Tuffaceous claystone with anomalously well-preserved radiolarian tests of Berriasian age was collected from the oceanward slope of the Mariana trench using the Japanese submersible Shinkai 6500. The rock is porous and light, slightly calcareous, and has experienced neither significant compaction nor deformation. Younger Cretaceous cherts are composed of recrystallized opal-CT and quartz silica phases, in which tests were destroyed, dissolved, and deformed. However, the oldest claystone still contains radiolarian tests of biogenic opal-A and dehydrated opal-A (opal-A'), showing slight dissolution. This anomalous preservation of reactive biosilica required isolation from ordinary chemical and physical diagenetic conditions since deposition. The unusually weak diagenesis might occur from either avoidance of deep burial or unique chemical and physical microenvironments controlled by the clay-rich composition. The latter would include the effect of high pore-fluid pressure due to low permeability, which may favor the anomalous preservation found in this rock.