ABSTRACT

Limestones, although rare in western Oregon, are important indicators of environmental conditions. The only significant bodies of unmetamorphosed limestone in the state are the Tertiary limestones of the northwestern area. Collectively, these are known as the Rickreall Limestone Member of the Yamhill Formation. These impure limestones form lenses that grade both laterally and vertically into fossiliferous sandstone and siltstone of the Yamhill Formation of middle and late Eocene age.

The Rickreall limestones are bioclastic, and consist of microfossils, rounded fragments of larger fossils, rounded fragments of basic volcanic rock, and minor amounts of lime mud (micrite). Foraminifers are the consistently most abundant fossils in the rocks; more than 50 species of benthonic foraminifers plus a few species of planktonic foraminifers were recovered from the limestones and interbedded muddy siltstones. Species of the hyaline Foraminifera Amphistegina are particularly common in both limestone and siltstone. In addition to the foraminifers, rounded fragments of calcareous algae and echinoderms (mainly echinoids) are common in all the limestones, and in some rocks exceed foraminifers in abundance. Fragments of other fossils, including bryozoa and pelecypods, genera ly occur in less abundance.

The low diversity of arenaceous foraminifers and paucity of planktonic foraminifers suggest deposition nearshore in shallow water. The prevalence of Amphistegina, which are generally restricted in modern environments to water depths less than a few meters, and the abundance of calcareous algae in all samples is also consistent with deposition in very shallow water. The rounding of the fossil fragments and associated volcanic rock fragments, together with their generally well sorted character, further suggests deposition under moderately high energy conditions, such as might be encountered on a submerged, but shallow, offshore bank.

The episodic accumulation of Eocene limestones and their intimate association with volcanic materials may imply some relationship with volcanic activity. It is suggested here that accumulation of these lenses of fossil detritus was controlled by the combined effects of increased productivity of CaCO₃ secreting organisms, possibly due to increased concentrations of calcium bicarbonate related to submarine weathering of volcanic materials, and to the inhibiting effects on terrigeneous sedimentation imposed by the presence of topographically high bank areas.