REFINING GEOLOGIC MAPPING OF THE MAD RIVER FAULT ZONE BETWEEN FIELDBROOK VALLEY AND MCKINLEYVILLE, CALFORNIA USING HIGH-RESOLUTION TOPOGRAPHIC DATA AND SPATIAL ANALYSIS TOOLS

Initial mapping of the Mad River Fault Zone, the onland fold and thrust belt of the southern Cascadia subduction zone, was at a scale of 1:24,000, at reconnaissance level with limited land access for field verification. 1m LiDAR hillshades and private timber company access provided improvements to mapping of the fault zone, while GPS and LiDAR improve map detail. I identified three Pleistocene marine terrace groups in coastal hills between McKinleyville, CA and Fieldbrook, CA. An assumed uplift rate of 1 m/ky correlates these three terrace flights to sea level high stands of Oxygen Isotope Stage (OIS) 5 (120 ka to 80 ka). Additionally, alluvial/fluvial terraces, north of Fieldbrook, are present at elevations consistent with sea level highs of OIS 5a and 5c. These terraces most likely developed from base level changes during the last inter-glacial period.

Using LiDAR, I mapped compression features near the identified trace of the McKinleyville fault. Evidence for the fault include linear scarps, swales and drainages and offset terraces. The OIS 5e and 5c terraces in the footwall of the fault are offset by secondary discontinuous 5 m scarps. These scarps may be related to compression along the Mad River fault to the southwest.

The Trinidad fault exists in the north part of the map area in Mather Creek, a drainage that flows into Fieldbrook Valley. The fault is characterized by low-lying, discontinuous linear ridges, juxtaposing Cretaceous-Jurassic Franciscan formation against Pleistocene marine sand. The fault appears to be an erosional remnant lacking "fresh" scarps in the young deposits of Mather Creek. To the northwest, surface expression of this trace diminishes in the OIS 5c marine terrace. Lack of fault expression may indicate a relative low level of activity on this portion of the Trinidad fault. The OIS 5 coastline was much different from that of today. Hills between McKinleyville and Fieldbrook may have persisted as an island during the initial OIS 5 sea level highs. Fieldbrook Valley was a marine embayment throughout OIS 5 time. The alluvial/fluvial terraces and surface inclinations indicate that Little River, flowed south into the north end of Fieldbrook Valley during OIS 5. Stream piracy redirected Little River to the west. Precise timing for this piracy event is unknown.