DEVELOPMENT OF UNUSUAL ROCK WEATHERING FEATURES ON GLACIAL DEPOSITS IN THE CORDILLERA BLANCA, PERU

Unusual rock weathering features, termed weathering posts, on granodiorite boulders in the Cordillera Blanca Peru were first recognized as a potential relative dating indicator for glacial deposits in the 1970s. A visit by Peter Birkeland in 1986 prompted this study: to determine the rate of weathering post development and the origin of the differential weathering rates that lead to post development in what appears to be homogeneous granodiorite. Major element and trace element concentrations do not show a significant or systematic difference between weathering posts and the host rocks on which they are found. However, thin sections reveal that host rocks surrounding weathering posts have veins of micro-crystalline quartz surrounded by ribbons of elongated biotite that flow around feldspar phenocrysts. In contrast, thin sections from weathering posts, feature larger, equant, interlocking grains. Thus, we attribute the development of weathering posts to grain morphology, texture and size. In the host rock, chemical weathering may loosen the bonds between mineral grains, possibly through the expansion of biotite, leading to enhanced erosion and wind removal of loosened grains. The larger grain size and lack of elongated biotite in the weathering posts may slow the migration of fluids and subsequent matrix breakdown. The height of weathering posts on late Quaternary moraines dated by radiocarbon and/or cosmogenic ¹⁰Be increases following the logarithmic equation Y=13.852*lnX-12.420; r²=0.88; n=14, where Y is weathering post height (cm) and X is time (ka). Though the rate of post development appears to slow with time, the absence of any geochemical trace of incongruent weathering implies that the linear rate of post development of ~1.1 cm per 1000 yr that we calculated for moraines deposited within the past ~30 ka may continue until post heights reach the diameter of the host rock. Given the paucity of boulders with diameters that significantly exceed 1 m, the maximum age of utility of weathering posts as a numeric age indicator is ~100 ka.