FAR-REACHING SEISMIC EFFECTS OF THE MANICOUAGAN IMPACT: EVIDENCE FROM THE FUNDY BASIN

The wide-reaching seismic effects of bolide impact have been cited often in connection with the Chicxulub structure. Evidence for similarly far-reaching effects of the Manicouagan impact exists in the Triassic strata of the Fundy rift basin, approximately 700 km distant from the impact site. The ~100 km-wide Manicouagan structure in northeastern Canada is one of the largest well-documented impact sites. Numerical modeling suggests that the impact released nearly 10⁸ megatons of energy, resulted in an earthquake of M > 10, and caused ground displacement of nearly 5 m at a distance of 700 km. Once considered a candidate for the cause of the end-of-Triassic extinctions, U-Pb zircon dating of the impact melt at 214 + 1 Ma establishes the age of the impact as much earlier Norian, which correlates with the age of a recently described ejecta layer in the UK.

The Quaco Formation (Fundy Group) comprises mostly clast-supported cobble conglomerate of probable Carnian age cropping out in coastal New Brunswick. The most distinctive feature of this formation is the pervasive occurrence of cm-scale circular to elliptical markings on the cobble surfaces. Although smooth-surfaced depressions on some cobbles are consistent with a pressure-solution origin, hackly markings, radial fracturing of the clasts, grain fracturing within the clast fabric, and spalling are explained as the result of concussion fracturing at points of clast contact during the passage of a strong shock wave. Elsewhere in the basin, the Norian-Hettangian-age Blomidon Formation, exposed along the Nova Scotia coast, contains a 10-m-thick zone of intensely deformed strata that is characterized by rubblization and step-wise faulting near the formation base. This zone, which correlates basin-wide, is interpreted as the result of a strong synsedimentary seismic event during the early Norian. No distinct ejecta layer has been found in the Fundy basin, but quartz grains with multiple sets of planar features, which appear partially annealed, and glassy spherulitic grains occur at the top of the deformed zone. Paleomagnetic data suggest that this zone correlates with established age of the Manicouagan impact. Stratigraphic constraint of the age of the impact allows an accurate assessment of the biotic effects of this event, which does not coincide with any major extinctions.