CORRELATION OF ALLUVIAL DEPOSITS TO THEIR SOURCE CANYONS ACROSS THE SAN ANDREAS FAULT IN SAN BERNARDINO, CALIFORNIA USING COMPARATIVE ANALYSIS OF CLASTS

Two early(?) Pleistocene alluvial deposits in San Bernardino are offset at least 3.3 km from their source canyons, as indicated by comparing their clast lithologies to those of several potential source canyons on the opposite side of the San Andreas fault. If more precise ages can be measured through cosmogenic isotope dating, then these offset deposits will provide a slip rate averaged over a longer time period than prior investigations. Studies of the slip rate of the San Andreas fault are significant because of the fault’s vicinity to both economic interests and to the large population of Southern California.

A total of 1000 clasts per site were counted within the modern channels at the mountain front from each of six major drainages (from northwest to southeast: Meyers Canyon,Bailey Canyon, Devil Canyon, Sweetwater Canyon, Badger Canyon and Sycamore Canyon). Felsic plutonic clasts, gneiss, and marble were the dominant rock types. The percentage of these rock types within any single canyon or fan remnant generally varied within less than 5% from one site to another. The proportion of marble was a major distinguishing feature between the different source canyons. However, neither of the two old alluvial fan remnants studied (one near Meyers Canyon and the other near Devil Canyon) contained any significant amount of marble. The absence of carbonates within the old fan deposits may be a result of chemical weathering. For the fan remnant near Devil Canyon, however, 400 of the 1000 clasts counted were from an old road cut into the side of the deposit, where the clasts have been exposed to chemical weathering for only a limited amount of time.

The data suggest that the old alluvial remnant near Meyers Canyon was most likely deposited at the mouth of Devil Canyon 3.6 (± 0.3) km away. On the other hand, the old fan remnant near Devil Canyon does not match well with any of the studied source canyons that are right laterally separated across the fault. Of the canyons studied, Sycamore Canyon (3.3 ± 0.1 km away) is the most likely source because it contains less marble (11%) than the others. However, it has a higher ratio of gneiss to plutonic rock than is present in the old fan remnant. Waterman Canyon (7 km away) is another possible source, but clast lithologies there have not yet been counted.