Abstract

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 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 resulted from the development of the Central American Isthmus.

Results from this study have shown that integrated biostratigraphic and magnetostratigraphic techniques can be successfully combined to provide refined age dating in shallow-marine sediments, even where some microfossil reworking has occurred. We propose the new Quebrada Chocolate Formation to represent latest early–early late Pliocene mixed reefal and siliciclastic deposits that overlie the early Pliocene Rio Banano Formation. The superjacent Moin Formation is expanded to include a second reefal interval deposited 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 siliciclastic 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 corals 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 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.

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