Silurian 13C Stratigraphy

Silurian ␦13C stratigraphy: A view from North America

Matthew R. Saltzman* Department of Geological Sciences, Ohio State University, Columbus, Ohio 43210, USA

ABSTRACT stones drilled from fresh rock surfaces (Saltz- Three positive shifts in ␦13C are recognized in well-dated carbonate successions in North man et al., 2000a, 2000b). Powders were America and can be con®dently correlated with shifts in the Baltic region by using co- roasted under vacuum at 380 ЊC to remove -nodont and graptolite zonations. The ␦13C excursions reach distinct peaks of ؉3½ to volatile contaminants, and samples were re ؉4½ at the ends of the amorphognathoides and siluricus conodont zones and within the acted with 100% phosphoric acid at 75 ЊCin ludensis graptolite zone. The three events are associated with the most signi®cant extinc- an online carbonate preparation line connected tion horizons in the Silurian and occur near clastic-carbonate transitions that mark eu- to a Finnigan Mat 251 mass spectrometer. The static or oceanographic changes. The magnitudes of the two oldest ␦13C peaks compare analytical precision based on duplicate anal- well with the Baltic region; however, the end-siluricus excursion reaches far greater values yses and on multiple analyses of NBS19 was .½of as much as ؉11½ in Sweden, likely re¯ecting local modi®cation of the global seawater Յ0.04 signature.

Keywords: Silurian, carbon isotope, Oklahoma, Nevada, sea level, extinction. Highway 77 Section The Highway 77 road cut in the Arbuckle INTRODUCTION (Berry and Murphy, 1975; Klapper and Mur- Mountains of Oklahoma provides nearly com- Over the past ®ve years, an impressive phy, 1975), and the Highway 77 section is plete exposures of Silurian strata in parts of string of independent publications has docu- among the most densely sampled and well- the Cochrane, Clarita, and Henryhouse For- mented large ¯uctuations in carbonate carbon studied Silurian conodont successions in the mations (Fig. 2). The Cochrane consists of 13 isotope (␦ C) values in the Wenlockian and world (Barrick and Klapper, 1976, 1992). As skeletal wackestone and packstone that yield Ludlovian Series of the Silurian in Europe a result of this richness of biostratigraphic in- conodonts of the celloni zone. The Clarita is (Wenzel and Joachimski, 1996; Kaljo et al., formation, both sections ®gure prominently in divided into the Prices Falls Member, a thin 1997; Bickert et al., 1997; Azmy et al., 1998; the development of the Global Composite shaly unit containing the distinctive Pteros- Wigforss-Lange, 1999). The three positive Standard for the Silurian Period using the sta- pathodus amorphognathoides zone conodont ␦13C excursions apparent in these studies are tistical method of graphic correlation (Kleff- fauna, and an upper Fitzhugh Member, a skel- not distributed randomly within the Silurian ner, 1995); in addition, the Highway 77 sec- etal wackestone and argillaceous mudstone successions, but instead appear to be closely tion serves as the primary source of unit containing faunas of the ranuliformis, linked with the major biotic, climatic, and eu- information in the Silurian 87Sr/86Sr curve amsdeni, stauros, and crassa zones. The Hen- static events of the time period (Quinby-Hunt (Ruppel et al., 1996). The biostratigraphic and ryhouse consists of marlstone and wackestone and Berry, 1991; Johnson, 1996; Barrick, geochemical framework in place in Nevada 1997; Berry, 1998; Jeppsson, 1998). A close and Oklahoma allows for the highest possible that yield faunas of the crassa, ploeckensis, correlation between the ␦13C excursions and con®dence level in making direct comparisons siluricus, snajdri, eosteinhornensis, and detor- 13 positive shifts in ␦18O is also observed in bra- with the previously examined Baltic succes- ta zones. The ␦ C values increase from Ϫ2½ chiopod studies, compelling some to conclude sions and provides evidence for the global sig- in the lower Cochrane to ϩ4½ in the Clarita, that glacial episodes likely occurred later in ni®cance of Silurian ␦13C shifts. before decreasing steadily to a low in the low- the Silurian than has been revealed by the geo- ermost Henryhouse (Fig. 2). They increase logic evidence (e.g., Kump et al., 1999). How- SILURIAN ␦13C DATA again to a distinct peak of ϩ3.5½ at 5 m ever, because the ␦13C shifts are thus far well In order to generate high-resolution curves, above the base of the Henryhouse. Values then documented only from the Baltic region, their ␦13C values were derived from micritic lime- decrease to a new baseline of ϳϩ1½ and are global signi®cance and causal links with climatic and biotic events of the Silurian world remain intriguing yet untested possibilities. The purpose of this paper is to present the ®rst high-resolution carbonate ␦13C curves from the Silurian of North America that can be tied to recognized faunal boundaries and sea-level changes worldwide by using conodont and graptolite biostratigraphic zonations. The samples come from two of the most well- dated and continuously exposed Silurian sections in North America: Pete Hanson Creek II in central Nevada and Highway 77 in southern Oklahoma (Fig. 1). These sections are com- plementary in the sense that Pete Hanson Creek II represents one of the thickest Silurian sequences worldwide in which both conodont and graptolite faunas have been described Figure 1. Map (after Witzke, 1990) showing positions of Pete Hanson Creek II section in central Nevada and Highway 77 section in southern Oklahoma, and previously studied *E-mail: [email protected]. localities in Europe discussed in text. Shading is land, triangle pattern is mountains.

᭧ 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400. Geology; August 2001; v. 29; no. 8; p. 671±674; 4 ®gures; Data Repository item 2001081. 671 Figure 2. Integrated ␦13C and conodont biostratigraphy (Barrick and Klap- per, 1976, 1992) and generalized measured section for Highway 77 section. Ar- rows point to end-siluricus and end-amorphognathoides zone ␦13C excursions.

notably stable for the remainder of the Lud- vada provide the ®rst evidence that the Silu- lationship of globally traceable graptolite and lovian and Pridolian1. rian Period in North America was character- conodont biostratigraphic zones to demon- ized by three carbonate ␦13C excursions, the strate the intercontinental correlation of these Pete Hanson Creek II Section same number of events recognized in Europe. ␦13C peaks (Fig. 4) and serves to underscore A thick late Llandoverian through Pridolian The following discussion utilizes the interre- the close temporal link between the excursions section (Roberts Mountains Formation) is exposed along Pete Hanson Creek in the Roberts Mountains of central Nevada and exhibits a unique association of graptolites and conodonts (Berry and Murphy, 1975; Klapper and Murphy, 1975). The Roberts Mountains For- mation consists of argillaceous wackestone and marlstone, and minor pelletal grainstone and skeletal packstone. Unlike the condensed and easily accessible Highway 77 section, dis- continuous biostratigraphic sampling at Pete Hanson Creek precludes precise placement of zonal boundaries. Nonetheless, key zonal in- dicators are present and allow for correlation with Highway 77 and the global standard (Fig. 3). The ␦13C values reach three distinct positive intervals of ϳϩ3½ in the lower, middle, and upper parts of the Roberts Mountains For- mation. The peaks appear to interrupt a steadily rising ␦13C baseline, from Ϫ1½ near the base of the formation to ϩ1½ at the top (Fig. 3) (see footnote 1).

DISCUSSION The combined data from Highway 77 in Oklahoma and Pete Hanson Creek II in Ne-

1GSA Data Repository item 2001081, ␦13C and Figure 3. Integrated ␦13C and conodont-graptolite biostratigraphy (Berry and Murphy, 1975; ␦18O data, is available on request from Documents Klapper and Murphy, 1975) and generalized measured section for Pete Hanson Creek II Secretary, GSA, P.O. Box 9140, Boulder, CO section. Arrows point to end-siluricus, ludensis, and end-amorphognathoides zone ␦13C ex- 80301-9140, [email protected], or at www. cursions. Question marks at boundaries of siluricus zone indicate uncertainty in fossil range geosociety.org/pubs/ft2001.htm. data and zonal determinations.

672 GEOLOGY, August 2001 Figure 4. Correlation of Silurian ␦13C shifts between North America and Europe using compiled curve of Azmy et al. (1998), which is based primarily on brachiopod calcite data from Gotland, Sweden. and the three major biological and eustatic distinct faunal boundary marking the most 1996), increased organic matter burial (Azmy events of the Silurian. signi®cant conodont extinction event in the et al., 1998), and advection of 13C-enriched SilurianÐknown as the Ireviken (end-amor- surface waters (Bickert et al., 1997). Ireviken (End-amorphognathoides) Event phognathoides) event (Talent et al., 1993; The oldest of the three Silurian ␦13C ex- Jeppsson, 1998)Ðwhich also marks turnover Late Wenlockian (ludensis) Event cursions can be shown to correlate between among graptolites, brachiopods, and trilobites Evidence for a late Wenlockian ␦13C ex- North America and Europe on the basis of (Chatterton et al., 1990). The Ireviken event cursion in North America comes from the Pete strata bearing the distinctive faunal elements coincides with a profound change in the pat- Hanson Creek section in Nevada, where a of the amorphognathoides conodont zone tern of sedimentation, from the shale-domi- shift to ϳϩ3½ is dated to the ludensis grap- (Fig. 4). In North America, the rising limb of nated units of the amorphognathoides zone tolite zone (Fig. 3) (Berry and Murphy, 1975). the positive ␦13C shift is dated to the amor- worldwide to the pure carbonates of the over- In Europe, sections in Gotland and the Anglo- phognathoides zone by collections containing lying ranuliformis zone, that has been inter- Welsh basin (Cor®eld et al., 1992) show a (1) P. amorphognathoides along with the zon- preted to re¯ect the largest magnitude eustatic seemingly broader Wenlockian ␦13C excursion ally restricted Pterospathodus procerus and event of the entire Silurian (Johnson, 1996; spanning the ludensis zone as well as the over- Ozarkodina hadra in the Prices Falls at High- Barrick, 1997). This lithologic change and co- lying and underlying lundgreni, nassa, and way 77 (Fig. 2) (Barrick and Klapper, 1976), nodont extinction event are unambiguously re- nilssoni zones (Fig. 4), and a section in Es- and (2) O. hadra in the lower part of the Rob- ¯ected in southern Oklahoma at the transition tonia records a shift that is no older than the erts Mountains Formation at Pete Hanson between the shaly Prices Falls Member of the nassa zone (Kaljo et al., 1997). The shift is Creek (Fig. 3). In Europe, a correlative shift Clarita Formation and the pure carbonates of truncated or missing in southern Oklahoma at is dated to the amorphognathoides zone based the Fitzhugh Member (Fig. 2). Jeppsson a cryptic disconformity (Fig. 2; Barrick, on specimens of the name giver along with P. (1998) proposed an alternative hypothesis in 1997). Despite signi®cant uncertainties in the procerus in the lower Riga Formation of Es- which the drop-off in shale deposition re¯ects timing and anatomy of this Wenlockian ␦13C tonia (Kaljo et al., 1997; Loydell et al., 1998) a change from humid to dry climatic condi- event that preclude a more detailed consider- and in the base of the Upper Visby Formation tions rather than eustasy. It would thus appear ation at this time, it appears to share important in Gotland, Sweden (Jeppsson, 1998; Bickert that the trigger for the extinctions and asso- similarities with the Ireviken event in that the et al., 1997). Thus, within the current level of ciated ␦13C excursion was related to a eustatic late Wenlockian marks a major faunal crisis biostratigraphic resolution, the onset of the highstand or the transition to a humid climate, when the graptolites nearly became extinct ␦13C excursion is synchronous between North both of which could alter oceanic circulation (Berry, 1998). America and Europe, and perhaps Australia patterns suf®ciently to bring anoxic, nutrient- (Talent et al., 1993). laden waters into the habitat zones of cono- Pentamerid (End-siluricus) Event The ␦13C excursion that began in the amor- donts and graptolites (Quinby-Hunt and Ber- The youngest of three ␦13C excursions in phognathoides zone peaks at ϳϩ4½ in the ry, 1991), and increase ␦13C through some the Silurian peaks at values near ϩ4½ in the overlying ranuliformis zone in Oklahoma, combination of enhanced oceanic surface- middle Ludlovian of North America and is as- Gotland, and Estonia, and thus cuts across a water production (Wenzel and Joachimski, sociated with the third major faunal event of

GEOLOGY, August 2001 673 the Silurian known as the Pentamerid, or end- columns. M. Murphy provided guidance in the Rob- isotope event markers through the Wenlock- siluricus, event (Talent et al., 1993). The ␦13C erts Mountains. The manuscript was improved by Pridoli sequence at Ohesaare (Estonia) and comments from W.B.N. Berry, M. Kleffner, and S. 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(Kump et al., 1999; Saltzman et al., 2000a, biogeography: Geological Society [London] Johnson, M.E., 1996, Stable cratonic sequences and Memoir 12, p. 57±73. 2000b). a standard for Silurian eustasy, in Witzke, B.J., et al., eds., Paleozoic sequence stratigraphy: Manuscript received December 8, 2000 ACKNOWLEDGMENTS Views from the North American craton: Geo- Revised manuscript received March 19, 2001 J. Barrick and G. Klapper provided helpful dis- logical Society of America Special Paper 306, Manuscript accepted April 9, 2001 cussion and guidance to the ®eld area in Oklahoma, p. 203±311. as well as copies of unpublished biostratigraphic Kaljo, D., Kiipli, T., and Martma, T., 1997, Carbon Printed in USA

674 GEOLOGY, August 2001