Title: Landslides, Freshwater Creek Watershed, California, 1999
- Line data
- 2002
- Not owned by MIT (Owned by Stanford)
Dates
- Issued: 2002
- Coverage: 1999
Publishers
- California Geological Survey
Summary
This line shapefile represents landslide activity within the Freshwater Creek watershed in Humboldt County, California at 1:24,000 scale. These data were derived from digital maps of landslide potential and geologic features in Humboldt County originally published in 1999 by the California Division of Mines and Geology (OFR 99-10 and OFR 99-10a). The data for the original mapping were compiled from multiple sources including published maps and reports, aerial photograph mapping, and reconnaissance geologic mapping. The maps/data are useful in preparation of Timber Harvesting Plans, and for use by land managers for identifying areas of potentially unstable ground. The maps/data serve as a guide to potential problem areas where more site-specific review is required. California Division of Mines and Geology and California Geological Survey. (2002). Landslides, Freshwater Creek Watershed, California, 1999. California Dept. of Conservation and California Geological Survey. Available at http://purl.stanford.edu/kt474zt6227. The discussion below was excerpted from OFR 99-10 and 99-10a. The geology of the drainage is dominated by the Freshwater Fault. This fault is considered a normal fault with blocks on either side stepping down to the west in a regional context. Multiple strands of the fault appear to be present in the study area. The fault is considered inactive because no evidence of movement after late Quaternary time has been observed along it. The Freshwater Fault splits the watershed into two sections having different geologic conditions, topographic styles and potentials for landsliding (Plate 1). Much of the western half of the drainage has relatively steep topography underlain by gently dipping, interbedded mudstone, siltstone, sandstone and minor conglomerate of the Wildcat Group (early Pleistocene to late Miocene). These rocks are typically prone to shallow debris flows/slides and occasional translational/rotational rock slides. The Wildcat Group in this part of the drainage rests uncomformably atop interbedded sandstone and shale of the Yager Formation (early Tertiary). The Yager Formation dips steeply to the northwest (Plate 1). Only one translational/rotational rockslide was identified in this unit. Several lineaments were noted on the aerial photographs throughout this part of the drainage, but no faults are indicated. The geology of the eastern half of the drainage is complex and most of the larger translational/rotational landslides and earthflows are found there. The western third of this area is underlain by Wildcat Group rocks that rest unconformably atop dense sandstones and shales of the Yager Formation and Coastal Belt of the Franciscan Assemblage (PLATE 1). The contacts between Yager and Franciscan rocks probably represent multiple strands of the Freshwater Fault (Keith Knudsen, May 1999, personal communication). The remainder of the eastern half is dominated by a series of northeast-dipping thrust faults. These faults are considered inactive at this time because they are not indicated on the State of California Earthquake Fault Zone map (State of California, 1983). The thrust faults juxtapose irregular slabs of dense sandstone/shale and melange of the Coastal Belt of the Franciscan Assemblage. The erosional contrast between the two units is seen as discrete bands of "hard" and "soft" topography that cut through the drainage in a north-westerly direction (Plate 1). The characteristics and properties of the various rock units found in this area were taken into account during the assessment of relative landslide potential. The primary sources of geological data used in preparing the relative landslide potential map are compiled on Plate 1 of Open-File Report 99-10. The Relative Landslide Potential Map has been prepared to aid in resource management and general land-use planning. It is not intended, nor should it be used for, evaluation of specific sites. Site specific evaluations often require detailed engineering geologic studies and soil engineering investigations of the underlying soil and bedrock for proper planning of specific projects. This map is derivative, based on the following: 1) the occurrence and distribution landslides, other types of slope failure, and features indicating slope instability (Plate 1, Open-File Report 99-10); 2) the geology of the area, including bedrock types and lithologic properties relative to slope stability and distribution of various earth materials, as well as the structural framework, such as the folded and faulted strata found throught the region; and 3) the relative behavior of slopes within the area as interpreted from analysis of historic aerial photographs and recent field observations. Studies of the stability of specific sites commonly require development of quantitative data through laboratory testing of field samples. This level of testing was not done for this regional evaluation. In producing this map, it was assumed that actively sliding material has the lowest relative strength, and thus, the highest relative potential for landsliding of all the geological materials underlying the slopes. Recent alluvial deposits in the valley bottom were assumed to possess the least potential for landsliding due to their flat slope. The relative potential for landsliding between these extremes was evaluated subjectively based on aerial photograph interpretation, field observations and the following principal factors: The broad apparent stability characteristics of geological materials underlying the slopes and adjacent lower-lying areas, as expressed in their natural exposures and their observed responses to alteration by land-use activities. For example, slopes that exhibit abundant evidence of landsliding or downslope creep of the soil are considered oversteepened relative to the strength of the materials that underlie them. Steepness of slopes, whether or not landslides are apparent. The presence of active or intermittent natural influences that tend to cause slope failure. These include gravity, climatic conditions, fluvial processes, and the tendency of certain soils to shrink and swell under varying moisture conditions. These criteria are combined to yield the five-value scale used on the Relative Landslide Potential Map to indicate the comparative capacity of the slopes within the map area to resist failure by landsliding. The map, Landslide Potential with Geologic and Geomorphic Features, Freshwater Creek, Humboldt County, California is a digital representation of the map, Geological and Geomorphic Features Related to Landsliding, OFR 99-10, combined with the map of Relative Landslide Potential, OFR 99-10a. Purpose: The maps/data are useful in preparation of Timber Harvesting Plans, and for use by land managers for identifying areas of potentially unstable ground. The maps/data serve as a guide to potential problem areas where more site-specific review is required. Data Capture Procedures: As noted previously, geomorphic features were mapped on stereo pair photographs and transferred to 7.5 minute quadrangle mylar overlays. These overlays were scanned and converted to ArcInfo vector coverages. This layer is presented in the WGS84 coordinate system for web display purposes. Downloadable data are provided in native coordinate system or projection.
Subjects
- California, Northern
- Humboldt County (Calif.)
- Freshwater Creek (Calif.)
- Faults (Geology)
- Landslides
- Watersheds
- Landslide hazard analysis
- Geoscientific Information
- Datasets
Geospatial coordinates
- Bounding Box: BBOX (-124.081374, -123.963779, 40.788003, 40.701731)
- Geometry: BBOX (-124.081374, -123.963779, 40.788003, 40.701731)
Provider
Stanford
Rights
- Access rights: Public
Citation
California Geological Survey, California. Division of Mines and Geology. Landslides, Freshwater Creek Watershed, California, 1999. California Geological Survey. Line data. https://purl.stanford.edu/kt474zt6227
Format
Shapefile
Languages
- English