Continental-slope instability triggered by seepage: An experimental approach

2020 ◽  
Vol 90 (8) ◽  
pp. 921-937
Author(s):  
Carolina H. Boffo ◽  
Tiago A. de Oliveira ◽  
Daniel Bayer da Silva ◽  
Rafael Manica ◽  
Ana Luiza de O. Borges
Keyword(s):  
Mass Transport ◽  
Sea Level ◽  
Continental Slope ◽  
Physical Simulation ◽  
Slope Instability ◽  
Shear Stresses ◽  
Relative Sea Level ◽  
Hypothetical Scenario ◽  
Experimental Apparatus ◽  
Continental Slopes

ABSTRACT Mass-transport complexes (MTCs), mass-transport deposits (MTDs), and associated facies and features are widely recognized in continental slopes around the world. In most current stratigraphic models of MTCs and MTDs, these submarine sediment failures are related to aquifer outflow (sapping, seepage) along continental slope fronts that originated during relative sea-level fall. We test a hypothetical scenario that is favored during early forced regression using reduced-scale physical simulation. A major underground subaerial hydraulic gradient is assumed to flow towards the basin depocenter as a function of relative sea-level fall. We developed an experimental apparatus with slope angles varying between 15 and 30° to test this concept. Hydraulic gradients, aquifer outflow velocities, and triggered collapses induced by the seepage effect were recorded at various positions of the slope. Analysis shows that steeper slope gradients require lower seepage velocities (and shear stresses) to trigger collapse, but gentler slopes remain unchanged. Experimental data are compatible with a seepage effect that could potentially trigger mass failure and the formation of MTCs during relative sea-level fall. The features produced in the experiment have geometries comparable to natural environments, and the experimental seepage velocities are of an order of magnitude similar to those monitored in submarine aquifers. The experimental results advance understanding of mass transport in continental slopes by introducing and testing new methods, and also provide new insights into potential submarine geohazard risks where tectonic uplift operates along some coastal regions.

Continental slope sedimentation in the Sheepbed Formation (Neoproterozoic, Windermere Supergroup), Mackenzie Mountains, N.W.T.

1996 ◽  
Vol 33 (6) ◽  
pp. 848-862 ◽  
Author(s):  
R. W. Dalrymple ◽  
G. M. Narbonne
Keyword(s):  
Sea Level ◽  
Continental Slope ◽  
Cold Water ◽  
Passive Margin ◽  
Slope Instability ◽  
Fine Grained ◽  
Windermere Supergroup ◽  
Mackenzie Mountains ◽  
Water Depths

The Sheepbed Formation (Ediacaran) is a 1 km thick, siliciclastic unit that overlies glacial deposits of the Ice Brook Formation and is overlain by carbonates of the Gametrail Formation. Observations in the Mackenzie Mountains indicate that the Sheepbed Formation accumulated in water depths of 1–1.5 km on a passive-margin, continental slope. The lower part of the formation consists predominately of dark mudstone. Fine-grained, turbiditic sandstone becomes more abundant upward, as does the scale and abundance of slope-instability indicators. Mesoscale facies successions (i.e., evidence of channels, lobes, and (or) compensation cycles) are developed in the upper half of the formation. The larger-scale changes are interpreted as reflecting a postglacial sea-level rise, followed by a relative fall and an increase in the rate of deposition. Contourites that may have been formed in response to the circulation of deep, cold water occur in the lowstand deposits. Their presence confirms previous speculation that the proto-Pacific Ocean was initiated at the beginning of Windermere deposition (ca. 780 Ma), not at the start of the Cambrian. The paleoflow direction toward the present-day northwest suggests that this part of Laurentia lay in the northern hemisphere. In situ Ediacaran megafossils are preserved on the soles of sandy turbidites; the deep-water setting indicates that these organisms were not photoautotrophs.

Mass-transport deposits and the onset of wedge-top basin development: An example from the Dinaric Foreland Basin, Croatia

2020 ◽  
Vol 90 (11) ◽  
pp. 1527-1548
Author(s):  
Katarina Gobo ◽  
Ervin Mrinjek ◽  
Vlasta Ćosović
Keyword(s):  
Mass Transport ◽  
Sea Level ◽  
Regional Scale ◽  
Foreland Basin ◽  
Combined Effect ◽  
Tectonic Activity ◽  
Relative Sea Level ◽  
Basin Development ◽  
Blind Thrust ◽  
Mass Transport Deposits

ABSTRACT Mass-transport deposits (MTDs) represent resedimentation phenomena triggered by the combined effect of seismic shocks of regional scale, structural tilting, basin-floor gradient, relative sea-level fluctuations, and/or excess pore-water pressure and can be useful in the reconstruction of basin development dynamics. The present study from the Dinaric Foreland Basin in Croatia documents several limestone blocks (olistoliths), carbonate debris, and associated bipartite carbonate megabeds as MTDs of exotic origin encased in deep neritic hyperpycnites, referred to as host deposits. Detailed facies and micropaleontological analyses indicate that host deposits were sourced from a fluvio-deltaic system located in the proximity of the uplifting orogen, while the MTDs originated from gravitational collapses of late Ypresian and early Lutetian limestones that were uplifted on blind-thrust anticline ridges on the opposite side of the basin. Mass wasting-produced carbonate blocks, debris, and gravity flows were probably triggered concurrently during the middle to late Eocene, but the blocks could have travelled faster downslope due to the lubricating effect of the underlying water “cushion,” overpressured mud, and the pull of gravity. Debrisflows and co-genetic turbidity currents that contributed to the formation of bipartite megabeds were likely mobilized deeper and moved slower than the carbonate blocks and could have been partly deflected by the previously emplaced olistoliths, resulting in megabed thinning along the olistoliths' down-dip edges. Those collapses were most likely triggered by the combined effect of relative sea-level changes associated with tectonic activity and seismic shocks of regional scale. The study suggests that progressive uplift of the frontal blind-thrust anticline ridge resulted in episodic emergence and collapses of progressively older limestone units, and marked the onset of development of the wedge-top basin. Conceptual models of olistolith emplacement and onset of basin development are suggested and may be applicable to both ancient and recent settings. The insights obtained from the integration of detailed facies analysis and micropaleontology may be useful in similar areas where such a level of detail cannot be obtained by conventional field methods.

Modelling salt-marsh vegetation dynamics through species dispersal and competition

2021 ◽  
Author(s):  
Alvise Finotello ◽  
Enrico Bertuzzo ◽  
Andrea D'Alpaos ◽  
Marco Marani
Keyword(s):  
Salt Marsh ◽  
Sea Level ◽  
Habitat Quality ◽  
Salt Marshes ◽  
Vegetation Dynamics ◽  
Vertical Direction ◽  
Tidal Marshes ◽  
Spatially Explicit ◽  
Shear Stresses ◽  
Relative Sea Level

<p>Salt marshes are widespread morphological features in coastal and estuarine tidal landscapes, and are ecologically and economically important as they significantly contribute to coastal primary production, support high biodiversity, and provide a broad range of valuable ecosystem services.</p><p>The ability of salt marshes to counteract changes in external forcings depends on the complex dynamic interactions between physical and biological processes acting at different spatial and temporal scales. In particular, the evolution of tidal marshes in the vertical direction results from the balance and feedbacks between organic and inorganic deposition, erosion, and changes in relative sea level. For example, colonization of salt marsh platforms by halophytic vegetation enhances both organic and inorganic deposition due to increased flow resistance, reduced bottom shear stresses, capture of sediment particles by plant stems, and direct biomass accumulation. Moreover, halophytes control soil aeration, which feeds back into vegetation zonation and the related biogeomorphic interactions typically observed in tidal marshes.</p><p>In spite of their importance, however, modeling vegetation dynamics in intertidal marshes remains a major challenge both at the theoretical and practical/numerical level. Improving our current understandings of the mechanisms that drive the zonation of halophytic species is of critical importance to enhance projections of salt-marsh response to changes in climate and relative sea level.</p><p>Here we present a new bi-dimensional, spatially explicit ecological model aimed to simulate the spatial dynamics of halophytic vegetation in tidal saline wetlands. Vegetation dynamics are treated differently compared to previous models, which employed relatively simple deterministic or probabilistic mechanisms, dictated only by the ability of different species to adapt to different topographic elevations. In our model, in contrast, spatial vegetation dynamics depend not only on the local habitat quality, but also on spatially explicit mechanism of dispersal and competition among multiple, potentially interacting species. The temporal evolution of vegetation biomass at each site depends on death and colonization processes, both local and resulting from dispersal. These processes are modulated for each species by the habitat quality of the considered site. The latter is synthesized only through the local elevation relative to the mean sea level, and is mathematically modeled using a logistic function that represents the theoretical niche of each considered species.</p><p>Results indicate that such a relatively simple model, where species have elevation-dependent fitness and otherwise neutral traits, can predict realistic diversity and species-richness patterns. More importantly, the model is also able to effectively reproduce the outcome of classical ecological experiments, in which a species is transplanted to an area outside its optimal (realized) niche. A direct comparison clearly shows how previous models not accounting for dispersal and interspecific competitions are unable to reproduce such dynamics.</p>

scholarly journals Late Cenozoic shelf delta development and Mass Transport Deposits in the Dutch offshore area – results of 3D seismic interpretation

2012 ◽  
Vol 91 (4) ◽  
pp. 591-608 ◽  
Author(s):  
A. Benvenuti ◽  
H. Kombrink ◽  
J.H. ten Veen ◽  
D.K. Munsterman ◽  
F. Bardi ◽  
...  
Keyword(s):  
Mass Transport ◽  
Sea Level ◽  
North Sea ◽  
Sediment Supply ◽  
Slope Instability ◽  
Shear Surface ◽  
Southern North Sea ◽  
Basal Shear ◽  
Mass Transport Deposits ◽  
High Sediment

AbstractIn this study, seismic stratigraphic criteria have been used to characterise the evolution of the Southern North Sea (SNS) shelf-delta system that progressively filled the Southern North Sea basin during Plio-Pleistocene times. Based on the prograding and down-stepping architecture of the shelf-delta sequence it is inferred that deposition occurred during a time of high sediment supply and overall sea-level lowering. During this time the delta slopes failed several times, creating at least 30 internally coherent Mass Transport Deposits (MTDs) mainly grouped in common areas, affecting the same clinoform set and partially sharing the basal shear surface (groups of MTDs). The most important features of the studied MTDs are 1) the dominance of brittle deformation; 2) the small amount of material removal from the headwall domain (lack of completely depleted areas above the basal shear surface); and 3) the lack of an emergent toe domain above the un-failed sediment located basinward, although proper confining geometries for the MTD are not detected. Therefore, the studied MTDs can neither be classified as frontally confined nor as frontally emergent but they are a new intermediate type of submarine landslides which has not been described before. These characteristics suggest that the mass movement ceased relatively soon after initiation of failure. Incisions on top of the MTDs suggest the presence of erosive flows. These flows were probably generated due to a concentration of the drainage in the negative morphology the failure event left behind in the upper sector of the slope. The stronger progradational character of the reflections on top of MTDs confirms a concentration of drainage after the erosional phase too.The interplay between high sediment supply and constant or even decreasing accommodation space (caused by constant or decreasing sea-level) is supposed to be the main precondition for slope instability for most of the MTDs in this study area. Slope failures themselves can also be considered a preconditioning factor by the creation of local very high sedimentation rates (see groups of MTDs). Salt-induced seismicity and storm waves' effect superimposed on high frequency sea level fall are considered the most important triggering factors.

Submergence, nutrient enrichment, and tropical storm impacts on Spartina alterniflora in the microtidal northern Gulf of Mexico

2020 ◽  
Vol 644 ◽  
pp. 33-45
Author(s):  
JM Hill ◽  
PS Petraitis ◽  
KL Heck
Keyword(s):  
Gulf Of Mexico ◽  
Sea Level Rise ◽  
Sea Level ◽  
Spartina Alterniflora ◽  
Anthropogenic Impacts ◽  
Interactive Effects ◽  
Relative Sea Level ◽  
Hurricane Disturbance ◽  
Submergence Stress ◽  
Relative Sea Level Rise

Salt marshes face chronic anthropogenic impacts such as relative sea level rise and eutrophication, as well as acute disturbances from tropical storms that can affect the productivity of these important communities. However, it is not well understood how marshes already subjected to eutrophication and sea level rise will respond to added effects of episodic storms such as hurricanes. We examined the interactive effects of nutrient addition, sea level rise, and a hurricane on the growth, biomass accumulation, and resilience of the saltmarsh cordgrass Spartina alterniflora in the Gulf of Mexico. In a microtidal marsh, we manipulated nutrient levels and submergence using marsh organs in which cordgrasses were planted at differing intertidal elevations and measured the impacts of Hurricane Isaac, which occurred during the experiment. Prior to the hurricane, grasses at intermediate and high elevations increased in abundance. After the hurricane, all treatments lost approximately 50% of their shoots, demonstrating that added nutrients and elevation did not provide resistance to hurricane disturbance. At the end of the experiment, only the highest elevations had been resilient to the hurricane, with increased above- and belowground growth. Added nutrients provided a modest increase in above- and belowground growth, but only at the highest elevations, suggesting that only elevation will enhance resilience to hurricane disturbance. These results empirically demonstrate that S. alterniflora in microtidal locations already subjected to submergence stress is less able to recover from storm disturbance and suggests we may be underestimating the loss of northern Gulf Coast marshes due to relative sea level rise.

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