2002 Volume 53 Issue 9-10 Pages 645-655
Diatom biostratigraphy is revised for the Sugata Formation (Miocene) in the Nadaura area,Toyama Prefecture, central Japan. The Sugata Formation is mainly composed of diatomaceous mudstone, containing diatom fossils throughout the formation. Crucidenticula nicobarica Zone (NPD 5A),Denticulopsis praedimorpha Zone (NPD 5B) and Thalassiosira yabei Zone (NPD 5C) are identified with diatom biohorizons of Yanagisawa and Akiba (1998) including D 51, D 52, D 53 and D 55 in the Sugata Formation. C. nicobarica occurs 10 to 20 percent of total assemblage in most of the samples from the upper part of NPD 5A to the lower part of NPD 5B, and decreases rapidly to less than 1 percent between the biohorizon D51 (first occurrence of Denticulopsis praedimorpha var. minor; 12.9 Ma) and D 52 (last occurrence of C. nicobarica ; 12.7-12.8 Ma). The sediment accumulation rate curve of the Sugata Formation suggests that a hiatus exists between the diatomaceous mudstone and the glauconite Sandstone in the uppermost part of the Sugata Formation.
