Hoko River Formation
Encyclopedia
The Hoko River Formation is a Late Eocene marine
sedimentary geologic formation
. 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.
.
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 1600 metres (5,249.3 ft) section which outcrops along the Hoko River
, for which the formation is named, and a 2300 metres (7,545.9 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. Magnetostratigraphy preformed 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.
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
, 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
. 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
.
Marine (ocean)
Marine is an umbrella term. As an adjective it is usually applicable to things relating to the sea or ocean, such as marine biology, marine ecology and marine geology...
sedimentary geologic formation
Geologic formation
A formation or geological formation is the fundamental unit of lithostratigraphy. A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties...
. The formation is exposed in outcrops along the Strait of Juan de Fuca
Strait of Juan de Fuca
The Strait of Juan de Fuca is a large body of water about long that is the Salish Sea outlet to the Pacific Ocean...
on the Olympic Peninsula
Olympic Peninsula
The Olympic Peninsula is the large arm of land in western Washington state of the USA, that lies across Puget Sound from Seattle. It is bounded on the west by the Pacific Ocean, the north by the Strait of Juan de Fuca, and the east by Puget Sound. Cape Alava, the westernmost point in the contiguous...
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.
Geology and stratigraphy
The Hoko River Formation consists of sediments deposited on the inner and middle slopes of a deep marine fan systemAbyssal fan
Abyssal Fans, also known as deep-sea fans, underwater deltas, and submarine fans, are underwater structures that look like deltas formed at the end of many large rivers, such as the Nile or Mississippi Rivers. Abyssal fans are also thought of as an underwater version of alluvial fans.- Formation...
.
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 1600 metres (5,249.3 ft) section which outcrops along the Hoko River
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 Little Hoko River, which joins at river mile . The lower of...
, for which the formation is named, and a 2300 metres (7,545.9 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. Magnetostratigraphy preformed 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 NautilusNautilus
Nautilus is the common name of marine creatures of cephalopod family Nautilidae, the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in two genera, the type of which is the genus Nautilus...
aff. N. cookanum
Nautilus cookanum
Nautilus cookanum is an extinct species of nautilus. It lived during the Eocene epoch. N. cookanum has been grouped into a single taxon together with extant species based on their shared shell characters. Fossils of the species from the Late Eocene Hoko River Formation are noted as one of the two...
, Aturia cf. A. alabamensis and an indeterminate Belosaepiidae
Belosaepiidae
Belosaepiidae is a family of cephalopods known from the Eocene, and bearing close similarity to the sepiid cuttlefish, whilst retaining the remnants of a belemnite-like guard....
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
Nautilus praepompilius
Nautilus praepompilius is an extinct species of nautilus. It lived during the Late Eocene through Oligocene epochs. Fossil specimens have been uncovered from the Chegan Formation of Kazakhstan. N. praepompilius has been grouped into a single taxon together with extant species based on their shared...
from the Republic of Kazakhstan.
Fossils of the extinct species Megokkos (ex Euphylax) feldmanni, a small crab
Crab
True crabs are decapod crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" , or where the reduced abdomen is entirely hidden under the thorax...
, 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
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...
subfamily Podophthalminae, as well as the earliest fossil record of this genus in the eastern North Pacific
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
. 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
Cheiragonidae
Cheiragonidae is a small family of crabs, sometimes called helmet crabs, placed in its own superfamily, Cheriagonoidea. It comprises three extant species, Erimacrus isenbeckii, Telmessus acutidens and Telmessus cheiragonus, and 13 extinct species in the three genera Karasawaia, Montezumella and...
.