Basin Development in an Accretionary, Oceanic-Floored Fore-Arc Setting; Southern Coastal Ecuador During Late Cretaceous-Late Eocene Time

Abstract

Southern coastal Ecuador is an accreted terrane underlain by an oceanic crust formed during the AptianAlbian. To the southeast, the oceanic crust is overlain by Cenomanian-Coniacian fine-grained pelagic deposits, coarse-grained volcaniclastic turbidites of Santonian-Campanian age, and Maastrichtian-middle Paleocene tuffaceous shales. Toward the northwest, late Campanian-Paleocene volcaniclastic beds and lava flows of island arc composition rest on the oceanic crust. This results from the opening of a marginal basin between an early Late Cretaceous island arc (Cayo arc) and a latest Cretaceous-Paleocene island arc (San Lorenzo arc). In the late Paleocene, the accretion of the Cayo remnant arc to the Andean continental margin caused a major deformation phase that affected only the southern part of coastal Ecuador. There, deformation was sealed by thick, coarse-grained, quartz-rich turbidites that constitute the infilling of an early fore-arc or slope basin. A subsequent tectonic event in the early Eocene is believed to have resulted in emergence of the entire area. At the early-middle Eocene boundary, new fore-arc basins were created that filled with mud and clastic shelf deposits. A marked disconformity is overlain by coastal to continental coarse-grained deposits of late middle-early late Eocene age. These express a major tectonic phase attributed to definitive collision of coastal Ecuador with the Andean margin. The entire area then emerged, until the formation of new fore-arc basins in the latest Oligocene-Miocene. The late Paleocene, earliest Eocene, and early late Eocene tectonic events are the most important deformation phases to affect southern coastal Ecuador and represent its progressive accretion to the margin. The creation of repeated fore-arc basins can be attributed to subsidence from crustal erosion of the upper plate because each subsidence event succeeded an important compressive phase that must have favored coupling and tectonic erosion. This complex geologic history has implications for burial and maturation of organic matter and must be taken into account in guiding oil exploration in coastal Ecuador.