title: The Depositional Environment, Petrography, and Tectonic Implications of Informally Named Middle to Late Eocene Marine Strata, Western Olympic Peninsula, Washington
Creators: name: Adams, Benjamin Nickolas
Publisher: name: Western Washington University
Year: 1988

description: <p>The informally named marine strata examined in this study comprise fault bounded slivers of middle to late Eocene (Ulatizian to Narizian) siltstone, sandstone, and conglomerate exposed on the northwestern Olympic Peninsula. The strata are divided into three lithofacies: the sandstone of Bahobohosh, the siltstone of Waatch Point and the siltstone and sandstone of Bear Creek by Snavley et al. (1986). Six facies have been identified consisting of strata deposited by high- and low-density turbidites, storm waves, slumping, tidal or littoral currents, and debris flows. Relationships among the facies indicate shallow marine deposition that shoaled from below to above storm wave-base on the outer shelf.</p>
<p>The sandstone and conglomerate of the study unit consist mainly of predominantly angular to sub-angular, moderately to very poorly sorted lithic arenites. Quartz and plagioclase are the most common monocrystalline grains with potassium feldspar and epidote present insignificant quantities. Lithic grains comprise 35% of all grains. The predominant types are polycrystalline quartz (including chert) followed by intrabasinal sedimentary clasts and basaltic volcanic lithics. A diverse source area consisting of dioritic, granodioritic, and granitic plutonic rocks or gneisses, basaltic volcanics, chert, and greenschist-grade metasedimentary and metavolcanic rocks is indicated.</p>
<p>The study unit most likely had a Vancouver Island source. Erosion of the Sicker, Vancouver, and Bonanza Groups and of the Island Intrusions and West Coast Complex, which together comprise Wrangellia on Vancouver Island, could generate the sediments of the study unit. The western position of the study unit argues against sources further east that would have required large amounts of littoral transport to reach the depositional site. The probable Vancouver Island source and the shallow marine facies of the study unit imply that it formed as a sequence onlapping Vancouver Island. This probable onlap, in conjunction with the petrologic similarities between the study unit and the western portion of the coeval Aldwell Formation, that indicate a possible coextensive relationship, have implications regarding the Paleogene tectonics of the Pacific Northwest, particularly the setting of the Crescent terrane.</p>
<p>The Paleogene was a tectonically dynamic period in the northeast Pacific Basin. Subduction rates of the Kula and Farallon plates beneath North America were very high (> 100 km/my on the average), and major reorganizations of the Kula-Farallon ridge were taking place. The basaltic basement of the Crescent terrane, which the Aldwell Formation overlies, was generated during the early Eocene in a setting that may have been allochthonous (accreted oceanic plate or oceanic island chain) or autochthonous to North America (continental-margin-rift). If the study unit developed as a sequence onlapping Vancouver Island that was coextensive with the western portion of the Aldwell Formation, the Crescent terrane basalts must have been proximal to Vancouver Island during the middle to late Eocene. Therefore, an allochthonous origin for the Crescent terrane is unlikely.</p>
<p>Coeval shoaling sequences along western North America suggest eustatic control of shoaling in the study unit. However, the shoaling could have resulted from the uplift of the southern edge of Vancouver Island due to the underthrusting of the Crescent terrane or from the approach of the Kula-Farallon ridge to the Pacific Northwest during the Paleogene. Immature sediments and rapid sedimentation rates on the shelf could have resulted from either process.</p>