Small shelly fossils and carbon isotopes from the early Cambrian (Stage 3-4) Mural Formation of western Laurentia

The extraordinary window of phosphatised and phosphatic Small Shelly Fossils (SSFs) during the early and middle Cambrian is an important testament to the radiation of biomineralising metazoans. While SSF are well known from most Cambrian palaeocontinents during this time interval, western Laurentia has relatively few SSF faunas. Here we describe a diverse SSF fauna from the early Cambrian (Stage 3-4) Mural Formation at three localities in Alberta and British Columbia, Canada, complemented by carbon isotope measurements to aid in a potential future bio-chemostratigraphic framework. The fauna expands the recorded SSF assemblage diversity in western Laurentia and includes several brachiopods, four bradoriids, three chancelloriids, two hyoliths, a tommotiid and a helcionellid mollusc as well as echinoderm ossicles and specimens of Microdictyon, Volborthella and Hyolithellus. New taxa include the tommotiid genus Canadiella gen. nov., the new bradoriid species Hipponicharion perforata sp. nov. and Pseudobeyrichona taurata sp. nov. Compared to contemporaneous faunas from western Laurentia, the fauna is relatively diverse, particularly in taxa with originally phosphatic shells, which appear to be associated with archaeocyathid buildups. This suggests that the generally low faunal diversity in western Laurentia may be at least partly a consequence of poor sampling of suitable archaeocyathan reef environments. In addition, the tommotiid Canadiella filigrana appears to be of biostratigraphic significance in Cambrian Stage 3 strata of western Laurentia and the unexpected high diversity of bradoriid arthropods in the fauna also suggests that this group may prove useful for biostratigraphic resolution in the region

The Occitan and Pyrenean domains of southwestern Europe: a new reference area for the subdivision of the Fortunian Stage (Terreneuvian) in West Gondwana

Trabajo presentado en el International Symposium on the Ediacaran-Cambrian Transtion (ISECT), celebrado en St. John's, Newfoundlan (Canadá), del 15 al 29 de junio de 2017In the Occitan and Pyrenean Domains of southwestern Europe, the Ediacaran-Cambrian transition is different from that exhibited in the neighbouring Iberian Peninsula. The former is characterized by conformable and fossiliferous carbonate beds episodically punctuated by volcanic interbeds. Despite the absence of Cadomian-related metamorphism and deformation, these strata comprise (i) the stepwise record of Cadomian felsic-dominated volcanosedimentary complexes that have yielded a complete U-Pb zircon age constraint (the Rivernous Formation for the Occitan Domain and the Fabert and Finestrelles members for the Pyrenean Domain), overlain by (ii) the development of carbonate platforms, locally associated with phosphogenesis (the Marcou Formation, including the Heraultia Member vs. the Puig Sec Member). The lack of Cadomian “molasses” in this margin of West Gondwana favoured the nucleation and development of both microbial and shelly carbonate factories. As a result, the Fortunian (Terreneuvian) is bracketed between the U-Pb zircon ages of the Rivernous (542.5+0.7/-0.6 Ma to 537.2+2.3/-1.2 Ma), Fabert (559+1.7/-1.3 Ma to 542.9+5/- 1.3 Ma) and Finestrelles (565±9 Ma to 532.9±6.9 Ma) volcanosedimentary complexes and the Watsonella crosbyi-Oelandiella korobkovi Zone erected in the phosphoritic Heraultia Member. A peak in biodiversity is recognized in this zone, which contains 28 species of skeletonized microfossils, such as molluscs (halkieriids, helcionellids,?polyplacophors, sachitids and other problematic taxa), orthothecid hyoliths, stem brachiopods and problematic taxa. The traditional image of West Gondwana as a Pan-African and Cadomian belt, resulted from the amalgamation of magmatic arcs and synorogenic basins, should be replaced by a more complex geodynamic framework, where some marine platforms were protected from active terrigenous input and recorded the establishment of carbonate platforms. The alternation of ash-tuff interbeds and shelly accumulations is key to subdivide the Fortunian in this Gondwanan margin.Peer reviewe

Sublacustrine hydrothermal seeps and silicification of microbial bioherms in the Ediacaran Oued Dar’a caldera, Anti‐Atlas, Morocco

This paper presents a case study of the sublacustrine precipitation of hydrothermal silica ± TiO2 in the Ediacaran Mançour Group of the Saghro inlier, Anti‐Atlas, Morocco. Lacustrine carbonates containing stromatolitic mats and bioherms occur in ephemeral ponds developed within the Oued Da'ra caldera. Its syn‐eruptive infill consists of pyroclastites, ashflow tuffs, and subsidiary lava flows and sills, whereas inter‐eruptive deposition is mainly represented by slope‐related debris‐flow breccias and landslides, alluvial fans and fluvial channels. Carbonate production took place in a mosaic of differentially subsiding, fault‐bounded intra‐caldera blocks controlled by episodic collapse‐induced drowning, pyroclastic blanketing and migration of alluvial/fluvial environments. After microbial carbonate production, the carbonates recorded several early‐diagenetic processes, punctuated by polyphase fissuring (controlling secondary permeability) locally linked to hydrothermal influx. Three generations of carbonate cements are recognisable: (i) fibrous, botryoidal and blocky/drusy mosaics of calcite; (ii) idiotopic mosaics of dolomite caused by flushing of hypersaline Mg‐rich brines; and (iii) euhedral to drusy calcite via dedolomitization. The δ13C and δ18O values from carbonate cements broadly become successively isotopically lighter, as a result of meteoric and hydrothermal influence, and were probably overprinted by the Panafrican‐3 phase that affected the top of the Mançour Group. Two mechanisms of silicification are involved: (i) early‐diagenetic occlusion of interparticle pores at the sediment/water interface of pyroclastic substrates and reefal core and flanks; and (ii) hydrothermal precipitation of silica ± TiO2 lining fissures and vuggy porosity encased in the host rock. Silica conduits cross‐cutting lacustrine mats and bioherms exhibit high potential of preservation in collapsed volcanic calderas. Primary fluid inclusions of hydrothermal silica contain brine relics with NaCl/CaCl2 ratios of 2·1 to 4·4, representing minimum entrapment temperatures of about 142 to 204°C, and abiotic hydrocarbons (heavy alkanes) related to serpentinization of the volcanic and volcanosedimentary basement of the Oued Dar'a caldera