Skip to main content

Devonian rocks

Thin alternating beds of cherty quartzite and argillite, occasional limestone lenses, volcanic rocks, sandstone, and graywacke on northwest shore of Orcas Island. Small outcrops of Devonian limestone included under Pzu in southeastern Stevens County and under PMPms in northwestern Whatcom County.
State Washington
Name Devonian rocks
Geologic age Devonian to Permian
Comments Brandon and others, 1988, maps this as the East Sound Group (see also WAPM), which is primarily andesitic to dacitic pyroclastic rocks, with rare lava flows, as well as minor limestone; he specifically distinguishes this rock group from the Deadman Bay Terrane which includes chert as well as basalt and limestone. May include incidental Turtleback Complex (see WApPMg and WApTm).
Primary rock type andesite
Secondary rock type dacite
Other rock types limestone
Lithologic constituents
Major
Igneous > Volcanic > Felsic-volcanic > Dacite (Pyroclastic)
Igneous > Volcanic > Mafic-volcanic > Andesite (Pyroclastic)
Minor
Sedimentary > Carbonate > Limestone (Bed)
Incidental
Igneous > Volcanic > Mafic-volcanic > Andesite (Flow)
Igneous > Volcanic > Felsic-volcanic > Dacite (Flow)
Map references
Huntting, M. T., Bennett, W. A. G., Livingston, V. E. Jr., Moen, W. S., 1961, Geologic Map of Washington: Washington Division of Mines and Geology, scale 1:500,000.
Unit references
Huntting, M. T., Bennett, W. A. G., Livingston, V. E. Jr., Moen, W. S., 1961, Geologic Map of Washington: Washington Division of Mines and Geology, scale 1:500,000.
Brandon, M. T., 1989, Geology of the San Juan-Cascade Nappes, northwestern Cascade Range and San Juan Islands, in Joseph, N. L. (Ed.), Geologic Guidebook for Washington and Adjacent Areas: prepared for the 85th Annual Meeting of the Cordilleran Section, Geological Society of America, May 8-11, 1989, Spokane, Washington: Washington Division of Geology and Earth Resource Infomation Circular 86, p. 137-162.
Brandon, M. T., Cowan, D. S., Vance, J. A., 1988, The Late Cretaceous San Juan thrust system, San Juan Islands, Washington: Geological Society of America Special Paper 221, 81 p. 1 pl.
Counties San Juan