{{Short description|Late Eocene marine sedimentary geologic formation}} {{Infobox rockunit | name = Hoko River Formation | image = Branchioplax washingtoniana 01 SR EC 07-01-13.jpg | caption = ''Branchioplax washingtoniana'', Hoko River Formation | type = Geological formation | age = {{Fossilrange|40|38|Late Eocene}} | period = Priabonian | prilithology = submarine fan system | namedby = Snavely ''et al'', 1978 | region = Clallam County, Olympic Peninsula Washington | country = United States | unitof = Twin River Group | overlies = Lyre Formation sediments | underlies = Makah Formation sediments | thickness = approximately {{convert|2,300|m|ft|-1}} | extent = over {{convert|100|km|mi}} }}
The '''Hoko River Formation''' is a Late Eocene marine sedimentary geologic formation.<ref name="Sanvely1978"/> The formation is exposed in outcrops along the Strait of Juan de Fuca on the Olympic Peninsula in Washington state, USA. It is known for containing numerous fossils of crabs. It overlies the older Lyre Formation and underlies the younger Makah Formation.<ref name="Sanvely1978"/>
==Geology and stratigraphy== The Hoko River Formation consists of sediments deposited on the inner and middle slopes of a deep marine fan system. <ref name="Sanvely1978">{{cite journal|last1=Snavely|first1=P.D. |last2=Niem |first2=A.R. |last3=Pearl |first3=J.E. |year=1978 |title=Twin River Group (upper Eocene to lower Miocene)–Defined to include the Hoko River, Makah, and Pysht Formations, Clallam County, Washington. |journal=U.S. Geological Survey Bulletin |volume=1457-A |pages=111–120}}</ref><ref name="Prothero2009">{{cite journal |last1=Prothero |first1=D.R. |last2=Draus |first2=E. |last3=Burns |first3=C. |year=2009 |title= Magnetostratigraphy and Tectonic Rotation of the Eocene-Oligocene Makah and Hoko River Formations, Northwest Washington, USA |journal= International Journal of Geophysics |volume=2009 |pages=1–15 |doi= 10.1155/2009/930612|doi-access=free }}</ref> It is composed primarily of siltstones and some sandstones exposed under and to the south of the main Makah Formation outcrops along the Strait of Juan De Fuca. The type section, as designated by Parke Snavely ''et al.'',<ref name="Sanvely1978"/> is a {{convert|1600|m|ft}} section which outcrops along the Hoko River, for which the formation is named, and a {{convert|2300|m|ft}} section which outcrops along Deep Creek. While the Hoko River formation overlies the Lyre Formation in many places, the two formations intertongue in others. The Makah and Hoko River formations are separated by a major unconformity. Calcareous clasts in the formation contain crab, gastropod, cephalopod, and wood fossils.<ref name="Prothero2009"/> Magnetostratigraphy performed in 2008 on samples taken from the type section of the Hoko River formation showed a correlation of age with either Chron C18r (40.0–41.2 Ma) or Chron C17r (38.0–38.2Ma). A closer correlation was not possible due to the limited sample size obtained for the testing.<ref name="Prothero2009"/> Of note is that the samples tested for both the Makah and Hoko River Formations showed a slight counterclockwise tectonic rotation. This is in contrast to many other formations of similar ages on the Olympic Peninsula with clockwise rotations. Similar results, however, from some formations on Vancouver Island and the northern Olympic Peninsula have been reported.<ref name="Prothero2009"/>
==Paleontology== Macrofossils are uncommon and occur approximately 300 m above the formation base. Crab fossils are common in the formation, while rare ''Nautilus'' aff. ''N. cookanum'', ''Aturia'' cf. ''A. alabamensis'' and an indeterminate Belosaepiidae shell have been found.<ref name="Squires1988">{{cite journal |last1=Squires |first1=R.L. |year=1988 |title= Cephalopods from the Late Eocene Hoko River Formation, northwestern Washington |journal= Journal of Paleontology |volume=62 |issue=1 |pages=76–82 |doi=10.1017/S002233600005890X |jstor= 1305257}}</ref> The fossil of ''Nautilus'' aff. ''N. cookanum'' is one of the two oldest occurrences for the genus ''Nautilus'' in the fossil record, the other occurrence being fossils of ''Nautilus praepompilius'' from the Republic of Kazakhstan.<ref name="Ryoji2008">{{cite journal |last1=Ryoji |first1=W. |year=2008 |title= First discovery of fossil ''Nautilus pompilius'' (Nautilidae, Cephalopoda) from Pangasinan, northwestern Philippines |journal= Paleontological Research |volume=12 |issue=1 |pages=89–95 |doi= 10.2517/1342-8144(2008)12[89:FDOFNP]2.0.CO;2|s2cid=129517075 |display-authors=etal|doi-access=free }}</ref>
Fossils of the extinct species ''Megokkos'' (ex ''Euphylax'') ''feldmanni'', a small crab, were found on this site. ''M. feldmanni'' was first described from the formation by Torrey Nyborg, Ross Berglund, and James Goedert in 2003 as the oldest member of the genus ''Euphylax''. These remains were considered the earliest fossil record of the Portunidae subfamily Podophthalminae, as well as the earliest fossil record of this genus in the eastern North Pacific.<ref name="Nyborg2003">{{cite journal|last1=Nyborg |first=T.G. |last2=Berglund |first2=R.E. |last3=Goedert |first3=J.L. |year=2003 |title= A new crab from the late Eocene Hoko River Formation, Olympic Peninsula, Washington: The earliest record of ''Euphylax'' (Decapoda: Portunidae). |journal=Journal of Paleontology |volume=77 |issue=2 |pages=323–330 |doi=10.1666/0022-3360(2003)077<0323:ANCFTL>2.0.CO;2}}</ref> In their 2006 paper on fossil decopods of the Caribbean, Carrie Schweitzer and her coauthors moved the species from ''Euphylax'' to the extinct genus ''Megokkos'' making it one of three species of ''Megokkos'' crabs found in the Washington state fossil record.<ref name="Schweitzer2006">{{cite journal|last1=Schweitzer |first=C.E. |last2=Iturralde-Vinent |first2=M. |last3=Hetler |first3=J.L. |last4=Velez-Juarbe |first4=J. |year=2006 |title=Oligocene and Miocene decapods (Thalassinidea and Brachyura) from the Caribbean. |journal=Annals of Carnegie Museum |volume=75 |issue=2 |pages=111–136 |doi=10.2992/0097-4463(2006)75[111:OAMDTA]2.0.CO;2|s2cid=53496359 }}</ref> Fossils of the genus ''Montezumella'' from the Hoko River Formation represent the oldest occurrence of the crab family Cheiragonidae.<ref name="Schweitzer2000">{{cite journal|last1=Schweitzer |first1=C.E. |last2=Salva |first2=E.W. |year=2000 |title=First Recognition of the Cheiragonidae (Decapoda) in the Fossil Record and Comparison of the Family with the Atelecyclidae. |journal=Journal of Crustacean Biology |volume=20 |issue=2 |pages=285–298 |doi=10.1651/0278-0372(2000)020[0285:FROTCD]2.0.CO;2|s2cid=198968801 }}</ref>
==References== {{reflist}}
{{Coord|48.1186|-123.7988|display=title}}
Category:Geologic formations of Washington (state) Category:Eocene Series of North America Category:Priabonian Stage