Tephrochronology of the upper Río Cisnes valley (44 S), southern Chile - PDF

Description
Andean Geology 42 (2): May, 2015 doi: /andgeoV42n2-a02 Andean Geology Tephrochronology of the upper Río Cisnes valley (44 S), southern Chile Charles R. Stern 1, María

Please download to get full document.

View again

of 17
All materials on our website are shared by users. If you have any questions about copyright issues, please report us to resolve them. We are always happy to assist you.
Information
Category:

Abstract

Publish on:

Views: 7 | Pages: 17

Extension: PDF | Download: 0

Share
Transcript
Andean Geology 42 (2): May, 2015 doi: /andgeoV42n2-a02 Andean Geology Tephrochronology of the upper Río Cisnes valley (44 S), southern Chile Charles R. Stern 1, María Eugenia de Porras 2, Antonio Maldonado 2,3 1 Department of Geological Sciences, University of Colorado, Boulder, Colorado , USA. 2 Centro de Estudios Avanzados en Zonas Áridas (CEAZA), Universidad de La Serena, Raúl Bitran 1305, La Serena, Chile. 3 Departamento de Biología Marina, Universidad Católica del Norte, Larrondo 1281, Coquimbo, Chile. ABSTRACT. Based on their petrography and chemistry, 18 tephra analyzed from two lake and bog cores and one outcrop in the upper Río Cisnes valley are believed to have been derived from nine different eruptions of the Mentolat volcano, four of the Melimoyu volcano, and one from the Hudson volcano. Some of these tephra correlate chronologically and petrochemically with previously documented large eruptions of these volcanoes, including the Late-Glacial Ho eruption of Hudson (17,340 cal yrs BP), the mid-holocene MEN1 eruption of Mentolat (7,710 cal yrs BP), and the Late-Holocene MEL2 eruption of Melimoyu (1,680 cal yrs BP). A Melimoyu-derived tephra from the outcrop occurs in glacial-lacustrine sediments and is considered to pre-date the Last Glacial Maximum ( 19,670 cal yrs BP). The data suggest that none of the tephra were produced by explosive eruptions of the Maca, Cay and Yanteles volcanoes. Keywords: Tephra, Tephrochronology, Tephrostratigraphy, Volcanism, Andes, Chile. RESUMEN. Tefrocronología en curso superior del valle del río Cisne (44 S), Chile Austral. Dieciocho tefras provenientes de testigos de un lago y un mallín, junto a un perfil expuesto en el alto valle del río Cisnes fueron caracterizadas sobre la base de su petrografía y química y corresponderían a nueve diferentes erupciones del volcán Mentolat, cuatro del volcán Melimoyu y una del volcán Hudson. Algunas de estas tefras se correlacionan cronológica y petroquímicamente con grandes erupciones de estos volcanes previamente documentadas, incluyendo la erupción Ho del volcán Hudson (Tardiglacial, años cal. AP), la erupción MEN1 del volcán Mentolat (Holoceno medio, años cal. AP) y la erupción MEL2 del volcán Melimoyu (Holoceno tardío, años cal. AP). Una tefra perteneciente a la erupción del volcán Melimoyu, hallada en un perfil expuesto en un contexto de depósitos glaciolacustres, tiene una edad ( años cal. AP) que precede al término del Último Máximo Glacial en Patagonia Central. Los datos sugieren que ninguna de las tefras fueron producidas por erupciones explosivas de los volcanes Maca, Cay y Yanteles. Palabras clave: Tefra, Tefrocronología, Tefrostratigrafía, Volcanismo, Andes, Chile. 174 Tephrochronology of the upper Río Cisnes valley (44 S), southern Chile 1. Introduction Tephra layers in sediment cores from lakes and bogs provide information on the history of explosive volcanic eruptions from nearby volcanoes, and thus a basis for evaluating the possibilities for and potential effects of future eruptions. The identification, petrochemical description and correlation of synchronous volcanic tephra layers over large geographic areas and in different environmental settings also provides a stratigraphic correlation tool for a broad range of disciplines (Lowe, 2011; Fontijn et al., 2014), including archaeology (Prieto et al., 2013), palaeoclimatology and palaeogeomorphology (García et al., 2015, this volume). Here we characterize both petrochemically and chronologically multiple Late-Glacial and Holocene tephra layers in two sediment cores from the area of the upper Río Cisnes valley (Figs. 1 and 2); one core from Lago Shaman (Fig. 3; de Porras et al., 2012), and one core from Mallín El Embudo (Fig. 4; de Porras et al., 2014). With this information we attempt to correlate tephra layers between the cores and to other previously described tephra in the region, and identify for each layer their possible source volcano, which potentially include the Yanteles, Melimoyu, Mentolat, Maca, Cay and Hudson stratovolcanoes, as well as numerous small monogenetic cones located between Puyuhuapi and Palena (Fig. 1). We also describe one sample of tephra that outcrops in glacial-lacustrine sediments, and pre-dates the Last Glacial Maximum. 2. Background Bedrock geology in the upper Río Cisnes valley consists of plutons of the Patagonia batholith and Lower FIG. 1. Regional map showing the location, in the box, of the area of Alto Río Cisnes (Fig. 2) from which tephra samples were studied, and both stratovolcanoes (larger triangles) and the Puyuhuapi and Palena groups of minor monogenetic mafic eruptive cones (MEC: smaller solid triangles) in the southern SSVZ (Stern, 2004; Stern et al., 2007). Stern et al. / Andean Geology 42 (2): , FIG. 2. Map of the location of the Lago Shaman and Mallín El Embudo cores in relation to the LGM moraine complexes along the Chile-Argentina border and the frontal moraine formed during a Late-Glacial glacial advance in the upper Río Cisnes valley (de Porras et al., 2012, 2014). Also shown is the location at Las Barrancas of the Late-Glacial glacial-lacustrine sediments containing the outcrop of the 19,670 BP tephra sample Cisnes 263A. Cretaceous sediments overlain by Quaternary deposits, which include the materials examined in this study. de Porras et al. (2012, 2014) describe in some detail the environmental setting of the two cores. Lago Shaman is located in the semi-arid forest-steppe ecotone just west of the Chile-Argentina border, which at this latitude corresponds to a moraine complex formed during the last Late-Glacial Maximum (LGM; Fig. 2). This area became ice free at or soon after 19,000 BP, and the deepest organic layer dated, from 599 cm depth in the 613 cm long Lago Shaman core (LS0604A; Fig. 3), yields an age of 18,950 cal yrs BP (Table 1). In contrast, the Mallín El Embudo core is located in a wetter forested area ~35 km to the southwest of Lago Shaman (Fig. 2), west of a small frontal moraine interpreted to have formed by a Late-Glacial glacial advance before approximately 13,000 BP (de Porras et al., 2014). The oldest age obtained, from 809 cm deep in this 844 cm long composite core (EE0110A and B; Fig 4) was 12,997 cal yrs BP. These two cores were collected with the purpose of providing a pollen record and its implications for the changing climate in this region since the end of the last glaciation (de Porras et al., 2012, 2014), as well as a charcoal record and its implication for the history of fires caused possibly by climate change, volcanic activity and/or the human occupation of the valley, which dates back to 11,500 BP (Méndez and Reyes, 2008; Méndez et al., 2009; Reyes et al., 2009). Both cores contain clastic layers which are in most cases tephra (Figs. 3 and 4). The Lago Shaman core contains more numerous tephra, which may possibly reflect the fact that Lago Shaman occurs in an open arid area with no vegetation to interfere with tephra fall and wind redistribution of tephra, while Mallín El Embudo occurs in a wetter environment with forest cover. One other tephra sample (Cisnes 263A; Fig. 5) was also collected from an outcrop of glacial-lacustrine sediment at Las Barrancas (Fig. 2). It occurs ~3 meters below the contact, dated as 176 Tephrochronology of the upper Río Cisnes valley (44 S), southern Chile FIG. 3. X-ray image of Lago Shaman core (de Porras et al., 2012). Bright white layers are either sand or tephra and darker areas are organic rich sediments. 19,670 cal yrs BP, between these and overlying fluvial sediments (Fig. 5). Previous tephrochronologic studies in this area of the southern Southern Volcanic Zone (SSVZ) of the Andes include those of Naranjo and Stern (1998, 2004), Mella et al. (2012) and Weller et al. (2014). These studies indicate that all the potential source volcanoes for the tephra in the upper Río Cisnes valley (Yanteles, Melimoyu, Maca, Cay and Husdon) have had Holocene explosive eruptions producing locally or regionally distributed tephra falls (Naranjo and Stern, 1998, 2004; Mella et al., 2012; Weller et al., Stern et al. / Andean Geology 42 (2): , FIG. 4. X-ray image of Mallín El Embudo core (de Porras et al., 2014). Bright white layers are either sand or tephra and darker areas are organic rich sediments. 2014). Melimoyu, Mentolat and Hudson have summit craters/calderas possibly formed in association with these events. Melimoyu and Hudson are two of the largest volcanic edifices in the SSVZ (Völker et al., 2012). Also the many small monogenetic cones in the region have produced basaltic scoria deposits as well as lava flows (López-Escobar et al., 1995a; Gutiérrez et al., 2005; Watt et al., 2013; Vargas et al., 2013), but the potential regional extent of distribution of tephra from these generally small volume mafic eruptions is uncertain The previously published interpretations of the source volcanoes of tephra in the SSVZ were made in part on the basis of tephra major element chemistry compared with published whole rock chemistry of samples of lavas from the volcanoes of the SSVZ (Naranjo and Stern, 2004; Weller et al., 2014). In a similar fashion, the possible sources of seven of the tephra in the Lago Shaman core were made on the basis of bulk tephra trace-element chemistry compared to published whole-rock trace-element analysis of lava samples from the SSVZ volcanoes to the west of the core site (de Porras et al., 2012). These trace-element data suggest Melimoyu, Mentolat and Hudson volcanoes as the sources for these seven tephra (de Porras et al., 2012; Weller et al., 2014). Since spatial coverage is still too restricted to allow for the construction of tephra isopach maps, which is the most conclusive way to identify source volcanoes for tephra, this paper also employs the geochemical approach of comparing tephra chemistry with the published data concerning the volcanic rocks associated with the different SSVZ centers (Fig. 6) to identify possible tephra source volcanoes of the tephra. Information concerning the chemistry of the magmas erupted from the volcanic centers in the SSVZ has been published by Stern et al. (1976), Futa and Stern (1988), López-Escobar et al. (1993, 1995a), D Orazio et al. (2003), Gutiérrez et al. (2005), Kratzmann et al. (2009, 2010), Watt et al. (2013) and Weller et al. (2014). Samples from SVZ volcanoes in south-central Chile, and specifically the SSVZ 178 Tephrochronology of the upper Río Cisnes valley (44 S), southern Chile TABLE 1. DEPTH IN CM OF TEPHRA AND 14 C AGE DATES IN CAL YRS BP FROM THE LAGO SHAMAN (DE PORRAS ET AL., 2012) AND MALLÍN EL EMBUDO (DE PORRAS ET AL., 2014) CORES. Lago Shaman tephra depth cm ~age* Source* eruption* age* a ,440 Melimoyu MEL2 1, ,827± b 94 2,140 Mentolat - - c 104 2,490 Mentolat ,111± d 160 3,720 Mentolat - - e 170 3,880 Mentolat ,275± f 207 4,610 Melimoyu - - g 255 8,280 Melimoyu ,357± h 261 8,400 Mentolat - - i 270 8,800 Mentolat MEN1 7, ,824± m ,140 Mentolat ,241± ,474± q ,665 Mentolat - - v ,820 Hudson Ho 17,340 y ,940 Mentolat MENo 17, ,951± Mallín El Embudo tephra depth cm ~age* source eruption* age* 25 94± ,453± ,743± a ,090 Melimoyu MEL2 1, ,492± b ,810 Melimoyu - - h 549 9,010 Mentolat MEN1 7, ± ± ,302± j ,450 Mentolat ,997± CIS 263-A - 19,670 Melimoyu - - *Measured 14 C age dates in cal yrs BP; ~ages for tephra interpolated from measured ages; possible sources based on tephra chemistry (Table 3); ages of previously documented eruptions from Naranjo and Stern (2004) and Weller et al. (2014). Stern et al. / Andean Geology 42 (2): , FIG. 5. A. Photo of the ~6 cm thick Cisnes 263A tephra in glacial-lacustrine sediment formed during the last glaciation; B. This tephra occurs ~3 meters below the contact, dated as 19,670 cal yrs BP, between the glacial-lacustrine clay and overlying fluvial sediments. volcanoes, fall into different and distinguishable chemical groups (Fig. 6). Two of these groups have previously been termed Type-1, or Low Abundance, and Type-2, or High Abundance magmas (Hickey et al., 1986, 1989, 2003; López-Escobar et al., 1993, 1995a, 1995b). These two different magma types are distinguished by their different concentrations of the incompatible elements K 2 O (Fig. 6a), Rb, Ti, Ba. Zr, Sr, Y, Nb and La, as well as La/Yb and Ba/ La ratios, over a large range of SiO 2 contents from basalts to dacites. In the SSVZ, Maca, Cay and Yanteles stratovolcanoes (Fig. 6), and the Palena group of monogenetic cones are Type-1 or Low Abundance volcanoes (López-Escobar et al., 1993, 1995a; D Orazio et al., 2003; Gutiérrez et al., 2005; Carel et al., 2011; Watt et al., 2013), while Hudson and Melimoyu volcanoes (Fig. 6) and the Puyuhaipi group of monogenetic cones are Type-2 or High 180 Tephrochronology of the upper Río Cisnes valley (44 S), southern Chile FIG. 6. A. SiO 2 versus K 2 O for samples of both lavas and tephra from Yanteles, Melimoyu, Maca, Cay and Hudson volcanoes (Futa and Stern, 1988; López-Escobar et al., 1993; Naranjo and Stern, 1998, 2004; D Orazio et al., 2003; Gutiérrez et al., 2005; Kratzmann et al., 2009, 2010), illustrating the separation of these samples into what have previously been termed High, Low and Very Low Abundance magma types (Hickey et al., 1986, 1989, 2003; López-Escobar et al., 1993, 1995a, 1995b; Sellés et al., 2004; Watt et al., 2011). Line separating the fields of High-, Medium-, and Low-K convergent plate boundary magmas are from Peccerillo and Taylor (1976); B. Ti versus Rb for the SSVZ volcanoes and each individual tephra from the upper Río Cisnes valley (Table 3); C. Sr versus Ba for the SSVZ volcanoes and tephra from the upper Río Cisnes valley (Table 3). Stern et al. / Andean Geology 42 (2): , TABLE 2. MAIN PETROGRAPHIC FEATURES OF THE TEPHRA FROM THE UPPER RÍO CISNES VALLEY. tephra components Glass color Vesicles Microlites a Gl Plag Cpx Opx Brown Few, round Plag b Plag Gl Opx Cpx Amph Ol Clear Abund, round - c Plag Gl Opx Cpx Amph Ol Clear Abund, round - d Plag Gl Opx Cpx Amph Ol Clear Abund, round - e Plag Gl Opx Cpx Amph Ol Clear Abund, round - f Gl Plag Cpx Opx Ol Brown Few, round Plag g Gl Plag Cpx Opx Brown Few, round Plag h Plag Gl Opx Cpx Amph Ol Clear Abund, round - i Plag Gl Opx Cpx Amph Ol Clear Abund, round - m Plag Gl Opx Cpx Amph Ol Clear Abund, round - q Plag Gl Opx Cpx Amph Ol Clear Abund, round - v Gl Plag Opx Brown to Tan Abund, stretched - y Plag Gl Opx Cpx Amph Ol Clear Abund, round - Mallín El Embudo tephra components Glass color Vesicles Microlites a Gl Plag Cpx Opx brown Few, round Plag b Gl Plag Cpx Opx Ol brown Few, round Plag h Plag Gl Opx Cpx Amph Clear Abund, round - j Plag Gl OPx Cpx Amph Clear Abund, round - CIS 263-A Gl Plag Cpx Opx brown Few, round Plag Gl: glass; Plag: plagioclase; Cpx: clinopyroxene; Opx: orthopyroxene; Amph: amphibole; Ol: olivine; Abund: abundant. Abundance centers (López-Escobar et al., 1993, 1995a; Naranjo and Stern, 1998, 2004; Kratzmann et al., 2009, 2010; Carel et al., 2011). The Palena and Puyuhuapi group basalts are not plotted in figure 6 because they both contain abundant olivine and lack orthopyroxene and amphibole, and are therefore petrologically distinct from the tephra in the upper Río Cisnes valley described below (Table 2). Although samples from different Type-1 Low Abundance volcanoes are generally similar to each other, a specific exception in this southern part of the SSVZ is the Mentolat volcano, which at any given SiO 2 content has lower K 2 O (Fig. 6a; López-Escobar et al., 1993; Naranjo and Stern, 2004; Watt et al., 2011), Rb, Ti (Fig. 6b), Sr, Ba (Fig. 6c) and La/Yb (Watt et al., 2011), similar to other unusually or Very Low Abundance samples from Nevado de Longaví (Sellés et al., 2004), Calbuco (López-Escobar et al., 1995b) and Huequi (Watt et al., 2011) volcanoes further to the north. Like Mentolat, all these other Very Low Abundance centers are characterized by the presence of amphibole in their eruptive products (López-Escobar et al., 1993, 1995b; Sellés et al., 2004; Watt et al., 2011). 3. Methods X-ray images of the cores (Figs. 3 and 4) were taken to allow for better visual identification of the tephra deposits and to provide a means of stratigraphic correlation of the tephra layers between the cores. The white layers in these images, arbitrarily termed a though z in the Lago Shaman core (Fig. 3) and a, b, g, h and j in the Mallín El Embudo core (Fig. 4), are the denser lithologies, often tephra deposits, but in some cases sand, and the darker layers are less dense organic-rich lacustrine sediments. The chronology of the tephra in the trenches and cores is constrained by AMS radiocarbon dates of organic material in the overlying and underlying sediments 182 Tephrochronology of the upper Río Cisnes valley (44 S), southern Chile (Fig. 7; Table 1; de Porras et al., 2012, 2014). Radiocarbon dates were converted to calendar years before present (cal yrs BP) using the CALIB 5.01 program (Stuiver et al., 1998). The tephra samples were washed to remove any organic matter, and then dried and sieved to remove any coarse fraction material not volcanic in origin. After cleaning, the bulk tephra samples were mounted on petrographic slides and examined under a petrographic microscope in order to identify petrographic characteristics such as tephra glass color and morphology (Fig. 8) and the proportion and identity of mineral phases (Table 2). Trace-element data for bulk tephra samples were determined using an ELAN D CR ICP-MS (Table 3; Saadat and Stern, 2011). Trace-element compositions are considered accurate to ±5% at the level of concentrations in these samples, based on repeated analysis of standard rock samples of known composition (Saadat and Stern, 2011). 4. Results A summary of some of the most obvious petrographic features of each of 13 tephra samples from the Lago Shaman core, four from the Mallín El Embudo core, and one other outcrop sample (Cisnes 263A), are presented in Table 2 and tephra trace-element chemistry are presented in Table 3. The chemical and petrologic characteristics of each tephra, and the reasons for suggesting a possible source volcano, are discussed below in chronological order from the youngest to the oldest Tephra a from both cores The youngest tephra in both cores, tephra a (Figs. 3 and 4; Tables 1-3), is approximately 6 cm thick in each core and in both consists dominantly of identical appearing brown glass with a few round and only rarely stretched vesicles and containing FIG. 7. Age versus depth in the core of the different tephra analyzed from the Lago Shaman and Mallín El Embudo cores. Also shown are the ages for tephra MEL2 from Melimoyu (Naranjo and Stern, 2004) which may correlate with tephra a in both cores, MEN1 from Mentolat (Naranjo and Stern, 2004; Stern et al., 2013) which may correlate with tephras i in Lago Shaman and h in Mallín El Embudo, and Ho from Hudson which correlates with tephra v in Lago Shaman (Weller et al., 2014). Stern et al. / Andean Geology 42 (2): , FIG. 8. Photomicrographs (2.2x2 mm) showing glass color and morphology and proportion relative to phenocrysts of: A. Tephra a from Lago Shaman, containing abundant dark brown glass with only a few round vesicles and occasional plagioclase microlites (Table 2), suggested to be derived from Melimoyu volcano (Table 1); B. Tephra c from Lago Shaman, containing clear glass with round vesicles, and abundant plagioclase and orthopyroxene phenocrysts (Table 2), suggested to be derived from Mentolat volcano; C. Tephra v from Lago Shaman, with abundant brown to tan glass containing stretched vesicles, correlated with tephra derived from the Late-Glacial Ho eruption of the Hudson volcano (Weller et al., 2014); and D. Tephra Cisnes 263A, with abundant dark brown glass containing only a few rounded vesicles (Table 2), suggested to be derived from the Melimoyu volcano. occasi
Related Search
Similar documents
View more...
We Need Your Support
Thank you for visiting our website and your interest in our free products and services. We are nonprofit website to share and download documents. To the running of this website, we need your help to support us.

Thanks to everyone for your continued support.

No, Thanks