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96-696: The mean annual flow of the Dolores prior to damming was approximately 1,200 cu ft/s (34 m/s), but due to diversions it has been reduced to about 600 cu ft/s (17 m/s). The Dolores River rises in a meadow called Tin Can Basin, near 12,520-foot (3,820 m) Hermosa Peak in the San Miguel Mountains , in Dolores County, Colorado . The headwaters are located about 5 miles (8.0 km) south of Lizard Head Pass in

192-664: A national monument encompassing 400,000 acres of the Dolores River Canyon and the surrounding region in Mesa and Montrose Counties. There are also proposals for a smaller 30,000-acre national conservation area in the northern portion (Mesa and Montrose Counties) and a 68,000-acre national conservation area and special management area in the southern portion (San Miguel, Dolores, and Montezuma Counties). [REDACTED] Media related to Dolores River at Wikimedia Commons San Miguel Mountains The San Juan Mountains

288-711: A 70 km diameter impact structure buried beneath the Kalahari desert in northern South Africa. The impact is dated to the Tithonian, approximately 146.06 ± 0.16 Mya. Another major structure is the Puchezh-Katunki crater , 40 kilometres in diameter, buried beneath Nizhny Novgorod Oblast in western Russia. The impact has been dated to the Sinemurian, 195.9 ± 1.0 Ma. At the beginning of the Jurassic, all of

384-509: A controversial topic. Currently, the Bureau of Reclamation operates McPhee on a "fill, then spill" policy, where the dam is filled first, and only in high water years when inflows exceed the reservoir capacity are larger flows released. This fact aggrieves recreational boaters, who claim it is difficult for anyone but a local resident closely watching the gauges to plan trips in advance on the river. The San Juan Citizens' Alliance has worked to start

480-567: A dialogue between Dolores River stakeholders in the hopes of shifting release policy to one that allows for greater, planned releases. The Dolores is navigable by rafts and kayaks (up to class IV) from McPhee Reservoir to its confluence with Colorado. When water is restricted from the reservoir, it may be possible to boat downstream from the San Miguel River. However, the river is extensively used for irrigation and, during low water years, can be wholly unnavigable. In high-runoff years,

576-499: A dominant component of Jurassic floras. The Late Triassic and Jurassic was a major time of diversification of conifers, with most modern conifer groups appearing in the fossil record by the end of the Jurassic, having evolved from voltzialean ancestors. Araucarian conifers have their first unambiguous records during the Early Jurassic, and members of the modern genus Araucaria were widespread across both hemispheres by

672-647: A kink in one of the plate boundaries, resulting in the formation of the Pacific Plate at the centre of the junction. During the Middle to early Late Jurassic, the Sundance Seaway , a shallow epicontinental sea , covered much of northwest North America. The eustatic sea level is estimated to have been close to present levels during the Hettangian and Sinemurian, rising several tens of metres during

768-607: A popular destination for endurance sports like trail running and mountain biking; Silverton is home to the Hardrock 100 Endurance Run one of the most difficult and technical ultra-marathons in the world. The Rio Grande drains the east side of the range. The other side of the San Juans, the western slope of the continental divide , is drained by tributaries of the San Juan , Dolores and Gunnison rivers, which all flow into

864-453: A result of high ocean acidity and temperature inhibiting its mineralisation into apatite; the abundance of phosphorus in marine environments caused further eutrophication and consequent anoxia in a positive feedback loop. The end-Jurassic transition was originally considered one of eight mass extinctions, but is now considered to be a complex interval of faunal turnover, with the increase in diversity of some groups and decline in others, though

960-599: A result, facilities in the towns and cities of the region are among the highest in the nation. Telluride Airport , at an elevation of 9,070 feet, is the highest in the United States with regularly scheduled commercial service. Human history at the eastern edge of the San Juan Mountains dates back thousands of years. Smithsonian archaeologists have identified paleo-Indian sites that are about 5,000 years old, located near Great Sand Dunes National Park and at

1056-492: Is Austrohamia minuta from the Early Jurassic (Pliensbachian) of Patagonia, known from many parts of the plant. The reproductive structures of Austrohamia have strong similarities to those of the primitive living cypress genera Taiwania and Cunninghamia . By the Middle to Late Jurassic Cupressaceae were abundant in warm temperate–tropical regions of the Northern Hemisphere, most abundantly represented by

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1152-755: Is a geologic period and stratigraphic system that spanned from the end of the Triassic Period 201.4 million years ago (Mya) to the beginning of the Cretaceous Period, approximately 145 Mya. The Jurassic constitutes the middle period of the Mesozoic Era as well as the eighth period of the Phanerozoic Eon and is named after the Jura Mountains , where limestone strata from the period were first identified. The start of

1248-608: Is a high and rugged mountain range in the Rocky Mountains in southwestern Colorado and northwestern New Mexico . The area is highly mineralized (the Colorado Mineral Belt ) and figured in the gold and silver mining industry of early Colorado. Major towns, all old mining camps , include Creede , Lake City , Silverton , Ouray , and Telluride . Large scale mining has ended in the region, although independent prospectors still work claims throughout

1344-447: Is a major part of the regional economy, with the narrow gauge railway between Durango and Silverton being an attraction mostly in the summer and fall. Off-roading is popular on the old trails which linked the historic mining camps, including the notorious Black Bear Road . Visiting old ghost towns is popular, as is wilderness trekking and mountain climbing . Many of the old mining camps are popular sites of summer homes . Though

1440-804: Is believed to have flowed south to join the San Juan River near the Four Corners in what is now northwestern New Mexico . The uplift of Sleeping Ute Mountain about 70 million years ago diverted the Dolores River to its present northward course, causing it to carve the Dolores River Canyon on its way to the Colorado River, creating unusual geologic features such as the Paradox Valley. The Dolores Canyon exposes rocks ranging from 300-million-year-old Pennsylvanian limestone to

1536-494: Is divided into three epochs : Early, Middle, and Late. Similarly, in stratigraphy , the Jurassic is divided into the Lower Jurassic , Middle Jurassic , and Upper Jurassic series . Geologists divide the rocks of the Jurassic into a stratigraphic set of units called stages , each formed during corresponding time intervals called ages. Stages can be defined globally or regionally. For global stratigraphic correlation,

1632-565: Is followed by the Kimmeridgian Warm Interval (KWI) between 164 and 150 million years ago. Based on fossil wood distribution, this was one of the wettest intervals of the Jurassic. The Pangaean interior had less severe seasonal swings than in previous warm periods as the expansion of the Central Atlantic and Western Indian Ocean provided new sources of moisture. A prominent drop in temperatures occurred during

1728-470: Is fourth order, with a periodicity of approximately 410,000 years. During the Early Jurassic the world's oceans transitioned from an aragonite sea to a calcite sea chemistry, favouring the dissolution of aragonite and precipitation of calcite . The rise of calcareous plankton during the Middle Jurassic profoundly altered ocean chemistry, with the deposition of biomineralized plankton on

1824-555: Is linked to the Jura Mountains , a forested mountain range that mainly follows the France–Switzerland border . The name "Jura" is derived from the Celtic root * jor via Gaulish *iuris "wooded mountain", which was borrowed into Latin as a name of a place and evolved into Juria and finally Jura . During a tour of the region in 1795, German naturalist Alexander von Humboldt recognized carbonate deposits within

1920-475: Is marked by a globally documented high amplitude negative carbon isotope excursion, as well as the deposition of black shales and the extinction and collapse of carbonate-producing marine organisms, associated with a major rise in global temperatures. The TOAE is often attributed to the eruption of the Karoo-Ferrar large igneous provinces and the associated increase of carbon dioxide concentration in

2016-577: Is named after the city of Oxford in England and was named by Alcide d'Orbigny in 1844 in reference to the Oxford Clay . The base of the Oxfordian lacks a defined GSSP. W. J. Arkell in studies in 1939 and 1946 placed the lower boundary of the Oxfordian as the first appearance of the ammonite Quenstedtoceras mariae (then placed in the genus Vertumniceras ). Subsequent proposals have suggested

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2112-707: Is the setting of the world's largest oil reserves, including the Ghawar Field , the world's largest oil field. The Jurassic-aged Sargelu and Naokelekan formations are major source rocks for oil in Iraq . Over 1500 gigatons of Jurassic coal reserves are found in north-west China, primarily in the Turpan-Hami Basin and the Ordos Basin . Major impact structures include the Morokweng impact structure ,

2208-622: Is used to mark the beginnings of stages, as well as smaller timespans within stages, referred to as "ammonite zones"; these, in turn, are also sometimes subdivided further into subzones. Global stratigraphy is based on standard European ammonite zones, with other regions being calibrated to the European successions. The oldest part of the Jurassic Period has historically been referred to as the Lias or Liassic, roughly equivalent in extent to

2304-519: The Colorado River . The San Juan and Uncompahgre National Forests cover a large portion of the San Juan Mountains. The Continental Divide Trail , a long-distance hiking trail follows the crest of the San Juan Mountains. The Weminuche Wilderness , Colorado's largest wilderness area, with an area of 499,771 acres (202,250 ha) is in the San Juans. The San Juan Mountains are also distinctive for their high altitude plateaus and peaks. As

2400-492: The Dolores River Canyon , which stretches north for over 40 miles (64 km) and averages 1,100 feet (340 m) deep. This section of the Dolores River is noted for its exposed sedimentary strata , desert wildlife, and during years of heavy snowmelt for its whitewater. Near Egnar the river crosses into San Miguel County and then from there into Montrose County . Continuing north, the Dolores cuts across

2496-513: The International Commission on Stratigraphy (ICS) ratify global stages based on a Global Boundary Stratotype Section and Point (GSSP) from a single formation (a stratotype ) identifying the lower boundary of the stage. The ages of the Jurassic from youngest to oldest are as follows: Jurassic stratigraphy is primarily based on the use of ammonites as index fossils . The first appearance datum of specific ammonite taxa

2592-603: The Paradox Valley which runs in an unusual transverse direction to the river. Immediately below Paradox Valley it is joined by the San Miguel River , its main tributary, from the east. (Incidentally, the Dolores and San Miguel have their headwaters to either side of Lizard Head Pass.) Due to diversions on the main stem, the San Miguel is typically the same size as the Dolores if not larger, providing most of

2688-728: The Redcar Mudstone Formation , and was ratified in 2005. The beginning of the Pliensbachian is defined by the first appearance of the ammonite Bifericeras donovani . The village Thouars (Latin: Toarcium ), just south of Saumur in the Loire Valley of France , lends its name to the Toarcian Stage. The Toarcian was named by Alcide d'Orbigny in 1842, with the original locality being Vrines quarry around 2 km northwest of Thouars. The GSSP for

2784-610: The San Juan National Forest . The river flows southwest in a canyon past Rico , receiving the West Dolores River , then flows into McPhee Reservoir near Dolores in Montezuma County . Formed by McPhee Dam, the reservoir is about 10 miles (16 km) long and diverts flows of the upper Dolores River for irrigation. Downstream from McPhee Dam, the river re-enters Dolores County and carves

2880-537: The Uncompahgre Plateau was formed, it diverted the larger Colorado northwards through what is now the Grand Valley , looping around through Westwater Canyon to the confluence with the Dolores in eastern Utah and leaving Unaweep Canyon as a huge dry gap across the plateau. However, some geologists contend that Colorado never flowed through Unaweep and the lower Dolores River, as the erosive force of

2976-470: The 140-million-year-old Entrada sandstone deposited during the Jurassic . A cap of Cretaceous Dakota sandstone forms most of the upper rim of the canyon. It also includes red Wingate Sandstone . The lower Dolores River may have once been the original course of the Colorado River, which flowed through the now dry Unaweep Canyon , currently occupied by West Creek, a small tributary of the Dolores. When

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3072-537: The 1990s when the liner of a cyanide -laced tailing pond began leaking heavily. Summitville is in the Summitville caldera , one of many extinct volcanoes making up the San Juan volcanic field . One, La Garita Caldera , is 35 miles (56 km) in diameter. Large beds of lava , some extending under the floor of the San Luis Valley , are characteristic of the eastern slope of the San Juans. Tourism

3168-600: The 27 years from October 1986 (the year McPhee Dam was completed) to October 2013, the river averaged only 599 cubic feet per second (17.0 m/s) due to the McPhee Dam diversions. Measured at Bedrock, Colorado , at the entrance to Paradox Valley (above the San Miguel confluence) the effect of the flow reductions is more obvious, with an average of 504 cubic feet per second (14.3 m/s) before September 1984 as compared to 240 cubic feet per second (6.8 m/s) between October 1984 and May 2014. The ancestral Dolores River

3264-636: The Aalenian, precessionally forced climatic changes dictated peatland wildfire magnitude and frequency. The European climate appears to have become noticeably more humid at the Aalenian-Bajocian boundary but then became more arid during the middle Bajocian. A transient ice age possibly occurred in the late Bajocian. The Callovian-Oxfordian boundary at the end of the MJCI witnessed particularly notable global cooling, potentially even an ice age. This

3360-748: The Bajocian Stage after the town of Bayeux (Latin: Bajoce ) in Normandy, France. The GSSP for the base of the Bajocian is located in the Murtinheira section at Cabo Mondego , Portugal; it was ratified in 1997. The base of the Bajocian is defined by the first appearance of the ammonite Hyperlioceras mundum . The Bathonian is named after the city of Bath , England, introduced by Belgian geologist d'Omalius d'Halloy in 1843, after an incomplete section of oolitic limestones in several quarries in

3456-593: The Black Jurassic and the overlying clayey sandstone and ferruginous oolite of the Brown Jurassic sequences of southwestern Germany. The GSSP for the base of the Aalenian is located at Fuentelsaz in the Iberian range near Guadalajara, Spain , and was ratified in 2000. The base of the Aalenian is defined by the first appearance of the ammonite Leioceras opalinum . Alcide d'Orbigny in 1842 named

3552-560: The Black Jurassic in England by William Conybeare and William Phillips in 1822. William Phillips, the geologist, worked with William Conybeare to find out more about the Black Jurassic in England. The French palaeontologist Alcide d'Orbigny in papers between 1842 and 1852 divided the Jurassic into ten stages based on ammonite and other fossil assemblages in England and France, of which seven are still used, but none has retained its original definition. The German geologist and palaeontologist Friedrich August von Quenstedt in 1858 divided

3648-655: The Callovian–Oxfordian boundary, peaking possibly as high as 140 metres above present sea level at the Kimmeridgian–Tithonian boundary. The sea levels falls in the late Tithonian, perhaps to around 100 metres, before rebounding to around 110 metres at the Tithonian–Berriasian boundary. The sea level within the long-term trends across the Jurassic was cyclical, with 64 fluctuations, 15 of which were over 75 metres. The most noted cyclicity in Jurassic rocks

3744-464: The Early Jurassic, but also including part of the preceding Rhaetian . The Hettangian Stage was named by Swiss palaeontologist Eugène Renevier in 1864 after Hettange-Grande in north-eastern France. The GSSP for the base of the Hettangian is located at the Kuhjoch Pass, Karwendel Mountains , Northern Calcareous Alps , Austria; it was ratified in 2010. The beginning of the Hettangian, and thus

3840-519: The Greek goddess of dawn . His name was chosen by Albert Oppel for this stratigraphical stage because the Tithonian finds itself hand in hand with the dawn of the Cretaceous. The base of the Tithonian currently lacks a GSSP. The working definition for the base of the Tithonian is the first appearance of the ammonite genus Gravesia . The upper boundary of the Jurassic is currently undefined, and

3936-792: The Jura Mountains as geologically distinct from the Triassic aged Muschelkalk of southern Germany , but he erroneously concluded that they were older. He then named them Jura-Kalkstein ('Jura limestone') in 1799. In 1829, the French naturalist Alexandre Brongniart published a book entitled Description of the Terrains that Constitute the Crust of the Earth or Essay on the Structure of

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4032-483: The Jurassic as a whole, is marked by the first appearance of the ammonite Psiloceras spelae tirolicum in the Kendlbach Formation exposed at Kuhjoch. The base of the Jurassic was previously defined as the first appearance of Psiloceras planorbis by Albert Oppel in 1856–58, but this was changed as the appearance was seen as too localised an event for an international boundary. The Sinemurian Stage

4128-655: The Jurassic was marked by the major Triassic–Jurassic extinction event , associated with the eruption of the Central Atlantic Magmatic Province (CAMP). The beginning of the Toarcian Age started around 183 million years ago and is marked by the Toarcian Oceanic Anoxic Event , a global episode of oceanic anoxia , ocean acidification , and elevated global temperatures associated with extinctions, likely caused by

4224-457: The Jurassic was warmer than the present, and there were no ice caps . Forests grew close to the poles, with large arid expanses in the lower latitudes. On land, the fauna transitioned from the Triassic fauna, dominated jointly by dinosauromorph and pseudosuchian archosaurs , to one dominated by dinosaurs alone. The first stem-group birds appeared during the Jurassic, evolving from a branch of theropod dinosaurs. Other major events include

4320-487: The Jurassic, both the North and South Pole were covered by oceans. Beginning in the Early Jurassic, the Boreal Ocean was connected to the proto-Atlantic by the "Viking corridor" or Transcontinental Laurasian Seaway, a passage between the Baltic Shield and Greenland several hundred kilometers wide. During the Callovian, the Turgai Epicontinental Sea formed, creating a marine barrier between Europe and Asia. Madagascar and Antarctica began to rift away from Africa during

4416-437: The Jurassic, the North Atlantic Ocean remained relatively narrow, while the South Atlantic did not open until the Cretaceous. The continents were surrounded by Panthalassa , with the Tethys Ocean between Gondwana and Asia. At the end of the Triassic, there was a marine transgression in Europe, flooding most parts of central and western Europe transforming it into an archipelago of islands surrounded by shallow seas. During

4512-510: The Jurassic–Cretaceous boundary is currently the only system boundary to lack a defined GSSP. Placing a GSSP for this boundary has been difficult because of the strong regionality of most biostratigraphic markers, and lack of any chemostratigraphic events, such as isotope excursions (large sudden changes in ratios of isotopes ), that could be used to define or correlate a boundary. Calpionellids , an enigmatic group of planktonic protists with urn-shaped calcitic tests briefly abundant during

4608-535: The Known Lands of the Earth. In this book, Brongniart used the phrase terrains jurassiques when correlating the "Jura-Kalkstein" of Humboldt with similarly aged oolitic limestones in Britain, thus coining and publishing the term "Jurassic". The German geologist Leopold von Buch in 1839 established the three-fold division of the Jurassic, originally named from oldest to the youngest: the Black Jurassic , Brown Jurassic , and White Jurassic . The term " Lias " had previously been used for strata of equivalent age to

4704-407: The Late Jurassic (Kimmeridgian) of Scotland, which remains the only known unequivocal fossil of the group before the Cretaceous. Despite being the earliest known member of the Pinaceae, Eathiestrobus appears to be a member of the pinoid clade of the family, suggesting that the initial diversification of Pinaceae occurred earlier than has been found in the fossil record. The earliest record of

4800-416: The Middle Jurassic. Also abundant during the Jurassic is the extinct family Cheirolepidiaceae , often recognised through their highly distinctive Classopolis pollen. Jurassic representatives include the pollen cone Classostrobus and the seed cone Pararaucaria . Araucarian and Cheirolepidiaceae conifers often occur in association. The oldest definitive record of the cypress family ( Cupressaceae )

4896-445: The Middle to Late Jurassic, corresponding to the increasing aridity of the region. Ginkgoales , of which the sole living species is Ginkgo biloba , were more diverse during the Jurassic: they were among the most important components of Eurasian Jurassic floras and were adapted to a wide variety of climatic conditions. The earliest representatives of the genus Ginkgo , represented by ovulate and pollen organs similar to those of

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4992-405: The North Atlantic Ocean with eastern Panthalassa. Palaeontological data suggest that the seaway had been open since the Early Jurassic. As part of the Nevadan orogeny , which began during the Triassic, the Cache Creek Ocean closed, and various terranes including the large Wrangellia Terrane accreted onto the western margin of North America. By the Middle Jurassic the Siberian plate and

5088-462: The North China-Amuria block had collided, resulting in the closure of the Mongol-Okhotsk Ocean . During the Early Jurassic, around 190 million years ago, the Pacific Plate originated at the triple junction of the Farallon , Phoenix , and Izanagi tectonic plates , the three main oceanic plates of Panthalassa. The previously stable triple junction had converted to an unstable arrangement surrounded on all sides by transform faults because of

5184-400: The San Juans are extremely steep and receive much snow, so far only Telluride has made the transition to a major ski resort . Purgatory Resort , once known as Durango Mountain Resort, is a small ski area 26 miles north of Durango. There is also skiing on Wolf Creek Pass at the Wolf Creek ski area . Recently Silverton Mountain ski area has begun operation near Silverton. The range is also

5280-502: The TOAE represented one of the most severe extinctions in their evolutionary history. While the event had significant impact on marine invertebrates, it had little effect on marine reptiles. During the TOAE, the Sichuan Basin was transformed into a giant lake , probably three times the size of modern-day Lake Superior , represented by the Da'anzhai Member of the Ziliujing Formation . The lake likely sequestered ~460 gigatons (Gt) of organic carbon and ~1,200 Gt of inorganic carbon during

5376-414: The Tithonian, known as the Early Tithonian Cooling Event (ETCE). The end of the Jurassic was marked by the Tithonian–early Barremian Cool Interval (TBCI), beginning 150 million years ago and continuing into the Early Cretaceous . The Toarcian Oceanic Anoxic Event (TOAE), also known as the Jenkyns Event, was an episode of widespread oceanic anoxia during the early part of the Toarcian Age, c. 183 Mya. It

5472-529: The Toarcian around 174 million years ago. During the Toarcian Warm Interval, ocean surface temperatures likely exceeded 30 °C (86 °F), and equatorial and subtropical (30°N–30°S) regions are likely to have been extremely arid, with temperatures in the interior of Pangea likely in excess of 40 °C (104 °F).The Toarcian Warm Interval is followed by the Middle Jurassic Cool Interval (MJCI) between 174 and 164 million years ago, which may have been punctuated by brief, ephemeral icehouse intervals. During

5568-489: The appearance of the earliest crabs and modern frogs , salamanders and lizards . Mammaliaformes , one of the few cynodont lineages to survive the end of the Triassic, continued to diversify throughout the period, with the Jurassic seeing the emergence of the first crown group mammals . Crocodylomorphs made the transition from a terrestrial to an aquatic life. The oceans were inhabited by marine reptiles such as ichthyosaurs and plesiosaurs , while pterosaurs were

5664-428: The atmosphere, as well as the possible associated release of methane clathrates . This likely accelerated the hydrological cycle and increased silicate weathering , as evidenced by an increased amount of organic matter of terrestrial origin found in marine deposits during the TOAE. Groups affected include ammonites, ostracods , foraminifera , bivalves , cnidarians , and especially brachiopods , for which

5760-407: The base of the Toarcian is located at Peniche, Portugal , and was ratified in 2014. The boundary is defined by the first appearance of ammonites belonging to the subgenus Dactylioceras ( Eodactylites ) . The Aalenian is named after the city of Aalen in Germany. The Aalenian was defined by Swiss geologist Karl Mayer-Eymar in 1864. The lower boundary was originally between the dark clays of

5856-454: The confluence with the Colorado River, the average unimpaired discharge of the Dolores River between 1906 and 1995 was 841,000 acre-feet (1.037 km), or about 1,160 cubic feet per second (33 m/s). The United States Geological Survey has operated a stream gage at Cisco from 1950 to the present. For the 36-year period December 1950 to September 1986, the river flow at Cisco averaged 845 cubic feet per second (23.9 m/s). By contrast, in

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5952-442: The dam has reduced and sometimes completely halted spring peak flows in the lower Dolores, it provides supplemental flows in late summer (August through October) in the range of 75 cubic feet per second (2.1 m/s), maintaining downstream fisheries. Before the dam was built, irrigators diverted nearly the entire river flow, leaving as little as 10 cubic feet per second (0.28 m/s) to flow downstream. Releases from McPhee Dam are

6048-447: The dominant flying vertebrates . Modern sharks and rays first appeared and diversified during the period, while the first known crown-group teleost fish appeared near the end of the period. The flora was dominated by ferns and gymnosperms , including conifers , of which many modern groups made their first appearance during the period, as well as other groups like the extinct Bennettitales . The chronostratigraphic term "Jurassic"

6144-428: The eight-hour day , which had been approved in a referendum by 72 percent of Colorado voters . The new association consolidated the power of thirty-six mining properties in San Miguel , Ouray , and San Juan counties. The SJDMA refused to consider any reduction in hours or increase in wages, helping to provoke a bitter strike. Jurassic The Jurassic ( / dʒ ʊ ˈ r æ s ɪ k / juurr- ASS -ik )

6240-616: The end of the Jurassic. The oldest unambiguous members of Podocarpaceae are known from the Jurassic, found across both hemispheres, including Scarburgia and Harrisiocarpus from the Middle Jurassic of England, as well as unnamed species from the Middle-Late Jurassic of Patagonia. During the Early Jurassic, the flora of the mid-latitudes of Eastern Asia were dominated by the extinct deciduous broad leafed conifer Podozamites , which appears to not be closely related to any living family of conifer. Its range extended northwards into polar latitudes of Siberia and then contracted northward in

6336-413: The end of the Triassic, the Peltaspermaceae became extinct in most parts of the world, with Lepidopteris persisting into the Early Jurassic in Patagonia. Dicroidium , a corystosperm seed fern that was a dominant part of Gondwanan floral communities during the Triassic, also declined at the Triassic–Jurassic boundary, surviving as a relict in Antarctica into the Early Jurassic. Conifers formed

6432-405: The eruption of the Karoo-Ferrar large igneous provinces . The end of the Jurassic, however, has no clear, definitive boundary with the Cretaceous and is the only boundary between geological periods to remain formally undefined. By the beginning of the Jurassic, the supercontinent Pangaea had begun rifting into two landmasses: Laurasia to the north and Gondwana to the south. The climate of

6528-417: The event. Seawater pH , which had already substantially decreased prior to the event, increased slightly during the early stages of the TOAE, before dropping to its lowest point around the middle of the event. This ocean acidification is the probable cause of the collapse of carbonate production. Additionally, anoxic conditions were exacerbated by enhanced recycling of phosphorus back into ocean water as

6624-449: The evidence for this is primarily European, probably controlled by changes in eustatic sea level. There is no evidence of a mass extinction of plants at the Triassic–Jurassic boundary. At the Triassic–Jurassic boundary in Greenland, the sporomorph (pollen and spores) record suggests a complete floral turnover. An analysis of macrofossil floral communities in Europe suggests that changes were mainly due to local ecological succession . At

6720-403: The first appearance of Cardioceras redcliffense as the lower boundary. The village of Kimmeridge on the coast of Dorset , England, is the origin of the name of the Kimmeridgian. The stage was named by Alcide d'Orbigny in 1842 in reference to the Kimmeridge Clay . The GSSP for the base of the Kimmeridgian is the Flodigarry section at Staffin Bay on the Isle of Skye , Scotland , which

6816-409: The first appearance of the ammonite Strambergella jacobi , formerly placed in the genus Berriasella , but its use as a stratigraphic indicator has been questioned, as its first appearance does not correlate with that of C. alpina . The Kimmeridge Clay and equivalents are the major source rock for the North Sea oil . The Arabian Intrashelf Basin, deposited during the Middle and Late Jurassic,

6912-589: The flow below the confluence in dry years. Below the confluence with the San Miguel, the Dolores enters Mesa County , flowing north-northwest past Gateway and then turning west into Utah . The last segment of the river, entirely within Grand County , joins the Colorado near the historic Dewey Bridge , about 30 miles (48 km) above Moab . The Hanging Flume is located on the canyon wall near Uravan, Colorado . Measured at Cisco, Utah , not far above

7008-402: The genus Elatides . The Jurassic also saw the first appearances of some modern genera of cypresses, such as Sequoia . Members of the extinct genus Schizolepidopsis which likely represent a stem-group to the pine family ( Pinaceae ), were widely distributed across Eurasia during the Jurassic. The oldest unambiguous record of Pinaceae is the pine cone Eathiestrobus , known from

7104-668: The late Early Jurassic in association with the eruption of the Karoo-Ferrar large igneous provinces , opening the western Indian Ocean and beginning the fragmentation of Gondwana. At the beginning of the Jurassic, North and South America remained connected, but by the beginning of the Late Jurassic they had rifted apart to form the Caribbean Seaway, also known as the Hispanic Corridor, which connected

7200-472: The late Sinemurian–Pliensbachian before regressing to near present levels by the late Pliensbachian. There seems to have been a gradual rise to a peak of ~75 m above present sea level during the Toarcian. During the latest part of the Toarcian, the sea level again dropped by several tens of metres. It progressively rose from the Aalenian onwards, aside from dips of a few tens of metres in the Bajocian and around

7296-465: The latest Jurassic to earliest Cretaceous, have been suggested to represent the most promising candidates for fixing the Jurassic–Cretaceous boundary In particular, the first appearance Calpionella alpina , co-inciding with the base of the eponymous Alpina subzone, has been proposed as the definition of the base of the Cretaceous. The working definition for the boundary has often been placed as

7392-428: The modern species, are known from the Middle Jurassic in the Northern Hemisphere. Several other lineages of ginkgoaleans are known from Jurassic rocks, including Yimaia , Grenana , Nagrenia and Karkenia . These lineages are associated with Ginkgo- like leaves, but are distinguished from living and fossil representatives of Ginkgo by having differently arranged reproductive structures. Umaltolepis from

7488-714: The northern edge of the San Luis Valley, close to Poncha Pass. This suggests that early human presence in the region dates back to ancient times, long before the establishment of modern landmarks. Mining operators in the San Juan mountain area formed the San Juan District Mining Association (SJDMA) in 1903, as a direct result of a Western Federation of Miners proposal to the Telluride Mining Association for

7584-422: The ocean floor acting as a buffer against large CO 2 emissions. The climate of the Jurassic was generally warmer than that of present, by around 5–10 °C (9–18 °F), with atmospheric carbon dioxide likely about four times higher. Intermittent "cold snap" intervals are known to have occurred during this time period, however, interrupting the otherwise warm greenhouse climate. Forests likely grew near

7680-460: The oceans, resulting in large areas of desert and scrubland in the lower latitudes between 40° N and S of the equator. Tropical rainforest and tundra biomes are likely to have been rare or absent. The Jurassic also witnessed the decline of the Pangaean megamonsoon that had characterised the preceding Permian and Triassic periods. Variation in the frequency of wildfire activity in the Jurassic

7776-611: The poles, where they experienced warm summers and cold, sometimes snowy winters; there were unlikely to have been ice sheets given the high summer temperatures that prevented the accumulation of snow, though there may have been mountain glaciers. Dropstones and glendonites in northeastern Siberia during the Early to Middle Jurassic indicate cold winters. The ocean depths were likely 8 °C (14 °F) warmer than present, and coral reefs grew 10° of latitude further north and south. The Intertropical Convergence Zone likely existed over

7872-630: The range. The last large-scale mines were the Sunnyside Mine near Silverton, which operated until late in the 20th century, and the Idarado Mine on Red Mountain Pass , which closed in the 1970s. Famous old San Juan mines include the Camp Bird and Smuggler Union mines , both located between Telluride and Ouray. The Summitville mine was the scene of a major environmental disaster in

7968-648: The region. The GSSP for the base of the Bathonian is Ravin du Bès, Bas-Auran area, Alpes de Haute Provence , France; it was ratified in 2009. The base of the Bathonian is defined by the first appearance of the ammonite Gonolkites convergens , at the base of the Zigzagiceras zigzag ammonite zone . The Callovian is derived from the Latinized name of the village of Kellaways in Wiltshire , England, and

8064-410: The reservoir is diverted, leaving only a small legally mandated minimum flow to pass downstream, as a result reducing the 150 mile (242 km) stretch between the dam and the confluence of the San Miguel River to a large creek. The dam's construction allowed local farmers to extend the irrigation season through September, whereas natural river flows would have been insufficient by July or August. While

8160-469: The river in Colorado, though permits are required from the Moab BLM office for boaters wishing to make the run from Gateway, CO, into Utah and to the confluence with the Colorado River near Dewey Bridge. In 2010, flash flooding altered Diversion Dam Rapid and Stateline Rapid on this section, making the latter very difficult to run in any vessel larger than a kayak. Advocates have proposed establishment of

8256-531: The river should have created a water gap here; instead, the canyon may have been formed by glaciation during the Paleozoic . The Dolores is dammed at McPhee Reservoir near Cortez, Colorado to irrigate about 61,660 acres (24,950 ha) of arid plateau land. The dam and diversion canals are operated by the Bureau of Reclamation as the Dolores Project. In some years, almost all the water entering

8352-664: The section from Bradfield Ranch near Cahone, Colorado down to Slick Rock, Colorado , offers scenery, camping, and rapids for inflatables and kayaks. The section from Slickrock to Bedrock, Colorado goes through the goosenecks of a sandstone canyon with several mostly class II rapids. The Bureau of Land Management recommends minimum flows of 200 cubic feet per second (5.7 m/s) for canoes, kayaks, and inflatable kayaks, 800 cu ft/s (23 m/s) for small rafts of up to 14 feet (4.3 m), and 1,000 cu ft/s (28 m/s) for larger rafts of up to 18 feet (5.5 m) in size. The BLM does not require permits for groups running

8448-573: The three series of von Buch in the Swabian Jura into six subdivisions defined by ammonites and other fossils. The German palaeontologist Albert Oppel in his studies between 1856 and 1858 altered d'Orbigny's original scheme and further subdivided the stages into biostratigraphic zones, based primarily on ammonites. Most of the modern stages of the Jurassic were formalized at the Colloque du Jurassique à Luxembourg in 1962. The Jurassic Period

8544-407: The world's major landmasses were coalesced into the supercontinent Pangaea , which during the Early Jurassic began to break up into northern supercontinent Laurasia and the southern supercontinent Gondwana . The rifting between North America and Africa was the first to initiate, beginning in the early Jurassic, associated with the emplacement of the Central Atlantic Magmatic Province . During

8640-585: The yew family ( Taxaceae ) is Palaeotaxus rediviva , from the Hettangian of Sweden, suggested to be closely related to the living Austrotaxus , while Marskea jurassica from the Middle Jurassic of Yorkshire, England and material from the Callovian–Oxfordian Daohugou Bed in China are thought to be closely related to Amentotaxus , with the latter material assigned to the modern genus, indicating that Taxaceae had substantially diversified by

8736-474: Was first defined and introduced into scientific literature by Alcide d'Orbigny in 1842. It takes its name from the French town of Semur-en-Auxois , near Dijon . The original definition of Sinemurian included what is now the Hettangian. The GSSP of the Sinemurian is located at a cliff face north of the hamlet of East Quantoxhead , 6 kilometres east of Watchet , Somerset , England , within the Blue Lias , and

8832-629: Was governed by the 405 kyr eccentricity cycle. Thanks to the breakup of Pangaea, the hydrological cycle during the Jurassic was significantly enhanced. The beginning of the Jurassic was likely marked by a thermal spike corresponding to the Triassic–Jurassic extinction and eruption of the Central Atlantic magmatic province. The first part of the Jurassic was marked by the Early Jurassic Cool Interval between 199 and 183 million years ago. It has been proposed that glaciation

8928-639: Was named by Alcide d'Orbigny in 1852, originally the base at the contact between the Forest Marble Formation and the Cornbrash Formation . However, this boundary was later found to be within the upper part of the Bathonian. The base of the Callovian does not yet have a certified GSSP. The working definition for the base of the Callovian is the first appearance of ammonites belonging to the genus Kepplerites . The Oxfordian

9024-490: Was present in the Northern Hemisphere during both the early Pliensbachian and the latest Pliensbachian. There was a spike in global temperatures of around 4–8 °C (7–14 °F) during the early part of the Toarcian corresponding to the Toarcian Oceanic Anoxic Event and the eruption of the Karoo-Ferrar large igneous provinces in southern Gondwana, with the warm interval extending to the end of

9120-687: Was ratified in 2000. The beginning of the Sinemurian is defined by the first appearance of the ammonite Vermiceras quantoxense . Albert Oppel in 1858 named the Pliensbachian Stage after the hamlet of Pliensbach in the community of Zell unter Aichelberg in the Swabian Alb , near Stuttgart , Germany. The GSSP for the base of the Pliensbachian is found at the Wine Haven locality in Robin Hood's Bay , Yorkshire , England, in

9216-430: Was ratified in 2021. The boundary is defined by the first appearance of ammonites marking the boreal Bauhini Zone and the subboreal Baylei Zone. The Tithonian was introduced in scientific literature by Albert Oppel in 1865. The name Tithonian is unusual in geological stage names because it is derived from Greek mythology rather than a place name. Tithonus was the son of Laomedon of Troy and fell in love with Eos ,

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