NEW INSIGHTS ON THE STRATIGRAPHY, SEDIMENTOLOGY AND DEFORMATIONAL EVOLUTION OF AN ANDEAN INTERMONTANE QUATERNARY PALEOLAKE

We present new results about the stratigraphy, sedimentology and syn-sedimentary deformation of a Pleistocene lacustrine sequence developed within an intermontane basin in the core of northern Andes orogen. We carried out a detailed facies analysis of the ~100 m-thick lacustrine succession, known as the San Miguel Fm. The lower part of the lacustrine deposits is characterized by a regressive sequence formed by volcanoclastic conglomerates and sandy deltaic progradational sequences. These coarse-grained sediments pass gradually to a transgressive sequence constituted by massive claystones and tuffs, contoured bedding, gullies and thick sandy debris flows. The upper part is mainly characterized by millimetre rhythmic laminated sediments composed of light-grey claystone–siltstone laminates alternating with light diatomite laminates. These fine sediments are interbedded with mass-transport volcanoclastic sandstones beds. The San Miguel Fm. was deposited in a tectonically and volcanically active strike-slip basin associated to a crustal-scale N–S dextral restraining-bend megashear. This megashear controlled the orogenic uplift and volcanic activity of the sector during the last ~5.5 Ma. The tectonic evolution of the basin was locally driven by the active NW–SE Quito faults’ system located in the south-western part of the basin and linked to the N–S megashear. The tectonic evolution of the basin can be divided into two stages: 1) normal movements of these faults system, which promoted the deepening of the lake to south-westward and 2) inverse movements related to the end of the lake, as well as soft-sediment deformation characterized by NE-verging folds, shallow thrusts and abundant sandy dikes and sills intrusion. The interandean San Miguel Fm. allows us to relate the tectonic evolution of the N–S trend strike-slip faults systems to the last Andean uplift pulse. Moreover, the volcanic, volcanoclastic and fine laminated sediments constitute an excellent record for the volcanic and paleoclimatic history of this region.