|
Hein et al, in press, Geology paper:
Methanogenic calcite, 13C-depleted bivalve shells, and gas hydrate from a mud volcano offshore southern California
Abstract:
Methane and hydrogen sulfide vent from a cold seep above a shallowly buried methane hydrate in a mud volcano located 24 km offshore southern California in 800 m water. Bivalves, authigenic calcite, and methane hydrate were recovered in a 2.1 m piston core. Aragonite shells of two bivalve species are unusually depleted in 13C (to -19‰ δ13C), the most 13C-depleted shells of marine macrofauna yet discovered. Carbon isotopes for both living and dead specimens indicate that they used in part C derived from anaerobically oxidized methane (AOM) to construct their shells. Although the 13C values are highly variable, most fall within the range -12 to -19‰. This variability may be diagnostic for identifying cold-seep/hydrate systems in the geologic record. Authigenic calcite is abundant in the cores down to about 1.5 m subbottom, the top of the methane hydrate. The calcite is depleted in 13C (δ13C = -46 to -58‰) indicating that C produced by the AOM was the main source. Likely methane sources include a geologic hydrocarbon reservoir from Miocene source rocks, and biogenic and thermogenic degradation of organic matter in basin sediments. Oxygen isotopes indicate that most calcite formed out of isotopic equilibrium with ambient bottom water, under the influence of gas hydrate dissociation and strong methane flux. High concentrations of Ag, Hg, Cd, Tl, and other elements in mud volcano sediment may indicate leaching of basement rocks by fluid circulating along an underlying fault, which also allows for a high flux of fossil methane to the seep/hydrate site.
Back to Reports
Top of Page
Baher et al, 2005, BSSA paper:
Upper-Crustal Structure of the Inner Continental Borderland near Long Beach, California
Abstract:
A new P-wave velocity/structural model for the inner Continental Borderland (ICB) region was developed for the area near Long Beach, California. It combines controlled-source seismic reflection and refraction data collected during the 1994 Los Angeles Region Seismic Experiment (LARSE), multichannel seismic reflection data collected by the U.S. Geological Survey (1998-2000), and nearshore borehole stratigraphy. Based on lateral velocity contrasts and stratigraphic variation determined from borehole data, we are able to locate major faults such as the Cabrillo, Palos Verdes, THUMS-Huntington Beach, and Newport Inglewood fault zones, along with minor faults such as the slope fault, Avalon knoll, and several other yet unnamed faults. Catalog seismicity (1975-2002) plotted on our preferred velocity/structural model shows recent seismicity is located on 16 out of our 24 faults, providing evidence for continuing concern with respect to the existing seismic-hazard estimates.
Forward modeling of P-wave arrival times on the LARSE line 1 resulted in a four-layer model that better resolves the stratigraphy and geologic structures of the ICB and also provides tighter constraints on the upper-crustal velocity structure than previous modeling of the LARSE data. There is a correlation between the structural horizons identified in the reflection data with the velocity interfaces determined from forward modeling of refraction data. The strongest correlation is between the base of velocity layer 1 of the refraction model and the base of the planar sediment beneath the shelf and slope determined by the reflection model. Layers 2 and 3 of the velocity model loosely correlate with the diffractive crust layer, locally interpreted as Catalina Schist.
Back to Reports
Top of Page
Fisher et al, 2005, BSSA paper:
Neotectonics of the Offshore Oak Ridge Fault near Ventura, Southern California
Abstract:
The Oak Ridge fault is a large-offset, south-dipping reverse fault that forms the south boundary of the Ventura Basin in southern California. Previous research indicates that the Oak Ridge fault south of the town of Ventura has been inactive since 200-400 ka ago and that the fault tip is buried by ~1km of Quaternary sediment. However, very high-resolution and medium-resolution seismic reßection data presented here show a south-dipping fault, on strike with the Oak Ridge fault, that is truncated at 80 m depth by an unconformity that is probably at the base of late Pleistocene and Holocene sediment. Furthermore, if vertically aligned features in seismic reßection data are eroded remnants of fault scarps, then a subsidiary fault within the Oak Ridge system deforms the shallowest imaged sediment layers. We propose that this subsidiary fault has mainly left-slip offset. These observations of Holocene slip on the Oak Ridge fault system suggest that revision of the earthquake hazard for the densely populated Santa Clara River valley and the Oxnard coastal plain may be needed.
Back to Reports
Top of Page
Bohannon et al, 2004, Tectonics paper:
Holocene to Pliocene tectonic evolution of the region offshore of the Los Angeles urban corridor, southern California
Abstract:
Quaternary tectonism in the coastal belt of the Los Angeles urban corridor is diverse. In this paper we report the results of studies of multibeam bathymetry and a network of seismic reflection profiles that have been aimed at deciphering the diverse tectonism and at evaluating the relevance of published explanations of the region's tectonic history. Rapid uplift, subsidence in basins, folds and thrusts, extensional faulting, and strike-slip faulting have all been active at one place or another throughout the Quaternary Period. The tectonic strain is reflected in the modern physiography at all scales. Los Angeles (LA) Basin has filled from a deep submarine basin to its present condition with sediment impounded behind a large sill formed behind uplifts near the present shoreline. Newport trough to the south-southeast of LA Basin also accumulated a large volume of sediment, but remained at midbathyal depths throughout the Period. There is little or no evidence of Quaternary extensional tectonism in either basin although as much as 6 km of subsidence, which mainly occurred by sagging, has been recorded in places since the middle Miocene. The uplifts include folded and thrust faulted terranes in the Palos Verdes Hills and the shelves of Santa Monica and San Pedro Bays. The uplifted areas have been shortened in a southwest-northeast direction by 10% or slightly more, and some folds are reflected in the bathymetry. Two large adjacent midbathyal basins, Santa Monica and San Pedro, show strong evidence of subsidence and slight west-northwest extension (10%) during the same time folding was taking place in the uplifts. The tectonic boundaries between uplifts and basins are folded, normal faulted, reverse-faulted, and strike-slip faulted depending on location. The rapid Quaternary uplift and subsidence, along with the filling of LA Basin, have produced a reversal in the regional physiography. In the early Pliocene, LA Basin was a submarine deep, Palos Verdes and the shelves comprised a northeast basin slope, and the present offshore basins and Catalina Island formed an emergent or shallowly submerged shelf. Since extensional, compressional, and lateral strains are all locally in evidence, simple notions that this part of southern California underwent a change from Miocene transtension to Quaternary transpression fail to explain our observations.
Back to Reports
Top of Page
Fisher et al, 2004, BSSA paper:
The Offshore Palos Verdes Fault Zone Near San Pedro, Southern California
Abstract:
High-resolution seismic-reflection data are combined with a variety of other geophysical and geological data to interpret the offshore structure and earthquake hazards of the San Pedro shelf, near Los Angeles, California. Prominent structures investigated include the Wilmington graben, the Palos Verdes fault zone, various faults below the west part of the San Pedro shelf and slope, and the deep-water San Pedro basin. The structure of the Palos Verdes fault zone changes markedly along strike southeastward across the San Pedro shelf and slope. Under the north part of the shelf, this fault zone includes several strands, with the main strand dipping west. Under the slope, the main fault strands exhibit normal separation and mostly dip east. To the southeast near Lasuen Knoll, the Palos Verdes fault zone locally is low angle, but elsewhere near this knoll, the fault dips steeply. Fresh seafloor scarps near Lasuen Knoll indicate recent fault movement. We explain the observed structural variation along the Palos Verdes fault zone as the result of changes in strike and fault geometry along a master right-lateral strike-slip fault at depth. Complicated movement along this deep fault zone is suggested by the possible wavecut terraces on Lasuen Knoll, which indicate subaerial exposure during the last sealevel lowstand and subsequent subsidence of the knoll. Modeling of aeromagnetic data indicates a large magnetic body under the west part of the San Pedro shelf and upper slope. We interpret this body to be thick basalt of probable Miocene age. This basalt mass appears to have affected the pattern of rock deformation, perhaps because the basalt was more competent during deformation than the sedimentary rocks that encased the basalt. West of the Palos Verdes fault zone, other northwest-striking faults deform the outer shelf and slope. Evidence for recent movement along these faults is equivocal, because we lack age dates on deformed or offset sediment.
Back to Reports
Top of Page
Greene et al, 2004, Canadian Geotech. paper:
Types and mechanisms of mass-wasting events that shape the California continental margin, USA
Abstract:
Multibeam bathymetric and seismic reflection profile data collected along selected parts of the California continental margin indicate that mass wasting is a significant process that has shaped the seafloor in the past and is presently active today. A variety of mass movement features mapped along the margin indicate that many different triggering mechanisms are at work to produce landslides. Some of these mass-wasting features cover extensive areas such as the 130 km2 Goleta landslide in the Santa Barbara Basin a complex compound slide that has been produced from different types of movements and mechanisms occurring at different times throughout its failure history. Potential triggering mechanisms for submarine landslides include fluid flow, tectonic oversteepening of slopes, earthquakes, sediment and rock undercutting in submarine canyons, sediment accumulations reaching the angle of repose, and terrestrial input from subarial mass-movement features. Many of these slides appear capable of producing a tsunami.
Back to Reports
Top of Page
Lee et al, 2004, OTC paper:
Timing and Extent of Submarine Landslides in Southern California
Abstract:
Submarine landslide deposits occur in many locations throughout the Southern California Borderland and indicate the potential for continued slope failure. Future landslide activity may constitute a direct hazard to offshore facilities and an indirect hazard to coastal communities through landslide-induced tsunamis. Evaluating the risk of these hazards requires information on the scale of landslides that can occur and their recurrence rate. In this study, five mass transport complexes are described and volumes are estimated. Two of these complexes, the Palos Verdes debris avalanche and the Goleta slide contain the remains of many past events. Using dated cores and tracing stratigraphy to nearby ODP borings, we have estimated ages of the most recent failures in the five complexes and some of the ages of earlier failures in the Goleta slide. These results show that the volumes of the failed masses vary over several orders of magnitude with the largest of the masses having volumes on the order of 0.5 km3. The ages of the failures range from a few hundred years to over 100,000 years. The two complexes that show repeated failure represent the largest landslides we evaluated and probably are the largest complexes on the mainland slope in Southern California. We estimate that these large failures on the mainland slope probably reoccur with an interval that has an order of magnitude of 10,000 years.
Back to Reports
Top of Page
Normark et al, 2004, S. Coast Geol. Soc. paper:
Late Quaternary sedimentation and deformation in Santa Monica and Catalina Basins, offshore southern California
Abstract:
The late Pleistocene history of sedimentation in Santa Monica Basin has been documented using seismic-reflection profiles with ground truth provided by drilling at Ocean Drilling Program (ODP) Site 1015 on the basin floor. High-resolution deep-tow boomer profiles together with both multichannel and single-channel seismic-reflection data provide a framework of 15 key horizons in the upper 200 m of basin fill. The uppermost 12 key reflectors, many of which have been traced across much of the basin in the upper 100 m of sediment fill, have been correlated with the sequence cored at ODP Site 1015. Recently completed radiocarbon dating of samples from Site 1015 on the floor of Santa Monica Basin confirmed a Holocene rate of nearly 3 m/ky, which is the highest yet documented for southern California deep-water basins and only slightly lower than the peak rate during sea-level lowstand. The radiocarbon dates provide stratigraphic age control for the upper 12 key reflectors back to 32 ka at ~100 meters below the sea floor (mbsf). The dated stratigraphic sequence is used to evaluate deformation along the linear southwestern margin of Santa Monica Basin that is formed by the Santa Cruz-Catalina Ridge (SC-CR). In the northwestern corner of the basin, turbidite deposits of Hueneme Fan show local evidence for flexure of sediment horizons as young as 6 ka with minor fault offsets as recently as 1.5 ka. Larger scale anticlinal folding (~5 km width and >100 m of relief) of the basin fill is observed for strata older than ~65 ka. Farther south in the basin, however, much of the western as well as the southern margin of Santa Monica Basin shows limited evidence for tectonic activity affecting the basin fill during the last 100 ka.
The Santa Cruz-Catalina Ridge (SC-CR) separates Santa Monica Basin and the Catalina Basin to the west. There is limited seismic-reflection data for study of the sediment fill of Catalina Basin compared to Santa Monica Basin. The effects of sea level on sources for sediment entering the San Gabriel Canyon system on the Long Beach shelf control the largest sediment inputs to Catalina Basin. We use sediment accumulation rates determined from 20 recently obtained piston cores in San Pedro Basin, the Gulf of Santa Catalina, and San Diego Trough as an analog for the rate of sediment input to Catalina Basin. Deformation within the Catalina Basin fill during the latest (<300 ky) Quaternary is recorded by successive tilting and fault offsets of deposits primarily deposited during lowstands of sea level.
Back to Reports
Top of Page
Normark et al, 2004, Marine Geology paper:
Age of Palos Verdes submarine debris avalanche, southern California
Abstract:
The Palos Verdes debris avalanche is the largest, by volume, late Quaternary mass-wasted deposit recognized from the inner California Borderland basins. Early workers speculated that the sediment failure giving rise to the deposit is young, taking place well after sea level reached its present position. A newly acquired, closely-spaced grid of high-resolution, deep-tow boomer profiles of the debris avalanche shows that the Palos Verdes debris avalanche fills a turbidite leveed channel that extends seaward from San Pedro Sea Valley, with the bulk of the avalanche deposit appearing to result from a single failure on the adjacent slope. Radiocarbon dates from piston-cored sediment samples acquired near the distal edge of the avalanche deposit indicate that the main failure took place about 7500 yr BP.
Back to Reports
Top of Page
Fisher et al, 2003, BSSA paper:
Geology of the Continental Margin Beneath Santa Monica Bay, Southern California, from Seismic-Reflection Data
Abstract:
We interpret seismic-reflection data, which were collected in Santa Monica Bay using a 70 in3 generator-injector airgun, to show the geologic structure of the continental shelf and slope and of the deep-water, Santa Monica and San Pedro basins. The goal of this research is to investigate the earthquake hazard posed to urban areas by offshore faults. These data reveal that northwest of the Palos Verdes Peninsula, the Palos Verdes fault neither offsets the seafloor nor cuts through an undeformed sediment apron that postdates the last sea level rise. Other evidence indicates that this fault extends northwest beneath the shelf in the deep subsurface. Other major faults in the study area, such as the Dume and San Pedro Basin faults, were active recently, as indicated by an arched seafloor and offset shallow sediment. Rocks under the lower continental slope are deformed to differing degrees on opposite sides of Santa Monica Canyon. Northwest of this canyon, the continental slope is underlain by a little-deformed sediment apron; the main structures that deform this apron are two lower-slope anticlines that extend toward Point Dume and are cored by faults showing reverse or thrust separation. Southeast of Santa Monica Canyon, lower-slope rocks are deformed by a complex arrangement of strike-slip, normal and reverse faults. The San Pedro Escarpment rises abruptly along the southeast side of Santa Monica Canyon. Reverse faults and folds underpinning this escarpment steepen progressively southeastward. Locally they form flower structures and cut downward into basement rocks. These faults merge downward with the San Pedro Basin fault zone, which is nearly vertical and strike-slip. The escarpment and its attendant structures diverge from this strike-slip fault zone and extend for 60 km along the margin, separating the continental shelf from the deep-water basins. The deep-water Santa Monica Basin has large extent but is filled with only a thin (less than 1.5 km) section of what are probably post-Miocene rocks and sediment. Extrapolating ages obtained from ODP site 1015 indicates that this sedimentary cover is Quaternary, possibly no older than 600 ka. Folds and faults along the base of the San Pedro escarpment began to form during 8 ka to 13 ka ago. Refraction-velocity data show that high-velocity rocks, probably the Catalina schist or Miocene volcanic rocks, underlie the sedimentary section. The San Pedro basin developed along a strike-slip fault, widens to the southeast, and is deformed by faults having apparent reverse separation and by folds near Redondo Canyon and the Palos Verdes Peninsula.
Back to Reports
Top of Page
Normark et al, 2002, Marine Geology paper:
Variability in form and growth of sediment waves on turbidite channel levees
Abstract:
Fine-grained sediment waves have been observed in many modern turbidite systems, generally restricted to the overbank depositional element. Sediment waves developed on six submarine fan systems are compared using high-resolution seismic-reflection profiles, sediment core samples (including ODP drilling), multibeam bathymetry, 3D seismic-reflection imaging (including examples of buried features), and direct measurements of turbidity currents that overflow their channels. These submarine fan examples extend over more than three orders of magnitude in physical scale. The presence or absence of sediment waves is not simply a matter of either the size of the turbidite channel-levee systems or the dominant initiation process for the turbidity currents that overflow the channels to form the wave fields. Both sediment-core data and seismic-reflection profiles document the upslope migration of the wave forms, with thicker and coarser beds deposited on the up-current flank of the waves. Some wave fields are orthogonal to channel trend and were initiated by large flows whose direction was controlled by upflow morphology, whereas fields subparallel to channel levees resulted from local spillover. In highly meandering systems, sediment waves may mimic meander planform. Larger sediment waves form on channel-levee systems with thicker overflow of turbidity currents, but available data indicate that sediment waves can be maintained during conditions of relatively thin overflow. Coarser-grained units in sediment waves are typically laminated and thin-bedded sand as much as several centimetres thick, but sand beds as thick as several tens of centimetres have been documented from both modem and buried systems. Current production of hydrocarbons from sediment-wave deposits suggests that it is important to develop criteria for recognising this overbank element in outcrop exposures and borehole data, where the wavelength of typical waves (several kilometres) generally exceeds outcrop scales and wave heights, which are reduced as a result of consolidation during burial, may be too subtle to recognise.
Back to Reports
Top of Page
Kennedy and Clarke, 2001, Calif. Geol. paper:
Late Quaternary faulting in San Diego Bay and hazard to the Coronado Bridge
Abstract:
Southern California is transected by numerous pervasive northwest-trending Quaternary fault zones. Together they form the broad transform-fault boundary along which the Pacific and North America crustal plates move irregularly past one another in a right-lateral sense at a rate of about 5 centimeters (cm)/year. The city of San Diego, which lies adjacent to the Pacific Ocean in the southwestern-most corner of California, is cut by one such fault zone-- the Rose Canyon Fault Zone. Oblique movement on faults within the Rose Canyon Fault Zone has, over time, led to the development of San Diego Bay, which separates the metropolitan area of San Diego from Coronado and North Island.
The Coronado Bridge spans San Diego Bay and connects the cities of San Diego and Coronado. The bridge is supported by 32 piers, 21 of which (piers 3-23) rest on footings anchored in the bay floor and rise above mean sea level to elevations ranging from 5 meters (m) at Coronado to more than 75 m in the main channel of the bay near San Diego. A principal concern regarding the bridge's earthquake safety involves its proximity, especially of its foundation piers, to potential shallow fault rupture. Our objectives in this study were 1) to identify and accurately locate Holocene faults (those younger than about 12,000 yrs); and 2) to determine the time of most recent movement on these faults and therefore their potential hazard to the Coronado Bridge.
Back to Reports
Top of Page
Piper and Normark, 2001, AAPG Bulletin paper:
Sandy fans --- from Amazon to Hueneme and beyond
Abstract:
Most submarine fans are supplied with both sand and mud, but these become segregated during transport, typically with the sand becoming concentrated in channels and channel-termination lobes. New data from high-resolution seismic reflection surveys and Deep Sea Drilling Project (DSDP)/Ocean Drilling Program (ODP) wells from a variety of fans allow a synthesis o the architecture of those submarine fans that have important sand deposits. By analyzing architectural elements, we can better understand issues important for petroleum geology, such as the reservoir properties of the sand bodies and their lateral continuity and vertical connectivity. Our analysis of fan architecture is based principally on the Amazon and Hueneme fans, generally perceived to be classic examples of muddy and sandy systems, respectively. We recognize depositional elements, for example, channel deposits, levees, and lobes, from seismic reflection data and document sediment character in different elements from DSDP/ODP drill cores. We show the utility for petroleum geology of evaluating sandy and muddy elements rather than characterizing entire fans as sand rich or mud rich. We suggest that fan classification should include evaluation of source-sediment volumes and grain size, as well as the probable processes of turbidity-current initiation, because these factors control the character of fan elements and their response to changes in sea level, sediment supply, and autocyclic changes in channel pattern. Basin morphology, controlled by tectonics, influences overall geometry, as well as the balance between aggradation and progradation.
Back to Reports
Top of Page
Marlow et al, 2000, Geology paper:
Using high-resolution multibeam bathymetry to identify seafloor surface rupture along the Palos Verdes fault complex in offshore southern California
Abstract:
Recently acquired high-resolution multibeam bathymetric data reveal several linear traces that are the surficial expressions of seafloor rupture of Holocene faults on the upper continental slope southeast of the Palos Verdes Peninsula. High-resolution multichannel and boomer seismic-reflection profiles show that these linear ruptures are the surficial expressions of Holocene faults with vertical to steep dips. The most prominent fault on the multibeam bathymetry is about 10 km to the west of the mapped trace of the Palos Verdes fault and extends for at least 14 km between the shelf edge and the base of the continental slope. This fault is informally called the Avalon Knoll fault for the nearby geographic feature of that name. Seismic-reflection profiles show that the Avalon Knoll fault is part of a northwest–trending complex of faults and anticlinal uplifts that are evident as scarps and bathymetric highs on the multibeam bathymetry. This fault complex may extend onshore and contribute to the missing balance of Quaternary uplift determined for the Palos Verdes Hills and not accounted for by vertical uplift along the onshore Palos Verdes fault. We investigate the extent of the newly located offshore Avalon Knoll fault and use this mapped fault length to estimate likely minimum magnitudes for events along this fault.
Back To Reports
Top of Page
ten Brink et al, 2000, JGR paper:
Geophysical evidence for the evolution of the California Inner Continental Borderland as a metamorphic core complex
Abstract:
We use new seismic and gravity data collected during the 1994 Los Angeles Region Seismic Experiment (LARSE) to discuss the origin of the California Inner Continental Borderland (ICB) as an extended terrain possibly in a metamorphic core complex mode. The data provide detailed crustal structure of the Borderland and its transition to mainland southern California. Using tomographic inversion as well as traditional forward ray tracing to model the wide-angle seismic data, we find little or no sediments, low (²6.6 km/s) P wave velocity extending down to the crust-mantle boundary, and a thin crust (19 to 23 km thick). Coincident multichannel seismic reflection data show a reflective lower crust under Catalina Ridge. Contrary to other parts of coastal California, we do not find evidence for an underplated fossil oceanic layer at the base of the crust. Coincident gravity data suggest an abrupt increase in crustal thickness under the shelf edge, which represents the transition to the western Transverse Ranges. On the shelf the Palos Verdes Fault merges downward into a landward dipping surface which separates "basement" from low-velocity sediments, but interpretation of this surface as a detachment fault is inconclusive. The seismic velocity structure is interpreted to represent Catalina Schist rocks extending from top to bottom of the crust. This interpretation is compatible with a model for the origin of the ICB as an autochthonous formerly hot highly extended region that was filled with the exhumed metamorphic rocks. The basin and ridge topography and the protracted volcanism probably represent continued extension as a wide rift until ~13 m.y. ago. Subduction of the young and hot Monterey and Arguello microplates under the Continental Borderland, followed by rotation and translation of the western Transverse Ranges, may have provided the necessary thermomechanical conditions for this extension and crustal inflow.
Back to Reports
Top of Page
Piper et al, 1999, Sedimentology paper:
Outcrop-scale acoustic facies analysis and latest Quaternary development of Hueneme and Dume submarine fans, offshore California
Abstract:
The uppermost Quaternary deposits of the Hueneme and Dume submarine fans in the Santa Monica Basin have been investigated using a closed-spaced grid of boomer seismic-reflection profiles, which give vertical resolution of a few tens of centimetres with acoustic penetration to 50 m. Acoustic facies integrated with geometry define six architectural elements, some with discrete subelements that are of a scale that can be recognized in outcrops of ancient turbidite systems. In the Santa Monica Basin, the relationship of these elements to fan morphology, stratigraphy and sediment source is precisely known.
The width of upper Hueneme fan valley has been reduced from 5 km since the last glacial maximum to 1 km at present by construction of laterally confined sandy levees within the main valley. The middle fan comprises three main subelements: thick sand deposits at the termination of the fan valley, low-gradient sandy lobes typically 5 km long and <10 m thick, and scoured lobes formed of alternating sand and mud beds with many erosional depressions. The site of thickest lobe sediment accumulation shifts through time, with each sand bed deposited in a previous bathymetric low (i.e. compensation cycles). The lower fan and basin plain consists of sheet-like alternations of sand and mud with shallow channels and lenses.
Variations in the rate of late Quaternary sea level rise initiated changes in sediment facies distribution. At lowstand, and during the approximately 11 ka stillstand in sea level, the Hueneme Fan was fed largely by hyperpycnal flow from the Santa Clara River delta, depositing high sediment waves on the right hand levee and thick sandy lobes on the middle fan. At highstand of sea level, most turbidity currents were generated by failure of silty prodelta muds. In contrast, the smaller Dume Fan was apparently always fed from littoral drift of sand through a single-canyon point source.
Back to Reports
Top of Page
Bohannon and Geist, 1998, GSA Bulletin paper:
Upper crustal structure and Neogene tectonic development of the California Continental Borderland
Abstract:
Multichannel seismic-reflection data, sonobuoy seismic-refraction data, and regional geology are used to define the upper crustal structure of the southern California continental borderland and to delineate the characteristics of the main lithotectonic belts of the region. The Catalina Schist belt is separated on its west side from the gently deformed Nicolas forearc belt by faults that have steep west dips and pronounced normal separations. On its east side the schist belt is bounded by a large detachment fault that dips gently to the east beneath the west edge of the Peninsular Ranges belt at the coastline near Oceanside. The Catalina Schist was uplifted from middle crustal depths and exposed during a major event of extensional tectonism that started in early Miocene time in conjunction with about 10° of clockwise rotation of the western Transverse Ranges belt. Part of the uplift of the Catalina Schist could have occurred on the detachment fault, but it is thought to have mostly occurred on the steep faults that bound the west edge of the schist belt. A large amount of uplift is required, and it probably involved strong footwall flexural deformation in the wake of the translating and rotating western Transverse Ranges and Nicolas forearc belts. Extension, accompanied by probable large amounts of right slip, continued in the borderland region during and after middle Miocene time. The later stage of extension was accompanied by rapid clockwise rotation of the western Transverse Ranges of at least 90°. Most of the borderland, including the belt of schist that was uplifted in early Miocene time, was further deformed into numerous basins and ridges during this stage of oblique extension. The primary driving force for the deformation is thought to have been derived from the rapid northwest motion of the Pacific plate after it had become coupled to the Farallon plate system, which had previously been subducted beneath the borderland.
Back to Reports
Top of Page
Normark et al, 1998, Sedimentology paper:
Sea level controls on the textural characteristics and depositional architecture of the Hueneme and associated submarine fan systems, Santa Monica Basin, California
Abstract:
Hueneme and Dume submarine fans in Santa Monica Basin consist of sandy channel and muddy levee facies on the upper fan, lenticular sand sheets on the middle fan, and thinly bedded turbidite and hemipelagic facies elsewhere. Fifteen widely correlatable key seismic reflections in high-resolution airgun and deep-towed boomer profiles subdivide the fan and basin deposits into time-slices that show different thickness and seismic-facies distributions, inferred to result from changes in Quaternary sea level and sediment supply. At times of low sea level, highly efficient turbidity currents generated by hyperpycnal flows or sediment failures at river deltas carry sand well out onto the middle-fan area. Thick, muddy flows formed rapidly prograding high levees mainly on the western (right-hand) side of three valleys that fed Hueneme fan at different times; the most recently active of the lowstand fan valleys, Hueneme fan valley, now heads in Hueneme Canyon. At times of high sea level, fans receive sand from submarine canyons that intercept littoral-drift cells and mixed sediment from earthquake-triggered slumps. Turbidity currents are confined to thalweg channels in fan valleys and to steep, small, basin-margin fans like Dume fan. Mud is effectively separated from sand at high sea level and moves basinward across the shelf in plumes and in storm-generated lutite flows, contributing to a basin-floor blanket that is locally thicker than contemporary fan deposits and that onlaps older fans at the basin margin. The infilling of Santa Monica Basin has involved both fan and basin-floor aggradation accompanied by landward and basinward facies shifts. Progradation was restricted to the downslope growth of high muddy levees and the periodic basinward advance of the toe of the steeper and sandier Dume fan. Although the region is tectonically active, major sedimentation changes can be related to eustatic sea-level changes. The primary controls on facies shifts and fan growth appear to be an interplay of texture of source sediment, the efficiency with which turbidity currents transport sand, and the effects of delta distributary switching, all of which reflect sea-level changes.
Back to Reports
|
CABRILLO Home | Tierra | Reports | Papers
