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Petrology and Diagenetic Effects of Lithic Sandstones: Paleocene and Eocene Umpqua Formation, Southwest Oregon

By
Lary K. Burns
Lary K. Burns
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Frank G. Ethridge
Frank G. Ethridge
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Published:
January 01, 1979

Abstract

The Eocene and Paleocene Umpqua Formation in the southern part of the Oregon Coast Range comprises a thick sequence of lithic arenites, siltstones, mudstones, conglomerates, coals, and, in the basal part, basaltic volcanic rocks intercalated with detrital sedimentary rocks. The sediments were deposited in a basin that developed during the evolution of the Mesozoic-Cenozoic arc-trench system of western North America. Environments ranging from deltaic to moderately deep marine are reflected in the rocks.

Lithic arenites, the dominant sandstone type, have framework constituents of quartz, feldspars, micas, microfossils, plant fragments, heavy minerals, and volcanic, metamorphic and sedimentary lithic fragments. Digenetic minerals include phyllosilicates (chlorite and clay minerals), calcite, iron oxides, quartz, and zeolites.

PhyQosilicate cements occur in three varieties: clay coats on framework grains, pore-filling chlorite with a radiating habit, and chlorite as unoriented microcrystalline aggregates. The radiating chlorite is found only in the upper and middle members of the formation, and the zeolite is found in only the lower member. Sandstone porosity has been reduced by the cements and by compaction and mechanical deformation of soft grains.

A progressive sequence of diagenetic features from youngest to oldest evident in the upper and middle members is: (1) calcite pore-fill cement and the development of clay coats around framework grains, (2) precipitation of radiating pore-fill chlorite, or alteration of volcanic fragments to form unoriented microcrystalline aggregates of phyllosilicates, and (3) precipitation of silica cement in the center of pores not already completely filled. In the lower member, pore-space was not present for precipitation of the radiating chlorite. Zeolites occur in the lower member, indicating that low-grade metamorphic conditions were attained.

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Contents

SEPM Special Publication

Aspects of Diagenesis

Peter A. Scholle
Peter A. Scholle
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Paul R. Schluger
Paul R. Schluger
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SEPM Society for Sedimentary Geology
Volume
26
ISBN electronic:
9781565761568
Publication date:
January 01, 1979

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