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Abstract Caldera lake sediments of the early Eocene Tufolitas Laguna del Hunco (Chubut Province, Argentina) host one of the world’s best-preserved and most diverse fossil plant assemblages, but the exceptional quality of preservation remains unexplained. The fossils have singular importance because they include numerous oldest and unique occurrences in South America of genera that today are restricted to the West Pacific region, where many of them are now vulnerable to extinction. Lacustrine depositional settings are often considered optimal for preservation as passive receptors of suspended sediment delivered, often seasonally, from lakeshores. However, caldera lakes can be influenced by a broader range of physical and chemical processes that enhance or decrease fossil preservation potential. Here, we use Laguna del Hunco to provide a new perspective on paleoenvironmental controls on plant fossil preservation in tectonically active settings. We establish a refined geochronological framework for the Laguna del Hunco deposits and present a detailed history of processes active during ∼ 200,000 years of lake filling from 52.217 ± 0.014 Ma to 51.988 ± 0.035 Ma, the time interval that encompasses nearly all fossil deposition. Detailed facies analysis shows that productive fossil localities reside within high-deposition-rate beds associated with high-energy density flows and wave-reworked lake-floor sediments, challenging traditional views that low-energy environments are required for well-preserved plant fossils. These results demonstrate that even delicate fossil components like fruits and flowers can survive high-energy transport, underscoring the importance of rapid burial as a primary control on fossil preservation. Short, steep sediment-transport networks may facilitate terrestrial fossil preservation by limiting opportunities for biochemical degradation on land and providing relatively frequent, high-energy depositional events, which quickly transport and bury organic material following events such as landslides from steep, wet, surrounding slopes. Our new model for plant taphonomy opens a path toward finding and understanding other exceptional biotas in environments once considered unlikely for preservation. 

ABSTRACT Buried channel sand bodies are important reservoirs of subsurface water and energy resources, but their arrangement and interconnectedness are difficult to predict. The dominant process that distributes channels and their sediments in alluvial basins is river avulsion, which occurs when a channel seeks a new location on the adjacent floodplain. Floodplain sedimentation, incision, and channel levee growth influence channel pathfinding during avulsion, and should control key aspects of the stratigraphic arrangement of channel bodies, including compensational (spatially and temporally even) deposition, stratigraphic completeness, and facies distributions; however, this impact has been difficult to isolate in natural and experimental basin fills. To test how different avulsion pathfinding parameters influence stratigraphic architecture, we use a numerical model of a fluvial fan to produce synthetic fluvial stratigraphy under seven different runs with progressively more complex channel pathfinding rules. In the simplest models where pathfinding is set by a random walk, the channel rapidly changes position and avulsions spread across the fan surface. The corresponding deposit is dominated by channel facies, is relatively incomplete, and the compensation timescale is short. As rules for pathfinding become more complex and channels can be attracted or repulsed by pre-existing channels, lobe switching emerges. Deposits become more diverse with a mix of channel and floodplain facies, stratigraphic completeness increases, and the compensation timescale lengthens. Previous work suggests that the compensation timescale is related to the burial timescale and relief across the depositional surface, yet we find that compensation approaches the burial timescale only for model runs with high morphodynamic complexity and relatively long topographic memory. Our results imply that in simple systems with limited degrees of freedom, the compensation timescale may become detached from the burial timescale, with uniform sedimentation occurring quickly relative to long burial timescales. 

Ancient river deposits are important archives of past landscape conditions on planetary surfaces. On Earth, they host valuable groundwater, energy resources, and carbon-storage potential. Reconstructing details of paleochannel forms and movements refines our understanding of the controls on river behavior under different climate, landcover, and tectonic conditions, and improves predictions and models of subsurface reservoirs. While studies have shown detailed connections between channel kinematics and bar-deposit architecture in meandering river systems, similar connections between braided river movements and preserved braided river deposits have not been established. Here we explore the potential for connecting braided river deposits to paleochannel movements, form, and flow conditions, and we evaluate the controls on bar preservation using synthetic stratigraphy generated with a numerical morphodynamic model. We investigate how attributes of channel morphodynamics, like channel widening or braiding intensity, impact bar deposits’ preservation, scale, geometry, and architecture. We then assess how the scale, preservation, and facies composition of bar deposits reflect formative flow conditions of the channel. Our results demonstrate that no diagnostic signature of braided channel morphodynamics is recorded in bar-deposit geometry, facies, or preservation patterns. Rather, the unique local history of thread movements combines stochastically to preserve or rework bar deposits, and the timing of channel avulsion is the dominant control on bar preservation. Our results also show that representative paleochannel flow conditions will likely be accurately reflected in aggregate observations of braid bar deposits within channel-belt sandbodies at a regional or member/formation scale. These results demonstrate the need for broad sampling and statistical approaches to subsurface prediction and paleo-flow reconstruction in ancient, braided river deposits. 

Abstract Reconstructing river planform is crucial to understanding ancient fluvial systems on Earth and other planets. Paleo-planform is typically interpreted from qualitative facies interpretations of fluvial strata, but these can be inconsistent with quantitative approaches. We tested three well-known hydraulic planform predictors in Cretaceous fluvial strata (in Utah, USA) where there is a facies-derived consensus on paleo-planform. However, the results of each predictor are inconsistent with facies interpretations and with each other. We found that one of these predictors is analytically best suited for geologic application but favors single-thread planforms. Given that this predictor was originally tested using just 53 data points from natural rivers, we compiled a new data set of hydraulic geometries in natural rivers (n = 1688), which spanned >550 globally widespread, sand- and gravel-bed rivers from various climate and vegetative regimes. We found that the existing criteria misclassified 65% of multithread rivers in our data set, but modification resulted in a useful predictor. We show that depth/width (H/W) ratio alone is sufficient to discriminate between single-thread (H/W > 0.02) and multithread (H/W < 0.02) rivers, suggesting bank cohesion may be a critical determinant of planform. Further, we show that the slope/Froude (S/Fr) ratio is useful to discriminate process in multithread rivers; i.e., whether generation of new threads is an avulsion-dominated (anastomosing) or bifurcation-dominated (braided) process. Multithread rivers are likely to be anastomosing when S/Fr < 0.003 (shallower slopes) and braided when S/Fr > 0.003 (steeper slopes). Our criteria successfully discriminate planform in modern rivers and our geologic examples, and they offer an effective approach to predict planform in the geologic past on Earth and on other planets. 

Abstract Reconstruction of active channel geometry from fluvial strata is critical to constrain the water and sediment fluxes in ancient terrestrial landscapes. Robust methods—grounded in extensive field observations, numerical simulations, and physical experiments—exist for estimating the bankfull flow depth and channel-bed slope from preserved deposits; however, we lack similar tools to quantify bankfull channel widths. We combined high-resolution lidar data from 134 meander bends across 11 rivers that span over two orders of magnitude in size to develop a robust, empirical relation between the bankfull channel width and channel-bar clinoform width (relict stratigraphic surfaces of bank-attached channel bars). We parameterized the bar cross-sectional shape using a two-parameter sigmoid, defining bar width as the cross-stream distance between 95% of the asymptotes of the fit sigmoid. We combined this objective definition of the bar width with Bayesian linear regression analysis to show that the measured bankfull flow width is 2.34 ± 0.13 times the channel-bar width. We validated our model using field measurements of channel-bar and bankfull flow widths of meandering rivers that span all climate zones (R2 = 0.79) and concurrent measurements of channel-bar clinoform width and mud-plug width in fluvial strata (R2 = 0.80). We also show that the transverse bed slopes of bars are inversely correlated with bend curvature, consistent with theory. Results provide a simple, usable metric to derive paleochannel width from preserved bar clinoforms. 

Abstract The StraboSpot data system provides field-based geologists the ability to digitally collect, archive, query, and share data. Recent efforts have expanded this data system with the vocabulary, standards, and workflow utilized by the sedimentary geology community. A standardized vocabulary that honors typical workflows for collecting sedimentologic and stratigraphic field and laboratory data was developed through a series of focused workshops and vetted/refined through subsequent workshops and field trips. This new vocabulary was designed to fit within the underlying structure of StraboSpot and resulted in the expansion of the existing data structure. Although the map-based approach of StraboSpot did not fully conform to the workflow for sedimentary geologists, new functions were developed for the sedimentary community to facilitate descriptions, interpretations, and the plotting of measured sections to document stratigraphic position and relationships between data types. Consequently, a new modality was added to StraboSpot—Strat Mode—which now accommodates sedimentary workflows that enable users to document stratigraphic positions and relationships and automates construction of measured stratigraphic sections. Strat Mode facilitates data collection and co-location of multiple data types (e.g., descriptive observations, images, samples, and measurements) in geographic and stratigraphic coordinates across multiple scales, thus preserving spatial and stratigraphic relationships in the data structure. Incorporating these digital technologies will lead to better research communication in sedimentology through a common vocabulary, shared standards, and open data archiving and sharing. 

Abstract The process of river avulsion builds floodplains and fills alluvial basins. We report on a new style of river avulsion identified in the Landsat satellite record. We found 69 examples of retrogradational avulsions on rivers of densely forested fluvial fans in the Andean and New Guinean alluvial basins. Retrogradational avulsions are initiated by a channel blockage, e.g., a logjam, that fills the channel with sediment and forces water overbank (dechannelization), which creates a chevron-shaped flooding pattern. Dechannelization waves travel upstream at a median rate of 387 m/yr and last on average for 13 yr; many rivers show multiple dechannelizing events on the same reach. Dechannelization ends and the avulsion is complete when the river finds a new flow path. We simulate upstream-migrating dechannelization with a one-dimensional morphodynamic model for open channel flow. Observations are consistent with model results and show that channel blockages can cause dechannelization on steep (10−2 to 10−3), low-discharge (~101 m3 s−1) rivers. This illustrates a new style of floodplain sedimentation that is unaccounted for in ecologic and stratigraphic models. 

Abstract The low temporal completeness of fluvial strata could indicate that recorded events represent unusual and extreme conditions. However, field observations suggest that preserved strata predominantly record relatively common transport conditions—a paradox termed thestrange ordinarinessof fluvial strata. We theorize that the self‐organization of fluvial systems into a morphodynamic hierarchy that spans bed to basin scales facilitates the preservation of ordinary events in fluvial strata. Using a new probabilistic model and existing field and experimental data sets across these scales, we show that fluvial morphodynamic hierarchy enhances the stratigraphic preservation of medial topography—ordinary events. We show that lower‐order landforms have a higher likelihood of complete preservation when the kinematic rates of evolution of successive levels in the morphodynamic hierarchy are comparable. We highlight how relative changes in kinematic rates of evolution of successive levels in the morphodynamic hierarchy can manifest as major shifts in stratigraphic architecture through Earth history.