Paleomagnetic Results from Archaeological Sites in Argentinean Patagonia: Evidence for the Holocene Geomagnetic Excursions in Southern South America and Its Chronostratigraphic Implications

Hugo G Nami

doi:10.4236/ad.2019.72008

Abstract

Detailed palaeomagnetic research performed in archaeological and paleontological sites in southern South America yielded a number of highly fluctuating paleosecular variation records with geomagnetic field excursion(s) during the Holocene. To assess this topic a variety of sections of recent sediments were sampled. In the case presented here they belong from diverse continental sedimentary sequences formed in coastal marine and cave’s environments. Paleomagnetic samplings were performed in several deposits in San Blas Bay and Pali Aike areas, respectively located in the northern and southern parts of Argentinean Patagonia region. To collect samples, cylindrical plastic containers 2.5 cm long and 2 cm diameter were carefully pushed into the sections, overlying each other by about 50 percent. Their strike and dip were measured using a Brunton compass and inclinometer; they were consolidated with sodium silicate after removal and numbered from top to bottom. All samples were subjected to progressive AF demagnetization in steps of 3, 6, 9, 12, 15, 20, 25, 30, 40 and 60 mT in a 3-axis static degausser attached to a 2 G cryogenic magnetometer. Additional steps from 80 to 120 mT were used in some samples. Characteristic remnant magnetization was calculated using principal-components analysis, with the best-fitting line going to the origin in the Zijderveld diagrams. In general, maximum angular deviations were generally within low values. Some samples had univectorial behavior, while some showed two or three components. Most of the sections show normal and intermediate polarity directions far from the present GF, while reversed polarities were recorded at San Blas 2, La Serranita 1 and Punta Rubia 2 sites in San Blas Bay as well Saenz cave in the Pali Aike area. They show similar directions observed in nearby sites from southern Patagonia in Argentina and Chile. In fact, previous results obtained at Mylodon, Cueva del Medio, Don Ariel and Las Buitreras caves yielded remanence directions corresponding to obliquely normal, obliquely reversed and reversed field polarity directions with similar VGPs. This situation strengthens the hypothesis of the existence of the Mylodon excursion in southern Patagonia. On the other hand, the presence of intermediate and reverse VGPs in San Blas records and other sites also supports its regional extent in the southern cone of South America. If the presented paleomagnetic features are true GMF behavior, the remarkably PSV record can serve to correlate regional stratigraphies, and to determine relative and absolute chronologies. Besides, if the anomalous directions represent excursions, they may be also used as dating devices, becoming excellent magnetostratigraphic markers for the time-span covered by the paleomagnetic record of the sites presented in this paper.

Highlights

Shared by the Republics of Argentina and Chile, Patagonia is a large geographic region of ~1,000,000 m2 located in the southern end of South America
Besides normal and intermediate polarity directions far from the present geomagnetic field (GF), most of the sections reported here shows reversed polarities. They were recorded at SB2, LS1 and PR2 sites in San Blas Bay as well Saenz cave in the Pali Aike area, showing similar directions observed in nearby sites from southern Patagonia in Argentina and Chile
There, clear reverse samples were dated during the middle Holocene at ~5.5 ky BP (Figure 17(d), Figure 17(e)); nearby, Cueva del Medio yielded a record from a section dated at ~10.0 ky BP with samples that recorded two magnetic components with normal and intermediate directions (Figure 18) indicating the presence of an anomalous GF behaviour after deposition (Nami & Sinito, 1995)

Summary

Introduction

Shared by the Republics of Argentina and Chile, Patagonia is a large geographic region of ~1,000,000 m2 located in the southern end of South America. The Chilean part is located south of Bío Bío River and comprises the southern provinces and regions of Aisén and Magallanes, including the west of Tierra del Fuego and Cape Horn, and Palena Province in Los Lagos Region. This region was subjected to a significant number of Cenozoic palaeomagnetic investigations (Ré et al, 2008). Detailed studies performed in archaeological and paleontological sites in Patagonia have yielded a number of results with stable and unstable records that showed intermediate and reverse geomagnetic field (GF) directions during the last ~11/10 ky, suggesting that GF excursion(s) occurred at different times through the Holocene (Nami, 1995a, 1999a, 2011, 2012; Nami & Sinito, 1991, 1993, 1995; Nami et al, 2017; Sinito et al, 2001). The identification and definition of geomagnetic excursions have several re-

Results

Discussion

Conclusion