The Frasnian-Famennian (mid-Late Devonian) boundary in the type section of the Long Rapids Formation, James Bay Lowlands, northern Ontario, Canada

摘要

Utilizing a combination of conodont-based biostratigraphy and geochemistry, the Frasnian-Famennian (F/F, mid-Late Devonian) boundary was located in the Long Rapids Formation, northern Ontario. It is present in a black shale sequence, 21.46 m above the base of the Long Rapids Formation, just above a carbonate bed that has similarities to the Upper Kellwasser Limestone, recognized as a lithological marker for the F/F boundary in Germany, Belgium, and France. There is no evidence of an impact event, and sedimentation across the boundary is continuous, although there is a discontinuity layer just below the boundary and the early part of the linguiformis Zone (of the standard Late Devonian conodont zonation) is missing. Strong positive delta ~(13) C (organic and carbonate) values were identified and are interpreted as evidence of mass mortality in anoxic waters at the F/F extinction; positive delta ~(18)O values suggest the extinction was also accompanied by a temperature drop. Negative delta ~(34)S values also indicate anoxia, and negative delta ~(15) N values provide evidence of reduced surface-water productivity. No iridium anomaly was found at the boundary, although higher than usual values were found about 85 cm below the F/F boundary. Sedimentological and biological evidence in the Long Rapids Formation indicates that the F/F extinction was coincident with major sea-level fluctuations in the late Frasnian, including a transgression beginning at the end of the linguiformis Zone and continuing through the Early triangularis Zone and beyond. Marine oscillations were accompanied by anoxia, which appears to be the killing mechanism for the conodont and other end Frasnian extinctions.