Abstract

Diagenesis in Eocene basinal lithic sandstones of the Oregon Coast Range is constrained by mineralogic composition and original depositional environment and various other chemical and physical factors, including fluid flow and temperature. Homogeneous volcanic and lithic sandstone compositions within the sequence of the Tyee, Elkton, and Coaledo Formations allow examination of diagenetic phases in relation to sedimentary environment. Authigenic minerals are controlled m part by depositional facies; well-developed clay and zeolite (laumontite and heulandite/clinoptilolite) cements are abundant in deltaic to shelf and inner-fan facies throughout the section. Carbonate cements occur in sandstones from many facies. Matrix is most common in sandstones of the outer-fan facies. Matrix-rich rocks had less original depositional porosity and permeability, hence zeolite and clay cements are not well developed in outer-fan (turbidite) sandstones. A paragenetic sequence of the diagenetic phases indicates that several cement generations and replacements reduced most of the depositional porosity. Minor dissolution of Ca-rich phases indicates sparse secondary porosity developments in these rocks. Statistical analyses support the distinction of a homogeneous petrofacies group and the correlations of diagenesis with facies.

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