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 coarse-grained highly amalgamated (HA), laterally-stacked, channel bodies, and finer-grained less amalgamated (LA), vertically-stacked, channels encased in floodplain deposits. Such patterns are usually ascribed to slower, respectively higher, rates of base level rise (accommodation). However, “upstream” factors such as water discharge and sediment flux also play a potential role in determining stratigraphic architecture, yet this possibility has never been tested despite the recent advances in the field of palaeohydraulic reconstructions from fluvial accumulations. Here, we chronicle riverbed gradient evolution within three Middle Eocene (~ 40 Ma) fluvial HA-LA sequences in the Escanilla Formation in the south-Pyrenean foreland basin. This work documents, for the first time in a fossil fluvial system, how the ancient riverbed systematically evolved from lower slopes in coarser-grained HA intervals, and higher slopes in finer-grained LA intervals, suggesting that bed slope changes were determined primarily by climate-controlled water discharge variations rather than base level changes as often hypothesized. This highlights the important connection between climate and landscape evolution and has fundamental implications for our ability to reconstruct ancient hydroclimates from the interpretation of fluvial sedimentary sequences.

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

“…These calculations require estimates for both channelforming discharges and longer term water budgets and similarly for sediment transport capacities for channelforming conditions, compared to long-term sediment flux rates. 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 (2023aSharma et al ( , 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.…”

“…Previously, several stratigraphic sequences consisting of laterally amalgamated and vertically stacked channels have been described by Labourdette and Jones (2007) and Labourdette (2011). This study focusses on three fining-upward sequences at Olson, described by Sharma et al (2023aSharma et al ( , 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(Sharma et al, , 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(Sharma et al, , 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(Sharma et al, , 2023b. In other localities more to the east of the Ebro Basin (northeastern Spain), other proxies such as palynological, pollen taxa and floral diversity (Cavagnetto & Anadón, 1996;Haseldonckx, 1972) suggest warm and humid climate.…”

“…Total water budget for the Escanilla Formation was estimated using two end members of MAP rates (ca 0.3 m year −1 and ca 1.0 m year −1 ). In the first instance, Sharma et al (2023aSharma et al ( , 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.…”

“…To study sediment fluxes and basin infilling over extended geological timescales, Paola et al (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(Lyster et al, , 2023McLeod et al, 2023;Sharma et al, 2023aSharma et al, , 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).…”