Continental Triassic-Jurassic Boundary in Central Pangea: Recent Progress and Discussion of an Ir Anomaly

Olsen, Paul E.; Koeberl, Christian; Huber, Heinz; Montanari, Alessandro; Fowell, Sarah J.; Et-Touhami, Mohammed; Kent, Dennis V.

The Triassic-Jurassic (Tr-J) boundary marks one of the five largest mass extinctions in the past 0.5 b.y. In many of the exposed rift basins of the Atlantic passive margin of eastern North America and Morocco, the boundary is identified as an interval of stratigraphically abrupt floral and faunal change within cyclical lacustrine sequences. A comparatively thin interval of Jurassic strata separates the boundary from extensive overlying basalt flows, the best dates of which (ca. 202 Ma) are practically indistinguishable from recent dates on tuffs from marine Tr-J boundary sequences. The pattern and magnitude of the Tr-J boundary at many sections spanning more than 10° of paleolatitude in eastern North America and Morocco are remarkably similar to those at the Cretaceous-Tertiary boundary, sparking much debate on the cause of the end-Triassic extinctions, hypotheses focusing on bolide impacts and climatic changes associated with flood basalt volcanism. Four prior attempts at finding evidence of impacts at the Tr-J boundary in these rift basin localities were unsuccessful. However, after more detailed sampling, a modest Ir anomaly has been reported (up to 285 ppt, 0.29 ng/g) in the Newark rift basin (New York, New Jersey, Pennsylvania, United States), and this anomaly is directly associated with a fern spike. A search for shocked quartz in these rift basins has thus far been fruitless. Although both the microstratigraphy and the biotic pattern of the boundary are very similar to continental Cretaceous-Tertiary boundary sections in the western United States, we cannot completely rule out a volcanic, or other nonimpact, hypothesis using data currently available.



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Catastrophic events and mass extinctions: impacts and beyond
Geological Society of America

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