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

Abstract

The characterization of carbonate production on rift basins is critical for understanding the nucleation and demise of reefs in tectonically active areas. A new petrographic and mineralogical analysis of Cambrian strata from the Avalon Zone in Newfoundland, based on scanning electron microscopy – back-scattered electron detector and Raman spectrometer analyses, facilitates recognition of several episodes of Terreneuvian–Miaolingian carbonate production and associated precipitation of ironstone and phosphorite. These distinct units mainly developed on uplifted rift shoulders and basaltic lava palaeoreliefs, and reflect amalgamated high-energy events, interrupted by scouring discontinuities (diastems) commonly lined by phosphatized and ferruginized microbial crusts. Mud-mounds, in contrast, nucleated under calm conditions episodically punctuated by high-energy episodes, where scattered thromboid structures occur as both clotted textures and distinct calcimicrobes. Precipitation of hematite/goethite versus chamosite couplets, both occluding primary porosities and replacing interlaminae and cortices of oncoids and coated aggregates, point to marine substrates close to the Fe-redox boundary. Upwelling of phosphate-rich ferruginous hydrothermal waters contributed to the precipitation of ironstone and phosphate interbeds. Ferruginous waters related to penecontemporaneous hydrothermal activity, reflected by the record of synsedimentary fissuring and stockwork ore bodies, were delivered to confined rift-related horst-and-graben settings, largely controlled by the development of specific Cambrian carbonate and associated ironstone facies. The influence of ferruginous waters necessarily affected the record of climatically sensitive evaporitic pseudomorphs, reefs/mounds and phosphorites, which are then not suitable criteria to discriminate palaeolatitude, as demonstrated by a comparison of low-latitude to middle-latitude margins fringing Baltica, and the Avalonian and Atlas – Ossa-Morena – Northarmorican rift transects of West Gondwana.