Flights of older, elevated marine wave-cut platforms on Santa Rosa Island, CA

The California Continental Borderland exhibits Neogene and Quaternary faults and folds with poorly constrained kinematics and associated surface uplift rates. We aim to infer a more detailed, long-term tectonic history from the spatial distribution, age, and deformation of uplifted marine wave-cut platforms on Santa Rosa Island. Composed mainly of Paleogene and Neogene subtidal to abyssal marine sediments and volcanic rocks, the island is mantled by late Neogene(?) and Quaternary intertidal marine and nonmarine terrace deposits. Previous research focused upon the widespread coastal platforms generated during 120 and 80 ka sea-level high stands correlated with marine oxygen isotope stages 5.5 and 5.1. Recent Quaternary geologic mapping identified several new topographically higher, presumably older terrace remnants on the island. These elevated, low-gradient, low-relief topographic surfaces display features characteristic of marine incursion including: topographic breaks at potential shoreline angles, thin fossiliferous strata containing diverse, shallow-water, well-preserved Pliocene (or) Pleistocene marine molluscan fauna [e.g., Calicantharus fortis (Carpenter), Epilucina californica (Conrad), Glans carpenteri (Lamy), and Callianax biplicata (Sowerby)]. Also observed were pholad-like borings in well-rounded clasts and in underlying planar bedrock surfaces, and gravel lags partly composed of rock types not present on the island. These marine sequences are typically overlain by younger alluvial and eolian deposits, including weakly to moderately cemented, thin, whitish micritic strata. Some of the most elevated platforms, however, may reflect either the i) reworking and preferential concentration of older fossils by Quaternary marine incursion, or ii) exhumation of older Miocene strata that coincidently correspond with the present ground surface. Paleontologic evidence indicates that some of the mollusks [e.g., Lyropecten crassicardo (Conrad), Pycnodonte? howelli (Wiedey), and Turritella inezana (Merriam)] are lower to middle Miocene in age, consistent with the Miocene bedrock. In a few locations, however, the fossils appear to be anomalously concentrated at the present ground surface and discordant with bedrock structure. One possible explanation is that the marine fossils were concentrated as a more recent erosional surface-lag deposit overlying older, marine strata. GIS analysis of 1-m airborne lidar data searching for low-gradient, low-relief areas identified distinct populations of topographic surface elevation. Although the analysis cannot identify older, dissected and steeply inclined surfaces as a result of tectonic deformation, areas identified by the model correlate well with field sites containing evidence of marine platforms. At present, we do not have quantitative dating control, nor have we assigned paleo-mean-sea-level heights, nor glacial isostatic adjustments to estimate rock uplift rates. That is, we do not have sufficient constraints to ascertain how these higher elevation terraces compare to global sea-level variations. Nonetheless, abundant evidence for markedly elevated marine wave-cut platforms is more extensive than previously recognized, potentially providing a longer term comparative surface uplift history with much of the island experiencing marine transgressions during the Pleistocene and possibly the Pliocene.