GEOLOGIC MAPPING OF THE SEQUIM, CARLSBORG AND MORSE CREEK 7.5-MINUTE QUADRANGLES, NORTHEAST OLYMPIC PENINSULA, WASHINGTON STATE

Bedrock in the Sequim, Carlsborg, and Morse Creek quadrangles consists of lower to middle Eocene oceanic basaltic rocks interbedded with Paleocene(?) to Eocene continental sediments overlain by middle Eocene to lower Miocene, faulted and folded, fossiliferous marine sedimentary rocks. Faults and folds trend and gently plunge eastward reflecting a post-early Miocene tectonic event of unknown duration. Local cross faulting appears to overprint the folds. Quaternary units at the surface are dominated by glaciomarine drift, recessional outwash, till, and advance outwash deposits of the Vashon Stade of the Fraser Glaciation. The Quaternary section is thickest along the Strait of Juan de Fuca, thinning to the south. Pre-Fraser deposits exposed on coastal bluffs represent possibly two nonglacial intervals (Olympia and Whidbey(?)) and one glacial (Possession?) period. Holocene fluvial and mass-wasting processes have locally modified bedrock and Pleistocene deposits. New data and conclusions from our mapping include:

· Delineation of bedrock exposures formerly mapped as “till over bedrock” (Othberg and Palmer, 1979) and undivided glacial drift (Brown and others, 1960; Tabor and Cady, 1978).

· New foraminiferal data for the previously undivided Tertiary Twin River Group allows assignment to the Miocene-Oligocene Pysht, Oligocene-Eocene Makah, and upper Eocene Hoko River Formations.

· New geochemical analyses enhance the database for the basaltic and rare rhyolitic rocks of the Eocene Crescent Fm.

· Recognition of a previously unmapped high-angle fault SE of Bell Hill, Sequim quadrangle.

· Seven new ¹⁴C age dates, combined with mapped stratigraphy, suggest that pre-Fraser nonglacial sediments belong to the Olympia nonglacial interval (northern Morse Creek and Carlsborg quadrangles). Another ¹⁴C date establishes the age of peat deposits nested in Vashon glacial drift at the surface (Morse Creek quadrangle).

· Possible subglacial flow and/or jökulhlaup deposits are identified in Fraser and/or pre-Fraser glacial sediments in the northern Morse Creek quadrangle.

· Mazama tephra (6,730±40 ¹⁴C yrs B.P.) interbedded with fluvial deposits in McDonald Creek suggests post-glacial fluvial incision of northward-draining streams after ~ 6,700 ¹⁴C yrs B.P.