PALEOMAGNETIC AND BIOSTRATIGRAPHIC CORRELATION OF THE MANICOUAGAN IMPACT: IMPLICATIONS FOR THE LATE TRIASSIC TIME SCALE

The Manicouagan structure in northeastern Canada, with an estimated pre-erosional diameter of ca.100 km, is one of the largest well-documented impact sites, exceeded in size only by the Chicxulub site among Phanerozoic structures. Once considered a candidate for the cause of end-of-Triassic extinctions, Ar-Ar dating of impact melt rock establishes the age of the impact as much earlier; the date of ~215.4 +/- 0.16 of Jaret et al. (2018) compares very well with earlier dating attempts and places the impact during the early to middle Norian rather than at the system boundary. In contrast to the clearly defined association between the Chicxulub structure and the K-T boundary, however, the Norian age of the Manicouagan structure is not connected to any major paleontological events, making its biostratigraphic correlation problematic. Temporal correlation to the Newark-Hartford astrochronological time scale places the Manicouagan impact date within the E-14n chron (e.g., Kent et al., 2017). However, the position of potential impact-derived debris above a seismically disrupted zone in the lower part of the Upper Triassic White Water Member of the Blomidon Formation at Dellhaven and Red Head in the Fundy Basin (Canada) suggests that the impact occurred during the Bl1 or lowermost Bl2 chron of the GAV-77-3 core drilled in the western Fundy Basin (Kent & Olsen, 2000), which is correlated to the Newark-Hartford uppermost E15r, or lowermost E16n chrons. Uno et al. (2015) also correlated the ejecta layer identified in the Mino Belt cherts of central Japan (Onoue et al., 2016), where conodont biostratigraphy places the impact layer near the base of the Epigondolella bidentata conodont zone (uppermost Alaunian), to the E15r chron of the Newark time scale. This correlation has significant implications for the astrochronologically-based time scale of the Late Triassic, indicating miscalibration of the astrochronology that may reflect an unidentified unconformity. However, close examination of the E15r interval in the Rutgers cores from the Newark Basin Coring Project has failed to produce any impact-related grains.

Jaret et al (2018) EPSL 501, 78–89; Kent et al (2107) Ear Sci Rev 166, 153–180; Kent & Olsen (2000) EPSL 179, 311-324; Onoue et al. (2016) Sci Rep 6:29609; Uno et al. (2015) Phys Earth Planet Inter 249, 59-67