Sedimentology of the Pliocene Upper Onzole Formation,an inner-trench slope succession in northwestern Ecuador

The Pliocene Upper Onzole Formation exposed in the vicinity of Punta Gorda, near Esmeraldas, Ecuador, is composed mainly of fine-grained mud turbidites, having regular vertical sequences of sedimentary structures associated with a positive grading, and bioturbation restricted mostly to the tops of beds. The remainder of beds measured consist of volcanic ash, mud pelagite, and glauconitic silt–sand turbidites. Vertical sequential analysis of stratigraphic sections for the most part show no pronounced trends in bed thickness or grain size. Volcanic ashes are crystal–vitric tuffs occurring in four bedding styles: (A) normally-graded ashes with burrowed gradational tops and sharp wavy bases; (B) ashes that form part of a complex microstratigraphy consisting of thinly-bedded mudstone, silt–sand turbidites, tuffaceous mudstone, and ash beds; (C) less conspicuous ash laminae and ash-filled burrows; and (D) a tuffaceous bed with ash and mud swirled together in convolute layers. Ash chemistry suggests an Andean high-K calc-alkaline provenance. Facies relations, paleontologic data and regional geologic setting suggest sediment accumulation on an inner trench slope in a basin situated oceanward of the Pliocene trench–slope break. Faulting is extensive and reflects two deformational episodes, the youngest involving Holocene marine and fluvial terraces. Faults group into an older, northerly trending, listric set, and a younger, west–northwest-trending high-angle set. Fault striations of both sets suggest dominantly dip–slip motion. The faults reflect an episode of latest Pliocene through Quaternary trench-normal extension in the forearc, followed by trench-parallel extension. Both sets of faults crosscut large northeast-trending regional folds. Post-Pliocene changes in stress regime, from generally northwest–southeast compression, to east–west tension, to north–northeast–south–southwest tension probably reflect change in stress vectors that resulted from the ongoing collision of the leading edge of the South America plate with the thick oceanic crust of the Carnegie Ridge.