THE MIOCENE COLLISION OF THE PANAMA ARC WITH NORTHWESTERN SOUTH AMERICA: STRATIGRAPHIC EVIDENCE FROM THE BORBON BASIN, NORTHERN ECUADOR

The Tertiary stratigraphy of the Borbón Basin, a forearc basin extending from northern Ecuador to southern Colombia, comprises a thick siliciclastic succession of deep-marine beds with minor intercalations of shallow-water strata. At Punta Verde, along the sea-cliff east of the mouth of Rio Verde (northern Ecuador), two lithostratigraphic units crop out. The oldest sediments are those of the Early to lower Middle Miocene Viche Fm, a monotonous succession of bathyal, bioturbated mudstones. This unit is separated from the overlying shallow-marine, markedly cyclothemic sediments of the late Middle to Late Miocene Angostura Fm, by a regionally-mappable angular unconformity. The abrupt shallowing of the basin from bathyal to inner neritic depths across the unconformity is inferred to be the effect of a regional uplift phase. Astrochronologically tuned biostratigraphic datums recorded both below and above the unconformity and correlation of the eight high-frequency cyclothems of the Angostura Fm with the high-resolution eustatic curves derived from the deep-sea oxygen isotope record, constrained the Angostura Fm to a period of 3.3 million years (from 12.4 to 9.1 Ma) and restricted the duration of the stratigraphic gap at its base between 0.45 million years (12.85 Ma through 12.4 Ma) and 1.39 million years (13,79 Ma through 12.4 Ma). Because other authors have reported the occurrence of similar unconformities and shallowing patterns in correlative deposits of Panama, Costa Rica, and Colombia and related them to the uplift due to the onset of collision of the Panama arc with the South American continent, we interpret the abrupt shallowing across the unconformity at the base of the Angostura Fm and the unconformity itself as compelling evidences that the effects of this collision extended at least as far as northern Ecuador. The uplift phase ended during the latest Middle Miocene, when renewed subsidence again became prevalent across the region.