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Development of unusual rock weathering features in the Cordillera Blanca, Peru

Published online by Cambridge University Press:  20 January 2017

Donald T. Rodbell ,
Holli M. Frey ,
Matthew R.F. Manon ,
Jacqueline A. Smith  and
Nicholas A. McTurk
Donald T. Rodbell*
Affiliation:
Geology Department, Union College, Schenectady, NY 12308, USA
Holli M. Frey
Affiliation:
Geology Department, Union College, Schenectady, NY 12308, USA
Matthew R.F. Manon
Affiliation:
Geology Department, Union College, Schenectady, NY 12308, USA
Jacqueline A. Smith
Affiliation:
Department of Physical & Biological Sciences, The College of Saint Rose, Albany, NY 12203, USA
Nicholas A. McTurk
Affiliation:
Geology Department, Union College, Schenectady, NY 12308, USA
*
*Corresponding author. Fax: + 1 518 388 6417. E-mail address:rodbelld@union.edu (D.T. Rodbell).
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Abstract

Mylonite textures in granodiorite boulders are responsible for higher rates of surface denudation of host rocks and the progressive development of unusual rock weathering features, termed weathering posts. These textures are characterized by smaller grain sizes, higher biotite content, and a higher biotite axial ratio in host rocks relative to weathering posts. Elemental concentrations do not show a significant difference between weathering posts and the host rocks in which they are found, and this reflects the absence of a weathering residue on the rock surfaces. Chemical weathering loosens the bonds between mineral grains through the expansion of biotite, and the loosened grains fall off or are blown off the boulder surface and continue their chemical alteration in the surrounding soil. The height of weathering posts on late Quaternary moraines increases at a linear rate of ~ 1.45±0.45 cm (1000 yr)– 1 until post heights reach the diameter of host rocks. Such a rate of boulder denudation, if unrecognized, would generate significant errors (> 20%) in cosmogenic exposure ages for Pleistocene moraines. Given the paucity of boulders with diameters that significantly exceed 1.5 m, the maximum age of utility of weathering posts as a numeric age indicator is ~ 100 ka.

Keywords

Rock weatheringMoraine datingExposure agesTropical glaciation
Type
Original Articles
Information
Quaternary Research , Volume 77 , Issue 1 , January 2012 , pp. 149 - 158
Copyright
University of Washington

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