Geology topics

W. R. Normark

Publications and source records attributed to W. R. Normark.

At least 37 records · Page 2Linked to original sources

USGS2003

Geology of the continental margin beneath Santa Monica Bay, Southern California, from seismic-reflection data

We interpret seismic-reflection data, which were collected in Santa Monica Bay using a 70-in3 generator-injector air gun, 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. However, 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 Ocean Drilling Program 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 8-13 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.

California↗

USGS2002

Outcrop-scale acoustic facies analysis and latest Quaternary development of Hueneme and Dume submarine fans, offshore California

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.

Sedimentology↗

USGS2002

Sea level controls on the textural characteristics and depositional architecture of the Hueneme and associated submarine fan systems, Santa Monica Basin, California

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 ‘underfit’ talweg 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.

California↗

USGS2002

Variability in form and growth of sediment waves on turbidite channel levees

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 burried 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 maintaned 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 modern 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. Crown Copyright ?? 2002 Published by Elsevier Science B.V. All rights reserved.

Marine Geology↗

USGS2001

Vertical tectonics in northern Escanaba Trough as recorded by thick late Quaternary turbidites

Escanaba Trough, the southernmost segment of the Gorda Ridge, is filled by as much as 500 m of late Quaternary turbidite and hemipelagic sediment. Coring at Deep Sea Drilling Project Site 35 and Ocean Drilling Program (ODP) Sites 1037 and 1038 together with 4.5-kHz deep-tow and 3.5-kHz surface-ship seismic reflection profiles enable a distinct pattern of reflections to be mapped throughout Escanaba Trough in the upper part of this sediment fill. The uppermost 80 m of turbidite sediment, which includes at least 11 turbidity current events, were deposited in <7 kyr. Nine of these turbidites are found throughout Escanaba Trough at water depths >3200 m. The turbidity currents were trapped upon entering Escanaba Trough, resulting in all of the sediment in suspension in the flows being deposited. The thickness of the turbidite layers reflects both the flow thickness and the vertical grain concentration within the flow that deposited the layer. Variations in the turbidite thickness with respect to water depth can be used to estimate the degree of relative vertical movement within the floor of Escanaba Trough. In the area of hydrothermal activity near ODP Site 1038, uplift of as much as 140 m has occurred over the past 8 kyr.

California↗

USGS2000

Turbidite megabeds in an Oceanic Rift Valley recording jokulhlaups of late Pleistocene glacial lakes of the western United States

Escanaba Trough is the southernmost segment of the Gorda Ridge and is filled by sandy turbidites locally exceeding 500 m in thickness. New results from Ocean Drilling Program (ODP) Sites 1037 and 1038 that include accelerator mass spectrometry (AMS) 14C dates and revised petrographic evaluation of the sediment provenance, combined with high-resolution seismic-reflection profiles, provide a lithostratigraphic framework for the turbidite deposits. Three fining-upward units of sandy turbidites from the upper 365 m at ODP Site 1037 can be correlated with sediment recovered at ODP Site 1038 and Deep Sea Drilling Program (DSDP) Site 35. Six AMS 14C ages in the upper 317 m of the sequence at Site 1037 indicate that average deposition rates exceeded 10 m/k.yr. between 32 and 11 ka, with nearly instantaneous deposition of one ~60-m interval of sand. Petrography of the sand beds is consistent with a Columbia River source for the entire sedimentary sequence in Escanaba Trough. High-resolution acoustic stratigraphy shows that the turbidites in the upper 60 m at Site 1037 provide a characteristic sequence of key reflectors that occurs across the floor of the entire Escanaba Trough. Recent mapping of turbidite systems in the northeast Pacific Ocean suggests that the turbidity currents reached the Escanaba Trough along an 1100-km-long pathway from the Columbia River to the west flank of the Gorda Ridge. The age of the upper fining-upward unit of sandy turbidites appears to correspond to the latest Wisconsinan outburst of glacial Lake Missoula. Many of the outbursts, or jokulhlaups, from the glacial lakes probably continued flowing as hyperpycnally generated turbidity currents on entering the sea at the mouth of the Columbia River.

Journal of Geology↗

USGS2000

Using high-resolution multibeam bathymetry to identify seafloor surface rupture along the Palos Verdes fault complex in offshore Southern California

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.

Geology↗

USGS1999

No spreading across the southern Juan de Fuca ridge axial cleft during 1994-1996

Direct-path acoustic measurements between seafloor transponders observed no significant extension (−10±14 mm/yr) from August 1994 to September 1996 at the southern Juan de Fuca Ridge (44°40′ N and 130°20′ W). The acoustic path for the measurement is a 691-m baseline straddling the axial cleft, which bounds the Pacific and Juan de Fuca plates. Given an expected full-spreading rate of 56 mm/yr, these data suggest that extension across this plate boundary occurs episodically within the narrow (∼ 1 km) region of the axial valley floor, and that active deformation is occurring between the axial cleft and the plate interior. A cleft-parallel 714-m baseline located 300 m to the west of the cleft on the Pacific plate monitored system performance and, as expected, observed no motion (+5±7 mm/yr) between the 1994 and 1996 surveys.

Geophysical Research Letters↗

USGS1999

Late Pleistocene channel-levee development on Monterey submarine fan, central California

Much of the modern upper (proximal) Monterey fan is a channel–levee complex, the Upper Turbidite Sequence (UTS), that was deeply eroded after the channel breached a volcanic ridge to reach a deeper base level. Ages of sediment samples collected with the ALVIN submersible from the deepest outcrop within the channel–levee system, 390 m below the adjacent western levee crest, indicate that the UTS deposits accumulated at ≥1 m ka −1 during the last 500 ka. Neogene and Early Pleistocene sediment accumulation on the fan prior to the UTS was much slower (<0.03 m ka −1 ), and underlying turbidite systems(?) had substantially different morphologic expression(s).

California↗

USGS1999

Deep-sea sedimentary record of the late Wisconsin cataclysmic floods from the Columbia River

New results from Ocean Drilling Program Site 1037 and U.S. Geological Survey high-resolution seismic-reflection profiles confirm the great thickness, fast deposition rate, distant source, and convolute path of turbidites that fill the Escanaba Trough, the rift valley of the southernmost segment of the Gorda Ridge. Accelerator mass spectrometry 14 C measurements provide the first direct dating of the Escanaba Trough turbidites, demonstrating an average deposition rate faster than 10 m/k.y. between 32 and 11 ka and as fast as 15 m/k.y. during the oxygen isotope stage 2 lowstand. In the upper 60 m of sediment, the petrology of turbidite sand beds, which are as much as 12 m thick, show that the dominant source for the turbidites is from the Columbia River, which is more than 800 km to the north, rather than from the much closer rivers of northern California. New high-resolution seismic-reflection profiles show that, except for areas of very recent volcanism, the entire Escanaba Trough below 3200 m water depth is floored by the turbidite sequence that was cored in the upper 60 m at Site 1037B. The ages of the upper 120 m of turbidites correspond with the ages of channeled scabland deposits associated with latest Quaternary jokulhlaups from glacial Lake Missoula. The age and source characteristics suggest that these megaturbidite beds in Escanaba Trough are most likely deposits formed by hyperpycnally generated turbidity currents as the largest of the Lake Missoula floods entered the sea.

Geology↗

USGS1997

Shifting sources and transport paths for the late Quaternary Escanaba Trough sediment fill (northeast Pacific)

Escanaba Trough, which forms the southernmost part of the axial valley of the actively spreading Gorda Ridge, is filled with several hundred meters of sediment of presumed late Quaternary age. Surficial sediment samples from gravity cores, deeper samples (as much as 390 m) from Site 35 of the Deep Sea Drilling Program (Leg 5), and the acoustic character of the sediment fill observed on seismic-reflection profiles indicate that much of the sediment fill is of turbidite origin. Gross composition and heavy- mineral analyses of sand samples show that two distinct petrofacies comprise the sediment fill. The lower part of the fill was derived primarily from the Klamath River source of northern California while the younger fill, including the surficial sand beds, are from the Columbia River drainage much farther north. The Escanaba Trough sediment provides an opportunity to evaluate concepts for paleogeographic and paleotectonic reconstructions that are based on facies analysis and compositional and textural data for the volcanic components because both intrabasinal and extrabasinal sources are present as well as coeval (neovolcanic) and non coeval (paleovolcanic) sourcre This study of a modern basin shows, that although the sediment sources could be identified, it was useful to have some knowledge of the sediment pathway(s), the effects of diagenesis, and the possible effects of sediment sorting as a result of long transport distances from the source area for some components. Application of these same techniques to ancient deposits without benefit of the additional parameters will face limitations.

Giornale di Geologia↗

USGS1995

Giant blocks in the South Kona landslide, Hawaii

A large field of blocky sea-floor hills, up to 10 km long and 500 m high, are gigantic slide blocks derived from the west flank of Mauna Loa volcano on the island of Hawaii. These megablocks are embedded in the toe of the South Kona landslide, which extends ∼80 km seaward from the present coastline to depths of nearly 5 km. A 10–15-km-wide belt of numerous, smaller, 1–3-km-long slide blocks separates the area of giant blocks from two submarine benches at depths of 2600 and 3700 m depth that terminate seaward 20 to 30 km from the shoreline. Similar giant blocks are found on several other major submarine Hawaiian landslides, including those north of Oahu and Molokai, but the South Kona blocks are the first to be examined in detail using high-resolution bathymetry, dredging, and submersible diving. Dredging of two of the giant blocks brought up pillowed tholeiitic lava. Observations from the U.S. Navy submersible Sea Cliff on the asymmetrically steep eastern flank of one block 10 km long and 300 m high revealed a succession of fractured massive basalt, laminar lava flows, hyaloclastite, and pillow lavas. Chemical analyses of dredged lava identified 19 units that overlap compositionally with lavas from the south rift-zone ridge of Mauna Loa. Sulfur content indicates that most of the lavas were erupted in subaerial and shallow submarine (<200 m depth) sites, but some were erupted in deeper submarine sites. These results indicate that the megablocks were carried by a late Pleistocene giant landslide 40–80 km west from the ancestral shoreline of Mauna Loa volcano before growth of the midslope benches by later slump movement.

Hawaii↗

USGS1994

Giant Hawaiian landslides

Sixty-eight landslides more than 20 km long are present along a 2200 km segment of the Hawaiian Ridge from near Midway to Hawaii. Some of the landslides exceed 200 km in length and 5000 km3 in volume, ranking them among the largest on Earth. Most of these giant landslides were discovered during a mapping program of the U.S. Hawaiian Exclusive Economic zone from 1986 to 1991 utilizing the GLORIA side-looking sonar mapping system. Two general types of landslides are present: slumps and debris avalanches. Many intermediate forms occur and some debris avalanches form from oversteepened slumps. The slumps are deeply rooted in the volcanoes and may extend back to volcanic rift zones and down to the base of the volcanic pile at about 10 km depth. Debris avalanches are thinner, longer, and move on lower gradients than slumps. Their rapid movement is indicated by the fact that some have moved uphill for tens of kilometers, and are believed to have produced major tsunamis. The debris avalanches left large amphitheaters at their heads and produce broad hummocky distals lobes at their toes. Commonly, major canyons have incised the amphitheaters. Giant landslides have recently been discovered on many other marine volcanoes where they also can be related to volcanic structure and eruptive activity.

Book chapter↗

USGS1993

Annotated bibliography; marine geologic hazards of the Hawaiian Islands with special focus on submarine slides and turbidity currents

Explore the source record for details and available documents.

Open-File Report↗

USGS1992

Effect of sea-level variation on upper-slope depositional processes offshore of Tiber delta, Tyrrhenian Sea, Italy

The upper slope and outer shelf in front of the Tiber River mouth (east-central Tyrrhenian Sea) exhibits a series of about 15 gullies that are not clearly related to present-day erosional or depositional processes. An extensive, high-resolution, seismic-reflection profiling survey of a 450 km 2 area in front of the Tiber delta shows several generations of similar features within the older depositional sequences underlying the outer continental shelf. The gully relief appears dominantly depositional, probably developed during lowstand periods when the Tiber River mouth was relatively near the shelf break. The position of the gullied intervals, including those on the modern slope relict from the last lowstand, shows a successive northward shift with time. This northward shift indicates continued tilting of this part of the eastern Tyrrhenian continental margin, probably resulting from a continued subsidence to the north.

Marine Geology↗

USGS1992

Map showing bottom topography of the Pacific continental margin, Point Conception to Point Loma

All contours, geographic outlines, and political boundaries shown on this map of the bottom topography, or bathymetry, of the Pacific continental margin between 34° and 41° N. latitudes were plotted from digital data bases in the library of the U.S. Geological Survey (USGS)-National Oceanic and Atmospheric Administration (NOAA) Joint Office for Mapping and Research (JOMAR). These digital data were obtained and compiled from many sources; consequently, data quality varies within particular data bases as well as from one data base to another. Bathymetric contours north of 32°N. were digitized from a map by Chase and others (1981) and from the unpublished large scale versions of that map (T. E. Chase, unpublished maps, 1981). South of 32°N, the contours are from unpublished maps (T.E. Chase and B.A. Seekins, unpublished maps, 1989). Chase and others (1981 and unpublished maps) obtained the data for the area seaward of the continental slope (-2000 m depth) primarily from the U.S. Coast and Geodetic Survey (C&GS) 1955 Pacific Exploratory Survey, a systematic and detailed (~8-nmi-trackline spacing) survey between Mexico and Canada. The USGS provided data from cruises S3-78-NC, L2-77-NC, L10-76-NC, K-73-NC, and Bartlett 72. Data were also obtained from Scripps Institution of Oceanography cruises Blue Flash, Kayak B, Scan I, and Seven Tow (Chase and Menard, 1971; Chase and others, 1975). The 200-m contour was derived from the National Ocean Survey charts 1206N-16 (1975a) and 1306N-20 (1975b), and C&GS charts 1206N-15 (1967a) and 1306N-19 (1967b). Sea-floor depths were corrected for sound velocity in sea water using Matthew's (1939) tables. Onshore topographic contours were generated by computer from a modified version of 3-arc-second elevation data provided by the Defense Mapping Agency. The United States digital shoreline was obtained from the NOAA, NOS, Nautical Charting Division, National Atlas files. The coastline of Canada was digitized from Canadian Hydrographic Service bathymetric maps. The primary source of names of the sea floor features was the "Gazetteer of Undersea Features" (Defense Mapping Agency, 1990). Acknowledgments Christopher Hines assisted in the construction of the digital data bases. Reviews and suggestions by Edward C. Escowitz and Florence Wong and advice provided by Will Stettner regarding the cartographic design substantially improved the quality of this map. References Cited Chase, T.E., and Menard, H.W., 1971, Bathymetric atlas of the northeastern Pacific Ocean: U.S. Naval Oceanographic Office Publication 1303, scale 1:2,000,000 at 33° latitude, 48 p. Chase, T.E., Wilde, Pat, and Normark, W.R., 1975, Oceanographic data of the Monterey Deep Sea Fan: San Diego, University of California, Institute of Marine Resources Publication TR 58, scale 1:898,524 at 35° latitude. Chase, T.E., Wilde, Pat, Normark, W.R., Miller, C.P., Seekins, B.A., and Young, J.D., 1981, Offshore topography of the Western United States between 32° and 49° North latitudes: U.S. Geological Survey Open-File Report 81-443, scale 1:864,518 at 38° latitude, 2 sheets. Coast and Geodetic Survey, 1967a, Santa Barbara to Huntington Beach : Coast and Geodetic Survey Bathymetric Map 1206N-15, scale 1:250,000. Coast and Geodetic Survey, 1967b, Southwest of Santa Rosa Island: Coast and Geodetic Survey Bathymetric Map 1306N-19, scale 1:250,000. Defense Mapping Agency, 1990, Gazetteer of undersea features (4th ed.): Washington, D.C., Defense Mapping Agency. Matthews, D.J., 1939, Tables of the velocity of sound in pure water and sea water (2d ed.): London, Admiralty, Hydrographic Department, H.D. 282, 52 p. National Ocean Survey, 1975a, Hungington Beach to Punta Sal Si Puedes: National Ocean Survey Bathymetric Map 1206N-16, scale 1:250,000. National Ocean Survey, 1975b, Cape San Martin to Point Conception: National Ocean Survey Bathymetric Map 1306N-20, scale 1:250,000. index map for I-2089-A

California↗

USGS1992

Map showing bottom topography of the Pacific Continental Margin, Cape Mendocino to Point Conception

All contours, geographic outlines, and political boundaries shown on this map of the bottom topography, or bathymetry, of the Pacific continental margin between 34? and 41? N. latitudes were plotted from digital data bases in the library of the U.S. Geological Survey (USGS)-National Oceanic and Atmospheric Administration (NOAA) Joint Office for Mapping and Research (JOMAR). These digital data were obtained and compiled from many sources; consequently, data quality varies within particular data bases as well as from one data base to another. Bathymetric contours were digitized from a map compiled by Chase and others (1981) and from the unpublished large scale versions of that map (T.E. Chase, unpub. maps, 1981). Data for the area seaward of the continental slope (~2000 m depth) were obtained primarily from the U.S. Coast and Geodetic Survey (C&GS) 1955 Pacific Exploratory Survey, a systematic and detailed (~8 -nmi trackline spacing) survey between Mexico and Canada. The USGS provided data from cruises S3-78-NC, S15-79-NC, L2-77-NC, Ll0-76-NC, and Bartlett 72. Data were also obtained from Scripps Institution of Oceanography cruises Merazine, Blue Flash, Kayak B, Scan I, and Seven Tow (Chase and Menard, 1971; Chase and others, 1975; Wilde and others, 1976; Wilde and others, 1978). The 200-m contour was derived from the National Ocean Survey charts 1306N-20 (1975), 1307N-l1B (1974a), 1307N-18B (1974b), and C&GS chart 1308N-12 (1969). Sea-floor depths were corrected for sound velocity in sea water using Matthew's (1939) tables. Onshore topographic contours were generated by computer from a modified version of 3-arc-second elevation data provided by the Defense Mapping Agency. The United States digital shoreline was obtained from the NOAA, NOS, Nautical Charting Division, National Atlas files. The coastline of Canada was digitized from Canadian Hydrographic Service bathymetric maps. The primary source of names of the sea floor features was the "Gazetteer of Undersea Features" (Defense Mapping Agency, 1990). Acknowledgments Christopher Hines assisted in the construction and verification of the digital data bases. Reviews and suggestions by Edward C. Escowitz and Florence Wong and advice provided by Will Stettner regarding the cartographic design substantially improved the quality of this map. References Cited Chase, T.E., and Menard, H.W., 1971, Bathymetric atlas of the northeastern Pacific Ocean: U.S. Naval Oceanographic Office Publication 1303, scale 1:2,000,000 at 33° latitude, 48 p. Chase, T.E., Wilde, Pat, and Normark, W.R., 1975, Oceanographic data of the Monterey Deep Sea Fan: San Diego, University of California, Institute of Marine Resources Publication TR 58, scale 1:898,524 at 35° latitude. Chase, T.E., Wilde, Pat, Normark, W.R., Miller, C.P., Seekins, B.A., and Young, J.D., 1981, Offshore topography of the Western United States between 32° and 49° North latitudes: U.S. Geological Survey Open-File Report 81-443, scale 1:864,518 at 38° latitude, 2 sheets. Coast and Geodetic Survey, 1969, Point St. George to Point Delgada: Coast and Geodetic Survey Bathymetric Map 1308N-12, scale 1:250,000. Defense Mapping Agency, 1990, Gazetteer of undersea features (4th ed.): Washington, D.C., Defense Mapping Agency. Matthews, D.J., 1939, Tables of the velocity of sound in pure water and sea water (2d ed.): London, Admiralty, Hydrographic Department, H.D. 282, 52 p. National Ocean Survey, 1974a, Vicinity Pt. Sur to Pt. Reyes: National Ocean Survey Bathymetric Map 1307N-11B, scale 1:250,000. National Ocean Survey, 1974b, Pt. Reyes to Tolo Bank: National Ocean Survey Bathymetric Map 1307N-18B, scale 1:250,000. National Ocean Survey, 1975, Cape San Martin to Point Conception: National Ocean Survey Bathymetric Map 1306N-20, scale 1:250,000. Wilde, Pat, Normark, W.R., and Chase, T.E., 1976, Oceanographic data off central California 37° to 40° North including the Delgada Deep Sea Fan: Berkeley, University of California, Lawrence Berkeley Laboratory Publication 92, scale 1 :864,581 at 38° latitude. Wilde, Pat, Chase, T.E., Holmes, M.L., Normark, W.R., Thomas, J.A., McCulloch, D.S., and Kulm, L.D., 1978, Oceanographic data off northern California-southern Oregon 40° to 43° North including the Gorda Deep Sea Fan: Berkeley, University of California, Lawrence Berkeley Laboratory Publication 251, scale 1:815,482 at 42° latitude.

IMAP↗

USGS1992

Map showing bottom topography of the Pacific Continental Margin, Strait of Juan de Fuca to Cape Mendocino

All contours, geographic outlines, and political boundaries shown on this map of the bottom topography, or bathymetry, of the Pacific continental margin between 34° and 41° N. latitudes were plotted from digital data bases in the library of the U.S. Geological Survey (USGS)-National Oceanic and Atmospheric Administration (NOAA) Joint Office for Mapping and Research (JOMAR). These digital data were obtained and compiled from many sources; consequently, data quality varies within particular data bases as well as from one data base to another. Bathymetric contours were digitized from a map compiled by Chase and others (1981) and from the unpublished large scale versions of that map (T.E. Chase, unpub. maps, 1981). Data for the area seaward of the continental slope (~2000 m depth) were obtained primarily from the U.S. Coast and Geodetic Survey (C&GS) 1955 Pacific Exploratory Survey, a systematic and detailed (~8 -nmi trackline spacing) survey between Mexico and Canada. The USGS provided data from cruises S3-78-NC, S15-79-NC, L2-77-NC, Ll0-76-NC, and Bartlett 72. Data were also obtained from Scripps Institution of Oceanography cruises Merazine, Blue Flash, Kayak B, Scan I, and Seven Tow (Chase and Menard, 1971; Chase and others, 1975; Wilde and others, 1976; Wilde and others, 1978). The 200-m contour was derived from the National Ocean Survey charts 1306N-20 (1975), 1307N-l1B (1974a), 1307N-18B (1974b), and C&GS chart 1308N-12 (1969). Sea-floor depths were corrected for sound velocity in sea water using Matthew's (1939) tables. Onshore topographic contours were generated by computer from a modified version of 3-arc-second elevation data provided by the Defense Mapping Agency. The United States digital shoreline was obtained from the NOAA, NOS, Nautical Charting Division, National Atlas files. The coastline of Canada was digitized from Canadian Hydrographic Service bathymetric maps. The primary source of names of the sea floor features was the "Gazetteer of Undersea Features" (Defense Mapping Agency, 1990). Acknowledgments Christopher Hines assisted in the construction and verification of the digital data bases. Reviews and suggestions by Edward C. Escowitz and Florence Wong and advice provided by Will Stettner regarding the cartographic design substantially improved the quality of this map. References Cited Chase, T.E., and Menard, H.W., 1971, Bathymetric atlas of the northeastern Pacific Ocean: U.S. Naval Oceanographic Office Publication 1303, scale 1:2,000,000 at 33° latitude, 48 p. Chase, T.E., Wilde, Pat, and Normark, W.R., 1975, Oceanographic data of the Monterey Deep Sea Fan: San Diego, University of California, Institute of Marine Resources Publication TR 58, scale 1:898,524 at 35° latitude. Chase, T.E., Wilde, Pat, Normark, W.R., Miller, C.P., Seekins, B.A., and Young, J.D., 1981, Offshore topography of the Western United States between 32° and 49° North latitudes: U.S. Geological Survey Open-File Report 81-443, scale 1:864,518 at 38° latitude, 2 sheets Coast and Geodetic Survey, 1969, Point St. George to Point Delgada: Coast and Geodetic Survey Bathymetric Map 1308N-12, scale 1:250,000. Defense Mapping Agency, 1990, Gazetteer of undersea features (4th ed.): Washington, D.C., Defense Mapping Agency. Matthews, D.J., 1939, Tables of the velocity of sound in pure water and sea water (2d ed.): London, Admiralty, Hydrographic Department, H.D. 282, 52 p. National Ocean Survey, 1974a, Vicinity Pt. Sur to Pt. Reyes: National Ocean Survey Bathymetric Map 1307N-11B, scale 1:250,000. National Ocean Survey, 1974b, Pt. Reyes to Tolo Bank: National Ocean Survey Bathymetric Map 1307N-18B, scale 1:250,000. National Ocean Survey, 1975, Cape San Martin to Point Conception: National Ocean Survey Bathymetric Map 1306N-20, scale 1:250,000. Wilde, Pat, Normark, W.R., and Chase, T.E., 1976, Oceanographic data off central California 37° to 40° North including the Delgada Deep Sea Fan: Berkeley, University of California, Lawrence Berkeley Laboratory Publication 92, scale 1 :864,581 at 38° latitude. Wilde, Pat, Chase, T.E., Holmes, M.L., Normark, W.R., Thomas, J.A., McCulloch, D.S., and Kulm, L.D., 1978, Oceanographic data off northern California-southern Oregon 40° to 43° North including the Gorda Deep Sea Fan: Berkeley, University of California, Lawrence Berkeley Laboratory Publication 251, scale 1:815,482 at 42° latitude.

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