Sedimentary Petrology, Depositional Environment and Paleogeographic Significance of the Upper Eocene Hoko River Formation, Northern Olympic Peninsula, Washington
The upper Eocene Hoko River Formation is the oldest formation of the Twin River Group. Exposures dipping to the northeast and north run across the northern Olympic Peninsula. The Hoko River Formation is composed of siltstone, turbidites, channel conglomerates and debris flows. The sandstones are lithic-arenites and -graywackes and are composed of fine to very coarse, moderately well sorted to poorly sorted sand. The dominant cement is calcite with minor polycrystalline quartz and zeolite cements. The average composition of the sandstones is quartz (Q) = 29%+14, feldspar (F) = 15%+9, and lithics (L) = 55%+12. Comparison of these data to the tectonic provenance fields of ternary diagrams indicates a mixed tectonic source.
The lithic population of the sandstones contains 21%+15 polycrystalline quartz (Qp)/ 43%+20 volcanic and metavolcanic lithics (Lvm) and 36%+14 sedimentary and metasedimentary lithics (Lsm). These lithics occur in decreasing order of abundance: metasediments, basalt, chert, polycrystalline quartz, felsic and intermediate plutonics, volcanic glass, felsic and intermediate volcanics, metavolcanics and gabbro and diabase. A slight increase in chert and polycrystalline quartz was found in the Elwah River and Morse Creek sections. The Morse Creek section also showed an enrichment of volcanic glass fragments.
Of the possible source areas for the Hoko River Formation, southern and central Vancouver Island are favored because all of the lithic types found in the Hoko River Formation are present. The lack of high pressure, low temperature metamorphic mineral assemblages typical of the San Juan Islands and the northwest Cascade mountains is the primary basis for excluding these areas as sediment sources of the Hoko River Formation. Sediment shed from the Coast Plutonic Complex would have contained much more potassium feldspar than the minor amounts found in the Hoko River Fomation (0.5 % + 1). The Olympic Core and Ozette terranes contain sufficient metasediment, basalt and polycrystalline quartz, but lack the other components. In addition, paleocurrent data suggest flow from the west and north to the east.
The Hoko River Formation was deposited in a submarine fan complex in middle-fan channel, outer middle-fan, and middle-fan depositional lobe environments, except at the most western section, Neah Bay, where a transgression of middle-fan channel deposits over inner-fan channel deposits occurs. Subsidence of that portion of the depositional basin is indicated.
The Hoko River Formation was deposited in the Juan de Fuca basin, separate from the basin in which the Chuckanut Formation and its distal equivalents were deposited. A comparison of petrologic data from the Chuckanut and Hoko River Formations shows they are clearly unrelated. Nor is the Hoko River Formation the distal equivalent of the Puget Sequence or the upper Eocene rocks of the Olympic Core. Based on similar depositional environments, Escalante Formation of the Caramanah Group may be a proximal equivalent of the Hoko River Formation
