Journal of Undergraduate Research
Volume 2, Issue 5 - February 2001

Late Quaternary Tephra Deposits of Mt. Hudson Volcano, Southern Argentina

Lucas Moxey

ABSTRACT

Five discrete tephra deposits from Mt. Hudson Volcano were identified at an outcrop 2 km north of the town of Perito Moreno (Argentina). Four tephra units correspond to in situ glaciolacustrine deposits. The youngest unit appears to have been deposited in a shallow-water fluvioglacial or paleolacustrine shoreline environment. All units seem to have originated from phreatomagmatic volcanic episodes. Statistical analysis has revealed two tephra units which have originated from the same volcanic pulse. By means of geochemical correlation it will be possible to confirm Mt. Hudson volcano as the source volcano of all tephra units.

INTRODUCTION

The pyroclastic product called tephra, which is comprised of volcanic glass, lithic and crystal fragments, has been deposited in proximal and downwind distal areas of Mt. Hudson volcano. Studies at an outcrop approximately 2 kilometers north of the Argentine town "Perito Moreno" (46º 33.52 S, 70º 57.6' W) in the Province of Santa Cruz (Fig. 1) during June 2000 have revealed the existence of five distinct tephra units.

Figure 1. Location of study.

Figure 1. Location of study.

Phreatomagmatic eruptions, which are often characterized by violent explosions, can often produce large volumes of tephra that can be emplaced in a very short period of time (Shane, 2000). Tephra has been used successfully in the past as a regional stratigraphic marker, because it can be quickly transported thousands of kilometers from its source and can be deposited over a large area without undergoing chemical alteration (Newton and Metcalfe, 1999).

Ancient tephras derived from Mt. Hudson should be found at Perito Moreno, because it lies 170 km to the east and downwind of the volcano. Such potential depositional areas were recognized during the 1991 eruption, when satellites (such as NOAA-11) recorded the paths of pyroclastic clouds.

Previous tephrochronologic studies (Naranjo and Stern, 1998) have examined tephra levels in modern lacustrine and terrestrial systems within Chilean territory, some at upwind localities from the volcano (Haberle and Lumley, 1998).

This paper presents the results of a detailed study performed on the Quaternary tephra deposits from Mt. Hudson Volcano proximal to Perito Moreno, Argentina.

BACKGROUND

The town of Perito Moreno is 17 km to the east from the glacially-fed Lake Buenos Aires (Fig. 2), and is 170 km from the active Mt. Hudson Volcano. This volcano is located in the southernmost part of the Southern Volcanic Zone, and lies near the triple point where the Nazca, South American and Antarctic plates meet (Naranjo et al. 1993).

Figure 2. Geologic map of Lake Buenos Aires

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

Mt. Hudson (45º 54' S, 72º 58' W) is a stratovolcano that has a 10 km diameter caldera, which was formed by the convergence of the Antarctic and South American plates (Stern, 1990). The volcanic structure, discovered in 1970 (Fuenzalida and Espinosa, 1973) rises to a height of 1905 m, and contains a 2.5 km2 glacier within the caldera (Naranjo et al. 1993). In August 1991, this volcano had two large eruptive pulses that blanketed over 100.000 km2 of the Argentine Patagonia with tephra fallout (Scasso et al. 1994), including Perito Moreno. In less than one week, during the duration of the eruption, Hudson's products varied in composition, ranging from basalt to dacite (Naranjo et al. 1993).

Mt. Hudson has had several volcanic episodes, at least eight in the last 8300 years (Naranjo and Stern, 1998). The five discrete ash layers found at Perito Moreno, of which four are primary airfall deposits, are believed to have originated from Mt. Hudson volcano.

TREATMENT AND DESCRIPTION OF TEPHRAS

The outcrop examined was found to contain five tephra layers (Fig. 3), denoted N5 (youngest) through N1 (oldest). Strata between all volcanic units (except N4 and N5) consist of a mud-clay sediment containing non-volcanic, well-rounded clasts. Individual tephra layers varied in thickness from 5 to 31.5 cm (Table 1). Level N5 contains ripple cross-lamination features, a unique characteristic of shallow-water depositional environments (Boggs, 1995). (Fig. 4)

Figure 3. Detailed stratigraphic section of the Perito Moreno outcrop.

Figure 3. Detailed stratigraphic section of the Perito Moreno outcrop.

Table 1
Tephra levels, characteristics and location within the outcrop
Tephra
Layer
Thickness (centimeters)
Height Relative to Base of Outcrop
(Meters)
N5 (youngest)
31.5
8.04
N4
24
7.80
N3
8.3
6.91
N2
6
5.75
N1 (oldest)
5
5.1

Figure 4: Ripple cross-lamination features observed within tephra unit N5 and contact regions with N4.

Figure 4. Ripple cross-lamination features observed within tephra unit N5 and contact regions with N4.

Tephra samples for each level were mounted and polished for electron microprobe analysis (EMPA). The geochemical analyses, which are currently in progress, will provide major element compositions of tephra in each layer.

Grain size data were obtained by wet sieving each tephra layer using nine different sieve sizes. Once the grain size data were collected, several statistical calculations were performed, following the guidelines established by Boggs (1995). Statistical measurements for grain size data for each tephra level were obtained through calculations instead of graphical methods.

RESULTS

Grain size analysis revealed the physical characteristics of each layer as displayed in Figure 5 and Table 2 and Table 3. Level N5 is poorly sorted, symmetrically distributed and the mean and mode fall within the silt and sand size-range, respectively. Calculations reveal a coarse, strongly skewed distribution, with a moderate peak. Level N4 is moderately well sorted, symmetrically distributed and has a mean and mode that are within the slit and sand-size range, respectively. The distribution is coarse and strongly skewed. Level N3 is moderately well sorted, and has a fine-grained mode and mean. The distribution is fine and strongly skewed, and has low kurtosis. Level N2 is poorly sorted and has a mode and mean in the silt-size range. The distribution is fine and strongly skewed, with a low kurtosis. The sample is characterized by a bimodal distribution. Level N1 is moderately well sorted with similar mean and mode. The distribution is also coarse and strongly skewed, and has a bimodal distribution.

Figure 5. Grain size distributions at the Perito Moreno outcrop.

Figure 5. Grain size distributions at the Perito Moreno outcrop.


Table 2
Descriptions of individual tephra units
Tephra Level Description
N5 Tephra is poorly sorted, has a sugary texture and contains no induration. Overall has a light brown colour. Large non-volcanic clasts (>0.5 cm) are contained within the sample, and are heavily weathered. These clasts do not seem to be related to the volcanic origin of the tephras.
N4 The tephra is moderately sorted, very unconsolidated and has a light-brown colour. It is sand-sized and has a sugary texture. There seems to be a very high presence of glass shards, all of which are highly angular and translucent. Lithic fragments are also present, and all show some moderate signs of weathering. Green amphiboles are abundant. Evidence of grading within the ash layer was observed, dominated by clasts of non-volcanic origin.
N3 The sample is well sorted, and has a very high degree of induration. Tephra is light brown, clay-sized with a talc-like texture. There are virtually no lithics present. Most sediments are clay-sized to silt-sized. Few glass shards observed. Clumps of sediments are common.
N2 The tephra is very well sorted, sand to clay sized, and has a brown-light brown colour and chalk-like texture. Small angular glass shards present, translucent in colour. Lithic fragments display moderate signs of weathering. Minerals observed include green amphibole, feldspars, biotite and opaque minerals. Few pumice observed. Clumps of sediments are present due to high degree of induration. Evidence of normal grading of lithics was observed.
N1 The tephra is poorly sorted, sand-sized, has an overall light brown colour and presents a sugary texture. Cognate lithic fragments are abundant in this level, while there is a low percent of glass. All glass shards are clear in colour and have a fresh appearance (angular and blocky). Some shards appear to be elongated and translucent. Lithic fragments display strong rounding. Identified minerals include feldspars, green amphiboles, biotite and other metamorphic rock fragments. Few pumice shards were observed.
Table 3
Statistical data of the five tephra units
Tephra Unit
N5
N4
N3
N2
N1
Mode
4
4
5
5
3.2
Bimodal
Yes
Yes
No
Yes
Yes
Mean
4.3
4.7
4.9
4.5
3.5
Standard Deviation
1.052
0.604
0.714
1.007
1.175
Skewness
-1.32
-1.11
0.82
0.334
-0.39
Kurtosis
5.18
9.09
2.22
3.06
2.28

DISCUSSION

The tephra units N1 through N4 all display sedimentary features that correspond to a closed-lake paleolacustrine depositional environment. All of these layers are primary (in situ) tephra fallout deposits. The presence of massive gravels in between the tephra units suggests that glacial deposits were possibly affected during deposition by the action of high-energy currents. The glacial sedimentary units seem to have strong similarities to the "rodados Patagónicos" (gravel unit), as described by Strelin et al. (1999). Given that this area was affected by several Pleistocene glaciation episodes (Rabassa and Clapperton, 1990; Sylwan, 1990), the possibility that Lake Buenos Aires suffered numerous advances and retreats, as well as the fluvial outflow levels, is likely.

Grain size data collected from units N4 and N5 suggest that both tephra layers originated from the same eruptive episode. Although both layers were deposited in a subaqueous environment, level N5 contains ripple cross-lamination features (Fig. 4) that are unique to shallow-water (< 1meter) fluvial or lacustrine shoreline environments (Boggs, 1995). Therefore, this may serve as evidence for climatic variations. Level N4 is likely to have been deposited at the time of the eruption in a paleolacustrine environment. Level N5 was likely formed by the local reworking of the N4 top layers and later deposited in a shallow-water fluvial or lacustrine shoreline environment. Level N5 was preserved as a result of rapid burial processes (Fig. 6).

Figure 6. Photograph displaying details of the stratigrapic section at Perito Moreno

Figure 6. Photograph displaying details of the stratigrapic section at Perito Moreno

According to Naranjo and Stern (1998), Mt. Hudson volcano had two eruptions (3600 and 6700 BP) that were strong enough to reach localities as far as Perito Moreno. Therefore, once the geochemical analysis is completed, we can expect to find that two of the five tephra layers correspond to the eruptions that occurred 3600 and 6700 years BP. Level N4 (24 cm thick) does not fit any of the models suggested by Naranjo and Stern (1998). Although it is very unlikely that the source for these tephras is a volcano other than Mt. Hudson, this possibility cannot be dismissed. It is clear that any correlation of this unit to either the 1971 or 1991 eruption must be rejected due to its stratigraphic position in the outcrop examined.

Levels N4 and N5, which are presumed to have originated from the same eruption, share many characteristics. Both tephra beds have a large amount of pumice, glass shards with coarse, strongly skewed bimodal distributions. These characteristics suggest a Plinian eruption style (Parfitt, 1998; Cas and Wright, 1987). Mt. Hudson's history of highly explosive eruptions, highly viscous lavas and stratovolcanic structure support this hypothesis.

In contrast, the characteristics of units N3, N2 and N1 suggest a Phreatoplinian volcanic origin, where a magma-water interaction existed. This may be inferred from several granulometric features, such as the fine grain sizes, poor sorting, angular and blocky glass shard morphologies and bimodal distributions. (Parfitt, 1998; Cas and Wright, 1987). Bimodality and poor sorting may be attributed to the effects of humid ash or clustered fallouts from an eruption plume (Cas and Wright, 1987).

CONCLUSIONS

The tephra deposits studied in the area of Perito Moreno have provided evidence of the existence of a glacially-fed paleofluvial or paleolacustrine system. Evidence of possible climatic changes has also been observed, originated by factors such as seasonal weather patterns or glacial and interglacial episodes.

Different physical characteristics within the tephra units suggest Plinian and Phreatoplinian origins. Phreatomagmatic episodes occurred in the August 1991 eruption, where a volcanic plume reached an altitude of 18 km and extended over 1000 km SE of the Islas Malvinas (GVN, Jul 1991), reaching Bird Island (South Georgia Island) (Smellie, 1999). Therefore, such reoccurrence can be expected in its eruptive past.

Geochemical fingerprinting will provide data to pinpoint Mt. Hudson volcano as the source for all five tephra units. Measurements of alkali abundances have been successfully used in the past for such specific tasks (Boygle, 1999). Based on findings of Naranjo and Stern (1998), it is expected that two of the volcanic units may correspond to the 3600 and 6700 years old eruptions. Geochemical analyses will allow for the construction of more detailed models and Mt. Hudson's eruptive history.

Because important geochemical compositional changes occurred during the 1991 eruption, similar variations may be present in the fallout units, and these may range from basaltic to dacitic composition. This may present difficulties when geochemical correlations are attempted.

The present study must be viewed as a preliminary contribution that may help to improve the understanding of the Quaternary volcanic history of Mt. Hudson.

ACKNOWLEDGEMENTS

I thank the University of Florida for their generous support through the 2000-2001 University Scholars Program research grant. I am especially thankful to my mentors, Dr. David Hodell and Dr. Mike Perfit. The following people also provided me with great assistance and support: Dr. Jason Curtis, Dr. Pablo Guerstein, Mary Palacios, Martín Muñoz, Andrea and Andrés Moxey, Elena Drabble, Dr. Norman Banks, Nicolás Ayling, José Luis Méndez and Sebastián Navarta.

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