COLLAPSE DEPOSITS OF THE PERMIAN SEAMOUNT ONTO A DEEP-SEA FLOOR IN THE DEADMAN BAY VOLCANICS, THE SAN JUAN ISLAND, NORTH WASHINGTON

The Deadman Bay Volcanics is a stratigraphic unit in the Deadman Bay terrane and is distributed on the west side of the San Juan Island, north Washington. It is composed of accreted oceanic rocks including basaltic rocks, chert, and Permian limestone yielding Thethyan fusulines. The stratigraphic reconstruction reveals that the oceanic rocks are divided into the three lithologic successions: the Permian shallow-marine limestone succession, the Permian volcaniclastic rocks succession, and the gravity-flow deposits succession.

The shallow-marine limestone succession consists of Permian fossiliferous limestone and is accompanied by basaltic rocks at the bottom. The volcaniclastic rocks succession comprises basaltic volcaniclastic rocks, basalt lava, and Permian limestone-conglomerate. It is stressed that no coarse terrigenous clastic grains are contaminated in the two successions. The oceanic rocks of the successions are reconstructed as Permian sediments on the top and the upper slope of a basaltic seamount in an open ocean realm.

The gravity-flow deposits succession comprises chert-sandstone, chert-breccia, and volcaniclastic rocks in which variously-sized blocks of Permian shallow-marine limestone and chert are chaotically embedded. The blocks are supported by the surrounding fine volcaniclastic rocks. The chert-sandstone and the chert-breccia are composed of a siliceous mud matrix and angular clasts of the chert, siliceous shale, basalt, and Permian shallow-marine limestone. The matrix contains a little amount of detrital quartz grains. The succession can be best explained by a collapse of a seamount. It is possible that the Permian seamount approaching a continent from an open ocean realm was collapsed and the collapse products were redeposited onto a deep-sea floor where chert and siliceous shale have accumulated.