Cretaceous sedimentary rocks, mostly marine

Cretaceous conglomerate, sandstone, shale, breccia, limestone, and gritstone in San Juan Islands.
State Washington
Name Cretaceous sedimentary rocks, mostly marine
Geologic age Late Cretaceous; possibly Eocene
Lithologic constituents
Sedimentary > Clastic > Sandstone (Bed)
Sedimentary > Clastic > Conglomerate (Bed)
Sedimentary > Clastic > Mudstone > Shale (Bed)
Sedimentary > Coal (Bed)
Comments Dragovich and others, 2002, maps the northernmost San Juan Islands (Patos, Sucia, and Matia Islands) in this unit as Eocene continental sedimentary rocks correlative with the Chuckanut Formation; Stuarts, Johns, Waldron, Flattop, Skipjack Islands, and the northern shore of Orcas Island consist of the Nanaimo Group of Cretaceous age. Both groups have similar lithology. Huntting and others, 1961, listed the Spieden group and Winthrop sandstone in this unit. The Spieden Formation mapped in the WAJK unit more recently (sole occurrance is on Spieden Island just north of San Juan Island) (Brandon, 1989). The Winthrop formation appears in northern Okanogan County (see unit WAKu), but not in this unit which is exclusive to the northern San Juan Islands.
Stratigraphic units Nanaimo Group

Huntting, M.T., Bennett, W.A.G., Livingston, V.E.Jr., and 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.

Johnson, S.Y., 1981, Stratigraphy, Sedimentology, and Paleogeographic Significance of Spieden Group, San Juan Islands, Washington: American Association of Petroleum Geologists (AAPG) Bulletin, v. 65, no. 5, p. 942.

Dragovich, J.D., Logan, R.L., Schasse, H.W., Walsh, T.J., Lingley, W.S., Jr., Norman, D.K., Gerstel, W.J., Lapen, T.J., Schuster, J.E., Meyers, K.D., 2002, Geologic Map of Washington-Northwest Quadrant: Washington Division of Geology and Earth Resources Geologic Map GM-50, scale 1:250,000.

NGMDB product
Counties San Juan