Abstract. The numerical age of the Jurassic–Cretaceous boundary has
been controversial and difficult to determine. In this study, we present
high-precision U–Pb geochronological data around the Jurassic–Cretaceous
boundary in two distinct sections from different sedimentary basins: the Las
Loicas, Neuquén Basin, Argentina, and the Mazatepec, Sierra
Madre Oriental, Mexico. These two sections contain primary and secondary
fossiliferous markers for the boundary as well as interbedded volcanic ash
horizons, allowing researchers to obtain new radioisotopic dates in the late Tithonian
and early Berriasian. We also present the first age determinations in the
early Tithonian and tentatively propose a minimum duration for the stage as
a cross-check for our ages in the early Berriasian. Given our radioisotopic
ages in the early Tithonian to early Berriasian, we discuss implications
for the numerical age of the boundary.
The Early Jurassic (late Pliensbachian to early Toarcian) was a period marked by extinctions, climate fluctuations, and oceanic anoxia. Although the causes of the early Toarcian Oceanic Anoxia Event (OAE) have been fairly well studied, the events that lead to the Toarcian OAE, i.e. the events in the late Pliensbachian, have not been well constrained. Scenarios of the driving mechanism of biotic and environmental changes of the late Pliensbachian have ranged from LIP volcanism (the Karoo-Ferrar LIP), ocean stagnation, and changing ocean circulation, to orbital forcing. The temporal relationship between the Karoo LIP and the late Pliensbachian (Kunae-Carlottense ammonite Zones) are investigated in an effort to evaluate a causal relationship. We present the first absolute timescale on the Kunae and Carlottense Zones based on precise high-precision U-Pb geochronology, and additional geochemical proxies, for a range of environmental proxies such as bulk organic carbon isotope compositions, Hg concentration, and Hg/TOC ratios, and Re-Os isotopes to further explore their causal relationship. The data presented here show that causality between the Karoo LIP and the late Pliensbachian events cannot be maintained.
Abstract. 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, 15Argentina, 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.
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