Abstract Fluvial fans are large, low-gradient depositional systems that occur in sedimentary basins worldwide. Fluvial fans can represent much of the geologic record of foreland basins, create hazards, and record paleoclimate and tectonic signals. However, we lack an understanding of how fluvial fans grow into the variety of shapes observed around the world. We explored this aspect using a cellular model of foreland basin landscape evolution with rules for sediment transport, river avulsion, and floodplain processes. We tested the hypothesis that avulsion dynamics, namely, avulsion trigger period and abandoned channel dynamics, are a primary control on fluvial fan development. We found that shorter trigger periods lead to rounder planform fluvial fan shapes because, between avulsions, channel aggradation (and thus avulsion setup) propagates shorter distances from the upstream boundary along channel pathways. This prioritizes lateral sediment dispersion, creating shorter, rounder fans, over sediment delivery further into the basin, which would create elongated fans. Modeled fans with abandoned channel attraction (but not repulsion) generated a commonly observed abrupt fan boundary marked by a transition from distributary to tributary channel patterns. While fluvial fans are thought to be linked to climate, they can occur anywhere that rivers aggrade, lose lateral confinement, and preserve alluvial topography. Instead, fluvial fans might be more recognizable in environments that frequently trigger avulsions and preserve abandoned channels that capture future avulsions.
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This content will become publicly available on September 26, 2025
A RECIPE FOR MEGAFANS
Large fan-shaped sediment distributary systems (103 – 105 km2), typically referred to as fluvial megafans, are found proximal to topographic barriers within terrestrial basins. Concepts regarding their formation have been focused on water-dominated processes associated with monsoonal climates, high sediment bedload, and flow rate. But are there other processes behind the erratic avulsive channel behavior of some of the largest fans on the planet? This study presents remotely mapped geomorphic observations from megafans including playa lakes, dunes, vegetation patterns, and basin elevation profiles of fans. Corroborating these concepts are field observations drawn from the Chaco Plain of Argentina. Active tectonics set the stage for multiple monsoon-affected eastward-flowing fluvial systems to interact with the northerly prevailing winds from the South American lower-level jet. The Chaco megafans exhibit low overall slope and limited drainage integration creating playas in both abandoned channels within the active fluvial belt and small depressions on the loess-covered floodplains. Chaco forebulge stratigraphy shows that distal from the main fluvial belt multiple loess-paleosol sequences are present up to four meters below the surface. This suggests that where subsidence outpaces fluvial avulsion rates, fine-grain wind-blown detritus aggrades in packages defined by soil profiles. Loessic paleosols are also located proximal to the orographic front, where the development of uplands relative to modern/recent fluvial incision protects the eolian sediments from erosion. Varying temporal cyclicity in aridity and seasonality of precipitation determines the climatic regime that the megafan sediments experience, resulting in megafans covered in complex fabrics of slope-oriented fluvial/pluvial features superimposed on wind-oriented eolian features and vice versa. The interaction of these depositional environments in the Chaco foreland basin highlights the smoothing effect of eolian transport and deposition, which may allow for high-avulsion rates that enhance megafan development. These geomorphic features can be observed on other globally notable megafan basin systems, highlighting the complexity of sediment transport pathways within a terrestrial realm.
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- Award ID(s):
- 1911340
- PAR ID:
- 10553775
- Publisher / Repository:
- Geological Society of America Abstracts with Programs
- Date Published:
- Volume:
- 56
- Issue:
- 5
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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ABSTRACT Natural river diversion, or avulsion, controls the distribution of channels on a floodplain and channel sandstone bodies within fluvial stratigraphic architecture. Avulsions establish new flow paths and create channels through several recognized processes, or styles. These include reoccupying existing channels, or annexation, downcutting into the floodplain, or incision, and constructing new channels from crevasse‐splay distributary networks, or progradation. Recent remote sensing observations show that avulsion style changes systematically moving downstream along modern fluvial fans but, to date, no studies have assessed the significance of these trends on fluvial fan stratigraphy. Here, spatiotemporal changes in avulsion stratigraphy are investigated within the Salt Wash Member of the Morrison Formation, deposited in the Cordilleran foreland basin during the Late Jurassic epoch. Measured sections and photographic panels were analysed from 23 locations across the Salt Wash extent. Avulsion style was identified in the stratigraphic record by the basal contact of a channel storey with underlying strata: channel–channel contacts indicate annexation, channel–floodplain contacts indicate incision and channel–heterolithic contacts indicate progradation. Contact types change downstream, such that channel–channel and channel–floodplain contacts dominate proximal locations, while channel–heterolithic contacts become increasingly prevalent downstream. Outcrop results were compared to a numerical model of fluvial fan formation and remote‐sensing analysis of avulsions on modern fans. In both additional datasets, channels in proximal fan positions tend to avulse via annexation, reoccupying abandoned channels, while channels in more distal positions tend to avulse via increasingly significant progradation. These findings suggest a relationship between newly recognized downstream changes in avulsion style and well‐established downstream changes in fluvial fan architecture. Furthermore, this suggests that fan architecture can inform interpretations of ancient fluvial dynamics, including avulsion behaviour, and that avulsions can cause stratigraphically significant and measurable changes to fan architecture.
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Permocarboniferous strata of basins proximal to the Central Pangaean Mountains in France archive regional paleoequatorial climate during a unique interval in geological history (late Paleozoic Pangaean assembly, ice age collapse, megamonsoon inception). The voluminous (estimated 2 km) succession of exclusively fine-grained redbeds that composes the Permian Salagou Formation (Lodéve Basin, France) has been interpreted as recording either lacustrine or fluvial settings. We present preliminary field data to explore the hypothesis that these deposits record eolian transport, and ultimate deposition as either loess or in a shallow lacustrine environment. Fieldwork includes ~1000 m of section described at dm-scale, and magnetic susceptibility measured at 0.5 m intervals, from sections strategically located in both proximal and distal areas, and from all stratigraphic levels of the unit to assess spatial and temporal variations. These data indicate that the lower and middle Salagou Formation is dominated by internally massive, red mud-siltstone with no evidence of channeling. Up-section, a higher frequency of ripples, rare hummocky cross stratification, and mudcracks record the presence of shallow water, but with no channeling, nor units of grain size exceeding very fine-grained sand. Randomly-oriented slickensides at various localities in the mid-upper Salagou may represent incipient pedogenesis. The lack of evidence for channels and other fluvial features casts doubt on a fluvial interpretation. A lacustrine interpretation is consistent with local evidence of shallow water. However, in the absence of fluvial transport indicators, large volumes of entirely fine-grained material that were delivered to the Lodéve basin call for eolian transport, and thus a loess or shallow lacustrine interpretation. The documentation of voluminous paleoloess in eastern equatorial Pangea during the Permian could reflect the influence of glaciation associated with the Variscan highlands. Together with previous studies that detail Permian loess in western equatorial Pangea, this work impacts our understandingof the global Late Paleozoic climate system and presents a need to reevaluate modeling parameters (e.g. equatorial mountain glaciation, atmospheric dust loading).more » « less
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Abstract Understanding the effects of flat slab subduction on mountain building, arc magmatism, and basin evolution is fundamental to convergent‐margin tectonics, with implications for potential feedbacks among geodynamic, magmatic, and surface processes. New stratigraphic and geochronological constraints on Cenozoic sedimentation and magmatism in the southern Central Andes of Argentina (31°S) reveal shifts in volcanism, foreland/hinterland basin development, sediment accumulation, and provenance as the retroarc region was structurally partitioned during slab flattening. Detrital zircon U‐Pb age distributions from the western (Calingasta basin), central (Talacasto and Albarracín basins), and eastern (Bermejo foreland basin) segments of the retroarc basin system preserve syndepositional volcanism and orogenic unroofing of multiple tectonic provinces. Initial shortening‐related exhumation of the Principal Cordillera at 24–17 Ma was recorded by the accumulation of distal eolian deposits bearing Oligocene–Eocene zircons from the Andean magmatic arc. The Calingasta basin chronicled volcanism and basement shortening in the Frontal Cordillera at ~17–11 Ma, as marked by an upward coarsening succession of fluvial to alluvial fan deposits with a sustained zircon U‐Pb age component that matches pervasive Permian‐Triassic bedrock in the hinterland. An ~450 km eastward inboard sweep of volcanism at 11 Ma coincided with the inception of flat slab subduction, and subsequent thin‐skinned shortening in the Precordillera fold‐thrust belt that exhumed wedge‐top deposits and induced cratonward (eastward) advance of flexural subsidence into the Bermejo foreland basin. This foreland basin was structurally partitioned as basement uplifts of the Sierras Pampeanas transformed a fluvial megafan sediment routing network into smaller isolated alluvial fan systems fed by adjacent basement blocks.
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Abstract. River avulsions are an important mechanism by which sediment is routed andemplaced in foreland basins. However, because avulsions occur infrequently,we lack observational data that might inform where, when, and why avulsionsoccur and these issues are instead often investigated by rule-basednumerical models. These models have historically simplified or neglected the effects of abandoned channels on avulsion dynamics, even though fluvialmegafans in foreland basins are characteristically covered in abandonedchannels. Here, we investigate the pervasiveness of abandoned channels onmodern fluvial megafan surfaces. Then, we present a physically basedcellular model that parameterizes interactions between a single avulsingriver and abandoned channels in a foreland basin setting. We investigate how abandoned channels affect avulsion setup, pathfinding, and landscapeevolution. We demonstrate and discuss how the processes of abandoned channel inheritance and transient knickpoint propagation post-avulsion serve to shortcut the time necessary to set up successive avulsions. Then, we address the idea that abandoned channels can both repel and attract future pathfinding flows under different conditions. By measuring the distance between the mountain front and each avulsion over long (106 to 107 years) timescales, we show that increasing abandoned channel repulsion serves to push avulsions farther from the mountain front, while increasing attraction pulls avulsions proximally. Abandoned channels do not persist forever, and we test possible channel healing scenarios (deposition-only, erosion-only, and far-field-directed) and show that only the final scenario achieves dynamic equilibrium without completely filling accommodation space. We also observe megafan growth occurring via ∼100 000-year cycles of lobe switching but only in our runs that employ deposition-only or erosion-only healing modes. Finally, we highlight opportunities for future field work and remote sensing efforts to inform our understanding of the role that floodplain topography, including abandoned channels, plays on avulsion dynamics.more » « less
