Magnetostratigraphy of the Upper Jurassic–Lower Cretaceous from Argentina: Implications for the J-K boundary in the Neuquén Basin
Graphical abstract
Introduction
The Vaca Muerta Formation is a thick rhythmic alternation of dark bituminous shales, marlstones and limestones deposited as result of a rapid and widespread Paleo-pacific early Tithonian to early Valanginian marine transgression in the Neuquén Basin (Fig. 1a), west-central Argentina (Legarreta and Uliana, 1991). It is famous for its important oil and gas resources, as well as its abundant fossil content and temporal continuity along several hundreds of meters thick that comprise the Jurassic/Cretaceous boundary (Leanza, 1981, Mitchum and Uliana, 1985, Leanza and Zeiss, 1992).
In contrast to most geological systems, the Jurassic/Cretaceous transition is characterized by the absence of a significant faunal turnover, as well as by the remarkable increase of faunal provinciality, especially in ammonites (e.g., Remane, 1991, Wimbledon, 2008, Michalík and Reháková, 2011, Wimbledon et al., 2011, Wimbledon et al., 2013, Ogg and Hinnov, 2012, and references cited there). The uncertainties in the inter-regional correlation constitute a major classic problem for this time interval across the world. The reliability and accuracy of chronocorrelation tools can be considerably improved however, by the combination of different methods of quantitative stratigraphy, among which magnetostratigraphy proved to yield most interesting results (Ogg and Lowrie, 1986; Ogg et al., 1991, Remane, 1991, Grabowski, 2011).
The geomagnetic polarity time scale (GPTS) spanning the Middle Jurassic and Cretaceous, is derived from paleomagnetic studies in the continents with detailed biostratigraphy as well as Deep Sea Drilling Project and Oceanic Drilling Program cores, which are correlated with the marine M-sequence magnetic anomalies. Since polarity reversals are recorded simultaneously in all type of rocks all over the world, they provide a distinctive pattern or finger-print for a certain time interval. Marine magnetic anomalies and their calibrations to biostratigraphy make up the reference against which magnetostratigraphic sequences, either on land or in deep-sea cores, are correlated (Ogg and Hinnov, 2012). Thus, one fundamental requisite to attempt a non-ambiguous paleomagnetic correlation between a section on-land and the GPTS is a good biostratigraphic definition (e.g. Ogg and Hinnov, 2012). In the Jurassic, biostratigraphic, magnetostratigraphic, chemostratigraphic and other events are calibrated typically to the standard ammonite zones in Europe, although during the Oxfordian and Tithonian other paleogeographic realms take place such as the Boreal (Arctic and northernmost Europe), sub-Boreal (northern Europe), sub-Mediterranean (southern Europe) and Tethyan (southernmost Europe). In fact, all chronostratigraphic data shown in the GPTS for this time come from basins located in these regions, and these Upper Jurassic-Lower Cretaceous ammonite zones from Europe have been directly calibrated to the M-sequence chrons.
The definition of the boundary and base of the Cretaceous System is still controversial. Historically, at least three definitions are considered (Remane, 1991, Wimbledon, 2008, Grabowski, 2011, Gradstein et al., 2012): 1) One is the base of the Grandis ammonite Subzone defined in the Colloque sur la Crétacé inferieur (1963), that corresponds to the lower part of the Calpionella Zone, almost coinciding with the base of magnetozone M18r. 2) The second is the base of the Jacobi ammonite Zone, defined in the Colloque sur la limite Jurassique-Crétacé (1973), which is often regarded as approximately equivalent to the base of the Calpionella Zone and correlated with the upper part of magnetosubzone M19n.2n. 3) The last definition corresponds to the base of the Occitanica ammonite Zone, correlated with the middle part of the Calpionella Zone and the lower part of magnetozone M17r (Hoedemaeker, 1990).
According to the magnetostratigraphic revision of Grabowski (2011), the integration of calpionellid and magnetic stratigraphy is fairly robust, although calcareous nannofossils still needs refinement and the correlation with ammonite stratigraphy is still poorly constrained. Recently, the Berriasian Working Group has defined the Jurassic/Cretaceous boundary at the base of the Calpionella alpina Zone in the middle part of magnetosubzone M19n2n (Leanza pers. comm. 2016).
In the Neuquén Basin, ammonites have an excellent biostratigraphic resolution, but their autochthony prevents a straightforward correlation with the Tethys, and prompts the occurrence of different correlation schemes between Andean and Tethyan ammonite Zones (e.g. Leanza, 1981, Riccardi, 2008, Riccardi, 2015, Vennari et al., 2014). The aim of this paper is to contribute to the calibration of such biostratigraphic correlations using magnetostratigraphy and previous cyclostratigraphic data.
Section snippets
Geological framework
The Neuquén Basin comprises a Mesozoic-Cenozoic sedimentary succession of at least 7 km thick covering an area of c.120,000 km2 that extends between 32° and 40°S in west central Argentina (Fig. 1a,b). It is a back-arc basin that was originated in the Triassic due to the break-up of Gondwana (Uliana and Biddle, 1988). Subsidence in the Neuquén Basin lasted at least 220 my, and started in the Late Triassic-Early Jurassic as the result of the extensional collapse of the Permian-Triassic orogenic
The Vaca Muerta Formation at the Arroyo Loncoche section
The Vaca Muerta Formation is part of the Mendoza Group or Mendoza Mesosequence, that is made up of three main shallowing upward sedimentary cycles: Lower Mendoza (upper Kimmerdgian–lower Valanginian), Middle Mendoza (lower Valanginian), and Upper Mendoza Mesosequence (lower Valanginian–lower Barremian). The Lower Mendoza Mesosequence begins with continental deposits (alluvial, fluvial and aeolian) from the Tordillo Formation (Kimmeridgian-lower Tithonian?), and is overlaid by mainly marine
Paleomagnetic methodology
For this study, 56 sampling sites or horizons were distributed along the 280 m-thick section. The average distance between them was c. 5 m, except in the upper part where they are approximately 2 m apart or less, due to condensation. At each sampling horizon, four oriented cores were drilled with a portable core drill, from which at least, two standard specimens were cut, making a total of 8 specimens per horizon. Altogether, c. 450 altogether have been obtained and processed.
Demagnetization
Anisotropy of magnetic susceptibility (AMS)
The magnetic anisotropy of a rock depends on the anisotropy of individual grains of its minerals and their spatial arrangement. By knowing the magnetic anisotropy characteristics of the rock-forming minerals, it is possible to determine the spatial distribution of the grains, which results from the various forces acting when the rock was formed. Magnetic properties vary according to direction and therefore there are various types of anisotropy (Lanza and Meloni, 2006). A few magnetic grains can
Magnetic mineralogy
Rock magnetic and petrographic studies were carried out only in basinal and outer ramp facies of the Vaca Muerta Formation. Rock magnetic studies were mostly performed in well cores from El Trapial block in northern Neuquén province, and they cannot be published. Yet, we can say that thermomagnetic curves show Curie temperatures between 570 and 580 °C, which is a robust evidence of the occurrence of titanomagnetite as the main carrier. In addition, these curves show the occurrence of pyrrhotite
Magnetostratigraphy
In the Arroyo Loncoche section, Virtual Geomagnetic Pole (VGP) were calculated from site mean directions, yielding 11 reverse and 10 normal polarity zones (Fig. 3, Fig. 9). One interval at c. 30 m from the base bears no polarity, and corresponds to a Cenozoic sill.
Based on the correlation between ammonite zones from the Andean and Tethys Regions, these polarities were calibrated according to the last Geomagnetic Polarity Time Scale (GPTS) compiled by Ogg and Hinov (2012). Results show a good
Paleomagnetic pole and tectonics
The paleomagnetic pole calculated from the primary component is located (Table 3) at: Lon = 191.6°E, Lat = 76.2°S, A95 = 3.5°, k = 33, N = 54. There exist only a few reliable reference PP for this age for South America, and one of them has been obtained recently precisely from the type section of the Vaca Muerta Formation at Puerta Curacao (Table 3). This PP is located at: Lon = 66.5°E, Lat = 79.8°S, A95 = 3.6° and coincides with reliable Early Cretaceous age paleopoles (Fig. 10). Considering
Discussion
Results from the study at Arroyo Loncoche provide a detailed magnetostratigraphy, which can help to solve many questions regarding the Upper Jurassic – Lower Cretaceous stratigraphy of the Neuquén Basin that are still unanswered. In particular, the magnetostratigraphy calibration of the Tithonian–Berriasian Andean succession brings in two key points: 1) the age of the Tithonian transgression (V. mendozanus Zone), and 2) position of the Jurassic–Cretaceous boundary.
Concluding remarks
Paleomagnetic data indicate the occurrence of three magnetic components: i) a secondary component that was removed at c. 300 °C/15 mT bearing northern declinations with negative inclinations that is interpreted to result from the remagnetization of the local dipolar field, and carried by MD titanomagnetite. The second component has both positive and negative inclinations, is removed between 450 °C and 500 °C/40 and 60 mT, passes several paleomagnetic field tests and is carried by SD or PSD
Acknowledgements
This research has been done under the framework of the UBACyT X-801 (Universidad de Buenos Aires) projects. We are especially indebted to Dr. A.C. Riccardi (Universidad Nacional de La Plata y Museo, Argentina) who not only provided us with the biostratigraphy but also shared his expertise. Also, we thank Dr. H.A. Leanza (Museo de Ciencias Naturales Bernandino Rivadavía, Argentina) for his helpful comments regarding the V. mendozanus Zone. We were honored with the valuable reviews by James Ogg
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2022, Journal of South American Earth SciencesUpper Jurassic–Lower Cretaceous calpionellid zones in the Neuquén Basin (Southern Andes, Argentina): Correlation with ammonite zones and biostratigraphic synthesis
2021, Cretaceous ResearchCitation Excerpt :In fact, N. kamptneri minor and N. steinmannii minor were also reported in such a high stratigraphic position (e.g., Lakova et al., 2017; Wimbledon et al., 2020), but the FO of N. wintereri is located close to the Jurassic/Cretaceous boundary. Nonetheless, the Argentiniceras noduliferum Zone comprises a reverse polarity zone which was correlated by Iglesia Llanos et al. (2017) with the M16r Subchron, and therefore, the FO of N. wintereri occurs most likely in a lower stratigraphic position. The Calpionellopsis Zone spans the upper part of the Argentiniceras noduliferum Zone and the Spiticeras damesi Zone and is correlated with mid M16r - M14r Subchrons (Iglesia Llanos et al., 2017) that corresponds to the upper Occitanica and Boissieri Zones.