SEDIMENTATION IN A TROPICAL SUPRAGLACIAL LAKE, CORDILLERA BLANCA, PERÚ

Tropical glaciers of the Cordillera Blanca, Perú are rapidly thinning and retreating as a result of climate warming. The retreat of these glaciers along narrow linear bedrock valleys has increased the number and size of moraine-dammed glacial lakes formed in the valleys and this has in turn increased the risk of catastrophic releases of water as glacial lake outburst floods (GLOFs). These floods have the potential to destroy local communities, such as Huaraz. Current understanding of glacier dynamics and the processes operating to enlarge and subsequently infill glacial lakes in this region is rudimentary but is extremely important to local communities that need to prepare for future GLOFs. This study focuses on identifying patterns of sedimentation in an enlarging supraglacial lake formed on the margins of the Llaca Glacier in the Cordillera Blanca in attempt to better understand lake infilling processes and the past frequency of flooding events. The Llaca Glacier is an actively retreating glacier in the Cordillera Blanca and has recently experienced a significant loss of ice volume. Llaca Lake lies on the downvalley margin of the glacier and is a supraglacial moraine-dammed lake approximately 1 km long with an area of area of 65,513 m², containing approximately 495,477 m³ water.

Sediments deposited within the supraglacial lake are now exposed on upstanding ice-cored hummocks at several locations. Eight facies types have been identified including massive gravels (Gm), rippled, horizontally laminated, graded and deformed sands (Sr, Sh, Sg, Sd), and laminated and deformed fine grained sediment (Fl, Fd). These sediments were delivered to the lake by traction and density underflow currents generated by sub and supraglacial meltwater streams and valley side gullies; the abundance of soft sediment deformation structures indicates rapid deposition on unstable substrates. Interbedded gravel horizons record past flood or iceberg calving events; abrupt vertical changes in facies types may record lake level changes caused by the release of floodwaters. Documentation of facies types and depositional processes from Llaca aids in the understanding of present and past environmental conditions within deglaciated valleys elsewhere in the Cordillera Blanca and may be used better predict the frequency of flooding events.