The Tumaco Basin is a part of the North Ecuador-South Colombia Pacific physiographic region. It is a large asymmetric structure, elongated in a N30°E direction and with a stepper western flank; besides, the basin deepens and becomes wider to the southwest.

Seismic, gravity and magnetic data reveal an onshore trend of structural depressions and highs. The Remolino Grande Paleohigh separates the Tumaco basin in two sectors of different structural styles: 1) an internal basin, mostly onshore, of transpressive origin; and 2), an external basin located entirely offshore, of compresional type (Figure 2). Structurally, the basin is supposed to be bounded to the north by Garrapatas fault; to the east by the piedmont fault-systems of the Western Cordillera. Westwards, the onshore limit of the basin is marked by basement ups and downs running near parallel the coast line. The Tumaco off-shore basin is bordered in the west by the Colombian- Ecuadorian trench in the Pacific Ocean. In general terms, Tumaco Basin has been interpreted as a fore arc structure containing a pre-Miocene mega-sequence and three early post-Miocene sequences, most of them deposited in deep to shallow marine environments, and rest on a Cretaceous basement composed of metasedimentary and volcanic rocks.

Analysis of recent acquired gravity data, in the onshore area, suggest that the thickness of the sedimentary cover reaches a maximum of 10 800 m.

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Figure 2