Concurrent with the final collision of South America and Central America, sediments of the Upper Miocene Tuira Formation (Darien Province, southeastern Panama, Pacific side) were deposited 11.2–8.6 Ma within one of the few remaining straits of the Central American Seaway that connected tropical Atlantic and Pacific waters. Samples were collected from the Tuira Formation and coeval Membrillo Formation along five river sections of the Chucunaque-Tuira (C-T) river basin and a river section of the Sambu Basin to the southwest. Benthic foraminiferal assemblages from samples of well-preserved specimens were analyzed with cluster analysis for similarities, and indicator taxa were applied to interpret paleobathymetries, which were used to trace the net uplift history of the Darien region.

Paleoenvironments of the upper Miocene Tuira Formation, interpreted from foraminiferal species’ ecologies, varied from middle to upper bathyal in the center of the C-T Basin to shallower, neritic depths on the margins. Sediments of the Sambu Basin, seaward of the C-T Basin, were deposited at middle to upper bathyal depths. Some benthic foraminiferal assemblages reflect low-oxygen conditions of silled basins or the oxygen-minimum zone. Considered together with paleobathymetries of the underlying and overlying formations, water depths in this strait were progressively shallower through the Miocene in the deep, central and southeast portions of the main (northwest) C-T Basin and in the Sambu Basin. Near the C-T Basin margins, waters under which Tuira Formation sediments were deposited shallowed to inner to middle neritic depths, then deepened to outer neritic/upper bathyal depths in the latest Miocene, the age of the overlying Chucunaque Formation. The deepening may correspond to a sea-level rise ca. 6 Ma. The apparent absence of Pliocene marine sediments in Darien suggests emergence by that time.

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