ARE THE CONTACTS BETWEEN THE SOUTH FORK MOUNTAIN SCHIST, VALENTINE SPRING, AND YOLLA BOLLY UNITS TRANSITIONAL OR FAULTED IN THE EASTERN BELT OF THE FRANCISCAN COMPLEX OF CALIFORNIA?

In the Eastern belt of the Franciscan, an inverse metamorphic gradient is defined by three units adjacent and west of the Coast Range fault: the South Fork Mountain Schist (SFMS), Valentine Spring (VS, largely semi-schistose) and Yolla Bolly (YB, non-schistose). The nature of the contacts between these units has been the subject of controversy for many years. Some workers believe the contacts are faults (Log Springs and Sulfur Creek) defined by abrupt breaks in metamorphic and textural grade. Other workers believe the contacts are transitional as they have been unable to document the abrupt breaks described by fault advocates. Recent workers appear to have accepted the fault interpretation, probably because the contacts have been enshrined as faults on the most recent geologic maps. However the issue remains unresolved in large part because, with one debatable exception, the fault traces have not been seen in outcrop despite their regional extent.

The mapped trace of the Sulfur Creek “fault?” was examined in Grindstone Creek, near Willows, CA. The “fault?” separates non-schistose YB rocks from VS semischists (~textural grade 1 to 2). The textural zone boundary can be seen to cut across steeply dipping beds at a low angle; it can be walked across along a trail, in the main stream, and up two side creeks. No sharp break in textural grade or lithology was observed. Instead, a gradual transition from non-schistose to semischistose rocks is clearly seen over an ~100 meter interval. The trace of the Log Springs “fault?” is well exposed in Grindstone and Salt Creeks; it roughly defines the boundary between semischists (VS) and schists (SFMS) (~textural zone 2 to 3). The change in textural grade appears to be transitional over several hundred meters. The transition appears to be related in part to a change in sandstone character, from thin-bedded in the SFMS to thicker bedded in the VS unit. Thin section studies confirm all textural changes described above. In summary, the boundaries between the YB, VS and SFMS units are transitional in the areas studied and are likely to be so throughout the Eastern Belt.