Sr-isotope chronostratigraphy of Paleogene–Neogene marine deposits: Austral Basin, southern Patagonia (Argentina)

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Abstract

The Paleogene to Neogene San Julián and Monte León formations underlie the central and eastern parts of the Austral Basin (Patagonia, Argentina), and represent coastal plain to shallow shelf systems within a retroarc foreland basin east of the Andes. The formations are characterized by prominent occurrences of thin (<1 m) densely compacted fossil concentrations, including the large oyster Crassostrea? hatcheri. Formation ages have been problematic given macrofaunal provincialism effectively precluding precise extrabasinal correlations. A Sr-isotope chronostratigraphy based on fossil shell calcite from oysters, pectinids and brachiopods is presented for a regional NE–SW-oriented transect extending ∼200 km along strike of the Austral Basin. The Paleogene–Neogene boundary (∼23 Ma) may occur within the uppermost San Julián Formation (22.68 Ma, with a range between 22.22 and 23.22 Ma), coincident with a prominent glauconitic interval, or in a meteorically altered erosional paleosurface that caps this formation and may correlate with an eustatic sea level fall coincident with the global boundary. The younger Monte León Formation is entirely of early Neogene (Aquitanian to early Burdigalian) age, with ages of 22.12 Ma (21.68–22.58) at the base to 17.91 Ma (17.51–18.29) at the top. The proposed new chronostratigraphic framework forms a foundation for future differentiation of local, regional, and global sedimentary patterns in the Austral Basin during the Paleogene/Neogene transition; in particular, with contemporaneous basins and tectonic blocks across the southern polar region, such as in Australia, New Zealand, and Antarctica.

Highlights

► New chronostratigraphic framework for the Patagonian Oligocene-Miocene succession is presented. ► The San Julián Formation is late Chattian (late Paleogene), spanning ∼25 to 23 Ma. ► The Monte León Formation is Aquitanian to early Burdigalian (early Neogene), spanning ∼22 to 18 Ma. ► Location of the Paleogene/Neogene boundary in Patagonia sedimentary rocks is discussed. ► Chronostratigraphic framework forms a foundation for correlation with contemporaneous basins.

Introduction

The Paleogene–Neogene boundary (23.03 Ma) corresponds approximately with the prominent Mi-1 cooling event that lasted for ∼200 ky, and may have arisen through superposition of different orbital (Milankovitch) cycles (Ogg et al., 2008). This event was accompanied by accelerated rates of turnover and speciation in certain groups of the biota, and forms part of a longer-lived transition from greenhouse to icehouse conditions, beginning in the Oligocene (Zachos et al., 2001; Miller et al., 2005), influenced by changing oceanic gateways and ocean-basin bathymetry, tectonic changes influencing geography and topography, and changes in the concentrations of atmospheric greenhouse gases (Zachos et al., 2001).

Well exposed high-latitude marine sediments of Oligocene-Miocene age from the Austral Basin in eastern Patagonia, Argentina (Fig. 1), are contained within the San Julián and overlying Monte León formations. These deposits may be important geological archives of tectonic, oceanographic, and climatic changes within this part of the Atlantic Ocean-basin, offering an important paleogeographic location (Fig. 1A) with respect to developing regional climate and oceanographic frameworks in the southern hemisphere. Such potential includes oceanographic and climatic records across the Paleogene–Neogene boundary, and may record components of more regional and global events: namely, the Austral Basin's oceanographic response to continued opening of Drake Passage, which started ∼34 Ma (Lagabrielle et al., 2009) and established a deep-water connection between oceans and triggered the onset of the Antarctic Circumpolar Current (ACC); sedimentary and diagenetic response of the basin to the Cenozoic compressional history of the Andean orogen (Blisniuk et al., 2005; Ghiglione et al., 2010); and, newly established biogeographic links with basins in New Zealand that suggest biotic communication across the vast Antarctic region (Fig. 1A; Casadío et al., 2010).

A precise chronostratigraphic framework for these strata is currently lacking such that the position of the Paleogene–Neogene (Oligocene–Miocene) boundary has remained poorly defined (Fig. 2). To better resolve this stratigraphy, we present a set of 87Sr/86Sr-based age dates from skeletal (oysters, brachiopods, pectinids) material from the San Julián and Monte León formations. Our database includes samples from ten sections from across the Austral Basin (Fig. 1) that enable improved delineation of the Paleogene–Neogene boundary and age range of the two formations. Sr-isotope stratigraphy is well suited for geochronology because of the rapid rate of change of marine 87Sr/86Sr during the period 26 to 18 Ma (McArthur, 1994; McArthur and Howarth, 2004), which encompasses the stratigraphic interval being studied, and the relative abundance of well-preserved calcitic macrofossils including the oyster Crassostrea? hatcheri (Ortmann), the pectinids Reticulochlamys proximus (Ihering), Jorgechlamys centralis (Sowerby) and Nodipecten sp., and the brachiopod Pachymagas sp. We support out dataset with cathodoluminescence microscopy and geochemical (trace element, C, O-isotope) tracers used to avoid altered material. The outcome is a new chronostratigraphic foundation for future regional biogeographic, paleoclimatic, tectonic and evolutionary histories of the southern South American continent during the Paleogene/Neogene transition, and improving extrabasinal correlations with contemporaneous basins in the South Polar Region (Fig. 1A).

Section snippets

Tectonic framework

Our study area encompasses part of the Austral Basin of eastern Argentina (Fig. 1B). The Austral Basin is located near the southern end of the South American plate, and underlies southern Patagonia, Tierra del Fuego Island, and the adjacent continental shelf of Argentina (Fig. 1B). From Ghiglione et al. (2010), the tectonic history of the Austral Basin involves: (1) Triassic to Early Cretaceous rifting during break-up of Gondwana; (2) thermal subsidence, followed by development of a

Field collection

Stratigraphic sections (Table 1; Fig. 1C) were measured for lithology, facies attributes, and stratigraphic position of biota. Some sections (Gran Bajo, Nido de Águila, La Colmena, Cabo Curioso, and Darwin Section) have been represented previously (Parras and Casadío, 2005, 2006; Parras et al., 2008; Parras and Griffin, 2009), whereas the remaining sections (Playa La Mina, Punta Asconapé, Restinga Norte, Cabeza de León, and Las Cuevas) are new. Ages from Gran Bajo, Nido de Águila, La Colmena,

Lithostratigraphy

Figs. 3 and 4 illustrate the lithostratigraphic correlation of the San Julián and Monte León formations based on ten sections (Table 1; for locations, see Fig. 1C). For each of the sections, positions of prominent skeletal carbonate rudstones and floatstones, and ages determined from Sr-isotope analysis (see below) are noted.

The San Julián Formation consists of ∼32 m of yellowish-brown sandstone and mudstone that form the Gran Bajo Member, and up to 15 m of yellowish-brown and green medium to

Marine depositional and diagenetic setting

Validity of ages determined via Sr-isotope depends on the source material being of marine origin, with no evidence for significant diagenetic contribution from fluids that may have contained more or less radiogenic 87Sr than the seawater source. For this reason, marine calcitic macrofossils (oysters, brachiopods, pectinids) are potentially useful tracers of seawater chemistry (McArthur, 1994).

All skeletal material used in our study is of marine origin. The San Julián Formation records an abrupt

Conclusions

The San Julián and Monte León formations in southern Patagonia record net regional transgressions within the Austral Basin, Patagonia, wherein the latter is related to the influx of Antarctic-derived waters during the early Neogene. Previous age estimates offered a poorly constrained chronostratigraphy, and, in particular, placement of the Paleogene–Neogene boundary had remained imprecise. Sr-isotope-determined ages from skeletal calcite (oyster, pectinid, brachiopods) provide a well defined,

Acknowledgements

This study was supported by Argentine and Canadian research grants: CONICET (PIP11420080100503), a CONICET external fellowship and SECyT (PI188 FCEyN-UNLPam) to A. Parras; and, an NSERC Discovery Grant to G.R. Dix. The authors are grateful to María José Julián, Gastón Cornachione, Martín Rodriguez Raising, María and Juan Griffin, and Santiago Genta Iturrería for the help in field work, and to Lizzy Ann Spencer (Carleton University) for guidance in Sr-isotope analysis. We express our gratitude

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