Geology topics

S. Jeffress Williams

Publications and source records attributed to S. Jeffress Williams.

77 records · Page 5Linked to original sources

USGS1989

Holocene sand shoals offshore of the Mississippi River delta plain

Offshore of the Mississippi River delta plain lies a series of Holocene sand shoals marking the position of ancient submerged shorelines associated with younger shelf-phase delta plains. These submerged shorelines represent positions when sea level stood lower than present. Short periods of rapid sea level rise during the Holocene transgression, in combination with subsidence, led to the submergence of these sandy shorelines, which can be recognized at the -33 ft (-10 m) and -66 ft (-20 m) isobaths on the Louisiana continental shelf The -33 ft (-10 m) shoreline trend is represented by Trinity Shoal and Ship Shoal, which are associated with the late Holocene Mississippi River delta plain. Trinity Shoal is derived from the Cypremont-Sale delta complex and is located 12 mi (20 km) offshore of Marsh Island. This shoal is 22 mi (35 km) long, 5 mi (8 km) wide, and 16 - 20 ft (5 - 6 m) thick. The facies relationships indicate that Trinity Shoal is a submerged barrier system in the initial stages of shoreface reworking. To the east is Ship Shoal which is associated with the Maringouin-Teche delta complex. This shoal is located 12 mi (20 km) offshore of the Isles Dernieres and is 31 mi (50 km) long, 5 - 6 mi (8 - 10 km) wide, and 13 - 20 ft (4 - 6 m) thick. The facies relationships indicate that Ship Shoal is a marine sand body derived from shoreface reworking of a submerged barrier island. The -66 ft (-20 m) shoreline trend is represented by the Outer Shoal and St. Bernard Shoals, which are associated with the early Mississippi River delta plain. The Outer Shoal is a low relief sand body, which lies seaward of Ship Shoal immediately west of the Mississippi Canyon. The eastern continuation of the -66 ft (-20 m) shoreline trend is the St. Bernard Shoals, which lie 16 mi (25 km) offshore of the Chandeleur Islands. In contrast to the other shoal systems, the St. Bernard Shoals form a shore-parallel zone of more than seven smaller sand shoals which, in many respects, are similar to a shore-oblique sand-ridge field. Collectively, these sand shoals represent a large potential source of aggregate for shoreline restoration and erosion control as well as possible hard mineral resources. Scientifically, these shoals provide insight into the processes which control coastal evolution and shelf sand development under the condition of relative sea level rise.

Mississippi River↗

USGS1989

Erosion and deterioration of the Isles Dernieres Barrier Island Arc, Louisiana, U.S.A.: 1853 to 1988

Using cartographic and aerial photography data from the years 1853, 1890, 1934, 1956, 1978, 1984, and 1988, shoreline change maps of the Isles Dernieres barrier island arc were constructed. These data were accurately superimposed, using a computer mapping system, which removed projection, datum, scale, and other cartographic inconsistencies. Linear, areal, and perimeter measurements indicate that the Isles Dernieres are suffering rapid rates of coastal erosion, land loss, and breakup. Bayside and gulfside erosion, in combination with sediment shortage and subsidence, have caused the Isles Dernieres to narrow through time. In addition, the core of the barrier island arc does not migrate landward and instead, breaks up in place as a result of inlet breaching and development. This is in contrast to other models of landward barrier island migration during transgression. If these trends continue, the Isles Dernieres will likely evolve into a subaqueous inner-shelf shoal by the early 21st century. Loss of the Isles Dernieres barrier island arc will severely impact the Terrebonne parish estuary, resulting in decreased environmental quality and increased public risk from storms and hurricanes.

Louisiana↗

USGS1989

Inner shelf deposits of the Louisiana-Mississippi-Alabama region, Gulf of Mexico

The late Quaternary morphology, shallow stratigraphy and sediment distribution of the Louisiana-Mississippi-Alabama inner shelf region are the product of transgressive and regressive sedimentary processes. Shelf sedimentary facies were deposited by deltaic progradation, followed by shoreface erosion and submergence. This information is based on interpretations and synthesis of more than 4,160 mi (6,700 km) of high resolution seismic profiles, 75 grab samples, and 77 vibracores. The shelf can be divided into two main depositional regions. The southwestern region, east and south of the Mississippi River plain, was formed by early Holocene delta complexes, overlying a late Wisconsinan delta. Deposits of the late Wisconsinan delta consist of well-defined coarsening-upward sequences and represent deltaic progradation during low sea level stands. The relatively recent Mississippi delta complexes have deposits which consist of fine-grained sands, silt and clay. With the late Holocene rise in sea level, asymmetrical sand ridges (16 ft, or 5 m, relief) have formed due to marine reworking of shoreline features. The northeastern region, offshore of the Mississippi-Alabama barrier islands, was formed by Pleistocene fluvial systems and Recent shoreface erosion and ravinement. Underlying the relatively thin Holocene sediment cover are relict fluvial sands which were deposited during the late Wisconsinan lowstand. Subsequent sea level rise allowed marine processes to rework and redistribute sediments forming the nearshore fine-grained facies and shelf sands sheet.

Gulf of Mexico↗

USGS1987

LOUISIANA BARRIER ISLAND EROSION STUDY.

During 1986, the U. S. Geological Survey and the Louisiana Geological Survey began a five-year cooperative study focused on the processes which cause erosion of barrier islands. These processes must be understood in order to predict future erosion and to better manage our coastal resources. The study area includes the Louisiana barrier islands which serve to protect 41% of the nation's wetlands. These islands are eroding faster than any other barrier islands in the United States, in places greater than 20 m/yr. The study is divided into three parts: geological development of barrier islands, quantitative processes of barrier island erosion and applications of results. The study focuses on barrier islands in Louisiana although many of the results are applicable nationwide.

Conference Paper↗

USGSDate not supplied

Louisiana coastal wetlands: a resource at risk

Approximately half the Nation's original wetland habitats have been lost over the past 200 years. In part, this has been a result of natural evolutionary processes, but human activities, such as dredging wetlands for canals or draining and filling for agriculture, grazing, or development, share a large part of the responsibility for marsh habitat alteration and destruction. Louisiana's wetlands today represent about 40 percent of the wetlands of the continental United States, but about 80 percent of the losses. The State's wetlands extend as much as 130 kilometers inland and along the coast for about 300 kilometers. Not all the wetlands are receding; in fact some wetlands are stable, and others are growing. But, at the present net rate of wetlands loss, Louisiana will have lost this crucial habitat in about 200 years. Considerable effort has been expended, and will continue to be expended, on understanding the processes that control wetlands evolution.

Louisiana↗