THE CRETACEOUS CAYO FORMATION AT PUERTO CAYO, WESTERN ECUADOR

The cliffs in the southern part of Puerto Cayo expose generally darker colored inclined strata, which are unconformably overlain by a generally light colored sequence containing marl, some silicified to chert, and other strata. The dark colored Cayo (DCC) formation consists of turbidites that are dominated by generally thick-bedded, conglomeratic greywacke containing volcaniclastics and interbeded with variegated reddish brown and green mudstone and gray siltstone. Massively bedded graywacke can be up to 10 meters thick, and stacks of greywacke sequences without intervening fine-grained strata may reach up to 30 meters. The lithic fragments indicate that the marine depocenter received reworked island arc-back arc volcanic suites. The DCC is crisscrossed by numerous faults of different attitudes and translations. Calcite crystals heal fault fractures, and lineation and steps of fracture walls help estimate sense of movement of rock fragments. Attitudes of strata vary due to fragmentation and rotation by faults. The DCC strike from N15°E to N45°W and dip 20° to 30° SE. The attitude of strata is also apparent on the coastal Pacific Ocean at low tide, where some of the exposures are covered by algae, barnacles with green snails in pools of water, and Heliaster starfish [not withstanding the story of the extinction of this genus] graze the algae. At the bottom of cliffs is a dark zone sprayed during high tide water and inhabited by periwinkles. Sand sized magnetite grains are predominant in dark sand piles near tidal berms below some parts of the cliff.

The Cayo formation is thrust generally westward at Puerto Cayo, and about 33 kilometers south of Puerto Cayo, the DCC is folded into a west vergent anticline as exposed at the northern cliff of Puerto Lopez. More likely the region was thrust over the continental margin that contained the de Plata Island. Apparently, the Cayo formation was deposited on a successor marine igneous plateau, which subsequent to the first one that composes the substructure of the western Cordillera, was similarly thrust to the east and accreted to Ecuador. Later, the western edge of the successor plateau was under thrust by an east-dipping marginal plateau (the current continental margin) before the development of the current Ecuadorian trench, beneath which the Nazca plate descends.