Fluvial-aeolian interaction at different scales in the Miocene Andean Foreland

Abstract

Fluvial and aeolian processes interact at different spatial and temporal scales in dryland environments. Different scales of interactions result in contrasting scales of sedimentary heterogeneity in the rock record and can be related either to short-term changes in available runoff, to large-scale flooding of aeolian systems, and eventually to long-term climatic changes. In this contribution, different scales of fluvial-aeolian interactions are explored in a sedimentary succession developed in the Andean Foreland during the Miocene, the period of maximum uplift and more dramatic changes of landscape and climate in the area. The studied unit constitutes a 1500 m-thick succession that records the long-term evolution of a large fluvial fan. It is greatly dominated by fluvial deposits composed of channelized sandstones and conglomerates that intercalate with thick fine-grained overbank deposits. Aeolian deposits comprise well-sorted, fine- to medium-grained sandstones that show a wide range of thickness and lateral development, suggesting contrasting processes. The arid climate and the high-accommodation setting in a foreland basin provide an ideal context to analyse a relatively complete record of the interaction of fluvial and aeolian processes at variable scales.Small- and intermediate-scale interactions between fluvial and aeolian processes are mainly related to autogenic dynamics and to the intrinsic variability of hydrological processes in dryland systems, subject to winds capable of transporting and accumulating sand when not affected by the water table or its capillary fringe. Small-scale examples include aeolian reworking of water-lain deposits, resulting in dm-scale units that show better sorting and overall finer grain than the equivalent fluvial units. Intermediate-scale heterogeneities are in the studied case the consequence of more widespread flooding episodes of aeolian systems. These could be related to greater floods developed in the order of 101-102 years and are usually associated with low-gradient interdune areas. Irrespective of the wet or dry nature of the interdunes, large-scale flooding results in local accumulations of fluvial deposits, leading to the development of 10´s-m thick and 100-200 m wide, lens-shaped intercalations of fine-grained deposits within well-sorted, sandy aeolian dune deposits. Finally, large-scale interactions are related to allocyclic processes, mainly climate change leading to wetter and dryer periods in the evolution of this system, effective over a larger temporal framework (105-106 years). In the case at hand, the effect on the record is stronger in terms of petrophysical changes and sedimentary architecture as 10-m to 100-m thick sandy aeolian units (with subordinate fluvial-flood deposits) intercalate with conglomerates, sandstones and fine-grained intervals related to fluvial-dominated intervals (with minor aeolian reworking). Understanding this complex interactions of fluvial and aeolian processes in dryland environments and contrasting these styles with other units in the rock record may help to clarify different hierarchies of sedimentary heterogeneity in the subsurface and provide key elements in order to optimise development strategies, especially for mature fields.