The glacial paleovalley of Vichigasta: Paleogeomorphological and sedimentological evidence for a large continental ice-sheet for the mid-Carboniferous over central Argentina
Introduction
The Late Paleozoic Ice Age (LPIA) represents the longest Phanerozoic glacial interval (Veevers and Powell, 1987; Frakes et al., 1992) and comprises diachronous episodes of glacial and non-glacial conditions across the Gondwana supercontinent (Eyles, 1993; López-Gamundí, 1997; Isbell et al., 2003; Fielding et al., 2008; Gulbranson et al., 2010). Several works about the timing, extent and type of glaciation that affected Western Gondwana were published in the last decade (López-Gamundí and Buatois 2010; Isbell et al., 2012; Montañez and Poulsen 2013; Limarino et al., 2014; (Valdez et al., 2019; 2020, among others). A discussion about the type of glaciation that affected this region of western Argentina persists to date. While some authors suggest a large glacial event of the sort observed in North America, or Scandinavia, other authors suggest the glacial centers were only located near the western margin, as hypothetical mountain-shadow small glacier systems, and suggested that alternatively considered glacial valleys as the Malanzán, are in fact, non-glacial (Andreis et al., 1986; Azcuy et al., 1987; Moxness et al., 2018; Pauls et al., 2019).
One of the best places to evaluate the type of glaciation that affected the western margin of Gondwana during the LPIA is the Western Argentina. Innovative information on the LPIA, new stratigraphic, sedimentologic, paleogeomorphological and palynological data from the Carboniferous Vichigasta paleovalley is presented. It lies in the eastern Paganzo Basin (Fig. 1 A and B) and it has been recognized in early works of this basin (Limarino and Gutierrez, 1990; Astini, 2010), but any stratigraphic study was attempted until today. However, this locality may contribute to shed light on the controversy of the type of glaciation that predominated in central-western basins of South America during the LPIA. Two main interpretations are postulated: a) alpine glaciers emanating from a Paleo-Precordillera roughly coincident with the modern Precordillera (Amos and Rolleri 1965; Rolleri and Baldis 1969; Limarino and Spalletti 2006; Limarino et al., 2014), and b) a larger continental ice-sheet to the east (Sterren and Martínez 1996; Kneller et al., 2004; Astini et al., 2009, 2010; Aquino et al., 2014; Socha et al., 2014; Valdez et al., 2017, 2020).
As recently suggested, the fill of the Malanzán paleovalley, a critical locality for the Carboniferous glacial history of South America, was not glacial in origin (Moxness et al., 2018; Pauls et al., 2019). Although, there are a plethora of papers suggesting the contrary (Sterren and Martínez, 1996; Enkelmann et al., 2014; Rabassa et al., 2014; Socha et al., 2014). The sedimentological and palaeontological information presented in this contribution allows us to discuss the original hypothesis of a large ice-sheet feeding glaciers that carved these valleys in the light of the recent non-glacial hypothesis. It is also important to note that the suggested alpine glaciation to the west is based on the presence of a hypothetical mountain chain (the Protoprecordillera) that was cutting the humidity flow to the east (continent interior) like the effect of Andean Range today. However, the presence of the Protoprecordillera is under debate and without it, this mountain shadow effect should be revised. The arguments supporting the absence of such a mountain chain have been recently discussed by Milana and Di Pasquo (2019 and references therein). Numerous information on sensitive climatic indicators (e.g. diamictites, coal deposits, fossils, content of organic matter) are widely used in paleoclimatic reconstructions (see Scotese et al., 2014). In Argentina, since the first studies carried on in the Pennsylvanian of the Paganzo basin, these indicators were recorded in different east-west localities (see Azcuy et al., 2000; Césari et al., 2007; Limarino et al., 2014, and references therein). In fact, an opposite gradient of humidity (increasing humidity eastwards) could be inferred based on the extent of coal seams and plant deposits in the eastern Paganzo Basin, which contrast to more recent interpretations (cf. Pauls et al., 2019).
In order to strengthen our hypothesis about the type of glaciation affecting the western Gondwana we have compared our results from the terrain analysis done at Vichigasta with those of the Malanzán and Olta paleovalleys. We demonstrate the paleogeomorphological link between the Vichigasta and Malanzán glacial paleovalleys despite being about 160 km apart (Fig. 1B). Under the ice-sheet hypothesis, both paleovalleys would have been probably draining the same ice body and hence, inheriting similar boundary conditions to transport ice to the Western Paganzo basin.
Section snippets
Local geology of Vichigasta
The Vichigasta locality has been mentioned by some authors (Limarino and Gutierrez, 1990; Astini 2010), but no detailed work has been reported. The first part of the valley is clearly modern due to its V-shape. There are some mentions of a possible glacial origin for this paleovalley (Astini, 2010), but no detailed study supported this interpretation. Thus, we surveyed the distinguishable late Paleozoic units present in this paleovalley that trends east to west (see Fig. 1C).
The sedimentary
Methods
The study is focused on three lines of work: 1) physical stratigraphy and sedimentology, 2) terrain analysis and 3) paleontology. The first part of the work is centered on outcrop descriptions, using conventional techniques of facies analysis. Disconnected outcrops were described and mapped. For facies characterization and vertical stacking of stratigraphic levels, a geological log is shown at 1:250 scale (Fig. 1D). The stratigraphic analysis allows the recognition of four main stages described
Lithostratigraphic analysis of the valley fill
Stage 1: Proglacial sediments.
This stage is composed of three units (A, B and C) and is the oldest recorded in this valley. It crops out locally and only preserved in a limited area of the southern margin of the paleovalley in its eastern section (see Fig. 1B). The unit is absent in other places and is interpreted to have been eroded by the action of the coarse-grained fluvio-alluvial systems of Stage 2.
Unit A- Thin-bedded sandstones and siltstones with outsized clasts: this unit is up to 10 m
The glacial origin of Vichigasta paleovalley
Evidence presented in this contribution strongly supports a glacial origin for the Vichigasta paleovalley. Dropstones could result from seasonal freezing of the lake surface or even, they can be a product of vegetal rafting and used to discard a glacial origin of some dropstone-bearing sedimentary units (cf. Pauls et al., 2019). However, reported grain sizes associated with these processes in modern lakes never exceed 2–3 cm in diameter, and they are an exceptional process rather than a
Conclusions
Vichigasta paleovalley is a key locality for understanding the glacial context of LPIA in western Gondwana given the high level of preservation of paleo-geomorphological features. On top of the geomorphological evidence, the erosive remnants left within this paleovalley add important information for the complete understanding of this and some other glacial localities. All the available evidence indicates a glacial origin for the valley, also suggesting that it was an outlet glacier draining a
Author statement
I attest that all authors contributed to the elaboration of this manuscript. Dr. Victoria Valdez Buso and Dr. Juan Pablo Milana mapped, and worked in the sedimentary record of the valley. Dr. Mercedes di Pasquo sampled and analyzed the palinologic content. Mr. José Espinoza Aburto build up the DEMs.
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
The authors thanks to Lic. Leonardo Silvestri for his help in sample processing at the CICYTTP (CONICET-ER-UADER) and Consejo Nacional de Investigaciones Científicas y Técnicas CONICET PIP 0812 (2015–2017). We thank Dr. Ben Kneller for his critical observations. We are also grateful to an anonymous reviewer and Dr. Claus Fallgatter for their comments that helped improved our work.
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