 0.0043 0.000076 0.22 4.227 0.13 0.086 8.874 0.1061 8.868 0.097 0.0015 0.099 0.0631 0.729 0.0637 0.0005 0.0005 1636 0.1124 0.0012 0.1126 21 0.0013 0.951 1733 0.972 0.015 25 0.015 0.0613 0.0626 0.0007 6 0.0007 0.690 0.727 687 688 7 7 651 694 25 23 –5 1 14.1 43 17 0.40 4 0.000239 0.42 9.212 0.231 0.0666 0.0025 0.1081 0.0027 0.941 0.057 0.0632 0.0035 0.418 662 1614.1 142 714 116 58 7 0.41 14 0.000115 0.20 8.772 0.105 0.0640 0.0007 0.1138 0.0014 0.978 0.019 0.0624 0.0010 0.618 695 8 687 33 –1 14.127.1 233 330 14422.1 194 0.62 265 2310.1 194 0.000027 0.73 48 33 0.05 27 0.000060 34 0.000057 8.568 0.11 0.71 0.095 0.10 0.0641 8.558 5 0.0005 0.092 8.510 0.0626 0.1167 0.094 0.0004 – 0.0013 0.0621 0.0005 1.025 0.1167 <0.01 0.0013 0.015 0.1174 0.994 0.0637 0.0013 8.459 0.0006 0.991 0.014 0.122 0.741 0.0617 0.015 0.0646 0.0006 0.0011 0.0612 711 0.755 0.0007 0.1185 0.714 7 0.0017 712 1.091 733 716 7 0.032 21 7 665 0.0668 0.0017 647 3 20 0.494 23 –7 722 –11 10 830 53 13 12.1 170 75 0.44 16 0.000116 0.20 8.977 0.105 0.0630 0.0007 0.1112 0.0013 0.940 0.022 0.0613 0.0012 0.507 680 8 651 43 –4 20.1 9715.1 99 51 1.02 37 10 0.74 0.000140 0.25 5 8.544 – 0.111 0.0621 0.0008 <0.01 0.1168 8.499 0.0015 0.124 0.968 0.0633 0.027 0.0011 0.0601 0.1181 0.0015 0.0017 0.475 1.086 712 0.035 0.0667 9 0.0019 0.457 607 53 720 –17 10 828 60 13 15.1 707 211 0.3012.1 6816.1 134 0.000078 86 43 0.14 45 0.32 8.931 0.53 20 0.108 0.0625 18 0.000000 0.0006 0.00 0.1118 – 0.0014 5.701 0.946 0.087 <0.01 0.0767 0.017 0.0012 0.0614 4.132 0.0008 0.071 0.1754 0.693 0.0905 0.0027 0.0015 1.856 683 0.2420 0.041 0.0041 8 0.0767 3.020 0.0012 652 0.693 0.071 27 0.0905 1042 0.0015 –5 15 0.729 1115 1397 32 22 1436 7 31 3 13.1 61 31 0.51 6 0.000517 0.94 8.422 0.180 0.0617 0.0041 0.1177 0.0028 0.891 0.104 0.0549 0.0058 0.553 717 16 408 235 –76 where it was 0.58% (not included in above errors but required when comparing Grain spot 63JK09 ( 15PL08 64JK09

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Age (Ma)

650 660 670 680 690 700 710 720 730 740 750 Frequency

5 0.3 687.4 ± 1.3 Ma* 10 04JK09 CA-ID-TIMS 0.25 n = 5 685.5 ± 0.4 Ma† 0.2 34PL05 ID-TIMS 0.15 n = 3 06PL00 ID-TIMS n = 3

Relative Probability 0.1 04JK09 SHRIMP n = 15 0.05 06PL00 SHRIMP n = 18 0 650 660 670 680 690 700 710 720 730 740 750 Age (Ma)

Figure 10. Normalized probability density plots for sensitive high-resolution ion microprobe (SHRIMP), chemical abrasion– isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS), and ID-TIMS analyses of the Oxford Mountain tufﬁ te originally sampled by Fanning and Link (2004). Samples 06PL00 and 04JK09, ~200 m along strike south of 06PL00, were analyzed ﬁ rst by SHRIMP. SHRIMP histograms are plotted on the inverted y-axis. Sample 04JK09 was reanalyzed by CA-ID- TIMS (this study). *ID-TIMS age reported by Condon and Bowring (2011) from samples 34PL05 (25 m away from 04JK09) and 06PL00. †New CA-ID-TIMS age from sample 16JK10 (not plotted), this study. See sample locations in Figure 4 and Table DR2 (see text footnote 1).

15PL08 145PL02 640 Pocatello, Idaho (SHRIMP) (SHRIMP) 800 m Portneuf Narrows area 650

660 64JK09 (SHRIMP) Single-grain 600 63JK09 4JK09 (SHRIMP) (SHRIMP) 670

Oxford 62JK09 (SHRIMP) Mountain 680

Fault 16JK10 206 Oxford tuffite (CA-ID-TIMS) Pb/

19JK10 4JK09 15JK10 13JK10 690 238 400 + 23JK10 U dates (Ma)

700 Fe Scout Mountain Member 710 Scout Mountain Member, Pocatello Fm. Scout Mountain Member, 200 Bannock Volcanic Member error bars are 2 sigma 720 locally scoured unconformity robust maximum age constraint Fault maximum age constraint 730 Bannock correlation tie line Volcanic indicates older Neoproterozoic grains Member not used in concordia age calculations 740 Figure 11. Compilation diagram showing the stratigraphic location of all sensitive high-resolution ion microprobe (SHRIMP) and chemical abrasion– isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) geochronologic samples and their single-grain 206Pb/238U analyses. SHRIMP and CA-ID-TIMS error bars are 2σ. Grain analyses shown by black error bars were used in concordia age calculations (SHRIMP) and weighted mean age calculations (CA-ID-TIMS). Grain analyses shown by gray-ﬁ lled bars were not used in age calculations. Data are listed in Tables 1 and 2. See sample locations in Figure 4 and Table DR2 (see text footnote 1).

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TABLE 2. SUMMARY OF CA-ID-TIMS U-Th-Pb ISOTOPIC DATA Radiogenic isotope ratios Isotopic ages Th 206Pb* Mol % Pb* Pb 206Pb 208Pb 207Pb 207Pb 206Pb Corr. 207Pb 207Pb 206Pb Sample c % err % err % err ± ± ± (×10–13 mol) 206Pb* (pg) 204 206 206 235 238 coef. 206 235 238 U Pbc Pb Pb Pb U U Pb U U (a)† (b)† (c)† (c) (c) (c) (d)† (e)† (e) (f)† (e) (f) (e) (f) (g)† (f) (g) (f) (g) (f) 23JK10 z3 0.564 0.2379 98.56% 21 0.29 1275 0.175 0.062939 0.627 0.990384 0.691 0.114126 0.131 0.563 706 13 698.9 3.5 696.7 0.9 z1 0.606 0.7506 98.94% 29 0.66 1738 0.188 0.062806 0.413 0.988684 0.449 0.114170 0.066 0.593 702 9 698.0 2.3 696.9 0.4 z4 0.722 1.2202 99.67% 96 0.34 5513 0.224 0.062806 0.166 0.993855 0.184 0.114769 0.053 0.462 702 4 700.7 0.9 700.4 0.4 19JK10 z1 0.581 0.2773 98.58% 22 0.33 1296 0.180 0.062777 0.594 0.991466 0.645 0.114546 0.103 0.556 701 13 699.5 3.3 699.1 0.7 z3 0.559 0.1276 96.66% 9 0.36 549 0.173 0.062639 1.424 0.989471 1.549 0.114567 0.226 0.604 696 30 698.5 7.8 699.2 1.5 z2 0.593 0.1228 97.32% 11 0.28 683 0.183 0.063199 1.142 1.018990 1.250 0.116938 0.215 0.566 715 24 713.4 6.4 712.9 1.5 17JK10 z4 0.965 0.3524 98.79% 28 0.36 1512 0.298 0.062645 0.508 0.991797 0.554 0.114824 0.082 0.607 696 11 699.6 2.8 700.7 0.5 16JK10 z8 0.649 0.7445 99.35% 48 0.40 2820 0.201 0.062480 0.260 0.966170 0.310 0.112153 0.092 0.645 691 6 686.5 1.5 685.2 0.6 z9 0.783 0.8314 99.23% 42 0.53 2383 0.243 0.062528 0.309 0.967044 0.357 0.112169 0.088 0.633 692 7 686.9 1.8 685.3 0.6 z5 0.506 0.2437 98.25% 17 0.36 1051 0.157 0.062572 0.771 0.967776 0.841 0.112175 0.129 0.602 694 16 687.3 4.2 685.4 0.8 z1 0.348 1.0857 99.54% 64 0.41 4014 0.108 0.062366 0.191 0.964660 0.210 0.112182 0.041 0.546 687 4 685.7 1.0 685.4 0.3 z13 0.858 0.6240 99.33% 49 0.35 2719 0.266 0.062396 0.274 0.965132 0.323 0.112183 0.090 0.637 688 6 686.0 1.6 685.4 0.6 z12 0.635 1.1251 98.15% 17 1.74 999 0.197 0.062402 0.428 0.965264 0.480 0.112188 0.081 0.685 688 9 686.0 2.4 685.4 0.5 z6 0.898 0.1740 97.84% 15 0.32 850 0.279 0.062573 0.922 0.968135 1.010 0.112214 0.164 0.593 694 20 687.5 5.0 685.6 1.1 z3 0.464 0.5985 99.15% 35 0.42 2169 0.144 0.062460 0.354 0.966445 0.387 0.112220 0.065 0.565 690 8 686.6 1.9 685.6 0.4 z14 0.888 0.5020 99.44% 59 0.23 3257 0.275 0.062383 0.241 0.965612 0.306 0.112263 0.130 0.654 687 5 686.2 1.5 685.9 0.8 z4 0.411 0.4858 99.13% 34 0.35 2117 0.127 0.062343 0.389 0.965572 0.438 0.112330 0.125 0.508 686 8 686.2 2.2 686.3 0.8 z2 0.884 0.1830 98.02% 17 0.30 926 0.273 0.062899 0.808 1.006723 0.885 0.116083 0.149 0.582 705 17 707.2 4.5 708.0 1.0 z11 0.772 0.0993 91.88% 4 0.72 226 0.242 0.064229 3.367 1.032387 3.662 0.116577 0.390 0.777 749 71 720.1 18.9 710.8 2.6 15JK10 z8 1.168 0.1833 98.28% 20 0.26 1068 0.362 0.062697 0.689 0.983750 0.766 0.113798 0.150 0.586 698 15 695.5 3.9 694.8 1.0 z6 0.588 0.6255 99.44% 55 0.29 3268 0.182 0.062659 0.261 0.984784 0.305 0.113987 0.123 0.532 697 6 696.1 1.5 695.9 0.8 z5 0.581 0.6168 99.38% 50 0.32 2954 0.180 0.062629 0.287 0.985701 0.315 0.114148 0.060 0.537 696 6 696.5 1.6 696.8 0.4 z9 0.456 1.5214 99.72% 107 0.35 6543 0.141 0.062819 0.133 0.992212 0.192 0.114555 0.097 0.773 702 3 699.8 1.0 699.1 0.6 z13 0.598 0.5337 99.28% 43 0.32 2543 0.185 0.062829 0.289 0.993917 0.340 0.114732 0.099 0.621 702 6 700.7 1.7 700.2 0.7 z3 0.571 0.1392 95.65% 7 0.52 422 0.178 0.063598 1.906 1.009694 2.076 0.115145 0.327 0.575 728 40 708.7 10.6 702.6 2.2 z10 0.571 0.4999 93.52% 4 2.85 285 0.177 0.062821 0.965 0.997781 1.057 0.115194 0.164 0.617 702 21 702.7 5.4 702.8 1.1 z7 1.337 0.9008 98.56% 25 1.08 1277 0.414 0.062891 0.516 0.999407 0.560 0.115254 0.059 0.765 705 11 703.5 2.8 703.2 0.4 z11 1.286 2.9412 99.80% 178 0.50 8960 0.397 0.062835 0.101 1.002264 0.165 0.115686 0.089 0.846 703 2 705.0 0.8 705.7 0.6 z12 0.573 0.9163 99.50% 62 0.38 3684 0.177 0.063052 0.217 1.005921 0.265 0.115708 0.092 0.651 710 5 706.8 1.4 705.8 0.6 z4 0.767 0.4812 99.00% 32 0.40 1840 0.238 0.063261 0.433 1.013246 0.472 0.116165 0.086 0.524 717 9 710.5 2.4 708.5 0.6 13JK10 z4 0.815 0.0504 93.97% 5 0.27 304 0.252 0.062350 3.522 0.967451 3.753 0.112536 0.445 0.562 686 75 687.1 18.7 687.5 2.9 z7 0.615 0.3512 97.80% 14 0.65 835 0.190 0.062345 0.896 0.978608 0.973 0.113842 0.109 0.728 686 19 692.9 4.9 695.0 0.7 z1 0.645 0.2973 98.84% 27 0.29 1587 0.200 0.062779 0.469 0.987643 0.519 0.114100 0.113 0.529 701 10 697.5 2.6 696.5 0.7 z10 0.501 1.2041 99.60% 75 0.40 4565 0.155 0.062648 0.172 0.988580 0.223 0.114447 0.087 0.714 696 4 698.0 1.1 698.5 0.6 z14 0.510 0.4573 99.14% 35 0.33 2126 0.158 0.062725 0.349 0.993319 0.402 0.114854 0.102 0.611 699 7 700.4 2.0 700.9 0.7 z5 1.114 0.2763 98.45% 22 0.36 1184 0.345 0.062774 0.649 0.996335 0.729 0.115113 0.200 0.516 701 14 701.9 3.7 702.4 1.3 z3 0.586 2.0153 99.78% 142 0.36 8380 0.181 0.062916 0.090 0.999434 0.102 0.115210 0.029 0.538 705 2 703.5 0.5 702.9 0.2 z9 1.115 1.0860 98.58% 24 1.29 1290 0.345 0.062898 0.552 0.999695 0.615 0.115273 0.116 0.608 705 12 703.7 3.1 703.3 0.8 z12 0.963 0.3826 99.07% 36 0.30 1971 0.298 0.063013 0.380 1.005390 0.442 0.115719 0.132 0.590 709 8 706.5 2.3 705.9 0.9 z11 0.775 1.1619 99.61% 82 0.38 4651 0.240 0.062965 0.169 1.006401 0.241 0.115923 0.132 0.741 707 4 707.1 1.2 707.1 0.9 z8 0.384 0.5156 98.09% 16 0.83 962 0.131 0.083577 0.549 1.823915 0.628 0.158276 0.128 0.679 1283 11 1054.1 4.1 947.2 1.1 04JK09 z1 0.626 1.3030 99.75% 125 0.27 7340 0.194 0.062611 0.104 0.981924 0.119 0.113743 0.040 0.520 695 2 694.6 0.6 694.4 0.3 z5 0.536 0.1514 95.89% 7 0.54 446 0.167 0.062891 1.679 0.986984 1.816 0.113820 0.211 0.683 705 36 697.2 9.2 694.9 1.4 z3 0.426 0.7072 99.49% 59 0.30 3615 0.132 0.062799 0.251 0.992007 0.270 0.114567 0.053 0.432 701 5 699.7 1.4 699.2 0.4 z2 0.628 0.2532 98.68% 24 0.28 1390 0.194 0.062867 0.574 0.998378 0.638 0.115179 0.157 0.509 704 12 703.0 3.2 702.8 1.0 z4 0.778 0.9514 99.59% 79 0.32 4460 0.241 0.062951 0.168 1.000791 0.186 0.115303 0.041 0.522 707 4 704.2 0.9 703.5 0.3 Note: CA-TIMS—chemical abrasion–thermal ionization mass spectrometry. †a—z1, z2, etc., are single zircon grains or fragments extracted from grain mounts, all annealed and chemically abraded after Mattinson (2005). b—Model Th/U 208 206 207 235 206 ratio calculated from radiogenic Pb/ Pb ratio and Pb/ U age. c—Pb* and Pbc represent radiogenic and common Pb, respectively; mol % Pb* with respect to radiogenic, blank, and initial common Pb. d—Measured ratio corrected for spike and fractionation only. Pb and U fractionation were corrected internally using the double spike composition. e—Corrected for fractionation, spike, and common Pb; all common Pb was assumed to be procedural blank: 206Pb/204Pb = 18.35 ± 1.5%; 207Pb/204Pb = 15.60 ± 0.5%; 208Pb/204Pb = 38.08 ± 1.0% (all uncertainties 1σ). f—Errors are 2σ, propagated using the algorithms of Schmitz and Schoene (2007). g—Calculations are based on the decay constants of Jaffey et al. (1971). 206Pb/238U and 207Pb/206Pb ratios and ages were corrected for initial disequilibrium in 230Th/238U using Th/U [magma] = 3.

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texture and range of ages (704–673 Ma) sup- TABLE 3. ZIRCON LUTETIUM-HAFNIUM ISOTOPE DATA

port incorporation of xenocrystic zircons during 176 –6 176 –6 ε ε σ Title/grain Age Hf ± (10 ) Lu ± (10 ) Hf (t0) Hf (t1) ± 2 tDM eruption and deposition. All 18 Neoproterozoic (Ma) 177Hf 177Hf (Ga)* grains were used in the age calculation to yield a 04JK09 206 238 weighted mean Pb/ U age of 691 ± 4 Ma (n 9 683 0.282 58 0.001 25 –17.5 –2.5 2.0 1.7 = 18, MSWD = 0.98), assuming a single erup- 4 691 0.282 20 0.001 7 –25.9 –10.8 0.7 2.2 tive age on the volcanic clast. Therefore, 691 ± 12 693 0.282 20 0.001 1 –15.7 –0.5 0.7 1.6 17 694 0.282 28 0.002 31 –20.4 –5.7 1.0 1.9 4 Ma is interpreted as a maximum depositional 8 698 0.282 45 0.004 96 –12.9 0.8 1.6 1.5 age for the cobble conglomerate. 18 700 0.282 37 0.001 23 –23.3 –8.1 1.3 2.1 2 702 0.282 15 0.001 17 –20.5 –5.1 0.5 1.9 Lutetium-Hafnium Isotopic Analysis 7 704 0.282 86 0.001 41 –23.7 –8.5 3.1 2.1 16 705 0.282 24 0.001 15 –15.7 –0.3 0.9 1.6 3 710 0.282 25 0.002 51 –20.7 –5.8 0.9 1.9 Lutetium-hafnium isotopic analyses for 50 14 731 0.282 37 0.001 7 –17.2 –1.3 1.3 1.7 ca. 700 Ma zircon grains from samples 4JK09, 21 767 0.282 25 0.001 51 –15.6 1.1 0.9 1.5 ε 62JK09 62JK09, 63JK09, and 64JK09 yield Hf(i) values that range from +2 to −17 (Fig. 12; Table 3). The 16 689 0.282 164 0.001 56 –34.2 –19.4 5.8 2.8 17 696 0.282 15 0.001 12 –21.2 –5.9 0.5 1.9 volcaniclastic diamictite sample (04JK09), the 5 699 0.282 15 0.000 6 –19.9 –4.6 0.5 1.8 Oxford Mountain tufﬁ te, and lower diamictite 9 699 0.282 16 0.001 7 –17.4 –2.0 0.6 1.7 samples from Portneuf Narrows (62JK09 and 10 701 0.282 13 0.001 8 –15.1 0.2 0.5 1.5 6 704 0.282 19 0.000 7 –19.1 –3.6 0.7 1.8 63JK09) show considerable variation due to 13 704 0.282 14 0.001 2 –20.2 –4.7 0.5 1.8 xenocrystic and detrital mixing, whereas the 13 704 0.282 14 0.001 2 –19.7 –4.3 0.5 1.8 691 Ma volcanic clast, 64JK09, shows a narrow 11 705 0.282 13 0.001 3 –19.7 –4.2 0.5 1.8 − − 11 705 0.282 13 0.001 3 –19.1 –3.7 0.5 1.8 range of 7 to 10.5, reﬂ ecting a less dispersed 14 711 0.282 34 0.001 86 –19.1 –3.6 1.2 1.8 initial ratio for this single eruptive event. 7 712 0.282 16 0.000 6 –19.0 –3.3 0.6 1.8 2 713 0.282 18 0.001 9 –18.4 –2.7 0.6 1.7 8 723 0.282 18 0.002 26 –14.4 1.0 0.6 1.5

20 63JK09 DMT2 6 684 0.282 18 0.001 27 –18.0 –3.2 0.6 1.7 10 11 688 0.282 28 0.001 35 –16.0 –1.0 1.0 1.6 CHUR 16 692 0.282 17 0.001 47 –28.6 –13.6 0.6 2.4 0 13 696 0.282 30 0.001 9 –31.9 –16.7 1.0 2.6 64JK09 12 700 0.282 15 0.001 10 –28.8 –13.6 0.5 2.4 -10 1 707 0.282 20 0.001 30 –15.9 –0.7 0.7 1.6 Hf ε 9 714 0.282 19 0.001 20 –16.4 –0.8 0.7 1.6 -20 ε Hf(i) 15 720 0.282 23 0.001 65 –17.5 –1.7 0.8 1.7 5 722 0.282 16 0.001 5 –23.0 –7.1 0.6 2.0 -30 10 722 0.282 28 0.001 32 –29.9 –14.0 1.0 2.4 660 680 700 720 740 760 780 800 4 728 0.282 18 0.001 44 –14.0 1.8 0.6 1.5 Age (Ma) 2 786 0.282 19 0.001 8 –24.9 –7.7 0.7 2.1 ε 64JK09 Figure 12. Age (Ma) vs. Hf of 50 ca. 700 Ma grains from samples 4JK09, 62JK09, 63JK09, and 5 673 0.282 21 0.001 6 –22.4 –7.9 0.8 2.0 64JK09. Data are listed in Table 3. CHUR—Chon- 19 687 0.282 24 0.001 35 –22.2 –7.5 0.9 2.0 dritic uniform reservoir; DM—depleted mantle. 8 688 0.282 21 0.002 23 –23.8 –9.2 0.7 2.1 17 688 0.282 22 0.002 62 –23.2 –8.8 0.8 2.1 1 689 0.282 17 0.001 20 –24.1 –9.1 0.6 2.1 6 690 0.282 33 0.001 7 –23.0 –8.0 1.2 2.0 DISCUSSION AND INTERPRETATION 7 692 0.282 19 0.001 11 –22.0 –7.0 0.7 2.0 2 694 0.282 18 0.001 15 –24.9 –10.0 0.6 2.2 16 698 0.282 19 0.001 14 –23.4 –8.3 0.7 2.1 Pocatello Formation at Oxford Mountain 3 699 0.282 24 0.001 10 –25.6 –10.5 0.9 2.2 15 699 0.282 18 0.001 8 –23.3 –8.1 0.6 2.1 4 704 0.282 18 0.001 4 –23.0 –7.7 0.6 2.0 The gradational upper contact of the Ban- 176 –11 nock Volcanic Member and the intercalated Note: Data calculated using Lu decay constant of 1.865 × 10 (Soderlund et al., 2004). 176Hf/177Hf and 176Lu/177Hf of CHUR (chondritic uniform reservoir) values of 0.282785 and metabasalt fl ows at the base of the Scout Moun- 0.0336 (Bouvier et al., 2008). Present day depleted mantle values of 176Hf/177Hf and 176Lu/177Hf tain Member provide evidence that diamic- of 0.283225 and 0.0385 (Vervoort and Blichert-Toft, 1999) and the average crust 176Lu/177Hf value of 0.015 (Goodge and Vervoort, 2006). tite deposition was synchronous with terminal *Uncertainties on tDM are ± 0.1 Ga, at minimum. basaltic volcanism and suggest that basaltic volcanism was temporally related to silicic volcanism represented by felsic volcanic clasts and the Oxford Mountain tuffi te. be the key in separating exclusively rift-related quartzose clasts suggest glacial transport inland The diamictite below the tuffi te may have processes from glacial processes. The local of the rifted margin. Facies associations between been deposited under glaciomarine conditions, increases of mafi c clast components within the the two major facies types in this unit support a but no direct evidence was noted in this study. diamictite are interpreted to indicate uplift and glacial origin. The stratigraphic columns (Figs. Noting the variation in provenance between reworking of the underlying Bannock Volcanic 7B–7D) generally begin with a thick section of intrabasinal and extrabasinal diamictites may Member, whereas increases in basement and unstratifi ed diamictite topped by thinner, weakly

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stratifi ed units, as shown in Figure 8E (04JK09, deconvolve the range of grain ages present in also only tentatively mentioned to call atten- 16JK10, and 23JK10). We interpret this facies these samples. The SHRIMP results do, how- tion to the apparent younging-upward analyses. change to mark the transition from proximal- ever, overlap with CA-ID-TIMS data. Both the However, this pattern may be coincidental as it medial to medial-distal glacial deposition. We 709 Ma and the 686 Ma age groupings reported may be affected by sample size, the proportions interpret the thin green wavy laminations (Fig. by Fanning and Link (2004, 2008) and Con- of different age-groupings in the samples, and 8E) to represent tuffaceous horizons deposited don and Bowring (2011), and the 700 ± 5 Ma selection-related bias of grains. by suspension settling through the water column grouping reported by Keeley et al. (2010) are Three felsic porphyritic volcanic cobbles during a time of relative quiescence and possi- within the bounds of the bell-shaped portion of from the overlying cobble conglomerate mem- ble ice rafting. This period is followed by the younger 206Pb/238U ages in Figure 10. ber and upper diamictite yield ages of 691 ± 4 deposition of the upper Oxford Mountain tuffi te The clast compositions within samples Ma, 701 ± 4 Ma, and 717 ± 4 Ma (Fanning and (sample 16JK10). 13JK10 and 15JK10 demonstrate a mixed sedi- Link, 2004, 2008; this study), demonstrating The detrital zircon provenance analysis mentary and volcanic provenance, potentially that the period of silicic volcanic activity lasted on the diamictite and volcanic sandstone on reworking the Pocatello Formation. The sedi- at least 25 m.y. and up to 32 m.y., considering Oxford Ridge shows a “mixed Laurentian” mentary textures of these samples are interpreted the 685 Ma age of the Oxford Mountain tuffi te. signature (i.e., a sedimentary provenance with to represent mixing of pyroclastic and epiclastic Maximum depositional ages of the upper ages matching known ages of composite base- components in a subaqueous debris fl ow that diamictite and cap carbonate succession are per- ment provinces of Laurentia). The prominent was medial-proximal to the volcanic source. missively younger (youngest population 672 ± 1.6–1.7 Ga Paleoproterozoic peak (Fig. 9) sug- The Oxford Mountain tuffi te itself is inter- 6.3 Ma [FHDZ1], and 665.8 ± 5.7 Ma [FHDZ2]; gests that the dominant source of detritus was preted as a marine volcaniclastic deposit that LA-MC-ICP-MS ages; Dehler et al., 2009, the composite 1.6–1.9 Ga Mojave-Yavapai- was proximal to medial to its volcanic source, 2011). A 667 Ma reworked tuff overlies the cap Mazatzal Province (Stewart et al., 2001). The and redeposited by sediment gravity fl ows. This carbonate succession (Fanning and Link, 2004). ε granitic clast (24JK09) with a Paleoproterozoic proximal to medial interpretation is based on its The Hf(i) data from zircons from the mafi c- to Neoarchean age peak (2450–2700 Ma) sug- (1) abundant plagioclase crystals, (2) rare angu- clast–bearing diamictite at Portneuf Narrows gests that the granitic component in the basal lar basaltic pyroclasts, (3) round and angular (62JK09 and 63JK09) cluster between +2 and conglomerate and diamictite came from the volcanic epiclasts that depress laminae in under- −8. It is possible, given the high zirconium mea- underlying Farmington Canyon Complex, the lying volcanic siltstone, and (4) consistent, sured in geochemical samples from the Bannock Wyoming Province, and/or the Grouse Creek albeit mixed, zircon age populations from 705 Volcanic Member (Keeley, 2011), that some zir- block (Foster et al., 2006; Mueller et al., 2011). to 685 Ma. It is unclear whether rounded basal- cons may be from the basalt, potentially causing The 1.3–1.5 Ga peak is interpreted to be from tic clasts are primary (e.g., volcanic bombs) or the relatively juvenile signature. The two-stage the midcontinent Granite-Rhyolite Province have been rounded during aqueous transport. depleted mantle model age calculations for (Goodge and Vervoort, 2006, and references Based on these relationships, we interpret the the 50 zircons of known ca. 700 Ma age range therein). The small group of grains between 0.9 youngest zircon age grouping (685 Ma) from from 2.7 to 1.4 Ga, with peaks at 2.1 Ga and ε and 1.2 Ga may have come from the Grenville the Oxford Mountain tuffi te to be close to its 1.9–1.6 Ga (Table 3). The initial Hf values of orogen from eastern or southeastern Laurentia depositional age. all 50 zircons range from +2 to −17. The wide (Dickinson et al., 2010), perhaps transported by variation and weakly positive values suggest that a transcontinental river system (Rainbird et al., Pocatello Formation at Portneuf Narrows the igneous zircons may refl ect a mixing line 1992, 2012). Successful K-S tests on diamictite, between evolved and slightly juvenile sources. sandstone, and quartzite clast samples (73JK09, SHRIMP concordia ages from two mixed Relatively juvenile sources include some com- 74JK09, 75JK09, and 68JK09) suggest that the volcaniclastic diamictites and one sandstone ponents of Grenville-age (Mueller et al., 2007; zircon spectra are statistically indistinguishable. from the Portneuf Narrows area (Fig. 2), as well Petersson, 2010) and Mesoproterozoic A-type We interpret the Scout Mountain Member units as the youngest single-grain SHRIMP analy- granitic magma (Goodge and Vervoort, 2006), on Oxford Ridge to have a common provenance ses from each sample, young upward (Table and a component of the underlying Farmington and to have been deposited in a single basin. The 1), suggesting syndepositional volcanism. As Canyon Complex (Spencer et al., 2012). The ε provenance of the quartzite cobbles (74JK09 the underlying assumption of a weighted mean Hf(i) values from Pocatello Formation zircons ε and 03JK09) suggests that some of the zircon calculation requires a single age population, the overlap with Hf(i) values of the Paleoprotero- grains in the diamictite and sandstone may have SHRIMP concordia ages from detrital samples zoic Farmington Canyon Complex in northern been recycled out of the “mixed Laurentian” (62JK09, 63JK09, and 15PL08) are only tenta- Utah (Spencer et al., 2012), suggesting the zir- Uinta Mountain Group and Big Cottonwood tive. The youngest single-grain ages from sam- cons crystallized from magma that incorporated Formation to the southeast (Dehler et al., 2010). ples 62JK09, 63JK09, and 15PL08 are 689 ± 8 underlying Farmington Canyon Complex. The span of concordant SHRIMP ages from Ma, 684 ± 8 Ma, and 662 ± 16 Ma (or 676 ± the Oxford Mountain tuffi te (04JK09) indicates 8 Ma), respectively (Table 1). However, a mini- Correlation between Oxford Mountain that the zircons within the sample did not crys- mum of three zircon grains with identical ages and Portneuf Narrows tallize during a single volcanic eruption. One is required to be considered a single popula- discordant, high-U zircon with a Tertiary age is tion, and only two overlapping populations of The SHRIMP weighted mean age of 691 ± interpreted as either contamination or one that 699 Ma and 700 Ma occur in samples 62JK09 4 Ma (MSWD = 0.98) on the porphyritic vol- was reset during Mesozoic thrusting. The data and 63JK09. Due to the insuffi cient analytical canic clast (64JK09) from Portneuf Narrows is suggest recurrent Neoproterozoic felsic volca- precision of SHRIMP data (Fig. 11), attempts interpreted to represent a single eruptive age, nism, with other extraneous detrital zircons also to deconvolve the complex zircon populations thus providing a maximum age constraint for present. The ~1% (1σ) analytical uncertainty of are not considered to be viable. Therefore, the the cobble conglomerate unit. This age also the SHRIMP analyses is not precise enough to youngest single-grain SHRIMP analyses are provides a loose geochronologic correlation

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between the cobble conglomerate at Portneuf nian “Sturtian” glaciations described by Hoff- and Kominz, 1984). Stewart and Suczek (1977) Narrows and the slightly younger 685 Ma man and Li (2009). originally suggested rifting began around 650 Oxford Mountain tuffi te (Fig. 11). Below this None of the tuffaceous strata sampled thus Ma. Further subsidence analysis of Cambrian line of correlation, the age components in the far from the Scout Mountain Member repre- to Permian strata along the Cordillera, using lower diamictite of the Scout Mountain Mem- sents primary air-fall tuffs or volcanic fl ows. the backstripping approach, suggests Rodin- ber at Portneuf Narrows are comparable to Furthermore, the “cap carbonate” above the ian breakup and initiation of a continental drift the age range of 709–685 Ma from the volca- upper diamictite has been described by Corsetti phase at 600–550 Ma (Bond et al., 1983, 1985; niclastic rocks of Oxford Mountain, suggest- and Lorentz (2006) as having “Marinoan-style” Bond and Kominz, 1984; Armin and Mayer, ing a similar volcanic provenance. The lack characteristics (similar to those dated at 635 Ma 1983; Levy and Christie-Blick, 1991; Christie- of ≤667 Ma zircons in the lower diamictite at in South China and Namibia). Therefore, sev- Blick and Levy, 1989). The Re-Os age of 607.8 Portneuf Narrows supports a lithostratigraphic eral workers (Macdonald et al., 2010; Petterson ± 4.7 Ma from organic-rich transgressive shale correlation to the diamictite on Oxford Moun- et al., 2011) have suggested the possibility that of the Old Fort Point Formation (Kendall et al., tain. Such grains have only been documented the 685 Ma (and 667 Ma) zircons represent 2004) in the Horsethief Creek Group in south- above the lower diamictite (three such grains detritus deposited into strata tens of millions of eastern British Colombia supports this (Fig. 3). in 15PL08) at Portneuf Narrows. years later than eruption, during, for example, Felsic volcanism in southern Idaho spanned the 635 Ma Marinoan glaciations. 717 Ma to 667 Ma, perhaps 70 m.y. before initi- Regional Correlations The tuffaceous rocks are, however, proxi- ation of thermal subsidence at 600–550 Ma and mal and little-reworked, volcaniclastic depos- at least 25 m.y. after an earlier phase of amag- The CA-ID-TIMS maximum depositional its. Their deposition 50 m.y. after eruption is matic, intracratonic extension at ≤766 Ma to 742 age from the Oxford Mountain section overlaps unlikely, based on the poor preservation poten- Ma, recorded by strata of the Uinta Mountain with SHRIMP U-Pb ages of ca. 685 Ma obtained tial of silicic volcanoes after kilometer-scale Group (Dehler et al., 2010). Our new data sug- from the Edwardsburg Formation 415 km to the uplift and denudation of volcanic rifted margins gest that separation of Laurentia from its west- northwest in central Idaho (Lund et al., 2003). (Bryan et al., 2002). The age range of the Neo- ern counterpart in Idaho and Utah occurred no The 667 Ma maximum depositional age of the proterozoic population and the ages of the sub- earlier than 685 Ma, with the rift-drift transition reworked tuff above the pink dolomite at Port- populations within it (709, 702, 700, 695, 690, after 667 Ma and thermal subsidence initiating neuf Narrows correlates well with the recent and 685 Ma) are demonstrated in Table 2. Con- at 600–550 Ma. ca. 664 Ma U-Pb zircon ages from the Tuff of sidering the oldest 717 Ma volcanic clast (Fan- The “Missing-Link” model for Rodinian Daugherty Gulch and the syenite-diorite suite of ning and Link, 2004), volcanism in SE Idaho rifting (Li et al., 1995, 2008) suggests a fi rst Acorn Butte (Lund et al., 2010). The maximum may have lasted 20–32 m.y. A long-lived vol- stage of rifting between western Laurentia and depositional ages from the Pocatello Formation canic center, or numerous episodic ones, would South China as early as 750 Ma. This was based are consistent with magmatic quiescence in cen- be expected to develop some topographic relief on ages of 780–750 Ma from mafi c and felsic tral Idaho between ca. 650 Ma and ca. 500 Ma and be incised, leading to the delivery of syn- intrusions and volcanics in South China (Lee et suggested by Lund et al. (2010). The new rift, juvenile, volcanic debris to the deposi- al., 1998; Li et al., 2003; Lin et al., 2007) cor- SHRIMP data from the Portneuf Narrows section tional system. This would support our preferred related with those of the Franklin large igneous also overlap with lower and upper SHRIMP ages hypothesis involving syndepositional middle province (Harlan et al., 2003; Ernst et al., 2008, of 708 ± 5 Ma and 671 ± 5 Ma from the forma- Cryogenian volcanism. and references therein). Recent revisions to zir- tion of Perry Canyon in northern Utah (Balgord et Alternatively, after uplift and erosion, the roots con and baddeleyite ages from gabbroic sills al., 2010; Balgord, 2011). The new CA-ID-TIMS of the volcanic center may have been exposed and dikes of the Franklin large igneous province age of 685 Ma is also consistent with the 688 (Bryan et al., 2002) and therefore been subject to indicate an age of 716 Ma (Macdonald et al., +9.5/–6.2 Ma U-Pb zircon age from felsic volca- reworking, possibly resulting in the multimodal 2010), bringing the correlation into question. niclastic rocks from Gataga Mountain, northern xenocrystic zircon population documented in Whereas dike swarms may be representa- British Colombia (Ferri et al., 1999). However, this paper. This scenario would be consistent tive of crustal extension, only the sedimentary the Idaho and Utah ages are ~25 m.y. younger with the Petterson et al. (2011) claim that the record of rift basins gives a clear indication of than the older 716 Ma age constraints from the Scout Mountain diamictites are late Cryogenian mechanical rifting. Sedimentation and volca- Mount Harper Group (Rapitan Formation) of the (Marinoan). However, the diamictites are volcanism in rift basins in South China have been northern Canadian cordillera (Macdonald et al., niclastic rather than containing plutonic rocks, as dated between 820 Ma and 750 Ma (U-Pb zir- 2010), implying diachronous Cordilleran rifting would be expected from eroded roots of a volca- con SHRIMP ages), with a fi nal nonvolcanic and diachronous middle Cryogenian (Sturtian) nic center. Further high-precision CA-ID-TIMS phase between 750 Ma and 690 Ma (Wang and glacial episodes, as discussed next. work searching for younger zircon grains in the Li, 2003). Wang and Li (2003) interpreted the oldest glaciogenic strata is under way to test this 750–690 Ma rift phase as the rift-drift transi- Correlation to the Sturtian Glaciation alternative hypothesis. tion, consistent with a 90° counterclockwise rotation of the South China block at 750 Ma The new ages from the Oxford Mountain Rodinian Rifting along the North (Li and Evans, 2011). A “rift-to-drift” transition tuffi te place a maximum age constraint of 685 American Cordillera and rotation of South China at 750 Ma is not Ma above the diamictite on Oxford Ridge. compatible with ages from western Laurentia, The ages presented in this paper suggest that Basal Neoproterozoic volcanic- and diamic- casting doubt on the “Missing-Link” model of the lower diamictite at Portneuf Narrows and tite-bearing strata underlying the Cordilleran Li et al. (1995), unless an intervening rift block the diamictite on Oxford Ridge and the upper miogeocline have long been thought to either existed (Colpron et al., 2002). diamictite at Portneuf Narrows correlate with a represent protracted rifting before the rift-drift Colpron et al. (2002) proposed a two-stage late phase of the 715–660 Ma middle Cryoge- transition or a separate rift altogether (Bond rift model for the Cordillera from southeastern

146 www.gsapubs.org | Volume 5 | Number 1 | LITHOSPHERE

British Colombia to Utah, with the fi rst stage of components at ca. 709, 702, 700, 695, 690, and and intervening glaciation at ≥685–660 Ma may rifting at 750 Ma (revised to 716 Ma by Mac- 685 Ma. This suggests protracted volcanism be required. donald et al., 2010) to 700 Ma. A second stage spanning 709–685 Ma. Inclusion of the ca. of rifting at 600–570 Ma is suggested by ca. 717 Ma porphyritic silicic clast from Portneuf ACKNOWLEDGMENTS 600 Ma xenocrystic and solitary detrital grains Narrows (Fanning and Link, 2004) increases the throughout the Cordillera, a U-Pb zircon age of range to ~32 m.y., which is comparable to the This research was supported by the National Sci- ca. 570 Ma from a trachyandesite fl ow above a duration of silicic volcanism in rift settings last- ence Foundation grant EAR-0819759. Thanks rift-related unconformity in the Hamill Group ing ~40 m.y. (Bryan et al., 2002). The age com- go to Carol Dehler, Adolph Yonkee, Elizabeth in southeastern British Columbia (Colpron et ponents of the Oxford Mountain tuffi te overlap Balgord, Robert Mahon, Esther Kingsbury- al., 2002, and references therein), and a K-Ar with SHRIMP age components of ca. 705 Ma, Stewart, and Dawn Hayes for enabling trips to age of ca. 580 Ma from the trachytic Brown’s 700 Ma, and 688 Ma, from the lower diamictite other Neoproterozoic successions in the region Hole Formation in northern Utah, which over- of Portneuf Narrows, providing a correlation to and for many fruitful discussions. We gratefully lies incised valleys within Ediacaran quartzite the diamictite on Oxford Ridge. The ages also thank Lithosphere Science Editor John Goodge, (Christie-Blick and Levy, 1989). Our ages and overlap with the ca. 708 Ma age (SHRIMP) from Wolfgang Preiss, and the anonymous reviewers available data require a revision of the fi rst the formation of Perry Canyon in northern Utah for their comments. Supplemental information phase to 717–660 Ma; otherwise, our ages are (Balgord et al., 2010), supporting regional cor- (Tables DR1–DR2; Figs. DR1–DR3) is avail- consistent with this model. relations with the fi rst of two diamictite intervals able online at http://www.geosociety.org/pubs The 717–660 Ma age of volcanism demon- recognized by Crittenden et al. (1983). Indis- /ft2013.htm. strated in southeast Idaho correlates with simi- tinguishable detrital zircon U-Pb age spectra larly aged volcanics documented in East Antarc- between Fivemile Canyon and Oxford Ridge REFERENCES CITED tica (Goodge et al., 2002; Cooper et al., 2011). suggest deposition within a single rift basin with Goodge et al. (2002) reported a U-Pb zircon age a mixed Laurentian source. 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