Submerged paleoshorelines and terraces surrounding Santa Catalina Island and the Pilgrim/Kidney Banks in the California Continental Borderland demonstrate late Quaternary tectonic subsidence, in contrast to the other islands of the California Continental Borderland that are experiencing tectonic uplift. We used high-resolution seismic-reflection profiles to map a terrace package containing 16 successive parasequences surrounding Santa Catalina Island, preserved at depths from 30 to 470 m below modern mean sea level. The Pilgrim/Kidney Banks are surrounded by a terrace package containing 13 successive parasequences preserved at 90–310 m depth. The presence of marine terrace (beach) deposits at >400 m depth, far below the lowest estimates of Quaternary lowstand sea level (90–130 m), requires significant tectonic subsidence. Within each terrace, we identified the transgressive surface separating subaerial deltaic and shallow-marine deposits originating during sea-level lowstand from overlying subaqueous deltaic deposits emplaced after the lowstand. Remotely operated vehicle samples of sediment recovered from submerged terrace deposits offshore Santa Catalina Island contain faunal assemblages typical of submerged insular terraces in southern California. The distribution of equivalent extant mollusks and benthic foraminifera indicates deposition in water depths between 25 and 45 m. Extinct taxa present within the samples provide coarse (Late Pleistocene) age constraints on Santa Catalina’s deepest subsided terraces. We identified the transgressive surface corresponding to the Last Glacial Maximum and its paleo–sea-level marker at modern depths between -85 and -95 m surrounding Santa Catalina Island. Terraces surrounding Santa Catalina Island and Pilgrim Banks were correlated to lowstands and interstadials on a glacio-isostatic–adjusted, ice-volume–equivalent sea-level curve in order to evaluate subsidence rates. Santa Catalina Island has been tilting north and subsiding together with its surrounding platform at 0.08–0.27 mm/yr since at least 1.15 Ma (marine oxygen isotope stage [MIS] 34). Pilgrim Banks has been subsiding at 0.3 mm/yr for at least 0.35 m.y. but must have subsided no faster than 0.12 mm/yr between 0.35 and 1.15 Ma. We interpret the subsidence and 1.5° northward tilt of Santa Catalina Island as showing continued, although reduced, activity of the Catalina fault system simultaneous with increasing activity on the southern San Pedro Basin–San Diego Trough fault zone.
Late Quaternary subsidence of Santa Catalina Island, California Continental Borderland, demonstrated by seismic-reflection data and fossil assemblages from submerged marine terraces
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Chris M. Castillo, Simon L. Klemperer, James C. Ingle, Charles L. Powell, Mark R. Legg, Robert D. Francis; Late Quaternary subsidence of Santa Catalina Island, California Continental Borderland, demonstrated by seismic-reflection data and fossil assemblages from submerged marine terraces. GSA Bulletin doi: https://doi.org/10.1130/B31738.1
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