Integrated paleontologic and paleomagnetic stratigraphy of the upper Neogene deposits around Limon, Costa Rica: A coastal emergence record of the Central American Isthmus

A chronostratigraphic study of mixed carbonate and siliciclastic sediments (Limon Group) along the Caribbean side of the Central American Isthmus was conducted to provide refined depositional ages on the uplifted, nearshore m arine sequence exposed near Limon, Costa Rica, These upper Neogene sediment s provide a rich faunal history that spanned the closure of the isthmus and cessation of marine circulation between the Pacific Ocean and Caribbean Se a. This faunal archive provides a critical link in regional assessment of e volutionary changes that resulted from the development of the Central Ameri can Isthmus, Results from this study have shown that integrated biostratigraphic and mag netostrati-graphic techniques can be successfully combined to provide refin ed age dating in shallow-marine sediments, even where some microfossil rewo rking has occurred. We propose the new Quebrada Chocolate Formation to repr esent latest early-early late Pliocene mixed reefal and siliciclastic depos its that overlie the early Pliocene Rio Banano Formation, The superjacent M oin Formation is expanded to include a second reefal interval deposited nea r the late Pliocene-Pleistocene boundary as well as fine-grained, mollusk-r ich deposits in the forereef and lagoon. The style of mixed-system reef geo metry between the two units is distinct, The Quebrada Chocolate Formation r eefs consist of a lower interval of alternating reefal units buried by sili ciclastic sediment and an upper unit comprised of stacked, reefal buildups with relatively minor siliciclastic matrix, These stacked reefs formed on a rising sea level. In contrast, the Moin Formation reefs are isolated coral s and patch reefs within a siliciclastic matrix that were deposited during the peak (maximum flooding) of the sea-level highstand, Our refined age model for the Limon Group sediments allows reassessment bf Pleistocene uplift rates. We calculate a rate of about 50 m/m.y., slightly less than previous rate estimates, attributed to the shallow i subduction i n the Pacific of the Cocos Ridge beneath the Central American island-ate sy stem. resolution Development of similar high-resolution models from other marine basins shou ld provide the chronostratigraphic control necessary to assess environmenta l events and evolutionary trends in shallow-marine faunas separated and iso lated by the Central American Isthmus.