Magma composition and viscosity as controls on peperite texture: an example from Passamaquoddy Bay, southeastern Canada

Silurian mafic lava flows and rhyolite sills in the Passamaquoddy Bay area, southeastern Canada, interacted with wet sediment to produce peperitic breccia. The pahoehoe-type mafic flows flowed over and bulldozed into wet, unconsolidated silt and sand to produce both blocky and fluidal peperite at their lower contacts. Soft-sediment deformation structures in the siltstone adjacent to the flows, irregularly shaped vesicles in the siltstone, and siltstone-filled vesicles in the mafic lava all indicate the sediment was wet at the time of flow emplacement. Sills of rhyolite emplaced into the same sedimentary sequence also interacted with the wet sediment and produced peperite at both the upper and lower contacts. Evidence for the rhyolitic magma interacting with wet sediment includes sediment-filled vesicles in rhyolite, quartz-filled vesicles in siltstone and angular, non-vesicular shards of rhyolite in clastic dykes. Peperite formed by this process is exclusively blocky. The rhyolitic magma in the Passamaquoddy Bay sequence and other examples of high-viscosity magma producing only blocky peperite suggest that in contrast to low-viscosity magmas, which produce a combination of blocky and fluidal peperite, only blocky peperite will form associated with highly viscous rhyolite magma. The sediment that interacted with both the mafic and rhyolitic lavas is of similar texture indicating that, in this case, sediment texture did not control peperite type. The change from fluidal to blocky peperite texture may also be controlled in part by the magma temperature and/or confining pressure, and the generation and maintenance of a steam film at the magma/wet sediment interface.