Implications of palaeosols in low net-to-gross fluvial architecture reconstruction: Reservoir analogues from Patagonia and Spain

Abstract The changes in the geometry, density and distribution of the fluvial reservoir sand-bodies have been modelled both in the laboratory and from outcrop analogues. On the other hand, fine-grained floodplain deposits, often called “background” sedimentation, have not been in the spotlight. However, they provide a very important database for fluvial architecture reconstruction and therefore constitute a key tool in characterizing, correlating and modelling heterogeneous fluvial reservoirs. The methodology used in this work combines classical sedimentological and palaeopedological data. Palaeosols were identified in outcrops of the Cretaceous of Patagonia and Triassic red beds of Spain based on the main pedofeatures such as colour, soil structures, mottles, nodules, clay illuviation cutans, slickensides and rhizoliths. Palaeosol horizons, thickness and types of contact were described. The palaeosols classification was made through the comparison with the Soil Taxonomy. For the characterization of lateral palaeosols variability, several palaeopedological logs were constructed following the same stratigraphic level at both margins of the main channels. Small-scale or high-frequency palaeosol variations, identified in both fluvial succession are represented by the lateral and vertical superimposition of Inceptisols/Entisols, Vertisols and hydromorphic Vertisols/swamp deposits. We recognized a decrease in the drainage conditions within the palaeosols catena moving away from the main channel bodies. The most waterlogged palaeosol is located at 200 to 300 m from the main channels. Lateral changes of palaeosols are interpreted as the result of intrinsic factors to the depositional systems, such as the relative position within the floodplain and the distance from the main channels, that condition the particle size of parent material, the sedimentation rate and the palaeotopographic position which state the drainage conditions of palaeosols. Vertical stacking of different palaeosols is linked to avulsion processes and the relatively abrupt change in the distance to main channels as the system aggraded. The study of these two environmentally similar examples (low-gradient meandering fluvial systems subjected to a high rate of aggradation), although located in different tectonic and chronostratigraphic contexts, shows that the type of fluvial system acted as the main control of sedimentary and pedogenic processes; as well as the scale at which they occur. Thus, when choosing an outcrop fluvial reservoir analogue, the type of fluvial system and climate should be more important than the age or the tectonic framework of the succession to be studied.