Geochronologic contributions to the tertiary sedimentary-volcanic sequences ("baucarit formation") in sonora, México

Abstract

K-Ar dating of volcanic flows undcrlying, intercalated with and overlying volcanogenic conglomerates and sandstones of continental origin in Sonora yielded ages from 33 to 5 m.y. Among these, volcanic rocks associated with conglomerates and sandstones known as the "Baucarit Formation" range in age from 23 to 10 m.y. Thc Oligocene and middle Miocene strata contain basaltic, andesitic and andesitic-rhyolitic volcanic ílows. A peak of silicic magmatism occurred betwecn 10 to 14 m.y. The upper Miocene through Pleistocene sequences are associated with fundamentally basaltic volcanism. Crustal extension succeeded the cessation of the Laramide Orogeny, and may have begun in Sonora in the latest Eocene (?), followed by the initial accumulation of Oligocene syntectonic conglomerates and volcanic flows (33 to 24 m.y. old ). Extension was apparently continuous through most of the Tertiary; nevertheless, a major, but short episode of extensional strnctural deformation occurrcd between 10 and 9 m.y. This tectonic event may be responsible for the actual physiography of the Basin and Range Province in Sonora.