Lava-water-sediment interaction : processes, products and petroleum systems

Lava-water-sediment interaction encompasses the processes and products created as lava is emplaced over, or into sediment (and/or water). The lithologies preserved at the lava-sediment interface include pillow lavas, hyaloclastite and peperite, which are well documented within the literature. However, little work captures the full scope of the interaction between sub-aerially emplaced, invasive lava and (clastic) sediment (+/-water). Furthermore, the scales and geometries of interaction at the lava-water-sediment interface are yet to be fully understood. This research uses four field localities from a variety of environmental and tectonic settings to assess the remarkably variable, complex and intricate fragmental textures and geometries preserved at the lava-sediment interface, many of which are documented for the first time. The processes and lava/sediment properties that influence interaction are then interpreted. 
This research identifies a continuum of lava-water-sediment interaction, from minimal and passive interactions, to dynamic and complex interactions, predominantly between basalt lavas and siliciclastic and volcaniclastic sediments. The contiuum recognises that the variability of sedimentary properties (e.g. saturation, grain size, cohesion, compaction), rather than lava properties (e.g. effusion rates/flux, composition, temperature, viscosity, shear strength etc.), is the dominant influence on interaction products. The variability of sedimentary properties can occur on the micro- to macro- scale, producing a range of scale-invariant lava-sediment products. When sediment is partially consolidated and compacted, with relatively little to no water content, loading and passive interaction, including the formation of passive peperite, occurs. Sediment that is very fine grained, compacted, semi-saturated and only slightly consolidated, is typically more cohesive and produces coherent sedimentary inclusions. Sediment inclusions within lava and peperite domains are abundant, and interpreted as the product of lava invading and entraining fragments of more cohesive, consolidated sediment. When sediment is saturated (with pore water), unconsolidated, and uncompacted, dynamic peperite forms and sediment fluidisation occurs. Sediment fluidisation is also the main product at the interface between pillow lavas and sediment. Measurement analysis of pillow-sediment contacts establishes that pillow invasion is scale invariant. 
An understanding of the lateral variability of the processes and products of lava-water-sediment interaction is developed, along with the concept of individual sedimentary ‘barrier’ layers that may impede lava-invasion, and influence the geometries of the system. The geometries of lava-water-sediment domains, particularly where dynamic interaction occurs, may be further influenced by palaeoenvironment (e.g. fluvial drainage systems may focus aggressive interaction and peperite formation in channels). The products and processes of lava-water-sediment interaction, and the geometries of the lava-sedimentary systems, are presented in a series of models, all of which highlight the variable sediment properties at the time of lava invasion. 
The results of this research are directly applicable to the petroleum industry in aiding exploration within volcanic-rifted margins. Application of these findings is of particular importance during the development of regional and basin-scale depositional environment models. The field data is applied to wireline and borehole image log interpretations, which provides greater understanding of how potential reservoir units may be disrupted by lavas, both physically and by “compartmentalization” of the reservoir. Together, these results demonstrate how lavas have the potential to considerably fragment on interaction with sediment and/or water, informing our understanding of the interplay of volcanic and sedimentary systems.