An U-Pb zircon chronology based correlation for forearc sediments trough the Ecuadorian - Peruvian border
Résumé
The forearc area of SW Ecuador and NW Peru offers high-quality outcrops of sedimentary rocks of Paleocene to Late Miocene age. This section of the South American margin is highly segmented and is constructed trough different geodynamic processes north and south of the border area, the most important of those being related to the accretion of oceanic terrains during the Late Cretaceous.
Historically, the few sedimentary correlations through the Ecuador - Peru border have been exclusively based on time constrains, fully derived from micropaleontology analysis. A new preliminary correlation is proposed and based on U-Pb (LA ICP MS) dating of more than 3000 zircon grains coming from: 1) the Santa Elena Peninsula, an ancient outer wedge related to the accretion of SW Ecuador against the continental margin; 2) The Eocene Talara basin, and 3) the Oligocene-Miocene Progreso and Zorritos basins. Besides few cases in which the zircon record is related to recycling processes, most of younger age clusters provide the age of sedimentation or an age that is very close to it. The new isotopic ages have, in few cases redefined the former paleontological ages, but in most cases, they have sharpened the time interval proposed for sedimentation in function of paleontological records.
Our main preliminary results show that the deformed Paleocene series in Ecuador (60-56 Ma; Azúcar Fm.) has no relative counterparts in North Peru where oldest forearc sediments are dated between 54 and 51 Ma (Salinas Fm.). Other differences and similarities are noted. Eocene sediments outcropping in the Santa Elena Peninsula in Ecuador (Ancón Gp.) were deposited between _~52 Ma to _~41 Ma after main activity leading to the formation of the outer accretionary wedge in a turbiditic slope fan setting. The Peninsula uplifts at 32-30 Ma coeval with the sedimentation of the Zapotal Fm., bringing about shallow marine, tide-dominated settings with minor coastal carbonates in the middle-late Miocene units of the Progreso basin. In Peru, the Eocene Talara Fm. exhibits westward (?) prograding shelf deltas, and the depositional setting might remain shallow (at least locally) until the formation of a major, subaerial unconformity related to the formation of the Gulf of Guayaquil during the Early Miocene. The Zorritos Fm. deposited above indicate a fluvial to coastal setting.
More zircon ages are required on the Peruvian side to refine the picture of infilling basins, in order to relate their boundaries to the activity of the major sliver-bounding faults of the margin.