Shrimp U-Pb Age Of The Radiolarian-Bearing Yoshiki Formation In Japan

The dating of radiolarian biostratigraphic zones from the Silurian to Devonian is only partially understood. Dating the zircons in radiolarian-bearing tuffaceous rocks has enabled us to ascribe practical ages to the radiolarian zones. To extend knowledge in this area, radiometric dating of magmatic zircons within the radiolarian-bearing Yoshiki Formation, Japan, was undertaken. The Yoshiki Formation is mainly composed of alternating beds of tuffaceous sandstones, tuffaceous mudstones, and felsic tuff. The felsic tuff and tuffaceous mudstone yield well-preserved radiolarian fossils. Radiolarians and zircon grains were collected from 21 tuffaceous mudstone and 30 tuff horizons of the Yoshiki Formation. The following radiolarian species which can be ascribed to the Zadrappolus yoshikiensis-Futobari solidus assemblage were identified: Oriundogutta(?) kingi, Oriundogutta(?) varispina, Zadrappolus yoshikiensis, Zadrappolus tenuis, Zadrappolus hitoeganensis, Zadrappolus lunaris, Zadrappolus(?) nudus, Zadrappolus(?) sp., Futobari solidus, Futobari morishitai, Rotasphaera(?) sp., and Ceratoikiscum armiger. U-Pb SHRIMP ages of 420.5 +/- 2.5 Ma and 421.0 +/- 1.9 Ma were obtained from the zircon grains. Thus the lower limit of the Zadrappolus yoshikiensis-Futobari solidus assemblage, that has previously been assigned to an age from Pridolian to Pragian, is in reality before Ludlowian. The technique in this paper has the capability of refining and making more accurate the dating of many radiolarian zones worldwide, and potentially of changing the direction of the entire study of radiolarian biostratigraphy.