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The AAPG/Datapages Combined Publications Database
Segments of the petroleum industry are actively exploring the Tertiary rocks of offshore Oregon and Washington; several wells are being drilled and others are projected. It is noteworthy (1) that amphibole and pyroxene occur in sandstone beds of one of the potential reservoir formations under conditions that require an authigenic origin and (2) that the rocks have not been metamorphosed. These minerals, which usually are presumed to have formed in conditions of much higher temperatures than those of diagenesis, are found in the Spencer Formation of late Eocene age at Corvallis, Oregon. 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 f agments are replaced by thomsonite which contains many idiomorphic crystals and tangled needles of actinolite and clinopyroxene.
Reconstruction of the diagenetic environment suggests a formation temperature near 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 have been caused by a supply of basic volcanic glass and molluscan calcite rather than acid glass and soda-rich brines.
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