Stratigraphic Problems of Pyroclastic Rocks

Abstract

Stratigraphic methods used to study volcanic rocks are similar to those used to study sedimentary rocks and have similar purposes: establishing correlations, vertical time sequences, determining facies changes and the like. Additional aid in pyroclastic stratigraphy comes from igneous petrology, studies of magma evolution, and studies of the growth history of volcanoes. We stress that stratigraphic analysis — (1) mapping and subdivision of volcanic sequences into members, formations and groups, (2) determining vertical and lateral facies changes and (3) interpreting the types of eruptions, and the origin and manner of transport of rock types and the environments of deposition — provides the necessary framework for petrological and geochemical work, and that pyroclastic and epiclastic volcanic rocks often provide the most important parameters for establishing the stratigraphic framework in volcanic areas. Because of the many ways that volcanic sequences originate, however, different stratigraphic and petrologic approaches may be necessary in different situations such as, for example, in areas of plateau basalts, ignimbrite plateaus, clusters of scoria cones, composite andesite volcanoes, deep-sea ash layers or thick nonmarine tuff accumulations. Moreover, stratigraphic problems on active or dormant volcanoes commonly differ from those that must be solved in ancient volcanic regions, such as greenstone belts, or zeolitized volcaniclastic sediments, where the volcanic record may be best preserved in sedimentary accumulations derived by erosion of primary volcanic deposits.