Stratigraphy, age and correlation of Lepué Tephra: a widespread c. 11 000 cal a BP marker horizon sourced from the Chaitén Sector of southern Chile

Abstract

We describe the stratigraphy, age and correlation of a prominent tephra marker, named Lepué Tephra, extensively distributed in north‐western Patagonia. Lepué Tephra is well dated at c. 11 000 cal a BP from numerous lake and soil cover‐bed sequences and its recognition is useful for assessing the rate and timing of deglaciation as well as associated environmental changes in this region during the last glacial termination and early Holocene. Lepué Tephra has attributes typical of a complex and compositionally zoned phreatomagmatic eruptive. While the initial rhyolitic phase can be readily distinguished from multiple eruptive products sourced from the adjacent Volcán Chaitén, the main erupted end member is of basaltic–andesitic bulk composition − similar to younger tephras sourced from Holocene monogenetic cones adjacent to the Volcán Michimahuida massif (tMim). Lepué Tephra can be correlated to an equivalent‐aged pyroclastic flow deposit (Amarillo Ignimbrite) prominently distributed in the south‐eastern sector of tMim. The source vent for these co‐eruptive events is obscured by an extensive ice field and is currently unknown. The widespread radially symmetrical distribution of Lepué Tephra centred on tMim cannot be attributed solely to volcanological considerations. Reduced Southern Hemisphere westerly wind influence interpreted from climate proxies at the time of eruption are also implicated. Copyright © 2017 John Wiley & Sons, Ltd.

Correlating weathered, microphenocryst-rich, intermediate tephra: An approach combining bulk and single shard analyses from the Lepué Tephra, Chile and Argentina

“…This similarity with its enclosing sediments allowed its distal extent to remain unrecognised in previous studies in the area (Naranjo and Stern, 2004;Watt et al, 2011). Further details of the stratigraphic context and variations within the Lepué Tephra, along with details of all localities sampled for analysis in this study are given by Alloway et al (2017a), which should be consulted for details of the eruption and deposition history, and major element chemical variation of the deposit, which are not reiterated here.…”

“…The Lepué Tephra is well constrained at c. 11000 cal a BP from 14 C dates from lake and soil cover-bed sequences in the area (Alloway et al, 2017a), where it occurs as a deposit ranging from less than a centimetre in thickness to the north of Lago Llanquihue, a few centimetres to a decimetre thick on Isla Grande de Chiloé or in the vicinity of Puerto Montt, and up to (and more than) ~ m in thickness near the town of Chaitén to the immediate WSW of Volcán Michinmahuida. Lake cores from across the region record between 2 cm to 28 cm of Lepué Tephra, and ODP core 1233D (Leg 202) (Tiedemann et al, 2007) contains 12 cm of the tephra, ~260 km NW of Volcán Michinmahuida.…”

“…The combination of these textural features and variable sorting of the deposit suggested to Alloway et al (2017a) that variable interaction with significant quantities of water during the eruption (Zimanowski, 2001) resulted in a complex intermixing of magmatic and phreatomagmatic eruptive components (Cas and Wright, 1987), with the water here largely derived from melting of the overlying ice cap. In places close to the source, the typical, massive accretionary lapilli-tuff overlies a basal rhyolitic ash and a surge unit containing moderately sorted, inclined planar to low-angle cross-bedded, scoriaceous ash and lapilli beds (not illustrated here, but see Alloway et al, 2017a). At intermediate distances (between ~30 km to 60 km from Volcán Michinmahuida), the Lepué Tephra is between ~30 cm to 100 cm in thickness and typically is characterized by a decimetre-thick weakly stratified, brownish grey, very poorly sorted cemented ash with indistinct centimetre-sized accretionary lapilli and scoriaceous lapilli-rich ashy intra-beds ( Figures 2B and 2C).…”

“…The Lepué Tephra, a prominent marker horizon described in detail by Alloway et al (2017a), is extensively distributed across the Los Lagos Region of Chile, and straddles the Chile-Argentina frontier in northwest Patagonia (Figure 1). The Lepué Tephra can be correlated to an equivalentaged >40 m thick pyroclastic flow deposit (Amarillo Ignimbrite) which is well exposed on the southeastern flanks of Michinmahuida.…”

Late Quaternary records and chronology of environmental change in the Southern Hemisphere – a contribution to SHAPE

“…The same is true for Patagonia, and illustrated exquisitely by Alloway et al . () in their detailed investigation of the ca . 11 cal ka Lepué Tephra, covering its genesis, geochemistry, age and stratigraphic relationships.…”