Charles E. Stelting

Influence of relative sea level changes on the construction of the Mississippi Fan

A conceptual sea-level-driven depositional model for individual fanlobes (channel-overbank systems) of the Mississippi Fan does not permit direct application of the sequence stratigraphic principles of Vail and colleagues. Deep Sea Drilling Project Leg 96 results suggest that, during initial relative lowering of sea level, the canyon and upper fan channel were formed; excavated fine-grained slope sediments may have formed a debris flow deposit base for the fanlobe. Continued lowering produced constructional channel-levee-overbank deposits. Rising relative sea level inhibited input of coarse clastics, and channel depressions filled with muds. A blanket of (hemi)pelagics represents relative high sea level stand.

Mississippi Fan, Gulf of Mexico

The Mississippi Fan is a Quaternary accumulation composed of more than seven elongated fanlobes. Isopach and structure maps show frequent shifting of these lobes. The Mississippi Canyon, formed by retrogressive slumping, connects to the youngest fanlobe. The upper fanlobe is characterized by a large, incised, partially infilled, leveed channel. The middle fanlobe is aggradational, convex in cross section, with a channel-levee complex on its apex. The lower fanlobe contains a recently active small channel and several abandoned ones. Depositional patterns can be explained by aggradational processes and shifting of depositional sites.

Migratory Characteristics of a Mid-Fan Meander Belt, Mississippi Fan

Seismic reflection profiles across the most recent Mississippi Fan mid-fan channel reveals high amplitude reflectors in the lower part of the channel fill. Subsequent drilling results show that these high amplitude reflectors correspond to coarse-grained channel-lag deposits. These lag deposits can be divided into at least three distinct channel-fill units which show channel migration with time. Isopach maps indicate that the dimensions of the channel-lag accumulations are up to 6.5-km wide, slightly more than 200-m (250-msec) thick, and that the northernmost meander belt has migrated about 2.0-km laterally, 1.2-km downfan, and has climbed 175-m (220-msec) stratigraphically. Evolution of the meander belt shows characteristics similar to meandering fluvial systems.

Mississippi Fan: Internal structure and depositional processes

The Mississippi Fan is a Quarternary accumulation composed of more than seven elongated fan lobes. Isopach and structure maps show frequent shifting of these lobes. The Mississippi Canyon, formed by retrogressive slumping, connects to the youngest fan lobe. The upper fan-lobe is characterized by a large, incised, partially infilled, leveed channel. The middle fan-lobe is aggradational, convex in cross section, with a channel-levee complex on its apex. The lower fan-lobe contains a recently active small channel and several abandoned ones. Depositional patterns can be explained by several processes: “fluvial,” debris flows, and turbidity currents.

Drilling Results on the Middle Mississippi Fan

The middle-fan area of the youngest Mississippi fanlobe is a convex-shaped aggradational body with a sinuous, migratory channel located along its apex. The middle-fan/lower-fan boundary corresponds to a change from the sinuous channel pattern to one of lateral switching and abandonment. Sites drilled in the mid-fan channel indicate an upward-fining fill, commencing with gravel and ending with clay. The overbank deposits are primarily muddy with some silty and very-fine sandy turbidites. Sedimentation rates during the Late Wisconsin age for both the channel fill and overbank areas are about 12 m/1000 yr.

Mississippi Fan Sedimentary Facies, Composition, and Texture

Eight different sedimentary facies recognized in the Mississippi Fan sediments drilled during DSDP Leg 96 are defined on the basis of lithology, sedimentary structures, composition, and texture. Pelagic biogenic sediments are of minor importance volumetrically compared with the dominant resedimented terrigenous facies. Clays, muds, and silts are most abundant at all sites, with some sands and gravels within the mid-fan channel fill and an abundance of sand on the lower fanlobe. Facies distribution and vertical sequences reflect the importance of sediment type and supply in controlling fan development.

Drilling Results on the Lower Mississippi Fan

The sinuous migratory channel of the middle Mississippi Fan changes to a pattern of frequent lateral shifting and abandonment on the lower fan, with one channel being active at any given time. Borings in the upfan part of the lower fan recovered vertical successions of thin channel fill and overbank deposits. Near the distal end of the fan, the channels often bifurcate before merging with a low relief area of assumed laterally extensive “sheet sand” deposition. A significant amount of all the sand-sized sediment that came from the source area onto the Mississippi Fan during the Late Pleistocene was apparently transported to the lower fan.

Framework of Mississippi Fan, Gulf of Mexico: ABSTRACT

The Mississippi fan is a broad, arcuate Pleistocene accumulation of displaced shallow-water sediments. The fan consists of elongate fan lobes that shift position at the onset of each active sedimentation event, probably related to the lowering of sea level and the outbuilding of the shelf. Isopach and structure maps, based on eight acoustical reflectors of regional extent, demonstrate this shifting as well as changes in the location of the source area, and a progradation basinward. The youngest fan lobe is a suitable model for the underlying ones. It can be divided into four major morphologic units: (1) canyon--the Mississippi canyon resulted from retrogressive slumping during the late Wisconsin and was nearly filled thereafter; (2) upper fan lobe--a large, nearly filled erosional channel with low levees and a recent active central channel; (3) middle fan lobe--convex upward in cross section with a sinuous 3-km (2-mi) wide migratory and aggradational channel on its apex; (4) lower fan lobe--the central channel becomes smaller, less sinuous, shifts position periodically (as indicated by the indistinct abandoned channels), bifurcates, and terminates. Fan lobes are primarily channel-levee-overbank complexes, erosional in the upper fan and aggradational basinward. The channel is an active conduit; deposition in the channel took place during and after an active transport period. Accumulation rates on the middle and lower fan are high, ranging from 6 to 12 m/1,000 yr (20 to 40 ft/1,000 yr). A major portion of the sand is transported to the lower fan area. End_of_Article - Last_Page 464------------

Sedimentary Processes in Restricted Gulf Coast Estuarine System: Corpus Christi Bay, Texas: ABSTRACT

Corpus Christi Bay is a shallow (< 5 m), restricted estuary that is typical of estuaries on the Texas Gulf Coast. The distribution pattern of bay-floor sediments indicates that the bays interior is a depocenter for mud derived from multiple sources; the bays marginal areas are composed mainly of residual sandy deposits derived from shoreline erosion of a Pleistocene substrate (Beaumont Formation) and a modern baymouth barrier (Mustang Island). Synoptic time-sequence measurements of the bays suspended sediment and hydrographic characteristics were taken by helicopter at 15 monitoring stations; the measurements represent 8 surveys conducted during a 2-year observation period. The measurements show a high degree of spatiotemporal variability and indicate a wind-dominated sediment-dispersal system. During all surveys, the bay was consistently turbid (mean baywide transmissivity < 15%/m); the mean baywide concentration of suspended sediment during individual surveys ranged from 11 mg/l to 52 mg/l. The suspended-sediment concentrations were primarily controlled by wind speed which determines the extent of wave-induced resuspension of bay-floor deposits; wind direction and tidal phase controlled the sediment-dispersal patterns Texturally, the baywide mean grain size of suspended sediments during individual surveys ranged from very fine silt to clay (7.63 to 3.22^phgr), and mean silt/clay ratios were within the 0.68 to 1.38 range; the sediment was consistently poorly sorted. In contrast to suspended-sediment concentrations, which largely appear to be a short term response to recent wind conditions, sediment textures appear to be a longer term response to earlier wind conditions, thus suggesting that finer grained particulate matter has substantial residence time within the shallow water column. End_of_Article - Last_Page 992------------