ANALYSIS OF PALEOCURRENT INDICATORS IN OLDER ALLUVIUM NEAR MISSION RIDGE FAULT, SANTA BARBARA, CALIFORNIA

An investigation of sedimentary features found within the Pleistocene Older Alluvium was undertaken on an alluvial fan located just north of the City of Santa Barbara. The fan is cut by the Mission Ridge fault, and other geologists interpret folding of the Older Alluvium south of the fault. The goal of this study was to determine if any post depositional rotations of the deposits had occurred to form folded features, as postulated by other workers. The estimated trend and plunge of the long axis of 182 elongated clasts was measured and recorded, as shown by Compton (1982, p. 227, fig 12-10B), and plotted on a Schmidt Equal Area stereonet.

One such channel is located 200 feet east of the intersection of Mountain Drive and Tremonto Road at GPS coordinates: 34.445N, -119.7055E. The measurement of 15 long axes trend and plunge within this channel deposit were compared, and contains a moderate to strong correlation coefficient. The correlation coefficient is 0.434, and indicates a moderate to strong correlation. The long axes moderately dip towards the north, and show a lack of rotation by structural forces such as faulting or folding.

An examination the internal fabric of the older Alluvium deposits within the Rattlesnake Fan with an interest in pebble imbrication. Imbrication of elongated clasts was noted in our study, as it was also noted by the US Geological Survey et.al. (2001, p. 3). The imbricated clasts are contained within pebble and boulder conglomerate deposits. These deposits typically form paleochannels with contacts between beds forming concave shaped contact facing upward. Chaotic deposits typical of debris flows are the dominant feature in the Rattlesnake fan. Debris flows are the downslope flow of relatively coarse material: more than 50 percent of the particles are coarser than sand (Keller, 2011, p. 182). A total of five outcrops on the Rattlesnake Fan were examined for elongated clasts, and most were found to lack imbrication because of the chaotic alignment of clasts.