Purpose and Scope
The purpose of this investigation was to evaluate and compare paleocurrent indicators in Rattlesnake Fan across the Mission Ridge Fault in Santa Barbara, California for features that would provide support for or against an inferred blind Mission Ridge fault zone shown on geologic maps published by Gurrola (2004) and Minor et al. (2006).
Methods of Investigation
The investigation evaluated the imbrication of pebbles and other depositional features within the Pleistocene Older Alluvium (or fanglomerate of Dibblee ) at sites in the Rattlesnake Alluvial Fan of Mission Ridge in the City of Santa Barbara, California. Imbrication is a depositional fabric found in conglomerates and pebble beds in which the long axes of elongated clasts tend to lie more or less parallel with one another leaning in the direction of current flow. Deposition occurred under the influence of a powerful current. The investigation included:
1. Review of existing literature including a paper indicating that the Older Alluvium clasts are commonly imbricated (USGS Preliminary Geologic Map of the Santa Barbara Coastal Plain Area, Santa Barbara County, California, USGS Open File Report 02-136, 2002, page 3).
2. The collection of trend and plunge data along the long axes on alluvial cobbles and boulders in the Rattlesnake Alluvial Fan from site throughout Mission Ridge.
3. Performing a structural analysis of the alluvial cobble trend and plunge data by stereonet, and developing conclusions and recommendations.
4. Evaluation of paleo-depositional features preserved in the Rattlesnake Alluvial Fan at an outcrop on Mountain Drive in the City of Santa Barbara for indications of bedding orientation, stream channels, and current flow direction.
The contemporary interpretation of Mission Ridge as an anticline requires that the north dipping limb of the anticline include a north-northeast dipping “bedding plane” in the Rattlesnake Fan where exposed in the eastern portion of an outcrop along Mountain Drive in the City of Santa Barbara. Gurrola (2002, revised 2004) mapped a north-dipping bedding plain attitude in the eastern portion of the outcrop, and mapped the western portion of the outcrop as landslide deposits. This presentation summarizes evidence for an alternative interpretation of the outcrop indicating:
- The north dipping attitude in the eastern portion of the outcrop is a paleo-landslide surface;
- The western portion of the outcrop represents an intact portion of the Rattlesnake Alluvial Fan that has not been subjected to land sliding;
- The outcrop is not tilted to the northeast; and
- Paleo-depositional structures preserved in the western outcrop indicate a southeast dipping structural regime consistent with normal deposition in an alluvial fan.
1. The 3-dimensional view of bedding preserved in the outcrop indicates undeformed gently south sloping deposition, similar to that implied by imbrication data.
2. The northwest plunge of the imbricated gravel clasts indicates a southeast flow direction during deposition, and by implication, southeast dipping structure, consistent with field measurements from other areas of the fan.
3. The geomorphic evidence for a paleochannel located due south of the Mountain Drive outcrop indicates an ancestral stream, possibly Mission-Rattlesnake Creek, flowed south through this area.
4. Further evaluation of Mission Ridge and other implied folds in the South Coast area should be performed to verify the validity of the supporting data.