Water discharge variations control fluvial stratigraphic architecture in the Middle Eocene Escanilla formation, Spain

Abstract

Ancient fluvial deposits typically display repetitive changes in their depositional architecture such as alternating intervals of laterally-stacked, high-amalgamation (HA) channels, and floodplain-dominated intervals with vertically-stacked, low-amalgamation (LA) channels. Such patterns are usually ascribed to slow and high rates of base-level rise respectively, but “upstream” factors such as water discharge and sediment flux have also been recognized for their potential role in controlling stratigraphic architecture but have not been tested in ancient fluvial systems. Here, we use palaeohydraulic reconstructions to document riverbed gradient evolution within three middle Eocene (~40 Ma) fluvial HA-LA sequences in the Escanilla formation in the south-Pyrenean foreland basin. We show, in an ancient fluvial system, that river slope was primarily driven by climate-controlled water discharge variations rather than base-level changes as commonly assumed. These results have fundamental implications for the interpretation of the fluvial stratigraphic record and for our ability to reconstruct ancient hydroclimates.

Water discharge and sediment flux intermittency in the fluvial Escanilla Formation, Spain: Implications for changes in stratigraphic architecture

“…For instantaneous discharge and flux conditions in the Escanilla Formation, the recently published palaeohydrological reconstructions for the HA and LA units of Sharma et al. (2023a, 2023b), which are based on field measurements of channel geometries and sediment calibre (see supplementary material for full details), were used. However, drainage area, precipitation and total volumetric sediment flux estimates are required to approximately estimate the total water and sediment budget available to the Escanilla system.…”

“…In the first instance, Sharma et al. (2023a, 2023b) estimated MAP using the chemical index of weathering (CIW; see supplementary material; Figure 2), which is based on the major elemental composition of palaeosol samples, gave values ranging between 0.25 m year −1 and 0.57 m year −1 (average value of 0.33 m year −1 ). It is important to note here that these MAP values are based only on samples from the LA interval due to the absence of floodplain in the HA intervals.…”

“…This study focusses on three fining‐upward sequences at Olson, described by Sharma et al. (2023a, 2003b), with each sequence having a thickness of 35 to 45 m. Each fining‐upward sequence consists of a high amalgamation (HA) interval, which is defined as a 5 to 12 m thick and 600 to 2000 m wide complex of laterally and vertically amalgamated channel bodies in multiple stories, and a low amalgamation (LA) interval, which is defined as a floodplain‐dominated interval consisting of isolated channel bodies (less amalgamated) that are 2 to 4 m thick and 100 to 500 m wide (Sharma et al., 2023a, 2023b). Recent work in the Middle Eocene fluvial Escanilla Formation, Spain, has documented cyclical variations in instantaneous water discharge and bedload sediment flux relative to changes in stratigraphic architecture, that is, from HA to LA intervals within several fining‐upward sequences which is indicative of an upstream climate control on fluvial stacking pattern (Sharma et al., 2023a, 2023b).…”

“…The Escanilla Formation at Olson encompasses the Middle Eocene Climatic Optimum (MECO), a global warming event at ca 40 Ma. Geochemical proxies from the Escanilla Formation suggest regional climate in the south‐central Pyrenees to be dry, arid and devoid of much vegetation (Sharma et al., 2023a, 2023b). In other localities more to the east of the Ebro Basin (north‐eastern Spain), other proxies such as palynological, pollen taxa and floral diversity (Cavagnetto & Anadón, 1996; Haseldonckx, 1972) suggest warm and humid climate.…”

“…(1992) introduced a specific dimensionless intermittency factor ranging from zero to one. This factor serves as a means of scaling instantaneous sediment transport rates which for ancient rivers can be reconstructed from palaeohydrological methods applied to channel outcrops (Lyster et al., 2021, 2023; McLeod et al., 2023; Sharma et al., 2023a, 2023b) to longer term sediment fluxes or depositional volumes which can be estimated for geological applications using mapping or from sediment thickness isopachs (Michael et al., 2013). The chosen averaging time should be considerably longer than individual channel‐forming events (instantaneous conditions) yet relatively shorter than the overall geological timescale (mean conditions).…”