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The Deadman Bay Volcanics is a Jurassic accretionary complex in the Deadman Bay terrane and is distributed on the west side of the San Juan Island, Washington State, USA. The rocks of the Deadman Bay Volcanics are grouped into three major coexistent stratigraphic units: the Permian Shallow Cmarine Limestone unit, the Permian Volcanics unit, and the Permian to Jurassic Chert unit.
The Permian Shallow-marine Limestone unit consists of Permian fossiliferous limestone and is accompanied by basaltic rocks at the bottom. The Permian Volcanics unit comprises basaltic volcaniclastic rocks, basalt lava, and Permian limestone-basalt breccia. The Chert unit is composed of Permian to Early Jurassic bedded chert intercalating clastic rocks containing grains of Permian shallow-marine limestone, basalt, and Permian to Triassic chert. These units are reconstructed as sediments on the top, the upper slope, and the lower flank of a Permian basaltic seamount.
The limestone-basalt breccia of the Permian Volcanics unit is composed of unsorted, angular lithoclasts of shallow-marine limestone and basalt, which are chaotically embedded in a basaltic volcaniclastic mud matrix. The limestone clasts yield Middle Permian fusulines. It is stressed that no coarse terrigenous clastic grains are contaminated in the Permian rocks of the unit.
The clastic rocks of the Chert unit include chert-sandstone, chert-conglomerate, and volcaniclastic rocks containing variously-sized blocks of Permian shallow-marine limestone and Triassic chert. The blocks are chaotically embedded in and supported by the surrounding matrix of unsorted basalt fragments. On the other hand, the chert-sandstone and the chert-conglomerate are graded sediments composed of a siliceous mud matrix and angular clasts of Triassic radiolarian chert and siliceous shale, basalt, and Permian shallow-marine limestone yielding Permian fusulines. The matrix contains a little amount of detrital quartz grains.
These clastic rocks of the Permian Volcanics and the Chert units can be best explained by gravity-flows caused by the collapse of a seamount. In the Permian time, the clasts of shallow-marine limestone and basalt were emplaced by gravity-flows related to the collapse of a seamount from the top to the upper slope of the seamount in a pelagic realm. In the Triassic time, the seamount was approaching toward a continent and reached a hemi-pelagic realm. Then larger collapse occurred and the collapse products were transported down onto the foot of the seamount by debris flows and turbidity currents, where chert had accumulated.
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