Geological mapping of volcanic terrains: Discussion on concepts, facies models, scales, and resolutions from New Zealand perspective

Abstract

Geological mapping in volcanic terrains is a challenging task as there needs to be input from various geologists' groups and only limited, often ambiguous guidelines exist. Volcanoes and volcanic successions are in many ways viewed and treated differently than normal sedimentary successions. Volcanic systems are very much a part of the normal sedimentary environment and as such need to integrated into the stratigraphic framework of the enclosing deposits. Most volcanoes supply large volumes of material (pyroclasts) into the sedimentary system. The type and style of eruption determines the time frame of the process, while the type of volcano defines the interaction between volcano and background sedimentation (e.g. monogenetic versus polygenetic volcanic systems). A sound lithostratigraphic framework for mapping geological units in volcanic terrains should also adopt the latest developments in volcanology and understanding of volcanic systems. The lithostratigraphic classification needs to define the mapping unit and apply it to the 1) proximal to distal volcanic facies, 2) eruption styles and their temporal changes, and 3) mode of pyroclast transportation (i.e. fallout versus pyroclastic density current). Differentiation of syn- and post-eruption re-sedimentation, during either within- or between eruptive phases, will determine the most useful stratigraphic nomenclature system to apply to the volcanic units. The approach to geological mapping in ancient and modern volcanic terrains can be different but the same basic stratigraphic principles and classification apply. The purpose of the geological mapping project, whether it be in ancient or modern volcanic systems, will define the style and scale of mapping. The type of output depends upon the objectives and available resources, including the volcanological experience of the mapping team. Here we provide some insight from a New Zealand perspective to geological mapping on active volcanic terrains in a convergent plate margin.