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The age mismatch between the GTS2016 on the Tithonian-Berriasian boundary and the new data from South America
Victor A. Ramos

Building: Muséum d'Histoire Naturelle de Genève
Room: Amphithéâtre
Date: 2018-12-06 11:40 AM – 12:00 PM
Last modified: 2018-11-24

Abstract


In the last five years, researchers from the Institute of Andean Studies and other colleagues who studied the Mesozoic marine sequences on both sides of the Andes mountain range using different methodologies, revealed a mismatch with the Geological Time Table. The chronological data used to define the Tithonian-Berriasian stage boundary do not coincide, either using the latest IUGS ICS proposal, or the GTS2016 or earlier time scales of Ogg et al. (2016). There is a 4 to 5 Ma difference in different sections of the Andes between the official boundary of these stages and the new data obtained by diverse methods.

The biostratigraphy of these basins, especially the Neuquén Basin, is based on several generations of biostratigraphic and paleontological studies. The first studies were carried out by German paleontologists at the end of the 19th century and the beginning of the 20th century, who were familiar with the faunas of the Tethys Ocean. These studies made the first correlations of the ammonite fauna between the two regions, establishing different assemblage zones for the Tithonian and Berriasian stages. New biostratigraphic studies and revisions of these ammonite faunas conducted in recent decades have shown that although part of the faunas of these basins have certain endemism, there are key fossils and assemblages that can be correlated with the Standard zonations of the Tethys (Riccardi, 1988, 1991, 2015; Leanza, 1945; Leanza et al., 2011; Vennari, 2016; Vennari et al., 2014; Lena et al. 2018, in rev.). Besides, these works were complemented by studies in calcareous nannofossils, calpionellids, radiolarians, and others (López-Martínez et al., 2017, 2018, and cites there in).

These biostratigraphic studies were performed in forearc and retroarc basins close to the Jurassic-Cretaceous volcanic arcs, and as a result of this proximity these basins contain numerous deposits of Plinian fall tuffs (Ramos and Folguera, 2005). Due to its position south of the tropic of Capricorn, even in the Jurassic, the prevailing winds were from the southwest sector, which is why the retroarc basins have more frequent levels of tuffs interbedded with the marine sediments. These tuffs were dated in the last years using U-Pb techniques such as laser-ablation MC–ICPMS, SHRIMP II ion microprobe, and more recently chemical abrasion ID-TIMS analyses. All these analyses gave coherent results indicating a difference in the different sections studied with the official limits, being the Tithonian-Berriasian stage boundary between 140 and 141 Ma (Vennari et al., 2014; Naipauer, 2016, Lena et al., 2018, in. rev.).

When these results are analyzed with the evolution in the last decades of the age between these two stage boundaries, interesting data emerge. The limit between these stages has varied from 130 to 145 Ma, highlighting the proposal of EXXON 88 where the limit was close to 141 million years (Gradstein et al., 2012). The big change occurred when the magnetic anomaly model of the Shatsky Rise was introduced (Gradstein et al., 2004), which brought the limit to 145.5 Ma (GTS2004). The studies of calcareous nannofossils and their correlation with the magnetic anomalies of Channell et al. (2010), brings this limit again to 141.5 Ma, age much more in line with our determinations. Apparently, the Hawaiian block model, even using constant spreading rates (Channell et al., 1995, 2010; Malinverno et al., 2012), gives values closer to the 140-141 Ma interval determined in the Andes.

The examination of the biostratigraphic constrains used on the Shatsky Rise, as well as the chronological Ar-Ar data from the altered basalts, indicate that the age of this proposed limit is about 4 to 5 Ma older than the new data, and needs to be deeply modified. The stage boundary identified in different sectors of the Andes, together with the new and precise U-Pb ages, leads to consider that the limit should be close to 140-141 Ma.

 

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