Foreset bed

Abstract: To obtain a better grasp of the litho-facies and bio-facies of the delta-to-marine transition, a study has been made of the sedimentary environments bordering the eastern, actively growing distributaries of the Mississippi birdfoot delta. Approximately 1,000 samples, mostly cores, were obtained from the distributary mouths, the continental shelf to the east of the delta, the continental slope to depths of 550 feet, and Breton Sound with its bordering Breton and Gosier islands. For comparative purposes samples have also been obtained in the vicinity of South and Southwest passes. The present paper discusses primarily the characteristics of the surface sediment, leaving for a future report a discussion of a relatively shallow third-dimensional phase of the study which is no yet complete. It has been found that the sedimentary environments follow several natural subdivisions. Thus, the subaerial delta and the surrounding shallow-water platform, both of which represent the topset beds of the classic type of delta, have distinctive sediment characteristics such as a dominance of silt over clay. Sand is moderately abundant in samples taken along most of the river channels and across the adjoining platforms, although silt is usually more common. Sand percentages on the platform apparently vary according to river stages, the finer sediments increasing during low water. The sediments of the marsh areas of at least the outer delta are easily differentiated from all other environments by the abundance of roots and the absence of the lamination which characterize the submerged ordering platforms. Laminations are also found to some extent in the levee and channel deposits. A general low content of Foraminifera and mollusks in the coarse fraction (sand) and an almost complete absence of echinoids and ostracods characterize all of these topset facies of the delta. An abundance of woody materials, aggregates (largely ferruginous), and mica is found as on the bordering slope. The deposits formed on the slopes beyond the platform margins are representative of the foreset bed type of environment in classical deltas, except that these slopes have very low angles of inclination, rarely exceeding one per cent. The slope deposits differ from platform deposits in their high clay content and in the marked decrease in lamination. Wood fibers, aggregates, and mica are even more abundant in the coarse fractions of the typical slope deposits than on the platforms. Faunally, the slopes represent a transitional zone between the comparatively sterile topset beds and the abundant organisms of the bottomset beds. Foraminifera become abundant near the base of the slope east of the delta and various types of mollusks appear in appreciable quantities. The shelf deposits beyond the slope, constituting the bottomset beds, are very largely a product of the present Mississippi Delta out to a distance of about 30 miles east-northeast of Pass a Loutre. These sediments are poorly stratified and consist predominantly of clay, but have an abundance of silt, and show a gradual increase in the sand content toward their eastern margin. The coarse-fraction End_Page 2537------------------------------ constituents of the bottomset beds show a surprisingly large quantity of Foraminifera, echinoids, and macroorganisms. Glauconite becomes relatively common in contrast to the zones nearer the delta. Bordering these bottomset beds there are old sediments, probably related to the sea-level lowering during Pleistocene glacial stages. These deposits, which are found both on the south and on the east, contain the eastern Gulf mineral assemblages derived from the southern Appalachians. Their faunas are a mixture of the present-day planktonic Foraminifera and macroorganisms, mostly worn, which are clearly related to the former lowered sea-levels. The percentages of silt and clay are far lower than in the deposits that are influenced by the present delta. Also the wood fibers, aggregates, and mica have virtually disappeared. North of the tongue of bottomset beds the relatively shallow platform, mostly under 10 fathoms, is covered by sandy sediments which represent the reworked deposits of recent Mississippi Delta lobes known as the Plaquemines and St. Bernard deltas. This is faunally a very barren area. The sediments consist almost entirely of terrigenous minerals of which mica, common directly off the delta, is very scarce. The present island sediments are virtually identical with those of the submerged part of the reworked Mississippi Delta. Inlet types of deposits are found in the tideways between Breton Island and the delta and between Breton and Gosier islands. These represent a combination of the sediments derived from the active passes of the delta with those carried down from Breton Sound, or intr duced from the island platforms. Breton Sound deposits are variable in character, but have many of the characteristics of bay sediments along the Texas coast. These Sound deposits differ from most of the deposits that are marginal to the delta by an absence of the ferruginous aggregates that are common in the latter. Faunally the Sound province is quite distinct from the areas in the open Gulf and most resembles those of the Texas bays on the west. The artificially controlled South and Southwest passes are ringed by sediment zones very much like those off the eastern passes. These delta-front slope sediments of the south delta are closely related to the slope deposits of the eastern delta, although at comparable depths the sand content is somewhat higher, particularly off Southwest Pass. /P>