Potential Global Standard Stratotype-section and Point (GSSP) for a Cambrian stage boundary defined by the first appearance of the trilobite Ptychagnostus atavus, Drum Mountains, Utah,

Potential Global Standard Stratotype-section and Point (GSSP) fora Cambrian stage boundary defined by the first appearance of the

trilobite Ptychagnostus atavus, Drum Mountains, Utah, USA

GSSP potentielle de l’étage cambrien définie par la première apparitiondu trilobite Ptychagnostus atavus, Montagnes Drum, Utah, EU

Loren E. Babcock a,*, Margaret N. Rees b, Richard A. Robison c,Elizabeth S. Langenburg d, Shanchi Peng e

a Department of Geological Sciences, Ohio State University, 125, South Oval Mall, Columbus, OH 43210, USAb Department of Geosciences, University of Nevada, Las Vegas, Las Vegas, NV 89145, USA

c Department of Geology, University of Kansas, Lawrence, KS 66045, USAd Department of Geology, Utah State University, 4505 Old Main Hill, Logan, UT 84322, USA

e Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing 210008, China

Received 20 September 2002; accepted 18 March 2003

Abstract

The base of the Ptychagnostus (or Acidusus) atavus Zone is one of the most clearly recognizable horizons on an intercontinental scale inthe Cambrian System, and would serve as an excellent position for the base of a new stage-level chronostratigraphic subdivision. Amongwell-exposed, readily accessible sections in Laurentia, the “Stratotype Ridge” section, Drum Mountains, western Utah, USA, is perhaps themost suitable for a Global Standard Stratotype-section and Point (GSSP) defined by the first appearance datum (FAD) of the cosmopolitanagnostoid trilobite P. atavus. In the “Stratotype Ridge” section, the FAD of P. atavus occurs near the base of a calcisiltite bed 62 m above thebase of the Wheeler Formation. A position corresponding closely to this horizon can be recognized with precision in Gondwana, Siberia,Kazakhstan, and Baltica using a combination of stratigraphic tools, the most useful of which are trilobite biostratigraphy, conodontbiostratigraphy, and sequence stratigraphy. Brachiopod biostratigraphy and chemostratigraphy provide general constraints on the position ofthe horizon intercontinentally.© 2004 Elsevier SAS. All rights reserved.

Résumé

La base de la Zone à Ptychagnostus (ou Acidusus) atavus est un des horizons les plus clairement reconnaissables du système cambrien àl’échelle intercontinentale. Il serait un excellent repère pour une nouvelle subdivision biostratigraphique à l’échelle de l’étage. Parmi lescoupes bien exposées et directement accessibles de la Laurentia, celle de « Stratotype Ridge », Drum Mountains, Utah occidental, EU, est sansdoute la plus adaptée pour servir de GSSP (Global Standard Stratotype – section and Point) défini par la première apparition (FAD) du trilobiteagnostoïdé cosmopolite P. atavus. Dans la coupe « Stratotype Ridge », le FAD de P. atavus se trouve près de la base d’un lit de calcisiltite, à62 m au-dessus de la base de la Formation Wheeler. Cet horizon peut être corrélé étroitement avec des horizons équivalents dans le Gondwana,la Sibérie, le Kazakhstan et Baltica, par l’utilisation combinée d’outils stratigraphiques, les plus utiles étant la biostratigraphie des trilobites,la biostratigraphie des conodontes et la stratigraphie séquentielle. La biostratigraphie des brachiopodes et la chemostratigraphie fournissentdes précisions générales sur la position de l’horizon à l’échelle intercontinentale.© 2004 Elsevier SAS. All rights reserved.

Keywords: Cambrian; Chronostratigraphy; GSSP; Trilobite; Laurentia; Utah

Mots clés : Cambrien ; Chronostratigraphie ; GSSP ; Trilobite ; Laurentia ; Utah

* Corresponding author.E-mail address: [email protected] (L.E. Babcock).

Geobios 37 (2004) 149–158

www.elsevier.com/locate/geobio

© 2004 Elsevier SAS. All rights reserved.doi:10.1016/j.geobios.2003.03.007

1. Introduction

Work is progressing on the development of subdivisionsof the Cambrian System that are applicable on a global scale.To date, relevant boundary positions ratified by the Interna-tional Commission on Stratigraphy (ICS) are: 1, the base ofthe Cambrian System, and the Palaeozoic Eonothem, at thebase of the Trichophycus (or Treptichnus, Phycodes) pedumZone in Newfoundland (Brasier et al., 1994; Landing, 1994;Gehling et al., 2001); 2, the base of the Ordovician System atthe base of the Iapetognathus fluctivagus Zone in Newfound-land (Cooper et al., 2001); and 3, the base of the upperCambrian Furongian Series and Paibian Stage at the base ofthe Glyptagnostus reticulatus Zone in South China (Peng etal., 2002, 2004a). For more than a century, the Cambrian wasusually subdivided into three parts, but recognition of a thickpre-trilobitic lower Cambrian (Landing, 1994, 1998; Land-ing et al., 1998; Geyer and Shergold, 2000), equivalent toroughly half of Cambrian time (Landing et al., 1998) pro-vides an incentive to subdivide the system into four parts(Landing, 1998; Palmer, 1998; Geyer and Shergold, 2000;Peng et al., 2002), with two series in the lower half, and twoseries in the upper half. Within each of those series, it isexpected that two to three stages will be recognized. Geyerand Shergold (2000) emphasized the need to subdivide thesystem according to practical, intercontinentally recogniz-able horizons instead of according to techniques carried overfrom traditional usage. While this approach will require somereinterpretation of ages of mapped stratigraphic units in cer-tain regions, introduction of well-conceived, globally appli-cable, chronostratigraphic terminology will ultimately en-hance our ability to communicate stratigraphic informationinternationally.

As summarized by Geyer and Shergold (2000), at least11 candidate horizons for global chronostratigraphic correla-tion are present in the upper half of the Cambrian System,although not all are equally useful for stage and series bound-aries. One position, the first appearance datum (FAD) of theintercontinentally distributed agnostoid trilobite Glyptag-nostus reticulatus, has been ratified as the base of the Furon-gian Series and Paibian Stage (Peng et al., 2002a, 2004a). Ifsubequal series boundaries are adopted, it can be expectedthat the remaining series boundaries to be adopted maycorrespond roughly to the FAD of trilobites, and the FAD ofan intercontinentally distributed guide fossil near the tradi-tional position of the lower-middle Cambrian boundary.Work in progress is aimed at identifying suitable positionsfor additional series boundaries. Series and stage boundariesbelow the base of the Furongian Series (Paibian Stage) havenot been determined, but the number of reasonable optionsfor stage boundaries is relatively limited (Geyer and Sher-gold, 2000). It is thus appropriate to document strong candi-dates for global boundary stratotype sections and points(GSSPs) that include those intervals having greatest andmost precise correlation potential globally. Among horizonshaving potential as stage boundaries (Shergold and Geyer,

2001), the FAD of the intercontinentally distributed agnos-toid trilobite Ptychagnostus (or Acidusus) atavus is one ofthe most clearly recognizable datum points in the Cambrian.A position corresponding closely to the first appearance ofP. atavus is recognizable in strata of Gondwana, Baltica,Kazakhstan, Siberia, and Laurentia (e.g., Geyer and Sher-gold, 2000), and can be identified with precision using mul-tiple lines of evidence.

The purpose of this paper is to provide updated and refineddocumentation of a stratigraphic section (Figs. 1 and 2) thathas been previously suggested as a GSSP candidate definedby the FAD of Ptychagnostus (or Acidusus) atavus (Rowellet al., 1982; Robison, 1999). Based on a combination ofpreviously available information (White, 1973; McGee,1978; Grannis, 1982; Robison, 1982, 1999; Rowell et al.,1982; Rees, 1986) and new information, this section, in theDrum Mountains, western Utah, USA, is put forward as astrong candidate for a GSSP for a yet-to-be-named stageboundary. The proposed GSSP for the base of the new stageis at the FAD of P. atavus, 62 m above the base of theWheeler Formation (Figs. 1 and 2) in the section knowninformally as “Stratotype Ridge”, Drum Mountains, northernMillard County, Utah, USA. This point (Fig. 1) fulfills all ofthe geological and biostratigraphic requirements for a GSSP(Remane et al., 1996). Among the methods that should begiven due consideration in the selection of a GSSP (Remaneet al., 1996), biostratigraphic, chemostratigraphic, palaeo-geographic, facies-relationship, and sequence-stratigraphicinformation is available (e.g., Randolph, 1973; White, 1973;McGee, 1978; Grannis, 1982; Robison, 1982, 1999; Rowellet al., 1982; Rees, 1986; Langenburg et al., 2002); thatinformation is summarized here. The section is accessible,and access for research is unrestricted. It is located on publicland under permanent protection by a federal agency, the USBureau of Land Management (BLM). This will ensure con-tinued free access to the site for research purposes. If thesection is ratified as a GSSP, it is expected that a permanentmonument marking the GSSP position will be erected.

2. Proposed GSSP – Geography and physical geology

2.1. Geographic location

The “Stratotype Ridge” section is situated in the DrumMountains, northern Millard County, western Utah, USA. Itsgeographic coordinates are latitude 39°30.70′ N, longitude112°59.49′ W of Greenwich, England. The section consistsof more than 100 m of continuous exposure along a NE-trending ridge-crest and through adjacent hillside outcrops(Fig. 1). The section is located approximately 39 km WNWof Delta, Utah, and approximately 21 km WNW of theformer Topaz Relocation Camp (Robison, 1999: Figs. 3, 4).The “Stratotype Ridge” section includes approximately 70 mof Swasey Limestone (Randolph, 1973) overlain by morethan 100 m of the Wheeler Formation (White, 1973; McGee,

150 L.E. Babcock et al. / Geobios 37 (2004) 149–158

1978; Rowell et al., 1982; Rees, 1986; Robison, 1999). “Stra-totype Ridge” is represented as an unmarked NE-trendingridge on the Drum Mts. Well 7.5′ topographic quadranglemap (US Geological Survey, 1971, 1:24 000 scale). The ridgelies approximately 0.9 km E of an unnamed peak marked6033 on the map, and approximately 0.8 km W of an un-named peak marked 5989 on the map. The proposed GSSP isat an elevation of approximately 1760 m.

2.2. Geological location

The Drum Mountains are located within the Basin andRange Province, which consists of a series of basins boundedby predominantly N-trending normal faults that delimitmountainous blocks, or ranges. The structural history of theBasin and Range Province includes, at a minimum, oneperiod of compression during the Devonian, three periods of

Fig. 1. “Stratotype Ridge”, Drum Mountains, Utah, USA, showing the lower part of the Wheeler Formation and the position of the proposed Global StandardStratotype-section and Point (GSSP) for a new Cambrian stage defined by the first appearance datum (FAD) of Ptychagnostus (or Acidusus) atavus. 1. Southeastside of ridge showing position of the proposed GSSP (labeled as FAD of P. atavus); the FAD of Ptychagnostus (or Triplagnostus) gibbus (labeled as FAD ofP. gibbus) occurs in the lowermost calcareous shale bed of the Wheeler Formation, which overlies the Swasey Limestone. 2. Stratigraphic interval between about58 m and 65 m along the crest of Stratotype Ridge showing FAD of P. atavus (marked by a white line). 3. Close-up view of resistant limestone ledges along thecrest of Stratotype Ridge showing FAD of P. atavus (marked by a white line) near the base of a relatively resistant calcisiltite bed 62 m above the base of theWheeler Formation.Fig. 1. « Stratotype Ridge », Drum Mountains, Utah, USA, montrant la partie inférieure de la Formation Wheeler et la position du GSSP proposé pour unenouvelle définition de l’étage cambrien basée sur la première apparition (FAD) de Ptychagnostus (ou Acidusus) atavus. 1. Côté SE montrant la position du GSSPproposé (identifié comme FAD de P. atavus). La première apparition de Ptychagnostus (ou Triplagnostus) gibbus (identifié comme FAD de P. gibbus) se produitdans le lit de schiste calcaire le plus bas de la Formation Wheeler, là où elle recouvre le Calcaire Swasey. 2. Intervalle stratigraphique entre 58 m et 65 m le longde « Stratotype Ridge », montrant le FAD de P. atavus (marqué par une ligne blanche). 3. Vue rapprochée des rebords résistants de calcaire le long de « StratotypeRidge », montrant le FAD de P. atavus (ligne blanche) près de la base d’un lit relativement résistant de calcisiltite à 62 m au-dessus de la base de la FormationWheeler.

151L.E. Babcock et al. / Geobios 37 (2004) 149–158

compression during the Mesozoic, and episodes of extensionduring the Cenozoic (e.g., Dickinson, 1981; Speed, 1982;Allmendinger et al., 1983; Oldow et al., 1989). Cambrianrocks of the Drum Mountains lie within an area that experi-enced little deformation compared to surrounding areas(Rees, 1984, 1986).

During significant intervals of the Cambrian, Laurentianshelf areas were encircled by three major depositional envi-ronments: a relatively shallow inner-shelf environment was

separated from relatively deeper-shelf and ramp environ-ments by carbonate platform environments (e.g., Palmer,1972, 1973; Robison, 1976; Osleger and Read, 1993). Dur-ing some intervals, carbonate platform facies extendedbroadly across former upper ramp environments (Kepper,1972, 1976; Rees, 1986), and the areal extents of inner-shelfsettings were considerably reduced. The Swasey Limestone,which is subjacent to the Wheeler Formation, represents sucha broad carbonate platform environment (Randolph, 1973;

Fig. 2. Observed stratigraphic distribution of trilobites in the lower Wheeler Formation near the base of the Ptychagnostus (or Acidusus) atavus Zone,“Stratotype Ridge” section, Drum Mountains, Utah, USA. Scale is in metres above the base of the Wheeler Formation. An interpretive sea level history,reflecting small-scale regional or eustatic changes, is added for comparison.Fig. 2. Distribution stratigraphique des trilobites dans la partie inférieure de la Formation Wheeler, près de la base de la Zone de Ptychagnostus (ou Acidusus)atavus, coupe de « Stratotype Ridge », Drum Mountains, Utah, USA. Une interprétation des variations du niveau marin, reflétant des changements régionaux depetite amplitude ou eustatique, est ajoutée pour comparaison.


Caldwell, 1980; Rees, 1984, 1986). The overlying WheelerFormation consists of a thick succession (approximately300 m) dominated by thinly bedded, medium to dark calcar-eous shales intercalated with medium to dark calcisiltites.The succession records sedimentation mostly in an outer-shelf and ramp-to-basin environment below storm wave base(Grannis, 1982; Rees, 1984, 1986; Robison, 1991, 1999;Langenburg et al., 2002).

2.3. Location of level and specific point

On “Stratotype Ridge” (Fig. 1), the Wheeler Formation isessentially a monofacial succession of calcareous shales andcalcisiltites. The base of the first calcisiltite layer containingthe cosmopolitan agnostoid trilobite Ptychagnostus atavus inthe Wheeler Formation in the “Stratotype Ridge” section(62 m above the base of the formation; Fig. 2) is proposed asthe GSSP of an unnamed Cambrian stage. In previous reports(Robison, 1982, 1999; Rowell et al., 1982), the first occur-rence of P. atavus in this section was listed as 71 m above thebase of the Wheeler Formation. Fieldwork during 2002,however, shows that rare specimens occur through a series ofbeds down to a level of 62 m (Fig. 2). Intensive searching hasnot produced P. atavus below the 62 m level. The bedcontaining the lowest occurrence of P. atavus in the “Strato-type Ridge” section begins approximately 2 cm above thebase of the first of three closely spaced, relatively resistant(ledge-forming), limestone layers, each about 1 m thick,beginning 62 m above the base of the Wheeler Formation(Figs. 1 and 2).

2.4. Stratigraphic completeness

Detailed bed-by-bed correlation of the middle Cambrianthrough western Utah, coupled with detailed biostratigraphy(Robison, 1964a, 1976, 1982, 1984; Randolph, 1973; White,1973; Rowell et al., 1982), sedimentology (McGee, 1978;Grannis, 1982; Rees, 1984, 1986; Langenburg et al., 2002),carbon-isotope chemostratigraphy (Montañez et al., 2000;Langenburg et al., 2002), and strontium-isotope chemos-tratigraphy (Montañez et al., 1996, 2000) clearly demon-strate the stratigraphic continuity of the basal interval of theunnamed stage in the “Stratotype Ridge” section. Biostrati-graphic studies within the Basin and Range Province andglobally demonstrate that the succession of trilobite species(e.g., Westergård, 1946; Öpik, 1979; Robison et al., 1977;Ergaliev, 1980; Egorova et al., 1982; Rowell et al., 1982;Robison, 1984, 1994; Laurie, 1988; Geyer and Shergold,2000) and brachiopod species (McGee, 1978; Rowell et al.,1982) in the “Stratotype Ridge” section is undisturbed. Thesection lacks synsedimentary and tectonic disturbance at theproposed GSSP boundary point, although minor bedding-plane slippage, which is expected in an inclined successionof strata, occurs along some shale beds elsewhere in thesection. Bedding-plane-slip surfaces do not appear to haveresulted in any loss or repetition of stratigraphic thickness,

and the biostratigraphic succession in the section is unaf-fected. Evidence of metamorphism and strong diageneticalteration is absent.

2.5. Thickness and stratigraphic extent

The basal contact of the unnamed stage defined by theFAD of P. atavus, proposed as the GSSP (Figs. 1 and 2),occurs in a mostly monofacial succession of light- to dark-grey, and lavender-grey, thin-bedded calcareous shales, inter-bedded with medium- to dark-grey, thin-bedded calcisiltiteand argillaceous calcisiltite beds. The contact where P. ata-vus first appears is subtle, occurring at the base of a layer ofdark-grey, thinly laminated calcisiltite overlying a layer ofthinly laminated, dark-grey calcisiltite (Figs. 1(2, 3)). Thebed underlying the FAD of P. atavus is approximately 2 cmthick on the ridge crest. The basal contact of this bed in the“Stratotype Ridge” section is observable through a series ofexposures in a comparatively resistant ledge cropping out onthe ridge crest (Figs. 1(2, 3)), and in adjacent hillsides alongthe SE side of the ridge (Fig. 1(1)). The total bedding planelength of the basal contact is more than 200 m.

2.6. Provisions for conservation, protection,and accessibility

The exposure containing the proposed GSSP is not subjectto building, landscaping, or other destruction. It is located onpublic land to be permanently managed by the US Bureau ofLand Management (BLM). The proposed GSSP is within theGreat Basin Desert, and not subject to cover by significantvegetative growth. The ridge-crest section is also not subjectto cover by slope debris or alluvium.

Access to the outcrop in the Drum Mountains is essen-tially unrestricted in all seasons, although winter snowfallcan hinder travel to the site. Travel to Utah is open to personsof all nationalities, and travel for scientific purposes is wel-comed.

3. Motivation for selection of the boundary leveland of the potential stratotype section

3.1. Principal correlation event (marker) at proposedGSSP level

The agnostoid trilobite Ptychagnostus atavus has one ofthe broadest distributions of any Cambrian trilobite (e.g.,Westergård, 1946; Öpik, 1979; Robison et al., 1977; Er-galiev, 1980; Egorova et al., 1982; Rowell et al., 1982;Robison, 1982, 1984, 1994; Laurie, 1988; Geyer and Sher-gold, 2000; Peng and Robison, 2000; Pham, 2001; Peng etal., 2004b; ), and its first appearance has been acknowledgedas one of the most favourable levels for a GSSP defining thebase of a global Cambrian stage (e.g., Robison et al., 1977;Rowell et al., 1982; Robison, 1999, 2001; Geyer and Sher-


gold, 2000; Shergold and Geyer, 2001). Agnostoid trilobitesprovide the best and most precise tools for intercontinentalcorrelation in the upper half of the Cambrian System (e.g.,Robison, 1984; Peng and Robison, 2000). Recent recalibra-tion of radiometric ages for the Cambrian (Grotzinger et al.,1995; Davidek et al., 1998; Landing et al., 1998, 2000),scaled against the number of agnostoid zones recognized inthe upper half of the Cambrian, indicates that the averageduration of an agnostoid-defined biochron is about one mil-lion years (Peng and Robison, 2000). P. atavus has beenidentified (Geyer and Shergold, 2000) from Australia, China,Vietnam, North Korea, Russia, Kazakhstan, Sweden, Den-mark, Norway, the United Kingdom, Greenland, Canada, andthe United States, and has been used as a zonal guide fossil indeposits from Baltica, Gondwana, Kazakhstania, and Lau-rentia (e.g., Westergård, 1946; Robison, 1976, 1984; Öpik,1979; Geyer and Shergold, 2000; Peng and Robison, 2000).The base of the Floran Stage in Australia corresponds to thebase of the P. atavus Zone (Öpik, 1967; Geyer and Shergold,2000). As originally defined in North America, the base ofthe Marjuman Stage, which is at the base of the BolaspidellaZone, coincides with the base of the P. atavus Zone (Ludvig-sen and Westrop, 1985). However, Palmer (1998) redefinedthe base of the Marjuman Stage to coincide with the base ofthe stratigraphically lower Ehmaniella Zone. Co-occurrences with other trilobites allow correlation into suchregions as Siberia and Baltica (Tomagnostus fissus Zone;Geyer and Shergold, 2000). In Avalonia, the base of theHydrocephalus hicksi Zone corresponds approximately tothe base of the P. atavus Zone (Geyer and Shergold, 2000).

Stratigraphically, the first appearance of Ptychagnostusatavus always succeeds the first appearance of Ptychagnos-tus (or Triplagnostus) gibbus, although the last appearancedatum (LAD) of P. gibbus is commonly above the firstP. atavus (e.g., Peng and Robison, 2000). It is desirable toselect the position of a GSSP in a section showing a completesuccession from the P. gibbus Zone through the P. atavusZone. In a complete succession, the LAD of P. gibbus shouldfall within the lowermost part of the P. atavus Zone. Selec-tion of the FAD of P. atavus as the base of a Cambrian stageensures that the boundary will fall within the stratigraphicinterval bearing ptychagnostid trilobites, and at a readilyidentifiable point in a series of phylogenetically related forms(Rowell et al., 1982; Laurie, 1988). Globally, the strati-graphic interval bearing the overlap between P. gibbus andP. atavus is relatively narrow but widely recognizable. Thisnarrow overlap allows the boundary to be tightly constrainedas long as ptychagnostid-bearing strata are present in a re-gion.

Selection of a GSSP in an open-shelf to basinal deposit,and particularly in one from a low-latitude palaeocontinentsuch as Laurentia, is desirable because it provides faunal tiesand correlation with low-latitude open-shelf areas, high-latitude open-shelf areas, and low- or high-latitude, slope-to-basinal areas. In the latter half of the Cambrian, stratificationof the world ocean according to temperature or other factors

that covary with depth (e.g., Cook and Taylor, 1975, 1976;Babcock, 1994b) led to the development of rather distincttrilobite biofacies in shelf and basinal areas. Low-latitudeshelf areas were inhabited mostly by endemic polymeroidtrilobites and some pan-tropical taxa. High-latitude shelfareas, and basinal areas of low and high latitudes, wereinhabited mostly by widespread polymeroid trilobites andcosmopolitan agnostoid trilobites. Slope areas are character-ized by a combination of some shelf-dwelling taxa and basin-dwelling taxa. A combination of cosmopolitan agnostoids,which have intercontinental correlation utility, shelf-dwelling polymeroids, which mostly allow for intracontinen-tal correlation, and pan-tropical polymeroids, which allowfor limited intercontinental correlation, provides for precisecorrelation of the base of the P. atavus Zone through much ofLaurentia. Likewise, the combination of these taxa providesfor precise correlation of the base of the zone into areas ofBaltica, Siberia, Kazakhstan, South China, and Australia,and reasonably good correlation into Avalonia (Geyer andShergold, 2000).

The base of the P. gibbus Zone, relatively close beneaththe P. atavus Zone (less than 100 m in most areas of theworld), has been suggested as a potential stage boundary(Robison et al., 1977; Rowell et al., 1982; Geyer and Sher-gold, 2000). This is regarded as less desirable for defining astage boundary because the FAD of P. gibbus in many areasis linked closely to a significant lithologic change inferred torepresent a major eustatic event (commonly initial marinetransgression over a carbonate platform; see Kepper, 1976;Rowell et al., 1982; Robison, 1999). Thus, on a global scale,the FAD of P. gibbus may not necessarily represent a timehorizon as precise as that of the FAD of P. atavus. Further-more, the FAD of P. gibbus is not as well constrained bysecondary correlation tools (see sections 3.2, 3.3) as is theFAD of P. atavus.

3.2. Potential stratotype section

The FAD of P. atavus in the “Stratotype Ridge” section,Drum Mountains, Utah (Fig. 1), occurs in the Wheeler For-mation at a level 62 m above the base of the formation(Fig. 2). At this section, the Wheeler Formation rests on theSwasey Limestone. The Swasey-Wheeler contact is inferredto be a sequence boundary representing a major eustatictransgressive event (Kepper, 1976). Agnostoid trilobite zona-tion of the Wheeler Formation in the section reveals a com-plete, tectonically undisturbed, marine succession beginningat the base of the P. gibbus Zone (in the basal WheelerFormation) through much of the P. atavus Zone (Rowell etal., 1984; Robison, 1999). The Wheeler Formation at the“Stratotype Ridge” section is a mostly monofacial succes-sion of interbedded calcareous shales and calcisiltites(Fig. 2). Soft-sediment deformation, truncation surfaces, andslide surfaces are rare in the section and absent near theproposed GSSP, suggesting deposition in distal shelf togentle slope environments. Overall, the Wheeler Formation


represents outer-shelf through ramp and basinal deposition ina marine environment along the Cordilleran margin of Lau-rentia (e.g., White, 1973; Grannis, 1982; Rees, 1984, 1986;Robison, 1999; Langenburg et al., 2002).

The proposed GSSP in the “Stratotype Ridge” section iswithin a stratigraphic succession containing a complex ofphylogenetically related ptychagnostid species. The phylo-genetic pathways have been subject to differing interpreta-tions (Öpik, 1979; Robison, 1982, 1994; Rowell et al., 1982;Laurie, 1988), but this does not affect our understanding ofthe stratigraphic succession of species. Successive strati-graphic levels show a succession beginning with Ptychag-nostus (or Triplagnostus) gibbus, and continuing throughPtychagnostus (or Acidusus) atavus. In the bed containingthe lowest P. atavus in the section, the species is rare. P. ata-vus becomes more abundant upsection, and reaches an acmeoccurrence at 72 m, where it occurs in extraordinary abun-dance in a thin limestone coquina, the allochems of which arealmost entirely P. atavus. The LAD of P. gibbus in thesection occurs at 66 m, and this position provides an impor-tant means of constraining the proposed GSSP level. Thebase of the bed containing the FAD of P. atavus in the“Stratotype Ridge” section is isochronous along its exposedlength, although lithologically it is essentially indistinguish-able from other layers in a succession of thinly bedded,medium-grey to dark-grey calcisiltites near the base of a4.5-m-thick limestone-dominated interval (Figs. 1 and 2).

Ranges of trilobites across the stratigraphic interval con-taining the proposed GSSP (revised from Rowell et al., 1982;Robison, 1999) are summarized in Figure 2. Besides P. ata-vus, a number of other guide fossils, which have utility forcorrelation on an intercontinental scale, help to constrain theboundary position. In addition to P. gibbus, which rangesinto the lower P. atavus Zone, they include the agnostoidsPtychagnostus intermedius, which ranges through much ofthe P. gibbus Zone, and Peronopsis segmenta, which appearsin the lower P. gibbus Zone and ranges to the P. punctuosusZone. Locally, species of the polymeroids Olenoides,Bathyuriscus, Bolaspidella, Modocia, Zacanthoides, andSpencella, some of them new, make their first appearancenear the base of the P. atavus Zone (White, 1973). All ofthese genera, however, have considerably longer strati-graphic ranges that begin below the FAD of P. atavus (e.g.,Robison, 1964a, b, 1976; Babcock, 1994a). The agnostoidtrilobites Peronopsis fallax and Peronopsis interstrictarange through the boundary interval, and do not help toconstrain the proposed boundary. The polymeroid trilobitesPtychoparella (incorporating Elrathina as a junior synonym)and Elrathia have long stratigraphic ranges (Robison, 1964a,b, 1976; Babcock, 1994a) that extend through the proposedboundary interval (White, 1973) and provide little help inconstraining the position.

3.3. Demonstration of regional and global correlation

A position at or closely corresponding to the FAD ofP. atavus in the “Stratotype Ridge” section is one of the most

easily recognizable horizons on a global scale in the Cam-brian (e.g., Geyer and Shergold, 2000). Suitability of theFAD of this species for marking a global stage and seriesboundary has been summarized principally by Rowell et al.(1982) and Geyer and Shergold (2000). Key correlation toolsare as follows:

3.3.1. Agnostoid trilobite biostratigraphyP. atavus is recognized worldwide (e.g., Westergård,

1946; Öpik, 1979; Robison et al., 1977; Ergaliev, 1980;Egorova et al., 1982; Rowell et al., 1982; Robison, 1982,1984, 1994; Laurie, 1988; Geyer and Shergold, 2000; Pengand Robison, 2000; Ergaliev and Ergaliev, 2001; Pham,2001; Peng et al., 2004b), having been identified from rocksof Australia (Queensland and South Australia), Vietnam,China (Hunan, Guizhou, Xinjiang, and Zhejiang), North Ko-rea, Russia (Siberian Platform), Kazakhstan (Lesser Kara-tau), Sweden, Denmark, Norway, the United Kingdom,Greenland, Canada (western Newfoundland), Mexico (So-nora), and the United States (Alaska, Nevada, and Utah). Thespecies is used as a zonal guide fossil in Australia, SouthChina, Kazakhstan, and Laurentia (Geyer and Shergold,2000; Peng and Robison, 2000). Co-occurrences with othertrilobites allow precise correlations into Siberia and Baltica,as well as close correlations into Avalonia (near the base ofthe Hydrocephalus hicksi Zone).

3.3.2. Polymeroid trilobite biostratigraphyThe base of the P. atavus Zone coincides with a relatively

significant change in polymeroid trilobite faunas recognizedat the base of the Floran Stage in Australia (Öpik, 1967). Italso coincides, or nearly coincides, with a rather significantfaunal change associated with the base of the BolaspidellaZone in Laurentia (Robison, 1976; Palmer, 1998, 1999). Asoriginally conceived (Ludvigsen and Westrop, 1985), thebase of the P. atavus Zone corresponded to the base of theMarjumam Stage as used in Laurentia. The base of theMarjuman Stage, however, was revised downward to the baseof the Ehmaniella Zone with the introduction of a morecomprehensive nomenclatural system for series and stages inLaurentia (Palmer, 1998).

3.3.3. Conodont biostratigraphyA position near the base of the P. atavus Zone corresponds

closely with a turnover in conodont faunas, although thatturnover has not been documented in Utah. The base of theP. atavus Zone occurs just below the base of the Gapparodusbisulcatus-Westergaardodina brevidens Assemblage-zone(Dong and Bergstrom, 2001a, b; Dong et al., 2001). Theposition of the G. bisulcatus-W. brevidens Zone has beenwell documented in Baltica and South China (Dong andBergstrom, 2001a, b; Dong et al., 2001), but has not beenrecognized yet outside those areas. A conodont zonation hasnot been developed for western North America below theFurongian Series (see Miller, 1980, 1981; Dong and Berg-strom, 2001a).


3.3.4. Brachiopod biostratigraphyInarticulate brachiopods ranging across the P. gibbus-P.

atavus interval in Utah (McGee, 1978) provide only coarseconstraints on the zonation of strata. Species of Acrothyra(A. minor and A. urania) range through the lower part of theP. gibbus Zone, and one unnamed genus belonging to thesubfamily Linnarssoniinae ranges through most of the P. gib-bus Zone, with its LAD occurring just below the FAD of P.atavus. Acrothele subsidua, Prototreta, Pegmatreta bel-latula, Dictyonina, Micromitra, and Lingulella range fromthe Ptychagnostus (or Pentagnostus) praecurrens Zone,through the overlying P. gibbus Zone, and well into theP. atavus Zone. Linnarssonia ophirensis ranges from nearthe base of the P. gibbus Zone well into the P. atavus Zone.

3.3.5. ChemostratigraphyThe base of the P. atavus Zone, which corresponds to a

position near the base of the Bolaspidella Zone in Laurentia(Robison, 1976), corresponds closely with the onset of asignificant positive shift in d13C values (Montañez et al.,2000; Langenburg et al., 2002). The precise base of theexcursion is subjective, as the excursion follows a monotonicpositive shift in d13C values from ones that are indistinguish-able from background values. As recorded in the DrumMountains (Langenburg et al., 2002), the excursion reachespeak values of about +1.7 ‰ d13C at 112 m above the base ofthe Wheeler Formation, at a position corresponding roughlyto maximum flooding of the Cordilleran margin of the Lau-rentian shelf.

In addition to a carbon isotope excursion, the base of theP. atavus Zone corresponds closely with the onset of a longmonotonic 87Sr/86Sr isotopic shift (Montañez et al., 1996,2000). The 87Sr/86Sr ratio approximates 0.7091 near the baseof the Bolaspidella Zone (i.e., near the base of the P. atavusZone), but exceeds 0.7902 near the middle of the Bolaspi-della Zone, and reaches 0.7093 in the upper part of theBolaspidella Zone.

3.3.6. Sequence stratigraphyWork in the Cordilleran region of Laurentia shows that the

base of the P. atavus Zone is associated with the early part ofa transgressive event. Overall, the Wheeler Formation isinferred to have been deposited during a single third-ordercycle (Langenburg et al., 2002). Superimposed on this long-term transgressive phase (Montañez et al., 1996) is a series ofsmaller scale transgressive-regressive cycles (perhaps fifth-or sixth-order cycles). In the “Stratotype Ridge” section, theFAD of P. atavus is associated with one of the small-scaletransgressive events. The species first appears less than 1 mupsection of a surface inferred to represent a deepening eventof small magnitude. Comparative work on sections nearPaibi and Wangcun, Hunan Province, China (Peng and Robi-son, 2000; Peng et al., 2001a–c, 2004b), shows that P. atavusfirst appears along the Gondwanan slope in an early stage ofa transgressive event. In all likelihood, the transgression withwhich the FAD of P. atavus is associated is of intercontinen-tal, if not eustatic, scale.

4. Other regional candidates and reasons for rejection

Several sections exposing the Wheeler Formation, and thebase of the P. atavus Zone, are available in western Utah(e.g., White, 1973; Hintze and Robison, 1975; Grannis,1982; Rees, 1984, 1986; Langenburg et al., 2002). Besidesthe Stratotype Ridge section in the Drum Mountains, perhapsthe best of these is a section near Marjum Pass in the HouseRange (Rees, 1984, 1986). In the Marjum Pass section, thebase of the P. atavus Zone is well exposed. However, severalfaults and folds in the section, together with relatively poorexposure in some intervals, render this section less desirableas a potential GSSP.

Acknowledgements

We thank Robyn A. Howley for assistance with fieldworkduring 2002, and gratefully acknowledge the constructivecomments of members of the International Subcommissionon Cambrian Stratigraphy, notably John H. Shergold andGerd Geyer. This paper has benefited from the helpful re-views of John H. Shergold and Allison R. Palmer, and fromthe editorial work of Jean Vannier. This work was supportedin part by a grant from the U.S. National Science Foundation(EAR 0106883) to Babcock; and from the National ScienceFoundation of China (NSFC 40023002, 40072003), and theMinistry of Science and Technology of China(2001DEA20020-2, G200007700) to Peng.

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Potential Global Standard Stratotype-section and Point (GSSP) for a Cambrian stage boundary defined by the first appearance of the trilobite Ptychagnostus atavus, Drum Mountains, Utah,