Grain-size, coarse-fraction analyses and depositional environment as interpreted from microfauna are related to the character of sparker reflections at the location of core holes drilled by Exxon, Chevron, Gulf, and Mobil on the continental slope of the northern Gulf of Mexico. Continuous sparker reflections are correlated with slowly deposited, evenly bedded sediments containing bathyal faunas. The coarse fraction is dominated by the tests of foraminifera.
Discontinuous, discordant reflections and diffractions are correlated with sediments more rapidly emplaced in the bathyal environment of the continental slope by slumping and sliding from the continental shelf. Their coarse fraction is dominated by terrigenous sand grains. A large portion of the volume of continental slope sediments appears to consist of these "displaced" sediments, including an area 3 to 24. km wide and 80 km long, southeast of Corpus Christi, Tex.
Comparable processes of movement of sediments are interpreted on the continental shelf south of the Southwest Pass of the Mississippi River. Bathymetry in this area is characterized by a series of subaqueous "gullies" radiating from the river mouth leading to terraces at their southern extremities. Side scan sonar and PDR surveys show a rough bottom in these gullies and terraces as contrasted with a relatively smooth bottom elsewhere. The rough bottom is interpreted as indicative of slump and creep of the sediments from shallower water. Some foundation soil borings in this area south of Southwest Pass find a low strength material gradually increasing in strength with depth. Other borings find a "crust" of anomalously strong material B to 15 m below the mud line. The microfauna recovered from the crust has moved to its present position by slump or creep from shallower water along a pattern comparable with the gullies shown in the present-day bathymetry.
This paper discusses and interprets sediments and sedimentation of the continental slope of the northern Gulf of Mexico based upon data from core tests and sparker seismic surveys conducted by Exxon, Standard Oil Co. of California, Gulf, and Mobil in 1966, and relates these data to sediment movement south of the Southwest Pass of the Mississippi River. Sediment penetration of the core holes was restricted by government regulations to 305 m (1,000 ft) in water depths no leas than 200 m (650 ft). The general procedure was to cut a 4.6-m (15-ft) core and then to drill approximately 15 m (50 ft), followed by another 4.6-m core, and so on.
Environment of deposition, as determined by W. H. Akers, from the microfauna and other materials in washed residue; grain-size and coarse-fraction analysis, as interpreted by J. H. Spotts; lithologic descriptions by J. W. Low and N. E. Cygan; color photographs of the cores; bulk density measurements; and mechanical logs have all been used in varying degrees to interpret sedimentation on the continental slope.
