Preservation of Oxygen Isotope Stage 3 Marine Terrace Deposits along the Southwest Coast of the Osa Peninsula, Corcovado National Park, Costa Rica

Subduction of the aseismic Cocos Ridge at the Middle American Trench outboard of the Osa Peninsula results in rapid late Quaternary surface uplift. The distribution of surface uplift corresponds with the imaged bathymetric relief. On the Osa Peninsula, inboard of the northwest flank of the Cocos Ridge, exposures of the Late Pleistocene Puerto Armuelles Formation are recognized. The Puerto Armuelles Fm consists of poorly consolidated sands, silts, and muds from shallow marine, estuarine, and mangrove systems. Along the southwest coast, Puerto Armuelles Fm sediment infills paleo-topographic depressions. AMS radiocarbon dates obtained on 4 marine macrofossil samples yield ages ranging from 38.51 ka B.P. to 42.35 ka B.P. Dates obtained on multiple samples from individual sections are internally consistent, recording younger ages at higher stratigraphic levels within two fining upward deposits. The sections are displaced relative to one another across a northeast striking fault. The two measured stratigraphic sections are used to quantify a minimum Late Pleistocene to recent separation rate of 0.54 m ka-1. A suite of 15 radiocarbon dates on exposures of the Puerto Armuelles Fm from the eastern portion of the Osa Peninsula (Gardner et al., 1992) and 14 radiocarbon dates obtained from equivalent strata along the northwestern portions of the Osa Peninsula (Sak et al., 2004) clearly and consistently indicate a Late Pleistocene age of deposition during Oxygen Isotope Stage 3.