Discriminating ecological and evolutionary controls

Palaeogeography, Palaeoclimatology, Palaeoecology 476 (2017) 15–27

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Discriminating ecological and evolutionary controls during the Ediacaran–Cambrian transition: Trace fossils from the Soltanieh Formation of northern Iran Setareh Shahkarami ⁎, M. Gabriela Mángano, Luis A. Buatois Department of Geological Sciences, University of Saskatchewan, 114 Science Place Saskatoon, SK S7N 5E2, Canada

a r t i c l e

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Article history: Received 3 October 2016 Received in revised form 10 March 2017 Accepted 11 March 2017 Available online 19 March 2017 Keywords: Ichnology Bioturbation Cambrian explosion Evolutionary paleoecology Facies controls

a b s t r a c t The nature of the Cambrian Explosion, one of the most critical events in Earth history, has been a major topic of discussion. The trend of increasing morphological complexity and taxonomic diversity of trace fossils within this interval has been used as evidence of the true evolutionary nature of the Cambrian Explosion. However, the distribution of trace fossils is controlled by a complex interplay between evolutionary and ecological constraints. The pre-trilobite Soltanieh Formation of northern Iran displays a significant vertical facies recurrence, allowing comparison of identical environments through time; therefore, its study provides a test case for evaluating the role of evolutionary and ecological controls on lower Cambrian trace-fossil distribution. An integrated study of sedimentary facies, small shelly fossil zones, sequence stratigraphy, and distribution of associated trace fossils indicates a complex interplay of these two sets of factors. The distribution and preservation of diagnostic early Cambrian trace fossils in the Lower Shale Member of the Soltanieh Formation is very likely to have been controlled by facies, especially within storm-generated siltstone-shale intercalations providing better preservation potential for interfacial trace fossils, therefore delaying the stratigraphic occurrence of Treptichnus pedum. In contrast, evolutionary innovations exerted the primary control on trace-fossil distribution in the Upper Shale Member, where the vertical distribution of trace fossils strongly matches that observed worldwide. This result suggests that, although trace fossils provide a valid means of global correlation, placing trace fossil assemblages within a paleoenvironmental and sequence-stratigraphic framework is a prerequisite of any sound evaluation of the evolutionary and biostratigraphic implications of trace fossils. © 2017 Elsevier B.V. All rights reserved.

1. Introduction The Cambrian Explosion represents one of the most critical events in Earth history, and has been the topic of numerous studies (Brasier, 1992a; Brasier et al., 1994a; Butterfield, 2003; Dzik, 2005; Jensen et al., 2005; Marshall, 2006; Maloof et al., 2010; Landing et al., 2013; Erwin and Valentine, 2013; Mángano and Buatois, 2014, 2016). These studies have elicited a wide range of views on the nature of this event, with two main conflicting perspectives historically dominating the debate; one view takes the explosion simply as an artefact of changing fossilization potential of mineralized skeletons as a result of significant changes in the environmental conditions of Earth (Brasier, 1982, 1992b; Brasier and Lindsay, 2001; Maloof et al., 2010), whereas the alternative view sees the Cambrian Explosion as a real biological event indicating a rapid rate of animal evolution (Conway Morris, 2000, 2006; Mángano and Buatois, 2014, 2016). In recent years, an alternative perspective, which underscores an early Neoproterozoic divergence of animal clades ⁎ Corresponding author. E-mail address: [email protected] (S. Shahkarami).

http://dx.doi.org/10.1016/j.palaeo.2017.03.012 0031-0182/© 2017 Elsevier B.V. All rights reserved.

and implies a macroevolutionary lag between the initial establishment of the bilaterian developmental toolkit and the first appearance of bilaterians in the fossil record, seems to be emerging (Erwin et al., 2011). Seilacher (1956) was the first to introduce the term “explosive evolution” based on the trend of increasing morphological complexity and taxonomic diversity of trace fossils within this interval (Zhu, 1997; Jensen, 2003; Jensen et al., 2006; Mángano and Buatois, 2014, 2016). Seilacher, and many authors after him, employed the pattern in the distribution of trace fossils to argue that the fact that the evolutionary burst affected trace fossils as much as body fossils refutes the notion that the Cambrian Explosion only results from the higher fossilization potential (Budd and Jensen, 2000, 2003; Seilacher et al., 2005; Mángano and Buatois, 2014). However, this explanation is not without dispute, because the ichnological record can be tied to a number of extrinsic processes that controlled benthic colonization; as a case in point, facies control on the distribution of trace fossils is a widely recognized problem for this interval and has been noted by several authors (Mount and McDonald, 1992; Mount and Signer, 1992; Brasier et al., 1994b; Goldring and Jensen, 1996; Lindsay et al., 1996; Kouchinsky et al., 2007; Maloof et al., 2010; Landing

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S. Shahkarami et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 476 (2017) 15–27

et al., 2013). In general, trace fossils are more common in siliciclastics than in carbonates (Narbonne, 1984; Buatois and Mángano, 2011), show narrow environmental ranges (Frey, 1975; Mount and Signer, 1985, 1992; Buatois and Mángano, 2011), and have their own set of taphonomic constraints (Hallam, 1975; Seilacher, 1978). As a result, first appearances of ichnotaxa and increases in trace-fossil diversity and abundance in early Cambrian strata may represent a combination of evolutionary changes and the occurrence of suitable sedimentary facies as a reflection of paleoenvironmental conditions. However, despite the importance of this complex interplay of evolutionary and ecological constraints, detailed analysis of the contribution of both sets of factors are rare (see MacNaughton and Narbonne, 1999 for a notable exception). In this paper, we have integrated various datasets, namely sedimentary facies, small shelly fossil zones, sequence stratigraphy and distribution of associated trace fossils, through a ~1000-m-thick early Cambrian succession in the Alborz Mountains, northern Iran. The pre-trilobite Soltanieh Formation displays a significant vertical facies recurrence, allowing comparison of identical environments through time, and therefore provides a test case for evaluating the role of evolutionary and ecological controls on lower Cambrian trace-fossil distribution. In addition, the Soltanieh Formation siliciclastic strata represent deposition in relatively distal settings, allowing evaluating infaunal colonization in environments right below and above storm wave base.

were measured, analyzed from a sedimentary facies perspective, and sampled, with emphasis on the trace fossil content. Trace fossils illustrated in this study were all collected from the Garmab section, where trace fossils are more abundant and better preserved. Trace fossils were examined in the field and/or in the lab. Detailed descriptions have been presented elsewhere (Shahkarami et al., in press). In addition to the field observations, rocks from the carbonate members were sampled systematically, and 25 thin sections were prepared for microfacies analysis of carbonate members. Previous studies by Hamdi et al. (1989) and CiabeGhodsi (2007) provide data on the small shelly fossils and the carbonate facies. 4. Biostratigraphy

The lower Cambrian Soltanieh Formation of northern Iran is a thick succession of mixed carbonate–siliciclastic rocks, containing abundant trace fossils and small shelly fossils, formed on a low-angle, relatively low-energy homoclinal carbonate ramp, developed in the ProtoPaleotethys passive margin of northwestern Gondwana (Alavi, 1996). The Soltanieh Formation can be traced along the Soltanieh Mountains, south east Zanjan (Figs. 1A–B and 2A–B) and also to several other localities, including the Garmab and Vali-Abad areas in the Alborz Mountains, along the Tehran–Chalous road, north of Tehran (Figs. 1A–B–D and 2A–C). This formation consists of five members, from bottom to top: the Lower Dolomite, Lower Shale, Middle Dolomite, Upper Shale, and Upper Dolomite members. At the type section (also known as Chopoghloo Dolomite), the Lower Dolomite Member is a 165-m-thick succession of dolomite to cherty dolomite and thin-bedded shale, the Lower Shale Member is a 120-m-thick succession of black shale, the Middle Dolomite Member is a 180-m-thick succession of stromatolitebearing dolomite and grey limestone, the Upper Shale Member is a 90-m-thick succession of grey-green shale to silty shale interbedded with sandstone and phosphatic limestone, and the Upper Dolomite Member is a 580-m-thick succession of stromatolites and cherty dolomite. The lithology of the Soltanieh Formation in the Alborz Mountains is similar to that of the type section, albeit with the presence of phosphatic deposits at the base of the Upper Shale Member in the more distal Vali-Abad section (Ashkan, 1986; Salehi, 1986; Hamdi et al., 1989), and absence of the black chert in the entire Garmab section and Upper Dolomite Member of Vali-Abad section. The Soltanieh Formation overlies the Bayandor Formation in the Soltanieh Mountains and the Kahar Formation in the Alborz Mountains, both being of Ediacaran age based on stratigraphic relationships in the former and the presence of acritarchs in the latter (Hamdi et al., 1989). The Soltanieh Formation gradationally passes upwards into the overlying Barut Formation (Cambrian Age 3), a succession of greenish and reddish shale and siltstone alternating with dark dolomites and chert (Salehi, 1986) (Fig. 1C).

Shahkarami et al. (in press) proposed four ichnozones for the Soltanieh Formation in the Garmab section. Age for each ichnozone was based on an integration of trace fossil and small shelly fossil evidence. Ichnozone 1 characterizes the middle interval of Lower Shale Member (from 157 m to 171 m above its base). This ichnozone is of low ichnodiversity, containing only three ichnotaxa of simple horizontal trails, namely Helminthoidichnites tenuis, Helminthopsis tenuis and Cochlichnus anguineus, whose oldest record occurs 157 m above the base of Lower Shale Member. This ichnozone is of early Fortunian age. Ichnozone 2 represents the uppermost interval of the Lower Shale Member, the Middle Dolomite Member and the lower interval of the Upper Shale Member (from 171 m above the base of the Lower Shale Member to 80 m above the base of the Upper Shale Member). A total of six ichnospecies occur in this zone, including Cruziana isp., Gordia arcuata, Helminthoidichnites tenuis, Helminthopsis tenuis, Palaeophycus tubularis, and Treptichnus pedum. This ichnozone is regarded as late Fortunian in age. Ichnozone 3 characterizes the middle interval of the Upper Shale Member (from 80 m to 178 m above its base). This ichnozone is defined by a sudden increase in 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, Helminthoidichnites tenuis, Helminthopsis tenuis, Palaeophycus tubularis, Planolites montanus, Treptichnus pedum, and Treptichnus isp. Ichnozone 3 is interpreted as late Fortunian–Cambrian Age 2. Ichnozone 4 represents the uppermost interval of the Upper Shale Member (from 178 m above its base to the base of the Upper Dolomite Member). This zone is marked based on the first appearance of large back-filled trace fossil (Psammichnites gigas), together with the bilobate, trilobitelike resting trace (Rusophycus avalonensis), and large locomotion trace (Didymaulichnus miettensis). The other elements of this ichnozone are Cruziana isp., Curvolithus isp., Palaeophycus tubularis, Planolites montanus, Phycodes isp., and Treptichnus pedum. This zone is regarded as Cambrian Age 2–3. With respect to SSF biozones, the assemblage zone 1 (Anabarites trisulcatus–Protohertzina anabarica or SSF1) is of Fortunian age and correlates with Ichnozones 1 and 2 of the Soltanieh Formation. Assemblage zone 2 (Watsonella crosbyi or SSF2) is of Cambrian Age 2 and correlates with Ichnozone 3 of the Soltanieh Formation. Assemblage zones 3 (Paragloborilus subglobosus or SSF3) is of Cambrian Age 3 and correlates in part with Ichnozone 4 of the Soltanieh Formation (Shahkarami et al., in press). The Ediacaran–Cambrian boundary is placed either at the base of the Soltanieh Formation or within the Lower Dolomite Member. This is supported by the presence of the Anabarites trisulcatus–Protohertzina anabarica zone in the upper interval of the Lower Dolomite Member (Hamdi et al., 1989).

3. Methods

5. Sedimentary facies

Two stratigraphic sections (one close to the Garmab village, along the Tehran–Chalous road, north of Tehran, and another at the Chopoghloo village, along the Soltanieh Mountains, southeast Zanjan)

Eight facies have been recognized in this succession, which can be grouped into the following facies associations: (1) siliciclastic-dominated Facies Association, and (2) carbonate-dominated Facies Association

2. Geological setting


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Fig. 1. Location, geologic map, and regional stratigraphy of the study area. (A) General map of Iran. Insert shows location of Fig. 1B. (B) Map showing the location of studied sections. Red insert shows location of Fig. 1D, and blue insert shows location of Fig. 2A. (C) Late Ediacaran–lower Cambrian stratigraphy of northern Iran (based on Stöcklin et al., 1964; Hamdi et al., 1989). (D) Geologic map of the Tehran–Chalous road, Garmab area, north of Tehran, Iran (modified from Aghanabati et al., 2005).

(Table 1). Facies are described in order from proximal (nearshore) to distal (basinal). In the paleoenvironmental scheme adopted for the siliciclastic-dominated Facies Association, the offshore is located between the fair-weather wave base and the storm wave base, and the shelf below the storm wave base (Buatois and Mángano, 2011). Sedimentary facies of the Soltanieh Formation are characterized in terms of lithology, sedimentary structures, bed contacts, bed geometry, fossil content, color, and texture. Degree of bioturbation in cross-section is assessed following Taylor and Goldring (1993), who defined bioturbation indices (BI), ranging from 0 (no bioturbation) to 6 (complete bioturbation), based on a previous scale by Reineck (1963). The percentage of bioturbation on bedding planes was estimated using the bedding-plane bioturbation indices (BP-BI) proposed by Miller and Smail (1997), ranging from 1 (no bioturbation) to 5 (60% to full coverage). Both BI and BP-BI have been assessed for each bed and a range is provided for each facies following conventional ichnological practice. Because of poor preservation of shale deposits, BI is heavily based on measurements performed in the intercalated sandstone. Event sandstone beds typically tend to display lower bioturbation intensities than the associated fair-weather deposits

and, therefore, this may have resulted in underestimation of degree of bioturbation. 5.1. Siliciclastic-dominated Facies Associations 5.1.1. Sandy hale and sandstone (F1) This facies is represented by up to 6-m-thick, coarsening upward cycles of interbedded greenish-grey silty shale and erosionally based, very fine- to medium-grained sandstone and sandy dolomite (Fig. 3). Sandstone intercalations form 0.3–1.0-mm-thick laminae and 1.0–6.0-cmthick beds. Silty shale is parallel laminated, whereas sandstone beds are sharp-based, normally graded, and display low-angle cross lamination, parallel lamination, wavy bedding, and wave ripples (Fig. 3C, D). In the Garmab section, this facies occurs in the uppermost interval of the Upper Shale Member (from 174 m above its base to the base of the Upper Dolomite Member). The outcrops of Upper Shale Member at the type section are poor due to weathering and erosion, therefore detailed description of facies at this locality is not provided. The sandy shale contains nine ichnotaxa; Psammichnites gigas (Fig. 4A) and Didymaulichnus miettensis (Fig. 4E) are the most dominant. The less common elements

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Fig. 2. Geologic map of the Soltanieh Mountain area. (A) Geologic map of the Soltanieh region, Zanjan, Iran (modified from Geological map of Zanjan; Stöcklin and Eftekharnezhad, 1969). (B) Schematic cross-section displaying the stratigraphic framework of the Soltanieh region, Zanjan, Iran (modified from Geological map of Zanjan; Stöcklin and Eftekharnezhad, 1969).

include Curvolithus isp., Rusophycus avalonensis, Cruziana isp., Palaeophycus tubularis (Fig. 4I), Phycodes sp., Planolites montanus, and Treptichnus pedum (Table 2). The BI for this facies is 0–2, whereas the bedding plane coverage is approximately 0–35%, resulting in a BP-BI of 1–3. This facies records low-energy, suspension fallout deposition punctuated by relatively frequent storm events represented by the sandstone beds, which are interpreted as proximal storm beds (tempestites) (Reineck, 1974; Aigner and Reineck, 1982; Allen, 1982; Dott and Bourgeois, 1982; Aigner, 1985; Arnott and Southard, 1990; Seilacher and Aigner, 1991; Schieber, 1999). The local presence of sandstone beds with oscillatory structures indicates deposition above the storm wave base (Myrow et al., 2002; Buatois et al., 2009). This facies

is interpreted as having been deposited in an upper offshore environment.

5.1.2. Silty shale and sandstone (F2) This facies mainly consists of parallel-laminated, greenish-grey silty shale with millimeter-scale lenses and laterally discontinuous laminae of erosionally based, very fine-grained sandstone (0.5–3.0-mm-thick). The silty shale is composed of laterally continuous alternating, 0.5– 4.0-mm-thick laminae of silty and clayey shale. Individual sandstone laminae show internal parallel- or ripple-cross lamination. Millimeterscale tool marks (Figs. 4H and 5C), and flute casts (Fig. 5F) are common at the base of the sandstones.

Table 1 Summary of facies and facies associations from the Soltanieh Formation. Facies

Lithology

Sedimentary structure

Depositional environment

Sandy shale, and sandstone (F1) Silty shale and sandstone (F2) Silty shale (F3) Dark-grey shale (F4) Stromatolite-bearing cherty-dolomite (F5) Peloidal dolo-grainstone (F6) Limestone–shale (F7)

Interbedded greenish-grey silty shale and fine- to medium-grained sandstone Greenish-grey silty shale with millimeter scale lenses and thin beds of very fine-grained sandstone Dark-grey shale and fine- to very fine-grained silty beds Dark-grey shale Stromatolite-bearing, black chert and dolomite

Low angle cross lamination, parallel lamination, wavy bedding, and wave ripples Parallel or ripple-cross lamination, and less commonly tool marks Parallel lamination Parallel lamination and massive with wrinkle marks Desiccation cracks, peloids, and fenestral structures

Upper offshore

Grey to blue dolo-grainstone

Well bedded to massive dolomite, moderately sorted peloids and intraclastic

Phosphate-bearing carbonate (F8)

Dark-grey massive calcareous shale, silty shale, and rhythmically bedded limestone Black phosphatic limestone

Peloidal, intraclastic, bioclastic and oolitic

Lower offshore Proximal shelf Distal shelf Upper intertidal to supratidal Intertidal–Shallow subtidal Shallow subtidal Deep subtidal


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Fig. 3. Sedimentary facies from the Upper Shale Member of the Soltanieh Formation, Garmab section. (A) Outcrop photograph, showing general view of the trace-fossil bearing beds. (B) Close-up view of E, showing silty shale and sandstone layers of Facies 1. (C) Rippled bed top. (D) Wave ripple on top of sandstone bed. (E) Close-up view of A showing interbedded silty shale and sandstone of Facies 1. (F) General bedding-plane view of sandy shale deposits, Facies 1.

Fig. 4. Selected trace fossils from the Soltanieh Formation, Garmab section. (A) Palaeophycus tubularis, Facies 1, Upper Shale Member, positive hyporelief. (B) Didymaulichnus miettensis, Facies 1, Upper Shale Member, positive hyporelief. (C) Psammichnites gigas, Facies 1, Upper Shale Member, positive epirelief. (D) Rusophycus avalonensis, Facies 1, Upper Shale Member, positive hyporelief. (E) Treptichnus pedum on the base of sandstone bed, Facies 2, Upper Shale Member, positive hyporelief. (F) Cruziana problematica, Facies 2, Upper Shale Member, positive hyporelief. (G) Treptichnus pedum, Facies 3, Lower Shale Member, positive hyporelief. (H) Curvolithus isp., Facies 2, Upper Shale Member.

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Table 2 Environmental distribution of trace fossils in the Soltanieh Formation. Siliciclastic-dominated Facies Association F1 Trace-fossil Cruziana isp., Curvolithus isp., assemblage Didymaulichnus miettensis, Palaeophycus tubularis, Phycodes isp., Planolites montanus, Psammichnites gigas, Rusophycus avalonensis, Treptichnus pedum

F2

F3

F4

Cochlichnus anguineus, Cruziana problematica, Curvolithus isp., Gordia marina, Helminthoidichnites tenuis, Helminthopsis tenuis, Palaeophycus tubularis, Phycodes isp., Planolites montanus, Treptichnus isp.

Cruziana isp., Helminthopsis tenuis, Helminthoidichnites tenuis, Palaeophycus tubularis, Treptichnus pedum

Cochlichnus anguineus, Helminthopsis tenuis, Helminthoidichnites tenuis

In the Garmab section, Facies 2 comprises most of the Upper Shale Member (up to 129-m-thick), from 45 m to 174 m above its base (Fig. 5A, D, E). The silty shale and sandstone of Facies 2 contains twelve ichnospecies. The most common elements include Cruziana problematica, Helminthoidichnites tenuis, Helminthopsis tenuis (Fig. 4D), Palaeophycus tubularis (Fig. 4D, G), Treptichnus pollardi, Treptichnus pedum (Fig. 4C), and Planolites montanus. Phycodes isp., Curvolithus isp. (Fig. 4F), Cochlichnus anguineus, Gordia marina, Treptichnus isp. are also present in lower numbers (Table 2). The BI for this facies is 0–2, whereas the bedding plane coverage is approximately 0–60%, resulting in a BP-BI of 1–4 (Figs. 4D, G and 5A). This facies records low-energy, suspension fallout deposition punctuated by very rare storm events recorded by the sandstone beds. The thin sandstone beds with erosional bases and oscillatory structures are interpreted as distal tempestites (Gadow and Reineck, 1969; Aigner and Reineck, 1982; Aigner, 1985; Arnott and Southard, 1990; Seilacher

and Aigner, 1991; Bádenas and Aurell, 2001; Dumas and Arnott, 2006), representing storm deposition immediately above the storm wave base. These tempestites differ from proximal storm deposit of Facies 1 by being thinner, better sorted, and finer grained (Fürsich and Oschmann, 1993). This facies is interpreted as having been deposited in a lower offshore environment. 5.1.3. Silty shale (F3) This facies consists of up to 45 m of parallel-laminated to massive, dark-grey shale and thinly bedded shale-siltstone alternations (1.0– 3.0-mm-thick). In the type section, Facies 3 occurs in the lower interval of Lower Dolomite Member (from 27 m to 55 m above its base); this facies also comprise most of the Lower Shale Member (up to 95 m) in this section. In the Garmab section, Facies 3 occurs in the upper interval of Lower Shale Member (from 171 m above its base to its top), and in the lower interval of the Upper Shale Member (from its base to 45 m

Fig. 5. Sedimentary facies from the Lower and Upper Shale members of the Soltanieh Formation, Garmab section. (A) Outcrop photograph, showing general view of the trace-fossil bearing beds, Facies 2, Upper Shale Member. (B) Wrinkled bed surfaces, Facies 4, Lower Shale Member. (C) Tool marks on the base of sandstone bed, Facies 2, Upper Shale Member. (D) Silty shale deposit of Facies 2, Upper Shale Member. (E) Bottom view of storm beds, Facies 2, Upper Shale Member. (F) Flute cast on the base of sandstone bed, Upper Shale Member.


above it). The silty shale of Facies 3 contains six ichnotaxa: Palaeophycus tubularis, Helminthoidichnites tenuis, Helminthopsis tenuis, and Treptichnus pedum (Fig. 4B) are the most dominant ones, but Cruziana isp., and Gordia arcuata, are also present (Table 2). The BI for this facies is 0–1, while the bedding plane coverage is approximately 0–15%, resulting in a BP-BI of 1–3. The dominance of shale and siltstone together with the absence of oscillatory structures indicate low-energy, suspension fallout deposition, below storm wave base. The thin siltstone beds may represent the distal ends of storm-generated turbidity currents, which carried coarser-grained material into deeper water below storm wave base (Hamblin and Walker, 1979; Benton and Gray, 1981; Pedersen, 1985; Myrow and Southard, 1996). This facies is interpreted as having been deposited in a proximal shelf environment.

5.1.4. Dark grey shale (F4) This facies consists of laterally extensive, dark-grey parallel-laminated and massive shale. The thickness of individual bed ranges from a few millimeters (1–4 mm) to a few centimetres (4–10 cm). Wrinkle marks are present in this facies (Fig. 5B). This facies has been only observed in the Garmab section, where it comprises most of the Lower Shale Member, from 30 m to 171 m above its base (141-m-thick). This facies contains three ichnotaxa, Helminthoidichnites tenuis, Helminthopsis tenuis and Cochlichnus anguineus (Table 2). The BI for this facies is 0, whereas the bedding plane coverage is approximately 0–5%, resulting in a BP-BI of 1–2. The paucity of trace fossils in shelf deposits is commonly taken as evidence of reduced oxygen content (e.g., Bromley and Ekdale, 1984; Wetzel, 1991; Uchman and Wetzel, 2011). In the Soltanieh Formation, however, no evidence of drastic anoxia is apparent; pyrite is rare and these deposits include poorly diverse, non-specialized grazing trails. The absence of wave- and current-generated structures indicates deposition from suspension fallout in quietwater conditions well below the storm wave base (Arnott, 1991; Plint, 2010). This facies is interpreted as having been deposited in a distal shelf environment.

5.2. Carbonate-dominated Facies Associations Analysis of the carbonate-dominated Facies Association has been done mainly based on the observation in the field and microfacies analysis of thin sections. Carbonates from the Soltanieh Formation are largely affected by multiple episodes of dolomitization and dolomite recrystallization, which obscured or obliterated the primary fabrics and sedimentary structures in the dolomites, making recognition of the precursor lithology difficult. Previous descriptions and interpretations of the carbonate facies have been presented by CiabeGhodsi (2007) and his information is integrated within the framework of our study. Trace fossils are absent in the carbonate-dominated Facies Association. 5.2.1. Stromatolite-bearing cherty-dolomite (F5) This facies consists of rhythmic, laterally continuous, stromatolitebearing, black chert and dolomite with desiccation cracks, peloids, and fenestral structure (CiabeGhodsi, 2007) (Fig. 6D) forming up to 85-mthick unit. Chert intercalations occur as 10–15-mm-thick laminae and 5–15-cm-thick beds (Fig. 6C). In the type section, the cherty dolomite occurs in several intervals (up to 64-m-thick) within the Lower Dolomite Member, the upper most interval of Middle Dolomite Member (152 m above its base to the base of Upper Shale Member), and also through the Upper Dolomite Member (up to 85-m-thick). The chert layers are absent from this facies in the Garmab section. Features, such as fenestrae, desiccation cracks and peloids, indicate an upper intertidal to supratidal environment for this facies (Shinn, 1968, 1983; Lucia, 1972; Mountjoy, 1975; Tucker and Wright, 1990; Pratt, 2010).

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5.2.2. Peloidal dolo-grainstone (F6) This facies consists of well-bedded to massive, grey to blue, moderately sorted peloidal and intraclastic dolo-grainstone (CiabeGhodsi, 2007). The thickness of individual beds ranges from 2 to 20 cm. The peloids constitute the main components in this facies. Facies 6 occurs in the upper half of the Middle Dolomite Member in the two sections (CiabeGhodsi, 2007). Peloids are distinct components of low-energy shallow intertidal and subtidal carbonates (Flügel, 2004). This facies is common in protected shallow-marine environments with moderate water circulation in intertidal-shallow subtidal zones (James and Jones, 2016)

5.2.3. Rhythmic lime-mudstone facies (F7) This facies is characterized by parallel-laminated, current-rippled and low-angle cross-laminated limestone (packstone and wackestone) and intercalated dark-grey massive calcareous shale and silty shale, forming up to 30-m-thick intervals, with the thickness of the shale increasing upward, whereas the limestone thickness decreases (CiabeGhodsi, 2007). The thickness of individual beds ranges from 5 mm to 15 cm. Individual beds can be traced laterally without major thickness change (Fig. 6A, B). The limestone and shale interlayers commonly display planar or undulatory contacts. Bedding boundaries are sharp; irregularity of bedding boundaries where present might have been enhanced by compaction during early digenesis (Chen et al., 2010). This facies gradationally overlies the cherty-dolomite Facies and grades upward to the silty shale Facies. Chert nodules and lenses are relatively common. In the type section, this facies occurs in the middle interval of Upper Shale Member (from 45 m above its base up to 25 m), and at the base of the Upper Dolomite Member (from its base to 10 m above it). In the Garmab section, Facies 7 occurs in the lowermost interval of the Lower Shale Member (from its base to 30 m above), and at the base of the Middle Dolomite Member in the two sections. The limestone–shale couplets indicate cyclic deposition of siliciclastic and carbonate sediments in the transition zone between a clastic sedimentation-dominated zone and a carbonate sedimentationdominated zone (CiabeGhodsi, 2007). Deposition of the shale from suspension fall-out, and lateral continuity of beds indicate period of quiet condition, in the absence of erosion. This facies is interpreted as having been deposited by high-energy-storm currents in shallow low-energy subtidal setting, where carbonate production and siliciclastic input coexist (Chen et al., 2010; Bayet-Goll et al., 2015).

5.2.4. Phosphate-bearing carbonate (F8) This facies consists of black phosphatic limestone with diverse SSFassemblages. The phosphate contains pyrite, glauconite, goethite, clays (illite), and some siliceous, feldspathic and calcareous particles with peloidal, intraclastic, bioclastic and oolitic structures (CiabeGhodsi, 2007). Stromatolites are also present (Salehi, 1986; Sharifi, 2005). This facies has been only reported from the base of Upper Shale Member, in the Vali-Abad section (Ashkan, 1986; Hamdi et al., 1989; Sharifi, 2005). Lower Cambrian phosphorite deposits of the Soltanieh Formation show similarities to those of the Upper Phosphate Member in Yunnan and the Karatau Member in southeastern Kazakhstan (Shahkarami et al., in press). The Yunnan and Karatau deposits are characterized by associated shallow-water carbonates, particularly dolomite, overlain by or in places interdigitating with phosphorite and black shale (Zhu, 1997; Weber et al., 2013). The phosphatic beds are interpreted to be associated with condensed sections, formed during the onset of transgression under a period of slow sedimentation (Föllmi et al., 1994). This facies has been deposited in the deep subtidal region, probably under reducing conditions (e.g., Álvaro et al., 2010).

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Fig. 6. Sedimentary facies from the Lower Dolomite Member and the Upper Shale Member of the Soltanieh Formation, Garmab section. (A) Outcrop photograph, showing general view of the rhythmic beds of Facies 7. (B) Close-up view of A, showing the lowermost deposits of the Lower Shale Member, Facies 7. (C) Chert bed and dolomite, Facies 5, Lower Dolomite Member. (D) Stromatolitic cherty dolomite, Facies 8, Lower Dolomite Member.

6. Sequence stratigraphy

6.1. Depositional sequence 1 (DS1)

Based on sequence stratigraphic concepts and predictable geometric relationships between facies, lower Cambrian strata of the Soltanieh Formation have been subdivided into two third-order depositional sequences. The Lower Dolomite Member records the highstand systems tract of an older depositional sequence. Both depositional sequences identified are mixed siliciclastic–carbonate sequences. There are two main types of mixed-lithology sequences, lower carbonate–upper clastic sequences, in which the clastics, generally fine- to coarse-grained sandstone, occur above carbonates towards the top of a sequence, and lower mudrock–upper carbonate sequences, in which the clastics, dominantly mudrocks, occur in the lower part of the units, beneath the carbonates (Tucker, 2003; Catuneanu et al., 2011). Depositional sequences of the Soltanieh Formation are of the latter type, where the open marine clastic, lower part mostly represents transgressive facies, whereas the shallow subtidal to peritidal upper carbonates represent prograding shallow-water highstand deposits. In a mixed siliciclastic-carbonate system such as this, the sequence boundary is a co-planar surface, an amalgamated flooding surface/sequence boundary, and therefore is overlain not by lowstand systems tract deposits, but by transgressive systems tract deposits (MacEachern et al., 1992; Buatois and Mángano, 2011).

Drowning of the carbonate platform followed by deposition of rhythmic limestone–shale, marks the beginning of the DS1 as recorded by the top of the Lower Dolomite Member. The basinal shales of the Lower Shale Member mantled the sequence boundary on top of the Lower Dolomite Member (Fig. 7A–C). The upward change from the basinal shale Facies (F4) to silty shale Facies (F3) and progradation from distal to proximal shelf at the top of the Lower Shale Member followed by the gradual transition from Lower Shale Member to the Middle Dolomite Member suggests a decrease in the rate of relative sea-level rise resulted in shallowing and change from retrogradational to aggradational-to-progradational stacking patterns. Thus, this transition marks a change from transgressive to highstand systems tracts (Fig. 7A–C). Following the drowning of carbonate platforms and shale onlap in response to a rapid relative sea-level rise, carbonate deposition was re-established through start-up, catch-up, and keep-up phases (Kendall and Schlager, 1981; Glumac and Walker, 2000). Deposition of the mixed shale and carbonate strata at the base of the Middle Dolomite Member is equivalent to the start-up of carbonate platform deposition. The rest of the shallow subtidal succession of the Middle Dolomite Member is characteristic of a catch-up phase during which


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Fig. 7. Correlation of the three sedimentary successions and sequence-stratigraphic interpretation of the Soltanieh Formation. (A) Chopoghloo. (B) Garmab. (C) Vali-Abad. Vali-Abad log (modified from Hamdi et al., 1989 and CiabeGhodsi, 2007).

the carbonate accumulation rate exceeded the rate of sea-level rise and the platform aggraded to sea level. 6.2. Depositional sequence 2 (DS2) Likewise, the contact between the Middle Dolomite Member and the Upper Shale Member is interpreted as a sequence boundary. The sequence boundary is a flooding surface resulting in drowning of the carbonate platform, marked by the abrupt change from the shallow subtidal to intertidal peloidal dolo-grainstone carbonate Facies (F6) of the Middle Dolomite Member to the phosphate-bearing carbonate Facies (F8), and the proximal shelf Facies (F3) at the base of the Upper Shale Member (Fig. 7A–C). Thus, the lower part of the Upper Shale Member represents a retrogradational parasequence set of a

transgressive systems tract that reflects the backstepping of deeperwater shelfal siliciclastic deposits on top of the carbonate platform as a consequence of an increased rate of relative sea-level rise (Srinivasan and Walker, 1993; Glumac and Walker, 2000). As the transgression continued, phosphate nodules became smaller and carbonate sparser and grading into the silty shale Facies of the lower Upper Shale Member. Deposition of the proximal shelf, silty shale Facies (F3) of the Upper Shale Member corresponds to the period of maximum flooding of the carbonate platform. The upward change from silty shale Facies (F3) to the silty shale and sandstone Facies (F2), and sandy shale Facies (F1) at the top of the Upper Shale Member suggests shallowing, consistent with the early phase of highstand. The highstand systems tract of the DS2 is marked by deposition of upper offshore facies of the Upper Shale Member, and peritidal facies of the Upper Dolomite Member (Fig. 7A–C).

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Phosphates are usually interpreted as the result of an upwelling event (Brasier, 1990). However, marine transgressions are also commonly invoked as a mechanism of phosphate concentration; rising sea levels lead to higher biologic activity during flooding of shelf environments (Mitchell and Garson, 1981). Major phosphorite deposition appears to broadly correlate with elevated sea levels and, more specifically, with marine transgressions (e.g., McKelvey et al., 1959; Cressman and Swanson, 1964; Glenn and Arthur, 1990; Föllmi, 1990; Herbig and Trappe, 1994; Abed et al., 2007; Machalski and Olszewska-Nejbert, 2016). This process is associated with apparent marine hiatuses, representing omission surfaces or marine hardgrounds (Glenn et al., 1994). The phosphate-bearing carbonate of Facies 8 exhibits typical characteristics of condensed, sediment-starved deposits found directly above transgressive surfaces, including enrichment in organics and authigenic minerals (pyrite and phosphate) (Ashkan, 1986; Sharifi, 2005). 7. Evolutionary and ecological controls Crimes (1992), and Crimes and Anderson (1985) have suggested that the archetypal ichnofacies (Seilacher, 1967) had not fully evolved in the early Cambrian. These authors indicated that lower Cambrian trace fossils show a lower degree of environmental dependence than at any other time in the Phanerozoic. Narbonne et al. (1987) noted that the transition from the Harlaniella podolica Zone (Ichnozone I) to the Phycodes pedum Zone (Ichnozone II) in the GSSP at Fortune Head does not coincide with significant environmental or lithologic changes. Hence, they concluded that environmental factors were only a secondary control on the appearance of the Cambrian-type assemblage in that section. MacNaughton and Narbonne (1999) reached a similar conclusion for the Ediacaran–Cambrian succession in the Mackenzie Mountains of western Canada, proposing that depositional environment and facies exerted significant, but second-order controls on the vertical succession of ichnofossils and that evolution was the dominant control on the order of first appearances. Further work in these same strata by Carbone and Narbonne (2014) has essentially confirmed the primary role of evolutionary controls in trace-fossil distribution in the Ediacaran-Cambrian succession of the Mackenzie Mountains. However, in contrast to these studies, Mount and McDonald (1992), Mount and Signer (1992), Lindsay et al. (1996), and Goldring and Jensen (1996) have argued for significant environmental and facies control (as well as the influence of non-actualistic sedimentary factors) on the first appearance of trace fossils of earliest Cambrian aspect in Australia, California, and Mongolia. They therefore casted some doubts on the general utility of trace fossil in biostratigraphy of the Ediacaran– Cambrian boundary. In comparison with other Ediacaran–Cambrian successions worldwide, the Soltanieh Formation displays a recurrent stacking pattern of sedimentary facies and, therefore, the vertical repetition of sedimentary environments. This allows the tracking of changes in ichnofaunal composition in similar environments through this critical time of biotic changes, making it possible to discriminate between ecologic and evolutionary controls. The siliciclastic facies of the Soltanieh Formation bears trace fossils, and hence are directly relevant to the environmental interpretation of trace-fossil distribution, whereas the carbonate facies are barren of trace fossils, and are therefore excluded from the discussion below. The siliciclastic facies of the Soltanieh Formation represent three depositional environments, upper offshore (Facies 1), lower offshore (Facies 2), and shelf (Facies 3 and 4). The shelf is divided into two subenvironments, distal and proximal shelf, based on the presence of storm-induced turbidity currents in the latter. In the Lower Shale Member, the upward facies changes from fine-grained distal shelf deposits of Facies 4 to proximal shelf deposits of Facies 3 coincides with a notable change in the trace-fossil assemblages from low diversity, low abundance, “Ediacaran-like” assemblage including Helminthoidichnites tenuis, Helminthopsis tenuis and Cochlichnus anguineus in the distal shelf

facies (Fig. 8A) to a more diverse, more complex assemblage containing the earliest branching burrow Treptichnus pedum and the oldest bilobate, locomotion and grazing trace Cruziana isp. in the proximal shelf facies (Fig. 8B). In addition to changes in ichnotaxonomic composition, the passage from distal to proximal shelf facies was accompanied by a slight increase in bioturbation intensity (BI 0 in Facies 4 to BI 0–1 in Facies 3). However, these trace fossils are essentially restricted to bedding planes and no apparent change in BP-BI is detected through the Lower Shale Member. The co-occurrence between ichnofaunal and facies changes could indicate that environmental controls played a major role on benthic fauna during deposition of the Lower Shale Member of the Soltanieh Formation. The strong facies linkage is also responsible for the absence of the Cambrian index trace fossil Treptichnus pedum in these lower Fortunian distal shelf deposits and its first appearance within the silty shale deposits of proximal shelf at the top of the Lower Shale Member. Buatois et al. (2013) noted that, although Treptichnus pedum has a broad environmental tolerance, it shows high values of peak abundance in the upper offshore and lower intertidal sand flats, displaying a preference for sandy substrates. Therefore, the distal shelf shale of the Lower Shale Member may have represented the seaward limit of the T. pedum producer in the analyzed deposits. 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 T. pedum postdates the Ediacaran–Cambrian boundary. Two distinct trace-fossil assemblages are recognized within the lower offshore deposits of the Soltanieh Formation, represented by silty shale and sandstone of Facies 2. One is a low diversity trace-fossil assemblage, containing Treptichnus isp., Palaeophycus tubularis, Helminthoidichnites tenuis, Helminthopsis tenuis (Fig. 8C). Bioturbation in cross-section is negligible to minimum (BI 0–1), whereas density of trace fossils on bedding planes is remarkably low (BP-BI 1–2). This assemblage occurs in the lower interval of the Upper Shale Member (From 45 m to 80 m above its base), corresponding to strata representing ichnozone 2. The other is a more diverse trace-fossil assemblage, displaying an increase in complexity and abundance of trace fossils. This assemblage contains Cochlichnus anguineus, Cruziana problematica, Helminthoidichnites tenuis, Helminthopsis tenuis, Palaeophycus tubularis, Treptichnus pollardi, Treptichnus pedum, Planolites montanus, Phycodes isp., Curvolithus isp., Gordia marina, and Treptichnus isp. (Fig. 8D). Parallel to changes in ichnofaunal composition, an increase in bioturbation intensity in cross-section (BI 0–2) and on bedding plane (BP-BI 1–4) is apparent. This assemblage occurs in the middle interval of the Upper Shale Member (from 80 m to 174 m above its base) and corresponds to strata representing ichnozone 3. Trace fossils in both assemblages are preserved at the base of the thin sandstone beds included in Facies 2. This type of bed-interfaces generally results from event deposition, an interpretation further supported by the tool marks on the base of the sandstone beds, and indicates animals burrowing into the sediment to the mud–sand interface (Jensen et al., 2005). Heterolithic bedding characterized by moderately thin, generally centimeter-scale, commonly sharp-based sandstone and siltstone beds, separated by mudstone layers, tend to show greatest diversity of trace fossils in Cambrian rocks and elsewhere (Jensen et al., 2005). The upward increase in bioturbation intensity (both BI and BP-BI) and in diversity, abundance, and complexity of trace fossils in the lower offshore deposits of the Upper Shale Member occurs within the same facies, and hence, cannot be tied to environmental or lithological changes, reflecting instead evolutionary innovations. Likewise, two discrete trace-fossil assemblages characterize the upper offshore deposits of the Soltanieh Formation. One assemblage occurs in the upper interval of the Upper Shale Member (from 174 m to 178 m above its base), corresponding to strata that represent ichnozone 3. This assemblage consists of Cruziana problematica, Palaeophycus tubularis, Treptichnus pollardi, and Curvolithus isp. (Fig. 8E). Bioturbation


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Fig. 8. Schematic block diagram illustrating the trace-fossil assemblages associated with each depositional environment in the Soltanieh Formation. (A) Trace-fossil assemblage of Facies 4 (distal shelf, Lower Shale Member). (B) Trace-fossil assemblage of Facies 3 (proximal shelf, uppermost interval of the Lower Shale Member, and lower interval of the Upper Shale Member). (C) Trace-fossil assemblage of Facies 2 (lower offshore, lower to middle interval of the Upper Shale Member). (D) Trace-fossil assemblage of Facies 2 (lower offshore, middle to upper interval of the Upper Shale Member). (E) Trace-fossil assemblage of Facies 1 (upper offshore, upper interval of the Upper Shale Member). (F) Trace-fossil assemblage of Facies 1 (upper offshore, uppermost interval of the Upper Shale Member). Cochlichnus (Co), Cruziana (Cr), Curvolithus (Cu), Didymaulichnus (Di), Gordia (Go), Helminthoidichnites (Hi), Helminthopsis (He), Planolites (Pl), Palaeophycus (Pa), Phycodes (Py), Psammichnites (Ps), Rusophycus (Ru), Treptichnus (Tr), wrinkled structure (Wr). No upper offshore deposits occur in the Soltanieh Formation for strata representing ichnozone 1 and ichnozone 2, whereas no lower offshore deposits are present in strata corresponding to ichnozones 1 and 4, and no shelf deposits have been identified in strata representing ichnozones 3 and 4.

intensity is low in both cross-section (BI 0–1) and on bedding plane (BPBI 1–2). The other assemblage, however, shows very different characteristics in that it contains the first appearance of relatively large trace fossils, including the back-filled trace fossil Psammichnites gigas, the locomotion trail Didymaulichnus miettensis, and the trilobite resting trace Rusophycus avalonensis (Fig. 8F). A slight increase in degree of bioturbation is detected in these deposits (BI 0–2, BP-BI 1–3). This assemblage occurs in the uppermost interval of the Upper Shale Member (from 178 m above its base to the base of the Upper Dolomite Member), corresponding to strata representing ichnozone 4. The producer(s) of Psammichnites and Didymaulichnus, together with trilobites, are believed to be major early Paleozoic sediment disturbers (Jensen, 1997), and their first occurrence has been taken as evidence of macroevolutionary innovations resulting from the Cambrian Explosion, having been reported in the Chapel Island and Random formations, southeastern Newfoundland, Canada (Crimes and Anderson, 1985; Narbonne et al., 1987); the Wood Canyon Formation, eastern California (Jensen et al., 2002); the Arumbera Sandstone of central Australia (Walter et al., 1989); the Meishucun section, Yunnan Province, China (Crimes and Jiang, 1986; Zhu, 1997); the Mackenzie Mountains, northwest Canada (MacNaughton and Narbonne, 1999); the Uratanna Formation, Australia (Droser et al., 1999); and the Bayangol Formation, Mongolia (Goldring and Jensen, 1996; Smith et al., 2015). As in the case of the lower offshore, the differences between the two assemblages cannot be attributed to environmental factors because both occur in the same facies, therefore most likely reflecting evolutionary innovations.

To summarize, the Soltanieh ichnofauna was controlled by an interplay of environmental and evolutionary factors. Our study supports the conclusions by MacNaughton and Narbonne (1999) that evolution is a first rank control on trace-fossil distribution through the EdiacaranCambrian, with depositional environment and facies representing a second-order (albeit significant) control. In the Soltanieh Formation, the “Ediacaran” aspect of the lower Fortunian shelf trace-fossil assemblage and the delayed appearance of Treptichnus pedum in the stratigraphic succession most likely reflects environmental controls. Both features of these shelf environments may reflect the delayed colonization of sub-storm wave settings during the early Fortunian with respect to their more proximal counterparts. Although the absence of coeval deposits formed above storm wave base in the Soltanieh Formation precludes testing this hypothesis, the delayed colonization scenario is consistent with ichnological information from settings above storm wave base elsewhere (e.g., Mángano and Buatois, 2014, 2016). In addition, the lack of lithologic interfaces in shale may have been detrimental for trace-fossil preservation. On the contrary, the contrasting nature between upper and lower offshore trace-fossil assemblages throughout the Soltanieh Formation cannot be explained by environmental controls, reflecting instead evolutionary innovations within specific sub-environments along the depositional gradient. Our study on the Soltanieh ichnofauna underscores the importance of integrating multiple datasets (e.g. sedimentologic, sequence-stratigraphic) in evaluating the utility of biostratigraphic schemes and understanding evolutionary innovations through the Ediacaran-Cambrian.

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8. Conclusions An integrated study of sedimentary facies, small shelly fossil zones, sequence stratigraphy, and distribution of associated trace fossils in the Soltanieh Formation indicates a complex interplay between evolutionary and ecological factors. Distribution of trace fossils and preservation of diagnostic early Cambrian trace fossils in the Lower Shale Member of the Soltanieh Formation is very likely to have been controlled by facies, especially within storm-generated silt-shale intercalations providing better preservation potential for interfacial trace fossils. Also, the “Ediacaran” aspect of the shelfal lower Fortunian trace-fossil assemblage may reveal a delayed colonization of sub-storm wave environments. In contrast, evolutionary innovations exerted the primary control on trace-fossil distribution in the Upper Shale Member, where the vertical distribution of trace fossils strongly matches that observed worldwide. Environmental controls on trace-fossil distribution in these deposits are unlikely because identical sedimentary facies representing specific areas along the depositional gradient (e.g., upper offshore) were compared through time. Integration of the trace-fossil record in the context of facies analysis, sequence stratigraphy and SSF zonations, suggests that, although trace fossils could provide a valid means of global correlation, placing trace-fossil assemblages within a paleoenvironmental and sequence-stratigraphic framework is a prerequisite of any sound evaluation of the evolutionary and biostratigraphic implications of trace fossils.

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