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Geological Society, London, Special Publications; 1982; v. 10; p. 191-198;
DOI: 10.1144/GSL.SP.1982.010.01.12
© 1982 Geological Society of London

South America

The geology of the western part of the Borbón Basin, North-west Ecuador

C. D. R. Evans & J. E. Whittaker

Institute of Geological Sciences, Ring Road, Halton, Leeds LS15 8TQ, England
British Museum (Natural History), Cromwell Road, London SW7 5BD, England

The western part of the Borbón Basin, North-west Ecuador, contains some 5 km of predominantly thin-bedded Tertiary mudstones with rare shallow-water sand-stones, resting on a basement of Cretaceous basalts. The basin is floored by thin, discontinuous, turbiditic carbonates of middle Eocene age, which pass up into a 3 km thick sequence of mudstones, cherty at the base, extending into the mid-Miocene. Shallow-water sandstones accumulated during the late Miocene and early Pliocene and were succeeded by deeper water mudstones, which in the Pliocene, are over 1 km thick and contain resedimented sandstone units. The strata are folded into NE-SW-trending structures, parallel to the mid-slope basement high centred on the Río Verde area. Stratigraphic evidence suggests that much of this folding, and the emergence of the high, post-dates the late Pliocene (c.2.9 Ma).

Since the middle Eocene the area has occupied the arc-trench gap between the Ecuador-Colombia oceanic trench, and the volcanic arc of the Andes; it now lies on the oceanward side of the mid-slope basement high.