Enigmatic boulder trains, supraglacial rock avalanches, and the origin of “Darwin's boulders,” Tierra del Fuego

Abstract

Charles Darwin considered himself to be a geologist and published extensively on many geologic phenomena. He was intrigued with the distribution of erratic boulders and speculated upon their origins. In his accounts of the voyage of the HMS Beagle, Darwin described crystalline boulders of notable size and abundance near Bahía San Sebastian, south of the Strait of Magellan, Tierra del Fuego. Influenced by Charles Lyell's reflections upon slow, vertical movements of crust, submergence, and ice rafting to explain drift, Darwin proposed that the boulders of Bahía San Sebastian were ice-rafted. Benefiting from 170 years of subsequent study of the glacial history of Tierra del Fuego, petrography, and terrestrial cosmogenic nuclide measurements, we revisit the origin of "Darwin's Boulders" at Bahía San Sebastian. We suggest that they, as well as another train of boulders to the west, at Bahía Inútil, represent rock falls of Beagle-type granite from the Cordillera Darwin onto glacial ice flowing into the Bahía Inútil-Bahía San Sebastian lobe. These supraglacial rock avalanche deposits were subsequently elongated into boulder trains by glacial strain during transport and then deposited upon moraines. The cosmogenic nuclide exposure dates support the correlation of Andean glaciations with the marine oxygen isotope record and the glacial chronologies recently proposed for Tierra del Fuego.

Retreat of the Cordillera Darwin icefield during Termination I

“…Northern ice flow from Cordillera Darwin is characterized best along a flowline through Canal Whiteside to Bahia Inútil, where numerous, low-relief moraines enclose the eastern end of the bay (Caldenius, 1932;Meglioli, 1992;Clapperton et al, 1995;Bentley et al, 2005;McCulloch et al, 2005aMcCulloch et al, , 2005bEvenson et al, 2009;Darvill et al, 2014). The moraines can be traced southeastward across Isla Grande, where they are thought to correlate with those enclosing the eastern end of Lago Fagnano (Coronato et al, 2009).…”

“…2A). Known as Darwin's Boulders (Evenson et al, 2009), these erratics mark a former ice flowline from central Cordillera Darwin that, based on lithology, must have originated at the head of Marinelli and/or Brooks fjords (Fig. 1B).…”

The timing and cause of glacial advances in the southern mid-latitudes during the last glacial cycle based on a synthesis of exposure ages from Patagonia and New Zealand

“…(Kaplan et al, 2004Douglass et al, 2005Douglass et al, , 2006Glasser et al, 2012) Río Bayo valley -47 -73 3 (3) (Glasser et al, 2006) Nef valley -47 -73 6 (5) (Glasser et al, 2012) Pueyrredón -47/-48 -71/-73 19 (16) (Hein et al, , 2010(Hein et al, , 2011Glasser et al, 2012) San Martín valley -49 -72/-73 10 (10) (Glasser et al, 2011) Río Guanaco -50 -73 21 (21) (Murray et al, 2012) Lago Argentino -50 -73 30 (27) (Ackert et al, 2008;Kaplan et al, 2011) Torres del Paine -51 -73/-74 54 (45) (Fogwill, 2003;Moreno et al, 2009;García et al, 2012) Río Gallegos -51/-52 -71/-72 7 (6) (Kaplan et al, 2007;Evenson et al, 2009;Sagredo et al, 2011) Magellan -52/-53 -69/-71 17 (10) Kaplan et al, 2008Kaplan et al, , 2007 BI-SSb -53/-54 -68/-70 46 (34) Kaplan et al, 2007Kaplan et al, , 2008Evenson et al, 2009;Darvill et al, 2015) Total within Last Glacial Cycle 289 (241)…”

A glacial geomorphological map of the southernmost ice lobes of Patagonia: the Bahía Inútil – San Sebastián, Magellan, Otway, Skyring and Río Gallegos lobes

“…However, studies focussing on the intervening glaciations have proven problematic (Evenson et al, 2009;Kaplan et al, 2007), likely due to the exhumation and erosion of erratic boulders used for cosmogenic dating. Future investigations of these processes will be informed by a better understanding of the glacial history of the region, based on geomorphological mapping rather than solely drift characterisation.…”