Lincoln F. Pratson

Submarine landslide geomorphology, US continental slope

Abstract The morphometric analysis of submarine landslides in four distinctly different tectonic environments on the continental slopes of Oregon, central California, Texas, and New Jersey provides useful insight into submarine process, including sediment transport mechanisms and slope stability. Using Geographic Information System (GIS) software, we identify landslides from multibeam bathymetric and GLORIA sidescan surveys based solely on surficial morphology and reflectivity. This method provides useful data in a time- and cost-efficient manner. We measure various aspects of the failures, including landslide area, runout distance, and headscarp height, along with the slope gradient of the runout zone, the failures scar, headscarp, and adjacent slopes. The largest failures of the four study areas occur in the Gulf of Mexico, adjacent to Mississippi Canyon, and between salt withdrawal basins. Smaller landslides occur within the basins, and at the base of the Sigsbee Escarpment. These smaller landslides tend to have higher headscarps than the larger ones, and often have cohesive material at the base, suggesting a stronger rheology. Oregon has the steepest local slopes, but surprisingly few large failures for a seismically active margin (especially in the north), implying that slope angle and seismic activity may not be the most important slope stability controls. The California continental slope is heavily incised, which makes failure isolation difficult. Most of the landslides occur within the larger canyons (Vizcaino, Pioneer, Monterey) and adjacent to a pock mark field in the Point Arena basin. The majority of landslides offshore New Jersey occur on the open slope between Lindenkohl and Carteret Canyons. Morphometric statistics give us insight into where mass movements occur, how big they are likely to be, their relative importance as sediment transport mechanisms, and the overall slope stability of a given margin. Most landslides occur on slopes less than 10°. Curiously, the steepness of the slope adjacent to the failure tends to be inversely proportional to the runout length. In both California and Oregon, slope failures tend to make the local slope steeper, whereas failures in the Gulf of Mexico and offshore New Jersey will tend to make the local slope less steep. Landslides with rubble beneath the scar are mostly smaller than those without, are deep seated, and make the slope steeper. We use the ratio of headscarp height to runout length as a measure of the failures dynamic rheology. This ratio in the submarine case is orders of magnitude less than subaerial landslides. Hydroplaning of the failed mass may be responsible for the very long runout lengths. These morphometric relationships give us important insight into landslide dynamics and process in different sedimentary and tectonic environments.

Submarine canyon initiation by downslope-eroding sediment flows: Evidence in late Cenozoic strata on the New Jersey continental slope

Multibeam bathymetry and seismic reflection profiles of the New Jersey continental slope reveal a series of abandoned and now-buried submarine canyons that have apparently influenced the development of modern canyons. The buried canyons are infilled along nine slope-wide unconformities separating upper-middle Miocene to Pleistocene sediments that thin downslope. Canyons infilled during the Miocene occur in the southwest part of the study area where Miocene sediments are thickest. Other canyons, infilled during the Pleistocene, occur in the northeast part of the study area where Pleistocene sediments are thickest. When followed downslope, each of the buried canyons arrives at a confluence with a modern canyon, usually where the downslope-tapering sediment cover has failed to smooth over the buried canyon, leaving a sea-floor trough. Sea-ward of the confluences, the modern canyons have exhumed the buried canyons and use the older valleys to reach the base of the slope. Re-use of the lower slope reach of the buried canyons appears to have begun when the sea-floor troughs over the buried canyons captured sediment flows initiated along the upper slope and shelf break and confined them to follow the former path of the buried canyons to the base of the slope. The downslope erosion caused by the sediment.flows is proposed to have initiated the modern canyons, which eventually excavated and deepened the former routes of the buried canyons seaward of the sites of sediment flow capture. The occurrence of buried canyons where strata thickens alongslope suggests that infilling of the buried canyons occurred seaward of shelf-edge depocenters. The heightened sediment input to the slope in these regions may have also led to the initiation and growth of modern-day canyons. The temporal relation between modern canyon formation, sediment supply, and sea level, however, remains to be established.

A model for the headward erosion of submarine canyons induced by downslope-eroding sediment flows

A simple, physically based computer model of continental slope evolution is used to investigate the sequence of submarine canyon formation. The model simulates submarine canyons as evolving under the influence of sedimentation, slope failure, sediment flow erosion, and topography. Interactions between these factors are modeled as being governed by local sea-floor slope, which in the model determines the extent of sea-floor failures, directs the downslope path of sediment flows triggered by the failures, and scales the amount of sea-floor erosion caused by the sediment flows. Based on these interactions, the model simulates a three-stage sequence for submarine canyon formation: (1) the erosion of pre-canyon rills by sediment flows initiated at sites on the upper slope oversteepened by sedimentation; (2) localized slope failure of the walls and/or floor of the rills at one or more mid- to lower-slope sites destabilized by sediment flow erosion; and (3) evolution of the failure into a headward-eroding canyon that advances upslope along the rills by sediment-flow-driven retrogressive failure. Through this sequence, the model simulates canyon and intercanyon morphology that successfully reproduces crosscutting relations observed between Lindenkohl Canyon and adjacent erosional slope rills on the passive-margin New Jersey continental slope, and between slope failures and long, narrow dendritic tributaries that enter into the Aoga Shima Canyon on the convergent-margin Izu-Bonin fore arc. These results suggest that the model may be applicable in explaining submarine canyon formation along a variety of continental margins. More significantly, in illustrating how sediment flows might repeatedly trigger retrogressive failures, the model presents a new explanation for submarine canyon formation that reconciles morphologic evidence for headward canyon erosion by mass wasting with the stratigraphic evidence for canyon inception by downslope-eroding sediment flows.

Clinoform development by advection-diffusion of suspended sediment: Modeling and comparison to natural systems

Clinoforms are the building blocks of prograding stratigraphic sequences. These sig- moid-shaped surfaces can be found forming today on modem deltas. Sedimentation rate profiles over the clinoform surface of these deltas show low rates of sediment accumulation on both topset and bottomset regions, with a maximum accumulation rate on the upper foreset region. We pres- ent a model for the formation of clinoforms that relies on the interpretation of modem clinoform sedimentation as a result of the distribution of shear stresses at the mouth of a river. Model clino- form surfaces are generated using an equation for the conservation of suspended sediment concentration, together with a conservation of fluid equation for simple time-averaged flow velocity fields. In the model, suspended sediment is advected horizontally into a basin, and gravitational settling of sediment particles is counteracted by vertical turbulent diffusion. In shallow water, shear stresses are too large to allow deposition, and sediment bypasses the topset region. With increasing water depth, near-bed shear stresses decrease, and sediment is allowed to deposit at the foreset region, with gradually decreasing rates toward deeper water. This sedimentation pattern leads to progradation of the clinoform surfaces through time. The clinoform surfaces produced by the model capture the fundamental morphological characteristics of natural clinoforms. These include the gradual slope rollover at the topset and bottomset, steeper foreset slopes with increased grain size, and an increase in foreset slope through time as clinoforms prograde into deeper water. Because the parameters controlling the model clinoforms have a direct relation to physical quantities that can be measured in natural systems, the model is an important step toward unraveling the physical processes associated with these deposits.

Sustainable Product Indexing: Navigating the Challenge of Ecolabeling

There is growing scientific evidence that improving the sustainability of consumer products can lead to significant gains in global sustainability. Historically, environmental policy has been managed by bureaucracies and institutions in a mechanistic manner; this had led to many early successes. However, we believe that if policy concerning product sustainability is also managed in this way, negative unintended consequences are likely to occur. Thus, we propose a social-ecological systems approach to policy making concerning product sustainability that will lead to more rapid and meaningful progress toward improving the environmental and social impacts of consumer products.

Source of the great tsunami of 1 April 1946: a landslide in the upper Aleutian forearc

Abstract The Unimak (eastern Aleutians) earthquake of April 1, 1946 is an enigma. The earthquake ( M S =7.1) produced a disproportionately large tsunami ( M t =9.3) which killed 167 people. The tsunami was highly directional, and projected its largest waves along a beam perpendicular to the Aleutian arc. Those waves passed just east of the Hawaiian Islands, ran the length of the Pacific, and were still large when they ran ashore in Antarctica. In the near field, the tsunami had very high runup (42 m at Scotch Cap) but rapid lateral decay (6 m at Sanak Village, 120 km to the east). No earthquake source can simultaneously explain the narrow beam of large waves in the far field and the rapid variation in near-source runup. The slow rupture, the tsunami directivity, the rapid variation in near-source wave heights, the period of the waves, and the strong T -phase generation, together suggest an earthquake-triggered landslide rather than a purely tectonic source. From USGS GLORIA imagery we have identified a candidate landslide within the aftershock zone of 1946. The slide bites into the Aleutian shelf at a depth of only 120 m, is 25 km across, 65 km long, and has a volume of 200–300 km 3 . A slide with these dimensions would produce a tsunami matching the observations while still satisfying the seismic data. Such slope failures appear to be common along the Aleutian forearc, which has serious implications for tsunami warning.

The length‐scaling properties of topography

The scaling properties of synthetic topographic surfaces and digital elevation models (DEMs) of topography are examined by analyzing their “structure functions,” i.e., the qth order powers of the absolute elevation differences: Δhq(l) = E{|h(x + l) - h(x)|q}. We find that the relation Δh1(l) ≈clH describes well the scaling behavior of natural topographic surfaces, as represented by DEMs gridded at 3 arc sec. Average values of the scaling exponent H between ∼0.5 and 0.7 characterize DEMs from Ethiopia, Saudi Arabia, and Somalia over 3 orders of magnitude range in length scale l (∼0.1–150 km). Differences in apparent topographic roughness among the three areas most likely reflect differences in the amplitude factor c. Separate determination of scaling properties in the x and y coordinate directions allows us to assess whether scaling exponents are azimuthally dependent (anisotropic) or whether they are isotropic while the surface itself is anisotropic over a restricted range of length scale. We explore ways to determine whether topographic surfaces are characterized by simple or multiscaling properties. The difference between scaling exponents of Δh1(l) and Δh2(l) for the DEMs is small, but positive, and such divergence in the scaling exponents of the structure functions is consistent with multiscaling behavior. Exceedance and perimeter sets of fractional Brownian surfaces fail to yield the trivial fractal dimensions expected of sets of known monofractals, suggesting a practical limitation arising from the use of finite resolution data sets in the analysis. By comparing the hypsometry of “real” topography (represented as DEMs) with that of fractional Brownian surfaces, we show that synthetic surfaces based on Gaussian statistics are limited as models for natural topography. Hypsometric curves, which probably reflect the relative importance of tectonic and erosional processes in shaping topography, clearly show that statistical moments higher than the second are important in describing topographic surfaces. Scaling analysis is a valuable tool for assessing the quality and accuracy of DEM representations of the Earths topography.

Categorizing the morphologic variability of siliciclastic passive continental margins

The basic morphology of the continental shelf, slope, and rise on passive continental margins has many variations in the modern ocean environment. Through using a satellite-derived bathymetric data set of the global ocean, we have developed a classification for the shape of modern, siliciclastic-dominated, passive margins. The classification subdivides 50 margins into five distinct shape categories based on each margins pattern of sea-floor slopes as a function of depth. By comparing these shape categories with the underlying stratal architecture, sediment input, and present degree of canyon incision of the margins, we find apparent correlations between these factors and basic margin shape. In general, gently sloped margins tend to occur in regions with high sediment input, few modern canyons, and unstable substrates. Higher gradient margins tend to have lower sediment input, more modern canyons, and a variety of subsurface architectures. These results suggest two important conclusions: (1) morphology differences among modern siliciclastic passive margins can be objectively and systematically categorized, and (2) these shape differences seem to be related to aspects of the modern sedimentary environment despite the fact that the margins have evolved over geologic time.

Exponential approximations to compacted sediment porosity profiles

Sediment compaction and corresponding porosity variations can be modeled by a simple exponential with depth. The porosity solution is derived analytically as a complicated function of pore water pressure, but the underlying form is shown to approximate an exponential except near the surface where the behavior is linear. Even though the analytical simplifications ignore some of the detailed effects of sediment lithology, a large world-wide collection of porosity data from compacted shales, silts and sandstones supports the general exponential trend. For comparison, a numerical compaction routine (with fewer assumptions than the analytical solution) is used in conjunction with a previously developed process-based model of marine sedimentation. Together, the compaction and sedimentation models show that in a realistic depositional environment, porosity varies exponentially with depth, in agreement with both the empirical data and the analytical approximation.

The relative importance of gravity-induced versus current-controlled sedimentation during the Quaternary along the Mideast U.S. outer continental margin revealed by 3.5 kHz echo character

Abstract A detailed echo-character map based on more than 50,000 km of 3.5 kHz echograms reveals the relative importance of contour currents, turbidity currents and mass-wasting processes for the upper 50–100 m of sea-floor sediments along the U.S. continental margin between Southern Georges Bank and Cape Hatteras. Nineteen individual echo types are mapped and interpreted as deposits produced either by current-controlled or gravity-induced processes based on echo-type distribution, analysis of selected sediment core samples, and the published results of other workers. Turbidity currents and mass wasting have apparently been the primary depositional agents throughout 60% of the region. In contrast, contour currents alone appear to have influenced only 10% of the region. However, combined processes, i.e., non-separable interaction between downslope and parallel-to-contour processes have influenced an estimated 25% of the margin. The remaining 5% of the study area constitutes the outermost continental shelf; echo types here reflect the influence of fluvial-deltaic and shallow marine sedimentary processes. Changes in the relative distributions of various echo types with sea-floor depth indicate that the dominant sedimentary processes change downslope and are significantly different within each physiographic province. Deposits formed by mass wasting dominate the continental slope where 50% of the sea floor is eroded and gullied by slides, slumps and the heads of debris flows. Deposits developed under the influence of combined processes are most wide-spread on the upper continental rise and cover 40% of the sea floor, primarily in the form of deep-sea channel levees modified by contour currents. On the lower continental rise, turbidites form more than 50% of the sea-floor deposits. The downslope deposits (turbidity currents, slumps, debris flows, etc.) were emplaced primarily during the Pleistocene. Glacioeustatic sea-level lowering moved beaches and rivers across the exposed continental shelf to the shelf edge, where massive fluxes of terrigenous sediment could be delivered to the continental slope and rise by turbidity currents. Significant amounts of this sediment were transported through submarine canyons and leveed, deep-sea channels to the lower continental rise and formed the lower continental rise terrace, a flat turbidite basin ponded behind the Hatteras Outer Ridge. Southwest of the Wilmington Channel, this glacially derived influx was diminished; however, depositional patterns indicate that downslope processes continued to be dominant at least as far south as Cape Hatteras. Where rapid deposition on the continental slope and uppermost continental rise created instabilities, slope failure produced slumps, slides and debris flows and provided an additional, volumetrically important redistribution of sediment downslope. The influence of contour currents is clearly observed only in those areas topographically shielded from the masking effects of downslope processes, in particular along the seaward side of the buried Chesapeake Drift and the Hatteras Outer Ridge.