1192


Journal of Paleontology 91(6):1178–1198


Treptichnus pedum, indicating higher ichnodiversity than in the underlying ichnozone 1. The presence of Cruziana isp. in these strata is particularly significant because it reinforces evidence for the presence of arthropod-produced trace fossils in the Treptichnus pedum Zone. Most of the trace fossils appear in the lowermost interval of the zone (171–173m above the base of


regarded as Cambrian Age 2–3 (Buatois and Mángano, 2011). This is supported by the presence of Pelagiella lorenzi (Pelagiella subangulata; Steiner et al., 2007).


Lower Shale Member) (Fig. 7). This ichnozone is defined by the first appearance of T. pedum, and is regarded as Fortunian in age (Jensen, 2003; Buatois and Mángano, 2011), being referred to as Ichnozone II in the global ichnostratigraphic scheme or T. pedum Zone (Narbonne et al., 1987; MacNaughton and Narobonne, 1999; Jensen, 2003). This is consistent with the presence of the Anabarites trisulcatus-Protohertzina anabarica Zone (Hamdi et al., 1989). In strict ichnologic terms, the first occurrence of Treptichnus pedum together with the first bilobate trace Cruziana isp. may be taken as evidence that the Ediacaran- Cambrian boundary is placed within the Lower Shale Member, 171m above its base. As discussed above, however, integration with small shelly fossils and evaluation of environmental con- straints suggest placing the Ediacaran-Cambrian boundary at the base of the Soltanieh Formation or within the Lower Dolomite Member, rather than within the Lower Shale Member. Ichnozone 3 characterizes the middle interval of the Upper


Shale Member (80–178m above its base; Fig. 7). This ichno- zone is defined by a sudden increase in the abundance of trace fossils and the occurrence of more complex burrows, including Cruziana problematica, Phycodes isp., and Treptichnus pollardi. The other elements of this ichnozone are Cochlichnus anguineus, Curvolithus isp., Gordia marina, Helminthoi- dichnites tenuis, Helminthopsis tenuis, Palaeophycus tubularis, Planolites montanus, Treptichnus pedum, and Treptichnus isp. This ichnozone corresponds to ichnozone III of the global ichnofossil scheme, encompassing the lower half of the Rusophycus avalonensis Zone of the Chapel Island Formation, which represents the upper half ofMember 2 and higher strata of the Chapel Island Formation, Burin Peninsula (Narbonne et al., 1987), and the lower half of the Rusophycus avalonensis Zone of the Mackenzie Mountains, which characterizes the Backbone Ranges Formation and the lower interval of the Vampire Formation (MacNaughton and Narbonne, 1999). However, R. avalonensis has not been found at comparable levels in the Soltanieh Formation. Ichnozone 3 is interpreted as late Fortunian–Cambrian Age 2 (Buatois and Mángano, 2011). Ichnozone 4 represents the uppermost interval of the Upper


Shale Member (178m above its base to the base of the Upper Dolomite Member; Fig. 7). This zone is based on the first appearance of large back-filled trace fossils (Psammichnites gigas), together with bilobate, trilobite-like resting trace fossils (Rusophycus avalonensis), and large locomotion trace fossils (Didymaulichnus miettensis). The other elements of this ichno- zone are Cruziana isp., Curvolithus isp., Palaeophycus tubularis, Phycodes isp., Planolites montanus, and Treptichnus pedum. Ichnozone 4 corresponds to ichnozones III and IV of the global scheme, encompassing both the upper half of the Rusophycus avalonensis Zone of Burin Peninsula (Narbonne et al., 1987) and the Mackenzie Mountains (MacNaughton and Narbonne, 1999), and the Cruziana tenella Zone of the latter region, which characterizes the upper interval of the Vampire Formation (MacNaughton and Narbonne, 1999). This zone is


Discussion


The Ediacaran-Cambrian boundary is globally defined by a point placed with specific reference to the first appearance of T. pedum in the global stratotype section in Newfoundland (Narbonne et al., 1987) and in other regions, including the Mackenzie Mountains of Canada (MacNaughton and Narbonne, 1999; Carbone and Narbonne, 2014), the Flinders Ranges of South Australia (Jensen et al., 1998), eastern Finnmark, Norway (Føyn and Glaessner, 1979; Högström et al., 2013), Sonora, Mexico (Stewart et al., 1984; Sour-Tovar et al., 2007), and Death Valley, Eastern California (Jensen et al., 2002), among many other areas. In the Alborz Mountains of northern Iran, however, the first appearance datum of Treptichnus pedum occurs stratigraphically higher in the section. Although ichnozone 1 consists of grazing trails, which are common in Ediacaran strata elsewhere, these trace fossils are also abundant in Fortunian deposits in Burin Peninsula, in connection with microbially stabilized surfaces (Buatois et al., 2014). In contrast to the Fortunian of Burin Peninsula, arthropod trackways are absent in ichnozone 1 in the Soltanieh Formation. We hypo- thesize that the presence of an ichnofauna of “Ediacaran aspect” and the late appearance of Treptichnus pedum in the Soltanieh Formation is due to environmental constraints. Buatois et al. (2013) noted that this ichnotaxon has a broad environmental tolerance, albeit displaying a preference for sandy substrates. The shelf shales of the Soltanieh Formation may have represented the seaward limit of the T. pedum producer. Alternatively, because T. pedum is typically preserved along lithologic interfaces, its absence in the shelfal shale may simply reflect a taphonomic constraint. Therefore, ichnozone 1 in the Alborz Mountains may be understood as a distal expression of the Treptichnus pedum Zone, which is commonly recognized in settings above storm wave base (Buatois et al., 2013). A similar situation has been noted in the Ediacaran–Cambrian succession of eastern Yunnan Province, South China (Zhu, 1997), western Mongolia (Smith et al., 2015), Lesser Himalaya, India (Singh et al., 2014), and southeastern Kazakhstan (Weber et al., 2013), where the first appearance of the trace-fossil T. pedum postdates the Ediacaran–Cambrian transition. Ichnozone 2 of the Soltanieh Formation corresponds in part


to ichnozone III of global ichnofossil zones and the Treptichnus pedum Zone of the Chapel Island Formation, Burin Peninsula, which characterizes the lower part of Member 2, from 2.4 to 133m above its base (Narbonne et al., 1987; Landing, 1996). This ichnozone is also represented in the upper interval of the Ingta Formation in the Mackenzie Mountains (MacNaughton and Narbonne, 1999) and the Nomtsas Formation of Namibia (Crimes and Germs, 1982; Geyer and Uchman, 1995), among other areas. Integration of trace fossils and small shelly faunas suggests that ichnozone 2 of the Soltanieh Formation is best regarded as the upper half of the global Treptichnus pedum Zone to accommodate the shelf deposits of early Fortunian age that make up the lower interval of the Lower Shale Member.


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