Magnetostratigraphic correlation of the Oxfordian–Kimmeridgian boundary
Introduction
Ammonites and other marine fauna evolved during the Late Jurassic within separate climatic provinces in quasi-isolation. Each of these provinces has a high-resolution zonation based on ammonites, but it has been difficult to achieve an unequivocal correlation among these zonations except for rare intervals. This provincialism hindered the establishment of a Global boundary Stratotype Section and Point (GSSP) for the international Oxfordian–Kimmeridgian boundary, because such a GSSP should provide the means for precise global correlation. Traditionally, the base of the Kimmeridgian in the Boreal faunal province has been placed at the base of the ammonite Amoeboceras bauhini Zone, and in the Sub-Boreal province at the base of the Pictonia baylei Zone. However, in the Sub-Mediterranean province, the Kimmeridgian has a quite different definition and span, and traditionally began with the base of the ammonite Sutneria platynota Zone. Quantifying the offset between these competing regional definitions and an agreement on a standardized international Kimmeridgian has been a challenge.
A proposed international definition of the base of the Kimmeridgian within the British section in Flodigarry, at Staffin Bay, Isle of Skye (Wierzbowski et al., 2006, Matyja et al., 2006) was accepted in January, 2007, by over 70% of the Kimmeridgian Working Group of the Jurassic Subcommission of the International Commission on Stratigraphy (ICS). However, before a GSSP can be established, it is necessary to resolve how this level can be confidently correlated with other sections in the world. Therefore, in addition to biostratigraphy from unique evolutionary trends within multiple taxa groups, it is desirable to identify less-unique, but more global, non-biological stratigraphic tools, such as stable isotope excursions, major eustatic sea-level fluctuations or distinctive magnetic reversal horizons.
Magnetostratigraphy, combined with biostratigraphic constraints, can be a powerful tool for high-resolution correlation of stratigraphic sections. The M-sequence of marine magnetic anomalies spanning the Late Jurassic provides a reference polarity pattern and numerical time scale derived from seafloor spreading models (reviewed by Ogg and Smith in Gradstein et al., 2004). The presumed Kimmeridgian through Tithonian portion of this M-sequence (chrons M25r through M19n) is dominated by relatively long-duration polarity chrons. Magnetostratigraphic sections in condensed pelagic strata of southern Spain had indicated that the traditional placement of the “Oxfordian–Kimmeridgian boundary” within the Sub-Mediterranean faunal province was near polarity zone M25r (Ogg et al., 1984), but this level was suspected to be significantly younger than the classic “Oxfordian–Kimmeridgian boundary” of the Boreal faunal province. Marine magnetic anomalies older than Chron M25 are both closer spaced and lower in amplitude. Deep-tow magnetometer surveys in the Pacific Ocean have established a tentative polarity model extending into the Bathonian (Sager et al., 1998, Tivey et al., 2006, Tominaga et al., 2008), but this model remains uncalibrated owing to the lack of a complete and verified magnetic polarity time scale for the Bajocian through Oxfordian. Particularly vexing is the interval between anomalies M25r and M27r that has been interpreted from less precise sea-surface magnetic surveys and not yet verified by deep-tow magnetometer investigations.
We have utilized magnetostratigraphy based on over 900 paleomagnetic samples to establish an inter-regional correlation of the ammonite zones/subzones for Upper Oxfordian and Lower Kimmeridgian stages based on combination of the bio- and magnetostratigraphic results from the Boreal and Sub-Boreal temperate faunal provinces of Britain, and the Tethyan tropical faunal realm of Poland, France, Spain and Germany. Rare episodes of Boreal-fauna (cool) incursions into the Tethyan (warm) basins of Poland and Germany enable only a few inter-regional correlations limited to narrow stratigraphic intervals or horizons (e.g., Schweigert and Callomon, 1997, Matyja and Wierzbowski, 1997, Matyja and Wierzbowski, 2002, Matyja and Wierzbowski, 2006a, Matyja and Wierzbowski, 2006b, Matyja et al., 2006). Our correlation of magnetostratigraphies from these regions enables assignment of several precise horizons of inter-regional correlation, and provides a reference polarity pattern for the poorly-resolved marine magnetic anomalies between M25r and M27r. The proposed GSSP for the Oxfordian–Kimmeridgian boundary on the Isle of Skye (Wierzbowski et al., 2006, Matyja et al., 2006) nearly coincides with a distinctive polarity reversal that we correlate to the base of Chron M26r. The correlation to the M-Sequence chrons quantifies the approximate age, offset and duration of ammonite subzones and other datums among the different faunal provinces.
Section snippets
Magnetostratigraphic sections
Paleomagnetic minicores were drilled in most of the sections with an average spacing of 0.5 to 1 m. Denser samplings were near stage boundaries, within condensed intervals and close to hiatuses as determined by sequence stratigraphic analysis.
Demagnetization procedure
The suites of paleomagnetic samples from the British, Polish, French and German sections were analyzed in five laboratories (University of Michigan in USA, Lancaster University and Oxford University in Britain, GeoForschungsZentrum Potsdam and the University of Munich in Germany, and Polish Academy of Sciences). Remanent magnetization was measured using two- or three-axis cryogenic magnetometers.
The majority of samples from the Tethyan Realm (Poland, France and Germany) are limestones and
Calculation of paleomagnetic directions and paleopoles
Specimens rated as N, R, NP and RP were used to calculate mean directions, paleomagnetic poles and paleolatitudes, with their associated uncertainties. Mean directions were calculated separately for normal-, reversed- and combined-polarity directions using a weighted Fisher mean (Fisher, 1953) in which the NP and RP-rated specimens were given half-weight. In a second iteration, outliers of more than two circular standard deviations from the mean were excluded. For comparison, an additional mean
Boreal and Sub-Boreal composite
An integrated stratigraphy for each section was compiled with lithostratigraphy, magnetostratigraphy, selected sequence stratigraphy, and ammonite datums and associated zones (Fig. 4, Supplementary Figs. 2 through 4). The polarity pattern relative to ammonite subzones is generally quite consistent (Fig. 5). The uppermost Oxfordian is dominated by normal polarity. In contrast, the lowermost Kimmeridgian (sensu Boreal) is predominantly reversed polarity with two or three thin normal-polarity
Conclusions
The combined biostratigraphy and magnetostratigraphic results from sections in the Boreal, Sub-Boreal and Sub-Mediterranean outcrops allow a confident correlation of the Oxfordian–Kimmeridgian Stage boundary interval (as defined in Staffin Bay of the Isle of Skye) among these faunal provinces. The base of the Kimmeridgian, assigned in the GSSP as the base of the Amoeboceras bauhini and Pictonia baylei Zones, lies very close to the base of a reversed-polarity dominated interval. A similar
Acknowledgements
We thank Rob Van der Voo (University of Michigan, USA), Buffy McClelland (Oxford University, UK), Manfred Menning, Norbert Nowaczyk (GeoForschungsZentrum Potsdam, Germany) and Marek Lewandowski (Polish Academy of Sciences, Poland) for the use of their paleomagnetic laboratories, analytical software and technical support. We gratefully appreciate advice on paleomagnetic and lithostratigraphic interpretations by Piotr Ziόłkowski and Jacek Gutowski, and useful insights about the Plettenberg
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2016, GeobiosCitation Excerpt :This dinosaur-tracks interval of biolaminites is located in the upper part of the Morillon Formation (Énay, 1966) and attributed to the Hauffianum Subzone (Bimammatum Zone) of the submediterranean ammonite zonation (Fig. 4). Recent correlations between subboreal and submediterranean ammonite zonations show that the Hauffianum Subzone is equivalent to the Densicostata Subzone (Baylei Zone) of the subboreal chart (Przybylski et al., 2010; Wright, 2010; Matyja and Wierzbowski, 2000, 2003; Schweigert and Callomon, 1997; Énay et al., 2014). The dinosaur tracksite of Loulle can thus be located at the base of the Kimmeridgian Stage, ca. 157 M.a. ago (Fig. 4).
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