Ship Shoal Research Articles

Sedimentation processes and morphological changes in a dredge pit and surrounding environment on Ship Shoal in the northern Gulf of Mexico

The use of dredged material from continental shelves and estuaries for degraded barrier islands has become a common practice worldwide. South Pelto block on eastern Ship Shoal (SS) off the coast of Louisiana has been dredged for high-quality sand resources, including Caminada Dredge Pit (CDP) and multiple pits in East Timbalier borrow areas (ET) adjacent to CDP. To understand how these dredge pits change and sediments infill over time, multiple surveys were conducted, including bathymetry, sidescan sonar, subbottom CHIRP, and sediment samples were collected for grain size analysis.Although there is no nearby muddy sediment, sidescan and grain size data both show low-reflectivity areas inside CDP became larger over time, indicating continual supply from suspended mud. Subbottom data show decreasing pit wall slopes over time and degassing over patchy low reflectivity areas on the pit bottom. Arc-chord rugosity analysis was performed with emphasis applied to ET to examine the difference between cutter and hopper dredge techniques. It was revealed that dredging from the hopper created a higher rugosity value than the cutter, leading to a shorter recovery time for the benthic environment. Bathymetric results reveal approximately 20 m to 60 m of horizontal outward pit wall migrations over 3–5 years. Hurricanes Marco, Zeta, and Ida as well as Tropical Storm Claudette passed the study site (within 50 Km) during the study period which increased wind-wave energies sufficiently to develop new sand ripples discovered on sidescan sonar data near ET and CDP. By integrating various datasets and investigating the effects of tropical cyclones on a dredged area in SS, this study contributes valuable insights that fill existing knowledge gaps and enhance decision-making for future management strategies.

Spatial and temporal variations of seabed sediment characteristics in the inner Louisiana shelf

Although many studies have been performed in the northern Gulf of Mexico, there have been a paucity of high-resolution sediment data collected from seabed. New multicores were collected repeatedly at 4 fixed stations in the inner Louisiana shelf on March 12, 2021, July 7, 2021, November 20, 2021, and March 5, 2022, respectively, to study detailed spatial and temporal variations of surficial sediment. Being a very high sediment accumulation zone, Raccoon Island dredge pit and its nearby shelf were selected as our study area and additional six vibracores were collected inside and outside the pit for comparison. This study area is about 140 km west of the modern Mississippi Delta and 70 km southeast of the Atchafalaya Delta. A total of 624 samples from both vibracores and multicores were analyzed for this study. Spatially, our results show that sediment near Ship Shoal was the coarsest and sediment deposited in the pit was the finest. The organic matter was highest inside the pit and lowest near Ship Shoal. The carbonate content was the lowest near Ship Shoal. Temporally, surficial sediments were generally coarser in July and November of 2021 and finer in March of 2021 and March of 2022. Although the Mississippi and Atchafalaya Rivers supply a large amount of sediment during spring season, it takes time for the river-derived sediment to be delivered into the study area. Hurricanes Delta, Zeta and Ida passed this area and greatly impacted sediment transport in the years 2021 and 2022. The comparatively coarser sediment in July and November of 2021 were likely related to the resuspension of some coarse sediment by these hurricane events. Besides the river supply, Ship Shoal and barrier islands are nearby localized sandy sources and inner Louisiana shelf is a widespread mud source. This spatial heterogeneity of surficial sediment interplays with the timing of Mississippi and Atchafalaya River flood season and hurricanes and together dictates the sediment texture. This research on a filled-up dredge pit can benefit future evaluation of the safety of existing oil infrastructure and the feasibility of building new pipelines and platforms after dredging activities. Our results also shed light on the study of river- and hurricane-impacted sediment transport in muddy and mixed-texture passive continental margins around the world.

Contrasting modes of sediment infilling and geomorphic change within a sand dominated dredge pit on the inner Louisiana shelf

While many sediment transport studies have been performed on mud capped dredge pits (MCDP), there is a paucity of sediment infilling data for sand dominated dredge pits (SDDP) in the Northern Gulf of Mexico (NGoM). In this study, bathymetry, sidescan, and subbottom data were collected in 2018, 2019, and 2020 and sediment cores were collected between the 2019 and 2020 surveys at Block 88: a dredge pit sitting on the sand-dominated Ship Shoal of Louisiana's inner shelf. The crest of Ship Shoal was 5 m below mean sea level before dredging and was excavated to a depth of 10 m below mean sea level after the dredging of 8.4 million m3 of sediment. Two years after dredging, the northern end of the eastern pit wall accumulated ∼0.09 million tons (approximately 500 m long × 50 m wide × 2.5 m deep) of sediment, driven by westward longshore sediment transport and the deposition of bed load on the shoal crest. Sediment accumulation on the pit bottom was mainly driven by suspended load at a rate of ∼0.1 m yr−1 from 2018 to 2019 and ∼0.2 m yr−1 from 2019 to 2020. Thus, the sediment infilling in Block 88 was from both bed- and suspended-loads from contrasting sources. These pit-bottom sediment accumulation rates are approximately one order of magnitude lower than the predicted rate from a previous empirical modeling study conducted on Ship Shoal and lower than the rates (0.5–2 m yr−1) of three other MCDPs on the inner Louisiana shelf. The infilling sediment was patchy and predominantly silt and clay, with lower sidescan reflectivity. The slopes of Block 88's pit walls decreased from ∼20° in 2018 to ∼8° in 2020. These values were smaller than the values of pit wall slopes in MCDPs at a similar stage (i.e., 2–3 years after dredging). Our results indicate that the highest sediment accumulation rates occur immediately adjacent to the eastern pit wall of Block 88, which is down-current of the longshore transport direction. This study helps sand resource managers and policymakers to better plan and design future sediment dredging projects on sandy shoals. This study concludes that the longest axis of a dredge pit should be aligned perpendicular to the longshore transport direction to trap the most sediments for possible reuse. Additionally, both bed load and suspended load should be considered in future modeling to better predict sediment infilling.

Organoclastic sulfate reduction in deep-buried sediments: Evidence from authigenic carbonates of the Gulf of Mexico

The fate of organic matter in deep-buried marine sediments determines the formation of hydrocarbon reservoirs, shapes the deep biosphere, and affects the global carbon cycle. Several geochemical processes such as silicate weathering, iron and manganese (hydr)oxide reduction, and sulfate reduction influence the remineralization of organic matter in buried sediments. However, little information on deep-seated organoclastic sulfate reduction is available and the involved geochemical processes are largely unknown. Here, authigenic carbonates recovered from Ship Shoal Block 296 (SS296) of the Gulf of Mexico, all dominated by low-magnesium calcite, were investigated to constrain organoclastic sulfate reduction in deep-buried sediments. The SS296 carbonates had been previously suggested to be of a deep origin and transported upward by salt diapirism. The presence of cone-in-cone textures in samples 3110R1 and 3110R2 as well as vitrinite reflectance values of 0.48% to 0.58% indicate carbonate formation at a relatively deep burial. Likewise, clumped isotope thermometry yielded formation temperatures from 45 °C to 53 °C for sample 3110R1, 60 °C for sample 3110R2, and 11 °C to 37 °C for sample 3110R3. In spite of formation at greater depth, the temperatures of samples 3110R1 and 3110R2 are still within the temperature range of microbial sulfate reduction, reflecting formation depths between 1 km and 2.7 km. The presence of framboidal pyrite and its relatively low δ34S values (−27.8‰ to +11.5‰) agree with the occurrence of microbial sulfate reduction during carbonate precipitation. The carbon isotopic composition of the carbonates (δ13C: −16.6‰ to −8.9‰) and enclosed organic matter (δ13Corg: −26.8‰ to −20.5‰) suggests that organic matter is the dominant carbon source of carbonate minerals. This study confirms that microbial sulfate reduction occurs in deep-buried sediments, driving the degradation of organic matter. Our findings suggest that organoclastic sulfate reduction might be common in deep-seated sedimentary environments, controlling the fate of organic matter in deep subsurface environments and affecting the global carbon cycle.

E&P Notes (May 2022)

E&P Notes (May 2022)

E&P Notes (March 2022)

E&P Notes (March 2022)

Geomorphic and hydrodynamic impacts on sediment transport on the inner Louisiana shelf

To investigate the interactions among geomorphology, hydrodynamics, and sediment dynamics on the inner shelf offshore Louisiana, multiple acoustic and optical sensors were deployed during a 58-day intermediate-energy period from May 23 to July 22, 2016. Time series results show that an elongated bathymetric “trough” between Ship Shoal and Isles Dernieres partially confines flow in the E-W (shore-parallel) direction. Warm water with lower salinity was observed in the mid to upper water column with cool water with higher salinity in the lower water column. High sediment concentrations of 1–10 g/L were observed in the bottom boundary layer during intermediate-energy conditions in response to sustained winds of up to 11 m/s, significant waves heights of up to 1.5 m, occasional 8 s period swells, and a spring tidal range of 0.6 m. The dominant current and sediment transport directions were westward during the study period. About 77% of the sediment flux occurred during three 2-day-long periods (only 10% of the observation period), revealing the nonlinear and episodic nature of sediment transport in this study area. Although intermediate-energy conditions are less energetic than hurricanes and storms, they occur more often and contribute greatly to the long-term net sediment transport. Based on preliminary estimates, ~51.0 million tons of sediment passes along the Louisiana inner shelf annually, comparable with the annual sediment exiting the Mississippi Delta and sourced from marsh edge erosion in coastal Louisiana combined. The inner shelf sediment flux is an integral part of the coastal sediment budget and may provide important mineral sediment for wetland accretion if transported onshore during storms.

Geomorphologic change and patchy mud infilling in a sandy dredge pit in eastern Ship Shoal, Louisiana shelf, USA

To restore degraded barrier shorelines of the Mississippi River Deltaic Plain of Louisiana, sand is dredged from borrow areas in estuaries and continental shelves. The dredge pit in South Pelto Blocks 12 & 13 on eastern Ship Shoal used for Caminada-Moreau Headland restoration project is one of several sand dredge pits for barrier island restoration in Louisiana. Ship Shoal Sand Complex is in a sand-dominated, energetic, continental shelf environment. During energetic events, transient mud can blanket Ship Shoal, and then bypasses the shoal and deposits in deeper water. In this study, we collected and interpreted a suite of geophysical data from South Pelto Dredge Pit which is located in water depths of ∼9 m on eastern Ship Shoal. Repeat bathymetric survey data show that South Pelto Dredge Pit is presently infilling at an average rate of approximately 27,480 m3/year, which is slower than the predicted rate presented by an earlier numerical model. Side-scan mosaic maps indicate low-backscatter (muddy) sediments were transported into this pit and deposited in many patches within two years after dredging. The seabed inside this pit became smoother over time and the slope of the pit walls became gentler. Infilling of the South Pelto Dredge Pit seem to be controlled by sediment availability, which is volumetrically dominated by (a) far-field fluvial suspended sediment, (b) the adjacent seabed sediment resuspended by tides, waves, and currents, but presumably less impacted by (c) hurricanes or tropical storms during our study period of 2017–2018. The South Pelto Dredge Pit thus displays a unique mixing and redistribution of cohesive mud and non-cohesive sand. Initial spatial control on sediment infilling appears to be dictated by micro-bathymetry on the pit bottom created by the dredging activities. Bathymetric, side scan and grain-size data all indicate that mud was prone to deposit in the troughs.

Sandy Borrow Area Sedimentation—Characteristics and Processes Within South Pelto Dredge Pit on Ship Shoal, Louisiana Shelf, USA

Sandy Borrow Area Sedimentation—Characteristics and Processes Within South Pelto Dredge Pit on Ship Shoal, Louisiana Shelf, USA

Sediment infilling and geomorphological change of a mud-capped Raccoon Island dredge pit near Ship Shoal of Louisiana shelf

To restore degraded barrier shorelines of the Mississippi River Delta of coastal Louisiana, sand is dredged from high-quality borrow areas on the adjacent continental shelf. This dredging process generally creates a topographic low that facilitates the rapid capture of sediments. Our understanding of sedimentary processes and influences on seabed morphologic evolution is still limited. In this study, Raccoon Island dredge pit in a paleo-river channel on the Louisiana shelf was studied via multiple bathymetric surveys. Physical parameters include pit geometry and topographic change from post-dredging, and infilling sediment sources were analyzed. Raccoon Island pit was 100% filled up less than six years after dredging. The average infilling rate in the Raccoon Island pit was 1.10 m/year from 2013 to 2018. During the surveying period, the pit wall slope decreased by 6.8° within two years. The sediment infilling process in the Raccoon Island pit was likely under the combined influence of topography, wind-driven currents, storm waves, and the dynamic Atchafalaya River dispersal system. The Atchafalaya River reached high discharge in 2016, which likely contributed to a surprisingly high sediment infilling rate at Raccoon Island pit between 2015 and 2018. Raccoon Island pit is in a mixed texture environment in which the sandy pit was fully filled with mud. Mud is useful for marsh restoration but is not considered a quality resource for barrier island or beach restoration. Thus, the Raccoon Island pit is not regarded as a renewable resource for future barrier island restoration.

Sediment Transport near Ship Shoal for Coastal Restoration in the Louisiana Shelf: A Model Estimate of the Year 2017–2018

Ship Shoal has been a high-priority target sand resource for dredging activities to restore the eroding barrier islands in LA, USA. The Caminada and Raccoon Island pits were dredged on and near Ship Shoal, which resulted in a mixed texture environment with the redistribution of cohesive mud and noncohesive sand. However, there is very limited knowledge about the source and transport process of suspended muddy sediments near Ship Shoal. The objective of this study is to apply the Regional Ocean Modeling System (ROMS) model to quantify the sediment sources and relative contribution of fluvial sediments with the estuary and shelf sediments delivered to Ship Shoal. The model results showed that suspended mud from the Atchafalaya River can transport and bypass Ship Shoal. Only a minimal amount of suspended mud from the Atchafalaya River can be delivered to Ship Shoal in a one-year time scale. Additionally, suspended mud from the inner shelf could be transported cross Ship Shoal and generate a thin mud layer, which is also considered as the primary sediment source infilling the dredge pits near Ship Shoal. Two hurricanes and one tropical storm during the year 2017–2018 changed the direction of the sediment transport flux near Ship Shoal and contributed to the pit infilling (less than 10% for this specific period). Our model also captured that the bottom sediment concentration in the Raccoon Island pit was relatively higher than the one in Caminada in the same period. Suspended mud sediment from the river, inner shelf, and bay can bypass or transport and deposit in the Caminada pit and Raccoon Island pit, which showed that the Caminada pit and Raccoon Island pits would not be considered as a renewable borrow area for future sand dredging activities for coastal restoration.

Turning a tragedy into large-scale barrier island restoration in Louisiana: A three-project case study

Louisiana has successfully utilized the proceeds from the fines imposed for the Deepwater Horizon incident to significantly jump start barrier island restoration as identified in the Coastal Protection and Restoration Authority (CPRA) of Louisiana Coastal Master Plan (CPRA 2017). The Riverine Sand Mining/ Scofield Island Restoration (BA-40) project was the first to be implemented through a commitment of remaining funds in the initial emergency protective berms’ construction budget formulated into the Berms to Barrier Islands plan. The berm/ restoration conversion at Scofield Island was the first to utilize this funding mechanism. The Caminada Headland Beach and Dune Restoration–Increment II (BA-143) project was funded through the National Fish and Wildlife (NFWF) Gulf Environmental Benefit Fund and capitalized on a prior project constructed to the west completing the beach and dune restoration of the entire headland. Lastly, the Caillou Lake Headlands Restoration (TE-100) project was funded through the Natural Resource Damage Assessment (NRDA). The TE-100 project restored the entire degraded beach and dune system backed by a created marsh habitat to complement a prior restoration effort. Scofield Island is located west of the active Mississippi River bird’s foot delta in Plaquemines Parish, Louisiana. A primary objective of this project was the excavation and delivery of Mississippi riverine sand for beach and dune restoration; a first in our nation’s history. Multiple design and construction challenges arose requiring the CPRA, consulting team, and construction contractor to adapt. Construction of the beach and dune component of this project required approximately 22 miles (mi) of pipeline and four booster pumps along a sediment pipeline corridor that crossed two hurricane protection levees, went underneath two highways and a navigation channel, traversed the Empire Waterway, crossed Pelican Island, entered the Gulf of Mexico, and extended to Scofield Island. The restoration footprint length was approximately 2.4 mi, total volume placed was approximately 3.5 million cubic yards (MCY), and the benefit equaled 510 restored acres (CEC 2014). The pipeline corridor has subsequently been utilized for two other restoration projects, Shell Island East Berm Barrier Island Restoration (BA-110) and Shell Island West NRDA Restoration (BA-111). As a first in Louisiana’s restoration history, the Caminada Headland Beach and Dune Restoration–Increments I and II (BA-45 and BA-143) utilized sand dredged from Ship Shoal, an Outer Continental Shelf (OCS) sand resource located approximately 26-38 mi from the restoration areas. The 13.3 mi long headland was restored with approximately 3.7 MCY for BA-45 and 5.5 MCY for BA-143 from the borrow area (CEC 2015 and CEC 2017). A combination of cutterhead dredge/scow barges and hopper dredges were used to construct the project. A key goal of this project was restoring and protecting the fragile ecosystem which provides critical habitat for nesting shorebirds. The headland is of critical importance in serving as a defense of our national energy infrastructure. The western portion of the headland directly protects Port Fourchon, one of the nation’s most important energy ports. Caillou Lake Headlands (TE-100), known locally as Whiskey Island, is centrally located in the Isle Dernieres chain and it is a remnant of the single, larger Isle Dernieres (Last Island), which was segmented into multiple smaller islands by a major hurricane in 1856. The project included restoring the beach and dune along approximately 4.5 mi while simultaneously creating a marsh platform along approximately 5,500 feet (ft) utilizing 10.4 MCY of sand from the borrow area (CEC 2018). The borrow area lies within Ship Shoal OCS Lease Block 88 located over 10 mi along the conveyance corridor offshore of Whiskey Island. This project represents the largest barrier island restoration project to date in terms of volume per linear foot of shoreline with an average density of over 441 cubic yards per linear foot (CEC 2018).

Estimating blue crab (Callinectes sapidus) larval release sites in the Gulf of Mexico using an oceanographic particle-tracking model

Small, widely-dispersing, pelagic larvae are difficult to monitor through direct observation, but biophysical models can provide greater understanding of dispersal patterns. Here, we used a technique of reversing time in a biophysical model to estimate larval release sites of the blue crab, Callinectes sapidus, in the northern Gulf of Mexico by backtracking from known locations of postlarval settlement. We modeled surface ocean movement in reverse time from four settlement sites (Grand Isle, Louisiana; Ocean Springs, Mississippi; Dauphin Island, Alabama; and Pensacola, Florida). In the Gulf of Mexico, blue crab fisheries are managed at the state level. We found on average 73% of the larvae were traced back to larval release sites in the same state as settlement. Barrier island release sites accounted for 11% of settlement at Grand Isle and 17% at Dauphin Island. Less than 0.1% of larvae that settled in Grand Isle were tracked back to a known blue crab spawning area located to the south west, Ship Shoal. The distance traveled by larvae was highest in Pensacola and Grand Isle. The coefficient of variation of distance traveled was highest in Dauphin Island. The average distance traveled to each settlement location ranged from 31 to 124 km. Distance traveled explained 24% of empirical settlement numbers (F1,89 = 28.82, P < 0.001;R2 = 0.24). Settlement success was higher at release sites predicted by this study than at randomly generated sites suggesting female selection of larval release sites.

Sediment Identification Using Machine Learning Classifiers in a Mixed-Texture Dredge Pit of Louisiana Shelf for Coastal Restoration

Machine learning classifiers have been rarely used for the identification of seafloor sediment types in the rapidly changing dredge pits for coastal restoration. Our study uses multiple machine learning classifiers to identify the sediment types of the Caminada dredge pit in the eastern part of the submarine sandy Ship Shoal of the Louisiana inner shelf of the United States (USA), and compares the performance of multiple supervised classification methods. High-resolution bathymetry and backscatter data, as well as 58 sediment grab samples were collected in the Caminada pit in August 2018, about two years after dredging. Two primary features (bathymetry and backscatter) and four secondary features were selected in the machine learning models. Three supervised classifications were tested in the study area: Decision Trees, Random Forest, and Regularized Logistic Regression. The models were trained using three different combinations of features: (1) all six features, (2) only bathymetry and backscatter features, and (3) a subset of selected features. The best performing model was the Random Forest method, but its performance was relatively poor when dealing with a few mixed (sand and mud) surficial sediment samples. The model provides a new and efficient method to predict the change of sediment distribution inside the Caminada pit over time, and is more reliable when predicting mixed bed with rough pit bottoms. Our results can be used to better understand the impacts on biological communities by (1) direct defaunation after initial sand excavation, (2) later mud accumulation in topographic lows, and (3) other geological and physical processes. In the future, the deposition and redistribution of mud inside the Caminada pit will continue, likely impacting benthos and water quality. Backscatter, roughness derived from bathymetry, rugosity derived from backscatter, and bathymetry (in the importance order from high to low) were identified as the most effective predictors of sediment texture for mineral resources management.

Stone Crab Menippe spp. Populations on Louisiana's Nearshore Oil and Gas Platforms: Higher Density and Size at Maturity on a Sand Shoal

AbstractOil and gas platforms are common in the northern Gulf of Mexico. Platforms provide substrate for sessile organisms such as barnacles and oysters that enhance habitat for stone crabs Menippe spp. and other fouling‐associated organisms. In recent years, Louisiana's small nearshore platforms (<15 m) have been subjected to a high rate of removal, but little is known about their relative value as artificial reefs. During 2014, we compared stone crab populations at platforms on and off Ship Shoal in order to evaluate habitat conditions in the two areas. We hypothesized that platforms on the shoal would provide higher quality habitat than those in the surrounding area because the shoal supports higher diversity and abundances of benthic organisms. Stone crab populations were characterized by means of visual counts and by removing subsamples for species identification, sexing, and carapace measurements. Of the 378 stone crabs collected, 368 were Gulf stone crabs M. adina and 10 were Cuban stone crabs M. nodifrons. Stone crabs colonized every platform studied, but densities were higher (mean values: 4.0 crabs/m2 on the shoal versus 1.8 crabs/m2 off the shoal), and carapace width at 50% maturity (CW50) was 27.1 mm larger for crabs on than off the shoal. Mean carapace width, size‐class distribution, and sex ratio of crabs on and off the shoal were not significantly different. Platforms provided substrate for barnacles, which enhanced the structural complexity of platforms and were observed to be prey for stone crabs. Nevertheless, higher stone crab density and CW50 on the shoal suggested that shoal platforms are better habitat types for stone crabs.Received February 23, 2016; accepted January 9, 2017 Published online March 15, 2017

Spatial variation in basal resources supporting benthic food webs revealed for the inner continental shelf

We used stable isotopes to examine the relative importance of phytodetritus and microphytobenthos (MPB) in supporting benthic food webs on the Louisiana continental shelf. Primary producers and macroinfauna were collected from Ship Shoal (SS), a submerged, sandy, barrier island where sediment‐associated algae are primarily MPB and from silty, off‐shoal areas where sediment‐associated algae are primarily phytodetritus or a mixture of the two resources. Macroinfauna, as individual taxa and trophic guilds, were significantly more enriched in 13C on SS indicating greater dependence on MPB, which is enriched in 13C compared to phytoplankton. Using δ13C in a two‐source mixing model, the estimated MPB dietary contribution for SS macroinfauna averaged across trophic guilds, ranged from 53.4% to 83.0%, depending on the mixing‐model end‐members used, and 5 of 14 taxa had lower 95% confidence intervals (CI) > 20%, indicating that MPB was a primary resource for most SS macroinfauna. Off‐shoal, the dietary contribution of MPB ranged from14.5% to 47.7% and most taxa had a lower CI near 0%. Settled phytoplankton are the primary microalgal food source in muddy sediments, but the importance of MPB increases in more sandy sediments where MPB are the predominant microalgal resource. Sandy shoals play a unique food‐web role in deltaic shelf systems and support benthic food webs on the continental shelf.

Late Holocene chronology, origin, and evolution of the St. Bernard Shoals, Northern Gulf of Mexico, USA

Several shore-parallel marine sand bodies lie on the Louisiana continental shelf. They are Trinity Shoal, Ship Shoal, Outer Shoal, and the St. Bernard Shoals. These shoals mark the submerged positions of ancient shorelines associated with abandoned deltas. Three of these shoals are single elongate deposits. The fourth shoal, the St. Bernard Shoals, consists of a group of discrete sand bodies ranging in size from 44 to 0.05 km2, 25 km southeast of the Chandeleur Islands in 15–18 m of water. The St. Bernard Shoals are stratigraphically above the St. Bernard delta complex, which was active 2,500–1,800 years b.p. Understanding the evolution of the St. Bernard Shoals is necessary to reconstruct the Holocene chronology of the St. Bernard delta complex and the eastern Louisiana continental shelf. For this study, 47 vibracores and 400 km of shallow seismic reflection data collected in 1987 across the Louisiana shelf were analyzed. In June 2008, 384 km of higher-resolution seismic reflection data were acquired across the study area and appended to the preexisting datasets. Vibracores were integrated with seismic profiles to identify facies and their regional distribution. Our results demonstrate that the deltaic package stratigraphically below the St. Bernard Shoals is chronologically younger than the northern distributaries, but derived from the same trunk distributary channel (Bayou la Loutre). The river eventually bypassed the northern distributaries, and began to deposit sediment further onto the continental shelf. After abandonment, the overextended delta lobe was rapidly transgressed, creating a transgressive shoreline that eventually coalesced with earlier shorelines in the region to form the Chandeleur Islands. The St. Bernard Shoals formed by the reworking of the relict distributary deposits exposed on the inner to mid shelf during and subsequent to shoreface ravinement.

Diversity and composition of macrobenthic community associated with sandy shoals of the Louisiana continental shelf

Along the Louisiana, USA continental shelf, sandy shoals are shallow, possibly oxygen-rich “islands” surrounded by deeper muddy deposits prone to hypoxia. Shoals also contain significant quantities of fine sand that may be mined in the future for coastal restoration. The ecological role of shoals remains poorly understood and we hypothesized that shoals provide critical habitat for benthic invertebrates. Using Ship Shoal as a model system, we assessed the diversity and structure of macrobenthic assemblages and how community structure varies with season and environmental parameters. High biomass (averaging 26.7 g m−2) and high diversity (161 species) of macrobenthos was found in 2006. Polychaetes (45%—72 species) and crustaceans comprised most of the species (28%—46 species); spionids and amphipods dominated the polychaete and crustacean groups respectively, both in terms of number of species and abundances. Sharp decreases in diversity, abundance and biomass occurred from spring to autumn. Species diversity and total abundance significantly increased with decreasing sediment grain size and increasing bottom water dissolved oxygen. Across seasons, mole crabs Albunea paretii and amphioxus Branchiostoma floridae typified the community and contributed most of the biomass. The polychaetes Nephtys simoni, Neanthes micromma, Dispio uncinata, Mediomastus californiensis and Magelona sp. A, the amphipod Acanthohautorius sp. A and the burrowing shrimp Ogyrides alphaerostris also contributed to variation in community composition. Cluster analyses quantified seasonal variation, mainly based on sharp decreases in abundance, as well as spatial differences in species composition oriented along both east–west and north–south gradients. Variation in benthic assemblages was correlated with water depth and sediment characteristics (mean grain size and percentage of gravel-sized shell debris). We conclude that Ship Shoal is an unrecognized biodiversity hotspot and a hypoxia refuge compared to the immediate surrounding area where the benthic community is affected by seasonal hypoxia events and we discuss how sand-mining may influence this community.

Mineralogy of deepwater Gulf of Mexico salt formations and implications for constitutive behavior

A majority of the wells for the most significant announced developments in the deepwater Gulf of Mexico will penetrate salt formations thousands of feet thick. Assuring the integrity of these wells over the field lifetime is a major drilling engineering challenge. The effect of salt on long-term well integrity is tied to the constitutive behavior of the salt, and more specifically, to its creep rate. The constitutive behavior of salt has been well studied in two previous geotechnical engineering applications. A sophisticated constitutive model for salt that considers transient and steady state creep was developed under the Waste Isolation Pilot Plant, a licensed repository for transuranic waste. Further studies of salt creep were performed for the Strategic Petroleum Reserve (SPR), a series of leached storage caverns located in salt domes along the U.S. Gulf Coast. However, only a single set of laboratory creep data exists for salt from a deepwater Gulf of Mexico diapir. To constrain the constitutive response of deepwater Gulf of Mexico salt diapirs, the mineralogic composition of salt diapirs from several regions, including Ship Shoal, Ewing Bank, Mississippi Canyon, Garden Banks, Green Canyon, and Walker Ridge, was determined by quantitative X-ray diffraction analyses of drill cuttings. Water depths for the wells ranged from 718 ft to 7590 ft, and salt sections up to nearly 11,000 ft thick were penetrated. The database provides insight into the large-scale geographic variations in salt mineralogy, as well as insight into variations along vertical (or near-vertical) transects through massive salt bodies in the deepwater Gulf of Mexico. To enable comparison with on-shore domal salts with known constitutive response, mineralogic analyses were also conducted on salt cores recovered from various on-shore SPR sites. We show that that the range in composition of the salt diapirs in the deepwater Gulf of Mexico is essentially identical to the observed range in composition of the salt domes located along the U.S. Gulf Coast (to which the deepwater salt diapirs are geologically related). In light of the observed constitutive response of the sole deepwater salt diapir tested to date, we conclude that the expected constitutive response of deepwater Gulf of Mexico salt diapirs is likely to be bracketed by the observed range in constitutive response of the well-characterized Gulf Coast domal salts. We demonstrate with an example how this knowledge can be directly applied in well casing design analyses.

Ship Shoal as a Prospective Borrow Site for Barrier Island Restoration, Coastal South-Central Louisiana, USA: Numerical Wave Modeling and Field Measurements of Hydrodynamics and Sediment Transport

Ship Shoal, a transgressive sand body located at the 10 m isobath off south-central Louisiana, is deemed a potential sand source for restoration along the rapidly eroding Isles Dernieres barrier chain and possibly other sites in Louisiana. Through numerical wave modeling we evaluate the potential response of mining Ship Shoal on the wave field. During severe and strong storms, waves break seaward of the western flank of Ship Shoal. Therefore, removal of Ship Shoal (approximately 1.1 billion m3) causes a maximum increase of the significant wave height by 90%–100% and 40%–50% over the shoal and directly adjacent to the lee of the complex for two strong storm scenarios. During weak storms and fair weather conditions, waves do not break over Ship Shoal. The degree of increase in significant wave height due to shoal removal is considerably smaller, only 10%–20% on the west part of the shoal. Within the context of increasing nearshore wave energy levels, removal of the shoal is not significant enough to cause increased erosion along the Isles Dernieres. Wave approach direction exerts significant control on the wave climate leeward of Ship Shoal for stronger storms, but not weak storms or fairweather. Instrumentation deployed at the shoal allowed comparison of measured wave heights with numerically derived wave heights using STWAVE. Correlation coefficients are high in virtually all comparisons indicating the capability of the model to simulate wave behavior satisfactorily at the shoal.Directional waves, currents and sediment transport were measured during winter storms associated with frontal passages using three bottom-mounted arrays deployed on the seaward and landward sides of Ship Shoal (November, 1998–January, 1999). Episodic increases in wave height, mean and oscillatory current speed, shear velocity, and sediment transport rates, associated with recurrent cold front passages, were measured. Dissipation mechanisms included both breaking and bottom friction due to variable depths across the shoal crest and variable wave amplitudes during storms and fair-weather. Arctic surge fronts were associated with southerly storm waves, and southwesterly to westerly currents and sediment transport. Migrating cyclonic fronts generated northerly swell that transformed into southerly sea, and currents and sediment transport that were southeasterly overall. Waves were 36% higher and 9% longer on the seaward side of the shoal, whereas mean currents were 10% stronger landward, where they were directed onshore, in contrast to the offshore site, where seaward currents predominated. Sediment transport initiated by cold fronts was generally directed southeasterly to south-westerly at the offshore site, and southerly to westerly at the nearshore site. The data suggest that both cold fronts and the shoal, exert significant influences on regional hydrodynamics and sediment transport.