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

New results from integrated biochronologic and geochronometric studies on t he basal Jurassic section at Puale Bay (Alaska Peninsula) improve the calib ration of the Early Jurassic time scale, Previously, the interval around th e Triassic-Jurassic system boundary was poorly dated, which hampered our un derstanding of geologic and biotic events, e.g., the end-Triassic mass exti nction and subsequent recovery. Published suggestions for the presence of t he earliest Hettangian (Planorbis Zone) and a continuous boundary section a t Puale Bay are not substantiated. Although the Kamishak Formation is likel y to contain an uninterrupted sedimentary record, pre-middle Hettangian str ata are locally faulted, resulting in an apparent Rhaetian to early Hettang ian gap in the fossil record. The Hettangian ammonite zonal schemes develop ed locally for Nevada and the Queen Charlotte Islands permit reliable corre lation with Alaska, but have limited applicability. The faunal succession r ecorded at Puale Bay is useful in the development of a regional zonation fo r North America. We obtained three U-Pb zircon dates that are tied into an ammonite biochron ology 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 th e 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-Sinemu rian boundary be younger than 199 Ma. The Triassic-Jurassic boundary is lik ely to fall between 200 and 205 Ma. Similar studies are needed for the uppe rmost Triassic to obtain tighter constraints. Zircon U-Pb systematics of tw o samples revealed strong Proterozoic and late Archean inheritance patterns that require revised tectonic models to account for the proximity of the T alkeetna are to evolved crustal blocks.