Magnetostratigraphy of the Upper Jurassic–Lower Cretaceous from Argentina: Implications for the J-K boundary in the Neuquén Basin

Highlights

• First magneto and cyclostratigraphic study of the J-K boundary in the Southern Hemisphere.

• Refinement of the correlation between Andean and Standard ammonite zones.

• Precise definition of the J-K boundary in the Neuquén basin.

Abstract

A systematic sedimentologic and paleomagnetic study was carried out in the Vaca Muerta Formation, cropping out in the northern Neuquén Basin, west-central Argentina. The studied section is c. 280 m-thick and represents a carbonate ramp system bearing ammonites that indicate Late Jurassic–Early Cretaceous ages. The Vaca Muerta Formation is one of the most important unconventional hydrocarbon reservoirs in the world and its thorough study has become a relevant target in Argentina. The J-K boundary is comprised within this unit, and although it is well-dated through biostratigraphy (mainly ammonites), the position of particularly the boundary is yet a matter of hot debate. Therefore, the systematic paleomagnetic and cyclostratigraphic study in the Vaca Muerta Formation was considered relevant in order to obtain the first Upper Jurassic–Lower Cretaceous magnetostratigraphy of the southern hemisphere on the first place and to precise the position of the J-K boundary in the Neuquén Basin, on the other. Biostratigraphy is well studied in the area, so that paleomagnetic sampling horizons were reliably tied, particularly through ammonites. Almost 450 standard specimens have been processed for this study distributed along 56 paleomagnetic sampling horizons that were dated using ammonites. Paleomagnetic behaviours showed to be very stable, and their quality and primary origin have been proved through several paleomagnetic field tests The resultant magnetostratigraphic scale is made up of 11 reverse and 10 normal polarity zones, spanning the Andean Virgatosphinctes mendozanus (lower Tithonian) to Spiticeras damesi Zones (upper Berriasian). These polarity zones were correlated with those of the International Geomagnetic Polarity Time Scale 2012 and 2016 through the correlation between Andean and Tethyan ammonite zones. Cyclostratigraphy on the other hand, proved to be quite consistent with the magnetostratigraphy. Through the correlation of the resultant paleomagnetic and cyclostratigraphic data, it was possible to date the section with unprecedented precision, and therefore, to establish the position of the Jurassic-Cretaceous boundary. The paleomagnetic pole calculated from the primary magnetization is located at: Lon = 191.6°E, Lat = 76.2°S, A₉₅ = 3.5°, indicating a c. 24° clockwise rotation for the studied section, which is consistent with structural data of the region.

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.