Independent evidence from rocks, fossils, and genes converge on a cohesive narrative of isthmus formation in the Pliocene.

No abstract

The geology of the Darien province of eastern Panama is presented through a new geologic map and detailed biostratigraphic and paleobathymetric analysis of its Upper Cretaceous to upper Miocene sediments. The sequence of events inferred from the stratigraphie record includes the collision of the Panama arc (the southwestern margin of the Caribbean plate) and South American continent. Three tectonostratigraphic units underlie the Darien region: (1) PrecoUisional Upper Cretaceous-Eocene crystalline basement rocks of the San Bias Complex form a series of structurally complex topographic massifs along the northeastern and southwestern margins of the Darien province. These rocks formed part of a >20 m.y. submarine volcanic arc developed in a Pacific setting distant from the continental margin of northwestern South America. The northerly basement rocks are quartz diorites, granodiorites, and basaltic andésites, through dacites to rhyolites, indicating the presence of a magmatic arc. The southerly basement rocks are an accreted suite of diabase, pillow basalt, and radiolarian chert deposited at abyssal depths. PrecoUisional arc-related rocks, of Eocene to lower Miocene age, consist of 4000 m of pillow basalts and volcaniclastics, and biogenic calcareous and siliceous deepwater sediments. They consist of the Eocene-'Present address: 4193, Lower Cove Run Road,