The Montesano Formation, which ranges in age from late Miocene to questionable early Pliocene, is exposed in an area of approximately 250 square miles of Grays Harbor County in western Washington. It averages 2,500 feet in thickness and consists predominantly of fine- to medium-grained sandstone, with mudstone, pebbly sandstone, and conglomerate locally significant. The formation possibly represents the last marine incursion in a depositional basin that existed generally as a strongly negative feature through most of the Tertiary.
Paleoenvironmentally significant faunas from the Montesano Formation include: (1) rock-boring pelecypods, (2) Chione-Spisula molluscan assemblages, (3) a Miliammina fusca fauna, (4) a Buliminella elegantissima fauna, (5) a Nonionella fauna, (6) a Bolivina fauna, (7) a Uvigerina peregrina hispidocostata fauna, and (8) a Bolivina seminuda fauna. The succession of these assemblages, the associated quantitative microfaunal trends, and the sedimentary evidence indicate that the formation was deposited in a sea that first transgressed from west to east over Grays Harbor basin and then regressed. In the western part of the basin, water depths increased progressively from zero to more than 3,000 feet. On the east, deposition took place initially in the littoral zone, later the outer shelf, and finally under probable tidal sand-flat conditions. A local laminated mudstone unit contains an impoverished fauna suggestive of a partly closed basin about 2,000 feet deep with a sill at about 800 feet. Graded bedding, convolute structures, channels filled with shallow-water deposits, and a high percentage of displaced fauna indicate that much of the sediment was emplaced by turbidity currents and slumping. Planktonic Foraminifera indicate that late Miocene sea-surface temperatures in the Grays Harbor area were of the order of 10-15°C. A small terrestrial flora reflects a mild temperate climate.