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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. 

Climate, tectonics, and life influence the flux and caliber of sediment transported across Earth's surface. These environmental conditions can leave behind imprints in the Earth's sedimentary archive, but signals of climate, tectonic, and biologic change are not always present in the stratigraphic record. Deterministic and stochastic surface dynamics collectively act as a stratigraphic filter, impeding the burial and preservation of environmental signals in sedimentary deposits. Such impediments form a central challenge to accurately reconstructing environmental conditions through Earth's history. Emergent and self‐organized length and timescales in landscapes, which are themselves influenced by regional environmental conditions, define spatial and temporal sedimentation patterns in basins and fundamentally control the likelihood of environmental signal preservation in sedimentary deposits. Properly characterizing these scales provides a key avenue for incorporating the known “imperfections” of the stratigraphic record into paleoenvironmental reconstructions. These insights are necessary for answering both basic and applied science questions, including our ability to reconstruct the Earth system response to prior episodes of climate, tectonic, or land cover change.

 

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.

 

A transect of paleoshelf cores from Maryland and New Jersey contains an ~0.19‐ to 1.61‐m‐thick interval with reduced percentages of carbonate during the onset of the Paleocene‐Eocene Thermal Maximum (PETM). Outer paleoshelf cores are barren of nannofossils and correspond to two minor disconformities. Middle paleoshelf cores contain a mixture of samples devoid of nannofossils and those with rare specimens characterized by significant dissolution (i.e., etching). The magnitude of the decrease in carbonate cannot be explained by dilution by clastic material or dissolution resulting from the oxidation of organic matter during early diagenesis. The observed preservation pattern implies a shoaling of the calcite compensation depth and lysocline to the middle shelf. This reduced carbonate interval is observed during the onset of the PETM on other continental margins raising the possibility that extreme shoaling of the calcite compensation depth and lysocline was a global signal, which is more significant than in previous estimates for the PETM. An alternative scenario is that shoaling was restricted to the northwest Atlantic, enhanced by regional and local factors (eutrophication from rivers and microbial activity associated with warming) that exacerbated the impact of acidification on the shelf.