QUATERNARY UPLIFT OF SANTA CATALINA ISLAND FROM MARINE TERRACES AND TERRACE DEPOSITS

The California Channel Islands all have marine terraces—all except Santa Catalina Island. For over a century, the origin of low-relief, sub-horizontal surfaces and gravel deposits has been debated, as has the question of whether Catalina is uplifting or subsiding. These debates should now end. We identify pholad-bored clasts and previously unmapped wave-cut surfaces. Pholad borings—pit burrows made by marine bivalves—confirm the deposits and terrace surfaces as marine and confirm that Catalina has experienced late Quaternary uplift.

Terraces and terrace-capping deposits on Catalina Island are present in the Little Harbor embayment area. Rounded gravels and cobbles are present as diffuse to concentrated float across numerous low-relief surfaces. Deposits also include consolidated, in-place gravels, some of which overlie one or more wave-cut platforms cut into the underlying bedrock. At several sites, gravel deposits contain pholad-bored clasts concentrated on the wave-cut surfaces. A package of pholad-bored gravels overlying a broad paleo-platform, overlooked in previous mapping, is tentatively named the "Eagles Nest Gravels." Elevations of in-place marine gravels and concentrated float range from 100 m to 236 m.

Ongoing work includes terrace dating and topographic analysis of DEM and LiDAR data to explain the poor preservation of marine terraces on Catalina. In addition, we are comparing fossil and modern pholad borings to establish quantitative criteria for distinguishing pholads from other features such as weathering pits. Observations reveal that a distinct conical shape characterizes pholad borings, defined by an inverse relationship between the diameter of the boring opening and its distance to the point of maximum cross-sectional diameter.

Despite abundant evidence for long-term tectonic uplift on the other California Channel Islands, researchers have engaged in a century-long debate on whether Catalina Island has experienced long-term uplift or subsidence. Our results confirm Quaternary uplift of Catalina Island. Detailed mapping and measurements of these terraces will aid us in gaining a better understanding of the complex regional tectonics of the California Continental Borderland, allowing for more precise slip-rate and earthquake hazard estimates for the surrounding region.