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Late Quaternary Tephra Deposits of Mt. Hudson Volcano, Southern ArgentinaLucas Moxey
ABSTRACT
INTRODUCTION
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
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
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
DISCUSSION
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).
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
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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