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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 5. (May)

First Page: 558

Last Page: 558

Title: Progressive Stages of Diagenesis in Early-Middle Eocene Fore-Arc Delta, Shelf, Slope, and Basin Sandstones, Southern Coast Range, Oregon: ABSTRACT

Author(s): Marjorie A. Chan

Article Type: Meeting abstract

Abstract:

The Flournoy and Tyee formations (early to middle Eocene) of western Oregon are a model example of pervasive diagenesis in volcanic arenites, which extend over a full range of environments: delta, shelf, slope, and basin. Although younger Eocene units in Oregon prove to be good reservoir rocks, the Flournoy and Tyee lack porosity. This is due to fore-arc basin burial and subsequent mechanical and chemical diagenesis.

Evaluation of the diagenetic phases indicate deep burial and compaction at an early stage. Unstable volcanic rock fragments and plagioclase grains from all environments show alteration to mixed-layer clays and laumontite, making them more susceptible to mechanical plastic deformation.

Cementation and replacement were most common in coarse-grained deltaic and shelf sands, where the depositional porosity was high. An early stage of calcite cementation preserved open framework-supported textures in spherical concretions. A second stage of mixed-layer clays formed cement rims. Clinoptilolite filled remaining pore space. Locally, these two phases are reversed in Flournoy sands. Rare fractures were filled by stilbite. During a late stage, calcite replaced clay rims and zeolites, or filled remaining pores. In other samples, pervasive laumontite, together with minor clays, tightly cemented the coarse-grained sandstones.

Slope and basin sandstones are finer grained and contain more matrix than coarse-grained sandstones. Original composition, grain size, and original porosity (a function of depositional setting) controlled diagenetic development. These deeper water sandstones show some clay and calcite cements, but are generally lacking zeolite cements.

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