A high-precision U–Pb zircon age constraints the timing of the faunistic and palynofloristic events of the Carnian Ischigualasto Formation, San Juan, Argentina

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Highlights

  • Drastic paleoenvironment-biotic perturbation occurred at 310 m of Ischigualasto Fm.

  • This event dated at 228.91 Ma.

  • This is the first date for Carnian Tethyan palynomorphs in western Gondwana.

Abstract

The Late Triassic represents records many of the most critical biotic crises in life-history (from Carnian to the end-Triassic extinction, ETE). Some hypotheses propose that these life changes were linked with shifts in the environmental setting during the initial stages of the Pangea breakup. These crises were likely not sudden events but a series of step-changes that reached their peak at the end of the Triassic. The Carnian-Norian Ischigualasto Formation is a world-renown terrestrial sequence that records both the Triassic Dinosaur Diversification Event (DDE) and at least one crucial terrestrial environmental and biotic turnover. A notable increase in humidity and volcanism was the setting of critical faunal replacement represented by the passage of the Hyperodapedon-Exaeretodon-Herrerasaurus to Exaeretodon tetrapod biozones. That environmental modification, together with the paleofaunistic turnover, has been accurately constrained by a new high-precision CA-TIMS U/Pb zircon age of 228.91 ± 0.14 Ma obtained from a tuff layer located in the exact stratigraphic position of the major biotic-paleoenvironmental disturbance in the middle part of the Valle de la Luna Member, at ~310 m from the base of the Ischigualasto Formation. Palynological assemblages containing the first records in westernmost Gondwana of European Tethyan and Onslow palynological species are also recognized close to the dated tuff level. Thus, this age is the first isotopic record for the Onslow Microflora diagnostic species in Gondwana. Moreover, the CA-TIMS date promotes a discussion about the accurate chronostratigraphic position for the beginning (after the Carnian Pluvial Event) of a series of environmental-biological disturbances that culminated at the end of the Triassic.

Introduction

The Late Triassic was a time characterized by deep environmental stress and consequent biotic turnovers, climaxing with the End-Triassic-Extinction (ETE) (e.g., McElwain et al., 1999; Tanner et al., 2004; Marzoli et al., 2004; Huynh and Poulsen, 2005; Preto et al., 2010; Hautmann, 2012; Lucas and Tanner, 2015; Rigo et al., 2020). Among the crises recorded between the Permo-Triassic extinction and the ETE, there is a series of minor perturbations, mainly recorded during Carnian and Norian, linked to pulses of different large igneous provinces (LIPs,e.g., Wrangellia and CAMP; Preto et al., 2010; Mazza et al., 2010; Muttoni et al., 2014; Lucas and Tanner, 2015). These disturbances together led to sharp differences between the biocenosis of the Middle Triassic and those of the Early Jurassic.

The Ischigualasto Formation of central-western Argentina is a rich fossiliferous fluvial succession deposited within a Triassic continental rift basin (Ischigualasto-Villa Unión Basin; Fig. 1). Given its paleogeographic setting and paleontological content, this unit offers one of the best opportunities to study the coevolution of vertebrate, floral communities, and environmental perturbations in the Southern Hemisphere (Rogers et al., 1993; Spalletti et al., 1999; Artabe et al., 2001; Martínez et al., 2011, 2012; Martínez et al., 2011a, Martínez et al., 2011b, 2012 Césari and Colombi, 2013; Colombi et al., 2017). The Ischigualasto Formation transitionally overlies the lacustrine Los Rastros Formation, which characterizes a wet period that also encompasses the Ischichuca and Chañares Formations. Evidence of the Carnian Pluvial Event (CPE) has been recently recognized in the lower Los Rastros Formation at 234.47 ± 0.44 Ma (Mancuso et al., 2020; Dal Corso et al., 2020). However, the CPE in the Ischigualasto Basin is characterized by the permanence of most vertebrate groups (e.g., archosauromorphs, archosauriforms, cynodonts, dicynodonts) without recorded extinctions. The appearance and rapid diversification of dinosaurs in the overlying Ischigualasto Formation represent the only apparent major biotic event recorded in the basin (Martínez et al., 2011, 2012). The Ischigualasto Formation includes all dinosaur families already diversified, corresponding with the Dinosaur Diversification Event (DDE), as suggested by Benton et al. (2018). This unit possesses an exceptionally well-preserved record of Late Triassic vertebrates, including in addition to the early dinosaurs, archosauromorphs, crurotarsal archosaurs, therapsids, and amphibians. Utilizing >5000 tetrapod specimens found in the Ischigualasto Provincial Park (IPP), Martínez et al. (2011, 2012) characterized vertebrate biozones and recognized a crucial, post-CPE/pre-ETE paleofaunistic turnover in the passage between the Hyperodapedon (ex Scaphonix)-Exaeretodon-Herrerasaurus (HEH) to the Exaeretodon (E) biozones. This biota turnover occurs at 310 m from the base of the Ischigualasto Formation, in the middle portion of the Valle de La Luna Member (Martínez et al., 2011a, Martínez et al., 2011b, 2012; Colombi et al., 2012, 2017, Fig. 2).

The Ischigualasto Formation also comprises rich plant assemblages, including impressions, cuticles, and permineralized trunks referred to as the BNP Biozone (Yabeiella brackebuschiana/Scytophyllum neuburgianum/Rhexoxylon piatnitzkyi; Spalletti et al., 1999). These macrofloristic remains also occur from the same stratigraphic level of the transition from Hyperodapedon-Exaeretodon-Herrerasaurus to the Exaeretodon biozones. Additionally, near this same stratigraphic horizon, Césari and Colombi (2013, 2016) reported the first Argentinian occurrence of a rich palynofloral assemblage with European Tethys and Northwestern Australia characteristic species (Onslow Microflora).

A series of allogenic environmental perturbations were identified in the Ischigualasto Formation using sedimentology, paleopedology, and taphonomy in the outcrops of the Ischigualasto Provincial Park (Tabor et al., 2006; Colombi and Parrish, 2008; Currie et al., 2009; Colombi et al., 2012, 2017). These perturbations were linked to tectonism during the rift basin evolution, external volcanism, and global climate changes (Colombi et al., 2017). The environmental perturbation began at 170 m from the base of the unit, at the Valle de la Luna Member base, characterized by an increase in humidity (Tabor et al., 2006; Currie et al., 2009; Colombi et al., 2017). However, the most critical environmental disturbance occurred when volcanism added to the effect of humidity, at 310 m from the base of the Ischigualasto Formation (middle part of the Valle de la Luna Member; Colombi et al., 2017). These changes were simultaneous with the most critical biotic disturbances (HEH to E biozones and first appearance of Onslow Microflora), representing an event that has not been previously documented or related to the Late Triassic extinction. This episode could be one undocumented intermediate event or could represent the onset of Late Triassic biotic turnovers that culminate with the End Triassic Extinction (ETE). All these disturbances are similarly linked to global climate changes and extended developed volcanism (e.g., Hesselbo et al., 2002; Ruhl et al., 2009; Whiteside et al., 2010; Hautmann, 2012; Dal Corso et al., 2014; Davies et al., 2017; Percival et al., 2017; Rigo et al., 2020; Ruhl et al., 2020).

Previous 40Ar/39Ar datings of volcanic ash deposits sampled from near both the base and at the top of the Ischigualasto Formation have produced corrected ages of ~231.4 Ma and ~225.9 Ma, respectively (Rogers et al., 1993; Martínez et al., 2011a, Martínez et al., 2011b - Supporting Online Material; Fig. 2). The biotic perturbations referred previously occurred at ~229 Ma, before the likely Carnian-Norian boundary (~227.3 Ma; Gradstein et al., 2020), considering the thickness and the ages between the base and top of the formation. Recently, however, new U/Pb zircon ages were obtained by Desojo et al. (2020) from a section of the Ischigualasto Formation in the northern extreme of the Ischigualasto-Villa Unión Basin (Hoyada del Cerro Las Lajas; Fig. 1). Based on their chronostratigraphic reconstruction and a taphonomic trend related to the disappearance of paleofauna observed in that area, these authors proposed that the significant biotic perturbation observed in the IPP occurred at ~227 Ma and thus at the Carnian-Norian boundary.

In this contribution, we report a new age of high precision CA-TIMS U/Pb zircon for the outcrops of the Ischigualasto Formation in the IPP. The dated ash comes from the same stratigraphic level where the significant biotic disturbances described above occurred. Thus, the most crucial biotic and environmental turnover initially observed and studied in the Ischigualasto Basin is dated in situ and allows us to discuss the age of one of the older environmental-biotic crises in the Late Triassic.

Section snippets

Geological setting

The Ischigualasto Formation is exposed in the northern part of San Juan Province, in west-central Argentina. The formation was deposited in the Ischigualasto-Villa Unión Basin, an extensive continental rift basin during the Late Paleozoic-Early Mesozoic time (Fig. 1). The stratigraphy of this basin comprises two sections; the lower corresponds to a thick Upper Permian-Lower Triassic red bed succession and the upper to the Upper Triassic Agua de la Peña Group. The lower section includes the

Stratigraphy of the Ischigualasto Formation

The Ischigualasto Formation transitionally overlies the lacustrine and deltaic deposits of the Los Rastros Formation (Milana and Alcober, 1994; Currie et al., 2009; Colombi et al., 2007; Colombi et al., 2012), and it is overlaid by fluvial deposits of the Los Colorados Formation (Milana and Alcober, 1994; Caselli et al., 2001; Kent et al., 2014; Schencman et al., 2015, Fig. 2). The thickness of the Ischigualasto Formation ranges from 400 m on the margin of the basin, next to the El Alto fault,

Geochronological results

The radiometric analysis was performed at the Pacific Center for Isotopic and Geochemical Research housed in the Department of Earth and Ocean Sciences (EOS) at the University of British Columbia. The technique used is a modification of CA-TIMS procedures outlined in Mundil et al. (2004), Mattinson (2005), and Scoates and Friedman (2008). Six zircons from the tuff sample from the Valle de la Luna Member recorded the U/Pb dates listed in Table 2. An age of 228.91 ± 0.14 Ma is based on the

Significant paleofaunistic and paleofloristic turnover superimposed to paleoenvironment perturbation

Paleoenvironmental reconstructions of the Ischigualasto Formation in the area of PPI have been based on several different proxies, including fluvial architecture, paleosols, and plant and vertebrate taphonomy (Tabor et al., 2006; Colombi and Parrish, 2008; Currie et al., 2009; Colombi et al., 2011, 2012, 2017). After the humid conditions in the Los Rastros Formation (Mancuso et al., 2020), in the lower part of the Ischigualasto Formation (La Peña and Cancha de Bochas members), the climate

Conclusions

Based on the data offered in this contribution, it is possible to conclude that the most continuous, complete, and undisturbed succession of the Late Triassic Ischigualasto Formation is at the Hoyada de Ischigualasto, IPP. Throughout this section, is preserved the scientifically significant Carnian paleovertebrate collection of the Ischigualasto Formation. Most of the paleovertebrates are concentrated within the Cancha de Bochas Member and decrease in number toward the top of the unit. The base

Credit author statment

Carina Colombi: Conceptualization, Methodology, Investigation, Writing Original Draft, Visualization, Resources; Ricardo Martínez: Conceptualization, Investigation, Writing Original Draft; Silvia Césari: Conceptualization, Methodology, Investigation, Writing Original Draft, Resources, Visualization, Funding acquisition; Oscar Alcober: Conceptualization, Investigation, Writing Original Draft; Oscar Limarino: Conceptualization, Methodology, Investigation, Writing Original Draft, Resources,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Thanks are Luis Spalletti and the anonymous reviewer for constructive and detailed comments that improved the manuscript. We also thanks Richard Friedman for collaboration in the radiometric ages of the Ischigualasto Formation. Financial support was provided through grants from the Agencia Nacional de Promoción Científica y Tecnológica (PICT 2015–1312 and PICT 2015–2074) and Conicet (PIP 190).

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