GSA QUATERNARY GEOLOGY AND GEOMORPHOLOGY DIVISION KIRK BRYAN AWARD: QUATERNARY SEA-LEVEL HISTORY ON THE PACIFIC COAST OF NORTH AMERICA: EFFECTS OF LOW UPLIFT RATE AND GLACIAL ISOSTATIC ADJUSTMENT PROCESSES ON THE MARINE TERRACE RECORD

Along coastal California, each landform in a flight of marine terraces has long been thought to represent a single past high-sea stand. However, the last interglacial (marine isotope stage [MIS] 5) is often recorded on a single terrace by deposits containing fossils dating (U-series, coral) to both ~100 ka (MIS 5c; cool-water mollusks) and ~120 ka (MIS 5e; warm-water mollusks). Glacial isostatic adjustment (GIA) effects and a modest uplift rate explain much of this fossil reworking. Coastlines of North America are susceptible to GIA effects because of the nearby presence of large ice sheets during glacial periods: local paleo-sea levels are higher than what would be expected from a purely eustatic sea level rise. We hypothesize that pre-MIS 5 marine terraces should be similarly affected, particularly those of early Pleistocene age, a time dominated by the ~41 ka obliquity cycle. On San Nicolas Island, the 3rd, 4th, 5th, 6th, 8th, and 10th terraces have Sr-isotope ages correlating these deposits to MIS 11, 15, 17, 19, 21, and 31, respectively. The 5th, 8th, and 10th terraces host mixtures of warm-water and cool-water species of mollusks, analogous to the mixture of warm (120 ka) and cool (100 ka) species on the 2nd terrace. Thus, each terrace likely contains fossils from both a warm-water high-sea stand and a cool-water high-sea stand. The combination of low uplift rate and GIA effects indicates that a complex marine terrace record on the Pacific Coast is expected, and a simple, one-terrace-per-high-sea-stand scenario for the Quaternary is unlikely.