PRESERVATION OF OXYGEN ISOTOPE STAGE 3 TERRACE DEPOSITS ALONG THE PACIFIC COAST OF COSTA RICA

Subduction of the aseismic Cocos Ridge at the Middle American Margin outboard of the Osa Peninsula (SE Costa Rica) results in rapid late Quaternary surface uplift. The distribution of surface uplift corresponds directly with the distribution of imaged bathymetric relief on the incoming Cocos Plate. On the northwest coast of the Osa Peninsula, inboard of the northwest flank of the Cocos Ridge, the newly recognized shallow marine Marenco Formation is regionally extensive. Here the semi-lithified fossiliferious sands of the Marenco Formation infill paleo-topographic depressions in the underlying melange. Radiocarbon dating of a suite of 12 samples yields ages ranging from 48 ka B.P. to 34 ka B.P. Dates obtained on multiple samples from individual sections are internally consistent and record progressively younger ages at higher stratigraphic levels within the fining upward deposits. An AMS date of 40.7 ± 0.69 ka B.P. obtained on oyster shells collected from the contact with the subjacent planed-off marine abrasion platform at an elevation of 39 m is consistent with formation during oxygen isotope stage 3 (OIS 3). This facies constraint facilitates calculation of a late Quaternary uplift rate of ca. 2 m /ka assuming a 40 ka sea level of –30 m (Kuwabara et al., 1999). This uplift rate is consistent with a preliminary uplift rate calculated for a lower elevation Holocene abrasion surface exposed along the northwest coast of the peninsula. Thus the Osa Peninsula may prove suitable for further constraining OIS 3 sea level in time and elevation.