Julie C. Bernier

Recent Subsidence-Rate Reductions in the Mississippi Delta and Their Geological Implications

Abstract The Mississippi Delta has long been characterized as an area of rapid subsidence; however, recent subsidence rates are substantially lower than previously reported. Tide-gauge records indicate that rates of relative sea-level rise were slow from 1947 until the mid-1960s, relatively fast from the mid-1960s until the early 1990s, and then slow since the early 1990s. These trends and rates are independently verified by repeat benchmark surveys and height monitoring at continuously operating geographic positioning system stations. Subsidence rates for the slow periods were a few millimeters per year, comparable to rates averaged over geological time scales that are attributed to natural processes such as shallow sediment compaction and deep crustal loading. The decadal pattern of slow, then rapid, then slow subsidence may be caused by natural deep-basin processes (e.g., gravity gliding and salt migration), but it is more likely related to rates of hydrocarbon production that followed the same temporal trends. If accelerated subsidence was primarily induced by reservoir compaction and fault reactivation associated with fluid withdrawal that also accelerated in the 1960s and 1970s, then the recent reductions in subsidence rates likely reflect a balancing of subsurface stresses and a return to near preproduction conditions.

Simple Methods for Evaluating Accommodation Space Formation in Coastal Wetlands

Land-surface subsidence and erosion are the principal processes that form accommodation space in interior coastal wetlands when they are converted to open water. The relative contribution of subsidence and erosion to wetland loss can be estimated by comparing elevations and vertical offsets of stratigraphic contacts that are correlated between adjacent sediment cores. Accommodation-space measurements assume that wetland-sediment thicknesses and the elevation of stratigraphic contacts were originally nearly uniform over short horizontal distances (tens to hundreds of meters). The accommodation space attributable to erosion equals the difference in wetland-sediment thickness between wetland cores and adjacent open-water cores taken at formerly emergent wetland sites. The accommodation space attributable to subsidence equals the elevation difference of a stratigraphic marker correlated between the two cores using the wetland core as the reference standard. Together, subsidence plus erosion at an open-water core location equals the accommodation space created by land loss, which is the difference between the adjacent emergent wetland elevation and the existing water depth.

Wetland-Change Data Derived from Landsat Imagery, Assateague Island to Metompkin Island, Maryland and Virginia, 1984 to 2015

The U.S. Geological Survey has a long history of responding to and documenting the impacts of storms along the Nation’s coasts and incorporating these data into storm impact and coastal change vulnerability assessments. These studies, however, have traditionally focused on sandy shorelines and sandy barrier-island systems, without consideration of impacts to coastal wetlands. The goal of the Barrier Island and Estuarine Wetland Physical Change Assessment project is to integrate a wetland-change assessment with existing coastal-change assessments for the adjacent sandy dunes and beaches, initially focusing on Assateague Island along the Maryland and Virginia coastline. Assateague Island was impacted by waves and storm surge associated with the passage of Hurricane Sandy in October 2012, including erosion and overwash along the ocean-facing sandy shoreline as well as erosion and overwash deposition in the back-barrier and estuarine bay environments. This report serves as an archive of data that were derived from Landsat 5 and Landsat 8 imagery from 1984 to 2014, including wetland and terrestrial habitat extents; open-ocean, back-barrier, and estuarine mainland shoreline positions; and sand-line positions along the estuarine mainland and barrier shorelines from Assateague Island, Maryland to Metompkin Island, Virginia. The geographic information system data files with accompanying formal Federal Geographic Data Committee (FGDC) metadata can be downloaded from http://pubs.usgs.gov/ds/0968/ds968_data.html.

Single-Beam Bathymetry Data Collected in 2015 Nearshore Dauphin Island, Alabama

Dauphin Island, Alabama is a barrier island located in the Gulf of Mexico that supports local residence, tourism, commercial infrastructure, and the historical Fort Gaines. During the past decade the island has been impacted by several major hurricanes (Ivan, 2004; Katrina, 2005; Isaac, 2012). This, in addition to ongoing sea level rise, presents a continued threat to island stability. In an effort to properly restore and provide longer-term island resilience to future storms and sea-level rise, state and federal managers are taking a scientific investigative approach to identify the best options available to help formulate and implement a long-term plan for the Island s protection. Island morphology, including current seafloor and shoreline bathymetry data, is one of several aspects being investigated and funded through a grant from National Fish and Wildlife Foundation Gulf Environmental Benefit Fund. In August 2015, the United States Geological Survey Saint Petersburg Coastal and Marine Science Center (USGS SPCMSC) in cooperation with the United States Army Corps of Engineers (USACE) and the state of Alabama conducted bathymetric surveys of the nearshore waters surrounding Dauphin Island. This data release only includes the single-beam data collected by the USGS. This USGS data release provides 1,165-line kilometers (km) of processed single-beam bathymetry (SBB) data collected by the USGS SPCMSC under the Field Activity Number (FAN) 2015-326-FA. This FAN encompasses four subFANs each of which represents one survey vessel; the RV Sallenger (15BIM10), the RV Jabba Jaw (15BIM11), the RV Shark (15BIM12), and the RV Chum (15BIM13). This data release provides SBB point data (x,y,z) in three datums: 1) the International Terrestrial Reference Frame of 2000 (ITRF00) and ellipsoid height (-47.04 meters (m) to -29.36 m); 2) the North American Datum 1983 in the CORS96 realization (NAD83 (CORS96)) for the horizontal and the North American Vertical Datum 1988 (NAVD88) for the vertical (-0.24 m to -17.33 m); and 3) NAD83 (CORS96) for the horizontal, and Mean Low or Lower Water (MLLW) for the vertical (-0.12 m to -17.93 m). Additional files include trackline shapefiles, respective digital and handwritten field logs, and a comprehensive 50-meter Digital Elevation Model (DEM).

Application of Ground Penetrating Radar for Identification of Washover Deposits and Other Stratigraphic Features: Assateague Island, MD

ABSTRACT A combination of ground penetrating radar (GPR) data, core data, and aerial photographs were analyzed to better understand the evolution of two portions of Assateague Island, Maryland. The focus of the study was to investigate the applicability of using GPR data to image washover deposits in the stratigraphic record. High amplitude reflections observed in two shore-perpendicular GPR profiles were correlated to shallow (<1 m) lithologic contacts observed in sediment cores. At these contacts, deposits consisting primarily of quartz sand overlie sediments with organic matter that include degraded plant root or stem material. The underlying organic matter likely represents the vegetated portion of the barrier island that was buried by washover fans deposited during hurricanes Irene (2011) and Sandy (2012), as indicated in high-resolution aerial photographs. The GPR data were able to delineate the washover deposits from the underlying stratigraphic unit; however, the radar data did not resolve finer s...

Raw and processed ground-penetrating radar and postprocessed differential global positioning system data collected from Assateague Island, Maryland, October 2014

Scientists from the United States Geological Survey, St. Petersburg Coastal and Marine Science Center (USGS-SPCMS) acquired sediment cores, sediment surface grab samples, Ground Penetrating Radar (GPR) and Differential Global Positioning System (DGPS) data from Assateague Island, Maryland, in October (FAN 2014-322-FA) 2014. The objectives were to identify washover deposits in the stratigraphic record to aid in understanding barrier island evolution. The report associated with this metadata record serves as an archive of GPR and DGPS data collected from Assateague Island in October 2014. Data products and accompanying Federal Geographic Data Committee (FGDC) metadata can be downloaded from the Data Downloads page located at, http://pubs.usgs.gov/publication/ds989/ds_data_downloads.html.