EVIDENCE FOR SYN-SEDIMENTARY TECTONISM IN A PALEOPROTEROZOIC RIFT BASIN: THE ESPANOLA FORMATION, LOWER HURONIAN SUPERGROUP, CANADA

The 2.45 – 2.2 Ga Huronian Supergroup exposed along the north shore of Lake Huron, Ontario, Canada is inferred to have been deposited during a transition from continental rifting to passive continental margin development. The Espanola Formation is considered part of the lower Huronian Supergroup, which contains evidence of recurrent syndepositional to early post-depositional tectonic activity. Tectonically-induced deformation structures in the Espanola Formation are broadly grouped into two main categories: 1) structures formed by forceful intrusion, and 2) soft-sediment deformation structures. Forceful intrusion structures are represented by 10 to 100 cm thick clastic dykes, whereas soft-sediment deformation is indicated by large-scale slump structures, intraformational breccia, load casts, ball and pillow structures, convolute lamination and dish-and-pillar structures. All but the slump structures are restricted to discrete stratigraphic horizons, laterally traceable over long distances and confined between undisturbed strata of similar lithology. Such characteristics strongly indicate liquefaction and fluidization mechanisms possibly triggered by rift-related seismic activities during lower Huronian basin subsidence. The slump structures, presence of basin bounding faults, and continental flood basalts at the base of the Huronian Supergroup further support a rift basin interpretation. Strikingly similar deformation structures of different ages have been reported from other regions, including examples from South Africa, Australia and India. Geochemically, siltstones of the Espanola Formation plot in the continental arc field on Th-Sc-Zr/10 and La-Th-Sc ternary plots, which probably reflects the tectonic setting of the source rocks, rather than the Huronian basin itself. Geochemical comparisons with modern and Paleozoic continental rift basin sediments indicate similar discrepancies in tectonic setting discrimination.