Abstract
AbstractThe relationship between climate, landscape connectivity and sediment export from mountain ranges is key to understanding the propagation of erosion signals downstream into sedimentary basins. We explore the role of connectivity in modulating the composition of sediment exported from the Frontal Cordillera of the south‐central Argentine Andes by comparing three adjacent and apparently similar semi‐glaciated catchment‐fan systems within the context of an along‐strike precipitation gradient. We first identify that the bedrock exposed in the upper, previously glaciated reaches of the cordillera is under‐represented in the lithological composition of gravels on each of three alluvial fans. There is little evidence for abrasion or preferential weathering of sediment sourced from the upper cordillera, suggesting that the observed bias can only be explained by sediment storage in these glacially widened and flattened valleys of the upper cordillera (as revealed by channel steepness mapping). A detailed analysis of the morphology of sedimentary deposits within the catchments reveals catchment‐wide trends in either main valley incision or aggradation, linked to differences in hillslope–channel connectivity and precipitation. We observe that drier catchments have poor hillslope–channel connectivity and that gravels exported from dry catchments have a lithological composition depleted in clasts sourced from the upper cordillera. Conversely, the catchment with the highest maximum precipitation rate exhibits a high degree of connectivity between its sediment sources and the main river network, leading to the export of a greater proportion of upper cordillera gravel as well as a greater volume of sand.Finally, given a clear spatial correlation between the resistance of bedrock to erosion, mountain range elevation and its covariant, precipitation, we highlight how connectivity in these semi‐glaciated landscapes can be preconditioned by the spatial distribution of bedrock lithology.These findings give insight into the extent to which sedimentary archives record source erosion patterns through time.
Highlights
Sediment export from mountain ranges controls the morphodynamic behaviour of lowland rivers (Baynes et al, 2020; Pfeiffer et al, 2019) as well as the physical characteristics of their sedimentary records (Quick et al, 2019; Watkins et al, 2020)
The impact of past glaciation on the cordillera is evident from the cirque morphology of their headwaters, wide low-gradient valleys with steep tributaries, and the lateral and medial moraines that are preserved in their upper reaches (Figures 1, 2a)
As we demonstrate that connectivity and sediment export from catchments is sensitive to climate through its influence on sediment delivery to the river network, this has important implications for our understanding of how climate influences the evolution of river networks (Gasparini et al, 2007; Hodge et al, 2011), following glacial perturbation
Summary
Sediment export from mountain ranges controls the morphodynamic behaviour of lowland rivers (Baynes et al, 2020; Pfeiffer et al, 2019) as well as the physical characteristics of their sedimentary records (Quick et al, 2019; Watkins et al, 2020). To explore how the glacial conditioning of a landscape influences connectivity, sediment export and the construction of stratigraphic records, we draw on three lines of evidence; (i) river network morphology and its relationship with spatial variability in bedrock lithology and precipitation, (ii) channel–hillslope morphology and (iii) alluvial fan gravel composition Combined, these methodologies allow us to test a hypothesis that the lithological and climatic controls on landscape form are directly translated downstream into their sedimentary record. Based on their morphological features, deposits are grouped as (1) floodplain deposits, (2) alluvial and colluvial fans that feed sediment directly to the main river channel, and (3) slope deposits and moraine that form terraces and ridges along valley sides In combining these analyses, the role of climate in modulating connectivity and sediment export is explored within the context of the strong, along-strike precipitation gradient that prevails across the study area
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