Abstract
The thick (∼700 m) section of Upper Triassic (upper Norian) carbonates, cherts, and volcaniclastics exposed at Puale Bay, Alaska Peninsula, preserves an excellent record of early Mesozoic arc-related sedimentation on the Peninsular Terrane. The Puale Bay sedimentary basin was probably part of a back-arc basin with two stages of evolution: (1) an initial upward-deepening sequence, as reflected in shallow-water carbonates succeeded by spicular chert deposits; and (2) increased magmatic-arc activity and an upward-shallowing sequence near the Triassic-Jurassic boundary, as shown by increased influx of volcaniclastics and deposition of thick, coarse-grained turbidites. This deepening-shallowing cycle of the basin is recorded in kerogen types and in authigenic carbonates that precipitated under marine pore-water influence during the deepening phase, and under meteoric or hydrothermal pore-fluid influence during the upward-shallowing phase. The marine-dominant phase is characterized by types I and II kerogen, δ13C (calcite) ratios near 0.0‰ PDB, and Sr values near 700 ppm. The upward-shallowing phase is characterized by type III kerogen, δ13C (calcite) ratios with large negative excursions (up to -7.2‰ PDB), and low Sr values down to 33 ppm.
The increased volcanic activity, the transition to a terrestrial kerogen source, and the dominance of nonmarine pore fluids near the Triassic-Jurassic boundary are all suggestive of the development of a nearby volcanic arc. This early Mesozoic arc volcanism may reflect closure of ocean basins, between the Peninsular, Wrangellia, and Alexander terranes. Hence, this initiation of arc volcanism may represent an early phase of tectonic amalgamation in southern Alaska.
