Louis E. Garrison

Geological aspects of marine slope stability, northwestern Gulf of Mexico

Abstract The improvement of sensors such as various high‐resolution seismic and navigational systems and side‐scan sonar, of offshore shallow‐water drilling techniques, and of laboratory analyses has allowed the marine geologist to make more accurate identifications and maps of the distribution of numerous types of marine sediment instabilities, as well as to determine the mechanisms responsible for their occurrence. A large number of data on the continental shelf and upper continental slope off the modern delta of the Mississippi river have been compiled; these data will be used to document the major types of slope instabilities. The continental shelf and slope off the modern Mississippi river delta display various types of sediment instability. High rates of sedimentation (up to 80 m per century), weak, high‐water‐content clays, and differential weighting of clay sediments characterize this region. The major types of sediment instabilities that have been documented include (a) Peripheral slumping, with ...

Channel systems and lobe construction in the Mississippi Fan

Morphological features on the Mississippi Fan in the eastern Gulf of Mexico were mapped using GLORIA II, a long-range side-scan sonar system. Prominent is a sinuous channel flanked by well-developed levees and occasional crevasse splays. The channel follows the axis and thickest part of the youngest fan lobe; seismic-reflection profiles offer evidence that its course has remained essentially constant throughout lobe development. Local modification and possible erosion of levees by currents indicates a present state of inactivity. Superficial sliding has affected part of the fan lobe, but does not appear to have been a factor in lobe construction.

Digitally acquired undistorted side-scan sonar images of submarine landslides, Mississippi River delta.

New side-scan sonar images that are free from scale distortions have been acquired from an area of the Mississippi Delta, where a variety of subaqueous landslides are forming. Collapse depressions, diapiric intrusions, elongate sinuous and merging channels floored with chaotic blocks, and prominent parallel and subparallel scarps result from various types of subaqueous mass-movement processes. Scale-true, overlapping adjacent sonographs, combined with accurate offshore navigation, have yielded the first mosaics of the sea-floor morphology over a 70-km2 area. These mosaics can be used to infer a considerable number of associations in morphology, forcing mechanisms, and spatial internal characteristics. The new side-scan sonar method offers considerable potential for accurate remapping and assessment of active submarine landslides.

The SEASWAB experiment

Abstract SEASWAB is one element of the Delta Project of the U.S. Geological Survey, a cooperative effort with several universities and other governmental agencies to investigate the processes that cause marine‐sediment instability. The basic purpose of the SEASWAB experiment was to obtain field measurements of sediment motion and pore‐pressure variations in soft sediment affected by wave‐pressure perturbations. This article serves as an introduction to the six papers that follow and that together make up a report on the results of SEASWAB.