Submarine channels are important conduits for sediment in deep marine environments, and understanding their formation is critical to modeling basin fill processes. Most models describing channel evolution focus on turbidity currents as the erosive and constructive force in channel initiation. However, slope failure and slumping can be significant drivers of channelization, particularly in upper slope and ramp environments. Determining the relative roles of slumping and erosion by turbidity currents can provide important insight into the timing of channelization and the geometries of subsequent deposits. Samples were collected from Guadalupe Mountains National Park from two primary localities at Salt Flat Bench (Figure 2). Three vertical sections were measured at both locations. A total of 16 samples were collected for petrographic analysis and X-ray fluorescence (XRF) imaging. Spectacular outcrop quality makes the Middle Permian Brushy Canyon Formation in Guadalupe Mountains National Park an ideal location for the study of early channel evolution. A detailed facies analysis of fine-grained channel deposits was conducted in the Upper Brushy Canyon Formation in the Salt Flat Bench outcrops. After channelization, an interval of relative condensation dominated by hemipelagic settling of organic matter and silt was followed by an interval of incomplete sediment bypass by turbidity currents. This sequence of events suggests that sea level was at a relative highstand at the time of channel inception, whereas channel inception by turbidity currents is expected during a lowstand. Slumping rather than erosion by turbidity currents is the most likely mechanism to have initiated a channel at the study area. There is no evidence for the existence for high energy currents until after the interval of condensation. However, the action of weak contour currents during early channel evolution is observed in outcrop and microtextural features. Early carbonate cementation of channel-lining silts may have stabilized the slump surface with respect to erosion by later turbidity currents.

Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2003. / Includes bibliographical references (leaves 174-185). Also available in electronic version. Access restricted to campus users.

This dissertation examines how turbidity currents interact with submarine channels. Turbidity currents display exaggerated super-elevation at the outer banks of channel bends, because they have low excess densities relative to the ambient sea-water. Low-velocity zones form where flows separate from the inner banks. In a high-resolution seismic volume, I mapped 226 inclined surfaces associated with bank-attached bars in 16 channel bends of 2 buried sinuous channels. Position and geometries of bars indicate construction from suspended sediment in flow separation zones. Concave-bank benches, first identified in rivers where they are built from fully-suspended sediment deposited within flow separation zones in channel bends, comprise approximately 19% of this dataset. Bars have high median slopes (10°-11°) and occupy less than 30% of channel width. Associated channels migrated a median distance of less than 70% of the channel width and incised 20-30% of the channel depth. These bars are therefore interpreted to have formed during sediment bypass or weak erosion. I have analyzed the sedimentology and stratigraphy of a well-exposed channel complex, in the Permian Brushy Canyon Formation, west Texas. A steeply-inclined set of fine-grained sandstone beds (median dip=10°) at the margin of the channel complex is interpreted as deposits of a bank-attached bar. Beds are characterized by sub- to super-critically climbing ripple-lamination, planar stratification and trough cross-stratification. Paleo-transport directions are at high angles, 20-120°, to the dip azimuths of interpreted bar surfaces. Geometries of bounding surfaces, sedimentation styles and grain-size data were used to construct a facies model for suspension-dominated, bank-attached bars, built within flow-separation zones in submarine channels. I designed physical experiments to examine how erosional turbidity currents evolve channel- bend topography. Time-lapse bathymetry maps capture the evolution of raised benches tied to sedimentation within flow separation zones and erosion outside of separation zones. Erosional currents showed sensitivity to local conditions. The pattern of erosion was connected to roughness elements such as bend curvature and scours on the bed. Turbidity current run-up at the outside of bends produced a greater aerial extent of side-wall erosion than is commonly seen in incisional rivers.

Turbidity currents, both continuous flow and surge-type, have been detected with acoustic sounders operating at 42.5, 107 and 200, kHz. The turbidity currents are associated with the discharge of mine tailing into Rupert Inlet. A linear relation is obtained between the backscattered acoustic signal at 200 kHz and the one-half power of suspended particulate concentration from 10 to 1000 mg 1⁻¹. This relation is consistent with Rayleigh scattering theory in form and (relative to a standard target) amplitude, and is used to generate a cross-sectional profile of sediment concentration in the discharge plume. Estimates of surge speeds from the acoustic records based on a universal shape for density current heads range from 30 to 120 cm s⁻¹. The excess density of one surge was estimated from the reverberation amplitude to be 0.12 g cm⁻³. The additional attenuation of sound waves by suspended particles is important in turbidity currents and may be used to estimate suspended particulate concentration. Thermal processes contribute very little to the additional attenuation by particles with the grain densities of common minerals. A leveed submarine channel extended from the point of the tailing discharge (outfall) over the surface of the tailing deposit as early as 1974. The upper reach of this system was buried in 1978, and by late 1979 a new channel had developed. In 1976-77, the original channel consisted of: (1) a left-hooking upper reach with an average slope of 2.2°, (2) a middle reach (1° slope) with pronounced meanders (700-1100 m wavelengths) "increasing in curvature with distance downstream and (3) a straight lower reach (0.5° slope). The cross-sectional area of the channel decreased with distance downstream, excepting an increase in the first 100-200 m, until the channel disappeared about 5.5 km from the outfall. Acoustic records of the discharge plume in bends indicate overspill from the outer bank and an upward tilt of the upper interface away from the centre of bend curvature. The interfacial slope is steeper than indicated by the cross-channel difference in levee heights. These records together with observed tidal currents suggest that the left hook in the upper reach is caused by a mechanism similar to that which has been suggested for deep-sea channels. Turbidites in gravity cores from the levees are present as layers of vertically-graded, Cu-rich and Fe-poor sand and silt, some of which have load-casted flame-structures or load-pockets at their basai contacts. These layers comprise more of the sediment column with distance down-channel, suggesting that levee-building by overbank spillage from continuous flow becomes less important, and that most of the material transported through the lower reach is carried by turbidity surges. Surge recurrence intervals of 2-5 d are obtained from the number turbidites per core and the local deposition rate. The latter ranged from 0.3-4 m yr⁻¹, as given by changes in water depth, in tailing thickness from seismic reflection surveys, and in diatom frustule abundance in the cores. A model of continuous turbidity flow in submarine channels including entrainment is applied to the Rupert Inlet channel. Results are consistent with a sediment budget based on changes in the tailing deposit volume, and with turbidity surge recurrence intervals. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Bryant and Eastern Canyon systems are located on the northwest Gulf of Mexico, and they are characterized by a very complex sedimentological history related to glacioeustatic cycles, river discharges, and interactions of depositional and halokinetic processes. Both canyon systems were active during the low sea-level stand of Oxygen Isotope Stage 6, and provided the pathways for the transport of enormous amounts of sediments on the continental slope and abyssal plain of the northwest Gulf of Mexico. Right after their abandonment, at the beginning of Stage 5, salt diapirs encroached into the canyons, and resulted in their transformation into a network of intraslope basins. The transformation of the canyons resulted in the generation of massive sediment failures. The mid-shelf (Stages 4 and 3) to shelf edge (Stage 2) lowering of the sea-level during the last glacial episode resulted in: 1) extensive river-sourced deposits on the outer shelf and/or upper continental slope that contributed in a seaward mobilization of the underlying salt masses, and 2) the generation of numerous gravity flows and turbidity currents on the outer shelf/upper continental slope. The seaward mobilization of the salt masses resulted in the oversteepening of the flanks of the basins, and consequently in the generation of numerous and massive sediment failures. The turbidity currents were confined on the intraslope basins of the upper continental slope, depositing their coarsest material. However, their most diluted upper and end members were able to continue their downslope propagation depositing characteristic fine-grained turbidites. The frequency of the turbidity currents was highly increased during the last glacial maximum (Stage 2), and three short melt-water pulses centered at 30.5, 36, and 52 ky B.P. The last deglaciation event is characterized by the development of a major melt water event that resulted in the deposition of distinct organic rich sediments, similar to the sapropels of the Eastern Mediterranean. At about 11 ky B.P. the melt water discharges of the North America switched from Mississippi River to St Lawrence Seaway, causing the domination of hemipelagic sedimentation on the continental slope of the northwest Gulf of Mexico.

This dissertation advances our understanding of how turbidity currents interact with three-dimensional (3-D) minibasin topography and the resulting deposits that form. Conceptual Gulf of Mexico-centric models of minibasin fill development have become the foundation for exploring and identifying strategic deep-water hydrocarbon reserves on continental slopes around the world. Despite the abundance of subsurface data, significant questions remain about the 3-D physical processes through which minibasins fill and the relationship between these processes and the topography of the basin. To overcome this problem, I utilize techniques in physical laboratory modeling to query established models of the role that turbidity currents play in minibasin fill development, and observe the relationships between fill from the Lobster minibasin located in a proximal continental slope position in the Gulf of Mexico and from the Safi Haute Mer (SHM) minibasin located in the distal continental slope of offshore western Morocco. First, existing published literature are reviewed and assessed for the known state of minibasin development and fill processes, and the strengths and weaknesses of our current knowledge base. Second, results are presented from two series of experiments that document the interaction between steady, depletive turbidity currents and 3-D minibasin topography. Experimental results suggest that turbidity currents produce deposits that are more likely to drape pre-flow topography than pond within it. Turbidity current velocity data show a strong 3-D physical component in minibasin fill sedimentation that also influences extra-basinal sedimentation patterns. Details of these results provide insight into processes that have not been previously considered in published conceptual models of minibasin fill. Third, a comparison of the two subsurface datasets show that the types and abundance of architectural elements vary depending on the location of the minibasin on the continental slope (i.e. proximal vs. distal), and suggests key differences in the processes responsible for their infilling. Finally, a comparison of experimental results to preserved deposit architectures in the Lobster and SHM datasets suggest a more complex relationship of process-driven sedimentation than that derived primarily from suspension fallout. This improved understanding of minibasin fill is applicable to industry for increasing confidence in subsurface interpretations and reducing risk while exploring for quality reservoirs in deepwater regions. / text

京都大学 / 新制・課程博士 / 博士(理学) / 甲第23017号 / 理博第4694号 / 新制||理||1673(附属図書館) / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)准教授 成瀬 元, 准教授 堤 昭人, 教授 生形 貴男 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Continental slope channels, which serve as the primary conduits for sediment transport into the deep marine, occasionally become sites of sediment deposition with excellent reservoir potential. Increasingly reported in the literature are subsurface channel fills exhibiting shingled seismic reflectors that are interpreted to have formed by lateral channel migration. In lower Isaac Formation channels inclined strata are observed but at a lateral scale that is far below industry-seismic detection. Distinctively these flat-based channels are filled with coarse-grained sandstone that transitions abruptly and obliquely upwards into thin, fine grained turbidites. Like rivers, lateral accretion in Isaac channels is interpreted to be the result of the interaction of inertial and pressure forces, but in highly turbulent, highly density-stratified turbidity currents. This resulted in the formation of two superimposed secondary circulation cells that caused enhanced erosion on the outer bank and preferential deposition of coarse-grained sediment along the inner bank.

Thesis (S.M.)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering; and the Woods Hole Oceanographic Institution), 2012. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 77-79). / This study describes the spatial distribution of sediment deposition in the wake of a circular patch of model vegetation and the effect of the patch on turbulence and mean flow. Two difference types pf vegetation were used along with two different stem densities totaling four different case studies. The spatial location of enhanced deposition correlated with the steady wake zone, which has length, L1. The steady wake zone only occurred downstream of the rigid emergent patches of vegetation and was not seen downstream of the flexible submerged patches of vegetation. The enhanced deposition occurred when both turbulence and mean velocity was below the upstream, initial values. The enhanced deposition occurred when the mean velocity was less than or equal to half of the initial velocity. For the four cases studied, theses parameters of low velocity and low turbulence were primarily met within the steady wake region immediately downstram of the two rigid emergent patches of vegetation. In all four cases, large coherent structures are created in the flow due to the patch. Lateral vortices are formed downstream of the patch in a von-Karman vortex street that meets at the center of the flow a distance, Lw, downstream of the patch. For the flexible submerged cases, streamlines reattach to the bed of the flume a distance, Lv, downstream of the patch. In addition, for the flexible submerged cases, a secondary circulation is generated with flow moving laterally away from the patch at the surface and toward the centerline of the patch at the bed. / by Alejandra C. Ortiz. / S.M.

<p> Sedimentary structures, textures and fabric were studied in detail in eight turbidite greywacke beds from the Cloridorme Formation (Middle Ordovician), Gaspe, Quebec. The beds are exposed on the wave-cut platform near Grande Vallee and were traced for about two miles along the strike (276 degrees), which is parallel to the average current direction (274 degrees) as indicated by sole mark directions. </p> <p> The beds under study show the following systematic downcurrent changes along the strike from east to west: (i) Sole mark types show the following changes: (1) tool marks such as grooves, prod marks and isolated flute marks, to (2) longitudinal ridges, longitudinal ridges with overlapping flute, closely spaced flutes, to (3) poorly developed, shallow longitudinal ridges with occasional cuspate crossing bars, to (4) smooth bottom with occasional grooves. (ii) There is an increase in variance of sole mark directions without a large change in the mean direction in most beds, but the sole mark direction changes by nearly 90° as one of the beds is traced from east to west along the strike. (iii) Most of the beds are massive and are divided into two parts by a bedding joint. However, in the proximal region, if a bed is unusually thin, the bedding joint is absent and the bed shows slightly wavy, plane-laminated structure at places. (iv) Grain orientations are generally statistically non-significant in the upper massive part of the bed in the proximal region and significant in the rest of the bed. Significant grain orientations and graptolite orientations show large deviations from the sole in the distal region. Deviations increase towards the top of the bed. (v) The beds show a change from good grading to poor grading or slight reverse grading from the proximal to distal region. It is considered that the beds were deposited by low concentration, highly turbulent currents. Deposition of the beds took place in two distinct phases. First a 'quick' bed separated from the current was sheared extensively by the overflowing current and on consolidation formed the lower part of the bed. Later deposition in the proximal region from the upper part of the current and the part close to the tail was by settling of individual particles from the suspension. Separation and consolidation of another 'quick' bed in some cases formed the upper part of the beds in the distal region. </p> <p> The depositing currents produced a good vertical and lateral grain size grading in the proximal region, but poor vertical grading in the distal region due to the loss of coarse grains and possibly increased concentration of flocculated clay. Also, the turbidity currents tended to "meander" greatly on slowing down in the distal region. </p> / Thesis / Doctor of Philosophy (PhD)