Abstract
Dr. R. Bristow writes: Mr. Henderson is to be congratulated on the presentation of this stimulating paper. In it much new information is presented and it allows one of the first quantitative studies of are activity in Ecuador to be made. However, I have to disagree with the interpretation placed onsome of the facts contained in this paper. These are stratigraphical queries which should not detract from the author's main conclusions, but nevertheless they may have important implications concerning the timing and duration of arc activity in Ecuador. Basically, Mr. Henderson believes that there are three diachronous units-Macuchi Formation, Yunguilla Formation, and Cayo Rumi Member, which rangein age from Cretaceous to Eocene. I maintain that there is no evidence of diachronism, only that there are Eocene deposits, both marine and volcanic, which have a common lithology with Cretaceous deposits, but separated from them by a time gap of some 20 m.y. To take the Andean Yunguilla Formation, because it is the best documented, as an example: at a number of points (Loja, Cuenca, near Riobamba, and at the Yunguilla type locality in the N) the black flysch sediments of the formation are of Maastrichtian age; at one point only, near Pilaló, lithologically similar rockshave been dated as Middle Eocene in age (Bristow & Hoffstetter 1977, p. 350). Nowhere are there flysch sediments of Palaeocene or Lower Eocene age, and it is thus impossible to demonstrate that the Yunguilla Formation is diachronous from Maastrichtian to Middle Eocene.
Published Version
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