High-resolution Seismic Reflection Data Research Articles

Integration of high-resolution seismic and aeromagnetic data for earthquake hazards evaluations: An example from the Willamette Valley, Oregon

AbstractAeromagnetic and high-resolution seismic reflection data were integrated to place constraints on the history of seismic activity and to determine the continuity of the possibly active, yet largely concealed Mount Angel fault in the Willamette Valley, Oregon. Recent seismic activity possibly related to the 20-km-long fault includes a swarm of small earthquakes near Woodburn in 1990 and the magnitude 5.6 Scotts Mills earthquake in 1993. Newly acquired aeromagnetic data show several large northwest-trending anomalies, including one associated with the Mount Angel fault. The magnetic signature indicates that the fault may actually extend 70 km across the Willamette Valley to join the Newberg and Gales Creek faults in the Oregon Coast Range. We collected 24-fold high-resolution seismic reflection data along two transects near Woodburn, Oregon, to image the offset of the Miocene-age Columbia River Basalts (CRB) and overlying sediments at and northwest of the known mapped extent of the Mount Angel fault. The seismic data show a 100-200-m offset in the CRB reflector at depths from 300 to 700 m. Folded or offset sediments appear above the CRB with decreasing amplitude to depths as shallow as were imaged (approximately 40 m). Modeling experiments based on the magnetic data indicate, however, that the anomaly associated with the Mount Angel fault is not caused solely by an offset of the CRB and overlying sediments. Underlying magnetic sources, which we presume to be volcanic rocks of the Siletz terrane, must have vertical offsets of at least 500 m to fit the observed data. We conclude that the Mount Angel fault appears to have been active since Eocene age and that the Gales Creek, Newberg, and Mount Angel faults should be considered a single potentially active fault system. This fault, as well as other parallel northwest-trending faults in the Willamette Valley, should be considered as risks for future potentially damaging earthquakes.

Late glacial stratigraphy and history of the Gulf of St. Lawrence, Canada

The three-dimensional configuration of the Quaternary sediments in the Gulf of St. Lawrence is described based on analysis of 8000 km of high-resolution seismic reflection data complemented by analysis of seven piston cores. Till and or ice-contact deposits, glaciomarine sediments, postglacial basinal muds, and bank-top lagoonal sediments grading to sands and gravels make up the stratigraphic succession. Numerous thick accumulations of ice-contact - morainal deposits and "till tongues" indicate the position of former ice margins. The sequence of breakup of the last glacial ice in the gulf has been interpreted on the basis of these data and associated accelerator mass spectroscopy 14C dates. Grounded ice extended beyond Cabot Strait before 14.3 ka and retreated rapidly to north of Anticosti Island by 13.7 ka. Local residual piedmont lobes of glacial ice within the Cape Breton Channel and Baie des Chaleurs persisted longer, until about 12.2 ka. Ancestral hanging valleys formed the Cape Breton Channel and Baie des Chaleurs and served as conduits for ice lobes that flowed into the deeper Laurentian Channel. As many as four superimposed (Late Wisconsinan) glaciogenic sequences, up to 190 m in composite thickness, occur at the mouths of these (hanging) valleys. The thickest glacial sections were deposited on the southwestward slope of the Laurentian Channel. The surface of the till - ice-contact sediments between 440 and 100 m below present sea level has been extensively modified by iceberg scouring. The deeper limit (440 m) marks the maximum draft of icebergs, which was in large part determined by the thickness of the calving ice front. The disappearance of iceberg scour marks at 100 m is interpreted to have resulted from erosion of the glacial deposits by a Late Wisconsinan transgression whose low stand is suggested by a well-developed terrace on parts of the Magdalen Plateau at a present water depth of 110 m. A piston core that penetrated sediments overlying the uppermost (Late Wisconsinan) till in the Cape Breton Channel is interpreted to represent a deposit of lagoonal or shallow-marine sediments with localized deposits in excess of 65 m thickness. The depositional style of the postglacial deposits suggests sufficient bottom currents to erode sediments on the Magdalen Plateau.

Seismic stratigraphy of the Central Bransfield Basin (NW Antarctic Peninsula): interpretation of deposits and sedimentary processes in a glacio-marine environment

The Central Bransfield Basin is a deep narrow trough between the northern tip of the Antarctic Peninsula and the South Shetland Islands. Analyses of single-channel, high-resolution seismic reflection data are used to characterise the seismic stratigraphy of the Central Bransfield Basin. The tectonised acoustic basement is overlain by a 1-s-thick sedimentary cover composed of two main sedimentary sequences. The Lower Sequence, which shows synsedimentary deformation, has only been identified on the Antarctic Peninsula margin. The Upper Sequence is a complex sedimentary package composed of eight seismic units whose distribution, geometry and seismic facies allow two types of seismic units to be distinguished: slope and basinal units. The slope units, constituted by progradational stratified seismic facies, form a sedimentary wedge extending from the shelf edge. The basinal units fill the basin floor showing chaotic and undulated seismic facies that change basinward into stratified seismic facies. Both types of seismic units display an interfingering pattern at the base of the slope, suggesting an alternating shift of the sedimentary depocentre, from the slope to the basinfloor and vice versa. This alternate pattern indicates that the sedimentary processes responsible for the infilling of the Central Bransfield Basin followed a cyclic pattern, which has likely been associated with the advance and retreat of the ice sheets over the margins during glacial and interglacial episodes. During glacial periods, the ice sheets advanced, eroded the shallower sea floor areas and deposited diamicton and debris flow deposits along the moving grounding line, resulting in a progradational sedimentary wedge on the slope. At the end of glacial periods, coinciding with the retreat of the ice sheets, extensive sediment failures affected the continental margin. During interglacial periods the ice sheets remained restricted to coastal locations and glacial troughs, where processes of meltwater formation might have been significant. Sediment-laden underflows are generated within these troughs, from where they flow and spread over the shelf and down the slope to the basinfloor as sediment gravity flow deposits. The combined effect of these processes is a progradational build up of the shelf and an aggradational infilling of the basin floor, together with the development of the interfingering pattern at the base of the slope.

Neogene sedimentation history of the Congo Fan

High-resolution seismic reflection data were interpreted to develop a model for the Neogene sedimentation in the Congo Fan area. Identification of two reflectors representing marine isotope stages 6 and 12 allows a distinction between the Early and the Late Quaternary. A single sediment source system during the Late Paleogene⧹Early Neogene gradually changes into a two sediment source system in the Late Neogene. Since the Late Quaternary, the Congo River has been the prevailing sediment source. The evolving Benguela Current further modified the system and led to a high biogenic productivity. Evidence for gas in the sediments can be found in the whole area of investigation. Additionally, indications for a tectonic control of the Congo Canyon have been found in the distribution of sedimentary units and unit thicknesses.

Seismic stratigraphy and sedimentary processes at the Norwegian Sea margin northeast of the Faeroe Islands

Sedimentary processes on the continental slope north of the Faeroe Islands during the Cenozoic were studied in detail using high-resolution multichannel reflection seismic data. A wedge-shaped succession of Cenozoic sediments with a maximum thickness of approximately 2 km overlies the basaltic basement, which forms the underlying structure of the slope. The sedimentary succession represents four depositional sequences and a contourite deposition. Also some slump deposits are recognised. The oldest sequence, which is possibly of Eocene–Late Oligocene age, was prone to repeated mass movements during the deposition of the sequence. At the end of the Oligocene, the sediments on the slope stabilised, and the depositional pattern was strongly influenced by sea level changes. In the Late Pliocene, a renewed instability of the slope occurred, resulting in mass movements causing erosion at the middle of the slope and creating a major slide escarpment at the base of the slope. Subsequently, the sedimentary processes change from mainly downslope, progradational forms to along-slope, current-induced contourite deposition. This change most likely marks the formation of a Norwegian Sea Deep Water current pattern similar to the present. Variations of the current strength and sediment supply caused by glacial–interglacial cycles probably results in an alternating sedimentation pattern of the contourite deposit. Another impact from the glaciation of the Northern Hemisphere is the presence of iceberg turbate at the upper parts of the northeastern Faeroes slope.

The Frolikha Fan; a large Pleistocene glaciolacustrine outwash fan in northern Lake Baikal, Siberia

ABSTRACT New high-resolution seismic reflection data off the NE shore of Lake Baikal indicate intimate relations between recurrent late Pleistocene glaciations onshore and lacustrine sedimentation processes offshore. The basin floors bordering the most extensively glaciated regions of the lake margin are characterized by large sublacustrine fans. Extending over 20 km in diameter, the Frolikha Fan is among the largest fans in northern Lake Baikal. Morphologically the fan consists of three parts: the upper fan, which is marked by a prominent single-leveed channel, the middle fan, consisting of smaller distributary channels (without levees) associated with convex fan lobes, and the lower fan, which is poorly developed and characterized by a transition of lobate sedimentary units into basin-plain or bottom-current channel deposits. The extremely steep morphology of the Frolikha Fan, internal seismic characteristics, and in particular the depositional geometry and seismic signature of the upper fan division indicate that the fan formed during glacial periods in a proglacial depositional environment. In fact, large moraines onshore Frolikha Bay document multiple Pleistocene glaciations down to and beyond the present-day shoreline into the lake basin. It is therefore very likely that the Frolikha Fan developed in response to the recurrent late Pleistocene advances of valley glaciers into Lake Baikal.

Upper Quaternary strata of the upper continental slope, Northeast Gulf of Mexico; sequence stratigraphic model for a terrigenous shelf edge

ABSTRACT The shallow subsurface of the upper slope, northeastern Gulf of Mexico, shows abrupt changes in depositional environments, sequence stratigraphy, and sedimentation rates reflecting Pleistocene to Holocene glacioeustasy. Stratigraphic change is recorded in lithologic, sedimentologic, biostratigraphic, and oxygen isotopic data from a nearly continuous 245.5 m core from Viosca Knoll Block 774, a gamma-ray log of the borehole, and high-resolution seismic-reflection data. Mud with minor sand at the base of the boring (245.5 to ~ 136 m) was deposited during several sea-level cycles corresponding to isotope stages 13 to 8, and includes a thick, probable stage 10 unit. A thick, coarsening-upward deltaic mud and sand at ~ 136-59.9 m likely was deposited during oxygen isotope stage 8 glaciation. The section upward from ~ 59.9 m contains: a deltaic silt and sand probably deposited during stage 8; a transgressive sand shoal recording the stage 8-7 transition; highstand mud and sand of stage 7; a relatively thin stage 6 lowstand mud and sand; stage 5 highstand mud and sand; stage 4-2 mud and sand; and Holocene sandy silt. Almost all contacts between systems tracts are transitional in core. Inferred lowstand deposits are lithologically and sedimentologically variable, reflecting degree of delta progradation and subsequent erosion, and consist of sand and (or) mud. Thicker, coarser intervals at the site represent shelf-margin deltas likely deposited during falling sea level. Falling sea level and lowstand sediment is characterized by mostly middle to inner neritic benthic foraminifera, cool-temperature planktonic foraminifera, and high oxygen isotopic values of the planktonic foraminifera Globigerinoides ruber. Transgressive and highstand systems tracts are thin. Rising-sea-level sections and highstand condensed intervals consist of clay or silt with carbonate concretions and with numerous pyritized microfaunal tests and small burrows. Condensed sections also contain abundant microfauna, mostly warm-temperature planktonic foraminifera, outer neritic to upper bathyal benthic foraminifera, and transitional to interglacial 18O values of Globigerinoides ruber. Some sandy turbidites appear to be interbedded with highstand mud.

Volcanism offshore of Vesuvius Volcano in Naples Bay

High-resolution seismic reflection data are used to identify structural features in Naples Bay near Vesuvius Volcano. Several buried seismic units with reflection-free interiors are probably volcanic deposits erupted during and since the formation of the breached crater of Monte Somma Volcano, which preceded the growth of Vesuvius. The presumed undersea volcanic deposits are limited in extent; thus, stratigraphic relationships cannot be established among them. Other features revealed by our data include (a) the warping of lowstand marine deposits by undersea cryptodomes located approximately 10 km from the summit of Vesuvius, (b) a succession of normal step faults that record seaward collapse of the volcano, and (c) a small undersea slump in the uppermost marine deposits of Naples Bay, which may be the result of nuee ardentes that entered the sea during a major eruption of Vesuvius in 1631. Detection of these undersea features illustrates some capabilities of making detailed seismic reflection profiles across undersea volcanoes.

Reflection coefficient calculation from marine high resolution seismic reflection (Chirp) data and application to an archaeological case study

Chirp sub-bottom profilers produce high-resolution images of the near-surface. An attribute of the sea-bed reflection in chirp data are fluctuations in polarity between adjacent traces. Two models are proposed and presented to explain this: the first incorporates changes in an acoustic impedance gradient at the sea bed; the second uses changes in the thickness of the uppermost sediment layer. Mixing of adjacent traces produces a consistent polarity for the sea-bed reflector. Reflection coefficients are calculated, using amplitude information derived from single-traces, and polarity information from trace mixing, with application to a marine archaeological case study. The reflection coefficient calculated for the top of a buried 18th century wooden wreck is -0.26.

Optimal processing of marine high-resolution seismic reflection (Chirp) data

Chirp frequency-modulated (FM) systems offer deterministic, repeatable source-signatures for high-resolution, normal incidence marine seismic reflection data acquisition. An optimal processing sequence for uncorrelated Chirp data is presented to demonstrate the applicability of some conventional seismic reflection algorithms to high-resolution data sets, and to emphasise the importance of a known source-signature. An improvement of greater than 60dB in the signal- to-noise ratio is realised from correlating the FM reflection data with the transmitted pulse. Interpretability of ringy deconvolved data is enhanced by the calculation of instantaneous amplitudes. The signal-to-noise ratio and lateral reflector continuity are both improved by the application of predictive filters whose effectiveness are aided by the repeatability of the Chirp source.

Mapping of complex bedrock structure using the high-resolution reflection seismic technique

In May of 1996, four reflection seismic profiles and one VSP (vertical seismic profile) were acquired near Colonsay, Saskatchewan. This survey was conducted as part of a site-specific study of the high-resolution reflection seismic profiling, and shallow open-borehole VSP methods. The reflection seismic/VSP survey was designed with four specific objectives in mind. The sponsor wanted to determine: (1) whether high-resolution reflection seismic data could provide an essentially continuous structural image of the shallow subsurface (Quaternary alluvium and Cretaceous shale to a depth of ca 200 m); (2) if the Cretaceous bedrock and overlying Quaternary strata in the study area are extensively faulted; (3) whether useful VSP data could be acquired from shallow open-boreholes in the study area; and (4) optimum high-resolution reflection seismic acquisition parameters (given the target zone and site conditions) for future production-oriented work in the area. From a technical perspective the survey was successful. The acquired high-resolution reflection seismic data effectively image the shallow subsurface, and support the thesis that bedrock and overlying Quaternary strata are extensively faulted in the study area. The VSP data establish that sonic log velocities recorded above the water table in a fluid-filled borehole are anomalously high (relative to the seismic velocity of an equivalent thickness of unsaturated sediment). On the basis of the VSP control, the sonic log data were modified such that the corresponding synthetic seismograms correlate well with the reflection seismic profiles. Additionally, on the basis of field testing, optimum site/target specific field acquisition parameters were determined. These will be of use in designing any subsequent seismic reflection and VSP surveys in the study area.

Reflection Seismic Investigations In the Volcaniclastic Apron of Gran Canaria and Implications For Its Volcanic Evolution

High-resolution reflection seismic data obtained around Gran Canaria allow a detailed and consistent correlation of seismic reflectors of the northern and southern Canary Basins with the lithology drilled by DSDP Leg 47A SSE of Gran Canaria, as well as with major phases of volcanic activity on Gran Canaria as mapped onshore. Two prominent reflectors were chosen as marker horizons and correlated with the drilled lithology. the results indicate that reflector R7 above the Miocene volcaniclastic debris flows V1-V3 reflects the shield-building phase of Gran Canaria. Reflector R3 is interpreted as corresponding with the Pliocene Roque Nublo formation. The top of the massive island flank of Gran Canaria, defined by seismically chaotic facies, extends 44 to 72 km off the coast of Gran Canaria. West of Gran Canaria the flank of Tenerife onlaps the steeper and older flank of Gran Canaria, which, in turn, is onlapping the older flank of Fuerteventura to the east in a similar way. Erosional channels, which can also be traced up to 50 km from the area between Gran Canaria and Fuerteventura into the deeper northern basin, have been identified in the bathymetry. The data presented provide new detailed information for modelling the submarine and subaerial evolution of the central Canary Islands of Gran Canaria and Tenerife, i.e. the timing of their shield-building phases and later stages of major volcanic activity, as reflected by the position of prominent seismic reflectors in the seismic stratigraphy.

Geophysics and Shallow Faults in Unconsolidated Sediments

AbstractThis paper presents a case study using various geophysical techniques in unconsolidated sediments where shallow faulting is probable and most likely affects ground‐water flow. Wenner and dipole‐dipole resistivity, ground penetrating radar (GPR), and high resolution reflection seismic data are compared with cone penetrometer (CPT) information, existing monitoring well data and models of anticipated fault styles. The results of this study suggest that (1) the CPT study, combined with monitoring well data may suggest that discontinuities in corrrelatable zones indicate that faulting is present; (2) the addition of Wenner and dipole‐dipole data may further suggest that offset zones exist in the shallow subsurface but make it difficult to map specific faults; (3) the high resolution seismic data will image faults to within a few meters of the surface but cannot identify faulting on the scale of our models; (4) offset 100 MHz and 200 MHz common mid‐point (CMP) GPR will image features that may be fault planes and strands similar to the models; and (5) the combination of all the tools in this study, particularly the GPR and seismic, may allow for the mapping of small‐scale, shallow faults in unconsolidated sediments that affect ground‐water flow.

Effects of near‐surface waveguides on shallow high‐resolution seismic refraction and reflection data

Waveguides formed between the earth's free‐surface and sharp subsurface velocity discontinuities, such as the watertable, may have a significant impact on high‐resolution seismic refraction and reflection data. Energy trapped in such waveguides may propagate over long distances with little loss due to geometrical spreading. The dispersive nature of the guided waves results in pronounced shingling of the first arrivals. This narrows the “optimum reflection window” between the first arrivals and the ground roll, and may lead to mispicking of first breaks and difficulties in identifying and processing shallow reflections.

Site-response models from high-resolution seismic reflection and refraction data recorded in Santa Cruz, California : R. A. Williams, E. Cranswick, K. W. King, D. L. Carver & D. M. Worley, US Geological Survey Professional Paper, 1551-A, 1994, pp 217–242

Site-response models from high-resolution seismic reflection and refraction data recorded in Santa Cruz, California : R. A. Williams, E. Cranswick, K. W. King, D. L. Carver & D. M. Worley, US Geological Survey Professional Paper, 1551-A, 1994, pp 217–242

Latest Quaternary deposition on the outer shelf, northern Gulf of Mexico: Facies and sequence stratigraphy from Main Pass Block 303 shallow core

Sedimentologic, biostratigraphic, and isotopic geochemical data from a nearly continuous 91.4 m core and high-resolution seismic-reflection data from the middle to outer shelf east of the Mississippi River delta document deposition during changes in latest Quaternary sea levels. The data suggest that the shelf edge of the northern Gulf of Mexico is constructed from deltas deposited during falling sea level and lowstands, from sediment deposited in valleys during rising sea level, and from highstand clay. The Main Pass Block 303 core records sedimentation from the late middle Pleistocene (before ca. 135 ka) to the present. A lower delta-front interval was sampled at the core base (78.5–91.4 m). Delta sediment is truncated by a sequence boundary probably eroded during maximum glaciation corresponding to oxygen isotope stage 6 (late Illinoian?) and then modified during the following transgression. Above the sequence boundary is a transgressive shelf sand (77.2–78.5 m), overlain by burrowed, hemipelagic clay (∼63.4–77.2 m). The sand and clay were deposited during postglacial sea-level rise and a highstand. The rise and highstand correspond to oxygen isotope stage 5 and Ericson zone X and part of Ericson zone Y Sangamonian-“Eowisconsinan”). The clay interval from ∼51.8 to 63.4 m depth, in turn, was likely deposited during oxygen isotope stages 3 and 4 during the early and middle Wisconsinan. Overlying, with a transitional contact, is a relatively thick mud and sand (16 to ∼51.8 m) deposited during the subsequent sea-level fall to the maximum late Wisconsinan lowstand (oxygen isotope stage 2). The mud and sand interval correspond to steeply dipping clinoforms on seismic-reflection records. An interpreted sequence boundary, formed during fluvial incision of the shelf during the maximum late Wisconsinan lowstand, separates deltaic sediment from overlying fluvial, bay, and marsh deposits, which fill an incised valley. Thin, regressive delta and shelf facies overlie the valley fill. Sediment above the sequence boundary (above 16 m) was deposited during the rise in sea level from the maximum late Wisconsinan lowstand to the present. Late Wisconsinan deltaic and overlying incised-valley-fill sediment was derived from an ancestral river system that drained the southeastern United States.

Shallow Cenozoic seismic stratigraphy and structure: South Carolina lower coastal plain and continental shelf

Single-channel high-resolution seismic reflection data from the South Carolina lower Coastal Plain and continental shelf delineate no substantial deformation that could possibly be associated with the seismicity of the Charleston area. Correlation of the seismic data with available wells reveals seaward-dipping Paleocene through Quaternary depositional sequences representing carbonate and terrigenous sediments that were deposited in shallow to outer shelf paleoenvironments. Only middle Eocene and younger units are delineated on the lower Coastal Plain due to limited acoustic penetration. The seismic profiles also reveal thick Miocene and Pliocene progradational sediments within a broad north-northwest–south-southeast–trending depositional basin in the middle to outer shelf region. The variations in the thicknesses of the depocenters are attributed to an interrelationship of subaerial and marine processes, which are in turn governed by the shifting of the Gulf Stream, the Suwannee Channel, and sea level. The Cenozoic section in the study area indicates a stable structural framework except offshore Charleston where the Paleocene through Miocene reflectors are displaced by a northeast-southwest–trending monoclinal flexure downthrown to the southeast. The amount of displacement decreases from 20 m on the Paleocene surface to 10 m on the shallowest Miocene reflector seen 10 m below the sea floor. This flexure probably originated by reactivation of the deeply seated pre-Cretaceous Helena Banks fault revealed in multichannel profiles reported elsewhere.

Post glacial sea levels on the Western Canadian continental shelf: evidence for rapid change, extensive subaerial exposure, and early human habitation

Grounded piedmont type glaciers inundated and isostatically loaded the deep troughs which indent the Western Canadian continental shelf, as far west as the shelf edge. Glaciers do not appear to have covered the offshore banks east of the Queen Charlotte Islands (Haida Gwaii). Ice retreated from the shelf at approximately 13,500 14C yr B.P. Rapid emergence of the crust followed the ice retreat and resulted in a relative fall of sea level. At 10,400 14C yr B.P. relative sea level on the continental shelf was more than 100 m below that of today and large areas of the Queen Charlotte Basin were subaerially exposed. Eustatic sea-level rise, coupled with subsidence of a glacioisostatic forebulge, allowed sea levels to rise very rapidly, and reach the present shoreline on the Queen Charlotte Islands (Haida Gwaii) by about 9100 14C yr B.P. Dated shoreline deposits (shells), submerged wood remnants, and barnacles on stone tools at elevations between −110 m and +14 m suggest a sea-level rise of 6.3 cm per year between 12,200 and 11,000 calender years. Our reconstructions of the paleogeography and paleoenvironments suggest a hospitable environment for human habitation existed in areas that are now submerged. Stone tools excavated from intertidal deposits support this interpretation. Significant local variations in the depth of synchronous shorelines are described and attributed to localized differences in isostatic load. The documented rates of crustal adjustment are much greater than those used in conventional geophysical (forebulge) models. Regional high-resolution seismic reflection data (3400 line km) shows little evidence for post-glacial faulting and suggest that most crustal adjustments appear to have been isostatically rather than tectonically driven. Subaerial exposure and subsequent sea-level transgression were the dominant post-glacial processes that determined the morphology, texture and paleoenvironment of the Western Canadian continental shelf.

Sedimentary processes and variations in slope-current activity during the last Glacial-Interglacial episode on the Hebrides Slope, northern Rockall Trough, North Atlantic Ocean

Shallow cores and high-resolution seismic reflection data collected from two areas along the Hebrides Slope indicate that a variety of depositional processes have been active since the last Glacial interval. Seismic reflection profiles from the most northerly slope area, immediately south of the Wyville-Thomson Ridge, reveal a slope apron and an embayed area with a relict mid-upper slope channel. Five cores from this locality consist of distal muddy turbidites in the area of the channel, with muddy contourites interbedded with glaciomarine ice-rafted sediments and thin debris flow deposits. Most of the cores are topped by sandy contourites. The preserved sequence suggests that lowered sea-levels during the last glaciation (> 10 ka) resulted in a predominance of downslope and glaciomarine sedimentation, with bottom-current activity greatly suppressed. At the onset of deglaciation, alongslope activity increased as bottom currents gained in strength, with climatic fluctuation and the retreat of ice-sheets from the shelf, producing muddy and sandy contourites. Present-day current activity is still much in evidence with two current-meter stations on the mid and lower slope recording a northward-flowing current following the bathymetric contours. Recorded peak velocities at 468 m and 403 m were 26–48 cm/s. Some elements of reverse flow at the deeper site may be tidal oscillation compounded by short-duration high-intensity benthic storms. Cores from the more southerly area, in the vicinity of the Geikie Escarpment, penetrated hemipelagites erosively overlain by sandy contourites. Dinoflagellate and nannofossil dating of these deposits indicate that the lower hemipelagites are of late Glacial age (> 13 ka). The sandy contourites are probably of early Holocene age. The lack of the Allerød-Bølling interstadial and Younger Dryas stadial may be indicative of a hiatus in sedimentation. A possible cause of this may be the increasing slope angles (up to 28° on the escarpment), concentrating the effects of a thermohaline slope current. Current flow was also strengthened during deglaciation, producing erosion or non-deposition of the lower units leaving the coarser “top only” sandy contourites.

Mudflow Generated by Retrogressive Slope Failure, Santa Barbara Basin, California Continental Borderland

ABSTRACT The morphology and internal geometry of a mudflow deposit on the mainland slope of the Santa Barbara Basin are defined using high-resolution seismic-reflection data in combination with core samples. Sediment failure occurred on a 4° slope in the uppermost part of late Quaternary well-bedded slope deposits. The failure zone extends from water depths of 374-510 m near the base of slope, occupies an area of 4 km2, and involved the translation of 0.01-0.02 km3 of sediment. Major geomorphic features of the mudflow deposit include a headscarp 6-8 m high, a scar 50-700 m wide, and a main body 1 km long and 12 m thick. The hummocky surface of the mudflow deposits, their chaotic internal structure, and the bulbous toe tapering upslope to a thin tail are consistent wit mass flow involving extensive internal deformation. Sediment failed in stages, ending with upslope retrogressive retreat of the headwall along the east side of the failure zone. Known sedimentation rates of 0.8-1.4 m/k.y., as well as the presence of a thin (0.15-0.5 m thick) sediment cap resting atop the scar surface, indicate that the failure probably occurred within the past few centuries. A geotechnical analysis incorporating the results of both static and dynamic triaxial strength tests shows that the failure was probably caused by a strong (M 7.5) nearby earthquake. The weakened sediment that remained after earthquake shaking continued to flow down the gentle basin slope under the stresses generated by gravity alone. The analysis also shows that much of the slope sediment is marginally stable and that additional mudflows will probably occur during future strong seismic shaking.