Integrated ammonite biochronology and U-Pb geochronometry from a basal Jurassic section in Alaska

Abstract

New results from integrated biochronologic and geochronometric studies on the basal Jurassic section at Puale Bay (Alaska Peninsula) improve the calibration of the Early Jurassic time scale. Previously, the interval around the Triassic-Jurassic system boundary was poorly dated, which hampered our understanding of geologic and biotic events, e.g., the end-Triassic mass extinction and subsequent recovery. Published suggestions for the presence of the earliest Hettangian (Planorbis Zone) and a continuous boundary section at Puale Bay are not substantiated. Although the Kamishak Formation is likely to contain an uninterrupted sedimentary record, pre‐middle Hettangian strata are locally faulted, resulting in an apparent Rhaetian to early Hettangian gap in the fossil record. The Hettangian ammonite zonal schemes developed locally for Nevada and the Queen Charlotte Islands permit reliable correlation with Alaska, but have limited applicability. The faunal succession recorded at Puale Bay is useful in the development of a regional zonation for North America. We obtained three U-Pb zircon dates that are tied into an ammonite biochronology at the zonal level. A middle Hettangian tuff layer from near the top of the Kamishak Formation is dated at 200.8 +2.7 / ‐2.8 Ma. Tuffs from the overlying Talkeetna Formation are bracketed by middle and late Hettangian ammonites and yield crystallization ages of 197.8 +1.2 / ‐0.4 and 197.8 ± 1.0 Ma. These new calibration points require that the Hettangian-Sinemurian boundary be younger than 199 Ma. The Triassic-Jurassic boundary is likely to fall between 200 and 205 Ma. Similar studies are needed for the uppermost Triassic to obtain tighter constraints. Zircon U-Pb systematics of two samples revealed strong Proterozoic and late Archean inheritance patterns that require revised tectonic models to account for the proximity of the Talkeetna arc to evolved crustal blocks.