Basement rocks of eastern Panama, exposed around the Golfo de San Miguel, include dense well-preserved pillow basalt and diabase which are overlain by deformed chert and thin-bedded siliceous radiolarian-rich abyssal oceanic sedimentary rocks of Late Cretaceous (in part, early Campanian) age. This basalt-chert relation of the Late Cretaceous is approximately correlative with the lower reflector horizon of the deep oceans. A second abyssal volcanic phase with radiolarian-rich sediments occurs in the lower and middle parts of the Morti Tuffs, early to middle Eocene in age, which represents the equivalent of the upper reflector horizon in the deep oceans. A third abyssal volcanic phase associated with radiolarian-rich tuffaceous sediments, with some chert and agglomerate, occurs in the middle Oligocene to lower Miocene formations; this sequence has not been identified as a specific reflector in deep-sea sections. A deep-marine erosional or nondepositional interval occurs between the Late Cretaceous basement rocks and the Eocene Morti Tuffs; a second hiatus separates the Morti Tuffs from the overlying middle Oligocene to lower Miocene Pacific Tuffs and Clarita Limestone.

In eastern Panama, the development of a Panamanian ridge or block is shown by the progressively shallower water facies during the middle and later Neogene, leading to paralic and nonmarine facies of the Pliocene-Pleistocene. Cessation of interoceanic communication occurred gradually during this phase, terminating finally in the Pliocene-Pleistocene. This paleobathymetric development is likely similar to that for nearby northwestern Colombia; it is in distinct contrast to the developmental history of the Canal Zone region where deep-ocean sedimentation was terminated in the middle Eocene. These middle Eocene shallower water facies of the Canal Zone may have been the source for displaced shallower water facies that are intermixed in some cases with deep-water Eocene facies of eastern Panama.

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