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

摘要

A chronostratigraphic study of mixed car- bonate and siliciclastic sediments (Limon Group) along the Caribbean side of the Central American Isthmus was conducted to pro- vide refined depositional ages on the uplifted, nearshore marine sequence exposed near Limon, Costa Rica. These upper Neogene sediments provide a rich faunal history that spanned the closure of the isthmus and cessation of marine circulation between the Pacific Ocean and Caribbean Sea. This faunal archive provides a critical link in regional assessment of evolutionary changes that re- sulted from the development of the Central American Isthmus.Results from this study have shown that integrated biostratigraphic and magnetostrati- graphic techniques can be successfully combined to provide refined age dating in shallow-marine sediments, even where some microfossil reworking has occurred. We pro- pose the new Quebrada Chocolate Formation to represent latest early-early late Pliocene mixed reefal and siliciclastic deposits that overlie the early Pliocene Rio Ballano Formation. The superjacent Moill Formation is expanded to include a second reefal interval de- posited near the late Pliocene-Pleistocene boundary as well as fine-grained, mollusk- rich deposits in the forereef and lagoon. The style of mixed-system reef geometry between the two units is distinct. The Quebrada Chocolate Formation reefs consist of a lower interval of alternating reefal units buried by siliciclas-tic 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 For- mation reefs are isolated corals and patch reefs within a siliciclastic matrix that were de- posited during the peak (maximum flooding) of the sea-level highstand. Our refined age model for the Limon Group sediments allows reassessment of Pleistocene uplift rates. We calculate a rate of about 50 m/m.y., slightly less than previous rate estimates, attributed to the shallow subduction in the Pacific of the Cocos Ridge beneath the Central American island-arc system. Development of similar high-resolution age models from other marine basins should provide the chronostratigraphic control necessary to assess environmental events and evolutionary trends in shallow-marine faunas separated and isolated by the Central American Isthmus.