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The Manabí and Esmeraldas-Borbón forearc basins of Ecuador

Abstract:

Four selected geosections along the Ecuadorian Manabí and Esmeraldas-Borbón onshore basins, constrained by seismic lines, wells, and outcrop data, record the Late Cretaceous to Cenozoic stratigraphy and structural evolution of the Andean forearc basins. The stratigraphy is characterized by at least seven unconformity bounded sequences (SES1 to SES7). The spatial extent of SES1, SES2, and SES3 (Late Cretaceous to Eocene) was controlled by the development of half-grabens and related normal faults. SES4 (Oligocene), although a more tabular unit, was still controlled by extension as the section thick toward the footwalls of major faults and it was deformed by extensional roll-overs. The development of a major unconformity at the base of SES5, truncating previous sequences, suggests the end of the first tectonic extensional phase. The deposition of SES5 and SES6 (early to late Miocene) marks a switch in deposition setting and the establishment of a progradational shelf in the central part of the Manabí Basin. This was controlled by a western tilting basin subsidence that creates accommodation space for these sequences. The continues tilting of the basin was responsible for the exposure of the syn-extensional Eocene-Oligocene units to the east and their subsequent erosion as it is evident by the major truncation and unconformity surface at the base of SES7. Finally, a compressional phase resulted in a tectonic inversion of major basinal faults, and uplifting and anticline folding of their corresponding footwalls. This compressional event marks the exhumation of the Late Cretaceous to Miocene section as evident by inverted structure affecting SES1 through SES6 in the Esmeraldas-Borbón Basin. Also, it controls the accommodation space and depositional infill of the eastern SES7 depocenters from late Miocene-Pliocene to recent times.