Pillow Breccias and Their Aquagene Tuffs, Quadra Island, British Columbia

Pillow breccias consist of whole or disaggregated volcanic pillows in an abundant matrix of cogenetic basic tuff. They form in water and are thought to be common rocks, confused at times with other volcanic breccias. On Quadra Island, British Columbia, the breccias usually overlie ordinary pillow lava which rests in turn upon basalt flows or upon pelagic limestone. In the transition to isolated-pillow breccia the matrix increases gradationally upward to as much as 80 per cent; the pillows become isolated, diversiform, and ill-sorted. Some pillows were broken after settling but before compaction. The matrix, consisting of cogenetic globules, granules, shards, and basaltic fragments produced beneath water, is an aquagene tuff. Subaqueous shard formation is discussed. Broken-pillow breccia forming the top of the successions consists of unsorted pillow fragments set in similar aquagene tuff which is irregularly flow-laminated and somewhat resembles an ignim-brite. Laminated aquagene tuff beds with load casts in their upper surfaces occur within broken-pillow breccia. Ordinary pillow lava accumulated in relatively clear water, isolated pillows in increasingly turbid, vapor-charged water. Increasing thicknesses of incoherent, steam-laden pillows and tuff led to instability, subaqueous slumping, and fragmentation.