PALEOSHORELINES OFFSHORE OF THE SAN LUIS RANGE, SAN LUIS OBISPO COUNTY, CENTRAL COASTAL CALIFORNIA

Emergent marine terraces mapped in the San Luis Range between Morro Bay and Pismo Beach record a history of slow tectonic uplift. This flight of terraces consists of wave-cut platforms and paleo-seacliffs whose intersections (shoreline angles) record the relative sea level during middle to late Quaternary highstands. Terrace ages have been established based on multiple dating and correlation techniques (Hanson et al., 1994). These terraces are important datums for evaluating rates of Quaternary uplift and locations of active crustal structures in this area.

Similar to the emergent terraces, submerged wave-cut platforms and paleo-seacliffs offshore record paleosea-level lowstands, stillstands and highstands. These paleoshorelines are recorded in detailed multibeam bathymetry and shallow seismic reflection lines collected in 2008 and 2009. We have identified several gently-sloping platforms backed by steeper scarps (paleo-seacliffs), both as well expressed geomorphic features evident in the bathymetry of the rocky parts of the shelf and also buried beneath mobile sand sheets and thicker marine sediments evident in seismic reflection profiles. We estimate ages of submerged paleoshorelines based on correlation to global Quaternary sea-level curves. We recognize that wave erosion during the Holocene sea-level rise may have partly eroded the paleoshorelines; however, the (local) clarity of their geomorphic signature suggests that erosion during the short time (~hundreds of years) that the rise in sea level crossed any paleoshoreline was not sufficient to change its basic geomorphic character. We find that lateral correlation of individual features is complicated by the lack of continuity of many of the paleoshorelines due to fluvial erosion during sea-level lowstands, marine planation during younger sea-level high stands, burial of the features by younger sediment, and possible tectonic deformation. Precise dating of these shorelines will be difficult because the Holocene sea-level rise likely removed most of the older sediment and fossils that could be used to date the platforms and shorelines. Nonetheless we are using the offshore terrace sequence to help locate deformation known onshore as it projects offshore and to help estimate vertical slip rates.