SeaMARC 1A sidescan sonar mosaic, cores and depositional interpretation of the Mississippi Fan: ArcView GIS data release
No abstract available.
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Geology topics
Research about Mississippi Fan
Source-linked reports with geographic coverage including Mississippi Fan.
Breaching the levee of a channel on the Mississippi Fan
GLORIA images of the youngest channel on the Mississippi Fan indicate that it has not been a stable feature, but instead has shifted its course several times. A detailed study of a site of channel shifting found a complex stratigraphy that resulted from one episode of channel avulsion. The channel avulsion appears to have been initiated by a large mass flow that choked the channel below the point where the levee was breached and additionally spilled a large volume of material through the breach in the levee onto the adjacent fan. Subsequent flows were redirected through this breach in the levee and built a channel-levee complex over the mass movement deposits. A second phase of mass movement resulted from another large mass flow that came down the channel and triggered the collapse of part of the newly developed levee. In this case, locally derived levee sediment was mixed with allochthonous sediment from farther up the fan and was spread northward from the levee. This localized study suggests that fan stratigraphy is complex and variable at several scales, not just at the scale resolved in seismic stratigraphic studies (Weimer 1989) and that large mass flows capable of choking the channel system have been an important mechanism in redirecting sedimentation on the Mississippi Fan.
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Characteristics of a sandy depositional lobe on the outer Mississippi fan from SeaMARC IA sidescan sonar images
SeaMARC IA sidescan sonar images of the distal reaches of a depositional lobe on the Mississippi Fan show that channelized rather than unconfined transport was the dominant transport mechanism for coarse-grained sediment during the formation of this part of the deep-sea fan. Overbank sheet flow of sands was not an important process in the transport and deposition of the sandy and silty sediment found on this fan. The dendritic distributary pattern and the high order of splaying of the channels, only one of which appears to have been active at a time, suggest that coarse-grained deposits on this fan are laterally discontinuous.
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Upper Pleistocene turbidite sand beds and chaotic silt beds in the channelized, distal, outer-fan lobes of the Mississippi fan
Cores from a Mississippi outer-fan depositional lobe demonstrate that sublobes at the distal edge contain a complex local network of channelized-turbidite beds of graded sand and debris-flow beds of chaotic silt. Off-lobe basin plains lack siliciclastic coarse-grained beds. The basin-plain mud facies exhibit low acoustic backscatter on SeaMARC IA sidescan sonar images, whereas high acoustic backscatter characteristic of the lobe sand and silt facies. The depth of the first sand-silt layer correlates with relative backscatter intensity and stratigraphic age of the distal sublobes (i.e., shallowest sand = highest backscatter and youngest sublobe). The high proportion (>50%) of chaotic silt compared to graded sand in the distal, outer-fan sublobes may be related to the unstable, muddy, canyon-wall source areas of the extensive Mississippi delta-fed basin slope. A predominace of chaotic silt in cores or outcrops from outer-fan lobes thus may predict similar settings for ancient fans.
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Depositional patterns of the Mississippi Fan surface: Evidence from GLORIA II and high-resolution seismic profiles
GLORIA long-range side-scan sonar imagery and 3.5-kHz seismic-reflection profiles depict a series of nine elongate deposits with generally high-backscatter surfaces covering most of the latest fanlobe sequence of the Mississippi Fan in the eastern Gulf of Mexico. The youngest deposit is a “slump” that covers a 250 by 100 km area of the middle and upper fan. The remaining mapped deposits, termed depositional lobes, are long (as much as 200 km) and relatively thin (less than 35 m thick) bodies. Small channels and lineations on the surface of many of these depositional lobes radiate from a single, larger main channel that is the conduit through which sediment has been supplied to these surficial deposits on the fan. The 3.5-kHz profiles show that adjacent depositional lobes overlap one another rather than interfingering, indicating that only one lobe was an active site of deposition at a time. Shifting of the depositional sites appears to be caused by both aggradation and avulsion. The chronology developed from the overlapping relations indicates the oldest of the mapped depositional lobes are on the lowermost fan, and the youngest are further up the fan. Depositional lobes on the lower fan consist of a series of smaller, elongate features with high-backscatter surfaces (540 km in length) located at the ends of previously unrecognized small channels (< 5 m deep). These small channels extend as much as 120 km from the main channel. Small channelized turbidity currents and/or debris flows, sand flows, or mud flows appear to be the dominant transport process constructing these depositional lobes. Channelized flow is an important mechanism for transporting sediment away from the main channel on this fan and the resulting facies created by these small flows are laterally discontinuous.
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