THE DIRT ON CHANNEL ISLANDS NATIONAL PARK, CALIFORNIA: QUATERNARY GEOLOGIC MAPPING REVEALS NEW DETAILS OF ISLANDS’ TECTONIC AND PHYSIOGRAPHIC HISTORY

Through an agreement between the U.S. Geological Survey and the National Park Service, we are mapping the Quaternary geology of the five islands comprising Channel Islands National Park off the coast of southern California. As no detailed surficial geologic map exists for the islands, park staff requested a Quaternary map at 1:24,000 scale to aid in their ongoing natural resource assessments, including post-grazing disturbance recovery and identification of erosion and landslides.

Our mapping approach is to obtain numerous detailed geologic field observations throughout each island to characterize as much geologic diversity as possible. These field observations provide “ground-truth” for GIS-based map unit inferences using a variety of geospatial high-resolution (sub-meter) aerial photo imagery and LiDAR-based DEMs. Surficial map unit polygons are delineated and labeled according to a combined depositional mode-age classification scheme. Depositional modes include alluvial, fluvial, eolian, mixed eolian and alluvial, beach, marine terrace, hillslope, landslide, and anthropogenic. Tectonic structures (mainly faults) that cut Quaternary deposits are also mapped along their full extents.

The first two years of our five-year mapping campaign have focused on Santa Rosa Island, the second largest in the park. Significant discoveries made during this initial phase include: (1) Flights of older Pleistocene (>120 ka) and possibly Pliocene marine terraces have been identified beneath alluvial and eolian deposits at elevations as much as 275m above modern sea level. Such elevated older terraces suggest that Santa Rosa was a much smaller, more submerged island in the late Neogene and (or) early Pleistocene prior to Pleistocene tectonic uplift. (2) Structural and geomorphic observations made along the potentially seismogenic Santa Rosa Island fault spanning the island indicate a protracted slip history during the late Neogene and Quaternary involving early normal slip, later strike slip, and recent reverse slip. These changes in slip mode explain a marked contrast in island physiography across the fault. (3) Many of the steeper slopes in the island’s interior are dramatically stripped of their regolith presumably due to the effects of overgrazing in the twentieth and late nineteenth centuries.