JURASSIC PERIOD

Table 4: Geologic time scale. To see more information about a period, select one from the chart. second of three periods of the Mesozoic Era. On the basis of radiometric measurements, it extended from about 208 to 144 million years ago (see Table). It is often divided into the Early Jurassic Epoch (208 to 187 million years ago), the Middle Jurassic Epoch (187 to 163 million years ago), and the Late Jurassic Epoch (163 to 144 million years ago). The rocks that originated during this interval of time compose the Jurassic System. In many regions of the world, the Jurassic lies directly over the Triassic strata and, in turn, is often overlain by those of the Cretaceous. The Jurassic was named early in the 19th century by the French geologist and mineralogist Alexandre Brongniart for the carbonate terrane of the Jura Mountains between France and Switzerland. The presence of similar fossils was used to establish a correlation between the Jura carbonates and the oolite limestones of England. interval of geologic time from 208 to 144 million years ago. It is often divided into the Early Jurassic Epoch (208 to 187 million years ago), the Middle Jurassic Epoch (187 to 163 million years ago), and the Late Jurassic Epoch (163 to 144 million years ago). The Jurassic Period is one of the three major divisions of the Mesozoic Era and is preceded by the Triassic Period and followed by the Cretaceous Period. The rocks that originated during this period constitute the Jurassic System. Continental rifting had set in near the end of the Triassic Period, signaling the breakup of Pangaea, the enormous supercontinent made up of all the major present-day landmasses. At the outset of the Jurassic, the constituent continental masses were still connected or situated very close together. Australia, Antarctica, and India were joined together against the eastern coast of Africa, and South America was joined to the southwestern coast of Africa. North America was still joined to western Africa and Europe, and a deep seaway called the Tethys extended from Spain eastward across southern Asia. Most of these continents were farther south than their present positions; South Africa lay within the Antarctic Circle, and the equator intersected southern North America and Europe. As a result, most landmasses currently in the Northern Hemisphere had warmer climates than they do at present. Because of higher temperatures across the globe, there were probably no polar ice caps, and in some areas there were extensive deserts. By the Middle Jurassic Epoch, North America had begun pulling away from Eurasia and Africa, and the Atlantic Ocean opened. Shallow seas slowly advanced over many continental areas of the world, reached a maximum in Late Jurassic times, and retreated at the close of the period. In the Late Jurassic, the Nevadan Orogeny began on the western margin of North America. This major mountain-building event resulted when the western edge of the tectonic plate bearing North America collided with a complex of island arcs. The southern continents also had begun to separate by the Late Jurassic. South America and Africa split away from each other, and India separated from Australia and Antarctica, which were still joined together. Among the rocks of the Jurassic System, marine deposits are widespread. These deposits include both shallow-water reef and deep-water pelagic limestones, which are extensive in Europe (where the main stages of the Jurassic were first described and defined), North America, eastern Africa, and western Australia. Tracing the strata from region to region reveals that in some areas these limestones may grade into shales and sandstones. Continental deposits, including coal, are found throughout Asia. The fossil record of the period indicates that marine invertebrates, notably the ammonites (now extinct cephalopods), flourished and new species evolved rapidly. The shell forms, particularly the suture (line or furrow marking the junction of two adjacent body parts) of the ammonites became more complex, and each species was so characteristic of its time and so widespread that ammonites are now used as guide fossils to define Jurassic stratigraphic zones. The coccolithophores (variety of free-floating marine organisms) appeared on a smaller scale and attained major importance in the later Cretaceous Period (144 to 66.4 million years ago). Coral and sponge reefs were prominent in many areas, and, though fish continued to be of importance, the larger reptiles dominated many marine habitats. These included the plesiosaurs, descendants of the land reptiles; they had long necks and paddle-shaped limbs similar to those of turtles or seals, and they remained near the surface of the water. The plesiosaurs and related species attained great size toward the end of the period. The ichthyosaurs, which resembled the modern dolphin in profile, were better adapted to the seas. On land, ferns, mosses, cycads, and conifers thrived, some developing flowerlike structures in place of cones. The dinosaurs, evolving from their smaller ancestors of the Triassic, rose to supremacy on land. By the end of the Jurassic the largest species had evolved; the spectacular sauropods were vegetarians, probably favouring a shallow-water habitat; and the theropods included the well-known land carnivore the tyrannosaur. The pterosaurs were another common form. These flying reptiles of the Jurassic were small, some species being only about the size of sparrows or thrushes. Their muscles were not so well developed as those of present-day birds, and they must have glided leisurely on air currents. The archaeopteryx, the first primitive bird, appeared before the end of the period. The first mammals, tiny shrewlike creatures that appeared near the close of the Triassic, managed to survive and evolve throughout the Jurassic Period. Additional reading Discussions of notable geologic features of the period include David G. Howell, Terranes, Scientific American, 253(5):116126 (November 1985), a treatment of the origin and nature of microplates, with an analysis of western North American terranes that provides evidence of ancient oceanic crust; John C. Lorenz, Triassic-Jurassic Rift-Basin Sedimentology: History and Methods (1988), an overview of basin research from the 19th century to the present, emphasizing the Hartford Basin in Connecticut; and John McPhee, Basin and Range (1981), a discussion of the basin and range structure of North America. The results of significant stratigraphic research are presented in Ralph W. Imlay, Correlation of the Jurassic Formations of North America, Exclusive of Canada (1952), a clarification of the biostratigraphic basis for the correlations. Life-forms of the period are addressed in the appropriate sections of William A. Berggren, Treatise on Invertebrate Paleontology, part A, IntroductionFossilization (Taphonomy), Biogeography, and Biostratigraphy (1979); Raymond C. Moore, Cecil G. Lalicker, and Alfred G. Fischer, Invertebrate Fossils (1952), dealing with the biology, morphology, ecology, classification, and biostratigraphy of the invertebrate phyla; Robert T. Bakker, The Dinosaur Heresies: New Theories Unlocking the Mystery of the Dinosaurs and Their Extinction (1986); and John H. Ostrom, A New Look at Dinosaurs, National Geographic, 154(2):152185 (August 1978), intended to dispel old myths about dinosaur evolution and to substitute an ecosystem-based extinction scenario for the formerly proposed climatic, volcanic, dietary, or extraterrestrial explanations. Les Sirkin Jurassic environment Paleogeography The continents were still grouped closely in the Pangaean configuration during the Early Jurassic, although divergence in the Atlantic axis was under way at this time. The formation of Pangaea had caused geosynclines to develop on all the coastal margins of the constituent continents. Passive continental margins existed in the Arctic, with a clastic depositional regime, along eastern North America and western Europe, both of which acquired miogeoclinal basins as separation took place, and along the Tethyan geosyncline between Eurasia and Gondwana, which, with its medial deep-sea component, persisted through this interval. Active plate collisions took place around the Pacific rim: the western margins of the North American and South American plates collided with the eastern edge of the Pacific Plate and its microplates and exotic terranes, while Asia, from Japan to Indonesia, and New Zealand collided against the western margin of the Pacific Plate. Neither of the Jurassic poles was occupied by a landmass. The South Pole lay south of Antarctica, only part of which extended south of 60 S. Most of Antarctica and Australia, as well as the southern tip of South America and Africa, were north of 60 S. The North Pole lay engulfed in the Arctic Sea, and only northernmost North America, along with Greenland and Siberia, extended north of 60 N. The paleoequator cut through the Pacific Ocean and the Tethys Sea, dissecting Gondwana and slicing across the northern edge of South America. As a result of this geographic distribution, tropical and temperate conditions (i.e., those typical of areas between 60 N and 60 S) prevailed on most of the landmasses. Reefs existed between 30 N and 30 S; most red-bed, evaporite, and dune deposits also occurred in this region. Coal deposits were extensive in northwestern North America, southwestern Europe, much of Siberia, Asia (including China), and Australia. Many of the continental deposits resulted from fluctuations along the margins of the epicontinental seas during the Middle and Late Jurassic, as did the development of multiple interfingering transitional and continental sediments, which in some areas formed cyclothems. Jurassic spreading centres and mid-oceanic rifts formed between North America and Eurasia, North America and Gondwana, and Eurasia and Gondwana. Such centres and rifts also developed between the various segments of Gondwana itself. Paleoclimate Jurassic climates can be deduced from paleogeographic reconstructions that reveal the location of reefs, red beds, evaporites, and dune sandstone. The first three are all indicative of tropical to subtropical conditions, while the presence of the latter suggests semiarid to arid regions and generally subtropical climate. It has been determined that widespread sand sheets, or ergs, covered the desert plains adjacent to the epicontinental seas of the mid-latitude continental interior of North America from Utah to Arizona. Such vast accumulations of sand gave rise to thick eolian sandstone typified by the Early Jurassic Wingate Sandstone. Extensive salt deposits are associated with regions of great aridity, such as the sabkhah (also spelled sebkha) areas that are found near the Red Sea today. Distribution of landmasses, mountainous regions, shallow seas, and deep ocean basins during the The presence of these indicator rock types in modern temperate, boreal, or polar latitudes indicates that the continental plates moved away from the paleoequator after their deposition during the Jurassic. There is no evidence of glaciation or polar ice caps in the Jurassic, perhaps owing to the lack of a continental landmass in a polar position (see figure). Warm, sun-lit oceans, including the epicontinental seas, abounded in marine life, especially in reef-building invertebrates and the extensive reef infaunas. The widespread development of terrestrial forests and the thick deposits of coal show that most of the existing landmasses were in temperate to tropical climatic regimes with adequate moisture to support the development of plant life. Jurassic life Protists and invertebrates Among the prominent marine life-forms of the Jurassic were the Protista, including the foraminiferans, and radiolarians. Foraminiferans, particularly the large-sized varieties in the family Lituolidae, were abundant. Benthic protozoans, planktonic foraminiferans (including the first globigerinids), and radiolarians formed deep-ocean oozes and have provided evidence of the latitude of origin and age of some exotic terranes. Other abundant protists were the flagellates, coccolithophores (calcareous platelet-forming organisms that appeared in the Early Jurassic), dinoflagellates (a Middle Jurassic arrival), hystrichosphaerids (protists of uncertain origin but linked to the dinoflagellates), and the ciliophorans. The latter include the tintinnids, an important group of limestone-secreting organisms of the Late Jurassic particularly in the Tethyan region, and the calcareous calpionellids, important zone fossils, especially for the Upper Jurassic. Reefs of the Tethys Sea were formed of siliceous sponges and stromatolites, as well as of corals, and had an extensive infauna. Sponge spicules, along with stromatolites, are prevalent in some limestones in the continental interior of Europe during the Early Jurassic. Corneyella, a genus of thick-walled calcareous sponge, emerged in the Upper Jurassic of Germany, and Pachyterchisma, a genus of siliceous sponge, also appeared in the Upper Jurassic. Coral reefs were widespread throughout the Tethyan region, with the Anthozoa, mainly the order Scleractinia, dominant during the Late Jurassic in Europe and Africa. A number of scleractinian corals, such as the genera Actinarea and Comophyllia, were named by d'Orbigny. Scyphozoans, the medusae or jellyfish, are represented in the inland seas of Europe. Genera include Medusina and Rhizostomites. Large scyphozoan jellyfish are common in Upper Tethyan reef sequences. The contorted, calcareous tubes of serpulid worms are often found clustered in reef-forming masses in the Jurassic carbonates of northwestern Europe. Few brachiopod families, such as the spiriferids, survived the Paleozoic. Nevertheless, two groups, the rhynchonellids and the terebratulids, evolved in the shallow marine environments and survive today. The spiriferids, on the other hand, became extinct during the Jurassic. Rhynchonellids were widespread in the Jurassic of Europe and North America. Some genera are Homoeorhynchia from the Lower to Middle Jurassic of the Alps and western North America, Costirhynchia of the Middle Jurassic in Europe, and the Upper Jurassic (Portlandian and Volgian) form Rhynchonella. Examples of the terebratulids are Epithyris of the Middle Jurassic in Europe and Somalithyris of the Oxfordian in Somaliland. Bryozoans are represented by the encrusting cyclostomes of the warm Jurassic seas and include such genera as Entalophora, Spiropora (a spiral-shaped colonial form), and Idmonea. Members of another order of bryozoa, the cheilostomes, first appeared in the Jurassic and rapidly became abundant. The bivalves, or pelecypods, normally a slow-changing group of mollusks, showed rapid expansion during the Jurassic, adding a dozen new families. Jurassic assemblages tended to include the schizodonts, characterized by few and distinct hinge teeth, and dysodonts, distinguished by weak or absent teeth; included in the latter group are the pectinid and oysterlike forms. Heterodonts were relatively unimportant during this time. Oysters and pectinids, such as the genus Lima, with eulamellibranch gills (specialized gills in which the lamellae consist of solid sheets of tissue), continued to develop toward modern varieties. Pelecypods and gastropods, particularly the prosobranchs, were prominent in the shallow seas of the Jurassic geosynclines. The high-spired nerineids evolved during the Jurassic. Pulmonate gastropods, such as Limonaea and Helisoma, are found in lake beds of Purbeckian age, while others, like Valvata, are marine. In terms of varying shell morphology Itieria, a gastropod from the Upper Jurassic of France, has a complex umbilicate shell. Another group, the archaeogastropods, expanded markedly during the Late Jurassic as well. The ammonoid cephalopods, like the brachiopods, recovered remarkably from near extinction in Late Triassic time on the evolutionary success of two relict genera, Phylloceras and Lytoceras. Complex suture patternsthe trace of the chamber partitions or septa edges on the shell wallsand varying shell morphology and ornamentation provide the variety of index fossils for the more than 50 ammonoid zones that delineate Jurassic stratigraphy. While the Lower Jurassic is characterized by tightly coiled ammonoid shells, loosely coiled forms appear in the Middle Jurassic. Upper Jurassic ammonoids are more distinctively ornamented, and Jurassic ammonoids in general are larger than their predecessors. Later varieties demonstrated regressive shell form, tending toward uncoiling and linear types. Only one group of nautiloid cephalopods survived the Late Triassic extinctions. Coleoid cephalopodsthose lacking external skeletons like the belemnoids (or belemnites), are represented in the Jurassic fossil record by abundant mineralized phragmocones (conical internal shells). Arthropods, mainly the tiny aquatic crustaceans of the subclass Ostracoda, figure prominently in the stratigraphic zonation of the Jurassic. Decapods, crabs, and the first lobsters, which appear in the Upper Jurassic of Europe, are found in Jurassic benthic communities. The modern king crab Xiphosura, commonly called the horseshoe crab, also originated in the Jurassic. Isopods and insects constitute the terrestrial forms. Some insect groups that are represented in the fossil record of the period include the Odonata (dragonflies), Coleoptera (beetles), Neuroptera (lacewings), Diptera (flies), and Hymenoptera (bees, ants, and wasps). Prominent Jurassic echinoids include both stalked and unstalked varieties of pelmatozoans, such as the crinoids and the free-moving echinoids (sea urchins) and stelleroids. The Mesozoic crinoids are principally in the subclass Articulata, which arose in the Triassic. Stem-bearing forms of this group include Isocrinus and Pentacrinites. Holothuroid (sea cucumber) impressions and spicules, which take the form of wheels, crosses, and hooks, have been found in the Lias of Germany. Ophioglypha is a representative genus of Jurassic sea stars, while Plesiocidaris constitutes a representative fossil form of the so-called regular echinoids (those with radially symmetrical bodies). Irregular echinoids (those with bilaterally symmetrical bodies) first appeared in the Jurassic and include Clypeus, a European variety. Correlation between European and North American Jurassic strata has been facilitated through biostratigraphic zonations using primarily ammonoid index fossils. The ammonoid zones, supplemented by other invertebrate and protozoan index fossils, have been used to identify the stages of the Jurassic rocks found throughout the western interior of North America, as well as in Alaska, Mexico, and Cuba. Lower to Middle Jurassic ammonoid assemblages that indicate Toarcian and Bajocian ages are characterized by Sonninia and Tmetoceras and are found in the Kialagvik and Tuxedni formations in Alaska. Defonticeras and Stemmatoceras occur in the Twin Creek Limestone of Bajocian age in Wyoming, and Arnioceras dates the Barranca Formation of the Mexican Sonora as Lias (Early Jurassic) in age. The macrocephalitid ammonite Arcticoceras, which marks the Callovian Stage in Greenland, is found in the Curtis Formation of Utah, the Rierdon Formation of Montana, and the Sundance and Carmel formations of Wyoming. Arcticoceras and Gowericeras occur in the Lower Callovian in the northern interior of North America, as well as in the Lower Sundance Formation of Wyoming. The Cadoceras fauna identifies the Shelikof Formation and the Chinitna Siltstone in Alaska as Callovian, which is the equivalent of the Proplanuites to Erymoceras zones in Europe. Cardioceras and its associated fauna signify the Oxfordian in both Europe and North America; C. cardiforme marks the lower Oxfordian, as, for example, in the Stump Sandstone and the Upper Sundance Formation of Utah and Wyoming. Cardioceras, together with Goliathiceras and Pachycardioceras, represent the Late Oxfordian in the Swift Formation of Montana and the Jagua Formation of Cuba. Phylloceras and other related ammonites form a zonal equivalent to the Oxfordian Peltoceras zone of central Europe. In the Gulf Coast of the United States, the Smackover Formation contains Dichotomosphinctes and an associated fauna considered to be Late Oxfordian in age; Amoeboceras in the Mariposa Slate in the Sierra Nevada range indicates an Oxfordian to Early Kimmeridgian age; and the presence of Cardioceras, along with Amoeboceras, in the Naknek Formation in the Cook Inlet area of Alaska suggests that it is of Oxfordian age. In central and southern Mexico, ammonite zones show the presence of stages between the Bathonian and the Portlandian. Marls of Bathonian age are identified by a Strenoceras assemblage. The occurrence of Idoceras in the Zuloaga Limestone in southern Mexico and the Olvido Formation in the Sierra Madre Oriental, on the other hand, points to an Oxfordian origin. The Tamn Formation is shown to be Kimmeridgian through a Haploceras and Aspidoceras assemblage, and the Pimienta Formation is Portlandian, based on the Parodontoceras fauna. In the Alaskan Range, Oxfordian to Kimmeridgian beds contain Aucella, and Kimmeridgian to Portlandian rocks have Aucella, along with Amoeboceras. A similar fauna is found in the Kupreanof and Gravina islands in the Alaskan panhandle and in the Kingak Shale in the Canning River area of the Arctic slope. Vertebrates Chondrichthyes, mainly sharks, and Osteichthyes, the bony fish, including teleosts, ray-finned fish (or actinopterygians), and holosts (or ganoid fish), were the principal vertebrate swimmers of the Jurassic seas. The teleosts developed ossified vertebrae at this time and showed considerable change in bone structure, fins, and tail. The teleosts are the predecessors of the most prevalent modern fish. Early amphibian groups, such as the labyrinthodonts, became extinct by the Late Triassic and were succeeded by the first Anura (frogs and toads) and salamanders in the Jurassic. The dominant land animals of the Jurassic were reptiles, the most significant of which belonged to the superorder known as Archosauria. The archosaurs included the thecodonts of the Triassic from which the dinosaurs descended, as well as the crocodiles and pterosaurs (flying reptiles). The dinosaurs are divided into two principal groups on the basis of pelvic and hip structure: the saurischians and the ornithischians. The pelvis and hip of saurischians were reptilian (or lizardlike), whereas those of the ornithischians were birdlike. The sauropods, one of the basic types of saurischians, appeared in the Early Jurassic and became abundant in the Late Jurassic. They included both carnivorous and herbivorous forms. The latter were among the largest of the dinosaurs, with certain varieties reaching up to 30 metres in length. These gigantic vegetarians included the genera Apatosaurus, Brachiosaurus, and Diplodocus. (The Apatosaurus was long referred to as Brontosaurus, which was in actuality a form created by workers who inadvertently placed the wrong skull on the body of an Apatosaurus.) Another basic saurischian type was the meat-eating theropod, an early representative of which was Epanteria. This genus was succeeded by Tyrannosaurus and the smaller Allosaurus. The apatosaurs presumably could fend off the allosaurs but not the other larger carnivores. The theropods were all bipedal; they had powerful hind legs and shortened forelegs with sharp claws. The ornithopods, the first major suborder of ornithischians, appeared in the Late Jurassic and were much smaller, ostrichlike forms with large brains. They included the hadrosaurs and the anatosaurs, the bipedal duckbill forms formerly known as trachodons. The anatosaurs were hollow-boned and probably warm-blooded and may have been the ancestors of the birds. It is thought that other dinosaur groups may also have been warm-blooded. Armoured dinosaurs emerged during the Jurassic. One notable variety that roamed North America during the Middle Jurassic was the stegosaur, a relatively large ornithischian type characterized by a double row of vertical bony plates along the dorsal midline and a spiked tail. (For a detailed treatment of the dinosaurs, see the article dinosaur.) The reptiles of the Jurassic had diverse habitats. The plesiosaurs, with paddle-shaped limbs and generally long necks, shared the seas with the ichthyosaurs, highly specialized marine reptiles that have often been likened to such fast-swimming modern fish as tuna and marlin. Two other reptile groups, the crocodiles and marine turtles, are also known from the Early Jurassic, while the lizard made its appearance during Late Jurassic time. The pteropods, airborne flyers or perhaps gliders, were common throughout the Jurassic. The Jurassic pteropods were very small, comparable in size to sparrows and somewhat larger birds. They had hollow bones and were thought to be warm-blooded. Archaeopteryx skeleton, cast made from a fossil found in limestone matrix. Birds evolved in the Late Jurassic and closely resembled some small dinosaurs. Fossils of the first bird, the crow-sized Archaeopteryx (see photograph), were found in the lithographic limestones at Solenhofen in Germany. This bird was very reptilian in appearance, having teeth, a vertebrae-supported tail, three claws at the wing tips, and scales. The specimens, however, have feathers. By Jurassic time, six orders of mammalsmainly small, shrewlike creaturesexisted. Multituberculates, pantotheres, and triconodont mammals were important stock from which later forms developed. These early mammals have been classified on the basis of tooth morphology and were probably omnivorous in diet. Jurassic rocks Occurrence and distribution Pangaea began to break up as North America separated from Eurasia and Gondwana, with Africa subsequently splitting off from India, Australia, and Antarctica. As a result of this intense tectonic activity, alkalic igneous rocks were emplaced in the New England region of North America, and mafic volcanics were extruded in southern Africa during the Early Jurassic (Sinemurian). By Late Jurassic time, Africa had separated from South America and Australia and Antarctica had broken away from India. At the same time, the Iberian Peninsula rotated away from Europe, while Alaska rotated away from Canada. Evidence of these plate movements comes from radiometrically dated igneous intrusions, oceanic sediments (the oldest sediments on the oceanic crust in the Atlantic Basin are of Callovian ageabout 169 to 163 million years oldand the oldest geosynclinal sediments between North America and Africa are approximately 182 million years old), and magnetic anomalies, the oldest of which in the Atlantic is dated at about 153 million years. In the North Atlantic, the separation of North America and Greenland began in Late Jurassic (Kimmeridgian) time, as shown by the emplacement of dikes between 162 and 138 million years ago. The Late Jurassic continental breakup is associated with tectonic activity in North America during the Nevadan orogeny. The fact that South America separated from Africa between 150 and 130 million years ago has been determined by age measurements of Late Jurassic mafic volcanics, magnetic anomalies, geosynclinal sediments, and alkalic igneous intrusions. India began separating from Australia and Antarctica during the Kimmeridgian. North America In eastern North America, Late TriassicEarly Jurassic extensional basins became filled with synrift deposits (red beds and others produced during continental rifting), and pillow lavas were extruded into lake basins. The Upper Newark Supergroup of these basins is Early Jurassic in age, based on potassiumargon ages of 185- to 194-million-year-old basaltic flows like the Watchung Flows of the Newark Basin. More than 150 metres of Lower Jurassic cyclic fluvial and lacustrine beds were deposited in the Culpeper Basin in northern Virginia. Middle Jurassic volcanoclastic rocks have been found beneath continental shelf sediments on the New England margin of North America. Upper Jurassic marine sediments include clastics interfingering with carbonates in the Atlantic and Gulf Coast miogeoclines that formed on the initial continental margins of North America after its separation from Gondwana. Middle Jurassic strata include evaporites (e.g., Louann Salt), red beds, carbonates, and shelf-margin reefs. The Smackover Formation of the Gulf Coast sequences is a geosynclinal sedimentary unit typical of this interval. Plate activity continued with seafloor spreading and the development of the Caribbean microplate. Paleomagnetic studies of Early Jurassic rocks in eastern North America indicate normal polarity and a correlation with similar magnetostratigraphy in Africa and present-day Russia. An Early Jurassic marine transgression, the so-called Sundance sea, covered the western continental interior of North America, depositing thin sandstone, shale, and limestone beds from the Arctic almost to the Gulf of Mexico with accumulations of generally a few hundred metres. Distinctive stratigraphic units include the Early Jurassic Navajo Sandstone, a variable coastal plain sequence that incorporates cross-bedded, dune-formed sandstone and red beds, along with the Late Jurassic Morrison Formation, which is a continental clastic wedge of lacustrine and fluvial mudstone, siltstone, sandstone, and conglomerate that was built to the northeast over the deposits of the epicontinental sea. Uplift of the continental interior occurred between central Arizona and southern California during the interval from the Late Triassic to the Middle Jurassic. Dinosaur fossils (including some tracks) and associated plant and invertebrate fossil remains are especially abundant in the Morrison Formation. Deposition on the Arctic margin of North America was principally of fine-grained marine clastics and a thin continental clastic wedge of sandstone and conglomerate. Early Jurassic deformation was primarily compressional, and deposition continued through Portlandian timethat is to say, into the Late Jurassic. On the western continental margin of North America, volcanic island arccontinent collisions took place along what is now the foothills of the Sierra Nevada on the eastern edge of California during the Nevadan orogeny. Deformation of the Foothills Terrane in the Sierras occurred from 160 to 150 million years ago. Jurassic ophiolites are dated between 200 and 163 million years, as are intrusive plutons (batholiths and various other large igneous bodies) like the granodiorites of the western Sierras. Plutons in the central Sierras are dated at 150 million years. Volcanics, including pillow lavas, also were emplaced at this time. During the Middle Jurassic, accretion of oceanic crust in the Coast Range geosyncline involved a basaltic-to-ultramafic arc terrane and ophiolites above an east-dipping subduction zone in the Klamath Mountains of the OregonCalifornia boundary. This formed Upper Jurassic slate, graywacke, and chert, as exemplified by the Galice Formation and ophiolites of the western Klamath Mountains in southwestern Oregon and the Mariposa Formation in northern California. The Franciscan Formation of the Coast Ranges represents an accretionary wedge of sediments that accumulated in trenches in the plate-collision zone. It is composed largely of deep-water marine deposits called flysch, graywacke, banded radiolarian chert, volcanoclastics, mlange (a mixture of rock fragments of different type, character, and origin), and low-grade metasediments and metavolcanics (metamorphosed sediments and metamorphosed igneous rocks, respectively), together with pillow basalts and greenstone. An Early Jurassic island arc existed in southern Alaska, and a Middle Jurassic back-arc basin and sedimentary mlangenamely, the Coloradito Formationis found along the western coast of Baja California between the Vizcano Peninsula and Cedros Island. The Eugenia Formation of the Late Jurassic represents a younger back-arc basin sequence in the same region. Mafic dikes in northern California also may very well indicate a Late Jurassic volcanic arc. Oceanic sediments from the flat expanse of the deep seafloor known as the abyssal plain and from the continental margin were mixed with carbonates from Pacific atolls to form the accretionary molasse (i.e., thick shore deposits) of the Late Jurassic. East of the collision zone, deposition of sediments in fore-arc basins of the continental margin gave rise to the Great Valley Group of fine-grained sedimentary rocks. The western margin of the continent evolved into a more extensive system of fore-arc and interarc basin structures of late Kimmeridgian time with an accumulation of up to 16,000 metres of sediment. Subduction of a microplate, known as the Farallon Plate, resulted in Late Jurassic accretion of oceanic crust, arcarc and arccontinent collisions, emplacement of granitic intrusions, and east-directed thrust faults from northern California to British Columbia due to compressional movement related to the Nevadan orogeny. This process of microplate accretion has incorporated more than 50 Jurassic exotic terranes (see above) to the western Cordilleran orogenic belt. These exotic terranes, which are typified by the Cache Creek Terrane and the Wrangellia Terrane of the Middle Jurassic, consist of segments of oceanic crust that may have originated in tropical regions of the Pacific and were accreted to western North America between southern Alaska and eastern Oregon.