Pre-collision evolution of the Piñón oceanic terrane of SW Ecuador: stratigraphy and geochemistry of the “Calentura Formation”

The stratigraphic revision of the southern coastal Ecuadorian series makes possible the reconstruction of the pre-collision history of the Caribbean plateau accreted to the Ecuadorian margin. The Coniacian age of the oceanic basement (Piñón Fm) indicates that the latter is part of the Caribbean oceanic plateau. It is overlain by the Calentura Fm, which comprises from base to top: (i) 20 to 200 m of lavas and volcanic breccias of arc affinity (Las Orquídeas Mb), (ii) siliceous, organic rich black limestones of (middle?) Coniacian age, (iii) red, radiolarian rich, calcareous cherts ascribed to the Santonian-early Campanian, and (iv) marls, greywackes and island arc tuffs of Mid Campanian age. The latter are overlain by volcaniclastic turbidites of Mid to Late Campanian age (Cayo Fm), coeval to the Campanian-Maastrichtian island arc series locate farther west (San Lorenzo Fm). The Las Orquídeas magmatic unit is interpreted as resulting from the melting of the Caribbean plateau, rather than from an ephemeral subduction process. The transition from coniacian limestones to santonian red cherts would be related to the thermal subsidence of the Caribbean plateau. The uplift of the latter and the development of the San Lorenzo island arc in the Middle Campanian would be due to the collision of the Caribbean plateau with the Mexican margin. Early in the Late Maastrichtian, the collision of the Caribbean plateau with the Ecuadorian margin would have triggered the cessation of the San Lorenzo arc activity. In the Late Paleocene, the Caribbean plateau was split into two terranes: the western Piñón terrane, which collided with the eastern Guaranda terrane.