First Advisor

Robert O. Van Atta

Date of Publication


Document Type


Degree Name

Master of Science (M.S.) in Geology


Earth Science




Stratigraphy, Sedimentology, Oregon, Sedimentary Rocks



Physical Description

1 online resource (117 p.)


The Spencer Formation in Yamhill and Washington Counties, Oregon, is exposed in a narrow belt 27 km long, from 1/4 to 3 km wide and with a maximum thickness of about 400 m. The formation is composed entirely of sandstone with interbedded thin layers of mudstone in the uppermost member. The sedimentary structure and paleoecology indicate a shallow marine depositional environment. The upper member of the Spencer Formation contains more quartz, plagioclase, and hornblende than does the lower member, but K-feldspar is less than that of the lower member. Shallower water conditions for the deposition of the upper member are indicated by sedimentary structures and the abundance of pebbly lenses and coaly material. Eighteen species of megafossils collected from the formation indicate that the Spencer Formation is of the Tejon stage (late Eocene of the West Coast).


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Plates 1 & 2 are available for download in the Additional Files below.

Persistent Identifier

al-azzaby1.pdf (5670 kB)
Plate 1. Geologic Map of Portions of Central North Yamhill County and Southern Washington County, Oregon

al-azzaby2.pdf (3148 kB)
Plate 2. Geologic Sections (a) and (b)