The Ediacaran-Cambrian Transition: Sedimentary Facies Versus Extinction La Transición Ediacárico-Cámbrica: Facies Sedimentarias Versus Extinción

Estudios Geológicos julio-diciembre 2019, 75(2), e099 ISSN-L: 0367-0449 https://doi.org/10.3989/egeol.43601.554

The Ediacaran-Cambrian transition: sedimentary facies versus extinction La transición ediacárico-cámbrica: facies sedimentarias versus extinción

J.G. Gehling1, D.C. García-Bellido1, M.L. Droser2, M.L. Tarhan3, B. Runnegar4 1Earth Sciences Section, South Australian Museum, North Terrace, Adelaide, 5000 South Australia, Australia. Email: jim. [email protected]; ORCID ID: https://orcid.org/0000-0001-8459-5440, http://orcid.org/0000-0003-1922-9836 2Department of Earth Sciences, University of California at Riverside, Riverside, California, USA. ORCID ID: http://orcid. org/0000-0001-7112-5669 3Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA. ORCID ID: https://orcid. org/0000-0002-5878-929X 4Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, CA 90095-1567, USA. ORCID ID: http://orcid.org/0000-0002-7412-7982

ABSTRACT

Recent analysis of the terminal Ediacaran, Rawnsley Quartzite, in the Flinders Ranges of South Australia, demonstrates that key taxa of the Ediacara biota are restricted to certain sedimentary facies and stratigraphic levels. The Rawnsley Quartzite consists of three members separated by disconformities: (i) the basal, shallow marine Chace Sandstone Member is unfossiliferous, but replete with textured organic surfaces; (ii) the overlying Ediacara Sandstone Member fills submarine incisions cut through the underlying Chace Quartzite Member and paralic Bonney Sandstone below the Rawnsley Quartzite; and (iii) the Ediacara Sandstone Member is incised by the less fossiliferous Nilpena Sandstone Member that caps the Rawnsley Quartzite. Keywords: Facies control; Flinders Ranges; Ediacaran; Australia.

RESUMEN

Un estudio reciente de la Cuarcita de Rawnsley, en el Ediacárico terminal Ediacaran de la Cordillera de Flinders, Australia meridional, demuestra cómo algunos taxones clave de la biota de Ediacara están restringidos a ciertas facies sedimentarias y determinados niveles estratigráficos. La Cuarcita de Rawnsleycomprende tres miembros separados por discotinuidades: (i) el Miembro basal de la Arenisca de Chace es somera y azoica, aunque desta- can las superficies con texturas orgánicas; (ii) el Miemrbo de la Arenisca de Ediacara rellena un Sistema de inci- siones submarinas que recortan el miembro inferior de Chace y la Arenisca parálica de Bonney, infrayacente a la Cuarcita de Rawnsley; y (iii) el Miembro de la Arenisca de Ediacara es asimismo recortada de forma erosiva por el Miembro de la Arenisca de Nilpena, menos fosilífera. Palabras clave: Control de facies; Cordillera de Flinders; Ediacárico; Australia.

Recibido el 16 de mayo de 2019; Aceptado el 1 de septiembre de 2019; Publicado online el 18 de noviembre de 2019

Citation / Cómo citar este artículo: Gehling, J.G. et al. (2019). The Ediacaran-Cambrian transition: sedimentary facies versus extinction. Estudios Geológicos 75(2): e099. https://doi.org/10.3989/egeol.43601.554.

Introduction traces with scalloped levees (Fig. 1a, b), are found on both bed-tops and bed soles. As demonstrated in Claims about global extinction of the Ediacara the Nilpena SS Member (Gehling & Droser, 2018) biota, and the origins and duration of the “Cambrian bed sole grooves and levees were likely made by explosion” of marine animal life, have dominated tiny bilaterians, burrowing beneath thins sand lami- the literature for decades. In most cases, emphasis nae, separated from sand below by microbial mats. is given to key geological sections with datable ash These characteristics are exclusive to bed soles of beds and either diverse assemblages of body fossils very thin sandstone layers usually less than 5mm or trace fossils. However, apart from Buatois et al. thick, but never thicker than 15mm. Identified on (2013) and Gehling & Droser (2013), little atten- both bed-soles and bed-tops of excavated beds, tion has been paid to the role of the succession of Helminthoidichnites grooves are bordered by scal- sedimentary facies responsible for these samples of loped levees (Fig. 1 a, b, c). They appear to have the fossil record spanning the Ediacaran-Cambrian been produced by small, peristaltic-burrowing ani- boundary. mals, exploiting organic, mat-bound sands, both on Decades of analyses of the terminal Ediacaran, clearly identified bed tops (Fig. 1c), but also on bed Rawnsley Quartzite, in the Flinders Ranges of bases (Fig. 1a, b) where mats were smothered by South Australia (Gehling, 2000), demonstrates that thin sand cover that facilitated sufficient levels of key taxa of the Ediacara biota are restricted to cer- dissolved oxygen (Fig. 1a, b) for the survival of bur- tain sedimentary facies and stratigraphic levels. rowing organisms. These Helminthoidichnites bur- The Rawnsley Quartzite consists of three members rows are common in oscillation rippled sandstone separated by disconformities. The basal, shallow facies of the Ediacara SS Member, where crossing marine, Chace Sandstone Member is unfossilifer- avoidance is common, except where the burrow- ous, but replete with textured organic surfaces. The ing organisms were scavenging previously buried overlying, richly fossiliferous, Ediacara Sandstone bodies (Gehling & Droser, 2018). Bed-sole casts of Member fills submarine incisions cut through the regional base of the Rawnsley Quartzite, including the underlying tidal sand-flats of the Chace Quartzite Member and the otherwise basal, paralic Bonney Sandstone (Gehling & Droser, 2012; Counts et al., 2016). In turn, the Ediacara Sandstone Member was secondarily incised by the less fossiliferous, Nilpena Sandstone Member (new name), that caps the Rawnsley Quartzite. In the Nilpena area, the base of the Cambrian is marked by a sharp change to the Early Cambrian U-shaped Diplocraterian burrow sands of the Parachilna Formation.

Results and discussion

The oldest recorded fossils, in the Ediacara SS Member of the Rawnsley Quartzite, are Helminthoidichnites isp. groove and levee traces within thinly laminated silty sandstones that inter- tongue with sandstone breccias within a canyon Fig. 1.—a, b: Crossing-avoidance of Helminthoidichnites fill in the Nilpena region (Gehling & Droser, 2012) groove trace fossils on bed soles of thinly bedded, oscillation and also in the central Flinders Ranges, where rippled sandstone of the Ediacara SS Mbr; c: Bed top with Helminthoidichnites crenulated levees; d: Bed-sole casts of the Chace SS Member is missing. Higher in the bilobate furrows in deeper water, flat laminated, reddish, fine Ediacara SS Mbr., Helminthoidichnites groove grained sandstone. Scale in centimetres.

Estudios Geológicos, 75(2), julio-diciembre 2019, e099, ISSN-L: 0367-0449. https://doi.org/10.3989/egeol.43601.554 The Ediacaran-Cambrian transition: sedimentary facies versus extinction 3

3mm wide, bilobed trackways (Fig. 1c), that occur Member that fills a second incision and caps the within red, laminated, fine-grained sandstone facies, Rawnsley Quartzite in South Australia. However, low in the Ediacara SS Member, suggest furrowing many characteristic genera, like Tribrachidium, activity by 3 mm wide bilaterian organisms. Parvancorina, Spriggina, Kimberella, Rugoconites At both the Nilpena and the Ediacara Hills fossil and smaller species of Dickinsonia, are common in sites, three-walled, palisade constructed specimens the oscillation-rippled sandstone facies of both these of Pteridinium simplex (Fig. 2a) are preserved in fossil-bearing members (Droser & Gehling, 2015; mass-flow sandstones, low in these submarine val- Droser et al., 2017; Evans et al., 2018). This dem- ley incisions. Pteridinium is absent in overlying onstrates that classic, large Ediacaran body forms shallow marine facies of the Ediacara Sandstone are unlikely to have been missed in early Cambrian Member, which records the majority of classic sediments of similar environments; rather, with very members of the Ediacara biota in South Australia. few exceptions, they did not survive the onset of The absence of Pteridinium in the overlying Nilpena penetrative burrowing and predation that appears to SS Member, in South Australia, is in contrast to have characterized the Cambrian explosion in shal- its stratigraphic range of some 1300m of section low marine, higher energy environments. in the latest Ediacaran, Schwarzrand Subgroup of Fine-grained and thinly bedded sandstones of both southern Namibia (Narbonne et al., 1997). There, members, at Nilpena and the Ediacara Conservation Pteridinium ranges from the Kliphook Member Park, preserve tiny, relatively featureless, rice-grain- (Dabis Formation) to the Spitskop Member (Nomsas sized body moulds, small enough to have been the Formation) just below the early Cambrian discon- animals responsible Helminthoidichnites isp. groove formity. This emphasizes the role of environment in traces (Gehling & Droser, 2018), in addition to very determining the record of large, distinctive body fos- small external moulds of taxa, with affinities to sils let alone casts and moulds of tiny organisms that, Spriggina, Praecambridium and Kimberella. These until recently, have been overlooked. levee and groove traces (Fig.1a, b), found on both Conversely, gregarious taxa, like Phyllozoon han- tops and bases of thinly bedded sandstone laminae, seni (Fig. 2b) and Eoandromeda octobrachiata, in show crossing-avoidance and also preserve evidence addition to Dickinsonia rex, and several, as-yet, unde- of either peristaltic or appendicular displacement of scribed taxa, are exclusive to the fine-grained, sand- sediment (Figs 1c and 2b). Arguably, several small, stone facies of the newly named Nilpena Sandstone and as yet un-named, body-fossil taxa are candidate Ediacaran “survivors”. External moulds of such organisms are unlikely to be identified from better- oxygenated Cambrian sandy seafloors where active predation and scavenging may have erased their record before the advent of mineral skeletons. The Rawnsley Quartzite is disconformably overlain by the Uratanna Formation in the northern Flinders Ranges and, in turn, by the Parachilna Formation. The Uratanna Formation spans the Ediacaran-Cambrian boundary, as defined by the facies-controlled appearance of the characteristic early Cambrian, branching burrows of Treptichnus pedum (Fig. 2c) and at least one bilateral, soft- bodied “holdover” taxon described by Jensen et al. (1998) (Fig. 2d). The Cambrian Diplocraterion Fig. 2.—a: Pteridinium simplex, Ediacara SS Member; burrow-beds of the Parachilna Formation over- b: Phyllozoon hanseni, Nilpena SS Member.; c: Treptichnus lie the Uratanna Formation. Where the Uratanna pedum, Uratanna Form; d: Unnamed body fossils, Uratanna Form., Castle Rock, Angepina Syncline, Flinders Ranges. Castle Fm. is not preserved, these Parachilna Fm. burrow Rock, Angepina Syncline, Flinders Ranges. beds sharply overly the unfossiliferous Ediacaran

Estudios Geológicos, 75(2), julio-diciembre 2019, e099, ISSN-L: 0367-0449. https://doi.org/10.3989/egeol.43601.554 4 J.G. Gehling et al. sandstone facies of the Rawnsley Quartzite. Since Assemblages recorded by late Ediacaran fossil- the fossiliferous members of the Rawnsley Quartzite bearing successions from several continents, show are capped by 50 to 400m of shallow marine, high differing arrays of Ediacara body and trace fos- flow regime sandstone facies, there is no possibility sils in these transitions into Early Cambrian strata, of a record of taxa that may or may not have spanned depending on the preserved sedimentary settings the Ediacaran-Cambrian boundary. (see Narbonne et al., 1997; Jensen & Runnegar, The records of these key body and trace fossil taxa 2005; Darroch et al., 2015; Smith et al., 2017). are a function of appropriate sedimentary facies, There are no two, well-separated, successions that rather than an exclusive record of true biostrati- show a simple pattern of megascopic species that graphic ranges of taxa, and thus resulting in place- can be used to infer a specific extinction event at the ment of the Ediacaran\Cambrian boundary in South Ediacaran-Cambrian boundary. The preservation of Australia within a disconformity. Globally, it is trace fossils, on one hand, and body fossils of mil- unlikely that any one succession holds a true record limetric-scale taxa, such as Praecambridium, large of the precise base of the Phanerozoic. Consequently, numbers of juvenile specimens of Parvancorina there may be no single record of extinction of the (Coutts et al., 2018), and other as yet undescribed, Ediacara biota, or any record of the specific taxa tiny bilateral forms, is limited by both the grain- that ignited the Cambrian explosion of cuticle body- size resolution and chemical composition of these armoured and appendage-bearing marine animals. Ediacaran and early Cambrian siliciclastic units, Large, soft-bodied taxa of the Ediacara biota, such prior to the onset of effective bioturbation in the as Dickinsonia, Yorgia, Phyllozoon and Pteridinium, Early Cambrian. are unlikely to have survived in Cambrian shallow The newly defined Nilpena Sandstone Member, marine settings. capping the Rawnsley Quartzite in South Australia, is However, recent statistical analysis of the evolu- characterized by Phyllozoon hanseni, Eoandromeda tion of Cambrian trilobites (Edgecombe et al. 2019, octobrachiata, Dickinsonia rex and a new species has promoted previously voiced rejection of claims of Tribrachidium, together with a number of, as that small-scale, bilaterial Ediacaran taxa, such as yet, undescribed taxa. The new Nilpena Sandstone Parvancorina, Spriggina, Praecambridium and Member is restricted to the central and southern Kimberella, were likely Ediacaran precursors to the Flinders Ranges, south of the Ediacara Conservation Cambrian explosion of bilaterians, simply because Park, where the transitional facies of the Uratanna such taxa have not been discovered in dated Early Formation appears to include the Ediacaran- Cambrian strata of appropriate sedimentary com- Cambrian transition in the northern Flinders Ranges. position. The earliest Cambrian fossil Lagerstätten, The bioturbated sands of the Flinders Ranges wide, being limited to deeper water, fine grained sedimen- Parachilna Formation, cap the succession before the tary settings, document the diversification of arthro- transition to early Cambrian carbonates recording a pods and other megascopic animals with mineralized regional rise in sea level. These facies-fossil relation- and cuticular skeletons. These appear well above the ships in South Australian, act as a reminder that spe- base of the Cambrian in sections described glob- cies ranges in most time-equivalent late Ediacaran ally. The absence of strong evidence of Ediacaran sedimentary successions, world-wide, reflect similar body-fossil holdovers is likely a function of a lack facies control, and thus cannot be utilized as evi- of suitable shallow-marine preservational environ- dence of global origins and extinctions in any one ments in the Cambrian that were not also subjected succession. to bioturbation. Soft-bodied animals, small shelly In the youngest Ediacaran strata of South fossils, organic-walled microfossils and trace fos- Australia, the stratigraphic range of Pteridinium sils are each confined to markedly different sedi- (Fig. 2a), as in Namibia, is determined by the avail- mentary facies. This reduces our resolution of the ability of suitable sedimentary facies. Specimens ranges of late-Ediacaran marine animal assemblages of Pteridinium at Swartpunt (Namibia), just below and increases the difficulty of assessing taxonomic the Cambrian erosional incision, are flattened. ranges. However, three-dimensional Pteridinium specimens

Estudios Geológicos, 75(2), julio-diciembre 2019, e099, ISSN-L: 0367-0449. https://doi.org/10.3989/egeol.43601.554 The Ediacaran-Cambrian transition: sedimentary facies versus extinction 5

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Estudios Geológicos, 75(2), julio-diciembre 2019, e099, ISSN-L: 0367-0449. https://doi.org/10.3989/egeol.43601.554