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Abstract: Abstract: Petrology and Diagenetic Effects of Graywacke Sandstones--Eocene Umpqua Formation, Southwest Oregon

Lary K. Burns, Frank G. Ethridge

The Umpqua Formation (Group of Baldwin) of early and middle Eocene age crops out in the southern Oregon Coast Ranges. Detrital sediments of the Umpqua Formation were deposited in either a fore-arc or an interarc basin that developed during the evolution of the western North American Mesozoic-Cenozoic arc-trench system. Detrital sedimentary rocks in the lower part of the formation probably were deposited as deep-water turbidites. Sedimentary rocks and interbedded coals in the middle and upper parts indicate deposition under shallow subtidal-shelf to deltaic conditions.

Two sections of the Umpqua constituting 3,475 and 2,725 m, respectively, were measured and sampled for detailed petrographic and textural examination. Sandstone and coarse-siltstone samples are classified as lithic arenites and lithic wackes. Common framework constituents of these samples include monocrystalline and polycrystalline quartz, oligoclase to andesine feldspars, muscovite and biotite, microfossils, plant fragments, heavy minerals (epidote, chlorite, garnet, hornblende, olivine, and zircon) and volcanic, metamorphic, and sedimentary lithic fragments. In many sandstone samples, lithic fragments constitute the majority of the framework grains. Intermediate to mafic volcanic-glass fragments with vesicules are abundant in some samples. These fragments appear to be especially rea tive and are commonly devitrified to chlorite and/or clay minerals. Detrital matrix is common only in silt-size samples.

Diagenetic minerals include phyllosilicates such as chlorite and clay minerals, calcite, and less commonly silica. Phyllosilicate cements occur in two varieties--as clay coats on detrital framework grains or as chlorite with a radiating fibrous habit filling pores. Despite the fact that most samples examined were buried to depths sufficient to produce temperatures and overburden pressures high enough to develop well-crystallized phyllosilicate cements, only in certain sandstone samples were these phyllosilicate cements developed. Conditions necessary for the development of these cements, in addition to depth of burial, temperature, and overburden pressure, include a source for the cementing clay minerals and favorable conditions for pore-fluid movement. The source materials are the in ermediate to mafic volcanic-rock fragments. Pore channels were well developed in those sandstone samples with phyllosilicate cements; however, several rock types apparently lacked pore channels. The low permeability in these sandstones is due to (1) the presence of soft lithic fragments that, under compaction, have filled pores to form a pseudomatrix; (2) precipitation of early calcite cement in void spaces; and (3) poor sorting and/or fine-grained framework constituents.

Origin of the two distinct varieties of phyllosilicate cements may be somewhat similar to that described for red beds by Walker. The clay coats are formed by early mechanical infiltration of clay-rich waters into the permeable sediments. The resulting texture is one of discrete platelets oriented parallel with grain surfaces. The authigenic chlorites are precipitated from pore solutions after burial and compaction of the rocks. They are easily distinguished, in thin section, from the infiltrated clays, by their fibrous radiating habit.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado