Kyle M. Straub

Interactions between turbidity currents and topography in aggrading sinuous submarine channels: A laboratory study

We present results from a laboratory experiment documenting the evolution of a sinuous channel form via sedimentation from 24 turbidity currents having constant initial conditions. The initial channel had a sinuosity of 1.32, a wavelength of 1.95, an amplitude of 0.39 m, and three bends. All currents had a densimetric Froude number of 0.53 and an initial height equal to the channel relief at the start of the experiment. Large superelevation of currents was observed at bend apexes. This superelevation was 85%–142% greater than the value predicted by a balance of centrifugal and pressure-gradient forces. An additional contribution to the superelevation was the runup of the current onto the outer banks of bends. This runup height is described by a balance between kinetic and potential energy. Runup resulted in deposition of coarse particles on levee crests that were indistinguishable from those deposited on the channel bottom. Deposit thickness and composition showed a strong cross-channel asymmetry. Thicker, coarser, steeper levees grew on the outer banks relative to the inner banks of bends. Zones of flow separation were observed downstream from bend apexes along inner banks and affected sedimentation patterns. Sedimentation from currents caused the channel to aggrade with almost no change in planform. However, channel relief decreased throughout the experiment because deposition on the channel bottom always exceeded deposition at levee crests. The first bend served as a filter for the properties of the channelized current, bringing discharge at the channel entrance into agreement with the channel cross-sectional area. Excess discharge exited the channel at this filtering bend and was lost to the overbank surface.

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.

Rapid and widespread response of the Lower Mississippi River to eustatic forcing during the last glacial-interglacial cycle

The Lower Mississippi Valley provides an exceptional fi eld example for studying the response of a continental-scale alluvial system to upstream and downstream forcing associated with the large, orbitally controlled glacialinter glacial cycles of the late Quaternary. However, the lack of a numerical chronology for the widespread Pleistocene strata assemblage known as the Prairie Complex, which borders the Holocene fl oodplain of the Lower Mississippi River, has so far precluded such an analysis. Here, we apply optically stimulated luminescence (OSL) dating, mainly on silt-sized quartz from Prairie Complex strata. In total, 27 OSL ages indicate that the Prairie Complex consists of multiple allostratigraphic units that formed mainly during marine isotope stages 7, 5e, and 5a. Thus, the aggradation of the Prairie Complex is strongly correlated with the sea-level highstands of the last two glacialinterglacial cycles. Fluvial incision during the sea-level fall associated with the MIS 5a–MIS 4 transition extended as far inland as ~600 km from the present-day shoreline, testifying to the dominant downstream control of fl stratigraphic architecture in the Lower Mississippi Valley. In addition, the short reaction time of the Lower Mississippi River suggests that large fl uvial systems can respond much more rapidly to allogenic forcing than is commonly believed.

Storage thresholds for relative sea-level signals in the stratigraphic record

The tug of relative sea level (RSL), set by climate and tectonics, is widely viewed as the most important boundary condition for the evolution of deltas. However, the range of amplitudes and periodicities of RSL cycles stored in deltaic stratigraphy remains unknown. Experimental results presented here suggest that extraction of RSL cycles from the physical stratigraphic record requires their magnitudes and periodicities to be greater than the spatial and temporal scales of the internal (autogenic) dynamics of deltas. These results predict stratigraphic storage of information pertaining to RSL cycles during icehouse Earth conditions. However, these thresholds commonly overlap with the magnitudes and periodicities of RSL cycles for major river deltas during greenhouse Earth conditions, suggesting stratigraphic signal shredding. This theory suggests quantitative limits on the range of paleo-RSL information that can be extracted from stratigraphy, which could aid the prediction of deltaic response to climate change.

Connecting the backwater hydraulics of coastal rivers to fluvio-deltaic sedimentology and stratigraphy

Fluvial channels encounter a backwater reach when they approach a standing body of water, and recent studies have shown that the transition from normal flow to backwater-influenced flow is associated with sediment mass extraction through deposition. Here we test the hypothesis that systematic changes in the geometry of channel-belt deposits and sedimentary architecture occur across this transition, using data from the late Holocene Mississippi (southern USA) and Rhine (The Netherlands) fluvio-deltaic systems. We use the estimated backwater length and average channel width as characteristic length scales to non-dimensionalize the downstream trends in channel-belt width for these systems. The collapsed data follow similar trends, suggesting that the observed variations in channel-belt geometry and fluvio-deltaic stratigraphy are tied to the location of the backwater transition zone. These findings suggest a unifying hydraulic control on fluvio-deltaic channel belts and provide a new framework for predicting and understanding the properties of ancient rivers in the coastal zone.

Influence of sediment cohesion on deltaic shoreline dynamics and bulk sediment retention: A laboratory study

While boundary and forcing conditions influence the average location of a shoreline in deltaic systems, internal morphodynamics can drive high-magnitude deviations from the long-term trend. Here we explore the role of sediment cohesion on these morphodynamics using physical experiments. Specifically, we explore the role of sediment cohesion on the scales of autogenic shoreline transgressions and regressions. Results indicate that sediment cohesion enhances the time and space scales associated with autogenic cycles of channel formation, elongation, and abandonment. In systems with high sediment cohesion, this cycle can drive shoreline transgressions that produce flooding surfaces in the resulting stratigraphy which could be confused with surfaces produced by increases in sea level rise or subsidence rates. Enhanced channelization resulting from sediment cohesion also increases the pumping of fine-grained sediment into the marine realm, where it can bypass the delta foreset, thus decreasing total delta sediment retention rate.

Autogenic geomorphic processes determine the resolution and fidelity of terrestrial paleoclimate records

Mesoscale geomorphic processes impose regularity in deposition, allowing quantitative resolution of proxy-based climate reconstructions. Terrestrial paleoclimate records rely on proxies hosted in alluvial strata whose beds are deposited by unsteady and nonlinear geomorphic processes. It is broadly assumed that this renders the resultant time series of terrestrial paleoclimatic variability noisy and incomplete. We evaluate this assumption using a model of oscillating climate and the precise topographic evolution of an experimental alluvial system. We find that geomorphic stochasticity can create aliasing in the time series and spurious climate signals, but these issues are eliminated when the period of climate oscillation is longer than a key time scale of internal dynamics in the geomorphic system. This emergent autogenic geomorphic behavior imparts regularity to deposition and represents a natural discretization interval of the continuous climate signal. We propose that this time scale in nature could be in excess of 104 years but would still allow assessments of the rates of climate change at resolutions finer than the existing age model techniques in isolation.

Geomorphic stasis and spatiotemporal scales of stratigraphic completeness

Alluvial stratigraphic records are notoriously incomplete. All stratigraphic sections within sedimentary basins experience varying episodes of erosion and geomorphic stasis during their accumulation. This has detrimental effects on the completeness of paleoclimatic and paleobiologic records. Here we evaluate the resultant stratigraphic incompleteness using a physical experiment with self-organized vertical scales of topography and lateral scales of geomorphic stasis. First, we document how stratigraphic completeness improves as the temporal discretization interval coarsens, and show that the primary cause of the missing time shifts from stasis to erosion as the discretization is coarsened. Second, we demonstrate that the debilitating effect of finer temporal resolution can be predictably offset by compositing records across a wider area, and present a new two-dimensional formulation of stratigraphic completeness. These findings imply systematic shifts in taphonomic preservation, and by extension, the quality of paleobiologic and paleoclimatic proxy records.

Influence of sediment cohesion on deltaic morphodynamics and stratigraphy over basin-filling time scales

Results from physical and numerical experiments suggest that sediment cohesion influences deltaic morphodynamics by promoting the development and maintenance of channels. As a result, cohesion is thought to increase the magnitude and time scales of internally generated (autogenic) processes and the dimensions of their stratigraphic products. We test these hypotheses by examining the surface processes and stratigraphic products from a suite of physical experiments where the influence of cohesion is isolated over temporal and spatial scales important for basin-filling. Given the stochastic nature of autogenic sediment transport processes, we develop and employ a range of statistical tools and metrics. We observe that 1) an increase in sediment cohesion decreases lateral channel mobility and thus increases the time necessary to regrade deltaic surfaces; 2) enhanced channelization, due to sediment cohesion, increases the time necessary for the deposits of autogenic processes to average together and produce stratigraphic products with shapes set by the generation of regional accommodation; 3) cohesion promotes the transport of suspended sediment to terrestrial overbank and marine environments, which decreases the volume of channel, relative to overbank and marine deposits in the stratigraphic record. This increase in overbank and marine deposition changes the spatial distribution of sand in stratigraphy, with higher cohesion linked to enhanced segregation of fine particles from coarse sand in the experimental deposits. Combined, these results illustrate how the cohesion of sediment is fundamental in setting autogenic spatial and temporal scales and needs to be considered when inverting stratigraphic architecture for paleo-environmental history.

Turbidity Current Flow Out of Channels And Its Contribution to Constructing the Continental Slope

We combine analysis of shallow seismic data from industry grade 3D volumes with results from physical models that resolve channel-to-overbank sedimentation by turbidity currents to explore how regional surfaces are constructed by unconfined flows. Depositional patterns measured from seismic and laboratory data are used to define properties of proximal versus distal overbank sedimentation. Both data sets reveal a significant drop in variability of depositional thickness associated with the transition to distal sedimentation. This drop in standard deviation is a useful metric for defining the transition from levee to distal overbank deposits.