Journal of Paleontology, 91(6), 2017, p. 1178–1198 Copyright © 2017, The Paleontological Society 0022-3360/17/0088-0906 doi: 10.1017/jpa.2017.72
Ichnostratigraphy of the Ediacaran-Cambrian boundary: new insights on lower Cambrian biozonations from the Soltanieh Formation of northern Iran
Setareh Shahkarami, M. Gabriela Mángano, and Luis A. Buatois
Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E2, Canada ⟨
setareh.sh@
usask.ca⟩, ⟨
gabriela.mangano@
usask.ca⟩, ⟨
luis.buatois@usask.ca⟩
Abstract.—Strata in the Central AlborzMountains, northern Iran, are interpreted to show continuous sedimentation from Ediacaran through Cambrian times. The Soltanieh Formation consists of five members: Lower Dolomite, Lower Shale, Middle Dolomite, Upper Shale and Upper Dolomite members. The clastic units (Lower and Upper Shale members) represent sedimentation in distal marine settings, ranging from the shelf to offshore, and contain abundant trace fossils of biostratigraphic utility. Four ichnozones have been recognized. Ichnozone 1, containing Helminthoidichnites tenuis, Helminthopsis tenuis,and Cochlichnus anguineus, is early Fortunian based on small shelly fossils, and is interpreted as a distal expression of the Treptichnus pedum zone. Ichnozone 2, comprising the first occurrence of T. pedum, is middle Fortunian, and is best regarded as the upper half of the Treptichnus pedum Zone. Ichnozone 3 is late Fortunian–Cam- brian Age 2, characterized by a sudden change in abundance and complexity of trace fossils. Main elements in this ich- nozone include Cruziana problematica, Curvolithus isp., Phycodes isp., Treptichnus pedum, Treptichnus pollardi,and Treptichnus isp. Ichnozone 4 is of Cambrian Age 2–Age 3 and marked by the first appearances of Psammichnites gigas, Rusophycus avalonensis,and Didymaulichnus miettensis. Integration of trace fossils with small shelly fossils suggests that the Ediacaran–Cambrian boundary should be placed at the base of the Soltanieh Formation or within the Lower Dolomite Member. The delayed appearance of T. pedum and the low ichnodiversity in the Lower Shale and lower inter- val of the Upper Shale reflect limited colonization of settings below storm wave base during the early Fortunian.
Introduction
Defining the Ediacaran-Cambrian boundary and the precise timing of the Cambrian explosion has occupied the attention of paleontologists for decades. A very good measure of this inter- est is the considerable amount of work performed by the IUGS Cambrian Subcommission to settle a reference standard for the base of the Cambrian and internationally accepted series and stage subdivisions of the Cambrian system, which still are in progress (Narbonne et al., 1987; Brasier et al., 1994; Terfelt and Ahlberg, 2010; Babcock et al., 2011; Harvey et al., 2011; Peng and Babcock, 2011; Ahlberg and Terfelt, 2012; Ahn et al., 2012; Moczydlowska and Yin, 2012; Peng et al., 2012a, 2012b; Landing et al., 2013). Defining reference standards for the base of Cambrian and its subdivisions has promoted understanding of the successive stages of the Cambrian evolutionary radiation (Landing, 1998; Landing et al., 2013). A number of biostratigraphic and geochronologic techni-
ques are used for correlation of the Ediacaran-Cambrian boundary and subdivision of pre-trilobitic lower Cambrian. Traditionally, biostratigraphy of the lower Cambrian has been based on small shelly fossils (SSFs) and trace fossils. Small shelly fossils are largely found in carbonates, whereas trace fossils are more common in siliciclastics (Narbonne et al., 1987). Despite that, both groups are common, diverse, and show
rapid changes during this critical time of Earth history (Narbonne et al., 1987), Accordingly, they represent important biostratigraphic tools for the subdivision and correlation of pre-trilobitic strata of the Cambrian (e.g., Steiner et al., 2007). In spite of the various attempts to settle internationally
accepted series and stage subdivisions of the Cambrian system, the Ediacaran-Cambrian boundary and the subdivision of pre-trilobitic lowerCambrian strata require continued attention. In northern Iran, the Ediacaran–Cambrian Soltanieh Formation is well exposed across the Alborz Mountains, and thus has the potential to illumi- nate some of these issues. In comparisonwith Ediacaran–Cambrian successions elsewhere, the ichnologic content of the Soltanieh Formation is poorly known. The aims of this study are to: (1) document the Soltanieh Formation ichnofauna, (2) propose a bio- zonation scheme for the Soltanieh Formation based on trace fossils, (3) suggest themost plausible position for the Ediacaran-Cambrian boundary in northern Iran by using integration of trace fossil and SSF biostratigraphy, and (4) assess the implications of this ich- nostratigraphic scheme with respect to global schemes establishing biostratigraphic zonations for the lowermost Cambrian.
Geological setting
The Soltanieh Formation of the Alborz Mountains, northern Iran, regarded as Ediacaran–lower Cambrian, formed in the
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