Middle Pleistocene palaeolimnology of a dammed tropical river: The Zarzal Formation, Cauca Valley, Colombia

The Plio/Pleistocene Zarzal Formation (ZF) offers one of the few examples of alternation between fully lacustrine to fluvial conditions in a major, tropical, intramontane river in the Northern Andes, i.e., the Cauca River. The ZF is mainly composed of diatomites, mudstones, and tuffaceous sandstones deposited in an intramontane depression located between the Western and Central cordilleras of Colombia and associated with the Cauca-Romeral Fault System. Stratigraphic, sedimentological, micropalaeontological (diatoms), and geochronological analyses (independent (U-Th)/He dating of apatite and zircon) have been performed on two sections of the ZF. These analyses reveal a fluctuation between volcanic, fluvial and lacustrine deposits during the Middle Pleistocene. In addition, the presence of soft-sediment deformation structures reflects the synergetic effect of tectonism and volcanism on the sedimentation patterns in the Cauca River Valley. Our data show that between about 0.48-0.52 Ma (Ionian) depositional facies varied from a fluvial-dominated environment to a palaeo-lake, suggesting the impoundment of the river. Once the lake was formed, full lacustrine conditions were frequently interrupted by river pulses bringing terrigenous and previously deposited volcanic material into the lake. Diatom-based palaeolimnological reconstructions and comparisons with the sedimentological records indicate that river episodes played a decisive role on nutrient supply and sediment input, controlling important ecological dynamics in the newly formed lake. Although we cannot unambiguously pinpoint to the influence of volcanic activity on nutrient cycling within the lake, our data exhibit a strong relationship between volcanic deposits and changes in flora. The latter are possibly linked to the increase in phosphorous availability. The combined effects of tectonism and volcanism had major environmental implications in the Quaternary evolution of the Cauca River causing unprecedented damming of a one of the major torrential tropical rivers in northern South America. Therefore, are of utmost importance when considering risk assessment and management for riverine communities and infrastructure such as dam buildings.