Elizabeth Hajek

Significance of channel-belt clustering in alluvial basins

The distribution of channel deposits in alluvial basins is commonly used to interpret past changes in climate, tectonics, and sea level. Here we present preliminary evidence that longtime scale (~10 3 –10 5 yr) self-organization in fl uvial systems may generate structured stratigraphic patterns spontaneously, in the absence of, or independent from, changing basin boundary conditions. A physical experiment and an ancient alluvial succession (Ferris Formation, latest Cretaceous–Paleogene, south-central Wyoming) both show stratigraphy where clusters of many closely spaced channel deposits are separated from each other by extensive intervals of overbank mudstones. Analysis using spatial point process methods shows that channel deposits in both basins are statistically clustered over intermediate basin length scales. In the experiment, external controls (base level, subsidence rate, and sediment/water supplies) were not varied, and therefore not factors in cluster formation. Likewise, the ancient system lacks stratigraphic and sedimentologic evidence of external controls on channel clustering. We propose that channel clusters, as seen in this study, refl ect a scale of flself-organization that is not usually recognized in ancient deposits. This type of internally generated stratigraphy should be considered when reconstructing tectonic, climate, and sea-level changes from

Scale-dependent compensational stacking: An estimate of autogenic time scales in channelized sedimentary deposits

Recent studies show that paleoenvironmental (allogenic) signals preserved in the stratigraphic record may be contaminated or overprinted by internally generated (autogenic) sedimentation. This is problematic, but it is unclear over what temporal and spatial scales autogenic patterns are most prevalent. We propose that scale breaks in basin-fi lling trends can be used to identify the transition between allogenic and autogenic stratigraphy. Using data from numerical and physical experiments and an ancient outcrop, we explore how compensation, the tendency for sediment transport systems to preferentially fi ll topographic lows, varies with stratigraphic scale. Object-based models demonstrate the temporal scales at which stratigraphy changes from being partially infl uenced by autogenic processes to being completely determined by allo- genic forcings and suggest that this transition occurs at a time scale set by the maximum scale of surface roughness in a transport system divided by the long-term aggradation rate. This hypothesis is validated in a physical experiment where delta topography was monitored along fl ow-perpendicular transects at a high temporal resolution relative to channel kine matics. The strength of compensation in the experiment changes at the predicted time scale, where the maxi mum surface roughness is equal to the depth of the experimental channels. Above this compensation time scale deposits stack purely compensationally, but below this time scale deposits stack somewhere between randomly and deterministically. Similar scale-dependent stacking is also observed in the Ferris Formation (Cretaceous-Paleogene, Hanna Basin, Wyoming, United States). This study demonstrates that scale-dependent compensational stack- ing may be useful for isolating allogenic and autogenic stratigraphy in sedimentary basins.

Field test of autogenic control on alluvial stratigraphy (Ferris Formation, Upper Cretaceous-Paleogene, Wyoming)

Internally generated (autogenic) sedimentary processes can obscure signals of tectonic movements, climate conditions, or sea-level fl uctuations in alluvial basins. The stratigraphic effects of autogenic dynamics have been considered negligible on time scales of 10 4 –10 6 yr. However, recent physical and numerical models have shown that over basin-fi lling time scales, self-organization in sedimentary systems can produce stratigraphic patterns similar to those resulting from changing basin boundary conditions. Here, we present the fi rst fi eld-based test for autogenic control on stratigraphy in an ancient alluvial basin. The Ferris Formation (Upper Cretaceous–Paleogene, Hanna Basin, Wyoming) is composed of clusters of closely spaced channel deposits separated by intervals dominated by overbank material—a pattern that has been proposed as an example of a potentially autogenic stratigraphy. In order to evaluate controls on Ferris Formation stratigraphy, spatial patterns of key channel properties were analyzed. These variables (including sand-body dimensions, paleofl ow depth, maximum clast size, paleocurrent direction, and sediment provenance) were chosen because they can change in response to climate, tectonic, or sea-level forcing and are commonly measurable in ancient alluvial successions. No clear statistical trends were detected in the measured variables, which suggests that external forces did not likely control stratigraphic organization in the unit. This study demonstrates that autogenic dynamics in natural, fi eld-scale systems can produce organized stratigraphic patterns over much larger spatial and temporal scales than is typically presumed. This fi nding emphasizes the need for further understanding of autogenic stratigraphy and greater care interpreting climate, tectonic, and eustatic changes from alluvial basins.

Conceptual model for predicting mudstone dimensions in sandy braided-river reservoirs

Sandy braided-river deposits with high net-to-gross sand ratios are commonly attractive reservoirs, yet internal lithologic heterogeneities, particularly the presence of low-permeability mudstone deposits, significantly complicate the development of such units. Previous work has focused on measuring the scale and distribution of mudstone deposits in outcrop analogs; however, because of extreme differences in scale, discharge, sediment load, and geologic history, the results of these studies are difficult to apply with confidence to a wide range of sandy braided-river reservoirs. Based on work in modern braided rivers (Niobrara and North Loup rivers, Nebraska) and ancient braided-river deposits (Kayenta Formation, Jurassic and lower Castlegate Sandstone, Cretaceous, Colorado and Utah), we propose a process-based conceptual model for understanding and predicting the distribution and geometries of fine-grained (mudstone) intervals in sandy braided-river deposits. This model is an idealized channel-fill unit composed of five fine-grained lithofacies (mud plugs, channel-lining muds, interbar muds, inclined heterolithic strata, and flood-plain and overbank material) that scale proportional to channel-thread dimensions, including depth, cross-stream width, and downstream length. Each lithofacies is found in a different region in an individual channel fill, and lithofacies found low in a fill may be preferentially preserved. Within braided-river deposits, extrinsic depositional factors, such as aggradation rate, available accommodation, and avulsion-return time, produce different channel-fill stacking arrangements, preserving fine-grained lithofacies in different, relative proportions. This conceptual model provides an approach to reservoir characterization that deductively constrains the dimensions and distribution of fine-grained barriers to flow and may help account for the inherent variability in sandy braided-river deposits.

Is river avulsion style controlled by floodplain morphodynamics

River relocation, or avulsion, is a fundamental process that fills alluvial basins. However, it is difficult to predict avulsion patterns because the landscape conditions associated with different avulsion styles are currently unknown. Two end-member avulsion styles have been documented in modern rivers: progradational avulsions, during which significant floodplain deposition occurs as a new channel is built, and incisional avulsions, during which floodplain erosion captures flow from a parent channel. Here we propose that avulsion style is related to the tendency for overbank flows to erode or deposit sediment (i.e., floodplain morphodynamics). We present a scaling comparison of floodplain erosion and deposition rates, test it with morphodynamic modeling, and show field data from ancient deposits that are consistent with modeling results. These results demonstrate that floodplain morphodynamics may determine how rivers relocate and that assessing the relative influence of floodplain erosion and deposition may be useful for predicting sedimentation patterns in avulsive systems.

Avulsion flow-path selection on rivers in foreland basins

River avulsions help distribute sediment across the floodplain, and the frequency and flow path selected by each avulsion determines flooding and sedimentation patterns. However, rivers avulse infrequently, making it difficult to quantify flow-path behavior and test hypotheses posed by experimental and numerical studies. We used Google Earth Engine to create a novel data set of 55 avulsions that occurred from A.D. 1984 to 2014 in the Andean and Himalayan foreland basins. On each avulsion we measure hop length (the displacement of the avulsion across the floodplain orthogonal to the parent channel) and avulsion length (the length of the parent channel reach). Without controlling for external factors, such as climate or geology, we find that both hop length and avulsion length scale with parent channel-belt width. Avulsion channels tend to hop 2.5 channel-belt widths away from their parent channel and avulsion length is 13.4 parent channel-belt widths. We use these data to test between end-member scenarios where flow paths are randomly selected or steered by floodplain topography. Observed avulsions are inconsistent with flow-path predictions derived from a directed random walk model, and a scaling analysis of alluvial ridge size shows that hop lengths are set by alluvial ridge widths. These results suggest that avulsion flow-path selection in the Andean and Himalayan basins is driven by alluvial ridge topography, which promotes evenly spaced (or compensational) channel sand bodies over decadal time scales. These data and results are important for understanding controls on avulsion flow-path selection and how avulsion processes are represented in the stratigraphic record of foreland basins.

Shallow marine response to global climate change during the Paleocene‐Eocene Thermal Maximum, Salisbury Embayment, USA

The Paleocene-Eocene Thermal Maximum (PETM) was an interval of extreme warmth that caused disruption of marine and terrestrial ecosystems on a global scale. Here we examine the sediments, flora and fauna from an expanded section at Mattawoman Creek-Billingsley Road (MCBR) in Maryland and explore the impact of warming at a nearshore shallow marine (30-100 m water depth) site in the Salisbury Embayment. Observations indicate that, at the onset of the PETM, the site abruptly shifted from an open-marine to prodelta setting with increased terrestrial and fresh water input. Changes in microfossil biota suggest stratification of the water column and low oxygen bottom water conditions in the earliest Eocene. Formation of authigenic carbonate through microbial diagenesis produced an unusually large bulk carbon isotope shift, while the magnitude of the corresponding signal from benthic foraminifera is similar to that at other marine sites. This proves that the landward increase in the magnitude of the carbon isotope excursion measured in bulk sediment is not due to a near instantaneous release of 12C-enriched CO2. We conclude that the MCBR site records nearshore marine response to global climate change that can be used as an analog for modern coastal response to global warming.

Identifying autogenic sedimentation in fluvial-deltaic stratigraphy: Evaluating the effect of outcrop-quality data on the compensation statistic: Identifying Autogenic Sedimentation

Stratigraphy preserves an extensive record of Earth-surface dynamics acting over a range of scales in a variety of environments. To take advantage of this record, we first must distinguish depositional patterns that arise due to intrinsic (i.e., autogenic) landscape dynamics from sedimentation that results from changes in climate, tectonic, or eustatic boundary conditions. The compensation statistic is a quantitative tool that has been used to estimate scales and patterns of autogenic sedimentation in experimental deposits; it has been applied to a few outcrop studies, but its sensitivity to data limitations common in natural deposits remains unconstrained. To explore how the compensation statistic may be applied to outcrop data, we evaluate the sensitivity of the tool to stratigraphic data sets limited in extent and resolution by subsampling an autogenic experimental deposit to create pseudo-outcrop-scale data sets. Results show that for data sets more than 3 times thicker than a characteristic depositional element (e.g., channel or lobe), the compensation statistics that can be used reliably constrain the maximum scale of autogenic sedimentation even for low-resolution data sets. Additionally, we show that autogenic sedimentation patterns may be characterized as persistent, random, or compensational using the compensation statistic when data sets are high resolution. We demonstrate how these measurements can be applied to natural data sets with comparative case studies of two fluvial and two deltaic outcrops. These case studies show how the compensation statistic can provide insight into what controls the maximum scale of autogenic sedimentation in different systems and how landscape dynamics can produce organized sedimentation patterns over long time scales.