Derek E. Sawyer

Seismic geomorphology, lithology, and evolution of the late Pleistocene Mars-Ursa turbidite region, Mississippi Canyon area, northern Gulf of Mexico

The interplay between sedimentation and erosion during the late Pleistocene in the Mars-Ursa region, northern Gulf of Mexico, resulted in a complex compartmentalized reservoir. Rapid deposition, directly downdip of the Mississippi River beginning about 70 k.y., quickly filled antecedent topography in the Mars-Ursa region with a thick accumulation of sand and mud called the blue unit. This permeable reservoir was rapidly and asymmetrically buried by thick, mud-rich levees of two channel-levee systems. Both systems plunged from north to south with a steeper gradient than the underlying blue unit. Rotated channel-margin slides present in both channel-levee systems rotated low-permeability, mud-rich levee deposits beneath the sand-rich channel fill. As a result of the channel-levee systems, the east-west hydraulic connectivity of the blue unit decreases progressively from north to south until its eastern and western halves become completely separated.

Elevated shear strength of sediments on active margins: Evidence for seismic strengthening

Earthquakes are a primary trigger of submarine landslides, yet some of the most seismically active areas on Earth show a surprisingly low frequency of submarine landslides. Here we show that within the uppermost 100 m below seafloor (mbsf) in previously unfailed sediment, active margins have elevated shear strength by a factor of 2–3 relative to the same interval on passive margins. The elevated shear strength is seen in a global survey of undrained shear strength with depth as well as a normalized analysis that accounts for lithology and stress state. The enhanced shear strength is highest within the uppermost 10 mbsf. These results indicate that large areas of modern day slopes on active margins have enhanced slope stability, which may explain the relative paucity of landslides. These findings lend support to the seismic strengthening hypothesis that the repeated exposure to earthquake energy gradually increases shear strength by shear-induced compaction.

Continuous deep-seated slope failure recycles sediments and limits levee height in submarine channels

Channel-levee systems are responsible for constructing deep sea fans, among the largest sedimentary deposits on Earth. Levee height plays a key role in defining the volume and texture of the material that is deposited in the bounding levees, and thus the morphology of submarine fans. Models of channel formation and evolution generally assume that the levees aggrade in response to the cumulative overspill of turbidity flows, and that their height is controlled by these flows. In contrast, we show that levee growth in the Ursa Basin (Gulf of Mexico) is limited by the mechanical strength of the levee, not the flow behavior. While many studies document sidewall failures in levee systems, our poro-mechanical model is the first to demonstrate that collapse of levees is a large-scale, deep-seated process driven by the interaction of levee formation and high fluid pressure. Rapid deposition of a regional sand unit induced large fluid overpressure in the underlying mud, which preconditioned the system for levee failure, which then fed a large volume of sediment back into the channel-levee system. Long-lived levee failures continually reintroduced previously deposited levee material back into the channel system. This implies that a large volume of sediment is continuously recycled, which has not been previously understood. Turbidite flow models generally assume that flows progressively lose their fine-grained component due to levee overspill as they traverse the channel. In contrast, we show a mechanism by which fine-grained material can re-enter the system in large quantities, and this has significant and broad importance for models of channel and fan evolution. We also show that that levee failure introduces significant unconformities, in contrast with the common assumption that levees offer complete and high-resolution records of climate, tectonics, and sea level.

Ecohydrological disturbances associated with roads: Current knowledge, research needs, and management concerns with reference to the tropics

Las carreteras son una forma generalizada de perturbacion con potencial para afectar negativamente los procesos ecohidrologicos. Algunos de los crecimientos mas rapidos en las redes de carreteras se producen en los paises en desarrollo, particularmente en los tropicos, donde las agendas politicas a menudo se centran en fortalecer la economia, mejorar la infraestructura y reforzar la seguridad nacional., lograr la autosuficiencia y aumentar el bienestar de los ciudadanos, a menudo a expensas del medio ambiente. Revisamos lo que se sabe sobre los impactos de las carreteras en los procesos ecohidrologicos, centrandonos en los sistemas acuaticos, tanto templados como tropicales. Presentamos siete casos que representan las tendencias mas amplias del desarrollo vial y los impactos en entornos tropicales. Muchos de estos procesos dinamicos e impactos no son diferentes de los experimentados en entornos templados, aunque la magnitud de los impactos en los tropicos puede amplificarse con lluvias intensas y la falta de mejores practicas de gestion aplicadas a la construccion / mantenimiento de carreteras. Los impactos de las carreteras en entornos tropicales tambien pueden ser unicos debido a los organismos o ecosistemas afectados en particular. Delineamos unconjunto de mejores practicas para mejorar la gestion de la red vial y proporcionar recomendaciones para adoptar una agenda de investigacion y gestion vial en entornos tropicales. Es importante destacar que pedimos la incorporacion de enfoques transdisciplinarios para estudiar mas a fondo los efectos de las carreteras en los procesos ecohidrologicos en los tropicos. Tambien se debe hacer hincapie en la colaboracion con los gobiernos y los desarrolladores que estan promoviendo el desarrollo vial para ayudar a identificar los impulsores de la expansion de la carretera y los umbrales de impacto negativo, asi como los metodos de construccion y mantenimiento sostenibles de la carretera. Copyright

Submarine landslide and tsunami hazards offshore southern Alaska: Seismic strengthening versus rapid sedimentation

The southern Alaskan offshore margin is prone to submarine landslides and tsunami hazards due to seismically active plate boundaries and extreme sedimentation rates from glacially enhanced mountain erosion. We examine the submarine landslide potential with new shear strength measurements acquired by Integrated Ocean Drilling Program Expedition 341 on the continental slope and Surveyor Fan. These data reveal lower than expected sediment strength. Contrary to other active margins where seismic strengthening enhances slope stability, the high-sedimentation margin offshore southern Alaska behaves like a passive margin from a shear strength perspective. We interpret that seismic strengthening occurs but is offset by high sedimentation rates and overpressure. This conclusion is supported by shear strength outside of the fan that follow an active margin trend. More broadly, seismically active margins with wet-based glaciers are susceptible to submarine landslide hazards because of the combination of high sedimentation rates and earthquake shaking.

Linking Land & Sea: Watershed Evaluation and Mineralogical Distribution of Sediments in Eastern St. John, USVI

Abstract Tropical islands such as St. John in the U.S. Virgin Islands are naturally susceptible to terrigenous (land-based) sediment erosion due to their high-relief slopes, fast weathering rates, and intense precipitation events. Nearshore ecosystems that exist near these islands tend to thrive in static conditions, and are especially stressed by increases in terrigenous input. In the last few decades, island development and population have increased dramatically in some areas of St. John. We conducted a detailed characterization of watersheds and their sediments from ‘source to sink’ in eastern St. John. To accomplish this we combined field observations and sampling with a digital elevation model. Our research was focused on several morphologically similar embayments in eastern St. John; three impacted by anthropogenic development (Coral Harbor, Johnson Bay, and Sanders Bay) and an adjacent, virtually undeveloped bay within the Virgin Islands National Park and Virgin Islands Coral Reef National Monument (Otter Creek). We found a large disparity in upslope watershed size between Otter Creek and Coral Harbor: Otter Creek (0.09 km2) is ∼73× smaller than Coral Harbor (6.54 km2). As expected, watersheds transport terrigenous volcaniclastic sediments directly to the marine environment where shallow-water marine carbonates precipitate. Terrigenous volcaniclastic sediments persist furthest from the source in the basin of the largest watershed with the most development (Coral Harbor), and decay closest to the source in the basin of the smallest watershed with the least development (Otter Creek). Due to large disparities in watershed size, further research is required in order to determine the relative contribution of development on the distribution of terrigenous sediments.

Shear Strength of Siliciclastic Sediments from Passive and Active Margins (0–100 m Below Seafloor): Insights into Seismic Strengthening

Submarine geohazards threaten coastal communities and global economies. Submarine debris flows are the largest mass-wasting events observed on the Earth’s surface, comprising of up to 50 % of basin fill. Further insight can be gained into these important processes by understanding in-situ preconditioning factors that lead to slope destabilization. We examine two locations from the International Ocean Discovery Program data archive to determine how external effects on sediment properties compare between passive margins and active margins. We select representative passive margin (Amazon Fan) and active margin sites (Nankai Trough), and analyse peak shear strength, void ratio, and composition from the uppermost 100 m below seafloor. This depth corresponds to a depth range in which most submarine mass movements originate. However, it is not appropriate to directly compare shear strength and void ratio of samples from different settings due to differing stress histories, sedimentary composition, and consolidation properties. We focus on ideal locations on both margin types that have solely undergone one-dimensional burial, no diagenesis/cementation, and no unroofing. We find that active margin sediments exhibit an increase in shear strength when compared to their passive margin counterparts, while void ratio tends to be higher on active margins. We are currently conducting a focused lab program to better understand compositional effects and determine the intrinsic properties of each site to more definitively normalize the in-situ sediment profiles. Our results suggest a potential link between shear strength and margin seismicity.

Sedimentary filling characteristics of the South China Sea oceanic basin, with links to tectonic activity during and after seafloor spreading

ABSTRACT Based on approximately 11,000 km of seismic reflection data collected across the South China Sea oceanic basin, we describe the sedimentary filling characteristics of the basin since its Oligocene opening, as well as connections between this history and contemporaneous regional tectonic events. The seismic lines are spaced ~50 km apart, and the data are tied to International Ocean Discovery Program (IODP) Expedition 349 drilling data. Basin filling occurred in three phases, with basin-wide mean sedimentation rates increasing through time. During the Oligocene to middle Miocene, sediments accumulated primarily in the northern East and Northwest Sub-basins, with a mean basin-wide sedimentation rate of 8 m/m.y. The presence of these deposits over deep basement floor indicates that seafloor spreading initiated in these northern regions. During the late Miocene, deposition occurred primarily in the Northwest Sub-basin and partly in the southern East Sub-basin, with a mean basin-wide sedimentation rate of 30 m/m.y. Basin filling during this time seems to have been linked to slip reversal of the Red River Fault and collision of the North Palawan Block with the Luzon Arc. During the Pliocene and Pleistocene, sediments accumulated rapidly in the northeastern and southern East Sub-basin and the Southwest Sub-basin. The mean basin-wide sedimentation rate was 70 m/m.y. Basin filling during this phase seems to have been associated with the Taiwan and North Palawan collisions, SCS subduction along the Manila Trench, and Tibetan Plateau uplift. Gravity flow deposits predominate throughout the basin fill.

Deriving the Rate of Salt Rise at the Cape Fear Slide Using New Seismic Data

Recently acquired 2-D seismic data from offshore North Carolina provides images of salt diapirs and landslides in the Carolina Trough that give insight into the interaction between slope sediments and intruding salt from below. The best example of this is the Cape Fear Slide Complex in which at least two salt diapirs are surrounded by the lower headwall of the slide. Here, we present seismic images that were collected for the Eastern North American Margin Community Seismic Experiment. We describe the morphology of the slide and diapirs in order to infer rates of salt rise. We have tentatively estimated a post-slide growth rates of 517 m per million years (m/Ma). However, as the analysis continues, it is possible this estimate will change. This research provides significant insight into the interplay of salt and slope failure processes in an ocean basin setting.

Tiny Fossils, Big Impact: The Role of Foraminifera-Enriched Condensed Section in Arresting the Movement of a Large Retrogressive Submarine Landslide in the Gulf of Mexico

A 3.4-m thick condensed section enriched in foraminifera formed the final detachment horizon of a retrogressive submarine landslide, in the Ursa Basin, northern Gulf of Mexico. The high concentration of foraminifera produces a high porosity (up to five porosity units) layer distinct from the background clay. Integrated Ocean Drilling Program Expedition 308 Site U1324 cored and logged this layer. We conducted a sedimentological analysis on 31 samples across this zone and the overlying and underlying background clay. CT images show that foraminifera are individuals dispersed within the clay, unbroken, and have retained a significant amount of intraskeletal void space. The assemblage is expected for this time interval in the Late Pleistocene (~24 kya). We interpret the layer is a result of a pause in terrigenous sedimentation. The condensed section was a preferred detachment horizon but only minimal sliding occurred before further movement ceased. One possible mechanism to explain this is the presence of foraminifera results in a dilational shear strengthening behavior, which arrested movement. Further work will be required to test this, however. On a broader scale, condensed sections with abundant microfossils, may play a key role in landslide mechanics because they can alter the shearing properties of the background material.