High-resolution Seismic Study Research Articles

Origin of Icelandic basalts: A review of their petrology and geochemistry

The petrology and geochemistry of Icelandic basalts have been studied for more than a century. The results reveal that the Holocene basalts belong to three magma series: two sub-alkaline series (tholeiitic and transitional alkaline) and an alkali one. The alkali and the transitional basalts, which occupy the off-rift volcanic zones, are enriched in incompatible trace elements compared to the tholeiites, and have more radiogenic Sr, Pb and He isotope compositions. Compared to the tholeiites, they are most likely formed by partial melting of a lithologically heterogeneous mantle with higher proportions of melts derived from recycled oceanic crust in the form of garnet pyroxenites compared to the tholeiites. The tholeiitic basalts characterise the mid-Atlantic rift zone that transects the island, and their most enriched compositions and highest primordial (least radiogenic) He isotope signature are observed close to the centre of the presumed mantle plume. High-MgO basalts are found scattered along the rift zone and probably represent partial melting of refractory mantle already depleted of initial water-rich melts. Higher mantle temperature in the centre of the Iceland mantle plume explains the combination of higher magma productivity and diluted signatures of garnet pyroxenites in basalts from Central Iceland. A crustal component, derived from altered basalts, is evident in evolved tholeiites and indeed in most basalts; however, distinguishing between contamination by the present hydrothermally altered crust, and melting of recycled oceanic crust, remains non-trivial. Constraints from radiogenic isotope ratios suggest the presence of three principal mantle components beneath Iceland: a depleted upper mantle source, enriched mantle plume, and recycled oceanic crust. The study of glass inclusions in primitive phenocrysts is still in its infancy but already shows results unattainable by other methods. Such studies reveal the existence of mantle melts with highly variable compositions, such as calcium-rich melts and a low- 18 O mantle component, probably recycled oceanic crust. Future high-resolution seismic studies may help to identify and reveal the relative proportions of different lithologies in the mantle.

Detection and Evaluation of Gas Hydrates in the Eastern Nankai Trough by Geochemical and Geophysical Methods

Abstract: Interstitial waters extracted from the sediment cores from the exploration wells, “BH‐1” and “MITI Nankai Trough”, drilled ∼60 km off Omaezaki Peninsula in the eastern Nankai Trough, were analyzed for the chloride and sulfate concentrations to examine the depth profiles and occurrence of subsurface gas hydrates. Cored intervals from the seafloor to 310 mbsf were divided into Unit 1 (∼70 mbsf, predominated by mud), Unit 2 (70–150 mbsf, mud with thin ash beds), Unit 3 (150–250+ mbsf, mud with thin ash and sand), and Unit 4 (275–310 mbsf, predominated by mud). The baseline level for Cl “concentrations was 540 mM, whereas low chloride anomalies (103 to 223 mM) were identified at around 207 mbsf (zone A), 234–240 mbsf (zone B), and 258–265 mbsf (zone C) in Unit 3. Gas hydrate saturation (Sh %) of sediment pores was calculated to be 60 % (zone A) to 80 % (zones B and C) in sands whereas only a few percent in clay and silt. The total amount of gas hydrates in hydrate‐bearing sands was estimated to be 8 to 10 m3 of solid gas hydrate per m2, or 1.48 km3 CH4 per 1 km2. High saturation zones (A, B and C) were consistent with anomaly zones recognized in sonic and resistivity logs. 2D and high‐resolution seismic studies revealed two BSRs in the study area. Strong BSRs (BSR‐1) at ∼263 mbsf were correlated to the boundary between gas hydrate‐bearing sands (zone C) and the shallower low velocity zone, while the lower BSRs (BSR‐2) at∼289 mbsf corresponded to the top of the deeper low velocity zone of the sonic log. Tectonic uplift of the study area is thought to have caused the upward migration of BGHS. That is, BSR‐1 corresponds to the new BGHS and BSR‐2 to the old BGHS. Relic gas hydrates and free gas may survive in the interval between BSR‐1 and BSR‐2, and below BSR‐2, respectively.Direct measurements of the formation temperature for the top 170 m interval yield a geothermal gradient of ∼4.3d̀C/ 100 m. Extrapolation of this gradient down to the base of gas hydrate stability yields a theoretical BGHS at∼230 mbsf, surprisingly ∼35 m shallower than the base of gas hydrate‐bearing sands (zone C) and BSR‐1. As with the double BSRs, another tectonic uplift may explain the BGHS at unreasonably shallow depths. Alternatively, linear extrapolation of the geothermal gradient down to the hydrate‐bearing zones may not be appropriate if the gradient changes below the depths that were measured. Recognition of double BSRs (263 and 289 mbsf) and probable new BGHS (∼230 mbsf) in the exploration wells implies that the BGHS has gradually migrated upward. Tectonically induced processes are thought to have enhanced dense and massive accumulation of gas hydrate deposits through effective methane recycling and condensation. To test the hypothetical models for the accumulation of gas hydrates in Nankai accretionary prism, we strongly propose to measure the equilibrium temperatures for the entire depth range down to the free gas zone below predicted BGHS and to reconstruct the water depths and uplift history of hydrate‐bearing area.

Late Quaternary history of the Marmara Sea and Black Sea from high-resolution seismic and gravity-core studies

Lithologic and multi-proxy paleoenvironmental data from 21 dated cores have been used to define three allostratigraphic units (allounits) within the late Quaternary successions of the Marmara Sea and Black Sea. Allounits are bounded by unconformities and their correlative conformities. In both regions, Allounit A extends from the seafloor downward to a ∼12–11-ka sequence boundary, which is a major shelf-crossing unconformity in water depths less than ∼100–110 m. In deep basins of the Marmara Sea, the lower part of Allounit A, designated Subunit A2, is a laminated sapropel, M1. On the shelf, Subunit A2 consists of backstepping delta lobes and early-transgressive barrier islands and sand sheets. Allounit B has only been recovered in Marmara Sea cores collected at water depths greater than ∼90 m, and represents basinal or prodeltaic deposition during the 23–12-ka late Pleistocene lowstand. During the last glacial maximum, the shelves surrounding the Marmara Sea were subaerially exposed, and deltas of Allounit B accumulated along the present-day shelf edge. Following the post-glacial rise of global sea level to −75 m at ∼12 ka, the Marmara Sea quickly became inundated and thereafter rose in synchroneity with the Mediterranean. By ∼10 ka, the Black Sea rose to start spilling into the Marmara Sea, leading to establishment of a brackish-water lid that has persisted to the modern day. The strongest Black Sea outflow began at ∼10 ka and persisted to ∼6 ka, promoting the accumulation of sapropel M1 in the deep Marmara Sea, and progradation of an overflow delta just south of the exit from the Bosphorus Strait. Allounit C is a laminated sapropel (M2) in basinal cores, dated at ∼30–23 ka. Like M1, it is believed that M2 accumulated during a period of increased brackish-water input into the Marmara Sea mainly from the Black Sea. In the Black Sea, wave erosion kept the shelf stripped of unconsolidated sediments during the falling sea level associated with the last glaciation and subsequent early stages of the post-glacial Holocene transgression. This erosion created a major unconformity, α. Shelf-edge deltas of Allounit B received their sediment during the last lowstand from small rivers that likely coalesced into a single system toward the shelf edge, at modern water depths of −100 to −110 m. These deltas were active until ∼11–10.5 ka. Subsequently, sea level in the Black Sea rose to −40 m by ∼10 ka, and a set of backstepping barrier islands developed on the shelf as part of the associated transgressive systems tract. Once water level reached −40 m, continued sea-level rise stalled until ∼9 ka as the Black Sea began to spill across the Bosphorus Strait into the Marmara Sea.

A high-resolution seismic study in Sakarya Delta and Submarine Canyon, southern Black Sea shelf

High-resolution seismic reflection profiles obtained from the Sakarya Delta have revealed the existence of three seismic units. Unit 1 is the subaqueous prodelta deposits of the Sakarya River and is separated from Units 2 and 3 by a major erosional surface. It is an oblique progradational delta sequence and best developed to the west of the Sakarya River mouth. The erosional surface extends over the whole shelf and was produced during the lowstand of sea level before the latest rise in the Black Sea. Unit 2 represents the lowstand sedimentary wedge deposited at the shelf edge during that time and is covered by a thin drape of Unit 1. The basement Unit 3 consists of folded–faulted acoustically reflective strata and is cut by an erosional surface at its upper boundary where it is overlain by Unit 1 on the shelf. The boundary between Units 2 and 3 delineates the paleo-shoreline during the lowstand period and occurs in a water depth of about 105 m. The rise of the sea drowning the shelf area of the Black Sea is recorded by the thin drape deposited over the erosional surface separating Unit 1 from Units 2 and 3 at around 7–8 kyr BP. The submarine canyon in front of the Sakarya Delta extends to 1500–2000 m water depth and includes two canyon heads. The occurrence of steep scarps of the canyon walls and the lateral displacement of canyon heads from the main canyon axis coincide with a major fault zone. Tectonic and submarine erosion processes appear to have been important in the initiation and development of this canyon system.

Fast static correction methods for high-frequency multichannel marine seismic reflection data: A high-resolution seismic study of channel-levee systems on the Bengal Fan

Very high-frequency marine multichannel seismic reflection data generated by small-volume air- or waterguns allow detailed, high-resolution studies of sedimentary structures of the order of one to few metres wavelength. The high-frequency content, however, requires (1) a very exact knowledge of the source and receiver positions, and (2) the development of data processing methods which take this exact geometry into account. Static corrections are crucial for the quality of very high-frequency stacked data because static shifts caused by variations of the source and streamer depths are of the order of half to one dominant wavelength, so that they can lead to destructive interference during stacking of CDP sorted traces. As common surface-consistent residual static correction methods developed for land seismic data require fixed shot and receiver locations two simple and fast techniques have been developed for marine seismic data with moving sources and receivers to correct such static shifts. The first method – called CDP static correction method – is based on a simultaneous recording of Parasound sediment echosounder and multichannel seismic reflection data. It compares the depth information derived from the first arrivals of both data sets to calculate static correction time shifts for each seismic channel relative to the Parasound water depths. The second method – called average static correction method – utilises the fact that the streamer depth is mainly controlled by bird units, which keep the streamer in a predefined depth at certain increments but do not prevent the streamer from being slightly buoyant in-between. In case of calm weather conditions these streamer bendings mainly contribute to the overall static time shifts, whereas depth variations of the source are negligible. Hence, mean static correction time shifts are calculated for each channel by averaging the depth values determined at each geophone group position for several subsequent shots. Application of both methods to data of a high-resolution seismic survey of channel-levee systems on the Bengal Fan shows that the quality of the stacked section can be improved significantly compared to stacking results achieved without preceding static corrections. The optimised records show sedimentary features in great detail, that are not visible without static corrections. Limitations only result from the sea floor topography. The CDP static correction method generally provides more coherent reflections than the average static correction method but can only be applied in areas with rather flat sea floor, where no diffraction hyperbolae occur. In contrast, the average static correction method can also be used in regions with rough morphology, but the coherency of reflections is slightly reduced compared to the results of the CDP static correction method.

Neogene-Quaternary contourite and related deposition on the West Shetland Slope and Faeroe-Shetland Channel revealed by high-resolution seismic studies

The Neogene and Quaternary sediments of the Faeroe-Shetland Channel and West Shetland shelf and slope rest upon a major regional unconformity, the Latest Oligocene Unconformity (LOU), and have been deposited through the interaction of downslope and parallel-to-slope depositional processes. The upper to middle continental slope is dominated by mass-transport deposits (debris flows), which progressively diminish downslope, and were largely generated and deposited during glacial cycles when ice sheets supplied large quantities of terrigeneous sediment to the upper slope and icebergs scoured sea-floor sediments on the outer shelf and uppermost slope. Large-scale sediment failures have also occurred on the upper slope and resulted in deposition of thick, regionally extensive mass-transport deposits on portions of the lower slope and channel floor. In contrast, large fields of migrating sediment waves and drift deposits dominate most of the middle to lower slope below 700 m water depth and represent deposition by strong contour currents of the various water masses moving northeastward and southwestward through the channel. These migrating sediment waves indicate strong northeastward current flow at water depths shallower than ∼700 m and strong southwestward current flow at water depths from ∼700 to >1,400 m. These flow directions are consistent with present-day water-mass flow through the Faeroe-Shetland Channel. The Faeroe-Shetland Channel floor is underlain by thin conformable sediments that appear to be predominantly glacial marine and hemipelagic with less common turbidites and debris flows. No evidence is observed in seismic or core data that indicates strong contour-current erosion or redistribution of sediments along the channel floor.

Lowermost Amazon River: evidence of late Quaternary sea-level fluctuations in a complex hydrodynamic system

Abstract Sedimentological, geochemical, and high-resolution seismic studies on the lowermost Amazon River (Xingu Mouth–Macapa–Estreitos de Breves) allow reconstruction of the sediment dynamics of this large river system in response to late Quaternary sea-level changes. Areas of sediment transport and deposition on an older river bottom are delineated by seismic reflectors and are related to inundation and incision caused by late Quaternary rising and falling of sea level. Older substrates have been identified as the Alter do Chao Formation (Cretaceous) and the Barreiras Formation (Tertiary). The initial AMS-radiocarbon dates yielded late Pleistocene to Holocene ages, and indicate a high sedimentation rate (0.6 cm/yr) for the last 800 yr. Three stages in the late Quaternary evolution of the Amazon mouth system can be distinguished: (i) relative sea-level fall, probably related to the Last Glacial Maximum with channel incision; (ii) deposition and consolidation of mud found near Afua during relative sea-level rise between late Pleistocene and middle Holocene, corresponding to the “freshwater lake” or the evolution of large floodplain areas in Amazonia; and (iii) stable sea-level conditions since 6000 yr BP resulting in the modern pattern of deposition and erosion, controlled by complex interactions of fluvial and oceanic processes.

Land-Ocean Systems in the Siberian Arctic, Dynamics and History

Land-Ocean Systems in the Siberian Arctic, Dynamics and History

High-Resolution Seismic Study as a Tool for Sequence Stratigraphic Evidence of High-Frequency Sea-Level Changes: Latest Pleistocene-Holocene Example from the Korea Strait

ABSTRACT Sequence stratigraphic analysis of high-resolution seismic reflection profiles and sediment data reveals that the latest Pleistocene-Holocene deposits in the Korea Strait shelf off the southeastern Korean Peninsula form a high-frequency sequence consisting of a set of lowstand, transgressive, and highstand systems tracts that corresponds to a fifth-order (20 kyr) sea-level cycle. Eight depositional systems, each with different seismic facies, constitute the systems tracts. The lowstand systems tract, consisting of sandy mud, forms a deltaic wedge that pinches out updip near or at the relict shelf edge. The transgressive systems tract, consisting mainly of sands, includes six depositional systems: (1) inner-shelf transgressive layer, (2) transgressive estuarine-deltaic complex, (3) transgressive sand ridge, (4) mid-shelf transgressive layer, (5) incised-channel fill, and (6) beach-shoreface complex. Although the transgressive systems tract is widely distributed, it is much thinner than the lowstand and highstand systems tracts. The highstand systems tract is composed of a prodelta-shelf complex consisting almost exclusively of Holocene muds. The distribution and geometry of the systems tracts in the latest Pleistocene-Holocene sequence in the Korea Strait shelf is different from that of Vail's model in that: (1) the lowstand systems tract consists only of the lowstand deltaic wedge, which forms an elongated sediment body along the paleoshelf edge, and (2) the highstand systems tract is completely confined to the inner shelf and forms a nearshore belt parallel to the coastline. High-frequency (20 kyr), high-amplitude (magnitude of about 130 m) sea-level change, together with strong currents flowing northeastward along the southeastern coast of the Korean Peninsula and varying sediment supply, have resulted in this rather unusual stratigraphic architecture.

High-Resolution Seismic Study as a Tool for Sequence Stratigraphic Evidence of High-Frequency Sea-Level Changes: Latest Pleistocene-Holocene Example from the Korea Strait

High-Resolution Seismic Study as a Tool for Sequence Stratigraphic Evidence of High-Frequency Sea-Level Changes: Latest Pleistocene-Holocene Example from the Korea Strait

High-resolution seismic studies of gas hydrates west of Svalbard

A strong bottom-simulating reflection (BSR) with high-amplitude variations is detectable in high- resolution reflection seismic profiles west of Svalbard. Above the BSR, anomalously high velocities up to 1840 m/s, calculated from high-frequency ocean-bottom hydrophone (HF-OBH) data, indicate the existence of gas-hydrated sediments. Below the BSR, a low-velocity layer, interpreted as gas-bearing sediments, shows thickness variations from 12 to 25 m. In addition, two other low-velocity layers clearly containing free gas are detected within the classic hydrate stability zone (HSZ) where, a theoretical viewpoint, free gas cannot exist.

Abstract: Forced Regression in Upper Jurassic Sediments (Boulonnais, N France): Comparative Outcrop and High Resolution Seismic Study 

Abstract: Forced Regression in Upper Jurassic Sediments (Boulonnais, N France): Comparative Outcrop and High Resolution Seismic Study 

Abstract: High-Resolution Seismic Study of Gas Hydrate Areas in the Mississippi Canyon, Northern Gulf of Mexico 

Abstract: High-Resolution Seismic Study of Gas Hydrate Areas in the Mississippi Canyon, Northern Gulf of Mexico 

Seismicity at Somma-Vesuvius and its implications for the 3D tomography of the volcano

A research project to define the feeding system of the Vesuvius volcano and the upper crustal structure of the area was started in 1993. The core of the project is the high resolution seismic tomography study by using explosive sources. Results of a preliminary 2D seismic profile have been discussed by Zollo et al. (1998) [J. Volcanol. Geotherm. Res., this volume]. The study of local seismicity at Vesuvius has many implications for the determination of the substructure of the volcanic area. The shape and size of the seismic volume and the study of the focal mechanisms put important constraints on the stress field of the area. The local seismicity can greatly improve the knowledge of the tridimensional velocity distribution of both P and S seismic waves inside the volcano, using passive tomographic techniques. In this paper we obtain a first three-dimensional tomographic image of the volcanic structure, obtained by inversion of first P and S arrival times of local earthquakes. The results show the existence of a sharp velocity contrast, along a lineation oriented NW–SE, cutting the crater along the line separating the relict of the Somma caldera from the southwest part of the volcano. This study confirms, also for S wave velocities, the absence of indications for a magma chamber in the first 4–5 Km below the sea level, as already evidenced, just for P waves, by 2D tomography. Furthermore, we discuss the improvement of resolution obtainable by deploying a small set of three component seismic stations to get a better coverage of the area.

Abstract: High-Resolution Seismic and Core Studies of Quaternary Depositional Processes in Texas-Louisiana Intraslope Basins: The Use of Modern Analogs to Understand Depositional Processes and Facies of Intraslope Basin Reservoirs

Abstract: High-Resolution Seismic and Core Studies of Quaternary Depositional Processes in Texas-Louisiana Intraslope Basins: The Use of Modern Analogs to Understand Depositional Processes and Facies of Intraslope Basin Reservoirs

A shallow high-resolution seismic reflection study of dudley ridge, south-east missouri

Dudley Ridge is an arcuate, north-trending terrace remnant of Wisconsinan (late Quaternary) outwash in the Western Lowlands of southeast Missouri. The ridge, an elongate topographic bulge, is approximately 12 km long and 2 km wide with up to 5 m of relief. Previous workers have attributed the ridge to fluvial erosion, or to a combination of fluvial erosion and tectonic deformation during the late Quaternary. The Commerce geophysical lineament, which is a major northeast-trending basement feature, is located a few kilometers to the southeast, and swarms of paleoliquefaction and possible neotectonic deformations have been reported in nearby areas of the Western Lowlands. To test the tectonic hypothesis, in 1994 and 1995 we acquired two west–east high-resolution seismic reflection profiles along the crest and western flank of Dudley Ridge to determine whether Paleozoic, Cretaceous and shallow Quaternary strata are faulted or folded. Profile SPB-1 shows several near-vertical faults indicative of at least three periods of movement: Paleozoic to middle Mesozoic; late Cretaceous to Tertiary; and early to late Wisconsinan (late Quaternary). Profile SPB-2 shows three ambiguous fault-like features which cut Cretaceous and Quaternary reflectors, but cannot be interpreted unequivocally to extend down into the Paleozoic section. These new seismic reflection data are interpreted to show that at least three major periods of faulting have disrupted Paleozoic, Cretaceous and Quaternary strata beneath Dudley Ridge, with the most recent faulting being early to late Wisconsinan in age and producing about 6–8 m (8–10 ms) of near-vertical displacement. Because Quaternary reflectors cannot be resolved above about 30–40 ms, the seismic data alone do not reveal whether or not the Wisconsinan deformation affected the surface of Dudley Ridge or whether the faults may have died out as they propagated up through the soft, saturated Wisconsinan outwash. However, the presence of the Farmdale Geosol beneath lower terraces adjacent to Dudley Ridge and the geosol’s apparent absence, or near-total truncation, beneath modern soils on Dudley Ridge suggest that one or more periods of tectonic bulging and accelerated erosion may have occurred between the Farmdale Interstadial and the Holocene about 25,000–10,000 years before present.

97/00002 Age estimation of the cottage grove fault system (Illinois, U.S.) from high-resolution seismic reflection studies of coal measures

97/00002 Age estimation of the cottage grove fault system (Illinois, U.S.) from high-resolution seismic reflection studies of coal measures

Age estimation of the Cottage Grove Fault System (Illinois, U.S.A.) from high-resolution seismic reflection studies of coal measures

Geologic and high-resolution seismic reflection studies of Pennsylvanian coal deposits have provided new insights concerning the Cottage Grove Fault System (CGFS), a major east-west oriented wrench fault system in southern Illinois. The age of faulting has previously been estimated to be late Pennsylvanian to early Permian, based upon KAr dating of igneous intrusive rocks associated with subsidiary faults. Although these dates are accurate for intrusions and faulting at certain locations, the duration of Cottage Grove master fault zone or subsidiary faulting is still unclear. High-resolution seismic reflection profiles were collected approximately 6.4 km north of the master fault zone in the vicinity of mapped subsidiary faulting to study coal deposits. Several small-offset, high-angle normal and reverse faults were interpreted from the seismic records. Two small-offset fault zones observed on the seismic records, interpreted to be subsidiary faults of the CGFS, bound a gentle asymmetric anticline. Several small sandstone channels located on the down-thrown sides of these faults are interpreted from the seismic data. Vertical stacking of these sandstone channels and other sands within the coal measures is also observable in the correlated well data. Location of channel deposits suggests depositional control by these faults, indicating that subsidiary faulting was contemporaneous with sand deposition. Thus, subsidiary faulting of the CGFS occurred as early as middle Pennsylvanian time.

2D seismic tomography of Somma- Vesuvius. Description of the experiment and preliminary results.

A multidisciplinary project for the investigation of Mt. Vesuvius Structure was started in 1993. The core of the project is represented by a high resolution seismic tomography study by using controlled and natura1 sources. The main research objective is to investigate the feeding system of the vo1cano and to retrieve details of the upper crustal structure in the area. A first 2D using seismic experiment was performed in May 1994, with the aim of studing the feasibility of lIsing tomographic techniques for exploring the vo1cano interiors. Particularly, this experiment was designed to obtain information on the optimal sources-receivers configuration and on the depth extension of the volume sampled by shot-generated seismic waves. 66 three-component seismic stations and 16 single-component analogue instruments were installed by several Italian and French groups to record signals generated by three on-land, underground explosions. Sources and geophones were deployed along a 30-km NW-SE profile passing through the volcano crater. Receivers were placed at an average spacing of 250 m in the middle of the recording line and at 500 m outside. The arrival time data base was complemented by first P and S readings of micro earthquakes which occurred in the recent past within the volcano. The first arrival data set was preliminary used to determine the shallow structure of the volcano by applying Thurber's (1983) tomographic inversion technique. This analysis shows evidence for a high-velocity body which extends vertically from about 400 m below the crater down to at least 3000 m and for a shallow 300-500 m thick low-velocity cover which borders the edifice. Data from the distant shot show evidence for arrivals of deep reflected/converted phases and provide information on the deeper structure under the volcano. The results from the interpretation of 2D data are used for planning a 3D tomographic survey which will be cauied out in 1996.

Neogene Depositional Processes on the Texas-Louisiana Continental Slope Revealed by High-Resolution Seismic Studies

Systematic interpretation of high-resolution seismic profiles (3.5-10 kHz echograms and 100 Hz air gun) from the continental slope and rise of offshore Texas and Louisiana from the shelf break across the highly deformed zone of salt diapirs, folds, and intraslope basins to water depths greater than 3,500 m seaward of the Sigsbee Escarpment reveals the geometries, scales, distributions, and depositional processes of deep-water deposits in this region. The 3.5-10 kHz data indicate that redistribution of sediment by mass-transport processes a ubiquitous, and a wide spectrum of slumps, slide blocks, and debris flows are observed from small, isolated deposits in intraslope basins to large, complex deposits covering hundreds of kilometers. The reflection character (echo character) of many intraslope basin and canyon floors indicates sand-rich deposits; however, aggradational leveed-channel systems, characteristic of deep-sea fans, are not observed in the steep-walled intraslope basins. In contrast, some large submarine-fan channel-levee complexes do extend seaward from the mouths of these large canyons and basins at the base of the Sigsbee Escarpment to form one or more large deep-sea fans on the continental rise. Apparently, large quantities of terrigenous sediments can be transported by turbidity flows through the intricate network of rugged intraslope basins and canyonsmore » to feed these fans. Evidence of sediment redistribution by bottom currents (contour currents) is also observed in the form of sediment waves and drift deposits across a large area of the continental rise seaward of the southeastern base of the Sigsbee Escarpment.« less