--> ABSTRACT: Triassic-Jurassic Boundary Section at Puale Bay, Alaska Peninsula: Comparative Diversity Patterns of Skeletal Faunas and Trace Fossils, by Cathryn R. Newton; #91003 (1990).

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ABSTRACT: Triassic-Jurassic Boundary Section at Puale Bay, Alaska Peninsula: Comparative Diversity Patterns of Skeletal Faunas and Trace Fossils

Cathryn R. Newton

The Triassic-Jurassic systemic boundary is of fundamental paleobiological significance because it has been identified as the horizon of one of the five first-order marine mass extinctions in the Phanerozoic record. At Puale Bay, on the Alaska Peninsula, a thick (^sim700 m) stratigraphic succession of carbonates, cherts, and shales straddles the systemic boundary and provides new evidence concerning the nature of end-Triassic environments and extinctions. Although the section is locally interrupted by strike-slip faults and small-scale normal faults, the Puale Bay section nonetheless represents one of the most complete boundary sections in western North America. Overall, the Norian-Hettangian boundary succession at Puale Bay consists of an upper Norian deepening-upward seq ence, followed by a long-term shallowing-upward sequence that is continuous across the boundary and extends well into the Jurassic section. This shallowing-upward sequence resulted principally from the progradation of volcanigenic sediments associated with the development of a volcanic arc on the peninsular terrane. There is no sedimentological evidence of a short-term sea level drop at or near the systemic boundary in the Puale Bay section.

Latest Norian and Hettangian intervals are both well represented by shelly fossils at Puale Bay; index ammonoids indicative of upper Norian and Hettangian intervals are present, and in the lower and medial portions of the upper Norian section the bivalves Monotis subcircularis and Monotis alaskana also occur. As in many other Triassic-Jurassic boundary sites, a barren interval exists between uppermost Norian and Hettangian occurrences of shelly fossils. However, at Puale Bay this approximately 50-m interval is rich in trace fossils, with particularly high concentrations of Thalassinoides and Chondrites. This finding is consistent with extinction models that invoke dysaerobic conditions during the boundary event, although it does not suggest truly anoxic conditions within the Puale Bay sedimentary basin.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990