Ordovician graptolites of the Phi Kappa Formation at Trail Creek, Central Idaho, USA: a revised biostratigraphy
E. Mitchell1, Daniel Goldman2, Miles R. Cone1, Jörg. Maletz1 and Hilary Janousek2
1State University of New York at Buffalo, Dept. of Geological Sciences, Buffalo, NY 14260. E–mail: firstname.lastname@example.org
2 University of Dayton, Dept. of Geological Sciences, 300 College Park, Dayton, OH 45469.
Key words: Graptolites. Biostratigraphy. Phi Kappa Formation. Ordovician. Idaho.
The Trail Creek Summit section within the Phi Kappa Formation of central Idaho, northwestern U.S.A, exposes an approximately 200–meter–thick, homoclinal succession of predominantly black, and siliceous, graptolitic shale that is nearly continuously exposed. Despite high degrees of deformation near several faults and a weak but pervasive cleavage, this reference section contains an abundant graptolite fauna that ranges in age from Early Ordovician through Middle Silurian (Carter and Churkin, 1977; Dover et al., 1980). Surveys of the local biostratigraphy in this area, conducted by Churkin (1963), Carter (1972), and Carter and Churkin (1977), established a set of zones that have been employed as a standard reference for the North American Cordillera. However, our recent analysis (August 2001) and faunal reassessment of extensive new graptolite collections from Trail Creek indicates that this succession not only shows a profound similarity to the Australasian and South China succession, but is more incomplete than previously thought. A brief revision of the Ordovician biostratigraphy is presented here.
The first 15 meters of black shale above the base of the section overlying the Park Creek Thrust contain an interrupted sequence of Lower and Middle Ordovician graptolites (Figure 1). Our updated collections at 1.0 and 1.8m above the lower thick quartzites contain previously undocumented specimens of Xiphograptus lofuensis, Tetragraptus reclinatus, T. serra, Didymograptellus bifidus, and Pseudotrigonograptus ensiformis, a late Early Ordovician fauna referable to Chewtonian–2. Collections at 2.15m, in contrast, contain Arienigraptus gracilis and Isograptus maximodivergens (also recorded from this level and referred to an ‘Isograptus Zone’ by Carter and Churkin, 1977), taxa indicative of Castlemainian–4. Middle Ordovician faunas from the majority of the Castlemainian, Yapeenian, and Darriwilian stages are mostly missing from the Phi Kappa Formation at Trail Creek. That faunas of this age range are present in other sections in the region (see Dover et al., 1980) suggest omission by faulting at Trail Creek.
The subsequent 135m of strata extending to the base of the Silurian System contain a more complete succession of Upper Ordovician zones. Carter and Churkin (1977) located the base of the Upper Ordovician, Nemagraptus gracilis Zone, within the sparsely fossiliferous lower part of an approximately 75m–thick quartzite and silty shale interval (Figure 1). We recovered specimens of Nemagraptus gracilis and several other taxa from the basal beds of this interval that are indicative of the Gisbornian–1, or early Caradoc. Fifteen m higher these collections are followed by the FADs of Cryptograptus tricornis, and Climacograptus bicornis, nearly 60m lower than previously reported. The C. bicornis Zone fauna recurs at widely separated levels through the entire sandy interval in the middle part of the section. Thus the majority of what had formerly been thought to be Gi1 instead belong to the Gi2.
Figure 1. Stratigraphic section and graptolite species range chart for the lower portion of the Trail Creek. All studied collection levels numbered consecutively. New collections denoted by an (*). Correspondence between numbered collections and Carter and Churkin’s (1977) collection codes available by request. Left-most column is the revised biozonation discussed in the text (compare to that of Carter and Churkin, 1977, shown next to their lithostratigraphic column). Note that the Lower and Middle Ordovician succession is highly incomplete.
Figure 2. Stratigraphic section and graptolite species range chart for the upper portion of the Trail Creek section. The thickness of Upper Ordovician strata at this site is greatly expanded compared to that of the underlying units, although this interval also contains some relatively more condenses units. See Figure 1 explanation for other details.
The upper part of the C. bicornis Zone of Carter and Churkin (1977) and the lowermost part of their succeeding "Passage Beds" contain a sparse early Eastonian fauna (Ea1–2) that includes Climacograptus spiniferus and Orthograptus quadrimucronatus (Figure 2). Dicellograptus and Leptograptus species are entirely absent from this interval. The base of the Ea3 corresponds to the FADs C. tubuliferus and Leptograptus eastonensis (=L. demissus Carter and Churkin 1977), which occur near the base of the "Passage Beds." This fauna also contains specimens referable to Normalograptus pilatus (Williams 1982), formerly reported only from Scotland. The uppermost Eastonian Dicellograptus gravis Zone (Ea4) appears just above the level formerly identified at Trial Creek as the base of the C. tubuliferus Zone and includes the eponymous species (=Dicellograptus alector Carter 1972), Climacograptus nevadensis, Anticostia macgregorae (formerly reported only from Anticosti Island; Stewart and Mitchell 1997), and near the top of the unit, the FAD of A. fastigata and Pararetiograptus pulcherrimus. As for the Gisbornian, the upper Eastonian at Trail Creek is considerably thicker than the lower Eastonian.
Bolindian–1 faunas, marked buy the FADs of Climacograptus? uncinatus, Paraorthograptus prominens, and Dicellograptus ornatus, appear slightly below and range through the interval that Carter and Churkin (1977) identified as the Pleurograptus linearis Zone (Figure 2). Although Leptograptus eastonensis is abundant at some levels in this interval we have not found any material referable to P. linearis. Dicellograptus gravis ranges upward from the Ea4 into the Bo1. We refer collections above the LAD of C.? uncinatus and below the FAD of Paraorthograptus pacificus as the Bolindian–2. Appendispinograptus hvalross, Climacograptus hastatus, and Arachniograptus laquaeus make their first appearance in this interval, but the first two at least are known to range downward into Bo1 strata elsewhere. Finally, the diverse Bo3, Paraorthograptus pacificus Zone assemblages, are present in the former Dicellograptus ornatus Zone interval. This zone is truncated by a sheared and faulted interval in which L. eastonensis–bearing faunas are repeated and these are again followed by Bo3 samples. Collectively, the Bo3 samples at Trial Creek contain several species not previously reported from the region and rarely from outside China: Paraorthograptus brevispinus, Paraplegmatograptus uniformis, Pararetiograptus sinensis, and Sunigraptus regularis. Hirnantian and Rhuddanian graptolites appear to be entirely absent from this section.
The results of our recollection of the Trail Creek section have dramatically altered the range of many taxa and produced a very different distribution of rock thickness relative to time than was formerly apparent. The revised fauna lists (which are probably still incomplete) expand the known geographic range of several species and suggest much stronger affinities with the Australasian and Chinese successions than was apparent formerly.
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Received: February 15, 2003
Accepted: June 15, 20