Abstract

Sedimentary basins characterized by continental to marine shelf depositional regimes are common adjuncts to active lithospheric plate junctions of the northeast Pacific area. Most of these basins are elongate troughs and may form in both fore-arc and back-arc positions.

Andesitic composition of volcanic and plutonic rocks that form the major source lithologies in active or recently active arcs produces petrologically distinctive sands that are dominated compositionally by volcanic rock fragments, plagioclase feldspar, and mafic heavy minerals. Quartz content of the sands is low and rarely exceeds 50 percent. Such sands are mineralogically unstable and react upon shallow to moderate burial and consequent increase in temperature and pressure to produce a recurrent sequence of authigenic cements. Three progressive stages of diagenesis are differentiated in sample suites from the Bristol, Queen Charlotte, Grays Harbor, and Chehalis basins: stage 1, early diagenetic calcite pore-filling cement; stage 2, authigenic clay rims and coats around detrital grains; stage 3, authigenic phyllosilicate and (or) laumontite pore-filling cement.

Development of silicate overgrowths with chlorite and calcite replacement of rock fragments and plagioclase becomes more pronounced as burial is further increased.

As the depth of burial increases, higher temperature, overburden pressure, and fluid pressure result. Of these three factors, geologic evidence favors temperature as the primary control of burial diagenetic reactions. Petrographic data support the concept of a diagenetic origin for graywacke matrix. The second and third diagenetic stages recognized here result in the authigenesis of clay minerals and micas and can reasonably be considered precursors of a graywacke lithology.

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