The Hoko River Formation is a

Late Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', "daw ...

marine

sedimentary Sedimentary rocks are types of rock (geology), rock that are formed by the accumulation or deposition of mineral or organic matter, organic particles at Earth#Surface, Earth's surface, followed by cementation (geology), cementation. Sedimentati ...

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

. The formation is exposed in outcrops along the

Strait of Juan de Fuca The Strait of Juan de Fuca (officially named Juan de Fuca Strait in Canada) is a body of water about long that is the Salish Sea's outlet to the Pacific Ocean. The international boundary between Canada and the United States runs down the centre ...

on the

Olympic Peninsula The Olympic Peninsula is a large arm of land in western Washington that lies across Puget Sound from Seattle, and contains Olympic National Park. It is bounded on the west by the Pacific Ocean, the north by the Strait of Juan de Fuca, and the ...

Washington state Washington (), officially the State of Washington, is a state in the Pacific Northwest region of the Western United States. Named for George Washington—the first U.S. president—the state was formed from the western part of the Washington ...

, USA. It is known for containing numerous fossils of

crabs Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...

. It overlies the older Lyre Formation and underlies the younger Makah Formation.

Geology and stratigraphy

The Hoko River Formation consists of sediments deposited on the inner and middle slopes of a deep marine fan system. 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.'', is a section which outcrops along the

Hoko River The Hoko River is a river in the U.S. state of Washington. It originates in the foothills of the Olympic Mountains, and runs about to the Pacific Ocean through a rugged landscape that has been heavily logged. Its largest tributary is the Littl ...

, for which the formation is named, and a 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 Clastic rocks are composed of fragments, or clasts, of pre-existing minerals and rock. A clast is a fragment of geological detritus,Essentials of Geology, 3rd Ed, Stephen Marshak, p. G-3 chunks, and smaller grains of rock broken off other rocks ...

in the formation contain crab, gastropod, cephalopod, and wood fossils. 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. 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.

Paleontology

Macrofossils are uncommon and occur approximately 300 m above the formation base. Crab fossils are common in the formation, while rare ''

Nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in t ...

'' aff. '' N. cookanum'', '' Aturia'' cf. '' A. alabamensis'' and an indeterminate Belosaepiidae shell have been found. 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. Fossils of the extinct species '' Megokkos'' (ex ''Euphylax'') ''feldmanni'', a small

crab Crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (abdomen) ( el, βραχύς , translit=brachys = short, / = tail), usually hidden entirely under the thorax. They live in all the ...

, 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 A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in ...

of the

Portunidae Portunidae is a family of crabs which contains the swimming crabs. Description Portunid crabs are characterised by the flattening of the fifth pair of legs into broad paddles, which are used for swimming. This ability, together with their strong ...

subfamily Podophthalminae, as well as the earliest fossil record of this genus in the eastern

North Pacific The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...

. 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. Fossils of the genus '' Montezumella'' from the Hoko River Formation represent the oldest occurrence of the crab family Cheiragonidae.

References

{{coord missing, Washington Geologic formations of Washington (state) Eocene Series of North America Priabonian Stage