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The aim of the present research program, is to elaborate a stratigraphic-sedimentologic evolutionary model of the Pliocene-Holocene sedimentary basins by means of detailed analysis of all the factors which, through their interaction, have influenced spatio-temporal evolution (tectonics, eustasy, and sedimentation) in the period of time considered. This will be achieved by evaluating the interrelationships between tectonics, eustasy, and sedimentation, and facing up to some of the numerous aspects still not perfectly clarified, or not at all understood, regarding the tectonic-sedimentary evolution of the area in question. This aim will be reached by means of an integrated and multidisciplinary type of methodological approach, seeing that the operative unit is made up of sedimentologists, stratigraphers, geomorphologists, and structural geologists.
The research will be organized in the following way: a reexamination of the data already acquired, looking in greater depth at previous studies and extending the investigations currently under way in other zones of the Ecuador Pacific coast; elaboration of a basic mapping, something indispensable for the correct correlation in space and time of the deposits in question; a detailed sedimentary and stratigraphic analysis of the outcropping deposits; geomorphological and structural surveying of the surrounding areas; recognizing the depositional sequences on the basis of identifying discontinuity surfaces and their correlations; identifying those guidelines useful for spatio-temporal correlations at a regional scale; evaluating the role played by Quaternary climatic phases in the sedimentation by comparing the stratigraphic data with the paleoclimate curves.
The sedimentary-stratigraphic studies so far carried out in the section of Camarones, on the Canoa Formation, and on the Rio Verde and Las Penas area have allowed recognition of the effects of synsedimentary tectonics and of the eustatic and climatic variations which have conditioned the sedimentary environments, the modes of sedimentation, and thus the depositional systems. In the various sedimentary successions, depositional sequences (Canoa Formation, Tablazo Formation) have been recognized; these are characterized in the marginal parts of the basin by geometric unconformities and sedimentary hatuses, sometimes very extended in time. In the models initially proposed by Wilgus et al. (1988), the relations existing between sequence stratigraphy and tectonic are simplified by the interaction between absolute variations in sea level on a global scale (tectono-eustasy and glacio-eustasy) and thermal subsidence of divergent continental margins. Other factors such as short-term variations of subsidence, climate, sedimentary supply were recognized as modifiers rather than dominant controls of this pattern. If applied to crustal suture zone basins, these models must, however, necessarily take into account regional- and local-scale deformations which are able to produce lateral migrations of the tectonic hinge and the related point of equilibrium in very short geological times and at a rate comparable to those of the high-frequency eustatic cycles (Mutti, 1990, 1996; Flint et al., 1991). In this way, in fact, local and regional discontinuity surfaces are produced which go together with those produced by the eustatic cyclicity, but which are in no way useful for correlations outside the basins themselves. The distinction of the effects produced on the realization of the discontinuity surfaces and the internal architecture of the depositional sequences by active tectonics (uplifting and subsidence), from those attributable to sedimentation and to glacio-eustatic variations in the sea level, is the fruit of careful evaluation of the geometric relations between the various sequences (angular unconformities, paraconformities, etc.), of the type of internal cyclicity (small sequences sensu Mutti, 1990 and/or parasequences sensu Van Wagoner et alii, 1988), and of the breadth of the correlation radius displayed by the unconformities. In this context, the Plio-Pleistocene marine deposits of the Canoa Formation and Tablazo Formation cropping out along the central-southern coast of Ecuador, provides a good natural laboratory and give us the opportunity for creating efficient depositional models in situations of converging margins with intense synsedimentary tectonic activity. In detail, deposition of the Canoa Formation occurred during a period of strong subsidence, probably as conseguence of the arrival of the aseismic Carnegie Ridge at the Ecuador Trench, that partially obscured the effects of the contemporary high-frequence sea-level fluctuations. These deposits unconformably overlie the Miocene ones of the Tosagua Formation, and crop out extensively along the cliff, starting from Punta Canoa until the locality of "la Cotera", where they dip under the Pleistocene Tablazo Formation deposits, which in their turn are in unconformity. Analysis of the facies and the study of the foraminifers, molluscs, and otoliths (Bianucci et al., 1997; Cantalamessa et al., 1999) have allowed these sediments to be referred to a depositional environment located between the shoreface and the middle shelf, while detailed sedimentological observations of their architecture have evidenced a progressive southward migration of the basin depocenter, which is ascribable to the syndepositional uplifting of its northern margin. The following deposition of the Tablazo Formation (Pleistocene)occurred during a period in which the subsidence/uplift rates were of the same order of those of eustatic sea-level variations. The study of this formation, might allow us to distinguish the tectonic from the glacio-eustatic contribution in the creating of the depositional sequences. In the Cabo S. Lorenzo area (Manta), this formation is composed of terraced sediments which accumulated during the progressive uplifting, due to the subduction of the Carnegie Ridge beneath the South American plate. These sediments crop out extensively along the cliff and represent the late transgressive phases which, in the coeval ones of the Isla de la Plata lying offshore (situated near the shelf-slope break), have their correlatives of the early transgression. Thus they allow a reconstruction to be made of the longitudinal and transversal geometry of the whole basin at a scale, that of outcrops, which also evidences those peculiarities which, are not feasible with seismic stratigraphy. Thanks to its geographic position (being on the edge of the shelf in the space lying offshore the stretch of coast already investigated), the Isla de la Plata is of considerable importance for interpreting the evolution of the area. The scant geological bibliography concerning it describes it as being composed essentially of the bedrock basalt lithotypes (Pinon Formation) on which lie the Quaternary Tablazo Formation. Starting from this premise, an evaluation of the topographic altitudes of each of the terraces and the stratigraphic relationships existing between each of the units cropping out in the stretch of coast facing the island, would allow us to know the evolutionary phases, beginning from Late Pliocene, it had in common with the coast, as well as with the shelf edge. A coeval and also intensely cyclothemic succession crops out in the Jama area between Punta Venado to the south and Punta Ballena to the north. A comparison of these depositional sequences, with those previously described would allow the breadth of the correlation radius of the unconformities to be assessed, and hence the amount of "tectonic" (local and/or regional) and eustatic (global) contribution that went into their making.
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