DEFORMED MARINE TERRACES ON SANTA CATALINA ISLAND, CALIFORNIA: NEW MEASUREMENTS FROM TERRACE-PLATFORM SURFACES AND PHOLAD-BORED CLASTS

Emergent marine terraces have been documented on all of the California Channel Islands, with the sole exception of 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. We identify previously unmapped terrace surfaces overlain with gravel deposits and pholad-bored clasts. Pholad borings—fossil burrows made by marine bivalves—confirm the marine origin of the terrace surfaces and terrace-capping gravels.

Terraces and terrace-capping deposits on Catalina Island are present on the western side of the island in the Little Harbor embayment area. Rounded, pholad-bored gravels and cobbles are present as diffuse to concentrated float across low-relief surfaces, and as consolidated packages overlying wave-cut platforms. One such deposit, which we have named the “Eagles Nest Gravels”, extends 350 m across a ridgetop and is nearly 8 m thick. We report RTK GPS-derived measurements of the wave-cut surface underlying the Eagles Nest Gravels. The broad paleo-platform, overlooked in previous mapping, is continuously exposed at 200 m in elevation and has experienced approximately 3° of NE tilting.

We establish quantitative criteria for identifying pholad borings as indicators of rocky intertidal depositional environments. We compare the inner dimensions of Catalina terrace borings to those of borings found on modern beaches on mainland California. We also compare these to the inner geometry of other features, such as tafoni weathering pits. Results from Principal Component Analysis (PCA), Linear Discriminant Analysis (LDA), and PERMANOVA statistical tests indicate a significant difference between boring and tafoni dimensions. Additionally, larger pholad borings and tafoni pits appear to be more morphometrically distinct than smaller ones.

Our findings are unequivocal evidence of marine terraces on Catalina Island and confirm uplift of the island. Detailed mapping and measurements of these terraces will help us better understand the complex regional tectonics of the California Continental Borderland, allowing for more precise slip-rate and earthquake hazard estimates for the surrounding region.