Origin and chronology of Pleistocene marine terraces of Isla de la Plata and of flat, gently dipping surfaces of the southern coast of Cabo San Lorenzo (Manabı̀, Ecuador)
Using stratigraphic and geomorphological studies, we investigate and propose a model for the genesis of Quaternary staircase morphologies that occur at two different places along the central coast of Ecuador. In the study area, located landward of where the Carnegie Ridge impinges on the Ecuador trench, ridge subduction favored spatially nonuniform uplift rates that, combined with Pleistocene glacioeustatic sea-level fluctuations, led to punctuated forced regression. In turn, depending on the amount of sediment supply and the average amplitude of uplift rates, this led to the development of a cyclothemic shallow-marine sedimentary succession or of a flight of marine terraces. Isla de la Plata displays a sequence of four marine terraces that each mark a period of marine encroachment during uplift. Their age determination, based on inferences drawn about the amount of long-term tectonic uplift and heights of the eustatic peaks, has quantified vertical tectonic activity and correlated marine terraces with glacioeustatic variations in sea level. During the last 500 ka, the chronostratigraphy of the marine terraces of Isla de la Plata suggests a constant uplift rate of ca. 0.4 m/ka. Along the coast, on the basis of its relation to the late Pliocene and Middle Pleistocene units of the Canoa Formation, the highest surface present east of Cabo San Lorenzo is inferred as early Pleistocene in age and not, as recently suggested, early Pliocene. Quaternary uplift has been relatively slower with respect to Isla de la Plata, and synsedimentary uplift decreased from north (ca. 0.3 m/ka) to south (0.1 m/ka). This trend explains the angular and composite progressive unconformities that generated two, southward-dipping flat surfaces. Deposition of sediments associated with these landforms was accompanied by glacioeustatic sea-level fluctuation and tectonism that influenced the internal arrangement of the depositional sequences. The amount of uplift between successive highstands of sea level was not sufficient to create single marine terraces and instead resulted in repeated terrace reoccupation and the creation of a relatively thick, vertically stacked succession of shallow-marine and sometimes estuarine and coastal-plain sediments. From this study emerges the importance of local factors in the depositional response to forced regression, including the fundamental role played by the magnitude of uplift rates in generating detached or attached depositional sequences.