Abstract

The geomorphology and stratigraphy of massive debris flows on the Eastern Andean Cordillera, Colombia, indicate two distinct deposits can be recognized. The lower Chinauta deposit covers 14 km2 and has a thickness of ∼60 m, whereas the upper Fusagasugá deposit covers 20 km2 and has a thickness of ∼20 m. The lower Chinauta section consists of matrix-supported gravels, with isolated boulders and massive to moderately bedded structure and local inverse grading. The upper section displays sequences of inversely graded, clast-supported gravels, with boulders >2 m in axial length, capped by massive, matrix-supported fine gravels. The latter are dissected by coarse, channelized gravels. We interpret these facies as a series of debris and hyper-concentrated flows dissected by river channels. The Fusagasugá deposit is dominated by massive to inversely graded matrix-supported gravels with isolated boulders. Single-grain, optically stimulated luminescence dates of the sandy–silty matrix of debris and hyper-concentrated flows constrain the timing of deposition of the Chinauta debris flow deposits between 38.9 and 8.7 ka. We postulate that millennial-scale climate variability is responsible for causing these massive debris flows, through a combination of elevated temperatures and increased rainfall that triggered runoff and sediment transport.

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