LAST INTERGLACIAL SEA-LEVEL HISTORY ON SANTA BARBARA AND ANACAPA ISLANDS, CHANNEL ISLANDS NATIONAL PARK, CALIFORNIA

The two smallest (each ~2.6 km sq.) islands in Channel Islands National Park, California are Anacapa Island (AI) and Santa Barbara Island (SBI). Both have a rich history of Quaternary interglacial periods and tectonics as marine terraces that dominate the insular landscapes. SBI has at least four terraces (~7.5 m, ~23 m, ~50 m and ~80 m) and AI has at least two (~11 m and ~83 m). Fossils are abundant in some of these terraces. Amino acid ratios in fossil mollusks (primarily Tegula) can be used to correlate the terraces with U-series-dated terraces elsewhere in the Channel Islands and mainland California. Both the ~11 m terrace on AI and the ~7.5 m terrace on SBI likely date to the last interglacial complex (marine isotope stage 5, or MIS 5). However, both may contain fossils from two high-sea stands of this complex, one at ~120 ka (MIS 5.5) and the other at ~100 ka (MIS 5.3). Evidence of similar reoccupation of 120 ka marine terraces by the 100 ka high stand is also found on San Nicolas Island, Point Loma, and Cayucos, California. These three localities and the ~7.5 m terrace on SBI contain mixtures of both extralimital southern (warm water) and extralimital northern (cool water) mollusks. We hypothesize that the warm-water forms date from the ~120 ka high sea stand and the cool-water forms date from the ~100 ka high sea stand. Correlation of the ~11 m terrace on AI and the ~7.5 m terrace on SBI with the ~120 ka high-sea stand allows calculation of average late Quaternary uplift rates. If we assume a +6 m sea level at ~120 ka (a commonly accepted value), then AI has been uplifted only ~5 m (average rate of 0.042 m/ka) and SBI has been uplifted only ~1.5 m (average rate of 0.012 m/ka) in the late Quaternary. These low uplift rates, if correct, explain why the ~120 ka terrace was reoccupied by the ~100 ka high-sea stand. The very low uplift rate for AI is consistent with little or no uplift in the late Quaternary documented for nearby Santa Cruz Island (SCI) reported by Pinter et al. (1998, GSA Bull. 110: 711-722). Low uplift rates for both SCI and AI are not consistent, however, with very high uplift rates (~1.5 m/ka) reported for the northern Channel Islands crustal block (including SCI and AI) by Chaytor et al. (2008; GSA Bull. 120: 1053-1071). The reason for these vastly different estimates of late Quaternary uplift rate is not understood and requires further study.