The distribution of marine limestones in the Oregon Tertiary is an important facet of the paleoenvironmental setting. Although the most extensive deposits of Tertiary limestones in the state are found in the middle Eocene Yamhill Formation in the coast range, several smaller exposures of late Eocene and Oligocene limestones occur in the Western Cascades east of Salem, Oregon.

All of these accumulations are of limited stratigraphic and areal extent. Much of the carbonate content is invertebrate shell material which accumulated in shallow-water environments during periods of limited terrigenous sedimentation. A significant amount of the limestones, however, reflects an offshore or open-ocean environment, such as a bank or seamount.

Porosity in the carbonates ranges from extremely low values, where sparry calcite has precipitated in pore spaces, to values of 5% where solution has extensively removed shell calcite.

A recently drilled well near Lebanon, Oregon, produced gas for a short time before being abandoned. That well was completed at 3,000 ft (914 m) near the top of the Spencer Formation at approximately the same stratigraphic level as Eugene carbonate-bank deposits in northern Marion and southern Clackamas Counties.

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