Facies interpretation and geochronology of diverse Eocene floras and faunas, northwest Chubut Province, Patagonia, Argentina

Abstract

The Eocene Huitrera Formation of northwestern Patagonia, Argentina, is renowned for its diverse, informative, and outstandingly preserved fossil biotas. In northwest Chubut Province, at the Laguna del Hunco locality, this unit includes one of the most diverse fossil floras known from the Eocene, as well as significant fossil insects and vertebrates. It also includes rich fossil vertebrate faunas at the Laguna Fría and La Barda localities. Previous studies of these important occurrences have provided relatively little sedimentological detail, and radioisotopic age constraints are relatively sparse and in some cases obsolete. Here, we describe five fossiliferous lithofacies deposited in four terrestrial depositional environments: lacustrine basin floor, subaerial pyroclastic plain, vegetated, waterlogged pyroclastic lake margin, and extracaldera incised valley. We also report several new 40Ar/39Ar age determinations. Among these, the uppermost unit of the caldera-forming Ignimbrita Barda Colorada yielded a 40Ar/39Ar age of 52.54 ± 0.17 Ma, ∼6 m.y. younger than previous estimates, which demonstrates that deposition of overlying fossiliferous lacustrine strata (previously constrained to older than 52.22 ± 0.22 Ma) must have begun almost immediately on the subsiding ignimbrite surface. A minimum age for Laguna del Hunco fossils is established by an overlying ignimbrite with an age of 49.19 ± 0.24 Ma, confirming that deposition took place during the early Eocene climatic optimum. The Laguna Fría mammalian fauna is younger, constrained between a valley-filling ignimbrite and a capping basalt with 40Ar/39Ar ages of 49.26 ± 0.30 Ma and 43.50 ± 1.14 Ma, respectively. The latter age is ∼4 m.y. younger than previously reported. These new ages more precisely define the age range of the Laguna Fría and La Barda faunas, allowing greatly improved understanding of their positions with respect to South American mammal evolution, climate change, and geographic isolation.

Cunoniaceae infructescences from the early Eocene Laguna del Hunco flora, Patagonia, Argentina

“…The taxa described in this paper are based on fossil infructescences collected from five of 32 quarried localities of the Tufolitas Laguna del Hunco, Huitrera Formation, Chubut Province, Patagonia, Argentina, paleolatitude ~47°S (Wilf et al, 2003(Wilf et al, , 2005; Figure 1). The Tufolitas Laguna del Hunco are fossiliferous caldera-lake deposits within a volcaniclastic depositional sequence consisting of tuffaceous mudstones to sandstones (Wilf et al, 2003(Wilf et al, , 2005Gosses et al, 2021). The fossils occur throughout the most densely sampled and fossiliferous 60-m interval of the 170-m local stratigraphic section (Wilf et al, 2003(Wilf et al, , 2005Hermsen and Gandolfo, 2016).…”

“…Other fossil material is from the lower part of the section, below and thus older than Ash 2211A. The maximum age of the fossils comes from the youngest beds of the underlying, caldera‐forming Barda Colorada Ignimbrite, which yielded an 40 Ar/ 39 Ar age on sanidine of 52.54 ± 0.17 Ma (Gosses et al, 2021). Thus, the age of all the fossils is close to 52 Ma, within the Early Eocene Climatic Optimum.…”

Contribución a La Diversidad Del Registro Fósil De Tortugas (Testudinata) De La Provincia De Chubut (Argentina) Y Su Significado en La Comprensión De La Evolución De Las Tortugas en El Sur De Sudamérica

“…Chronostratigraphic packages: J, Jurassic; K, Cretaceous; K-Pg, Cretaceous-Paleogene; Pg, Paleogene; Ng, Neogene. References: (1) Bridge et al, 2000; (2) Aberhan et al, 2007;Barreda et al, 2012;Scasso et al, 2012;Vellekoop et al, 2017; (3) Cúneo et al, 2013a;(4) Clyde et al, 2014;(5) dozo et al, 2014;(6) Casal et al, 2015;(7) Carballido et al, 2017;(8) Krause et al, 2017;(9) del Río et al, , 2018Scasso et al, 2001;(10) Bucher et al, 2019;(11) Gosses et al, 2020;(12) Krause et al, 2020;(13) Clyde et al, 2021. Cenomanian-Turonian lapse (Ar-Ar ~96-91 Ma; Bridge et al, 2000).…”

“…Taxa from the Huitrera Formation, in Laguna del Hunco locality (Fig. 2), are contained within lacustrine facies deposited within a volcanic caldera (Gosses et al, 2020). Specimens from Sarmiento Formation, in this package, come from two different stratigraphic levels, the oldest restricted to the Eocene Cañadón Vaca Member, and the youngest to an Oligocene bed.…”

“…On the other hand, the Sarmiento Formation at Cabeza Blanca is about 30 Ma (post-Tinguirirican to pre-deseadan land mammal age), on the basis of faunal correlations with beds from 'La Cantera' (i.e., Gran Barranca) (dozo et al, 2014). The portion of the Huitrera Formation containing turtles is restricted to a lacustrine setting inside the Laguna del Hunco calderalake, constrained through Ar-Ar ages as younger than 52.54±0.17 Ma and older than 49.19±0.24 Ma (Gosses et al, 2020).…”