The late Oligocene Yaquina Formation is exposed in an arcuate outcrop pattern which extends along the central Oregon Coast from
Siletz Bay on the north to Seal Rock on the south. Maximum inland extent of the Yaquina outcrop is :five miles. Maximum thickness of the Yaquina Formation is 2, 000 feet although thicknesses are highly variable along strike. Three distinct deltaic units are recognized: a lower marine member is 410 feet thick at Yaquina Bay; a 172 feet thick non-marine member erosionally overlies the lower marine member; an upper marine member gradationally
overlies the non-marine member and is 817 feet thick. The upper and lower marine members are composed of medium to fine-grained, arkosic to volcanic arenites and wackes as well as glauconitic and tuffaceous sandstones and siltstones. Grain size, sorting, roundness of grains, sedimentary structures, and fossils suggest that the sandstones were probably deposited in moderate to high energy shallow marine environments; the siltstones were deposited
in lower energy conditions such as found in bay or lagoonal environments. The non-marine member contains pebbly volcanic arenites and wackes, carbonaceous mudstones, and tuffaceous siltstones which apparently were deposited in.fluvial and swamp environments.
The sedimentary petrography of the sandstones and conglomerate § indicate four source areas for the Yaquina strata: (1) pumice and acidic to andesitic volcanics from the western Cascades, (2) basalt from Eocene flows in the Coast Range, (3) metamorphic and plutonic
minerals and rock fragments from the Klamath Mountains, and (4) recycled minerals and rock fragments from the early Tertiary
sedimentary strata of the Coast Range.
Paleocurrent measurements made on trough and planar cross-bedding and channel-fills indicate a predominant westward sediment
dispersal pattern which is in agreement with the proposed provenances. Paleontological and sedimentological evidence suggest that the
three members of the Yaquina were deposited in a deltaic setting. The lower marine member represents a marine regression. The
non-marine member represents a progradation of a deltaic coastal plain. The upper marine member is a marine transgressive sequence
over the deltaic plain. Lateral fades changes in the non-marine member indicate shallow marine conditions prevailed along the margins of the Yaquina delta. Slow subsidence and rapid sedimentation of freshly erupted
volcanic detritus derived from the western Cascades are thought to have been the main controlling factors in the progradation of the Yaquina delta. Increased subsidence, slower sedimentation rates, and/or uplift of the central Coast Range in latest Oligocene and Miocene time cut off the source of sediments and resulted in a marine transgression of the Newport area.
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