Abstract The pacific border of the South American plate presents a more or less symmetrical sinuosity, with a central concave curvature (the Arica Angle located between two side rays along Chile and Peru) and ending in convex arcs (the Patagonian and Talara arcs, respectively). The width of the continental and coastal margins varies significantly according to the geometry of the border. The continental margin of Ecuador corresponds to the northern part of the Talara Arc. Three different segments showing different coastal geomorphology and continental platform characteristics are identified from north to south: the northern segment (Mataje River–Galera Point) shows a wide continental shelf and slope, the upper subducted slab of the subduction plane presents a low dip; the central segment (Galera Point–Santa Elena) stands in front of the Carnegie Ridge, and presents a moderate uplift in the Manta Peninsula, in front of the Carnegie Ridge, and the upper subduction plane is subhorizontal; the southern segment includes the side and inner coasts of the Gulf of Guayaquil, below the gulf the subduction plane shows a low dip. A comparison with published 3D numerical modelling of curved subduction suggests that the geometry of the continental boundary has a significant effect on uplift or subsidence along the continental margin. Also, the subduction of asperities in the trench, such as the Carnegie Ridge, may change the coastal motion from subsiding to uplifting, as is observed in the Esmeraldas area. There is no clear evidence of a shelf developed during the Last Glacial Maximum (LGM) sea-level lowstand, probably due to the vertical motion – uplift or subsidence – observed all along the coastal margin.

Abstract The Pacific basins of Ecuador have excellent geologic characteristics for the accumulation of hydrocarbons. Geologic, seismic, gravity, and magnetic surveys provided a basis for exploratory drilling in the provinces of Esmeraldas, Manabi, and Guayas. Exploratory drilling in the Santa Elena Peninsula has resulted in the discovery of the Ancon, Petropolis, Santa Paula, Concepción, Certeza, and other oil fields. In 1970, the Amistad gas field was discovered in the Gulf of Guayaquil. Shows of gas and oil were obtained in stratigraphic units penetrated in wildcats elsewhere in Guayas Province. The hydrocarbons are found in the Atlanta Formation (Atlanta sandstone) and the Socorro Formation. Shales of the San José Formation are considered to be the source rocks. The best petroleum prospects of the Ecuadorian littoral zone are placed offshore in the Gulf of Guayaquil. Important gas accumulations are present at depths between 9,000 and 14,000 ft (2,740–4,270 m), principally in the Miocene Subibaja Formation (Fig. 1), Cumulative production of oil in this area to June 1974 was 106,300,000 bbl. Although data on gas reserves are scarce, proved reserves in the Gulf of Guayaquil are estimated at 145,111 MMcf, and probable reserves at 373,536 MMcf.