Cross-continental age calibration of the Jurassic/Cretaceous boundary

The age of the Jurassic/Cretaceous boundary has remained elusive for the past decades. In this study we evaluate how well the determined boundary age agrees between two distinct sections from different sedimentary basins, and whether we can constrain a globally valid Jurassic/Cretaceous boundary age. Here we present high-precision U-Pb zircon age determinations on single grains of volcanic zircon of two sections that span the Jurassic/Cretaceous: the Las Loicas section, Argentina, and the Mazatepec section in Mexico. These two sections display well-established primary and secondary stratigraphic markers as well as interbedded volcanic horizons that allow bracketing the age of the Jurassic/Cretaceous boundary at 140.22 ± 0.13 Ma. We also present the first age determinations in the early Tithonian and tentatively propose a minimum duration of ~ 7 Ma for the Tithonian stage.

Jurassic–Cretaceous boundary ammonite Blanfordiceras (Mollusca: Cephalopoda) from Fortissimo-1 Wildcat Well, Browse Basin, Northwest Shelf, Australia

Jurassic–Cretaceous ammonites are particularly robust fossil tools in global stratigraphy and correlation. The successive evolution and extinction of these cephalopod mollusks was so rapid that many ammonite zones are no more than one million years in duration. A well-preserved ammonite specimen from the Fortissimo-1 core, Browse Basin, NW Australia is assignable to the widespread latest Jurassic dimorphic berriaselline genus, Blanfordiceras Cossmann, recorded previously from the Spiti area, Nepal, Tibet, Madagascar, Papua-New Guinea, Antarctica, and southern South America. This is the first report of ammonites of this age in the Australian region. The evolute shell of an estimated 90-100 mm diameter (when extrapolated) and pronounced ornamentation of variably bifurcating, curvilinear and flexuous ribs, intercalated with simple, non-bifurcating ribs, is consistent with Blanfordiceras wallichi (Gray, 1832), which has traditionally been restricted to the uppermost Tithonian Stage, ca. 146.5-145.5 Ma, but may well have survived into the earliest part of the Berriasian. The first recorded occurrence of this ammonite in Australia fills an anomalous absence in the paleobiogeographic distribution of Jurassic–Cretaceous boundary ammonites in the Indo-SW Pacific Subrealm with important implications for the calibration of offshore rocks and wells in Australia.