New molluscan assemblages from the upper member of the Twin River Formation, western Washington: Significance in Neogene chronostratigraphy

New molluscan assemblages of provincial early Miocene age from a previously unrecognized fossiliferous interval of 400 to 500 metres in the uppermost part of the Twin River Formation, western Washington, are referable to the upper part of the "Blakeley" [or Matlockian] Stage. The new biostratigraphic data provide, for the first time, definition of the uppermost part of the "Blakeley" Stage and the boundary between it and an unnamed stage of late early Miocene age represented by the Clallam Formation and its megafauna. In contradistinction to earlier studies, the upper boundary of the "Blakeley" Stage does not coincide with the upper boundary of the Zemorrian Stage of the foraminiferal chronology, the "Blakeley" boundary being at least 45 m higher and possibly much more. Nearly 60 species of mollusks from the upper few hundred metres of the Twin River Formation are recognized herein, and 40 of these are illustrated. Six species are previously undescribed.