Neogene stratigraphy, paleoceanography and paleobiogeography in northwest South America and the evolution of the Panama seaway

Abstract

The Atrato basin in the pacific coastal region of NW South America (Colombia) exhibits stratigraphic and biostratigraphic characteristics similar to other Neogene Pacific coastal basins of southern Central America and northern South America. An evaluation of the Neogene stratigraphy and foraminiferal biostratigraphy with emphasis on paleoceanographic and paleobiogeographic phenomena indicate the following. (1) Previous to the middle Miocene, well aerated deep and open oceanic conditions, and free and active water circulation prevailed along the steep continental margins of NW South America. (2) During the early middle Miocene, right after the Neogene Hiatus NH 2 (15.2 Ma), tectonic disturbances triggered the initial uplift of the Panama sill. Bottom water circulation, foraminiferal and sedimentation changes are observed in both the Pacific and Caribbean coastal areas of NW South America. (3) During the middle Miocene, coinciding with the Neogene Hiatus NH 3 (12.9–11.8 Ma), an abrupt paleobathymetric change from lower to middle bathyal depths indicates a major uplift of the Panama sill to about 1000 m, reflecting the middle Miocene tectonic disturbances in NW South America. (4) During the late middle Miocene, immediately following this uplift a distinctive Pacific benthic foraminiferal fauna abruptly appeared and extended from Ecuador to California. Because these assemblages do not occur in the adjacent Caribbean region, a circulation barrier between the Atlantic and Pacific Oceans as a result of intensification of the cool, marginal California Current is proposed. Anoxic middle to upper bathyal environments prevailed under well aerated cool surface waters in the Pacific coastal areas of NW South America. (5) During the latest Miocene, immediately following the Neogene Hiatus NH 6 (7.0–6.3 Ma), surface water circulation between the Caribbean and the Pacific was re-established and the previous influence of the California Current disappeared. A rapid filling and shallowing of the basin to mostly neritic depths is indicated along the Pacific and Caribbean coastal basins of Colombia as well as a progressive increase in similarity between the coastal Pacific and Caribbean benthic biotas. This shallowing in the early late Miocene, also appears to have been associated with the earliest terrestrial interchange of ground sloths to North America and raccoons and their allies to South America. (6) Immediately following early Pliocene time, the Panamanian isthmus became completely emergent providing a terrestrial environment favourable for intermingling of terrestrial faunas and floras between North and South America. I correlate this Pliocene uplift with the Neogene Hiatus NH 8 (3.7-3.1 Ma). Four regional unconformities associated with tectonic disturbances in the NW corner of South America, have also been recognized and correlated with Neogene hiatuses recorded from the oceans: early-middle Miocene, middle Miocene, late Miocene and early Pliocene.