During the Early Jurassic, animals and plants living both on land and in the seas recovered from one of the largest mass extinctions in Earth history. Many groups of vertebrate and invertebrate organisms important in the modern world made their first appearance during the Jurassic. Life was especially diverse in the oceans—thriving reef ecosystems, shallow-water invertebrate communities, and large swimming predators, including reptiles and squidlike animals. On land, dinosaurs (dinosaur) and flying pterosaurs (pterosaur) dominated the ecosystems, and birds made their first appearance. Early mammals also were present, though they were still fairly insignificant. Insect populations were diverse, and plants were dominated by the gymnosperms (gymnosperm), or “naked-seed” plants.

The Jurassic Period was named early in the 19th century, by the French geologist and mineralogist Alexandre Brongniart (Brongniart, Alexandre), for the Jura Mountains between France and Switzerland. Much of the initial work by geologists in trying to correlate rocks and develop a relative geologic time scale was conducted on Jurassic strata in western Europe.

To accommodate the production of new seafloor along the proto-Atlantic Ocean, significant subduction zones (subduction zone) (where seafloor is destroyed) were active along virtually all the continental margins around Pangea as well as in southern Tibet, southeastern Europe, and other areas. All along the west coast of North, Central, and South America, plate tectonic activity in the subduction zones brought on the initial formation of north-south mountain ranges such as the Rocky Mountains and the Andes (Andes Mountains). Along western North America, several terranes (islands or microcontinents riding on a moving plate) were brought east on oceanic crust and collided with the continent, including parts of a microcontinent that collided into the Alaskan and Siberian regions in the northern Pacific. These collisions added to the growth of the North American continent and its mountain chains. One mountain-building event, known as the Nevadan orogeny, resulted in the emplacement of massive igneous and metamorphic rocks from Alaska to Baja California. Granites (granite) formed in the Sierra Nevadas during this time can be seen today in Yosemite National Park, California.

Records of sea level changes can be found on every continent. However, because of the significant tectonic activity occurring around the world, it is not clear which of these local changes can be correlated to global sea level change. Because there is no evidence of major glaciations in the Jurassic, any global sea level change must have been due to thermal expansion of seawater or plate tectonic activity (such as major activity at seafloor ridges). Some geologists have proposed that average sea levels increased from Early to Late Jurassic time.

Jurassic climates can be reconstructed from the analyses of fossil and sediment distribution and from geochemical analyses. Fossils of warm-adapted plants are found up to 60° N and 60° S paleolatitude, suggesting an expanded tropical zone. In higher paleolatitudes, ferns and other frost-sensitive plants indicate that there was a less severe temperature difference between the Equator and the poles than exists today. Despite this decreased temperature gradient, there was a marked difference in marine invertebrates from northern higher latitudes—the boreal realm—and the tropical Tethyan realm. Decreased latitudinal temperature gradients probably led to decreased zonal winds.

Large salt deposits dating from the Jurassic represent areas of high aridity, while extensive coal deposits suggest areas of high precipitation. It has been suggested that an arid belt existed on the western side of Pangea, while more-humid conditions existed in the east. These conditions may have been caused by large landmasses affecting wind and precipitation in a manner similar to that of modern continents.

Analyses of oxygen isotopes in marine fossils suggest that Jurassic global temperatures were generally quite warm. Geochemical evidence suggests that surface waters in the low latitudes were about 20 °C (68 °F), while deep waters were about 17 °C (63 °F). Coolest temperatures existed during the Middle Jurassic and warmest temperatures in the Late Jurassic. A drop in temperatures occurred at the Jurassic-Cretaceous boundary.

It has been suggested that increased volcanic and seafloor-spreading activity during the Jurassic released large amounts of carbon dioxide—a greenhouse gas (greenhouse effect)—and led to higher global temperatures. Warm temperatures and decreased latitudinal gradients also may be related to the Tethys Sea, which distributed warm, tropical waters around the world. Ocean circulation was probably fairly sluggish because of the warm temperatures, lack of ocean density gradients, and decreased winds. As stated above, there is no evidence of glaciation or polar ice caps in the Jurassic. This may have been caused by the lack of a continental landmass in a polar position or by generally warm conditions; however, because of the complex relationships between temperature, geographic configurations, and glaciations, it is difficult to state a definite cause and effect.

The Triassic-Jurassic boundary is marked by one of the five largest mass extinctions on Earth. About half of the marine invertebrate genera went extinct at this time; whether land plants or terrestrial vertebrates suffered a similar extinction during this interval is unclear. In addition, at least two other Jurassic intervals show heightened faunal turnover affecting mainly marine invertebrates—one in Early Jurassic time and another at the end of the period.

Jurassic rock strata preserve the first appearances of many important modern biological groups. In the oceans, life on the seafloor became more complex and modern, with an abundance of mollusks (mollusk) and coral reef builders by Middle Jurassic time. While modern fishes became common in Jurassic seas, they shared the waters with ammonites (Ammonite) and other squidlike organisms as well as large reptiles that are all extinct today. On land a new set of plants and animals was dominant by the Early Jurassic. Gymnosperms (gymnosperm) (“naked-seed” plants such as conifers (conifer)) replaced the seed ferns (seed fern) that dominated older ecosystems. Similarly, dinosaurs and mammals, as well as amphibians and reptiles resembling those of modern times, replaced the ancestral reptiles and mammal groups common in Late Triassic times. The earliest bird fossils were found in Jurassic rocks. However, although groups now living were present in Jurassic terrestrial ecosystems, Jurassic communities would still have been very different because dinosaurs were the dominant animals.

The earliest Jurassic marine ecosystems show signs of recovery from the major mass extinction that occurred at the Triassic-Jurassic boundary. This extinction eliminated about half of marine invertebrate genera and left some groups with very few surviving species. Diversity increased rapidly for the first four million years (the Hettangian Age (Hettangian Stage)) following this extinction and then slowed through the next five million years. Another extinction event occurred among benthic (bottom-dwelling) invertebrates at the Pliensbachian (Pliensbachian Stage)-Toarcian (Toarcian Stage) boundary in the Early Jurassic, interrupting the overall recovery and diversification. The last spiriferid brachiopod (lamp shells) (abundant during the Paleozoic Era) went extinct at this time, and in some regions 84 percent of bivalve species went extinct. Although best documented in Europe, biodiversity during this period seems to have decreased around the globe. The extinctions may be related to an onset of low-oxygen conditions in epicontinental seas, as evidenced by the presence today of layers of organic-rich shales (shale), which must have been formed in seas with so little oxygen that no burrowing organisms could survive and efficient breakdown of organic matter could not occur. Full recovery from this extinction did not occur until the Middle Jurassic. It has been proposed that a final interval of heightened extinction took place at the end of the Jurassic, although its magnitude and global extent are disputed. This final turnover may have been limited to Eurasian regions affected by local sea level decreases, or it may be related to a decrease in the quality of fossil preservation through the Late Jurassic.

Except for the extinction events outlined above, in general, marine invertebrates increased their diversity and even modernized through the Jurassic. Some previously abundant Paleozoic groups were extinct by the Jurassic, and other groups were present but no longer dominant. Moreover, many important modern groups first appeared in the fossil record during the Jurassic, and many important groups experienced high levels of diversification (a process known as evolutionary radiation).

Among the plankton—floating, single-celled, microscopic organisms—two significant new groups originated and radiated rapidly: coccolithophores (coccolith) and foraminifera (foraminiferan). In addition, diatoms (diatom) are considered by some scholars to have originated in the Late Jurassic and radiated during the Cretaceous. The skeletons of all three groups are major contributors to deep-sea sediments. Before the explosion of skeletonized planktonic organisms, carbonates were mainly deposited in shallow-water, nearshore environments. Today the tests (shells) of coccolithophores and foraminifera account for significant volumes of carbonate sediments in the deep sea, while diatom tests create silica-rich sediments. Thus, the advent of these groups has significantly changed the geochemistry of the oceans, the nature of the deep-sea floor, and marine food webs.

Mollusks (mollusk) became dominant in marine ecosystems, both among swimmers in the water column (nekton) and organisms living on the seafloor (benthos). Nektic cephalopods (cephalopod), such as shelled ammonites (Ammonite) and squidlike belemnites with internal skeletons, were very common. Although only one group of ammonites survived the Triassic-Jurassic mass extinction, they radiated rapidly into many different forms. Because their shells have elaborate suture lines, they are easily identifiable; this quality, along with their abundance and rapid evolution, make them useful as index fossils for correlating and sequencing rocks. Thus, ammonites are a major tool for developing relative time scales and dividing the Jurassic into finer time intervals. Other common mollusks include bivalves (bivalve) (pelecypods) and snails (gastropods (gastropod)). These forms diversified into a number of different niches. Among the bivalves, scallops (pectinids) and oysters show marked radiation. Some bivalves also are used as index fossils.

Common echinoderms include crinoids (crinoid) (sea lilies), echinoids (sea urchins (sea urchin)), and sea stars (starfish (sea star)). Jurassic crinoids are descendants from the one group that survived the Permian-Triassic mass extinction. Their circular or star-shaped stem ossicles (plates) can be quite abundant in Jurassic sediments. Under special circumstances, articulated Jurassic crinoids are preserved; some of these fossils suggest that some species may have lived on floating logs and not on the seafloor. One group of regular sea urchins, radially symmetrical and living on the surface of the seafloor, radiated into a number of irregular echinoid groups (heart urchins (heart urchin)) that could burrow into sediment.

Some lophophorates (brachiopods (lamp shells), or lamp shells) and bryozoa (moss animal) (moss animals) underwent recovery and diversification in the Jurassic but never became as dominant as they were in the Paleozoic Era. Spiriferid brachiopods went extinct during the Early Jurassic extinction event, but rhynchonellid and terebratulid brachiopods can be found throughout the period.

Among bryozoans that survived into the Jurassic, cyclostomes are found encrusting hard substrates; cheilostomes (the most common modern bryozoan) appeared in the Late Jurassic. With the extinction of trilobites, a new set of arthropods (arthropod) developed. The first true crabs and lobsters appeared, bearing large front claws adapted for predation. Shrimp burrows are not uncommon in Jurassic sediments, and fossil shrimp are occasionally preserved. Ostracods—small crustaceans—radiated during the Jurassic and are used today as index fossils.

Unlike today's world, where virtually all reefs are formed by scleractinian corals, Jurassic reefs and mounds were constructed by a variety of invertebrate organisms. Buildups were constructed by siliceous sponges and serpulid tube worms as well as corals. stromatolite mounds were formed by communities of algae, bacteria, and other microorganisms. These reefs also had a diverse set of fauna associated with them.

The ecology of the seas was changed by the diversification of marine fauna and by the adaptations of these new organisms. With the evolution and radiation of more-effective predators (crabs, snails, echinoderms, and marine vertebrates), predation pressures began to increase rapidly. For this reason, the Jurassic marks the start of the “Mesozoic Marine Revolution”—an arms race between predators and prey that led to increased diversification of marine fauna. For example, increased levels of burrowing are found in Jurassic sediments, along with an increase in the maximum depth of burrowing. These increases may have developed as a predator-avoidance adaptation, with organisms evolving that were capable of burrowing into sediment, but the activity had far-reaching effects. Burrowing changed the nature of the seafloor, the utilization of resources and space, and sedimentation style.

Large marine reptiles were common denizens of Jurassic seas. Ichthyosaurs (ichthyosaur) had sleek profiles similar to those of modern fast-swimming fish and had large eye orbits, perhaps the largest of any vertebrate ever. Jurassic pliosaurs (short-necked plesiosaurs (plesiosaur)) could be about 15 metres (50 feet) long and are some of the largest carnivorous reptiles ever found—even rivaling Tyrannosaurus (tyrannosaur), which lived during the subsequent Cretaceous Period. Fossils of large crocodiles and elasmosaurs (long-necked plesiosaurs) are also found in Jurassic marine rocks.

Insects constitute the most abundant terrestrial invertebrates found in the Jurassic fossil record. Groups include the odonates (Odonata) (damselflies and dragonflies), coleopterans (coleopteran) (beetles), dipterans (dipteran) (flies), and hymenopterans (hymenopteran) (bees, ants, and wasps). The discovery of Jurassic bees—which today are dependent upon flowering plants (angiosperms (angiosperm))—suggests either the early presence of angiosperms or that bees were originally adapted to other strategies. Snails, bivalves, and ostracods are preserved in freshwater deposits.

Because of poor preservation of terrestrial deposits and their fossils, it is unclear whether the mass extinction at the end of the Triassic had the same impact on terrestrial ecosystems as it did in the oceans. However, there was a distinct change in vertebrate fauna by the Early Jurassic. In Triassic terrestrial ecosystems, synapsids and therapsids (therapsid)—ancestors of modern mammals and their relatives, often called “mammal-like reptiles”—were dominant. They occupied several ecological niches (niche) and grew to large sizes. By the start of the Jurassic, these groups became rare, a minor component of fossil assemblages; individuals were very small—no larger than squirrel-sized—and their teeth and skeletal anatomy show that the early mammals were probably omnivorous (eating plants and animals) or insectivorous. Instead, the archosaurs (archosaur) (dinosaurs, crocodiles, and pterosaurs) were the dominant terrestrial vertebrates. It is not clear why this change from synapsid-dominated to archosaur-dominated faunas occurred; it could be related to the Triassic-Jurassic extinctions or to adaptations that allowed the archosaurs to outcompete the mammals and mammalian ancestors (at least until the end of the Mesozoic Era). In the Late Jurassic, while some marine invertebrates were going extinct, terrestrial vertebrates may also have experienced a drop in diversity, but the evidence here, too, is inconclusive.

Pterosaurs were common throughout the Jurassic. With light skeletons and wing structure supported by a single digit on each “hand,” they were adapted to flying and gliding.

The dinosaurs are divided into two groups based on a number of skeletal characteristics: the saurischians (saurischian) (lizard-hipped) and the ornithischians (ornithischian) (bird-hipped). The pubic bone of the saurischians pointed forward, while the ornithischians had an extension that pointed backward.

Other reptiles, including turtles, were present throughout the Jurassic, while modern forms of lizards made their appearance in the Late Jurassic. Amphibians present during the Triassic Period declined drastically by the Jurassic, and more modern forms developed, such as the first frog with the type of skeletal characteristics seen today.

The first undisputed fossil evidence for angiosperms (angiosperm) (flowering plants) is not found until the Cretaceous Period. However, some pollen material similar to that of angiosperms has been reported in rocks of Jurassic age. Also present are fossils of insects whose present-day descendants depend upon angiosperms, suggesting that angiosperms may indeed have been present by Jurassic times.

Jurassic igneous rocks have yielded uranium and gold in the Sierra Nevada range of North America, including placer deposits that were mined during the California Gold Rush of the mid-1800s. Some of the diamonds in Siberia were emplaced during Jurassic times. The shallow seas inundating Jurassic continents allowed for extensive deposition of sedimentary rocks (sedimentary rock) that have provided important resources in many regions. For example, clay (clay mineral) and limestone have been used for brick, cement, and other building materials in various areas of Europe; iron ore is prevalent in western Europe and England; and Jurassic salt is mined in both the United States and Germany.

Energy resources have also been derived from Jurassic deposits. Jurassic coals are found throughout Eurasia; one significant example is from the Late and Middle Jurassic Yan'an Formation in the Ordos Desert (Ordos Plateau) of China. A significant amount of American petroleum production comes from deposits trapped against salt domes of Jurassic age in the Gulf Coast of the United States. The North Sea and Arabian oil fields can also be traced back to organic-rich deposition in restricted Jurassic marine basins. Oil also is found in northern Germany and Russia.

The Early Jurassic rock system has four stages—the Hettangian (Hettangian Stage), Sinemurian (Sinemurian Stage), Pliensbachian (Pliensbachian Stage), and Toarcian (Toarcian Stage). The Middle Jurassic also has four stages—the Aalenian (Aalenian Stage), Bajocian (Bajocian Stage), Bathonian (Bathonian Stage), and Callovian (Callovian Stage). The Late Jurassic has three stages—the Oxfordian (Oxfordian Stage), Kimmeridgian (Kimmeridgian Stage), and Tithonian (Tithonian Stage).

Jurassic rocks are widely distributed and include sedimentary (sedimentary rock), igneous (igneous rock), and metamorphic (metamorphic rock) rocks. Because of continuous subduction (subduction zone) and destruction of ocean crust in trenches (deep-sea trench), Middle Jurassic oceanic crust and sediments are generally the oldest sediments remaining in the deep sea. The Jurassic was a time marked by a high level of plate tectonic (plate tectonics) activity, and igneous (igneous rock) rocks of Jurassic age are concentrated in the areas of activity, such as spreading centres (ocean) (rifts and oceanic ridges) and mountain-building areas near subduction zones (ocean). In the areas where the Atlantic Ocean was opening and other continents were splitting apart, basalts (basalt) that make up oceanic crust today accumulated in the basins. Notably, basalts are found along the east coast of North America and in southern Africa where it was connected to Antarctica. Volcanic (volcano) ash can also be found near active margins; for example, many ash beds occur in the Late Jurassic Morrison Formation in western North America. granite batholiths (igneous rocks that were emplaced at depth) can be found along the western margin of North and South America where subduction was occurring during the Jurassic.

Jurassic sedimentary (sedimentary rock) rocks can be found on all modern continents and include marine, marginal marine, and terrestrial deposits. Jurassic marine sediments are also found on the modern seafloor. Because sea levels were high enough to cover large portions of continents, seaways formed on landmasses throughout the Jurassic. Thus, marine sandstones (sandstone), mudstones (mudstone), and shales (shale) often alternate with terrestrial conglomerates (conglomerate), sandstones, and mudstones. Marine carbonate limestones (limestone) are mostly found in the tropics and the midlatitudes, where waters were warm and faunal productivity high. In Europe, black shales (black shale) are common where restricted circulation in shallow marine basins caused bottom waters to become oxygen-deficient. Red beds, windblown sands, lake deposits, and coals can be found in terrestrial systems. Deltaic (delta) sands and salt deposits are found in what were once marginal marine environments.

The geologic profile of Jurassic North America is best separated into three different zones: the east coast, where rifting opened the Atlantic Ocean; the western interior, where continental sediments and epicontinental seaway sediments accumulated; and the west coast, where deformation occurred because of the presence of offshore subduction trenches.

In eastern North America, Late Triassic–Early Jurassic extensional basins were filled with red beds and other continental sediments, and pillow lavas were extruded into lake basins. The basaltic Watchung Flows of the Newark Basin are Early Jurassic in age, based on potassium-argon dating techniques that show them to be 185 to 194 million years old. More than 150 metres (500 feet) of Lower Jurassic lake beds were deposited in various basins on the east coast; some of these bedded sediments may reflect orbital cycles. Middle Jurassic volcanoclastic rocks have been found beneath sediments on the continental shelf of New England. Upper Jurassic marine sediments include clastics interfingering with carbonates in the Atlantic and Gulf Coast basins. Middle Jurassic strata include evaporites (evaporite), red beds, carbonates, and shelf-margin reefs. The Smackover Formation of the Gulf Coast sequences is a sedimentary unit typical of the Middle Jurassic.

In the western interior of North America, the Middle Jurassic is characterized by a series of six marine incursions. These epicontinental seaways are referred to collectively as the Carmel and Sundance seas; the Carmel Sea is older and not as deep as the Sundance. In these epicontinental seaways, marine sandstones, mudstones, limestones, and shales were deposited—some with marine fossils. Fully marine sequences interfinger with terrestrial sediments deposited during times of low sea levels and with marginal marine sediments that accumulated in environments bordering the seaways.

In the Late Jurassic, sea levels dropped in North America, and terrestrial sedimentation occurred across much of the continent. The Morrison Formation, a clastic deposit of lacustrine and fluvial mudstone, siltstone, sandstone, and conglomerate, is famous for fossil-rich beds that contain abundant plant and dinosaur remains. Uplift of the continental interior occurred between central Arizona and southern California from the Late Triassic until the Middle Jurassic.

Throughout the Jurassic the western margin of North America was bounded by an active subduction zone. This led to very complex geology and much plate tectonic activity, including collisions between terranes and North America, creation of volcanoes, and mountain-building episodes. Accretion of microcontinents and volcanic island arcs to the continent occurred along the entire coast of North America; more than 50 Jurassic terranes have been incorporated onto the continent. Some of the terranes may have originated from tropical areas and traveled far before colliding into North America. During the Nevadan orogeny, volcanic island arcs, including the Sierra Nevada, collided with the continent from northern California to British Columbia, and this resulted in the development of faults and emplacement of igneous intrusions. Deformation of the Foothills Terrane in the Sierra Nevada occurred 160 to 150 million years ago. Jurassic deep-sea rocks now uplifted and exposed in California are between 150 million and 200 million years old, as are intrusive igneous bodies such as the granite batholiths of Yosemite and the High Sierra. During the Jurassic, sediments accumulating off the continental margin were accreted along with the terranes. Many formations in the region are composed of ophiolites (oceanic crust), basalts, and deepwater marine sediments such as cherts, slates, and carbonates. Such a variety of rock types, deposited in a number of different environments, makes this region a geologic patchwork.

Similar to those in North America, Jurassic rocks in the rest of the world can be divided into three types: igneous rocks associated with continental rifting and seafloor spreading, sedimentary rocks associated with epicontinental seaways and terrestrial systems, and deformed deposits associated with subduction and mountain-building (orogenic) zones. Continental rifting between the regions of the Gondwana continent resulted in vast outpourings of basalts similar to those in the Newark Basin (although not as large in extent). These flood basalts are most notable in southern Africa, though thick volcanic sequences are also found on other landmasses that were breaking up at the time—Australia, South America, and India. Other rift-related sedimentary rocks also accumulated in these spreading centres.

The warm, shallow trough of the Tethys Sea between Eurasia and Gondwana accumulated thick sequences of Jurassic sediments. Carbonates (carbonate rock) are predominant and include fossiliferous shallow-water marls, limestones, and reefs. Siliceous limestones are fairly common, suggesting that an abundance of sponges were available to provide the silica. Evaporites formed along marginal environments around the seaway, while fine sandstones and mudstones are present mainly in nearshore environments near highlands. Deformation of these sediments began in the Late Jurassic, but most of the folding and faulting occurred after the Jurassic. The deformed sediments are exposed today in the Alps.

As seafloor spreading continued, Tethys widened further. Deeper-water sediments present within the Tethyan realm suggest that deepening basins developed during this time. The interiors of continents experienced different levels of marine inundation. As sea levels rose, Tethys expanded and at times covered large parts of the continental interior of Eurasia, allowing for the deposition of the sediments discussed above. Jurassic carbonates can be found in the Jura Mountains and southern France and in England. Fossiliferous, fine-grained lithographic limestones of Germany (see Solnhofen Limestone) were deposited in lagoonal and marginal marine environments adjacent to the seaway. Clastic facies include the Early Jurassic shales of western Europe, the Late Jurassic clays of England and Germany, and the clays of the Russian Platform. The Arctic region was primarily a clastic province dominated by clay-rich rocks, shale, siltstone, sandstone, and conglomerate.

There are many examples of Jurassic black shales in Europe that represent intervals of low oxygen conditions at the seafloor. These conditions may have been developed because of restricted circulation and high levels of productivity. Some of the black shales contain exceptionally preserved vertebrate and invertebrate fossils that provide much of the paleontological information about the Jurassic.

On most of the southern landmasses (India, Antarctica, Africa, Australia, and New Zealand), marine deposits are generally restricted to the edges of continents because the continents were mainly above sea level for much of the Jurassic. Continental deposits consist mainly of red beds, sandstones, and mudstones that were deposited under fluvial, lacustrine, and eolian (wind-dominated) environments. Many parts of Eurasia also were dominated by terrestrial environments, accumulating coal beds and other continental sediments.

The Pacific margin of Asia, which was surrounded by subduction zones such as those along the west coast of North America, developed volcanic island arcs and associated basins from Japan to Indonesia. As the Pacific plate subducted under New Zealand during the Late Jurassic, terrane accretion, volcanic activity, and deformation occurred. Subduction zones off the west margin of South America resulted in igneous activity, deformation, and mountain building similar to that occurring in North America.

The oldest oceanic evidence for seafloor spreading (and magnetic anomalies) dates from about 147 million years ago, and the oldest oceanic sediments date from the Middle Jurassic. The Indian Ocean began to open at this time as India separated from Australia and Antarctica. The oldest crust of the Pacific basin dates from the Late Jurassic.

By the Early Jurassic, much of the flood basalts associated with the opening of the Atlantic Ocean had already been formed, and some significant basalts are found in the Newark Basin. In the early stages of formation of the Atlantic, nonmarine deposits such as fluvial (river), deltaic, and lacustrine (lake) sediments accumulated within the basin. In other cases, as on the Gulf Coast, marginal marine deposits such as evaporites (salt deposits) accumulated. The Jurassic Gulf Coast salt domes are huge (200 metres, or 660 feet, tall and 2 km, or 1.2 miles, in diameter), suggesting prolonged intervals of seawater evaporation. As the basins grew larger, connections were made with the open ocean, and the basins filled with marine waters and normal marine sediments. However, because these new oceans were still restricted and did not have vigorous circulation, oxygen content was low, allowing for the deposition of organic-rich shales. The source of North Sea oil comes from organic material buried during the Jurassic.