A PHOTOGEOLOGICAL ANALYSIS OF THE STRUCTURE OF THE WESTERN GUAYAS PROVINCE, ECUADOR: WITH DISCUSSION OF THE STRATIGRAPHY AND TABLAZO FORMATION, DERIVED FROM SURFACE MAPPING

The geology of an area in the extreme western part of the Guayas Province of Ecuador is described, chiefly from a study of aerial photographs. The exposed succession extends from the Cretaceous to the Miocene, overlain unconformably by Pleistocene tablazo. The stratigraphy is discussed and a sequence suggested which is thought to improve on previous published accounts. The tablazos are discussed and it is concluded that only one deposit exists, faulted to different levels by recent movements ; this is at variance with the traditional view that tablazo is developed at three horizons. The aerial photographs show the important effect that faults have had on the distribution of tablazo and indicate how they may have caused the anomalous distribution of recent raised beaches. Some speculation is added about the tablazos in northern Ecuador and north-western Peru. The structure of the area is described and is shown to be dominated by faulting on three main lines, N.–S., NW.–SE. and NE.–SW. It is suggested that the faults consist essentially of intersecting sinistral and dextral wrenches which disrupt less prominent N.–S. high-angled thrusts ; such a pattern seems plausible in this area, which is at the top of the westward-facing arc in the Andean chain. By analogy with the observations on land it is suggested that the Gulf of Guayaquil was caused by an intersecting system of sinistral and dextral wrenches and further that similar fault-patterns and tectonic systems may occur elsewhere in coastal Ecuador and north-western Peru.