Petrology and Diagenetic Effects of Lithic Sandstones: Paleocene and Eocene Umpqua Formation, Southwest Oregon
Lary K. Burns, Frank G. Ethridge, 1979. "Petrology and Diagenetic Effects of Lithic Sandstones: Paleocene and Eocene Umpqua Formation, Southwest Oregon", Aspects of Diagenesis, Peter A. Scholle, Paul R. Schluger
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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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Aspects of Diagenesis
There are a number of gaping holes in accumulated knowledge within the discipline of sedimentology. Perhaps one of the largest holes has been the general subject of diagenesis in clastic rocks. It was therefore fortuitous that two symposia covering various aspects of diagenesis (mainly in clastics) were presented a year apart in different parts of the country but with the same motivation – to contribute to the closing of that knowledge gap. Sedimentologists now have a fairly good idea of the what and the how of sediment deposition. What happens after the sediments are lithified has frequently been ignored. It was the aim of both editors of this publication to approach the subject from two different viewpoints. Schluger directed a symposium which looked mainly at clastic reservoirs, and Scholle presented a symposium which examined various aspects of paleotemperature control of diagenesis.