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159-526: Siletzia is a massive formation of early to middle Eocene epoch marine basalts and interbedded sediments in the forearc of the Cascadia subduction zone , on the west coast of North America . It forms the basement rock under western Oregon and Washington and the southern tip of Vancouver Island . It is now fragmented into the Siletz and Crescent terranes . Siletzia corresponds geographically to

318-478: A subduction zone . This change of composition reflects a change from marine to continental volcanism that becomes evident around 48 to 42  Ma (millions of years ago), and is attributed to the accretion of Siletzia against the North American continent. Various theories have been proposed to account for the volume and diversity of Siletzian magmatism , as well as the approximately 75° of rotation, but

477-481: A Northern, Central, and Southern. The oldest portions of the range are over 60 million years old, with volcanics and a forearc basin as the primary mountain building processes responsible for the range. It is part of the larger grouping known as the Pacific Coast Ranges that extends over much of the western edge of North America from California to Alaska. The range creates a rain shadow effect for

636-406: A chain of seamounts much longer than observed, and too far away from the continent to explain the continentally derived sediments. This objection is attenuated somewhat in that the newer ages show a smaller range of ages. Various models have Siletzia forming inshore, on or near the continental margin. While all current models have Siletzia rifting away from the continent after accretion or formation,

795-470: A change in plate direction, or by kinematic effects as the Kula-Farallon ridge migrated along the continental margin. One such effect is the formation of a slab window (or slab gap) which would allow increased upwelling of magma. That spreading ridges could be subducted was recognized early in the development of plate tectonic theory , but there was little consideration of the ensuing effects. In

954-609: A change in the convergence of the Kula plate with the North American Plate. As subduction waned so did the force that had clamped Siletzia against the continent, and the tectonic regime shifted from compressional to extensional. Deposition of sand from the then proximal Idaho Batholith into the Tyee Formation in southern Oregon may have continued as late as 46.5 Ma, but was interrupted when Siletzia rifted from

1113-582: A different rotational history. Second, in Washington there is more variation in the amount of rotation and more faulting, which has led to a speculation that the Crescent terrane has broken up into eight or nine crustal blocks. At Bremerton , on the east side of the Olympics, the measured rotations are less, and within the statistical error bounds of being zero; while further north, near Port Townsend ,

1272-431: A large body of water is also present. In an attempt to try to mitigate the cooling polar temperatures, large lakes were proposed to mitigate seasonal climate changes. To replicate this case, a lake was inserted into North America and a climate model was run using varying carbon dioxide levels. The model runs concluded that while the lake did reduce the seasonality of the region greater than just an increase in carbon dioxide,

1431-635: A layer of alkalic volcanics deposited subaerially. All this suggests these formations were initially deposited in an oceanic environment, possibly as seamounts or an island arc. On the Olympic Peninsula the Blue Mountain unit at the base of the Crescent Formation includes sediments (including large boulders of quartz diorite ) of continental origin, suggesting that the continent was close by; other sediments were eroded from

1590-604: A major extinction event called the Grande Coupure (the "Great Break" in continuity) or the Eocene–Oligocene extinction event , which may be related to the impact of one or more large bolides in Siberia and in what is now Chesapeake Bay . As with other geologic periods , the strata that define the start and end of the epoch are well identified, though their exact dates are slightly uncertain. The term "Eocene"

1749-540: A marine ecosystem)—one of the largest in the Cenozoic. This event happened around 55.8 Ma, and was one of the most significant periods of global change during the Cenozoic. The middle Eocene was characterized by the shift towards a cooler climate at the end of the EECO, around 47.8 Ma, which was briefly interrupted by another warming event called the middle Eocene climatic optimum (MECO). Lasting for about 400,000 years,

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1908-538: A narrower range of ages from 56 Ma in the south to 50 or 49 Ma in the north. Subsequent high-precision U-Pb dates from northern Siletzia show a narrowly constrained age of 51 Ma for the Metchosin complex on Vancouver Island. Of particular interest is the somewhat broader range of approximately 53 to 48 Ma for Crescent Formation basalts on the east side of the Olympic Peninsula that overlie

2067-511: A role in triggering the ETM2 and ETM3. An enhancement of the biological pump proved effective at sequestering excess carbon during the recovery phases of these hyperthermals. These hyperthermals led to increased perturbations in planktonic and benthic foraminifera , with a higher rate of fluvial sedimentation as a consequence of the warmer temperatures. Unlike the PETM, the lesser hyperthermals of

2226-476: A section of the larger Pacific Border province, which in turn are part of the larger Pacific Mountain System physiographic division. A mild maritime climate prevails throughout the range with temperature and precipitation varying due to elevation and distance from the coastline. Characteristics of the climate include cool dry summers followed by mild and wet winters. The majority of precipitation accumulates in

2385-809: A shift in the subduction zone, termination of the Laramide orogeny that was uplifting the Rocky Mountains , and major changes in tectonic and volcanic activity across much of western North America. The rock of Siletzia has been exposed in various places by tectonic uplift (as around the periphery of the Olympic Mountains ), anticlinal folding (such as the Black Hills and Willapa Hills in southwestern Washington), and overthrusting onto other formations (along various faults in central and southern Oregon). These exposures have been variously named

2544-533: A significant amount of water vapor is released. Another requirement for polar stratospheric clouds is cold temperatures to ensure condensation and cloud production. Polar stratospheric cloud production, since it requires the cold temperatures, is usually limited to nighttime and winter conditions. With this combination of wetter and colder conditions in the lower stratosphere, polar stratospheric clouds could have formed over wide areas in Polar Regions. To test

2703-477: A slab window — and a single subducted ridge can give rise to multiple slab windows — can provide adequate magmatism without having to invoke a hotspot (mantle plume). (So much so that it has been suggested that the Yellowstone hotspot may have been initiated by a slab window.) Mantle plumes and slab windows both feature voluminous magmatism; the main difference is that slab windows would form only where

2862-768: A slab window, the Siletzian basalts were laid down on a subducting oceanic plate: the Siletz terrane on the Farallon Plate , and the Crescent terrane most likely on the adjoining Resurrection plate (previously broken away from the Kula Plate , which had previously broken away from the Farallon Plate). In both cases the Siletzia mass was drawn toward the subduction zone, which possibly ran diagonally across what

3021-536: A small exposure on the north side of the Olympic Peninsula near Port Crescent . Though this exposure is small, he recognized as very likely that much more of it was buried under younger deposits. With recognition that similar rock exposed at other outcrops is part of the same formation, the name Crescent Formation is now generally applied to all early and middle Eocene basalts on the Olympic Peninsula and Puget lowlands . The Metchosin Igneous Complex at

3180-613: A strong correlation with the age of the rock: about one and a half degrees of rotation per million years. These paleomagnetic rotations and other evidence show that Siletzia — or the part of it constituting the Siletz terrane ("SZ" on map, above ), from the Klamath Mountains to the Columbia River  — has rotated clockwise as a single, coherent block. Did Siletzia pivot about its northern end or southern end? This question has attracted considerable attention, with

3339-477: A study of the Clarno Formation by Grommé et al. (1986) and illustrated with a palinspastic reconstruction as of 38 Ma. An early and widely cited paper by Duncan (1982) (drawing on features of the fairly new theory of plate tectonics) exemplifies the off-shore or " seamount " type of models. It featured a set of radiometrically determined (K-Ar and Ar-Ar) ages that were younger in the center (for

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3498-474: A subclass of "rifted" models consider the rifting to have caused the Siletzia eruptions. Wells et al. 1984 proposed that the Siletzia basalts might have "leaked" through transform faults (perpendicular to a spreading ridge) during changes in direction of the tectonic plates. The size of these eruptions and their location in this region is attributed to proximity to the Yellowstone hotspot. This "leaky transform" theory seems to be largely rejected, likely because

3657-775: A wide variety of climate conditions that includes the warmest climate in the Cenozoic Era , and arguably the warmest time interval since the Permian-Triassic mass extinction and Early Triassic, and ends in an icehouse climate. The evolution of the Eocene climate began with warming after the end of the Paleocene–Eocene Thermal Maximum (PETM) at 56 Ma to a maximum during the Eocene Optimum at around 49 Ma. During this period of time, little to no ice

3816-694: Is a mountain range , in the Pacific Coast Ranges physiographic region , in the U.S. state of Oregon along the Pacific Ocean . This north-south running range extends over 200 miles (320 km) from the Columbia River in the north on the border of Oregon and Washington , south to the middle fork of the Coquille River . It is 30 to 60 miles (48 to 97 km) wide and averages around 1,500 feet (460 m) in elevation above sea level . The coast range has three main sections,

3975-543: Is a similar situation, where the St. Helens Fault Zone (SHZ) is believed to be the eastern edge of the Crescent Formation, but the pre-Cenozoic continental basement is near Mount Rainier . Separating these is the marine sedimentary formation known as the Southern Washington Cascades Conductor (SWCC); it is possible that it was deposited over a fragment of Siletzia. Or not: the oldest parts of

4134-459: Is an important factor in the creation of the primary Type II polar stratospheric clouds that were created in the early Eocene. Since water vapor is the only supporting substance used in Type II polar stratospheric clouds, the presence of water vapor in the lower stratosphere is necessary where in most situations the presence of water vapor in the lower stratosphere is rare. When methane is oxidized,

4293-638: Is considered to be primarily due to carbon dioxide increases, because carbon isotope signatures rule out major methane release during this short-term warming. A sharp increase in atmospheric carbon dioxide was observed with a maximum of 4,000 ppm: the highest amount of atmospheric carbon dioxide detected during the Eocene. Other studies suggest a more modest rise in carbon dioxide levels. The increase in atmospheric carbon dioxide has also been hypothesised to have been driven by increased seafloor spreading rates and metamorphic decarbonation reactions between Australia and Antarctica and increased amounts of volcanism in

4452-594: Is conventionally divided into early (56–47.8 Ma), middle (47.8–38 Ma), and late (38–33.9 Ma) subdivisions. The corresponding rocks are referred to as lower, middle, and upper Eocene. The Ypresian Stage constitutes the lower, the Priabonian Stage the upper; and the Lutetian and Bartonian stages are united as the middle Eocene. The Western North American floras of the Eocene were divided into four floral "stages" by Jack Wolfe ( 1968 ) based on work with

4611-603: Is derived from Ancient Greek Ἠώς ( Ēṓs ) meaning "Dawn", and καινός kainos meaning "new" or "recent", as the epoch saw the dawn of recent, or modern, life. Scottish geologist Charles Lyell (ignoring the Quaternary) divided the Tertiary Epoch into the Eocene, Miocene , Pliocene , and New Pliocene ( Holocene ) Periods in 1833. British geologist John Phillips proposed the Cenozoic in 1840 in place of

4770-600: Is divided into three separate sections: North, Central, and South. In the south is the oldest portion of the range with formation beginning in the Paleocene era with the Roseburg volcanics, while the newest section is the northernmost portion formed first with the Siletz River Volcanics. The Central and Northern sections contain more sedimentary rocks from the mud, silt, sand, and other volcanic debris than

4929-549: Is due to loss of material from the center after uplift by the Olympic Mountains, and how much reflects oroclinal bending. Siletzia's origin is not yet determined, and (as of 2017) remains controversial. Theories are still being developed, and even the details the theories depend on "have remained enigmatic". Following are several of the most notable models. Models of how Siletzia formed are of two general types: (1) Formation well offshore (possibly as seamounts , like

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5088-531: Is now Washington, approximately at the position of the Olympic–Wallowa Lineament . However, Siletzia was too big to be subducted, and it accreted to the continent. Accretion is sometimes called "docking," but is more akin to a collision: various peripheral structures are first folded or crushed, then the main structures are deformed when they come into contact, and various parts get pushed over other parts, all this playing out over several million years. To

5247-720: Is short lived, as benthic oxygen isotope records indicate a return to cooling at ~40 Ma. At the end of the MECO, the MLEC resumed. Cooling and the carbon dioxide drawdown continued through the late Eocene and into the Eocene–Oligocene transition around 34 Ma. The post-MECO cooling brought with it a major aridification trend in Asia, enhanced by retreating seas. A monsoonal climate remained predominant in East Asia. The cooling during

5406-490: Is the highest peak at 4,097 feet (1,248 m). Logging is a major industry in the range in both private and government owned forests. Both the state and federal government manage forests in the Oregon Coast Range. The mountains are home to a variety of wildlife including black bear, elk, deer, beaver, many species of birds, and bats among others. Fish, including salmon and trout, and other aquatic life inhabit

5565-535: Is the period of time when the Antarctic ice sheet began to rapidly expand. Greenhouse gases, in particular carbon dioxide and methane , played a significant role during the Eocene in controlling the surface temperature. The end of the PETM was met with very large sequestration of carbon dioxide into the forms of methane clathrate , coal , and crude oil at the bottom of the Arctic Ocean , that reduced

5724-622: The Basin and Range Province and asthenospheric flow around the southern edge of the subducting Juan de Fuca Plate. North of the Columbia River, matters are more complicated. First, in southwestern Washington there is only half as much rotation as seen in rocks of similar age in Oregon. This is the basis for believing the Crescent terrane has broken from the Siletz terrane (perhaps because they formed on different oceanic plates), and undergone

5883-527: The Coast Range Volcanic Province (or Coast Range basalts ), but is distinguished from slightly younger basalts that erupted after Siletzia accreted to the continent and differ in chemical composition. The Siletzia basalts are tholeiitic , a characteristic of mantle -derived magma erupted from a spreading ridge between plates of oceanic crust. The younger basalts are alkalic or calc-alkaline , characteristic of magmas derived from

6042-685: The Coquille River ’s middle fork provides the general dividing line between the Central Range and the Klamath Mountains to the south and east. Birds living in the Southern Coast Range include a variety of smaller and larger bird species. Species include peregrine falcons , pileated woodpeckers , olive-sided flycatcher, and western bluebirds among others. The threatened northern spotted owl also inhabit

6201-521: The Hawaiian-Emperor seamount chain , or a hotspot at a spreading ridge, like Iceland ) and then accretion to the continent; (2) formation inshore, on or near the continental margin (perhaps as a result of transcurrent extension, or of a slab window). All current models then have Siletzia rifting away from the continental edge about a northern pivot. Studies of Siletzia's origins have generally focused on accounting for two principal observations:

6360-871: The Mesozoic Klamath Mountains of southern Oregon along the Klamath—Blue Mountain Lineament (KBML). Near Roseburg this contact is exposed at the Wild Safari Fault where the Late Jurassic Dothan Formation has been thrust over the Roseburg Formation. Off the coast of southern Oregon, the western edge of Siletzia is the Eocene Fulmar Fault. This is a strike-slip fault, where part of Siletzia has been split off;

6519-713: The Metchosin Formation of Vancouver Island , the Crescent Formation , Black Hills Formation (Washington) , and Willapa Hills volcanics of Washington, and the Siletz River Volcanics and Roseburg Formation of Oregon. (See map . The Grays River Volcanics of Washington and Tillamook Volcanics of Oregon are now considered post-Siletz.) Elsewhere Siletzia is covered by younger volcanic and sedimentary deposits. The discovery of Siletzia began in 1906 with Arnold's description and naming of

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6678-713: The Middle Eocene Climatic Optimum (MECO). At around 41.5 Ma, stable isotopic analysis of samples from Southern Ocean drilling sites indicated a warming event for 600,000 years. A similar shift in carbon isotopes is known from the Northern Hemisphere in the Scaglia Limestones of Italy. Oxygen isotope analysis showed a large negative change in the proportion of heavier oxygen isotopes to lighter oxygen isotopes, which indicates an increase in global temperatures. The warming

6837-761: The Puget Group fossils of King County, Washington . The four stages, Franklinian , Fultonian , Ravenian , and Kummerian covered the Early Eocene through early Oligocene, and three of the four were given informal early/late substages. Wolfe tentatively deemed the Franklinian as Early Eocene, the Fultonian as Middle Eocene, the Ravenian as Late, and the Kummerian as Early Oligocene. The beginning of

6996-506: The Rocky Mountains , and triggered the ignimbrite sweep , a wave of large-volume silicic magmatism that swept over much of western North America between 20 and 50 Ma. This undoubtedly affected the enigmatic and controversial Challis Arc (stretching from southeastern British Columbia to the Idaho Batholith, roughly parallel with the Olympic–Wallowa Lineament ), but the details of this are unknown. Subduction, having ceased at

7155-865: The Salmon River where Oregon Route 18 crosses the range from the Willamette Valley to the Oregon Coast with width roughly 35 miles (60 km). Located between the Salmon River and the Umpqua River on the north and south, the Central range is bounded by the Willamette Valley on the east and the Pacific Ocean to the west. This approximately 90-mile (140 km) long mountain range contains mountains as high as 4,097 feet (1,248 m) for Marys Peak . Portions of

7314-475: The Strait of Juan de Fuca , possibly connecting with the southeast striking Southern Whidbey Island Fault (SWIF). This extends to the Rattlesnake Mountain Fault Zone (RMFZ), some 25 kilometers east of Seattle , which is believed to be the western edge of the pre-Cenozoic basement. However, gravity data indicates that at this latitude the Crescent Formation (at least near the surface) extends no further east than Seattle. Further south, near Mount St. Helens ,

7473-406: The Willamette Valley that lies to the east of the mountains, creating a more stable climate and significantly less rain than the coastal region of the state. To the west where the range over-shadows the Oregon Coast , the range causes more precipitation to fall on that side of the mountains, contributing to the numerous rivers that flow to the Pacific Ocean. Marys Peak in the Central Coast Range

7632-406: The amount of oxygen in the Earth's atmosphere more or less doubled. During the warming in the early Eocene between 55 and 52 Ma, there were a series of short-term changes of carbon isotope composition in the ocean. These isotope changes occurred due to the release of carbon from the ocean into the atmosphere that led to a temperature increase of 4–8 °C (7.2–14.4 °F) at the surface of

7791-417: The proxy data . Using all different ranges of greenhouse gasses that occurred during the early Eocene, models were unable to produce the warming that was found at the poles and the reduced seasonality that occurs with winters at the poles being substantially warmer. The models, while accurately predicting the tropics, tend to produce significantly cooler temperatures of up to 20 °C (36 °F) colder than

7950-473: The southeast United States . After the Paleocene–Eocene Thermal Maximum, members of the Equoidea arose in North America and Europe, giving rise to some of the earliest equids such as Sifrhippus and basal European equoids such as the palaeothere Hyracotherium . Some of the later equoids were especially species-rich; Palaeotherium , ranging from small to very large in size, is known from as many as 16 species. Established large-sized mammals of

8109-457: The 1980s came realization that the magma welling up from the asthenosphere through the subducted ridge would not reach seawater, and thus not be quenched to form rock and close the gap. Continued spreading would lead to a widening gap or "window" in the subducting plate through which there could be increased flow of magma. The implications of this for Siletzia were first shown by Thorkelson & Taylor (1989) and Babcock et al. (1992) (following

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8268-479: The Azolla Event. This cooling trend at the end of the EECO has also been proposed to have been caused by increased siliceous plankton productivity and marine carbon burial, which also helped draw carbon dioxide out of the atmosphere. Cooling after this event, part of a trend known as the Middle-Late Eocene Cooling (MLEC), continued due to continual decrease in atmospheric carbon dioxide from organic productivity and weathering from mountain building . Many regions of

8427-421: The Blue Mountain unit, reliably dated at 48 Ma or younger. This structural relationship was previously understood as showing that Siletzia – or at least its northern end – was built on the continental margin. It is now argued that the unconformity of ages can be explained by the Blue Mountain unit being thrust under Siletzia some time after 44.5 Ma, and shows that Siletiza was not necessarily emplaced along

8586-406: The Coast Range is located in the southwest portion of Oregon between the middle fork of the Coquille River in the south and the Umpqua River on the north. Oregon Route 38 is the general divide between the Central and Southern portions of the Coast Range. Approximately 55 miles (90 km) long, the section contains mountains as high as 3,547 feet (1,081 m) for Bone Mountain . On the south

8745-404: The EECO. Relative to present-day values, bottom water temperatures are 10 °C (18 °F) higher according to isotope proxies. With these bottom water temperatures, temperatures in areas where deep water forms near the poles are unable to be much cooler than the bottom water temperatures. An issue arises, however, when trying to model the Eocene and reproduce the results that are found with

8904-404: The Early Eocene had negligible consequences for terrestrial mammals. These Early Eocene hyperthermals produced a sustained period of extremely hot climate known as the Early Eocene Climatic Optimum (EECO). During the early and middle EECO, the superabundance of the euryhaline dinocyst Homotryblium in New Zealand indicates elevated ocean salinity in the region. One of the unique features of

9063-440: The Earth including the poles. Tropical forests extended across much of modern Africa, South America, Central America, India, South-east Asia and China.  Paratropical forests grew over North America, Europe and Russia, with broad-leafed evergreen and broad-leafed deciduous forests at higher latitudes. Polar forests were quite extensive. Fossils and even preserved remains of trees such as swamp cypress and dawn redwood from

9222-457: The Eocene have been found on Ellesmere Island in the Arctic . Even at that time, Ellesmere Island was only a few degrees in latitude further south than it is today. Fossils of subtropical and even tropical trees and plants from the Eocene also have been found in Greenland and Alaska . Tropical rainforests grew as far north as northern North America and Europe . Palm trees were growing as far north as Alaska and northern Europe during

9381-503: The Eocene include the Uintatherium , Arsinoitherium , and brontotheres , in which the former two, unlike the latter, did not belong to ungulates but groups that became extinct shortly after their establishments. Large terrestrial mammalian predators had already existed since the Paleocene, but new forms now arose like Hyaenodon and Daphoenus (the earliest lineage of a once-successful predatory family known as bear dogs ). Entelodonts meanwhile established themselves as some of

9540-544: The Eocene's climate as mentioned before was the equable and homogeneous climate that existed in the early parts of the Eocene. A multitude of proxies support the presence of a warmer equable climate being present during this period of time. A few of these proxies include the presence of fossils native to warm climates, such as crocodiles , located in the higher latitudes, the presence in the high latitudes of frost-intolerant flora such as palm trees which cannot survive during sustained freezes, and fossils of snakes found in

9699-426: The Eocene, and compression was replaced with crustal extension that ultimately gave rise to the Basin and Range Province . The Kishenehn Basin, around 1.5 km in elevation during the Lutetian, was uplifted to an altitude of 2.5 km by the Priabonian. Huge lakes formed in the high flat basins among uplifts, resulting in the deposition of the Green River Formation lagerstätte . At about 35 Ma, an asteroid impact on

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9858-399: The Eocene-Oligocene transition is the timing of the creation of the circulation is uncertain. For Drake Passage , sediments indicate the opening occurred ~41 Ma while tectonics indicate that this occurred ~32 Ma. Solar activity did not change significantly during the greenhouse-icehouse transition across the Eocene-Oligocene boundary. During the early-middle Eocene, forests covered most of

10017-480: The Farallon-Kula spreading ridge (such as at Iceland ) to generate a chain of islands. These islands were then accreted to the continent as the underlying oceanic crust was subducted. This study has been criticized on multiple grounds, particularly regarding the ages. Duncan himself noted that measurement of the northern ages may have been affected by loss of argon due to low-grade metamorphism, and that there might be bias in respect of stratigraphic position. The latter

10176-405: The Grays River volcanics) and older at the ends. This dihedrally symmetric age progression strongly suggested the pattern seen at spreading ridges , where the older rock is carried away on both sides from where the new rock erupts. Duncan considered five models (but none involving rifting or ridge subduction), favoring one where a hotspot — presumably the Yellowstone hotspot  — intersected

10335-441: The Gulf of Alaska towards Kodiak Island , and the former K-F (now R-F) ridge reaching Washington. Subduction of this plate under western Canada was rapid, and it disappeared entirely with subduction of the K-R ridge about 50 Ma. This scenario then permits rapid transport north of crustal blocks such as the Yakutat terrane . Now lying southeast of Cordova at the head of the Gulf of Alaska, paleomagnetic evidence indicates it

10494-439: The Kummerian was refined by Gregory Retallack et al (2004) as 40 Mya, with a refined end at the Eocene-Oligocene boundary where the younger Angoonian floral stage starts. During the Eocene, the continents continued to drift toward their present positions. At the beginning of the period, Australia and Antarctica remained connected, and warm equatorial currents may have mixed with colder Antarctic waters, distributing

10653-409: The Leech River Schists (resulting from the Metchosin/Crescent Formation being thrust under Vancouver Island ) and the end of strike-slip motion on the Straight Creek Fault ; these may reflect the last movement of Siletzia relative to North America. On a broader scale, there was a change in absolute direction of the Pacific Plate (marked by the end of the bend in the Hawaiian-Emperor seamount chain), and

10812-426: The MECO was responsible for a globally uniform 4° to 6°C warming of both the surface and deep oceans, as inferred from foraminiferal stable oxygen isotope records. The resumption of a long-term gradual cooling trend resulted in a glacial maximum at the late Eocene/early Oligocene boundary. The end of the Eocene was also marked by the Eocene–Oligocene extinction event , also known as the Grande Coupure . The Eocene

10971-625: The MECO. Both groups of modern ungulates (hoofed animals) became prevalent because of a major radiation between Europe and North America, along with carnivorous ungulates like Mesonyx . Early forms of many other modern mammalian orders appeared, including horses (most notably the Eohippus ), bats , proboscidians (elephants), primates, and rodents . Older primitive forms of mammals declined in variety and importance. Important Eocene land fauna fossil remains have been found in western North America, Europe, Patagonia , Egypt , and southeast Asia . Marine fauna are best known from South Asia and

11130-405: The North American continent, and it reduced the seasonal variation of temperature by up to 75%. While orbital parameters did not produce the warming at the poles, the parameters did show a great effect on seasonality and needed to be considered. Another method considered for producing the warm polar temperatures were polar stratospheric clouds . Polar stratospheric clouds are clouds that occur in

11289-404: The Olympics prior to their uplift, resembling a dome where top and western end has been removed. Siletzia's actual thickness, and the estimates of that thickness, vary. Under Oregon, the Siletz terrane appears to extend 25 to possibly 35 km into the trough between the subducting Juan de Fuca Plate and the edge of the continent, where it is slipping over sediments accumulated in the bottom of

11448-415: The PETM event in the sea floor or wetland environments. For contrast, today the carbon dioxide levels are at 400 ppm or 0.04%. During the early Eocene, methane was another greenhouse gas that had a drastic effect on the climate. Methane has 30 times more of a warming effect than carbon dioxide on a 100-year scale (i.e., methane has a global warming potential of 29.8±11). Most of the methane released to

11607-550: The SWCC likely predate Siletzia, and the nature and location of the contact between these two formations is unknown. In central Oregon, Siletzia forms a platform on which the older, now defunct volcanoes of the Western Cascades rest. The younger High Cascades to the east are believed to rest on sediments that accumulated in the basin between Siletzia and the continent. In southern Oregon, Siletzia has been thrust against

11766-713: The Tertiary subdivided into the Paleogene and Neogene periods. In 1978, the Paleogene was officially defined as the Paleocene, Eocene, and Oligocene epochs; and the Neogene as the Miocene and Pliocene epochs. In 1989, Tertiary and Quaternary were removed from the time scale due to the arbitrary nature of their boundary, but Quaternary was reinstated in 2009. The Eocene is a dynamic epoch that represents global climatic transitions between two climatic extremes, transitioning from

11925-581: The Tertiary, and Austrian paleontologist Moritz Hörnes introduced the Paleogene for the Eocene and Neogene for the Miocene and Pliocene in 1853. After decades of inconsistent usage, the newly formed International Commission on Stratigraphy (ICS), in 1969, standardized stratigraphy based on the prevailing opinions in Europe: the Cenozoic Era subdivided into the Tertiary and Quaternary sub-eras, and

12084-405: The actual determined temperature at the poles. This error has been classified as the "equable climate problem". To solve this problem, the solution would involve finding a process to warm the poles without warming the tropics. Some hypotheses and tests which attempt to find the process are listed below. Due to the nature of water as opposed to land, less temperature variability would be present if

12243-410: The addition of a large lake was unable to reduce the seasonality to the levels shown by the floral and faunal data. The transport of heat from the tropics to the poles, much like how ocean heat transport functions in modern times, was considered a possibility for the increased temperature and reduced seasonality for the poles. With the increased sea surface temperatures and the increased temperature of

12402-422: The amount of polar stratospheric clouds. While the polar stratospheric clouds could explain the reduction of the equator to pole temperature gradient and the increased temperatures at the poles during the early Eocene, there are a few drawbacks to maintaining polar stratospheric clouds for an extended period of time. Separate model runs were used to determine the sustainability of the polar stratospheric clouds. It

12561-456: The atmosphere during this period of time would have been from wetlands, swamps, and forests. The atmospheric methane concentration today is 0.000179% or 1.79 ppmv . As a result of the warmer climate and the sea level rise associated with the early Eocene, more wetlands, more forests, and more coal deposits would have been available for methane release. If we compare the early Eocene production of methane to current levels of atmospheric methane,

12720-528: The atmosphere may have been more important. Once the Antarctic region began to cool down, the ocean surrounding Antarctica began to freeze, sending cold water and icefloes north and reinforcing the cooling. The northern supercontinent of Laurasia began to fragment, as Europe , Greenland and North America drifted apart. In western North America, the Laramide Orogeny came to an end in

12879-524: The atmospheric carbon dioxide. This event was similar in magnitude to the massive release of greenhouse gasses at the beginning of the PETM, and it is hypothesized that the sequestration was mainly due to organic carbon burial and weathering of silicates. For the early Eocene there is much discussion on how much carbon dioxide was in the atmosphere. This is due to numerous proxies representing different atmospheric carbon dioxide content. For example, diverse geochemical and paleontological proxies indicate that at

13038-448: The better known Columbia River Basalts . Snavely et al., recognizing at least 10,000 feet of thickness, and as much as 20,000 feet under eruptive centers, estimated the volume to be in excess of 50,000 cubic miles (over 200,000 km). Duncan (1982) estimated around 250,000 km (about 60,000 cubic miles), which exceeds the volume of most continental rift zones, and some flood basalt provinces. A recent estimate put

13197-488: The coast range formation extend from southwest Washington state in the north to around the Coquille River in the south where the older and taller Klamath Mountains begin. In the east the mountains begin as foothills forming the western edge of the Willamette Valley and continue west to the coastline and beyond where the basalt formation tapers off into the continental shelf and ends at the continental slope with several banks and basins off shore. Physiographically, they are

13356-509: The coastal range. Additional basalt flows originated from Eastern Oregon and added to the layers that were uplifted, as the newer Cascade Mountains had not yet been formed. By the Early Oligocene period c. 30 million years ago the current coastline was in place and erosion has continued to shape the range. primarily through rivers cutting deep valleys through the igneous and sedimentary rocks. The geologic boundaries of

13515-492: The continent and began rotating away. What initiated rifting is unknown. Wells et al. (1984 , p. 290) suggested that as the continent overrode the Yellowstone hotspot, the upwelling plume tore away a previously accreted terrane. Babcock et al. (1992) suggested a change in the rate at which the plates were converging, or the "kinematic effects" (such as a slab window) from the passage of the Kula-Farallon ridge (or Resurrection-Farallon ridge). Eocene The Eocene spans

13674-529: The continent and rotated about a northern pivot near the Olympic Peninsula. Because sediments also show the Klamaths in close contact from the start, this requires the Klamaths to have moved with Siletzia. Originally there were conflicts in the understanding of when the Klamaths moved, and with the age and amount of rotation of the Clarno Formation in central Oregon. These were largely cleared up in

13833-477: The continental margin along what is now the Olympic–Wallowa Lineament (OWL; a zone of topographical features of unknown age and tectonic significance), with the southern end of Siletzia and the Klamath Mountains (joined to Siletzia) near the Idaho Batholith in central Idaho. Further evidence for this comes from the sand of the Tyee Formation that overlie the Roseburg Formation. Not only does this sand have

13992-400: The continental margin. Siletzia is massive: over 400 miles (600 kilometers) long, almost half that much across (and likely further at depth). The original deposits were from 16 to 35 kilometers thick. Weaver, reckoning a minimal thickness of only 3,000 feet, still estimated "nearly 10,000 cubic miles of rock"; he put the total volume to be as great, if not greater, than

14151-413: The decline into an icehouse climate and the rapid expansion of the Antarctic ice sheet . The transition from a warming climate into a cooling climate began at around 49 Ma. Isotopes of carbon and oxygen indicate a shift to a global cooling climate. The cause of the cooling has been attributed to a significant decrease of >2,000 ppm in atmospheric carbon dioxide concentrations. One proposed cause of

14310-437: The deep ocean water during the early Eocene, one common hypothesis was that due to these increases there would be a greater transport of heat from the tropics to the poles. Simulating these differences, the models produced lower heat transport due to the lower temperature gradients and were unsuccessful in producing an equable climate from only ocean heat transport. While typically seen as a control on ice growth and seasonality,

14469-410: The deep ocean. On top of that, MECO warming caused an increase in the respiration rates of pelagic heterotrophs , leading to a decreased proportion of primary productivity making its way down to the seafloor and causing a corresponding decline in populations of benthic foraminifera. An abrupt decrease in lakewater salinity in western North America occurred during this warming interval. This warming

14628-411: The early Eocene would have produced triple the amount of methane. The warm temperatures during the early Eocene could have increased methane production rates, and methane that is released into the atmosphere would in turn warm the troposphere, cool the stratosphere, and produce water vapor and carbon dioxide through oxidation. Biogenic production of methane produces carbon dioxide and water vapor along with

14787-901: The early Eocene, although they became less abundant as the climate cooled. Dawn redwoods were far more extensive as well. The earliest definitive Eucalyptus fossils were dated from 51.9 Ma, and were found in the Laguna del Hunco deposit in Chubut province in Argentina . Cooling began mid-period, and by the end of the Eocene continental interiors had begun to dry, with forests thinning considerably in some areas. The newly evolved grasses were still confined to river banks and lake shores, and had not yet expanded into plains and savannas . The cooling also brought seasonal changes. Deciduous trees, better able to cope with large temperature changes, began to overtake evergreen tropical species. By

14946-639: The eastern coast of North America formed the Chesapeake Bay impact crater . The Tethys Ocean finally closed with the collision of Africa and Eurasia, while the uplift of the Alps isolated its final remnant, the Mediterranean , and created another shallow sea with island archipelagos to the north. Planktonic foraminifera in the northwestern Peri-Tethys are very similar to those of the Tethys in

15105-544: The end of the period, deciduous forests covered large parts of the northern continents, including North America, Eurasia and the Arctic, and rainforests held on only in equatorial South America , Africa , India and Australia . Antarctica began the Eocene fringed with a warm temperate to sub-tropical rainforest . Pollen found in Prydz Bay from the Eocene suggest taiga forest existed there. It became much colder as

15264-508: The enhanced burial of azolla could have had a significant effect on the world atmospheric carbon content and may have been the event to begin the transition into an ice house climate. The azolla event could have led to a draw down of atmospheric carbon dioxide of up to 470 ppm. Assuming the carbon dioxide concentrations were at 900 ppmv prior to the Azolla Event they would have dropped to 430 ppmv, or 30 ppmv more than they are today, after

15423-472: The enhanced carbon dioxide levels found in the early Eocene. The isolation of the Arctic Ocean, evidenced by euxinia that occurred at this time, led to stagnant waters and as the azolla sank to the sea floor, they became part of the sediments on the seabed and effectively sequestered the carbon by locking it out of the atmosphere for good. The ability for the azolla to sequester carbon is exceptional, and

15582-471: The evidence is insufficient to determine Siletzia's origin; the question remains open. The accretion of Siletzia against the North American continent approximately 50 million years ago (contemporaneous with the initiation of the bend in the Hawaiian-Emperor seamount chain ) was a major tectonic event associated with a reorganization of Earth's tectonic plates . This is believed to have a caused

15741-406: The evidence long suggesting a northern pivot. A key piece of evidence is that the Crescent Formation is laid over sediments (the Blue Mountain unit) derived from the continent, including boulders of quartz diorite some 65 million years old. This was previously interpreted as requiring the Crescent Formation to have formed close to the continent. However, new high-precision U-Pb dating shows that

15900-479: The existing zone, eventually reinitiated further to the west as the current Cascadia subduction zone . Volcanism from the new subduction zone (such as the Grays River Volcanics and Northcraft Volcanics) reached the surface about 42 Ma, thereby initiating the rise of the ancestral Cascade Range . Several other significant events occurred around 42 Ma, including cessation of metamorphism of

16059-494: The expansion of the ice sheet was the creation of the Antarctic Circumpolar Current . The creation of the Antarctic circumpolar current would isolate the cold water around the Antarctic, which would reduce heat transport to the Antarctic along with creating ocean gyres that result in the upwelling of colder bottom waters. The issue with this hypothesis of the consideration of this being a factor for

16218-412: The exposures (black) and inferred near-surface extent (pink) of Siletzia, the latter being what can be detected in the upper crust by aeromagnetic, gravitational, or seismological studies. There are only two exposed contacts of Siletzia with the older (pre- Cenozoic ) North American basement . One is near Roseburg, Oregon , where it is thrust against formations of the Klamath Mountains (discussed below),

16377-602: The extant manatees and dugongs . It is thought that millions of years after the Cretaceous-Paleogene extinction event , brain sizes of mammals now started to increase , "likely driven by a need for greater cognition in increasingly complex environments". Oregon Coast Range The Oregon Coast Range , often called simply the Coast Range and sometimes the Pacific Coast Range ,

16536-531: The extent that the accretion of Siletzia to North America can be given a definite date most studies give it as about 50 Ma. This date has added significance as it is also the start of a change in direction of the Pacific plate, as seen in the bend in the Hawaiian-Emperor seamount chain , and also a change in the Pacific Northwest from compressional to extensional tectonics . This may also be when

16695-586: The form of rain , with snow during the winter months at the higher elevations, but no permanent snow pack . Annual precipitation differs from 60 inches (1,500 mm) in some parts to up to 120 inches (3,000 mm), with the higher amounts coming in the higher elevations. The average high temperature in January is 36.3 °F (2.4 °C), and the average high in July is 61.9 °F (16.6 °C) with temperatures also varying by elevation. The further inland and

16854-427: The formations are the result of pillow basalt formations created when a hot basalt flow rapidly cooled upon meeting the salt water of the ocean. These deposits offshore were then pushed into the continental plate as a forearc basin rotating slowly over millions of years. This tectonic collision forced the basalt formations (and newer sedimentary rock formations that include marine terrace deposits) upward and created

17013-701: The head of the Gulf of Alaska (along the Alaska panhandle) have ages and compositions corresponding to the Siletz volcanics, suggesting that the K-F ridge was offshore of the Yukon at the same time it was offshore of Washington. This can be resolved by assuming that by about 56 Ma the eastern part of the Kula plate had broken away to form the Resurrection plate, with the new Kula-Resurrection (K-R) spreading ridge running up

17172-399: The heat around the planet and keeping global temperatures high. When Australia split from the southern continent around 45 Ma, the warm equatorial currents were routed away from Antarctica. An isolated cold water channel developed between the two continents. However, modeling results call into question the thermal isolation model for late Eocene cooling, and decreasing carbon dioxide levels in

17331-402: The hot house to the cold house. The beginning of the Eocene is marked by the Paleocene–Eocene Thermal Maximum , a short period of intense warming and ocean acidification brought about by the release of carbon en masse into the atmosphere and ocean systems, which led to a mass extinction of 30–50% of benthic foraminifera (single-celled species which are used as bioindicators of the health of

17490-588: The initial stages of the opening of the Drake Passage ~38.5 Ma was not global, as evidenced by an absence of cooling in the North Atlantic. During the cooling period, benthic oxygen isotopes show the possibility of ice creation and ice increase during this later cooling. The end of the Eocene and beginning of the Oligocene is marked with the massive expansion of area of the Antarctic ice sheet that

17649-463: The large paleorotation (described above), and the voluminous output (over 50,000 cubic miles, exceeding the volume of most continental rift zones, and some flood basalt provinces). Accounting for the observed volumes of basalt requires an enhanced magmatic source, for which most models invoke either the presence of the Yellowstone hotspot , or slab windows . The latter would have resulted from

17808-516: The largest omnivores. The first nimravids , including Dinictis , established themselves as amongst the first feliforms to appear. Their groups became highly successful and continued to live past the Eocene. Basilosaurus is a very well-known Eocene whale , but whales as a group had become very diverse during the Eocene, which is when the major transitions from being terrestrial to fully aquatic in cetaceans occurred. The first sirenians were evolving at this time, and would eventually evolve into

17967-462: The last of the Resurrection plate was subducted under British Columbia . Initiation of the north-striking right-lateral Straight Creek Fault at ~48 Ma likely resulted from strain accumulated during the accretion of Siletzia. As Siletzia accreted it also jammed the existing subduction zone, halting subduction of the Farallon plate. This terminated the Laramide orogeny that had been uplifting

18126-430: The lower Southern section. Also, the Oregon Coast Range is home to over 50 mammal species, over 100 species of birds, and nearly 30 reptile or amphibian species that spent significant portions of their life cycle in the mountains. Located in the northwest portion of Oregon this section of the range has peaks as high as 3,706 feet (1,130 m) for Rogers Peak . Forests here are considered to be some of

18285-556: The lower stratosphere at very low temperatures. Polar stratospheric clouds have a great impact on radiative forcing. Due to their minimal albedo properties and their optical thickness, polar stratospheric clouds act similar to a greenhouse gas and trap outgoing longwave radiation. Different types of polar stratospheric clouds occur in the atmosphere: polar stratospheric clouds that are created due to interactions with nitric or sulfuric acid and water (Type I) or polar stratospheric clouds that are created with only water ice (Type II). Methane

18444-610: The maximum of global warmth the atmospheric carbon dioxide values were at 700–900 ppm , while model simulations suggest a concentration of 1,680 ppm fits best with deep sea, sea surface, and near-surface air temperatures of the time. Other proxies such as pedogenic (soil building) carbonate and marine boron isotopes indicate large changes of carbon dioxide of over 2,000 ppm over periods of time of less than 1 million years. This large influx of carbon dioxide could be attributed to volcanic out-gassing due to North Atlantic rifting or oxidation of methane stored in large reservoirs deposited from

18603-530: The members of the new mammal orders were small, under 10 kg; based on comparisons of tooth size, Eocene mammals were only 60% of the size of the primitive Palaeocene mammals that preceded them. They were also smaller than the mammals that followed them. It is assumed that the hot Eocene temperatures favored smaller animals that were better able to manage the heat. Rodents were widespread. East Asian rodent faunas declined in diversity when they shifted from ctenodactyloid-dominant to cricetid–dipodid-dominant after

18762-480: The methane, as well as yielding infrared radiation. The breakdown of methane in an atmosphere containing oxygen produces carbon monoxide, water vapor and infrared radiation. The carbon monoxide is not stable, so it eventually becomes carbon dioxide and in doing so releases yet more infrared radiation. Water vapor traps more infrared than does carbon dioxide. At about the beginning of the Eocene Epoch (55.8–33.9 Ma)

18921-569: The middle Lutetian but become completely disparate in the Bartonian, indicating biogeographic separation. Though the North Atlantic was opening, a land connection appears to have remained between North America and Europe since the faunas of the two regions are very similar. Eurasia was separated in three different landmasses 50 Ma; Western Europe, Balkanatolia and Asia. About 40 Ma, Balkanatolia and Asia were connected, while Europe

19080-534: The missing piece may be the Yakutat terrane now at the head of the Gulf of Alaska . Further north, the terrane boundary is believed to come ashore near the Columbia River . The way the Crescent Formation wraps around the Olympic Mountains ("Oly" on the map ) may reflect oroclinal bending as a result of being crushed against Vancouver Island . It has also been attributed to loss of the deposits originally overlying

19239-462: The modern mammal orders appear within a brief period during the early Eocene . At the beginning of the Eocene, several new mammal groups arrived in North America. These modern mammals, like artiodactyls , perissodactyls , and primates , had features like long, thin legs , feet, and hands capable of grasping, as well as differentiated teeth adapted for chewing. Dwarf forms reigned. All

19398-460: The more southerly portions have a more Mediterranean climate that is more similar to the climate of the Willamette Valley. The Coast Range creates a rain shadow effect by forcing moisture laden clouds to rise by expelling moisture. This shields the Willamette Valley and causes a less maritime climate with hotter summers and less precipitation than the Oregon Coast. The Oregon Coast Range

19557-450: The most productive timber land in the world. Trees include primarily Sitka spruce , western redcedar , Douglas-fir , and western hemlock . Other plants include huckleberry, salmonberry, salal, vine maple, Oregon grape , bracken fern, and thimble-berry among others. The northern boundary is the Columbia River , with some mountainous features on the north side of the river, and continues south for approximately 100 miles (160 km) to

19716-533: The northern and southern ends, and a date of 49 Ma for the Grays River volcanics near the center of Siletzia. This is suggestive of a spreading ridge (as previously noted by McWilliams 1980 ), and has been a strong constraint on models of how Siletzia formed. Other researchers have since found younger dates (50-48 Ma) for the Crescent basalts, removing much of the age symmetry. Dating from 2010 based on Ar-Ar, U-Pb (uranium-lead), and coccoliths shows

19875-664: The ocean. Recent analysis of and research into these hyperthermals in the early Eocene has led to hypotheses that the hyperthermals are based on orbital parameters, in particular eccentricity and obliquity. The hyperthermals in the early Eocene, notably the Palaeocene–Eocene Thermal Maximum (PETM), the Eocene Thermal Maximum 2 (ETM2), and the Eocene Thermal Maximum 3 (ETM3), were analyzed and found that orbital control may have had

20034-416: The orbital parameters were theorized as a possible control on continental temperatures and seasonality. Simulating the Eocene by using an ice free planet, eccentricity , obliquity , and precession were modified in different model runs to determine all the possible different scenarios that could occur and their effects on temperature. One particular case led to warmer winters and cooler summer by up to 30% in

20193-609: The other is along the Leech River Fault off the southern end of Vancouver Island , where it has pushed the pre- Cenozoic Pacific Rim formation beneath the Wrangellia Terrane ). Everywhere else the contact between Siletzia and the rest of the continent is concealed under younger deposits, especially the Cascade Range volcanics. The contact around the Olympic Mountains is actually the bottom contact with

20352-486: The overlying basalts are older, and therefore the Blue Mountain unit was not overlaid by the basalts, but thrust under them at a later date. Such under-thrusting implies that the northern end of Siletzia was initially further away from the continent, and permits radial motion about a more southerly or more easterly pivot near the Washington-Oregon border, as recently suggested. This model has Siletzia forming on

20511-416: The period progressed; the heat-loving tropical flora was wiped out, and by the beginning of the Oligocene, the continent hosted deciduous forests and vast stretches of tundra . During the Eocene, plants and marine faunas became quite modern. Many modern bird orders first appeared in the Eocene. The Eocene oceans were warm and teeming with fish and other sea life. The oldest known fossils of most of

20670-448: The pioneering work by Dickinson & Snyder 1979 ). Breitsprecher et al. (2003) subsequently identified the fan-shaped wake of volcanics of distinctive geochemistry left by the widening Kula-Farallon slab window across northeastern Washington and into Idaho. Madsen et al. (2006) showed that most of the Eocene and subsequent magmatism from Alaska to Oregon "is explainable in terms of ridge subduction and slab window tectonics". That is,

20829-406: The plate motion model it was based on was shown to be faulty. Wells, et al., alternately suggested that as a terrane at the margin of the continent was pushed over the Yellowstone hotspot, it was rifted away from the continent by the upwelling magma , which then formed the Siletzia basalts. This idea was further developed by Babcock et al. (1992) , who suggested rifting might have been initiated by

20988-438: The polar stratospheric clouds effects on the Eocene climate, models were run comparing the effects of polar stratospheric clouds at the poles to an increase in atmospheric carbon dioxide. The polar stratospheric clouds had a warming effect on the poles, increasing temperatures by up to 20 °C in the winter months. A multitude of feedbacks also occurred in the models due to the polar stratospheric clouds' presence. Any ice growth

21147-649: The pre-Cenozoic rock of Vancouver Island and the northern Cascade Range. At the southern end are sediments derived from the Klamath Mountains, while sand of the overlying Tyee Formation has an isotopic composition corresponding to rock of the Idaho Batholith . Eruption of the Siletzia basalts has been placed roughly in the late Paleocene through the mid Eocene ; more specific dates have been difficult to obtain and somewhat variable. Early K-Ar (potassium-argon) and Ar-Ar (argon-argon) radiometric dating by Duncan gave dates of 57 and 62  Ma (million years ago) to

21306-650: The range are inside the Siuslaw National Forest along with three designated wilderness areas: Drift Creek Wilderness , Cummins Creek Wilderness and the Rock Creek Wilderness . Larger animals that live in these section include deer, elk, bobcat, and bear. Bear are black bear while deer are mule and black-tailed deer species. Other mammals here are mountain beaver , beavers, coyote, mink, river otter , mountain lion, porcupines, skunks, and brush rabbit. The southernmost section of

21465-458: The reduction in carbon dioxide during the warming to cooling transition was the azolla event . With the equable climate during the early Eocene, warm temperatures in the arctic allowed for the growth of azolla , which is a floating aquatic fern, on the Arctic Ocean . The significantly high amounts of carbon dioxide also acted to facilitate azolla blooms across the Arctic Ocean. Compared to current carbon dioxide levels, these azolla grew rapidly in

21624-459: The region. One possible cause of atmospheric carbon dioxide increase could have been a sudden increase due to metamorphic release due to continental drift and collision of India with Asia and the resulting formation of the Himalayas ; however, data on the exact timing of metamorphic release of atmospheric carbon dioxide is not well resolved in the data. Recent studies have mentioned, however, that

21783-445: The removal of the ocean between Asia and India could have released significant amounts of carbon dioxide. Another hypothesis still implicates a diminished negative feedback of silicate weathering as a result of continental rocks having become less weatherable during the warm Early and Middle Eocene, allowing volcanically released carbon dioxide to persist in the atmosphere for longer. Yet another explanation hypothesises that MECO warming

21942-418: The rotation is slightly counter-clockwise. On Vancouver Island the paleorotations are counter-clockwise, and other evidence shows that the tip of the island has been bent, presumably as a result of the collision of Siletzia. The northwestern tip of the Olympic Peninsula also shows counter-clockwise rotation, of around 45 degrees. This raises a question of how much of the arcuate shape of the Crescent Formation

22101-469: The same isotopic composition of rock in the Idaho Batholith (and of sand now coursing down the Snake and Columbia Rivers), but it appears to have not been transported very far from its source. This implies that the Tyee Formation was much closer to the Idaho Batholith when it was deposited, and subsequently rotated away. Geodetic surveys show that the region continues to rotate, likely as a result of extension of

22260-545: The southern tip of Vancouver Island was described in a series of reports (1910, 1912, 1913, 1917) by Clapp, who recognized it as correlative with the Crescent formation on the other side of the Strait of Juan de Fuca . Weaver recognized that these "Metchosin volcanics" included various Eocene basalts in western Washington and the Oregon Coast Range as far south as the Klamath Mountains . The Siletz River Volcanics

22419-510: The spreading ridge is subducted. This implies formation at the continental margin, and then rifting, in the manner of the second class of models. Any model of the origin of Siletzia must account for interactions with plate boundaries that were being subducted under North America through the Eocene. Early studies were plagued by indeterminate locations for these boundaries, particularly of the Kula-Farallon (K-F) spreading ridge : basalts at

22578-541: The streams and rivers flowing through the range. Volcanic activity approximately 66 million years ago in the Cretaceous Period created offshore islands beginning in the southern portion of the current range. These Roseburg volcanics were followed by the Siletz River Volcanics in the northern portions of the range, and lastly a series of basalt flows from the Columbia River basalts also added to these formations with some smaller flows in-between. Much of

22737-427: The subduction of the Kula-Farallon (or possibly Farallon—Resurrection) spreading ridge . The relation with the Kula-Farallon spreading ridge is an important element in all models, though its location through that epoch is not well determined. Seeking to explain the observed clockwise paleorotation, and noting that Siletzia appeared to have rotated as a rigid block, Simpson & Cox (1977) proposed two models. First

22896-527: The time from the end of the Paleocene Epoch to the beginning of the Oligocene Epoch. The start of the Eocene is marked by a brief period in which the concentration of the carbon isotope C in the atmosphere was exceptionally low in comparison with the more common isotope C . The average temperature of Earth at the beginning of the Eocene was about 27 degrees Celsius. The end is set at

23055-399: The tropics that would require much higher average temperatures to sustain them. TEX 86 BAYSPAR measurements indicate extremely high sea surface temperatures of 40 °C (104 °F) to 45 °C (113 °F) at low latitudes, although clumped isotope analyses point to a maximum low latitude sea surface temperature of 36.3 °C (97.3 °F) ± 1.9 °C (35.4 °F) during

23214-467: The trough. The Crescent terrane (under Washington) is believed to be thinner, from as little as 12 and 22 km under the western and eastern ends of the Strait of Juan de Fuca , but possibly as much as 20 and 35 km thick. The various formations of Siletzia are characterized as marine tholeiitic pillow basalts and volcanic breccia , often interbedded with sedimentary layers of continental origin, lying on oceanic crust. These are usually capped by

23373-531: The underlying oceanic sediments, tilted up by the uplift of the Olympics and exposed by erosion of about 10 to 12 km of overlying deposits. The location of the near-surface contact between the Crescent Formation and the pre-Cenozoic metamorphic basement of the continent — what has been the termed the Coast Range Boundary Fault (CRBF) — is largely uncertain. The Leech River Fault extends southeast past Victoria, B.C. to cross

23532-484: The volume at 2 million cubic km. When lava solidifies and cools it retains an imprint of the Earth's magnetic field , thus recording how it was oriented. Measurements of such paleomagnetic fields in the Oregon Coast Range show rotations of 46 to 75°, all of it following the presumed accretion to the continent (alternately, formation) of the Siletz terrane at about 50 Ma. These rotations are all clockwise, and show

23691-463: The world became more arid and cold over the course of the stage, such as the Fushun Basin. In East Asia, lake level changes were in sync with global sea level changes over the course of the MLEC. Global cooling continued until there was a major reversal from cooling to warming in the Bartonian. This warming event, signifying a sudden and temporary reversal of the cooling conditions, is known as

23850-528: Was a major step into the icehouse climate. Multiple proxies, such as oxygen isotopes and alkenones , indicate that at the Eocene–Oligocene transition, the atmospheric carbon dioxide concentration had decreased to around 750–800 ppm, approximately twice that of present levels . Along with the decrease of atmospheric carbon dioxide reducing the global temperature, orbital factors in ice creation can be seen with 100,000-year and 400,000-year fluctuations in benthic oxygen isotope records. Another major contribution to

24009-476: Was caused by the simultaneous occurrence of minima in both the 400 kyr and 2.4 Myr eccentricity cycles. During the MECO, sea surface temperatures in the Tethys Ocean jumped to 32–36 °C, and Tethyan seawater became more dysoxic. A decline in carbonate accumulation at ocean depths of greater than three kilometres took place synchronously with the peak of the MECO, signifying ocean acidification took place in

24168-465: Was connected 34 Ma. The Fushun Basin contained large, suboxic lakes known as the paleo-Jijuntun Lakes. India collided with Asia , folding to initiate formation of the Himalayas . The incipient subcontinent collided with the Kohistan–Ladakh Arc around 50.2 Ma and with Karakoram around 40.4 Ma, with the final collision between Asia and India occurring ~40 Ma. The Eocene Epoch contained

24327-436: Was demonstrated by a recent study that showed, on the basis of geochemistry , that the Grays River volcanics followed the Siletzia eruptions, and thus are not representative of the initial phase of Siletz magmatism. Recent dating also shows a more monotonic trend of south to north age progression ("younging"). The range of the original ages was also a problem, as the rate of Kula-Farallon spreading over that time would produce

24486-608: Was described in 1948 by Snavely and Baldwin after exposures near the Siletz River, Oregon, and the Roseburg and related formations in southern Oregon described in various reports from the 1960s on. "Siletzia" was coined in 1979 by Irving, (based in turn on the Siletz River and the Siletz Reservation ), to describe the full extent of these Eocene basalts and interbedded sedimentary formations. The map shows

24645-402: Was determined that in order to maintain the lower stratospheric water vapor, methane would need to be continually released and sustained. In addition, the amounts of ice and condensation nuclei would need to be high in order for the polar stratospheric cloud to sustain itself and eventually expand. The Eocene is not only known for containing the warmest period during the Cenozoic; it also marked

24804-560: Was formed at a latitude corresponding to Oregon or northern California. Similarly, certain schists on Baranof Island are believed to have been contiguous with the Leech River Schists ( Leech River Complex ) on Vancouver Island around 50 Ma, and subsequently transported northward with other elements of the Chugach-Prince William terrane. Whether formed far offshore as seamounts, or close inshore by

24963-401: Was present on Earth with a smaller difference in temperature from the equator to the poles . Because of this the maximum sea level was 150 meters higher than current levels. Following the maximum was a descent into an icehouse climate from the Eocene Optimum to the Eocene–Oligocene transition at 34 Ma. During this decrease, ice began to reappear at the poles, and the Eocene–Oligocene transition

25122-511: Was rotation about a southern pivot in contact with the Klamath Mountains. This has various problems, especially because at the northern end sediments and even boulders from the continent are found at the base of the Crescent Formation, showing that it was near the continent from the beginning. In the second model (subsequently refined by Hammond 1979 ), Siletzia was originally adjacent to the Olympic–Wallowa Lineament, then rifted from

25281-437: Was slowed immensely and would lead to any present ice melting. Only the poles were affected with the change in temperature and the tropics were unaffected, which with an increase in atmospheric carbon dioxide would also cause the tropics to increase in temperature. Due to the warming of the troposphere from the increased greenhouse effect of the polar stratospheric clouds, the stratosphere would cool and would potentially increase

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