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.