Shahkarami et al.—Ediacaran–Cambrian ichnostratigraphy
dichotomic branching of this ichnogenus (Fu, 1991). The spe- cimen attributed to Diplichnites isp. appears to consist of a string of pits rather than appendage imprints and in fact, a sec- ond string of pits is apparent on the same slab, which may suggest an affinity with Saerichnites. The specimen attributed to Diplocraterion is only seen on bedding-plane view, so their U-shaped morphology cannot be confirmed. The ichnotax- onomy of Hormosiroidea is in need of revision (Gaillard and Olivero, 2009). In addition, the specimen illustrated lacks the morphologic features typically present in this ichnogenus. Based on the illustrations provided, the presence of scratch marks cannot be confirmed in the specimens assigned to Monomorphichnus lineatus. Neonereites uniserialis has been regarded as a preservation variant of Nereites (Uchman, 1995a). However, the specimen illustrated from the Soltanieh Formation lacks the diagnostic features of Nereites, actually resembling a
simple grazing trail, such as Helminthoidichnites. The specimen figured as Paleodictyon croaticum does not display the typical morphology of this ichnotaxon; an affinity with Multina cannot be disregarded. Plagiogmus arcuatus is now considered a pre- servational variant of Psammichnites gigas by some (McIlroy and Heys, 1997; Mángano et al., 2002a). The structures figured as Protovirgularia dichotoma? do not show the diagnostic chevronate pattern characteristic of Protovirgularia (e.g., Seilacher and Seilacher, 1994; Mángano et al., 1998).
Upper Dolomite Member.—Meyer (1967) noted that the stro- matolites Collenia spissa Fenton and Fenton, 1939 and Hadrophycus immanis Fenton and Fenton, 1939 are moderately common in the upper interval of this member. Problematic records of Salterella have also been reported (Assereto, 1963; Stöcklin et al., 1964). CiabeGhodsi (2007) and Tashayoee et al. (2012) also reported Hyolithellus filiformis Bengtson in Bengt- son et al., 1990; Conotheca subcurvata (Yu, 1974); Igorella sp.; Latouchella krobkovi Vostokova, 1962; Latouchella maidipingensis (Yu, 1974); and Obtusoconus rostriptutea Qian in Qian et al., 1978 from this unit.
Ichnostratigraphy of the Soltanieh Formation
Several ichnostratigraphic schemes have been proposed for the Ediacaran-Cambrian boundary (Alpert, 1977; Crimes, 1987, 1992; MacNaughton and Narbonne, 1999; Jensen, 2003). The latter two schemes have been recently adapted (Buatois and Mángano, 2011; Mángano et al., 2012), and two Ediacaran zones and three early Cambrian zones are considered. The lower Ediacaran zone includes simple grazing trails, such as Helminthoidichnites, Helminthopsis, Gordia, and Archae- onassa, together with Epibaion and Kimberichnus. The age of this interval is ca. 560–550 Myr (Jensen et al., 2006). The upper Ediacaran zone includes the oldest branching-burrow systems, represented by treptichnids. The age of this zone is 550–541 Myr (Grotzinger et al., 1995; Jensen et al., 2006; Schmitz, 2012). The lowermost early Cambrian zone, or the Treptichnus pedum Zone, is of Fortunian age and defined by the first appearance of T. pedum, as well as other complex forms (Narbonne et al., 1987; Landing et al., 2013, 2015). The Rusophycus avalonensis Zone contains the oldest bilobate, trilobite-type resting traces (R. avalonensis) and the bilobate
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correlation of Ediacaran-Cambrian boundary was first proposed at the Bristol plenary session in 1983 (Narbonne et al., 1987). Biozonations based on SSFs are available in Siberia and South China (Peng et al., 2012a). In Siberia, SSF zonation comprises, in ascending order, the Anabarites trisulcatus and Purella antiqua assemblage zones of the Fortunian, and Watsonella crosbyi and Aldanella operosa assemblage zones of Cambrian Age 2–Age 3 (Khomentovsky and Karlova, 1993, 2002; Peng et al., 2012a). The SSF zonation of south China comprises the assemblage zones Anabarites trisulcatus-Protohertzina anabarica (SSF1) of the lower Meishucunian Stage (= For- tunian), Paragloborilus subglobosus-Purella squamulosa (SSF2), and Watsonella crosbyi (SSF3) of the middle Meishu- cunian Stage (= lower part of Cambrian Age 2). The upper Meishucunian (=upper part of Cambrian Age 2) strata comprise the Sinosachites flabelliformis-Tannuolina zhangwentangi Assemblage Zone (SSF4) (Steiner et al., 2007), which directly underlies the trilobitic interval of the Cambrian in the shallow- water realm of the Yangtze Platform. Non-biostratigraphic techniques used for correlation of the
epichnial trail Taphrelminthopsis circularis (Narbonne et al., 1987; Jensen et al., 2006). The age of this zone ranges from Fortunian to Cambrian Age 2. The Cruziana problematica Zone (= C. tenella Zone), which contains the oldest bilobate, trilobite-like trails (Cruziana problematica) associated with Psammichnites gigas, is Cambrian Age 2. The application of the lowest appearance of SSFs for global
Ediacaran-Cambrian boundary include chemostratigraphic techniques, such as the measurement of carbon and sulfur isotopes (Magaritz et al., 1986; Tucker, 1986; Magaritz, 1989; Brasier et al., 1992; Brasier, 1993; Shen and Schidlowski, 2000; Ru et al., 2011). The carbon isotopic curve emerges as an increasingly important tool for intercontinental and intraconti- nental correlation, especially in regions where the primary biologic marker for a key horizon is absent. The results obtained by applying chemostratigraphic techniques, however, may be influenced by provenance and diagenetic alteration, and most importantly they still require biostratigraphy for final calibration (Rozanov et al., 2008). In any case, biostratigraphy remains a cost-effective means of correlating the Ediacaran–Cambrian transition. The biostratigraphy of the Soltanieh Formation is poorly
known and largely based on shelly fossils (Hamdi et al., 1989; Tashayoee et al., 2012) with very little published on ichnos- tratigraphy (CiabeGhodsi et al., 2006; Tashayoee et al., 2012). Small shelly fossils reported from the Soltanieh Formation are comparable with skeletal assemblages recorded in China and northern Siberia (Hamdi et al., 1989), containing elements of A. trisulcatus-P. anabarica andW. crosbyi assemblage zones of Meishucunian Stage, and the Pelagiella subangulata Taxon Zone of Qiongzhusian Stage (Steiner et al., 2007). Although the carbonate intervals (Lower, Middle and
Upper Dolomite members) do not contain trace fossils, the clastic deposits of the Soltanieh Formation (Lower and Upper Shale members) are host to trace fossils of biostratigraphic utility, which have largely been overlooked. The Lower Shale Member contains ichnotaxa that are known from both Ediacaran and early Cambrian rocks and those that are only known from the Phanerozoic. The former group includes the simple grazing
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