Magnetostratigraphy of U-Pb–dated boreholes in Svalbard, Norway, implies that magnetochron M0r (a proposed Barremian-Aptian boundary marker) begins at 121.2 ± 0.4 Ma

Yang Zhang,Daniel Minguez,James G Ogg,Selen Esmeray-Senlet,Mark W Hounslow,Snorre Olaussen,Felix M Gradstein

doi:10.1130/g48591.1

Abstract

Abstract The age of the beginning of magnetic polarity Chron M0r, a proposed marker for the base of the Aptian Stage, is disputed due to a divergence of published radioisotopic dates and ambiguities in stratigraphic correlation of sections. Our magnetostratigraphy of core DH1 from Svalbard, Norway, calibrates a bentonite bed, dated by U-Pb methods to 123.1 ± 0.3 Ma, to the uppermost part of magnetozone M1r, which is ∼1.9 m.y. before the beginning of Chron M0r. This is the first direct calibration of any high-precision radioisotopic date to a polarity chron of the M sequence. The interpolated age of 121.2 ± 0.4 Ma for the beginning of Chron M0r is younger by ∼5 m.y. than its estimated age used in the Geologic Time Scale 2012, which had been extrapolated from radioisotopic dates on oceanic basalts and from Aptian cyclostratigraphy. The adjusted age model implies a commensurate faster average global oceanic spreading rate of ∼12% during the Aptian–Santonian interval. Future radioisotopic dating and high-resolution cyclostratigraphy are needed to investigate where to expand the mid-Jurassic to earliest Cretaceous interval by the required ∼4 m.y.

Highlights

Rates of plate tectonic motions, biologic evolution, geochemical excursions, and other processes in Earth’s history depend on an accurate geologic time scale
The high-resolution time scale for the Late Jurassic through Early Cretaceous is compiled mainly from the correlation of biostratigraphy to the M sequence of magnetic polarity chrons, and the durations of many of those biozones and polarity chrons have been derived from cyclostratigraphy on reference sections (e.g., Channell et al, 1995; Sprovieri et al, 2006; Malinverno et al, 2012)
That age estimate was based on (1) the consistency of 40Ar/39Ar and U-Pb radioisotopic dating of oceanic basalts and volcanic ash beds from Ocean Drilling Program (ODP) sites, from the Great Valley Group in California (USA), and from Argentina (Fig. 1A); and (2) an assumed duration of 13 m.y. for the Aptian Stage according to cyclostratigraphic interpretation of the Piobbico core of central Italy (Huang et al, 2010) relative to a U-Pb date of 113.1 ± 0.3 Ma near the Aptian-Albian boundary (Selby et al, 2009)

Summary

Introduction

Rates of plate tectonic motions, biologic evolution, geochemical excursions, and other processes in Earth’s history depend on an accurate geologic time scale. That age estimate was based on (1) the consistency of 40Ar/39Ar and U-Pb radioisotopic dating of oceanic basalts and volcanic ash beds from Ocean Drilling Program (ODP) sites, from the Great Valley Group in California (USA), and from Argentina (Fig. 1A); and (2) an assumed duration of 13 m.y. for the Aptian Stage according to cyclostratigraphic interpretation of the Piobbico core of central Italy (Huang et al, 2010) relative to a U-Pb date of 113.1 ± 0.3 Ma near the Aptian-Albian boundary (Selby et al, 2009). A younger age was partly supported by 40Ar/39Ar dates of 122.0 ± 0.5 Ma on basalt flows yielding reversed polarity in northeastern China that were interpreted as belonging to magnetozone M0r (He et al, 2008), biostratigraphic constraints and bounding magnetozones were lacking

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Conclusion