쥬라기 시대 지리학

쥬라기 암석의 발생과 분포

쥬라기 암석은 널리 분포되어 있으며 퇴적암, 화성암 및 변성암을 포함합니다. 트렌치에서 해저 지각의 지속적인 섭입과 파괴로 인해, 중간 쥬라기 해양 지각과 퇴적물은 일반적으로 심해에 남아있는 가장 오래된 퇴적물입니다. 쥬라기는 높은 수준의 판 구조 활동으로 특징 지워졌으며, 쥬라기 시대의 화성암은 확산 센터 (소동과 바다 융 기부)와 섭입 구역 근처의 산 건물 영역과 같은 활동 영역에 집중되어 있습니다. 대서양이 열리고 다른 대륙이 갈라져있는 지역에서는 오늘날 해양 지각을 구성하는 현무암이 분지에 축적되었습니다. 특히 현무암은 북미의 동해안과 남극 대륙과 연결된 남부 아프리카에서 발견됩니다.화산재는 또한 활성 마진 근처에서 발견 될 수 있습니다. 예를 들어, 북아메리카 서부의 후기 쥬라기 모리슨 층에서 많은 재가 발생합니다. 쥬라기 동안 섭입이 발생한 북남미 서부 가장자리에는 화강암 욕석 (깊이에 위치한 화성암)이 있습니다.

쥬라기 퇴적암은 모든 현대 대륙에서 발견 될 수 있으며 해양, 한계 해양 및 지상 퇴적물을 포함합니다. 쥬라기 해양 퇴적물도 현대 해저에서 발견됩니다. 해수면은 대륙 전체를 덮을 정도로 충분히 높기 때문에 쥬라기 전역의 육지에 해수가 형성되었습니다. 따라서 해양 사암, 이암 및 셰일은 종종 육상 대기업, 사암 및 이암으로 번갈아 나타납니다. 해양 탄산염 석회석은 주로 물이 따뜻하고 동물 생산성이 높은 열대와 중위도에서 발견됩니다. 유럽에서는 얕은 해양 유역의 제한된 순환으로 인해 해수가 산소 부족 상태가되는 곳에서 검은 혈암이 일반적입니다. 빨간 침대, 바람에 날린 모래, 호수 퇴적물 및 석탄은 지상 시스템에서 찾을 수 있습니다.한때 소외된 해양 환경에서 델타 산 모래와 소금 침전물이 발견됩니다.

North America

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 million 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, 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 150 million to 160 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.

Eurasia and Gondwana

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 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.

Ocean basins

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.