--> ABSTRACT: Diagenesis of Sediments Under Thick Volcanic Strata, Ochoco Basin, Oregon, by Neil S. Summer and Kenneth L. Verosub; #91038 (2010)

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Diagenesis of Sediments Under Thick Volcanic Strata, Ochoco Basin, Oregon

Neil S. Summer, Kenneth L. Verosub

The subsurface Ochoco basin of Oregon contains a thick sequence of fine-grained Cretaceous and older sediments overlain by a thick accumulation of Tertiary volcanics. The unusual diagenetic history of the sediments began under the high regional heat-flow conditions of an active back-arc setting but reached a peak prior to the extrusion of the Columbia River basalts. The diagenetic history was reconstructed on the basis of an SEM study which indicated that the sediments were transformed to low-grade metamorphic rocks equivalent to the zeolite facies under the elevated temperatures and pressures of a fossil geothermal system. The presence of authigenic minerals formed as anhedral replacements implies a local redistribution of the released ionic species in a closed system. A later fracturation event opened the system to fluids that are still selectively leaching the potassium feldspars, resulting in a remarkable microporosity. This microporosity and the fractures which are partly to completely filled with iron-rich chlorite support the observation based on a sonic well log that porosities are present at depth in the Ochoco basin. No zeolites are present in the entire sequence, even though they are a common constituent of altered volcanogenic sediments elsewhere in the Pacific Northwest. This may be interpreted in two ways: first, zeolites were indeed formed but ongoing diagenesis has altered them; and second, they were never formed at all due to a siliceous-rich detritus. The diagenetic mineral assemblage consisting of sphene, chlorite, illite, and quartz corresponds to temperatures of about 170° to 190°C and is near constant throughout the 5,000 ft of sediments, implying an isothermal temperature field within the sedimentary sequence. Isothermal temperature fields are a common feature of geothermal systems in the Pacific Northwest and result from the interaction of the heat flow from a perched geothermal aquifer flowing in an overlying porous strata with the steady-state geothermal gradient.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.