Sediment Transport and Slope Stability in the Northern Gulf of Mexico

Sediment transport and slope stability are fundamental organizing agents of the geological record. These processes have been extensively studied along the northern margin of the Gulf of Mexico basin for both basic and applied purposes, but our knowledge of them is limited by the spatial and temporal sampling capabilities of traditional geologic oceanographic surveying tools such as coring, single-beam echosounders, and sidescan sonar. This dissertation seeks to update the state of knowledge regarding northern Gulf of Mexico sediment transport and slope stability from annual to millennial timescales, primarily using relatively high-resolution acoustic geophysical tools such as swath bathymetric echosounders and swept-frequency subbottom echosounders. There are three primary findings of this dissertation: (1) the subaqueous Mississippi River Delta Front is a zone of active downslope sediment flux in lieu of major hurricane passage, and the volume of sediment transported downslope during major hurricane and non-major hurricane containing intervals is comparable, (2) mud-capped dredge pits used for coastal restoration projects in Louisiana can be used as proxies for sediment deposition and slope stability along the Inner Continental shelf, and highlight the important role resuspension and slope failure play in decadal and longer-scale sediment accumulation in this environment, and (3) a drowned forest of age \u3e 40,000 years before present found offshore Gulf Shores, Alabama likely represents a unique or at least fairly localized depositional environment that preserved entire tree stumps during geologic periods that favor destruction of sedimentary fabric, including sea level lowstand and transgression

Analysis of the population structure of a gorgonian forest (Placogorgia sp.) using a photogrammetric 3D modeling approach at Le Danois Bank, Cantabrian Sea

The presence of gorgonian forests and deep-sea sponge aggregations in the Le Danois Bank promoted its declaration as the “El Cachucho” Marine Protected Area (MPA) by the Spanish Ministry of Environment, and its inclusion in the Natura 2000 network. Both habitats are considered vulnerable, so follow-up surveys are being performed to monitor their conservation in compliance with the EU Habitats Directive. The use of a non-invasive methodology, which does not cause damage or alterations on benthic communities, is particularly necessary in vulnerable ecosystem studies and MPA monitoring. This study analyzed the assemblage structure of a Placogorgia sp. population using a 3D photogrammetry-based method. The study was carried out through the analysis of the video transects obtained at the Le Danois Bank, using the Politolana underwater towed vehicle during the July 2017 ECOMARG survey. Recent developments in specific software of photogrammetric image analysis allowed extracting valuable information from these video transects. Using the Pix4D Mapper Pro software, 3D point clouds were obtained, and the size and morphometry of yellow fan-shaped gorgonian population structure could be evaluated. Due to gorgonian's high structural complexity, the use of length (i.e. height) as the morphometric descriptor of the real size of the colonies is not appropriate. Instead of length, the fan surface area covered by each gorgonian colony was selected as a suitable parameter of size. The direct measurement of this parameter was possible through a complete 3D reconstruction of the gorgonian forest. A total of 426 colonies of Placogorgia sp. were digitalized to obtain surface measurements and fan spread orientation calculations in 3D models. The results show that gorgonian populations were mostly composed of a high proportion of small colonies (0–0.10 m2). The population structure distribution shows a high proportion (~27%) of recruits (0.5 m2). In 78% of the gorgonian colonies, facing angles were grouped inside the first quadrant (0°-90°), in accordance with the main current direction in this zone. Colony distribution and fan orientation inside the gorgonian forest can be used as data sources to improve monitoring and management programs of these unique habitats in MPAs

Forces Driving the Morphological Evolution of a Mud-Capped Dredge Pit, Northern Gulf of Mexico

Sandy sediments preserved as paleo-channel fill on the inner shelf, some of which are overlain by modern muds, have been mined for barrier island restoration along the northern Gulf of Mexico. These mined areas have been termed “mud-capped” dredge pits. The processes governing the morphological evolution of the pits are poorly constrained due to limited observational data. Physical oceanographic (e.g., currents and waves) and sedimentary data were collected at Sandy Point dredge pit offshore Plaquemines Parish, Louisiana in summer 2015. Currents outside the pit flowed southward and/or southeastward at speeds of 8–20 cm/s, while currents inside the pit had speeds less than 2 cm/s with no clear dominant direction. Wave heights detected inside the pit were less than 0.4 m. A high turbidity layer with suspended sediment concentration around 4 g/L was observed above the pit floor, and its thickness was ~0.5 m. With observational data as input, three 2–D numerical models were employed to predict pit morphological responses, including pit infilling, margin erosion and slope change. The model results suggest that resuspension events were rare on the seafloor adjacent to the pit under summer fair weather conditions. Modeled pit margin erosion was very limited. With little resuspension of seafloor sediment locally, weak margin erosion and stable pit walls, the dominant process governing pit evolution was infilling sourced by the deposition of suspended sediments from the Mississippi River plume

A Machine Learning Approach Using Legacy Geophysical Datasets To Modeling Quaternary Marine Paleotopography

High-resolution subsurface marine mapping tools, including chirp and 3D seismic, enable the reconstruction of ancient landscapes that have been buried and subsequently submerged by marine transgression. However, the established methods for paleotopographic reconstruction require time consuming field and data interpretation efforts. Here we present a novel methodology using machine learning to estimate Marine Isotope Stage 2 (MIS2) paleotopography over a large (22 000 km2) area of the Northern Gulf of Mexico with meter-scale accuracy (2.7 m mean prediction error, 4.3 m 1-σ mean uncertainty). A relatively small area (3300 km2) of high-resolution (30 × 30 m) interpreted paleotopography is used as training and validation data, while modern bathymetry and MIS2 paleovalley location (binary deep/shallow paleotopography) are used as predictors. This approach merges the high-resolution of modern mapping techniques and the broad coverage of low-resolution legacy geophysical data. Machine learning-modeled paleotopography is not a substitute for precise high-resolution paleotopography reconstruction techniques, but it can be used to reasonably approximate paleotopography over large areas with greatly reduced expense and expertise

A machine learning approach using legacy geophysical datasets to model Quaternary marine paleotopography

High-resolution subsurface marine mapping tools, including chirp and 3D seismic, enable the reconstruction of ancient landscapes that have been buried and subsequently submerged by marine transgression. However, the established methods for paleotopographic reconstruction require time consuming field and data interpretation efforts. Here we present a novel methodology using machine learning to estimate Marine Isotope Stage 2 (MIS2) paleotopography over a large (22 000 km2) area of the Northern Gulf of Mexico with meter-scale accuracy (2.7 m mean prediction error, 4.3 m 1-σ mean uncertainty). A relatively small area (3300 km2) of high-resolution (30 × 30 m) interpreted paleotopography is used as training and validation data, while modern bathymetry and MIS2 paleovalley location (binary deep/shallow paleotopography) are used as predictors. This approach merges the high-resolution of modern mapping techniques and the broad coverage of low-resolution legacy geophysical data. Machine learning-modeled paleotopography is not a substitute for precise high-resolution paleotopography reconstruction techniques, but it can be used to reasonably approximate paleotopography over large areas with greatly reduced expense and expertise

Mud-capped dredge pits: An experiment of opportunity for characterizing cohesive sediment transport and slope stability in the northern Gulf of Mexico

© 2018 Elsevier Ltd Mud-capped dredge pits (MCDPs), defined as sand borrow sites from paleo river channels with cohesive sediment-capped walls, are used for northern Gulf of Mexico (NGoM) coastal restoration projects due to lower cost than more distal restoration resources. MCDP evolution, including infilling and pit wall slope stability, is poorly understood in relation to sandy borrow areas. We present and interpret bathymetric data from Sandy Point MCDP, which was constructed in 2012 about 20 km northwest of Southwest Pass, the most active Mississippi River distributary, in water depths of ∼11 m. Repeat bathymetric surveys show the pit is infilling ∼54 cm (or volumetrically 200,000 m3) per year, which is on the same order as seasonal-scale and an order of magnitude higher than the decadal-scale vertical accretion rates observed seaward of Southwest Pass. This infilling rate is higher than predicted based solely on excavated volume when compared with other dredge pits in sandy substrate, sediment-starved settings. MCDP infill is volumetrically dominated by far-field (here defined as river plume or shelf bed resuspension) sediment, with only ∼9% of pit infill from pit wall failure. Geomorphic comparison of Sandy Point MCDP walls with Southwest Pass mudflow gully walls shows that the MCDP is oversteepened relative to natural depression features in this setting; the convergence of MCDP and mudflow gully gradients towards a common value (4–5°) may represent an “angle of repose” in lieu of decadal-scale forcings. Findings from this experiment of opportunity show that MCDPs (1) are effective sediment traps, (2) have steep but relatively laterally stable walls (3) do not represent significant risk to infrastructure in a 3-year time scale, and (4) are predicted to infill within about 2 decades provided a proximal sediment source exists

Mississippi River subaqueous delta is entering a stage of retrogradation

© 2018 Elsevier B.V. The subaqueous delta of the Mississippi River, the largest river system in the conterminous U.S., has entered a stage of retrogradation caused by multiple natural and anthropogenic activities. Since the 1950s, the suspended sediment load of the Mississippi River has decreased by ~50% due primarily to the construction of \u3e50,000 dams in the Mississippi basin. The impact of this decreased sediment load has been observed in subaerial environments, but the impact on sedimentation and geomorphology of the subaqueous delta front has yet to be examined. To identify historic trends in sedimentation patterns, we compiled bathymetric datasets, including historical charts, industry and academic surveys, and National Oceanic and Atmospheric Administration hydrographic data, collected between 1764 and 2009. The progradation rate (measured at the 10 m depth contour) of Southwest Pass, which receives 69% of the suspended sediment load reaching Head of Passes, has decreased from ~67 m/yr between 1874 and 1940 to ~26 m/yr between 1940 and 1979, with evidence of further deceleration from 1979 to 2009. At South Pass and Pass a Loutre, the delta front has entered the destructive phase, with the 10 m contour retreating at rates \u3e20 m/yr at both passes since 1979. Advancement of the delta front also decelerated in deeper water (in some areas out to ~180 m depth). Except locally, where mudflow lobes are advancing, deeper contours show a pattern of decreasing progradation rate between 1874–1940 and 1979–2005 time periods. Furthermore, based on differences measured between available bathymetric datasets, the sediment accumulation rate across the delta front decreased by ~73% for the same period. The retention rate of Mississippi River sediment on the delta front ranged from 67 to 81% for the time periods assessed, with total sediment load stored on the delta front equal to 317 ± 54 Mt/yr from 1874 to 1940, 145 ± 25 Mt/yr from 1940 to 1979, and 87 ± 15 Mt/yr from 1979 to 2005. We document for the first time that the Mississippi River delta front has entered a phase of retrogradation, which will likely be accelerated by future upstream activities that divert a portion of the sediment load to the upper delta for coastal protection and restoration projects. The decline of the subaqueous Mississippi River Delta has critical implications for biogeochemical cycling, subaqueous mass wasting, and sediment dispersal to the coastal ocean