Journal of Undergraduate Research
Volume 4, Issue 1 - September 2002

Sedimentary Sequence From Southern Patagonia Serves as a Paleoclimatic Indicator

Luxas Moxey

ABSTRACT

A sedimentary sequence found at a roadcut in southern Argentina (46º 33.52' S, 70 57.6º W) presents distinct geologic features that were deposited under shifting environmental conditions. Detailed studies of the volcanic and glacial deposits suggest a glacial and interglacial origin. In addition, geochemical analyses revealed that the volcaniclastic deposits were formed during the Late Pliocene (3.5 Ma.) to Quaternary (recent). The climatic fluctuations that occurred throughout southern Patagonia during this period led to the formation of warm and cool paleoenvironments, resulting in the deposition of the glaciofluvial and glacio-lacustrine sediments observed in southern Argentina.

INTRODUCTION

In recent years global climatic trends have been the centre of attention, since they have aided scientists in developing prediction models of future climatic variations. Detailed worldwide geochemical and isotropic studies of sedimentary sequences have been used for identifying and understanding many of the warming and cooling trends that occurred in the past. Southern Patagonia has recently been the focus of numerous paleoclimatic investigations that attempted to unveil the paleoclimatic history of the region. During the last million years, Patagonia has had numerous glacial and interglacial cycles that resulted from warming and cooling episodes, leading to the advance and retreat of glaciers. As a result of these events, many sedimentary structures were formed throughout southern Chile and Argentina.

A roadcut located in the proximities of the town Perito Moreno (Santa Cruz &endash; Argentina) (46º 33.52' S, 70 57.6º W) (Figure 1) was found to contain a detailed sedimentary sequence that was deposited during the Plio-Quaternary (~3.5 million years ago (Ma.) &endash; recent) during glacial and interglacial periods. The stratigraphic record, located within the basin of Lake Buenos Aires, is composed of glacial till and epivolcaniclastic deposits (volcanic glass shards and rock fragments) (Leyrit and Montenat, 2000). The sequence is characterized by a variety of morphological and geologic features that provide information regarding their depositional history.

Figure 1. Location map for Lago Buenos Aires. The location of major urban centres are provided for geographic reference.

 

The purpose of this study is to identify and analyse the geologic structures within the sedimentary (epivolcaniclastic) record found in the proximities of Perito Moreno (southern Argentina) that suggest the existence of warming and cooling trends in southern Patagonia during the Plio-Quaternary. In addition, this paper discusses some of the implications that shifting environmental conditions may have had on early Patagonian population groups.

Geologic Setting

Dispersion Southern Patagonia consists primarily of a Late Devonian (115 Ma.) to Permian (245 Ma.) (Paleozoic) meta-sedimentary basement that was intruded by Jurassic (170 Ma.) lava flows that ranged from andesites to rhyolites. During the Cretaceous, a marine sedimentary unit 2500 metres thick was deposited over the Jurassic lavas. During the Late Miocene-Pliocene, basalts formed at the Meseta Buenos Aires located to the south of Los Antiguos (Figure 1). The remnant volcanic plugs and necks found near Perito Moreno provide evidence the Pliocene alkali basalts that existed in the region. The glacially-fed Lake Buenos Aires is currently enclosed in alternating layers of glacio-lacustrine and glacial deposits (Figure 2) and lies 90 kilometres SE from Mt. Hudson Volcano, a stratovolcano that erupted in August 1991.

 

Figure 2. Geologic map of Lake Buenos Aires (Adapted from Panza et al, 1994).

Previous Paleoclimatic Studies

The current landscape of Patagonia presents clear evidence of glacial erosion. Figure 3 delineates the geomorphology of the Lake Buenos Aires basin that resulted from at least ten glacier advances and retreat episodes. During the Last Glacial Maximum (30000-10000 ¹⁴C yr BP), glaciers occupied the totality of Lake Buenos Aires (Coronato et al., 1999). In addition, geochemical studies of different moraine deposits found throughout this area have yielded a variety of depositional dates, supporting the hypothesis of repeated glacier advances and retreats, as well as variable lake levels and the presence of inflow and outflow glaciofluvial systems.

Figure 3. Landsat-5 TM scene showing the glacial deposits in the Lake Beunos Aires region. The image was acquired 20 March 2001.

Several paleoclimatic studies of southern South America have yielded a variety of results that display poor agreement, leading to confusion and uncertainty within the literature. Nevertheless, recent studies have shown very strong Holocene paleoclimatic correlations (Markgraf, 1989; Rabassa and Clapperton, 1990; Stine and Stine, 1990; Ariztegui et al., 1997; Mancini, 1998; Thompson et al., 2000; McCulloch, 2000; Hodell et al., 2001a). During the Quaternary, southern Patagonia was affected by several glacial and interglacial periods that caused the repeated advance and retreat of glaciers. According to McCulloch et al. (2000), the glaciers at approximately 14600 ¹⁴ C yr BP covered large areas of southern Chile and a considerable portion of the eastern section of the southern Andes, including the Lake Buenos Aires region. By 10000 ¹⁴ C yr BP, after several warming episodes, the Patagonian Ice Field had separated and formed the Northern and Southern Patagonian Ice Fields that are presently observed in Chile and Argentina. Data from an ice core retrieved from Huascarán (Perú) by Thompson et al. (2000) shows a sharp decrease of d¹⁸O concentrations at ~10000 BP, indicating the existence of warm conditions in South America before this period. Other paleoclimatic studies of southern South America (Rabassa and Clapperton, 1990; Mancini, 1998; Thompson et al. 2000; Hodell et al., 2001a) have identified the existence of a strong cooling episode at 5000 BP, the Neoglaciation, that resulted in glacier advances.

DATASETS AND METHODOLOGY

Fieldwork was conducted in the Lake Buenos Aires region during June 2000. Geological exploration in Argentine territory allowed for the collection of epivolcaniclastic deposits at a roadcut approximately 2 kilometres north of Perito Moreno (Prov. of Santa Cruz &endash; Argentina).

The outcrop contained five epivolcaniclastic layers that included volcanic and clastic material (Figure 4) (Moxey, 2001). Each epivolcaniclastic unit was denoted N5 (youngest) through N1 (oldest) according to their stratigraphic position. The sediment layers found between each epivolcaniclastic layer are mud-clay units containing a high population of non-volcanic, well-rounded clasts. Each layer varied considerably in thickness from 5 to 31.5 cm (Moxey, 2001). The samples collected were wet sieved and statistical calculations carried out (Moxey, 2001).

Figure 4. Outcrop located approximately 2 km north of Perito Moreno.

The volcanic glass shards collected were analysed using an electron microprobe (EMP) from the US Geological Survey (USGS) at the Denver Geochemical Analysis Center. Five polished slides containing samples from each epivolcaniclastic level were analysed and their major and trace element compositions determined. Microscopic inspections of the thin sections did not reveal any pollen or microfossils.

RESULTS AND DISCUSSION

Quaternary Paleoclimate of Southern Patagonia

Field observations from the roadcut near Perito Moreno led to the identification of different geologic structures that suggest a closed-lake paleolacustrine depositional environment that resulted from a post-glacial melting episode. All of the observed volcanic layers, except N5, represent primary glaciolacusrtine deposits. It is likely that the deposits formed from the melt water influx provided by one of many fluvial melt water channels (high-energy environment) that flowed into a low-energy proglacial lake. The population of angular glass shards within the layers indicates the fluvial transportation from a proximal source. The geochemical analysis of the volcanic glass shards suggest that the sedimentary sequence represents a Late Pliocene-Quaternary deposit (Perfit and Moxey, unpublished data).

Given that the Lake Buenos Aires region was affected by glacial conditions, it is likely that the Patagonian gravels observed at the roadcut were deposited during glacier outwash episodes. Strelin et al. (1999) identified several locations throughout southern Santa Cruz that contain glacial till deposits overlaid by silty-clay sediments, and recognized a glaciolacustrine origin. Resulting from the warming environmental conditions and glacial melting, proglacial lake environments and fluvial channels were formed throughout southern Argentina. Alluvial plain deposits consisting of glacial till have been found in the region surrounding Lago Buenos Aires (Figure 5a) and extend at least 160 kilometres to the east, where thick deposits (at least 1 metre) can be observed (Figure 5b). Based on the descriptions by Strelin et al. (1999), the non-volcanic clasts interlaid with the epivolcaniclastic beds are believed to represent the Patagonian gravels ("Rodados Patagónicos"). Field inspections of the alluvial deposits detected the existence of stratified till. In proglacial settings such as southern Patagonia, alluvial fans develop as outwash fans in front of the melting glaciers.

 

Figure 5a. Glacial till deposits observed at the study site, comtaining well-rounded clasts.

 

Figure 5b. Thick glacier deposits found 160 km east of Perito Moreno (Sta. Crz, Arg).

 

On the contrary, the laminated epivolcaniclastic deposits (N1-N5) suggest that they were deposited under glaciofluvial-lacustrine conditions, resulting from the low-energy environment required for their formation and preservation. Therefore, the alternating epivolcaniclastic layers present within the alluvial fan deposits show that climatic trends changed to cooler conditions, resulting in the decreased glacier outflow and subsequently, decreased glacial till deposition. The well-defined parallel-laminae (<1 cm thick) that were identified throughout levels N1 to N4 (Figure 5c) also indicate the existence of a glacial lake environment.

 

Figure 5c. Fine laminations present in level N4 that resulted from a glacilacustrine depositional environment.

 

The ripple cross-lamination features observed at level N5 (Figure 6) suggest the existence of a shallow-water (<1 meter) fluvial or glaciolacustrine shoreline environment (Moxey, 2001) resulting from warm climatic conditions. Therefore, the sedimentary structures in level N5 can serve as paleoclimatic indicators that mark the existence of a warm period characterized by glacier retreats and higher freshwater outflow levels onto proglacial lakes. Moxey (2001) proposed that layers N1-N4 represented primary airfall tephra (ash) layers from Mt. Hudson, whereas level N5 represented a reworked layer that was identical to N4. Subsequent geochemical studies of each layer revealed identical compositions, demonstrating that they had originated from the same source. Furthermore, geochemical data (Perfit and Moxey, unpublished data) showed that the layers were not airfall deposits, but reworked epivolcaniclastic layers.

Figure 6. The shallow-water ripple cross-lamination structures present at N5 can serve as paleoclimatic indicators.

Human occupation of southern Patagonia

Archaeological studies performed throughout southern Chile and Argentina allowed researchers to establish a more complete record regarding the population migrations and development in southern South America. Other studies performed in Mesoamerica (Hodell et al., 2001b) have shown the strong impact that climatic change may have on early human cultures. In Central America, climatic change resulting from solar forcing caused a high environmental stress that affected the resources needed by the Maya civilization (Hodell et al., 2001b) at 2486 BP and 2286 BP.

Near the end of the Last Glacial Maximum, at 13000 ¹⁴ C yr BP, the first hunter-gatherers were found in southern Patagonia. They had migrated initially from Siberia and North America, and after many generations, from northern South America (Coronato el al., 1999). Archaeological research suggests that the early settlers of Patagonia were able to overcome the climatic changes that occurred during the Late Pleistocene-Early Holocene, allowing them to colonize specific regions within Extra-Andean Patagonia and the piedmont regions that contained adequate food, water and raw material supplies. Nonetheless, there is evidence that the population groups that existed during this period were small in size, and that they were scattered throughout Patagonia (Coronato et al., 1999).

Although the environmental conditions changed at 9000 ¹⁴C yr BP (Coronato et al., 1999), the causes and impacts of this change on the early human occupation remain unclear. At 6700 BP, Mt. Hudson Volcano had the largest eruption ever recorded in the southern Andres (Naranjo and Stern, 1998). Due to the magnitude of this eruption, the possibility that this episode had a strong impact on the environment and on the early settlers cannot be discarded. Furthermore, the large tephra deposits that blanketed southern Chile and Argentina would have forced population groups to migrate. The lack of detailed archaeological studies that address the impact of the 6700 BP eruption of Mt. Hudson on the early Patagonian population groups prevents any definite correlations. Nonetheless, it is clear that further studies in this topic must be made in an effort to have a better understanding of the geologic and human history of southern Patagonia.

CONCLUSIONS

This study demonstrated the value of sedimentary sequences for the identification of temperature fluctuations during the Plio-Quaternary epochs in southern South America. It identified and analysed the geologic structures found at a roadcut in southern Argentina that formed from glacial processes. The field observations conducted in southern Argentina yielded important clues regarding the warming and cooling (glacial and interglacial) paleoenvironmental conditions that occurred in the region. Sedimentary units and depositional structures observed at the roadcut present strong evidence of variable depositional environments that resulted from temperature fluctuations during the Plio-Quaternary in southern Patagonia. Furthermore, analyses of the volcaniclastic layers demonstrated that the older layers (N1-N4) were deposited in a glaciolacustrine environment, whereas the youngest epivolcaniclastic deposit (N5) formed in a shallow water lacustrine environment. In addition, this contribution also addressed some of the effects that shifting climates may have had on early Patagonian population groups.

ACKNOWLEDGEMENTS

The completion of this study was possible thanks to the support of my mentors, Dr. Mike Perfit and Dr. Dave Hodell. In addition, the invaluable logistical support provided by Dr. Jason Curtis, Dr. Mark Brenner, Dr. Pablo Guerstein (UNSL), Mary Palacios and John Chadwick greatly contributed to the success of the southern Patagonia campaign. The information and resources provided by Dr. Chuck Stern, Martín Muñoz, Nicolás Ayling, José Luis Méndez and Sebastián Navarta also made possible the success of this endeavour. The study was funded by the University of Florida through the 2000-2001 University Scholars Program research grant.

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