MID-HOLOCENE TEPHRA ON TIERRA DEL FUEGO (54°5) DERIVED FROM THE HUD50N VOLCANO (46°5): EVIDENCE FOR A LARGE EXPL051VE ERUPTION CHARLES R. STERN Department 01 Geological Science, University 01 Colorado, Boulder, Colorado, USA 80309 ABSTRAeT The Hudson volcano (46°8) is identified, on the basis of chemical data, as the source of a characteristically green-brown tephra within Holocene deposits on Tierra del Fuego. The 10-15 cm thickness of these ash layers, found over 900 km from their source volean o, suggest an explosive eruption of significant magnitude. This eruption has been dated as between 6,625 and 6,930 years B.P. Younger Hudson-derived tephra in Aisén, Chile, and Santa Cruz, Argentina, indicate another explosive eruption of this volcano occurred shortly after 4,830 years B.P. Key words: Tephra, Volcanism, Tierra del Fuego, Hudson volcano, Chile. RESUMEN El volcán Hudson (46°8) se identificó, sobre la base de información química, como la fuente de una tefra, de típico color verde-pardusco, intercalada en los depósitos del Holoceno en Tierra del Fuego. Estos niveles de ceniza, con espesores de 10-15 cm, situados a distancias mayores que 900 km de su fuente volcánica, sugieren una erupción explosiva de gran magnitud, datada entre 6.625 y 6.930 años A.P. Tefras más jóvenes en Aisén, Chile, y 8anta Cruz, Argentina, indican otra erupción explosiva del volcán Hudson poco después de 4.830 años A.P. Pelabres claves: Tetra, Volcanismo, Tierra del Fuego, Volcán Hudson, Chile. INTRODUCTION Auer (1974 and references therein) described 5- (AVZ ; Fig. 1) becauseof its relatively low8i02 and high 40 cm thick layers of distinctively green-brown tephra KP, as well as high FeO and Ti02(8tern, 1990). The from Holocene bog deposits on Tierra del Fuego , relatively high FeO and Ti02 compared to the white (Figs. 1, 2). Based on their stratigraphic position tephra of Tierra del Fuego (Table 1), is presumably above an earlierwhite tephra layer (Fig. 2), hetermed what imparts the characteristic darker green-brown this green-brown ash Tephra 11. color to this volcanic ash. Salmi (1941) determined a high-KP andesite Auer was mora interested in Patagonian tephra composition (8i02 = 59.4 and KP = 2.5 wt %; Table as Holocena stratigraphic marker horizons than he 1) for this green-brown tephra; both more mafic and was in their source volcanos, and he did not determi­ Kp-rich than white tephra from Tierra del Fuego. ne isopach maps fortephra. He attributed the green­ Based on Salmi's chemical analysis, I previously brown Tephra II of Tierra del Fuego to the aruption of concluded that the green-brown tephra of Tierra del "some young, unknown volcano" between 4,500 and Fuego could not be derived from any of the volcanos 5,OOOyears B.P. (Auer, p. 7-11,1974). Based onlyon that comprise the Andean Austral Volcanic Zone new chemical analysis of this tephra (Tables 1, 2), Revis/a Geológica de Chile. Vol. 18. No. 2. p. 139· 146, 7 Figs., 3 /ables. /991 . 140 MID-HOLOCENE TEPHRA ON TIERRA DEL FUEGO, DERIVED FROM THE HUDSON VOLCANO FIG. 1. Map showing the volcanic centers 01 the Austral Volcanic Zone (AVZ) and southernmost Southern VolcanicZone (SVZ) olthe southern Andes, including the recently documented Viedma Vol cano in the AVZ(R. Kilian, personalcommun., 1990), aswell as the inlerred distribution 01 tephra lor some 01 the major eruptions 01 these volcanos (dashed lines, approximate ages indicated in years B.P.; Stern, 1990 and unpublished data). The inlerred distribution 01 tephra Irom the 6,625 to 6,930 years B.P. (6,777) eruption 01 the Hudson volcano is indicated by the solid line. Locations 01 green-brown tephra described in this manuscriptare indicated by numbers enclosed in squares (04, 12, 14,22,23 lor samples listed in Table 2). Locations 01 tephra Irom Aisén, Chile, and Santa Cruz, Argentina, are indicated by numbers enclosed in circles (16 lar RI-16, 22 lor RI-22, and 3 lor Cueva 3 de los Toldos, Table 2). Locations 01 green-brown Tephra 11 as described by Auer (1974 and relerences therein) are indicated by small black . dots. - ~ FIG. 2. Photograph 01 green-brown tephra layerwithin a Holocene deposit cut by a stream south 01 Punta Delgada (site 23 , Fig. 1), Tierra del Fuego. The 15 cm thick green-brown tephra (arrow) overlies a 4 cm thick white tephra layer derived lrom ari eruption 01 Mount Burney (Stern, unpublished data), and underlies a white caliche-rich soillayer. C.R. Stern 141 TABLE 1. MAJOR ELEMENT COMPOSITIONS (VOLATILE-FREE AND NORMALlZED TO 100%) OF GREEN­ BROWN TEPHRA FROM SOUTHERNMOST PATAGONIA AND TIERRA DEL FUEGO, COMPARED TO WHITE TEPHRA DERIVED FROM HOLOCENE ERUPTIONS OF THE AGUILERA, RECLUS, AND MT BURNEY VOLCANOS OF THE AUSTRAL ANDES (STERN, 1990) Tephra Green-brown White Sample 1 2 3 4 5 6 Si02 59.36 63.04 62.58 66.37 70.27 73.00 Ti02 1.79 1.23 1.24 0.37 0.40 0.42 . AIP3 20.97 16.71 16.79 17.20 17.05 15,04 FeO· 5.12 4.66 4.55 3.14 3.04 1.75 MnO 0.11 - - 0.05 - 0.04 MgO 1.59 1.64 1.76 2.07 1.19 0.45 CaO 3.93 3.98 4.38 4.64 2.53 3.10 Nap 4.34 5.89 5.78 3.91 2.94 4.15 !<p 2.51 2.59 2.56 2.22 2.17 1.36 P 0 0.28 0.29 0.36 0.37 0.66 2 5 - No. 1: Green·brown Tephra 11 Irom southernmost Patagonia (No. 15; Salmi. 1941); No. 2: Sample 90-12 Irom Rio Rusphen 30 km south 01 Cameren. Chile (Location 12; Fig. 1); No. 3: Sarrple 90·23G Irom roadside river cut between Bahla Inútil and Punta Delgada (Location 23; Fig. 1); No. 4: Tephra 11 Irom shores 01 Lago Argentino (No. t3; Salmi. 1941) correlated w~h a late Holocene eruption 01 the Aguilera volcano (Slern. 1990) ; No. 5: Tephra I Irom Tierra del Fuego (No. 19; Salmi. 194t) correlated w~h a late glacial eruption ofthe Reclus volcano (Stern. 1990); No. 6: Tephra IlIlrom Tierra del Fuego (No.17; Salmi. 1941) correlated w~h a late Holocene eruptlon 01 the Mt Burney volcano (Stern. 1990) . FeO-: total Fe as FeO. and without pretending to even begin the process of B.P .. from a large explosive eruption of the Hudson producing an isopach map of its distribution, Isuggest volcano, the southernmost volcanic center in the that the green-brown tephra of Tierra del Fuego was Andean Southern Volcanic Zone, more than 900 km derived, some time between 6,625 and 6,930 years to the north (Fig. 1). SAMPLE DESCRIPTION Samples from 10-15 cm thick layers of green­ cleaned tephra were sieved to separate the 60 to 120 brown tephra were collected in four stream cuts into mesh fraction which was effective in removing large Holocene bogs both north and southof Bahía Inútilon extraneous lithic fragments. Tierra del Fuego, and from a layer observed in ? Two samples of bulk tephra (glass + minerals) trench dug as a watering hole for animals just west of were analyzed for major elements (Table 1). The Laguna Blanco, north of Punta Arenas (Figs. 1, 2). results are similarto Salmi's (1941 ) analysis of green­ T ephra samples were cleaned initially by repeated brown tephra in their high KP and Ti02 at relatively immersion in water, ultrasonicvibration, and decanting low Si02 compared to samples of both lava and to remove as much clay and organic material as tephra derived from volcanic centers of the Austral possible. This was followed bytwenty minute baths in Volcaníc Zone (Fígs. 3, 4; Table 1; Stern, 1990). The first 2N HCI, to dissolve clay and carbonates, and major element composition ofthe green-brown tephra then a 5.25% solution of NaOel (household bleach) from southernmost Patagonia are, however, similar to dissolve remaining organic material (Steen­ to intermediate rocks eruptedfrom the Hudson volcano Mclntyre, 1977). These steps also removed the finer (Figs. 3, 4, Table 3), the southernmost in the Andean fraction of the tephra and may have dissolved certain SVZ (Fig . 1). mineral components such as apatite grains. The so Five samples of bulk tephra were analyzed for 142 MID-HOLOCENE TEPHRA ON TIERRA DEL FUEGO, DERIVED FROM THE HUDSON VOLCANO 3.0 40 AGUILERA 35 2 . • 3.0 •" " •" " 25 • !J " / 10 I 20 / o N I 1/ V ~ 1.5 1 :3;2\~ I AVZ 6el ~ I ",,7 I ..... I ..... o 50 70 1. I ..... --- MT BURNEY I / ..... \~,~ ..... -­ a5 COOK ISLAND FIG. 4. Ti0 versus Si0 diagram, iIIustrating lields lorwhole­ 2 2 50 55 65 70 75 rock, tephra, and glass separates Irom the volcanos 01 the AVZ (Futa and Stern, 1988; Harambour, 1990;R. Kilian,personalcommun., 1990),compared to whole-rock samples Irom the Hudson volcano (solid triangles; Table 3). Green-brown tephra from FIG. 3. KP versusSi0 diagram, iIIustrating lields lorwhole­ 2 Tierra del Fuego (solid cirqles lor analyses No. 2 rock, tephra, and glass separates lor the volcanos and No. 3, and circled cross lor analysis No. 1, 01 the AVZ (Cook Island, Mount Burney, Aguilera Table 1) plot in the field 01 the Hudson volcano. and Lautaro lrom Futa and Stern, 1988; Stern, White tephra from southernmost Patagonia (circled 1990: Reclus Irom Harambour, 1989; Stern, 1990; crosses 4-6 lor analysis Nos. 4-6, Table 1) plot in Viedma Irom R. Kilian, personal commun.,1990), the field 01 the AVZ volcanos.