Sephton et al. (2002) reported a complex isotopic event, marked by a positive δ13Corg excursion and an initially positive δ15Norg excursion, from a Triassic-Jurassic boundary succession at Black Bear Ridge, Williston Lake, northeastern British Columbia, corresponding to the level of disappearance of abundant monotid bivalves. They interpreted this event as occurring at the Norian-Rhaetian boundary, thus preceding the Rhaetian-Hettangian (end-Triassic) isotopic excursions now known to occur at several other localities, but which apparently were absent in this section (Sephton et al., 2002, their Fig. 2). They then suggested that the Late Triassic was marked by more than one extinction event.
In the section at Black Bear Ridge, Sephton et al. placed the Norian-Rhaetian boundary at the level of disappearance of abundant Monotis bivalves and the Rhaetian-Hettangian (Triassic-Jurassic) boundary 9.0 m higher (Sephton et al., 2002, their Fig. 2), coinciding with the first recorded occurrence of the ammonite genus Psiloceras. As they indicated, the end-Triassic negative δ13Corg excursion should have been recorded somewhere within this 9 m sequence, in their samples numbered BBR39–BBR45 (Sephton et al., 2002), but it was apparently absent.
In the following discussion, all stratigraphic levels we refer to correspond to the measured stratigraphic section of Sephton et al. (2002, their Fig. 2). A more complete summary of new Hettangian ammonite faunas from the Black Bear Ridge section is found in Hall and Pitaru (2003). The lowest Jurassic ammonites occur at 63.4 m on their section, so that the Triassic-Jurassic boundary is at, or even possibly below, this level and the Rhaetian is represented by a maximum of 2.3 m of section; fossils are extremely rare in this interval. Orchard et al. (2001, their Fig. 3, p. 15–16) showed occurrences of the bivalve Monotis subcircularis, conodonts Neogondolella steinbergensis (Mosher), Epigondolella ex. gr. bidentata (Mosher), and elements resembling E. mosheri (Kozur and Orchard) “sitting immediately above the youngest bedding surface of Monotis.” They suggested that this indicated the presence of Rhaetian strata. They also recorded several lower Hettangian ammonites, Primapsiloceras? sp. (from GSC loc. 98871 at 63.4 m), Psiloceras calliphyllum (Neumayr) with phylloceratids (from GSC loc. 98531 at 70.1 m), and middle Hettangian genera at several higher levels. Our collections include the impression of a single Monotis valve in a siltstone bed at 62.4 m and two conodont ramiform elements (Epigondolella) from thin calcareous beds at 62.6 m and 62.8 m (C.M. Henderson, 2003, personal commun.).
The lowest Hettangian (Jurassic) ammonite in the section at Black Bear Ridge is Psiloceras plicatum (Quenstedt) at 63.4 m (Fig. 1A), cited in Orchard et al. (2001) as Primapsiloceras? sp. This specimen was collected by Tozer, and his original field note placed it five feet above the fibrous calcite bed, which occurs 0.8 m above the top surface of the Monotis beds. The genus Primapsiloceras was described from Russia and thought to come from the “pre-planorbis beds” (i.e., pre-Hettangian), but that stratigraphic level has been questioned (Guex and Rakus, 1991). The next highest psiloceratids in this section are two slightly flattened lateral impressions of Psiloceras majus (Neumayr) at 64.1 m (Fig. 1B), followed by Psiloceras cf. planocostatum (Hillebrandt) at 65.1 m (Fig. 1C), Psiloceras plicatum (Quenstedt) at 67.6 m and 68.4 m (Fig. 1D), Psiloceras cf. rectocostatum Hillebrandt at 69.4 m, and Psiloceras calliphyllum (Neumayr) at 69.6 m. The last specimen was initially recorded by Tozer (1982) and used by Sephton et al. (2002) to define the base of the Jurassic in their section. Other ammonites in this part of the section are ?Kammerkarites sp. between 67.3 m and 69.7 m and phylloceratids at 69.4 m and 72 m.
Thus, the apparent absence of the expected end-Triassic δ13Corg excursion at Black Bear Ridge within the stratigraphic interval beginning just below 64.6 m (sample BBR40; the level of their sample BBR39 is not given in their data table, GSA data repository item 2002131) up to 70.9 m (sample BBR45) is, we believe, explained by the fact that this interval lies entirely within the Hettangian. If the conodonts and monotid bivalve found above 61.1 m, where abundant monotids suddenly disappear, were reworked specimens, then this part of the section could also already be Hettangian, meaning the Rhaetian is absent in this section. It must be noted that the species of Psiloceras reported here from Black Bear Ridge are not the earliest known species of this genus. At most, the Rhaetian is represented by 2.3 m of section (61.1–63.4 m), in contrast to just over 100 m in the boundary sequence at Kennecott Point, Queen Charlotte Islands (Ward et al., 2001, their Fig. 1). The Rhaetian at Black Bear Ridge is represented either by a hiatus, an extremely condensed sequence, or a marine flooding event occurring in the latest Rhaetian.
We suggest, then, that the carbon and nitrogen isotopic excursions recorded in the Black Bear Ridge section by Sephton et al. (2002) actually approximate the anticipated Rhaetian-Hettangian (end Triassic) event as recorded at other localities.