Diatom diostratigraphy is established for themiddle Miocene Hatatate Formation exposed along theIwanosawa section, Sendai City, Miyagi Prefecture, northeasternJapan. The formation is composed mainly of muddy fine-grained sandstone intercalating a number of thin tuff beds. Thelower Hatatate Formation above the tuff bed Ht 10 is assigned tothe Denticulopsis praedimorpha Zone (NPD 5B) of Akiba(1986), and contains two diatom biohorizins, D52.5(the firstacme of Denticulopsis crassa )and D53(the first occurrence ofD.praedimorpha var.praedimorpha). The rapid decrease ofradiolarian Cyrtocapsella tetrapera lies just below the diatombiohorizon D53. Remarkable diachronism across latitude issuggested for the last occurrence of calcareous nannofossilCyclicargolithus floridanus which lies between the diatombiohorizone D52.5 and D53 in the Hatatate Formation. Theglauconite bed in the lowest Hatatate Formation is characterizedby a very slow sedimentation rate and was deposited during theinterval from 15 to 13 Ma.
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