Geology of the Mad River fault zone between Fieldbrook and McKinleyville, Calfornia based on field observations and high resolution spatial data analysis

Initial mapping of the Mad River fault zone (MRFZ) was at a scale of 1:24,000, over several years and at a reconnaissance level with limited access to land for field verification. My study area is within the MRFZ, approximately 15 km northeast of the city of Eureka, CA. I acquired LiDAR (1 m-resolution) hillshades and land access through an agreement with Green Diamond Resource Company. I used GPS and LiDAR in an effort to refine the original mapping. I identified three flights of late-Pleistocene marine terraces on the hills between McKinleyville, CA and Fieldbrook, CA. Assuming an average uplift rate of 1 m/ky, these terrace flights correlate to sea level highs of the last inter-glacial period, between approximately 120 ka and 80 ka. Several alluvial terraces are present north of Fieldbrook Valley that correlate to the ages of the marine terraces. I observed compression features related to the McKinleyville fault adjacent to the previously identified trace. Marine terraces on the footwall of the McKinleyville fault exhibit secondary scarps with up to 5 m of relief that are discontinuous along strike and may be related to compression along the Mad River fault, to the southwest. The Trinidad fault is identified in Mather Creek, northwest of Fieldbrook Valley. The fault is present as an eroded remnant of resistant ridges, with no apparent scarps cutting the young channel deposits. Expression of the fault diminishes in the 100 ka marine terrace.