STRATIGRAPHIC ARCHITECTURE OF THE EARLIEST RIFT (HALF-)GRABEN TROUGHS FROM EDIACARAN-CAMBRIAN TRANSITION IN THE AVALON ZONE (NEWFOUNDLAND) AND THE ATLAS MOUNTAINS (MOROCCO)

The beginning of rift conditions in the western Avalonian and Atlas rift transects, across the Ediacaran-Cambrian transition, is characterized by evenly distributed small extensional faults crosscutting an inherited Avalonian vs. Panafrican volcanosedimentary basement. Where fault displacements increased and faults grew laterally, they produced linked normal fault arrays featuring (half-)grabens that recorded a distinct change from slope-related and continental (alluvial and fluvial) to marine siliciclastic sedimentation (Rencontre Formation and members 1-3 of Chapel Island Formation vs. Agoundis and Tabia members of Adoudou Formation). The first episodes of carbonate production were not only climatically (or palaeolatitudinally) constrained, but also controlled by pyro- and epiclastic input, tectonic activity, microbial and shelly style of carbonate production, and influence (or lack) of upwelling ferruginous waters. In both areas, the earliest nucleation pulses of carbonate factories are associated with phosphogenesis (Member 4 of Chapel Island Formation vs. Taguedit Bed and Tifnout Member). Where stretching gradients (the horizontal distance over which stretching of continental lithosphere occurs) were high, brief episodes of carbonate production led to decimetre-scale build-ups and shoal barriers protecting back-barrier microbial consortia on syn-rift carbonate blocks limited to narrow branches (e.g., Avalonian and Moroccan High Atlas and Coastal Meseta rift transects); in contrast, where stretching gradients were low, the relative basement stability led to the growth of giant reef complexes as fault-bounded blocks progressively coalesced into larger composite intra-rift platforms (Anti-Atlas rift transect).