Cambrian syn‐rift tectonic pulses at unconformity‐bounded carbonates in the Avalon Zone of Newfoundland, Canada

Abstract

In the Avalon Zone of SE Newfoundland, several Terreneuvian to Cambrian Series 2 carbonate bodies occur associated with paraconformities and onlapping geometries. Their stratigraphic discontinuities and related gaps represent syn-rift episodes characterized by sharply tilted and downfaulted blocks and deposition of chaotic megabreccia beds and conglomerates on neighbouring footwall areas. Microbial and shelly carbonate production nucleated on tectonically unstable palaeohorsts, where stromatolitic crusts and mats commonly reflect the onset of hiatal stratigraphic diastems, and mud-mounds the episodic establishment of calm conditions. Short-term uplift episodes of rift shoulders led to unroofing of the Neoproterozoic basement (yielding granitoid clasts and input of reworked exotic clasts and allochthonous fossils from underlying Cambrian strata), localized subaerial exposure and karst features, and the development of synsedimentary fracture networks that provided favourable conduits and pathways for hydrothermal fluids. These carbonate beds host vein networks of carbonate-hosted polymetallic sulphide-sulphate-oxide mineral associations (Pb-Cu-Fe-barite stockworks). The mixture of parautochthonous and allochthonous bioclastic assemblages points to the record of event-concentration strata, which may clarify the long stratigraphic ranges of some involved microfossils. The Cambrian Avalonia rift and the offshore to basinal rift sector of the neighbouring Atlas-Ossa-Morena rift, preserved in the Moroccan northern High Atlas and Coastal Meseta, share common temperatewater carbonate facies and build-ups, lacking 'subtropical' indicators such as archaeocyathan-microbial reefs, ooidal shoals or evaporitic pseudomorphs. These diagnostic facies and minerals occur only in relatively stable, shallow-water rift branches, such as those preserved in the Anti-Atlas and the Ossa-Morena Zone (Iberian Massif).

Carbonate production and reef building under ferruginous seawater conditions in the Cambrian rift branches of the Avalon Zone, Newfoundland

“…Interpretation of the carbonate production and the record of climatically sensitive indicators along the margins of the subtropical to temperate Cambrian rifts that fringed West Gondwana (and probably Baltica), prior to and during the Ordovician opening of the Rheic Ocean (Van Staal et al, 1998;Nance et al, 2012;Waldron et al, 2022), is essential to the improvement of existing palaeogeographical models. This paper, which complements a former report of sedimentological features from the same palaeogeographical area ( Alvaro, 2021), presents: (i) an analysis and review of the sedimentary and oceanographic factors that controlled the nucleation, growth and demise of the microbial-shelly carbonate factories developed in a Cambrian rift transect preserved in the Avalon Zone of southeast Newfoundland; (ii) an interpretation of the palaeoecological patterns and evolution within the transect, according to the redox conditions and phosphate availability; and (iii) a comparison of reefal and non-reefal frameworks with neighbouring rift-related and passive-margin settings in similar low to mid-latitudes.…”

“…However, many questions remain, particularly regarding the interplay between coastal upwelling, microbial productivity, co-occurrence of various phosphate and Fe-redox indicators (i.e. chamosite, hematite and goethite), and potential links between reefal versus non-reefal structures from the Avalonian rift and other penecontemporaneous rift and passive-margin basins (see recent discussions in Alvaro, 2021;Alvaro et al, 2023). On other Cambrian rift basins fringing West Gondwana, different scales of tectonic deformation influenced the nucleation, growth and demise of carbonate factories.…”

“…Summary of facies associations described and interpreted in the text; afterAlvaro (2021) and this work.…”

U-Pb zircon dates from North American and British Avalonia bracket the Lower–Middle Cambrian boundary interval, with evaluation of the Miaolingian Series as a global unit

“…Cover sequence deposition was controlled by terminal Edicaran–Ordovician syndepositional faulting along the Avalon transform fault (Atf) (Landing, 1996; Landing et al 2022; Landing et al 2023). Activity on the Atf produced NNE-striking faults (modern coordinates) and elongated NNE-striking basins and uplifts in a transtensional regime that was not a ‘rift environment’ ( contra Álvaro, 2021; Álvaro et al 2022). The SE migration of Avalonian depocenters through the Early Cambrian (Fig.…”

Terrane history of the Iapetus Ocean as preserved in the northern Appalachians and western Caledonides

“…Their stratigraphy was summarized by Berry and Osberg (1989), based on compilations of earlier work (Neuman, 1967(Neuman, , 1984Hall, 1970). Van Staal et al, 2016, 2021 compared the Popelogan inlier with the inliers of northern Maine and showed that all have comparable stratigraphies.…”