Chemostratigraphy indicates a relatively complete Late Permian to Early Triassic sequence in the western United States

Matthew R. Saltzman* and Alexa R.C. Sedlacek School of Earth Sciences, The Ohio State University, Columbus, Ohio 43210, USA

ABSTRACT Collinson et al. (1976) assigned these beds to the basal “Thaynes” but Although the latest Permian mass extinction and associated δ13C noted that the contact with the underlying Gerster was diffi cult to discern excursion are well documented from the Tethys Ocean, carbonate and lacked relief. The age of the laminated, fenestral, and microgastropod- rocks preserving these events in the eastern Panthalassic Ocean (west- rich beds is controversial. Collinson et al. (1976) and Carr (1981) reported δ13 ern Pangea) are unknown. We present Ccarb from the Gerster and the Triassic (Smithian) conodont Parachirognathus ethingtoni from these Thaynes (Permian and Triassic) Formations in the western United beds in the Confusion Range. However, Wardlaw and Collinson (1986) States and document a negative excursion with no evidence for major later assigned these same basal “Thaynes” beds to the uppermost “Ger- δ13 breaks in continuity. To further constrain the age of the Ccarb excur- ster,” consistent with conodonts they identify as the Permian genus Mer- sion in the absence of index fossils, we analyzed the same samples for rillina in the microgastropod-rich wackestone facies (B. Wardlaw, 2012, 87Sr/86Sr. When examining our new carbon and Sr data in the context personal commun.). Because Merrillina is likely ancestral to Parachirog- of biostratigraphy and sequence stratigraphy, we conclude that parts nathus (Orchard, 2007), taxonomic uncertainty regarding the timing and of the western United States may preserve carbonate successions that morphological defi nition of this transition can explain the discrepancy in span the latest Permian extinction. age assignments of these key beds. Above the microgastropod-rich beds, the lower Thaynes Formation consists of siltstone, mudstone, and inter- INTRODUCTION bedded brownish-gray limestone beds typical of the region and containing The latest Permian mass extinction and corresponding negative δ13C abundant Meekoceras ammonites of Smithian age (Carr, 1981; Collinson excursion are extensively documented in the ancient Tethys Ocean (Korte et al., 1976; Lucas and Orchard, 2007). and Kozur, 2010), but comparatively little is known about the Panthalassic Ocean. Although middle Panthalassa is preserved (Musashi et al., 2001) METHODS and siliciclastic environments of eastern Panthalassa (western Pangea) We sampled the Gerster and lowermost Thaynes Formations in the are known (Sperling and Ingle, 2006; Grasby and Beauchamp, 2008), lat- Confusion Range, Utah. Due to structural complications, the remainder est Permian carbonate shelf deposits of western Pangea have long been of the Thaynes was sampled at Spruce Mountain, Nevada (see Field Sam- thought to be absent at an unconformity (Collinson et al., 1976). However, pling Methods and Figure DR1 in the GSA Data Repository1). Micritic this conclusion has been questioned due to a lack of diagnostic fossils limestone was preferentially drilled for analysis using a Kiel-III device (Alvarez and O’Connor, 2002). coupled to the dual inlet of a Finnigan MAT 253 mass spectrometer. We examined δ13C and 87Sr/86Sr trends across the Gerster and Thaynes Analytical precision based on analyses of reference material NBS19 was Formations in the western United States to address whether age refi ne- ≤0.04‰. A δ13C-δ18O crossplot is inconsistent with major resetting of δ13C ment of these carbonate shelf deposits was possible using chemostratigra- (see Table DR1 and Fig. DR2). δ13 87 86 δ13 phy. An integrated C-Sr isotope study was necessary because C excur- For Sr/ Sr, we used the same rock samples studied for Ccarb. Pre- sions alone cannot identify the latest Permian due to multiple maxima treatment of all samples with ammonium acetate buffered to a pH of 8 was and minima through the Middle Permian to Early Triassic (Payne et al., followed by dissolution in 4% acetic acid to minimize leaching of Sr from 2004; Bond et al., 2010). Seawater 87Sr/86Sr rises continuously through noncarbonate phases (Montañez et al., 1996) (see Analytical Methods in this time (Korte et al., 2003) and therefore may constrain hypotheses of the Data Repository). 87Sr/86Sr was measured using dynamic multicollec- δ13 δ13 C correlations. Here we report a negative Ccarb excursion across the tion with a Finnigan MAT 261A thermal ionization mass spectrometer Gerster-Thaynes transition in the Confusion Range (Utah) and assess its at the Ohio State Radiogenic Isotope Laboratory (Columbus, Ohio). The likely age. value for the SRM 987 standard was 0.710242 ± 0.000010 (1σ external reproducibility) during the study. BIOSTRATIGRAPHY AND LITHOSTRATIGRAPHY OF GERSTER AND THAYNES FORMATIONS RESULTS δ13 The youngest conodont fauna from the Gerster Formation contains Ccarb is steady near +3.0‰ through most of the Gerster Formation Neogondolella bitteri and Neospathodus divergens (Wardlaw and Col- (Fig. 1). Near the top of the Gerster, at a distinct red sandstone bed, δ13C linson, 1978, 1986), and could be as old as the Wordian stage (Guada- values decline to −0.3‰ at the highest chert bed (Fig. 2). δ13C continues to lupian) or as young as Wuchiapingian (Lopingian) (Henderson, 1997). decline in overlying fenestral, laminated, and microgastropod-rich lime- In the Confusion Range of Utah, which contains the thickest section in stone beds to −2‰. In succeeding brownish-gray, ammonite-bearing lime- the region, the uppermost ~80 m of the Gerster above this N. bitteri–N. stone beds of the typical lower Thaynes Formation (containing Smithian divergens fauna contain no index fossils, and a minimum age is uncertain fossils), δ13C is shifted to values as light as −4‰. Above a thick covered (Wardlaw and Collinson, 1978). Near the top of this ~80 m interval, a suc- cession with several meters of red sandstone followed by limestone with unusually large chert nodules is unique to the Confusion Range. 1GSA Data Repository item 2013110, fi eld sampling and analytical meth- These unusual uppermost Gerster beds are overlain by an ~8-m-thick ods, Figure DR1 (study map), Figure DR2 (δ13C vs. δ18O), Figure DR3 (paleo- interval containing a succession of carbonate lithologies that is also unique geography), Figure DR4 (δ13C, P-T boundary), Figure DR5 (δ13C, end-Guada- to the Confusion Range, including a lower 2 m of laminated, fenestral lupian), Figure DR6 (δ13C, Early Triassic), Figure DR7 (87Sr/86Sr, P-T boundary interval), Figure DR8 (biostratigraphy, geochronology), Table DR1 (δ13C and limestone with coated grains, and 6 m of microgastropod-rich wackestone. δ18O), and Table DR2 (87Sr/86Sr, Sr ppm), is available online at www.geosociety .org/pubs/ft2013.htm, or on request from [email protected] or Documents *E-mail: [email protected]. Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

GEOLOGY, April 2013; v. 41; no. 4; p. 399–402; Data Repository item 2013110 | doi:10.1130/G33906.1 | Published online 7 February 2013 ©GEOLOGY 2013 Geological | April Society2013 | ofwww.gsapubs.org America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 399 Confusion Range - Spruce Mountain, Great Basin, U.S.A. bio Sr all 87/86Sr ooids laminations an 0.70802

thi chert

n (i) ntain n a a echinoderms Sp

kia 0.70797 (h) ithi 50 m es Fm. ne bryozoans e ian Sm Triassic Smithian ammonites Ol ruce Mou p Thaynes Fm. Thayn S microgastropods Smith 0.70770 (g) ? see Fig. 2 mostly covered r.s.s. 0.70768 (f) shale-packstone limemud- nduan

pian ammonites 0.70766 (e) wackestone u e te I sic (Induan) brachiopods a fenestral limestone

ias recrystallized an l

shale and packstone h lime mudstone t

oolitic, fenestral rly Tr red sandstone a Permian

limestone, packstone E nger skeletal packstone Gerster Fm. red sandstone (r.s.s.) Confusion Rang latest Permian cherty limestone No you extinction horizon? No older than Guadal m. F 0.70743 (d)

-6 -4 -2 0 2 4 6 8 n 0.70735 (c) δ13C (‰ VPDB) Gerster

δ13 0.70730 (b) Figure 1. Ccarb for Confusion Range–Spruce Mountain composite (western United States) (see Table DR1 in the Data Repository [see 5m

footnote 1]). Brachiopods and conodonts suggest an age of the Ger- latest Permian ster Formation (lower ~150 m) no older than mid-Guadalupian. Am- sea level monites and conodonts indicate a Smithian to Spathian (Olenekian) lowstand age for the Thaynes Formation (see text and the Data Repository). Fm.—Formation. 0.70724 (a) No younger than early Indua -4 -2 0 2 interval, a positive excursion to +7.5‰ occurs in association with Spath- δ13C (‰ VPDB) ian fossils (Carr, 1981). 87 86 δ13 Sr/ Sr increases from 0.7072 in the upper Gerster to 0.7080 in the Figure 2. Plot of Ccarb for Gerster-Thaynes transition interval (shaded lower Thaynes (Fig. 3). We take a conservative approach and assume that interval in Fig. 1) in the Confusion Range (Utah, United States) and at 87 86 87 86 Spruce Mountain (Nevada). Sr/ Sr samples (tick marks at individual all of our Sr/ Sr values have been shifted to more radiogenic (higher sample horizons) are our least radiogenic values measured and pro- 87 86 Sr/ Sr) values to some degree by diagenesis (indicated by low Sr con- vide minimum age estimates for δ13C changes (see Fig. 3, and the Data centrations; see Table DR2 and Fig. DR7), and therefore report here only Repository [see footnote 1]). Black fi lled circles—Confusion Range the least radiogenic values to estimate minimum relative ages (e.g., Veizer, δ13C; gray circles—Spruce Mountain δ13C; Fm. —Formation. “Bio” and 1989). This method of relative age dating using 87Sr/86Sr is particularly “Sr” columns on left represent age constraints from biostratigraphy and Sr isotopes; “All” column is best age estimate using all avail- useful for this time period because of the large monotonic rise in the able data. Latest Permian extinction horizon is interpreted to lie within global seawater curve from ~0.7069 at the Middle Permian–Late Permian negative δ13C excursion near transition from cherty to fenestral lime- (Guadalupian-Lopingian) boundary to 0.7082 at the end of the Early Tri- stone. For more information on biostratigraphy and geochronology, assic (Martin and Macdougall, 1995; Korte et al., 2003, 2004; Twitchett, see Figure DR8 in the Data Repository. 2007). For example, because the Permian-Triassic (P-T) boundary is at ~0.7072 on the seawater curve, we assume that a sample with an 87Sr/86Sr of 0.7072 could date to any part of the Late Permian (with an earliest Lop- Xie et al., 2007; Korte et al., 2004; Payne et al., 2004; Horacek et al., 2007; ingian age requiring alteration by ~+0.0003) but is unlikely to be younger Yin et al., 2007; Bond et al., 2010; Tong and Zhao, 2011; Huang et al., than the P-T boundary. A post P-T boundary age would require the origi- 2012) indicate signifi cant negative excursions at the end-Guadalupian, lat- nal seawater value to have been altered to lower 87Sr/86Sr, which, while est Permian, or middle Olenekian (Smithian) that can all potentially cor- possible in the presence of nonradiogenic Sr sources such as volcanic ash relate to the Confusion Range (see Figs. DR4–DR7). Here we examine age from the McCloud Arc (Miller, 1987), is not supported by a lack of ben- constraints on δ13C based on our new 87Sr/86Sr, and then consider possible tonites in the area. ages in the context of sequence stratigraphy and facies analysis.

DISCUSSION 87Sr/86Sr Stratigraphy δ13C declines from +2.0 in the uppermost cherty beds of the Gerster Making the assumption that secondary alteration typically results in Formation to −2.0‰ in the overlying fenestral limestone sequence with increases in 87Sr/86Sr to ratios more radiogenic than the initial primary sea- no major discontinuities or offsets (Figs. 1 and 2). Although we interpret a water value (due largely to contamination by radiogenic strontium from steady δ13C decline as a refl ection of relatively continuous sedimentation, siliciclastic components; Veizer, 1989), we utilize least radiogenic values coarse biostratigraphic control and uncertainty in ranges of conodonts Mer- in our section to provide minimum age estimates on the timing of the rillina and Parachirognathus allow for the excursion to be as old as Middle δ13C excursion. In the Confusion Range, the least radiogenic 87Sr/86Sr near Permian (late Guadalupian) or as young as Early Triassic (Smithian). Pub- the onset of the negative δ13C excursion is 0.70730 (position b in Fig. 2). lished Middle Permian to Early Triassic δ13C curves for the Tethys (e.g., Because 0.70730 falls between best estimates of seawater Sr for the base

400 www.gsapubs.org | April 2013 | GEOLOGY 0.7082 δ13 i Biostratigraphically constrained Twitchett (2007) Dolomites negative C excursion in the Confusion Range (Figs. 2 and 3) because 87 86 a Correlated to Smithian Twitchett (2007) Meishan this would require assumptions about the Sr/ Sr of diagenetic fl uids i δ13 0.7080 Cexcursion Korte et al. (2003) and water-rock interaction pathways (Veizer, 1989). If we are correct that h a Correlated to P-T boundary 87 86 δ13C excursion Korte et al. (2004) Sr/ Sr stratigraphy precludes correlation of the Confusion Range excur- a Correlated to Guadalupian Martin & Macdougall (1995) sion with a negative δ13C excursion in the Early Triassic (Smithian) of the 0.7078 δ13 C excursion Tethys (see Figs. DR4–DR7), two remaining Permian possibilities exist. g g g fe f ef The fi rst is correlation with the well-known, global negative δ13C excur- 0.7076 sion associated with the latest Permian extinction, and the second is cor- 87 86 Sr/ Sr relation with an end-Guadalupian negative δ13C excursion described from d d d 0.7074 southern China (Bond et al., 2010; although see Chen et al., 2011). c c c b b b 0.7072 a a a Sequence Stratigraphy and Facies Analysis Both the end-Guadalupian and latest Permian negative δ13C excursions occur near prominent sequence boundaries (Yin et al., 2007; Bond et al., 0.7070 2010). However, while the end-Guadalupian sequence boundary postdates the δ13C minimum in the Jinogondolella prexuanhanensis–J. xuanhanensis 0.7068 conodont zones (Bond et al., 2010), the latest Permian sequence boundary ltuda. prexu. waag. prae. pakist. a isa.-ca. chan.

parvus 13 dieneri predates the δ C minimum in the Hindeodus praeparvus zone (e.g., Wignall Olenekian Induan Lopingian Guadalup. (Capit.) et al., 2009). In the Confusion Range, a prominent lowstand in sea level is Early Triassic Late Permian Middle Permian known from the upper Gerster Formation (Fig. 2) (Wardlaw and Collinson, 247.1 250 252.2 Age (Ma) 259.8 265.1 1978). This lowstand is represented by several meters of red-colored, high- angle cross-bedded, fi ne-grained sandstone recognized by previous workers Figure 3. 87Sr/86Sr data in relation to global seawater curve (solid line) modifi ed from Twitchett (2007) and Korte et al. (2003). Stratigraphic as a persistent marker bed above the thick (~200 m) open-marine, cherty positions of our new Sr points a–i are shown in Figure 2. Because of limestone of the Gerster (Collinson et al., 1976) (Figs. 1 and 2). Because the lack of good biostratigraphic control for points a–g, these were the lowstand predates the δ13C minimum in the Confusion Range, this is assigned ages (conodont zones) indirectly by correlating δ13C trends consistent with a latest Permian but not an end-Guadalupian age. in the Confusion Range (Utah, United States) (Fig. 2) with Tethyan sections that contain both conodonts and δ13C (see Figs. DR4–DR7 in Although not age-diagnostic, the facies succession associated with 13 the Data Repository [see footnote 1]). Circles, squares, and hexagons the δ C minimum in the Confusion Range shares similarities with those depict three alternative scenarios for correlation of the negative δ13C described from Tethyan sections that span the latest Permian δ13C excur- excursion in the Confusion Range with ages of three negative δ13C ex- sion and mass extinction. For example, the latest Permian extinction in cursions in the Tethys (see text for discussion). In contrast to Sr sam- many parts of the world is marked by disappearance of chert (Hender- ples a–g, samples h and i (triangles) are dated directly to Olenekian (Smithian) zones (see the Data Repository). Our 87Sr/86Sr data provide son, 1997; Isozaki, 1997; Wignall and Newton, 2003; Sperling and Ingle, minimum age estimates if we assume alteration to more radiogenic 2006) and in the Tethys by appearance of unique biofacies or lithofacies (higher) values during diagenesis; thus, the scenario depicted by including inorganically precipitated carbonate (Pruss et al., 2006; Groves the squares (Permian-Triassic [P-T] boundary interval) is considered et al., 2007; Kershaw et al., 2007; Algeo et al., 2007). In the Confusion more likely than the circles (Olenekian) (see text for discussion). An end-Guadalupian correlation (hexagons) cannot be ruled out using Sr Range, the last beds of cherty limestone of the Gerster Formation are 13 alone, but is viewed as unlikely based on stratigraphic context (see overlain by strata that record the δ C minimum within a succession of text). We also cannot rule out that samples a–d are separated from dominantly fenestral, laminated stromatolitic, and oolitic facies (Figs. 2 samples e–g by a hiatus, although this is not supported by the δ13C and 3) (although thin sections so far do not confi rm microbialites such as δ13 continuity (Fig. 2). Only conodont zones with negative C excur- those recorded in postextinction strata in the equatorial Tethys; S. Ker- sions and only published latest Permian–Early Triassic Sr are shown. Conodont zones: waag.—waageni; pakist.—pakistanensis; isa.-ca.— shaw, 2012, personal commun.). In addition, the lack of larger gastropod isarcica and carinata; chan.—changxingensis; prae.—praeparvus; species in beds of the δ13C minimum (Fig. 2) is consistent with a latest prexu.—prexuanhanensis-xuanhanensis; altuda.—altudaensis. Gua- Permian age, but less consistent with Guadalupian, which is dominated by dalup.—Guadalupian; Capit.—Capitanian. Age separation between larger gastropods in the western United States (Fraiser and Bottjer, 2004). individual samples within zones (x axis) is schematic. See Figure DR8 in the Data Repository for absolute ages and zones. IMPLICATIONS The evidence presented here taken as a whole suggests that the nega- δ13 tive Ccarb excursion in the Confusion Range is latest Permian, which Triassic (0.7071–0.7072) and middle Induan (~0.7074) (Korte et al., 2004; would represent the fi rst documentation of carbonates of this age in Twitchett, 2007), this suggests that the onset of the negative δ13C excursion western Pangea assumed to be missing at an unconformity (Fig. DR3). can be no younger than early Induan (Figs. 2 and 3). 87Sr/86Sr of 0.70766 Continuity of deposition across the latest Permian extinction interval in from a younger sample within the δ13C minimum (position e in Figs. 2 and the western United States is consistent with global transgression (Yin et 3) is less than the 0.70774 from the latest Induan N. pakistanensis conodont al., 2007). The magnitude of the δ13C excursion in the Confusion Range zone (Martin and Macdougall, 1995; Twitchett, 2007). Thus, based on con- (reaching values as light as −2‰) is greater than most equatorial Tethyan straints from 87Sr/86Sr (Fig. 3) we conclude that the negative δ13C excursion sections but similar to higher-latitude, outer-shelf sections in the Neote- δ13 in the Confusion Range can be no younger than Induan (see Figs. DR4– thys (Korte and Kozur, 2010) (Fig. DR4). This relatively light Ccarb min- DR7). This is consistent with the conodont sample in Wardlaw and Collin- imum in eastern Panthalassa may refl ect proximity of 12C-enriched, deep son (1986) that contains Merrillina sp. in the microgastropod-rich beds in anoxic water masses (e.g., Isozaki, 1997; Wignall and Newton, 2003). the Confusion Range (their top of the Gerster Formation). Ammonites and Lastly, the large positive δ13C shift to +7.5‰ in the Spathian part of the conodonts in the overlying Thaynes Formation indicate Smithian ages, in Thaynes Formation at Spruce Mountain confi rms the signifi cance of this agreement with our 87Sr/86Sr (0.70797 for point h in Figs. 2 and 3). particular event (Hauser et al., 2001; Tong and Zhao, 2011; Huang et al., Although 87Sr/86Sr stratigraphy is useful in providing minimum 2012) (see Fig. DR6) and more generally the global large Early Triassic age estimates, it is diffi cult to confi dently place a maximum age on the carbon cycle perturbations (Payne et al., 2004).

GEOLOGY | April 2013 | www.gsapubs.org 401 ACKNOWLEDGMENTS geochemistry and biostratigraphy across the Permian/Triassic boundary in We thank B. Wardlaw, J. Collinson, T. Carr, R. Paull, W. Sweet, C. Cowan, Abadeh, Iran: International Journal of Earth Sciences, v. 93, p. 565–581. and B. Pratt for discussions. We thank P. Marenco, E. Sperling, S. Kershaw, and Lucas, S.G., and Orchard, M.J., 2007, Triassic lithostratigraphy and biostratigra- anonymous reviewers for constructive comments. J. Linder, K. Foland, A. Howard, phy north of Currie, Elko County, Nevada, in Lucas, S.G., and Spielmann, Y. Matsui, and A. Grottoli at The Ohio State University, and G. Cane at the Uni- J.A., eds., Triassic of the American West: New Mexico Museum of Natural versity of Kansas, helped with analyses. A Geological Society of America research History and Science Bulletin 40, p. 119–126. grant provided funds. 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