PALEOENVIRONMENTS OF THE UPPER MIOCENE TUIRA FORMATION, DARIEN, PANAMA

Concurrent with the 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 in one of the few remaining straits that connected tropical Atlantic and Pacific waters. Compared to other Miocene formations of eastern Panama, the Tuira Formation crops out over a wider area and includes more facies, which can be related to the uplift history of the Isthmus of Panama. Benthic foraminiferal assemblages of the upper middle to late Miocene Chucunaque-Tuira Basin and Sambu Basin were analyzed for paleobathymetries used to trace the uplift history of this region.

Miocene sediment samples were collected from the Tuira Formation and coeval Membrillo Formation along five river sections of the Chucunaque-Tuira Basin (CTB) and two river sections of the Sambu Basin to the southwest. They contained abundant benthic foraminiferal species whose ecologies were used to indicate paleobathymetry. Paleoenvironments of the Upper Miocene Tuira Formation varied from middle to upper bathyal in the center of the CTB to shallower depths on the margins. Sediments of the Sambu Basin, seaward of the CTB, were deposited at middle to upper bathyal depths. Some benthic foraminifera of the Tuira Formation lived under low-oxygen conditions because of silled basins or the oxygen-minimum zone. The different water depths across the two basins reflect differential amounts of uplift of the isthmus. Considered together with previously determined paleobathymetries of the underlying and overlying formations, water depths in the tropical Atlantic-Pacific strait were progressively shallower through the Miocene in the deeper, center portion of the main (northwest) CTB, in the southeast CTB, and in the Sambu Basin. However, in several sections toward the margins of the CTB, waters shallowed to inner- middle neritic depths (Tuira Formation deposits), then deepened to outer neritic/upper bathyal depths in the latest Miocene as shown by paleobathymetries of the Chucunaque Formation. The deepening may correspond to a sea-level rise ca. 6 Ma. No Pliocene sediments have been found in Darien, presumably because it was fully emergent by that time.