Seismic Geomorphology, Architecture and Stratigraphy of Volcanoes Buried in Sedimentary Basins

Our ability to investigate both the intrusive and extrusive parts of individual volcanoes has evolved with the increasing quality of seismic reflection datasets. Today, new seismic data and methods of seismic interpretation offer a unique opportunity to observe the entire architecture and stratigraphy of volcanic systems, with resolution down to tens of meters. This chapter summarises the methods used to extract the geomorphic aspects and spatio-temporal organisation of volcanic systems buried in sedimentary basins, with emphasis on the utility of 3D seismic reflection volumes. Based on descriptions and interpretations from key localities worldwide, we propose classification of buried volcanoes into three main geomorphic categories: (1) clusters of small-volume (<1 km3) craters and cones, (2) large (>5 km3) composite, shield and caldera volcanoes, and (3) voluminous lava fields (>10,000 km3). Our classification primarily describes the morphology, size and distribution of eruptive centres of buried volcanoes, and is independent of parameters such as the magma composition, tectonic setting, or eruption environment. The close correlation between the morphology of buried and modern volcanoes provides the basis for constructing realistic models for the facies distribution of igneous systems buried in sedimentary strata, establishing the principles for a new discipline of seismic-reflection volcanology.

Subsurface fluid flow focused by buried volcanoes in sedimentary basins: Evidence from 3D seismic data, Bass Basin, offshore southeastern Australia

There is growing evidence that intrusive magmatic bodies such as sills and dikes can influence the migration of fluids in the deep subsurface. This influence is largely due to permeability contrasts with surrounding sedimentary rocks or because of interconnected open fractures within and around intrusions acting as conduits for migrating fluids. The role of buried volcanoes in influencing crossstratal fluid migration in sedimentary basins is less well-established. However, several studies have highlighted spatial linkages between extinct hydrothermal vent complexes and fluid seepage, suggesting that buried extrusive features can also influence subsurface fluid-flow pathways, potentially leading to migration of hydrocarbon fluids between the source and reservoir. We have developed 3D seismic reflection data from the Bass Basin in offshore southeastern Australia that image an early Miocene volcanic complex with exceptional clarity. This volcanic complex is now buried by [Formula: see text] of younger sediments. The largest volcano within this complex is directly overlain by a vertical feature interpreted to be a fluid escape pipe, which extends vertically for approximately 700 m across the late Miocene-Pliocene succession. We suggest that the buried volcanic complex was able to focus vertical fluid migration to the base of the pipe because its bulk permeability was higher than that of the overlying claystone sequence. The fluid escape pipe may have initiated through either (1) hydraulic fracturing following fluid expulsion from a deep, overpressured subvolcanic source region, (2) differential compaction and doming of the overlying claystones, or (3) a combination of these processes. Our results suggest a hitherto unrecognized role for buried volcanoes in influencing dynamic subsurface processes in sedimentary basins. In particular, our study highlights that buried volcanoes may facilitate cross-stratal migration of hydrocarbons from source to reservoir, or through sealing horizons.