SCANNING ELECTRON MICROSCOPE PETROGROGRAPHY OF TURBIDITES FROM THE CRESCENT VOLCANIC SEQUENCE (EOCENE) OF WESTERN WASHINGTON STATE

This paper describes BSE-imaging and microbeam analysis of sedimentary rocks from a section of turbidites which occur within the lower part of the Crescent formation in the southeastern portion of the Olympic Peninsula. This section comprises approximately 180 meters of red basaltic sandstones with individual beds averaging 4 cm in thickness. The section is bounded above and below by vesicular pillow basalts. The typical rock is a medium to fine grained litharenite primarily made up of angular basaltic fragments, plagioclase (An 50-75), pyroxene, Fe/Ti oxides, and minor amounts of calcite (shell fragments), albite, and anorthite. Some of the basalt fragments show quench textures (plumose pyroxenes). No quartz, mica, K-spar or any other continentally derived minerals were detected. Porosity is less than 5%. The rocks appear to have undergone a considerable amount of diagenesis as evidenced by the very low porosity, sutured grain contacts, and the occurrence of anorthite, albite (albitization), and skeletal and cruciform oxides. We conclude that this fan sequence was probably formed at moderate depths and isolated from continental sediment sources. This intrabasinal system exhibits properties that are distinctly different from other fan sequences that have been studied throughout much of the Olympic Peninsula. Other unmapped fan sequences of similar character have been observed in proximity to this section and these may help to define a stratigraphic boundary between the upper and lower Crescent in the near future.