Authigenic pyroxene and amphibole in sedimentary rock do not necessarily imply that the rock has undergone metamorphism. These minerals, which generally are presumed to represent much higher temperatures than those of diagenesis, are found in the Spencer Formation of late Eocene age at Corvallis, Oregon, under conditions that dictate an authigenic origin. The best-preserved examples of these minerals are found in a graded sedimentation unit rich in molluscan fossil fragments, basic volcanic glass, and zeolitic concretions. The marine shell fragments are replaced by thomsonite which contains many idiomorphic crystals and tangled needles of actinolite and clinopyroxene.

Reconstruction of the diagenetic environment indicates a temperature of little more than 140°F., solutions somewhat less saline than sea water, a pH of slightly less than 7.0, and an Eh near -0.2.

Formation of thomsonite, actinolite, clinopyroxene, and rare analcime, rather than a suite such as clinoptilolite or heulandite, orthoclase, and abundant analcime, is thought to be caused by a supply of basic volcanic glass and molluscan calcite rather than acid glass and soda-rich brine.