The marine geology of San Miguel Gap off Point Conception, California

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Frequently-anticipated questions:

What does this data set describe?

Title:
The marine geology of San Miguel Gap off Point Conception, California
Abstract:
"San Miguel Gap is a submarine basin which intersects the continental slope southwest of Point Conception. It lies at the northwest limit of the California continental borderland. The basin is bounded by straight, steep slopes, which separate it from Arguello Plateau to the north and the Channel Islands and Santa Rose-Cortes Ridge to the east. A gentler, less regular slope to the south leads to the Patton Ridge. The basin is 30 x 35 km at a depth of 1500 m; it slopes gently seaward to breach the continental slope to a depth of 2500 m. "Sub-bottom acoustic reflection profiles across the Gap show pre- and post-orogenic generations of sedimentary fill, distinguished by mild peripheral folding in the older section. Assuming a sediment velocity of 2 km/sec, the basin contains a maximum thickness of 270 m of post-orogenic and 400 m of pre-orogenic fill. Beneath the basin, forming the continental slope, and outcropping on the ridges there is a unit termed "acoustic bedrock." Where sampled, this is Miocene (?) basalt or sedimentary rock. Acoustic bedrock correlates with the Channel Islands section of Cretaceous to Miocene sediments and volcanics. Possible boundary faults lie to the east and north, while to the south there is a possible downfold to form the Gap basin. "The Gap region is an elongate continental slope sedimentary environment, under terrestrial influence to the east and oceanic influence to the west. Terrestrial sands, near the islands, grade into glauconitic sands on the deeper slopes and hemipelagic clayey silt in the basin. Both sediment production and distribution are controlled by oceanographic agencies. In particular, strong currents influence the entire water column. Surface flows often exceed 200 cm/sec, and velocities averaging 5 cm/sec, peaking over 20 cm/sec, were measured at 727 m. Terrestrial sands come from the islands; they are controlled by wave action. Silts and clays, largely derived from the mainland, are distributed by winds, surface and bottom currents, and are concentrated in the basin. Foraminifers are extremely common in the sediment; their abundance is a function of surface organic productivity and bottom current winnowing. Glauconite is ubiquitous and contemporary; it constitutes over one-half the sediment where other deposition is inhibited. A probable authigenic sequence form precipitation in small shells to typical glauconite pellets can be identified in Gap sediments. Relative contributions of the major sediment constituents were determined for each section of the Gap area: Santa Rosa Ridge-Cortes Ridge: 50% terrestrial, 42% organic, and 7% authigenic; Central Basin: 76% terrestrial, 22% organic, and 2% authigenic; Deep slopes and terraces: 30% terrestrial, 27% organic, and 42% authigenic. "Sedimentation rates range from 13-15 mm/10e3 years on the deep slopes to 31 mm/10e3 years at the head of the basin and 54 mm/10e3 years in the central basin. Assuming constant rates, the observed fill thickness in the basin could have accumulated in 13.4 x 10e6 years, since Early Pliocene. All dates used in the estimates are based upon the sinistral-to-dextral coiling direction shift in the foraminifer, Globigerina pachyderma, at the end of the Pleistocene. Study of cores in the Gap region indicates no significant change in the sedimentary or oceanographic regimes since Late Pleistocene. "Two pronounced east-west structures, the Murray Fracture Zone of the Pacific basin and the Transverse Ranges of California, are aligned with the Gap. It has been suggested that this break in the continental slope represents an interaction of oceanic and continental structures. All evidence, however, indicates that San Miguel Gap is a borderland structure formed during Late Miocene, which has subsequently undergone little disturbance. No relation with the east-west structures is clear."
Supplemental_Information:
Data digitized by the USGS for inclusion into usSEABED (<http://walrus.wr.usgs.gov/usseabed>)
  1. How should this data set be cited?

    Wright, Frederick Fanning , 1967, The marine geology of San Miguel Gap off Point Conception, California: University of Southern California, Los Angeles, CA.

    Other_Citation_Details:
    The Allan Hancock Foundation provided shiptime under a grant from the National Science Foundation. Much of the analysis and laboratory work was supported by a Penrose Bequest Research Grant from the Geological Society of America.

  2. What geographic area does the data set cover?

    West_Bounding_Coordinate: -121.100
    East_Bounding_Coordinate: -120.100
    North_Bounding_Coordinate: 34.136
    South_Bounding_Coordinate: 33.675

  3. What does it look like?

    Wright_PhD_1967 (.jpg)
    Sample distribution with coastline and bathymetry for reference.

  4. Does the data set describe conditions during a particular time period?

    Calendar_Date: 1967
    Currentness_Reference: Publication date

  5. What is the general form of this data set?

    Geospatial_Data_Presentation_Form: Paper

  6. How does the data set represent geographic features?

    1. How are geographic features stored in the data set?

      This is a Vector data set. It contains the following vector data types (SDTS terminology):

      • Entity point (88)

    2. What coordinate system is used to represent geographic features?

      Horizontal positions are specified in geographic coordinates, that is, latitude and longitude. Latitudes are given to the nearest 0.00028. Longitudes are given to the nearest 0.00028. Latitude and longitude values are specified in Decimal degrees.

  7. How does the data set describe geographic features?

    Point
    SDTS point (Source: Source report)

    Station numbers
    Station number (Source: Source report)

    ValueDefinition
    AHFCollected by the Allan Hancock Foundation
    DCollected for this study by USNOTS (U.S. Naval Ordnance Test Station)
    JD and MHSamples furnished by USNCEL (U.S. Naval Civil Engineering Lab)

    Interval
    Subsample depth interval (Source: Source report)

    Range of values
    Minimum:0
    Maximum:1.95
    Units:Meters
    Resolution:0.01

    Gravel
    Gravel (Source: Source report)

    Gravel was not specifically analyzed; however, in the petrographic analyses, three samples noted gravel presence and rough estimation of presence, up to 20 percent.

    Sand
    Sand (Source: Source report)

    Range of values
    Minimum:2.1
    Maximum:99.48
    Units:Percent
    Resolution:0.01

    Silt
    Silt (Source: Source report)

    Range of values
    Minimum:0.12
    Maximum:85.25
    Units:Percent
    Resolution:0.01

    Clay
    Clay (Source: Source report)

    Range of values
    Minimum:0
    Maximum:56
    Units:Percent
    Resolution:0.01

    Median
    Median grainsize distribution (Source: Source report)

    Range of values
    Minimum:2.22
    Maximum:8.2
    Units:Phi
    Resolution:0.01

    Lithologic descriptions
    Lithologic descriptions of rock samples (Source: Source report)

    Written descriptions

    Sand fraction analyses
    Petrographic analyses of the sand fraction (Source: Source report)

    Generally rough estimations (including trace presence) of planktonic, benthic, and arenaceous foraminifera, bryozoa, carbonaceous fragments, chiton fragments, diatoms, dark minerals, echinoid fragments, fish bones, glauconite, glauconite foraminiferal casts, mica, mollusk fragments, manganese fragments, phosphorite, radiolarian, sponge spicules, tar, terrestrial sand, volcanic rubble, volcanic debris, volcanic glass, and gravel.

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)

  2. Who also contributed to the data set?

    Frederick Fanning Wright at the University of Southern California for the collection and analysis of data. For inclusion into usSEABED: digitization: Carolynn Box and Jennifer Mendonca (USGS); Formatting corrections: Jane Reid (USGS) and Chris Jenkins (University of Colorado).

  3. To whom should users address questions about the data?

    University of Southern California
    Zumberge Hall
    Los Angeles, CA 90089

    213-740-6106 (voice)

Why was the data set created?

"The specific objectives of this study are (1) to describe the geology of an anomalous segment of the continental margin and to relate this to the regional geology, and (2) to explain the distribution of the contemporary and the sub-recent sediments of the area in terms of the observed and postulated physico-chemical environment."

How was the data set created?

  1. From what previous works were the data drawn?

  2. How were the data generated, processed, and modified?

    Date: Unknown (process 1 of 1)
    Samples were disaggregated by a treatment of 50 ml of 15 percent H2O2. Samples were desalted by alternate distilled water washing and centrifugation. Samples were wet sieved on a 62-micron screen to separate the sand fractions, which was dried and weighed. The washings, containing the finer fraction, were given an abbreviated pipette analysis. Grain counts were performed under a binocular scope on the coarse fraction on 150 to 250 individual grains. Rocks were thin-sectioned, and identified tentatively on the basis of textural and the more obvious mineral constituents.

    Person who carried out this activity:

    University of Southern California
    Zumberge Hall
    Los Angeles, CA 90089

    213-740-6106 (voice)

    Data sources produced in this process:
    • Wright_PhD_67

  3. What similar or related data should the user be aware of?

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?

    No estimate made for the accuracy of the data in the original report. Data digitized by the USGS and partners were visually compared to the source data, and corrected. Where appropriate, data were tested for completeness using MS Excel. Locations checked using GIS.

  2. How accurate are the geographic locations?

    For the Allan Hancock Foundation samples aboard the Velero IV, navigation was by radar and dead reckoning. Due to strong currents in the area, station locations are accurate to about 2 km (1 nautical mile) over most of the area, but there may be even greater errors in the positions of some of the more remote stations. For the USNOTS and USNCEL data, navigation was by Loran-A, Loran C, and some of the stations were positioned by the commercial Raydist system, which is accurate to a few tens of meters. Locations are given to minutes, seconds.

  3. How accurate are the heights or depths?

    Depth determination by Precision Depth Recorder echograms. Water depths to nearest meter. Some dredged samples had a range of depths associated; the midpoint of those depths are included in usSEABED.

  4. Where are the gaps in the data? What is missing?

    For usSEABED, Appendices I, III, IV, V, and VII were completely digitized. Foraminiferal data (Appendix VI) were not digitized. Some information from the report text may also be included in usSEABED.

  5. How consistent are the relationships among the observations, including topology?

    Data fall within normal ranges for analytical results. The locations listed for the rocks samples taken from 10097AHF (Appendix IV) are different from the locations given in Appendix I. The locations given in Appendix I were used.

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?

Access_Constraints:
None. Thesis is available at the USGS library and other libraries.
Use_Constraints:
Cite Frederick F. Wright and the University of Southern California as originators of the data.

  1. Who distributes the data set? (Distributor 1 of 2)

    University of Southern California
    Zumberge Hall
    Los Angeles, CA 90089

    213-740-6106 (voice)

  2. What's the catalog number I need to order this data set?

    Wright_PhD_1967

  3. What legal disclaimers am I supposed to read?

    Although this dataset has been used by the USGS, no warranty, expressed or implied, is made by the USGS as to the accuracy of the data. Users of the data should be aware of limitations of the data due to possible imprecision due to navigational inaccuracies and limitations of the statistical data.

  4. How can I download or order the data?

  1. Who distributes the data set? (Distributor 2 of 2)

    U.S. Geological Survey
    345 Middlefield Road
    Menlo Park, CA 94025

    650.329.5026 (voice)
    men_lib@usgs.gov

  2. What's the catalog number I need to order this data set?

    Wright PhD 1967 203 (050)W932m

  3. What legal disclaimers am I supposed to read?

    Although this dataset has been used by the USGS, no warranty, expressed or implied, is made by the USGS as to the accuracy of the data. Users of the data should be aware of the limitations of the data due to possible imprecision due to navigational inaccuracies, statistical limitations, and digitizing errors.

  4. How can I download or order the data?

  5. What hardware or software do I need in order to use the data set?

    None

Who wrote the metadata?

Dates:
Last modified: 2005
Metadata author:
U.S. Geological Survey
c/o Jane A. Reid
Geologist
400 Natural Bridges Drive
Santa Cruz, CA 95060

831-427-4727 (voice)
jareid@usgs.gov

Contact_Instructions: Email preferred
Metadata standard:
CSDGM Version 2 (FGDC-STD-001-1998)

Generated by mp version 2.8.17 on Thu May 18 21:41:53 2006