Coarse-Grained Volcaniclastic Sediments and Associated Penecontemporaneous Volcanism, Alisitos Group, Baja California, Mexico: ABSTRACT

Abstract

Volcaniclastic rocks of Cretaceous age make up the Alisitos group, which is exposed over large areas of northern Baja California, Mexico, and is inferred to have been deposited within an island arc. Intimately intermixed penecontemporaneous intrusive, extrusive, and fragmental volcanic rocks characterize many of the intra-arc deposits within the Alisitos. Peperites, formed by mixing of magma and wet sediment, are useful for demonstrating contemporaneity of volcanism and sedimentation. Peperites were recently discovered at the type locality of the Alisitos group at Punta China, only 25 km south of Ensenada, Mexico. These intrude a 50-m thick section, dominated by mafic lithic lapilli tuff breccias and lava flows, that accumulated in a nearshore to beach environment. Near Punta Maria (about 100 km north of Guerrero Negro), coarse-grained volcaniclastics interpreted to have been deposited on a north to northwest-facing submarine slope are exposed in sea cliffs. Ammonite (Pervinquieria.) and belemnite fossils suggest a Cretaceous age and marine deposition for the Punta Maria sequence. Horizontal burrows and lack of wave reworking indicate subwave-base depth for most of the section. The most common rock types in the sequence are andesitic lithic lapilli tuff and tuff breccia, deposited from nonturbulent high-concentration sediment gravity flows (density-modified grainmore » flow). Comparison of the Punta Maria and Punta China sections suggests that hydrostatic pressure and depositional slope play important roles in controlling the magma/sediment interaction. At Punta China, a shallow-water association was intruded by small sills, resulting in a highly dispersed peperite (explosive interaction). The Punta Maria section, comprising deeper water slope deposits, was intruded by a larger magma body that partly ingested a limestone and produced a thick carapace of hyaloclastite (nonexplosive interaction).« less