Volcaniclastic lacustrine sedimentation in the Pleistocene Guayllabamba intermontane basin in the Ecuadorian Andes

In this work, we present a description of the sedimentary fill of a well-exposed lacustrine succession in the Ecuadorian Andes. The Guayllabamba basin is an intermontane basin located in the Andean range of Ecuador, and part of its sedimentary history is represented by a volcanically influenced c. 100 m-thick lacustrine unit of Pleistocene age. We create a stratigraphic cross-section from the eastern to western lake margins and identify 19 facies that were used to carry out a palaeoenvironmental reconstruction. The Guayllabamba palaeolake was developed in a tectonic depression surrounded by volcanoes and it was filled with sediments derived from the erosion of the volcanic edifices, the reworking of unconsolidated pyroclastic deposits and deposition of pyroclastic currents into the lake. The lake shows a deepening trend, passing from shallow deltaic sedimentation to varved diatomites with turbidites. Abundant ash-fall beds, monolithological pumiceous deltaic sequences and pumice-dominated thick ignimbrites show the impacts of volcanism on lacustrine sedimentation within this basin. Soft-sediment deformation and gravity flow deposits are common owing to the intrabasinal tectonic activity and to the intrusion of a lava body. Aulacoseira-rich diatomites dominate the background lake sedimentation. The outcrops of the Guayllabamba basin are outstanding examples of the interaction between volcaniclastic and lacustrine sedimentation.