Quaternary deposits of the 9-county San Francisco Bay Region: an areally continuous digital map database prepared from Knudsen and others (2000) and Witter and others (2006)

This digital map database provides an areally continuous representation of the Quaternary surficial deposits of the San Francisco Bay region merged from the database files from Knudsen and others (2000) and Witter and others (2006). The more detailed mapping by Witter and others (2006) of the inner part of the region (compiled at a scale of 1:24,000), is given precedence over the less detailed mapping by Knudsen and others (2000) of the outer part of the area (compiled at a scale of 1:100,000).

The Quaternary map database is accompanied by a list of the map-unit names represented by polygon identities, a digital map index of the 1:24,000-scale topographic quadrangles of the region, and a figure illustrating the contents of the database. The Quaternary map database includes line work and the identity of the Quaternary map units, but no further description of the map units or how they were mapped. Use of the database should thus be accompanied by consultation with the original reports, which describe the map units and mapping procedures: citation of this database should be accompanied by citation of those original reports. As with all such digital maps, use of this database should attend to the compilation scales involved and not try to extract spatial detail or accuracy beyond those limits.

Database layers:
SFBQuat-lns: Quaternary map database: unit boundaries and their attributes
SFBQuat-pys: Quaternary map database: polygons and their attributes
SFBIndex-lns: Boundaries of 7.5-minute quadrangles for the map area, distinguishing those that form boundaries of 15-minute and 30x60-minute quadrangles
SFBIndex-pys: 7.5-minute quadrangles, and for those within map area, their names and the names of the 30x60-minute quadrangles that contain them.

The liquefaction ratings presented in the original reports for the various Quaternary map units remain valid and can be assigned to the units in this database if desired, with ratings of Witter and others (2006) given precedence.

Assembly of the Quaternary map database involved stripping out all the information from the source maps that dealt with liquefaction, a major component of the original reports, and adjusting line work at the common boundary between the two source maps to produce a nearly seamless spatial database. The common boundary between the two sources is retained. Mismatches remaining at that common boundary are of two types: (1) contrasts in the degree of subdivision of the deposits resulting from the different compilation scales, and (2) terminations of narrow bands of water and artificial fill and levees at quadrangle boundaries that resulted from differences in details shown on the 1:24,000-scale topographic maps used as a source of mapping information in the original reports.

The illustrative figure accompanying the database shows the content of the database plotted at a scale of 1:275,000, with the different map units distinguished by color and the different types of lines distinguished by symbol and color. An index map in that figure shows the 165 7½-minute quadrangles covering the region and the areas of the two source maps.

Knudsen, K.L., Sowers, J.M., Witter, R.C., Wentworth, C.M., Helley, E.J., Nicholson, R.S., Wright, H.M., and Brown, K.M., 2000, Preliminary maps of Quaternary deposits and liquefaction susceptibility, nine-county San Francisco Bay region, California: a digital database: U.S. Geological Survey Open File Report 00-444. http://pubs.usgs.gov/of/2000/of00-444/

Witter, R.C., Knudsen, K.L, Sowers, J.M., Wentworth, C.M., Koehler, R.D., Randolph, C. E., Brooks, S.K., and Gans, K.D., 2006, Maps of Quaternary Deposits and Liquefaction Susceptibility in the Central San Francisco Bay Region, California: U.S. Geological Survey Open-File Report 06-1037 (http://pubs.usgs.gov/of/2006/1037)