Darwin, o 'geólogo', errou feio em Geologia

GSA TODAY
Volume 19 Issue 12 (December 2009)
Article, pp. 4-10

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

Edward B. Evenson1, Patrick A. Burkhart2,*, John C. Gosse3, Gregory S. Baker4, Dan Jackofsky5, Andres Meglioli6, Ian Dalziel7, Stefan Kraus8, Richard B. Alley9, Claudio Berti10

1 Dept. of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA

2 Dept. of Geography, Geology, and the Environment, Slippery Rock University, Slippery Rock, Pennsylvania 16057, USA

3 Dept. of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada

4 Dept. of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA

5 Esso Australia, Victoria 3006, Australia;

6 Environmental Resources Management, 6455 South Yosemite Street, Suite 900, Greenwood Village, Colorado 80111, USA;

7 Institute for Geophysics, Jackson School of Geosciences, University of Texas, Austin, Texas 78713, USA;

8 Instituto Antarctico Chileno, Punta Arenas, Chile;

9 Dept. of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA;

10 Dept. of Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA

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.

Received 9 August 2009; accepted 9 September 2009.
doi: 10.1130/GSATG72A.1
*E-mail: patrick.burkhart@sru.edu

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