Early Middle Jurassic Time Research Articles

Foraminifera of the Toarcian-Aalenian boundary from the Lusitanian Basin (Portugal): a paleoecological analysis

The detailed study of 37 well preserved benthic foraminiferal assemblages sampled throughout the upper Toarcian-middle Aalenian in two well calibrated sections representing different facies within the Lusitanian Basin (Portugal) - the Maria Pares and the Zambujal de Alcaria sections - has been carried out, aiming at interpreting past environmental conditions. Several mathematical indexes related to diversity, which allows establishing the particular conditions to which the assemblages were submitted, were applied. All the studied assemblages are characteristic of the Jurassic carbonate platforms of the Boreal Realm, in which the Lenticulina genus usually dominates. The Lagenina Suborder is the most abundant, but taking the second more abundant suborder into account, the comparison between coeval sections in the Lusitanian Basin allows the recognition of a regional gradient ranging from the distal part of the platform (Murtinheira) towards the transitional and proximal parts of the shelf (Maria Pares and Zambujal de Alcaria sections respectively). The relative abundances values, as well as the obtained data from the diversity indexes indicate that the studied assemblages are relatively abundant and diverse, reflecting general appropriate paleoenvironmental conditions for their development. Detailed analysis of the data allows the identification in both sections of three different episodes, traducing particular environmental conditions: unstable (Mactra Subzone-lower part of the Opalinum Subzone), favorable and stable (lower part of the Opalinum Subzone-upper part of the Comptum Subzone), and unfavorable (upper part of the Comptum Subzone-lower Bradfordensis Subzone). These data are consistent with previous results obtained in the Murtinheira and São Gião sections, located in the Northern Lusitanian Basin and corresponding to the distal and transitional parts of the platform respectively, and they aimed at contributing to a better understanding of the paleoecological conditions prevailing within the Lusitanian Basin during the Early-Middle Jurassic times.

Early–Middle Jurassic intra-oceanic subduction in the İzmir-Ankara-Erzincan Ocean, Northern Turkey

The İzmir-Ankara-Erzincan suture zone near Çankırı comprises an ophiolite-related rock suite of amphibolite, pyroxene-amphibolite and epidote-amphibolite. The geochemical data reveal that the basaltic protoliths of the Çankırı amphibolite suite are derived from different mantle sources. Amphibolites showing a LREE enrichment in chondrite-normalized REE pattern, relatively high La N/Yb N (8.5–13.1), Sm/Yb (2.8–3.7), Ce/Sm (8.8–9.8), Nb/Yb (14.4–20.5), Th/Nb (1.1–2.6) originate from an OIB-like (Type I amphibolites) mantle source. Those with flat chondrite-normalized REE pattern, low La N/Yb N (~ 0.6), Sm/Yb (~ 1.0), Ce/Sm (~ 2.7) Nb/Yb (~ 0.6), suggest derivation from a depleted MORB-like (Type II amphibolites) mantle source. A third type of amphibolite is characterized by La N/Yb N (1.54), Sm/Yb (1.1), Ce/Sm (4.5), Nb/Yb (1.1) and Th/Nb (0.3). It is derived from an enriched MORB source and shows characteristics similar to island arc basalts. Amphiboles from these rocks yielded 40Ar/ 39Ar plateau ages between 177.08 ± 0.96 Ma and 166.9 ± 1.1 Ma. These dates are interpreted as metamorphic ages of the amphibolitic rocks. They differ significantly from previous ages of amphibolitic rocks (the metamorphic sole rocks) related to Turkish ophiolites. However, similar metamorphic ages have been reported for metamorphic sole rocks of the Hellenic-Dinaric ophiolites further west. The metamorphic sole rocks have been generated in intra-oceanic environments via intra-oceanic subduction/thrusting events. The new age data of the Çankırı amphibolite suite suggest that the İzmir-Ankara-Erzincan ocean was subducted around Early–Middle Jurassic times, i.e. roughly coeval with the oceanic basin(s) generating the Hellenic-Dinaric ophiolites. In the Early–Middle Jurassic time, the tectonic regime in the İzmir-Ankara-Erzincan ocean was compressional and the ocean was closing at that time. The new age data presented here could imply that Jurassic granites which cross-cut basement rocks of the Pontides are related to the northward subduction of the İzmir-Ankara-Erzincan ocean during Jurassic time.

Lake-mire deposition, earthquakes and wildfires along a basin margin fault; Rønne Graben, Middle Jurassic, Denmark

Regional uplift of structural highs took place in the greater North Sea region in early Middle Jurassic times, resulting in a change from marine shelf deposition in the Early Jurassic to widespread emergence, erosion and localized deposition in grabens. The Middle Jurassic Bagå Formation exposed on the island of Bornholm in the Baltic Sea was deposited in the Rønne Graben, which forms a dog-leg pull-apart basin linking the northern and southern branches of the Tornquist Zone. The studied succession is situated on the hanging wall immediately adjacent to the main eastern graben margin fault and the footwall consists of deeply weathered granite. It is the only locality along the northern branch of the Tornquist Zone where active tectonic influence on Middle Jurassic deposition can be demonstrated in outcrop. Small alluvial fans composed of poorly sorted washout sands and muddy debris flows with weathered granite boulders fringed the footwall scarp. Syn-depositional soft-sediment deformation and chaotic bedding structures interpreted as seismites caused by earthquake shocks are characteristic at several levels. On the hanging wall further away from the active fault the depositional environment was dominated by shallow freshwater lakes with deposition of clay. Lake deposition outpaced subsidence and open, oxygen-deficient freshwater mires were formed during periods of no or only incremental fault-movement. The mires were only occasionally isolated from clastic input due to doming and dense vegetation, resulting in accumulation of pure peat beds. The content of spores and pollen shows that the mires were covered by a vegetation of arborescent and herbaceous Filicopsida (ferns), Lycopsida (club mosses), Cycadales/Bennettitales, Ginkgoales, Caytoniales, Taxodiaceae or Cupressaceae, and Araucariaceae. Wildfires were common in the mires and also on the footwall probably followed by episodes of increased run-off and erosion of the upland. The presence of common washout sands, debrites, seismites and abundant lacustrine flooding surfaces indicating sudden drowning of the mires was caused by abrupt movements on the fault with associated earthquakes. The succession thus provides an example of temporal changes in deposition in response to changes in tectonic conditions. The study illustrates how regional uplift of structural highs caused a restriction of deposition to narrow grabens like the Rønne Graben. Facies patterns changed dramatically from laterally extensive uniform packages controlled by sea-level changes in the Early Jurassic to strongly localized deposition with marked facies changes controlled by faulting and earthquakes in the Middle Jurassic. The succession thus serves as an excellent example of tectonically controlled deposition under humid conditions in a continental graben.

Petromagnetic and paleomagnetic study of Jurassic sedimentary rocks of the southeast of Russia

In the present paper the new petro- and paleomagnetic data on the Jurassic terrigenous complexes of the Mesozoic sedimentary basins of the Amur River region, Trans Baikal region, and Yakutia are presented. The magnetic properties of the sedimentary rocks of coastal-marine (paleo-shelf) and lake genesis are investigated in the contemporary intracontinental riftogenic Mesozoic superimposed troughs of the Siberian and Amur plates: the Chulmansk, the Unda-Dainsk, the Sredne-Amur, the Amuro-Zeisk, and the Verkhne-Bureinsk troughs. The statistically significant differences in the magnetic (anisotropic) characteristics of continental and marine deposits were inferred. The correlation of the scalar and tensor characteristics of magnetic susceptibility anisotropy and the parameters of its linearity, which depend indirectly on the intensity of the folding, is established. The preferential directions of regional stress during the fold formation are determined based on the analysis of the distribution of the axes of the tensor ellipsoid of the magnetic susceptibility anisotropy. The Jurassic positions of the paleomagnetic pole, which are close to the Mesozoic section of the trajectory of its apparent motion for the North Chinese plate and which differ from the Jurassic poles of the Siberian plate, are defined more accurately. The intraplate rotations of geoblocks within the limits of the amalgamated to the end of the Jurassic-to the beginning of the Cretaceous terrains as a part of the Amur tectonic plate are inferred. The calculated coordinates of the paleomagnetic pole indicate the larger than the present-day difference of the latitudinal positions of the southern part of the Siberian plate, and also of the Amur and North Chinese plates in the Early-Middle Jurassic time. This can be indicative of the fact that the total width of the shallow residual basins of the Paleo-Asian Ocean, which separated the geoblocks indicated in the Early-Middle Jurassic, attained the first thousands of kilometers, and/or such a difference in the paleolatitudes reflects the total value of the shortening (crowding) of the crust during the accretion and the fold formation. The time of the completion of the accretion of the terrains of the Amur and the North-Chinese plates and their attachment to the Siberian plate is not earlier than the end of the Late Jurassic-the beginning of the Cretaceous.

Initiation of the Western Interior foreland basin

In this issue of Geology, Fuentes et al. (2009, p. 379) present a new subsidence plot and age data from detrital zircons that help to constrain the date of initiation of the foreland basin in Montana (United States). They offer these data as evidence for the resolution of a controversy regarding the initiation of foreland basin sedimentation in the Western Interior. However, their discussion raises the question of what geological data constitute the defi nitive evidence of the foreland basin style of sedimentation, and therefore the criteria that should be used to defi ne its commencement. A useful defi nition of a foreland basin is that it is a depression that develops adjacent and parallel to a mountain belt. Foreland basins form because the mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere to bend, by a process known as lithospheric fl exure. At convergent plate margins, foreland basins develop behind the marginal arc (retroarc or retroforeland type; Jordan, 1995) or above the downgoing continental plate adjacent to a collision zone (peripheral or proforeland type; Miall, 1995). The downward fl exure is accompanied by the uplift of a forebulge in the basement, typically a few hundred kilometers out from the fold-thrust belt, and beyond that there is a shallow depression called the backbulge basin. The relationship between crustal loading and basin formation was fi rst articulated by Price (1973) with respect to the foreland basin of southern Alberta, and subsequent modeling of the Alberta Basin by Beaumont (1981) demonstrated the quantitative link between fl exural loading, the formation of a crustal depression, and its fi lling by syntectonic sedimentation. Jordan (1981) demonstrated another key criterion for the defi nition of the foreland basin style of subsidence and sedimentation: the distinctive isopach pattern of the basin fi ll, elongated parallel to the mountain front and asymmetric in cross section, with a depocenter located close to the contemporaneous fold-thrust belt, the uplift of which serves as the proximal sediment source. Cross (1986) used this criterion to distinguish foreland basin subsidence from that due to subcrustal mantle loading, which yields a much broader, wider pattern of basinal subsidence. The Western Interior basin (of which the Alberta basin and the Rocky Mountain basin are parts) was initiated as a retroarc basin as a result of a fi rst-order change in the plate kinematics of Pangea. This supercontinent, assembled by multiple plate convergence and suture between Ordovician and Permian time, began to fragment by rifting in the Triassic, and by early Middle Jurassic time, oceanic crust was forming off what is now the Atlantic coast of the United States. The North American continent began a long process of northwestward drift, which carried the plate some 70° of longitude westward, relative to the Pacifi c plate, and, some 40° of latitude northward until Paleocene time, followed by a change in trajectory and a subsequent southward drift of ~10° up to the present day (Engebretson et al., 1985). There are numerous indicators of a fi rst-order change in magmatic, tectonic subsidence and sedimentation patterns associated with this change in plate kinematics. The fi rst appearance of westerly-derived detritus from a rising arc or orogen has long been accepted as providing the timing of initiation of the Western Interior foreland basin. Prior to the establishment of the basin, the western continental margin was miogeoclinal in character, with sediment derived from easterly sources (the craton and the Canadian Shield). Ron Blakey and I (in Miall, 2008), designated the

Spatio-Temporal Changes of Paleozoic-Mesozoic Radiolarian Faunas in Japan and Paleoenvironmental Changes

Abstract. The radiolarian faunal changes across the Permian/Triassic boundary and during Bajocian (early Middle Jurassic) time in Japan are briefly summarized, and the relationships between the radiolarian faunal changes and the paleoenvironmental changes are discussed in this paper. The cause of the radiolarian faunal change from the latest Permian to earliest Triassic time is not well understood, but it is believed due to rapid changes in the marine environment that were disadvantageous to radiolarians. The survival of radiolarians depends on temperature, depth (quantity of light), the amount of dissolved oxygen, nourishment, food and so on. Moreover, it is possible that difference in shell structure may have allowed survivors to adapt more easily to the rapid environmental change across the Permian/Triassic boundary. The Bajocian radiolarian fauna in the Unuma section, central Japan is composed of about 320 species in each horizon and the total reaches about 510 combined in 5 horizons. About 50 origina...

Petrology, geochemistry and geodynamic implications of Jurassic island arc magmatism as revealed by mafic volcanic rocks in the Mesozoic low-grade sequence, eastern Rhodope, Bulgaria

In the eastern Bulgarian Rhodope, mafic extrusive rocks and underlying greenschists are found in the Mesozoic low-grade unit, which represents the northern extension of similar sequences including the Evros ophiolites in Thrace (Greece). Both rock types define a suite of low-Ti tholeiitic basalts to transitional boninitic basaltic andesites and andesites and associated metapyroclastites (greenschists), intruded at its base by diorite dikes of a boninitic affinity. Mafic lavas and greenschists display large ion lithophile element (LILE) enrichment relative to high-field strength elements (HFSE), flat REE patterns of a slight light REE depletion, a strong island arc tholeiite (IAT) and weak MORB-like signature. All these rocks are characterized by negative Nb anomalies ascribed to arc lavas. They have positive ɛNd i values in the range of + 4.87 to + 6.09, approaching the lower limit of MORB-like source, and relatively high ( 207Pb/ 204Pb) i (15.57–15.663) at low ( 206Pb/ 204Pb) i (18.13–18.54) ratios. The Nd isotopic compositions coupled with trace element data imply a dominantly depleted MORB-like mantle source and a contribution of subduction modified LILE-enriched component derived from the mantle wedge. The diorite dike has a low ɛNd i value of − 2.61 and is slightly more Pb radiogenic ( 207Pb/ 204Pb) i (15.64) and ( 206Pb/ 204Pb) i (18.56), respectively, reflecting crustal contamination. Petrologic and geochemical data indicate that the greenschists and mafic extrusive rocks represent a magmatic assemblage formed in an island arc setting. The magmatic suite is interpreted as representing an island arc–accretionary complex related to the southward subduction of the Meliata–Maliac ocean under the supra-subduction back-arc Vardar ocean/island arc system. Magmatic activity appears to have initiated in the north during the inception of the island arc system by the Early–Middle Jurassic time in the eastern Rhodope that most likely graded to back-arc spreading southwards as represented by the Late Jurassic MORB-type Samothraki Island ophiolites. This tectonic scenario is further constrained by paleotectonic reconstructions. The arc–trench system collided with the Rhodope in the Late Jurassic times.

Stratigraphy and sedimentology of a basement-onlapping shallow marine sandstone succession, the Charcot Bugt Formation, Middle–Upper Jurassic, East Greenland

A rocky shore developed in early Middle Jurassic times by transgression of the crystalline basement in Milne Land at the western margin of the East Greenland rift basin. The basement is onlapped by shallow marine sandstones of the Charcot Bugt Formation, locally with a thin fluvial unit at the base. The topography of the onlap surface suggests that a relative sea-level rise of at least 300 m took place in Early Bathonian – Middle Oxfordian times. The sea-level rise was punctuated by relative stillstands and falls during which progradation of the shoreline took place. Palynological data tied to the Boreal ammonite stratigraphy have greatly improved time resolution within the Charcot Bugt Formation, and the Jurassic succession in Milne Land can now be understood in terms of genetically-related depositional systems with a proximal to distal decrease in grain size. The sequence stratigraphic interpretation suggests that translation of the depositional systems governed by relative sea-level changes resulted in stacking of sandstone-dominated falling stage deposits in the eastern, basinwards parts of Milne Land, whereas thick, remarkably coarse-grained transgressive systems tract deposits formed along the western basin margin. The bulk of the Charcot Bugt Formation consists of stacked sandstone-dominated shoreface units that prograded during highstands. The overall aggradational to backstepping stacking pattern recognised in the Charcot Bugt Formation is comparable to that in the contemporaneous Pelion Formation of the Jameson Land Basin and in correlative units of the mid-Norway shelf and the Northern North Sea. We suggest that the long-term evolution of the depositional systems may have been controlled by long-term eustatic rise acting in concert with relative sea-level changes reflecting regionally contemporaneous phases of rift initiation, climax and gradual cessation of rifting.

Early and Middle Jurassic mires of Bornholm and the Fennoscandian Border Zone: a comparison of depositional environments and vegetation

Suitable climatic conditions for peat formation existed during Early–Middle Jurassic times in the Fennoscandian Border Zone. Autochthonous peat and allochthonous organic matter were deposited from north Jylland, south-east through the Kattegat and Øresund area, to Skåne and Bornholm. The increase in coal seam abundance and thickness from north Jylland to Bornholm indicates that the most favourable peat-forming conditions were present towards the south-east. Peat formation and deposition of organic-rich muds in the Early Jurassic coastal mires were mainly controlled by a continuous rise of relative sea level governed by subsidence and an overall eustatic rise. Watertable rise repeatedly outpaced the rate of accumulation of organic matter and terminated peat formation by lacustrine or lagoonal flooding. Organic matter accumulated in open-water mires and in continuously waterlogged, anoxic and periodically marine-influenced mires. The latter conditions resulted in huminite-rich coals containing framboidal pyrite. The investigated Lower Jurassic seams correspond to peat and peaty mud deposits that ranged from 0.5–5.7 m in thickness, but were generally less than 3 m thick. It is estimated that on Bornholm, the mires existed on average for c. 1200 years in the Hettangian–Sinemurian and for c. 2300 years in the Late Pliensbachian; the Early Jurassic (Hettangian–Sinemurian) mires in the Øresund area existed for c. 1850 years. Aalenian uplift of the Ringkøbing–Fyn High and major parts of the Danish Basin caused a significant change in the basin configuration and much reduced subsidence in the Fennoscandian Border Zone during the Middle Jurassic. This resulted in a more inland position for the Middle Jurassic mires which on occasion enabled peat accumulation to keep pace with, or temporarily outpace, watertable rise. Thus, peat formation was sometimes sustained for relatively long periods, and the mires may have existed for up to 7000 years in the Øresund area, and up to 19 000 years on Bornholm. The combination of the inland position of the mires, a seasonal climate, and on occasion a peat surface above groundwater level caused temporary oxidation of the peat surfaces and formation of inertinite-rich coals. The spore and pollen assemblages from coal seams and interbedded siliciclastic deposits indicate that the dominant plant groups in both the Early and Middle Jurassic mires were ferns and gymnosperms. However, significant floral differences are evident. In the Lower Jurassic coals, the palynology testifies to a vegetation rich in cycadophytes and coniferophytes (Taxodiaceae family) whereas club mosses were of lesser importance. Conversely, in the Middle Jurassic coals, the palynology indicates an absence of cycadophytes, a minor proportion of coniferophytes (Taxodiaceae) and a significant proportion of club mosses. These variations are probably related to adaptation by different plants to varying environmental conditions, in particular of hydrological character.

The Jurassic of Kuhn Ø, North-East Greenland

The Middle–Upper Jurassic succession of Kuhn Ø, North-East Greenland accumulated in a major half-graben and is an excellent analogue for the subsurface of the mid-Norwegian shelf. On Kuhn Ø, peneplaned crystalline basement was incised by a drainage system during a major base-level lowstand, probably in late Early or early Middle Jurassic times. It was filled with fluvial conglomerates of the newly defined Middle Jurassic Bastians Dal Formation during subsequent base-level rise. As sea level continued to rise, precursor-peat of the coals of the Muslingebjerg Formation formed in swamps which covered the conglomerates and filled the remaining space of the incised valley system. The valley and interfluve areas were flooded in Late Bathonian – Callovian times and tidally-dominated, shallow marine sandstones of the Pelion Formation were deposited on top of the valley fill and over the adjacent basement peneplain. These sandstones are overlain by the newly defined shallow marine Oxfordian Payer Dal Formation which is subdivided into a lower unit and an upper unit, separated by a major drowning surface. The Payer Dal Formation sands were flooded in the Late Jurassic and organic-rich, offshore mudstones of the Bernbjerg Formation were deposited. The Jurassic succession of Kuhn Ø can thus be subdivided into large-scale sedimentary units separated by major drowning surfaces. They are of regional extent, and in combination with biostratigraphic and 87Sr/86Sr isotope data they allow the correlation of the sedimentary units on Kuhn Ø with more offshore deposits to the south in Wollaston Forland and more landwards successions to the north in Hochstetter Forland. Petrographically, the trough cross-bedded sandstones of the Pelion Formation and the lower unit of the Payer Dal Formation include both calcite-cemented and poorly cemented quartz sandstones. The calcite cement was derived from dissolution of abundant calcareous fossils and forms concretionary horizons. The upper unit of the Payer Dal Formation mainly consists of weaklycemented quartz sandstones with porosities around 30%. The sandstones of the Pelion and Payer Dal Formations on Kuhn Ø are petrographically very similar to Jurassic sandstones from the mid- Norwegian shelf and the Barents Sea with regard to original mineralogical composition, sorting and grain size. The Bernbjerg Formation mudstones are comparable to the Upper Jurassic source rock of the mid-Norwegian shelf and the Barents Sea, but have lower hydrogen index (HI) values due to terrigenous input in a relatively proximal setting. Coals of the Muslingebjerg Formation have significant source rock potential with measured HI values up to 700, kerogen types II–III and total organic carbon (TOC) values above 50%.

Jurassic lithostratigraphy and stratigraphic development onshore and offshore Denmark

A complete updated and revised lithostratigraphic scheme for the Jurassic succession of the onshore and offshore Danish areas is presented together with an overview of the geological evolution. The lithostratigraphies of Bornholm, the Danish Basin and the Danish Central Graben are described in ascending order, and a number of new units are defined. On Bornholm, the Lower–Middle Jurassic coal-bearing clays and sands that overlie the Lower Pliensbachian Hasle Formation are referred to the new Sorthat Formation (Lower Jurassic) and the revised Bagå Formation (Middle Jurassic). In the southern Danish Central Graben, the Middle Jurassic succession formerly referred to the Lower Graben Sand Formation is now included in the revised Bryne Formation. The Lulu Formation is erected to include the uppermost part of the Middle Jurassic succession, previously referred to the Bryne Formation in the northern Danish Central Graben. The Upper Jurassic Heno Formation is subdivided into two new members, the Gert Member (lower) and the Ravn Member (upper). The organic-rich part of the upper Farsund Formation, the former informal ‘hot unit’, is established formally as the Bo Member. Dominantly shallow marine and paralic deposition in the Late Triassic was succeeded by widespread deposition of offshore marine clays in the Early Jurassic. On Bornholm, coastal and paralic sedimentation prevailed. During maximum transgression in the Early Toarcian, sedimentation of organic-rich offshore clays took place in the Danish area. This depositional phase was terminated by a regional erosional event in early Middle Jurassic time, caused by uplift of the central North Sea area, including the Ringkøbing–Fyn High. In the Sorgenfrei–Tornquist Zone to the east, where slow subsidence continued, marine sandy sediments were deposited in response to the uplift. Uplift of the central North Sea area was followed by fault-controlled subsidence accompanied by fluvial and floodplain deposition during Middle Jurassic time. On Bornholm, deposition of lacustrine muds, fluvial sands and peats dominated. The late Middle Jurassic saw a gradual shift to shallow marine deposition in the Danish Central Graben, the Danish Basin and Skåne, southern Sweden. During the Late Jurassic, open marine shelf conditions prevailed with deposition of clay-dominated sediments while shallow marine sands were deposited on platform areas. The Central Graben received sand by means of sediment gravity flows. The clay sediments in the Central Graben became increasingly rich in organic matter at the Jurassic–Cretaceous transition, whilst shallow marine coarse-grained deposits prograded basinwards in the Sorgenfrei– Tornquist Zone.

Mesozoic–Paleogene sedimentary facies and paleogeography of Tibet, western China: tectonic implications

AbstractIn Early–Middle Triassic time, an abyssal sea covered most of the Songpan–Ganzi area, whereas a Central Tibetan Landmass, up to 400 km wide, may have stretched across the Lhasa and Western Qiangtang terrains. In Late Triassic time, the Songpan–Ganzi sea closed, the Central Tibetan Landmass receded westwards away from southern Western Qiangtang, a littoral environment dominated Eastern Qiangtang, middle Western Qiangtang, and southeastern Lhasa, a shelf environment existed only in northern and southeastern Western Qiangtang and northwestern Eastern Qiangtang, and abyssal flysch was spread along the eastern Bangonghu–Nüjiang zone. In Early–Middle Jurassic time, Songpan–Ganzi had become part of the Eurasian continent, abyssal flysch sediments stretched throughout the Bangonghu–Nüjiang zone, the Central Tibetan Landmass was only locally present in southwestern Lhasa, and the Tethyan epicontinental sea nearly covered all Tibet southwest of the Jinsajiang suture. In Late Jurassic time, oceanic flysch deposition existed only along the westernmost Bangonghu–Nüjiang zone, nearly all of Tibet was covered by coastal deposits, and shelf deposits existed only in northern Western Qiangtang and westernmost Lhasa. In the early stage of Early Cretaceous time, the majority of Qiangtang had become dry land, and a supralittoral environment dominated across the entire Lhasa terrain. However, during the late stage of the Early Cretaceous time, platform–shelf carbonates prevailed on southern Western Qiangtang and northern Lhasa. In Late Cretaceous time, the majority of Qiangtang had become emergent land, and a supratidal environment dominated Lhasa, the western rim of Western Qiangtang, and Tarim. In Paleogene time, the majority of Tibet became emergent land, and a supratidal environment existed only on the southern and western rims.The dominance of Upper Triassic–Jurassic shelf carbonates on the northwestern Eastern Qiangtang corner and the northern Western Qiangtang rim suggests a diachronous closing of the Jinsajiang paleo‐Tethys ocean, first during latest Triassic time when the Eastern Qiangtang terrain collided with Asia and finally in Jurassic time when the Western Qiangtang terrain was amalgamated to Asia. Rich picotites in Upper Triassic sandstones of middle Qiangtang suggest that the Shuanghu suture could have extended along the middle of Qiangtang, and stable shelf sedimentation during Late Triassic–Middle Jurassic time in the Western Qiangtang terrain shows that the suture probably could not have formed until Middle Jurassic time. The opening time of the Bangonghu–Nüjiang mid‐Tethys ocean could be Late Triassic time due to the existence of the Central Tibetan Landmass across Western Qiangtang and Lhasa during Early–Middle Triassic time. However, its opening was diachronous, at Late Triassic time in the east and at Early–Middle Jurassic time in the west. Furthermore, its closing was also diachronous, first in the east at the beginning of Late Jurassic time and later in the west in latest Jurassic to earliest Cretaceous time. Widespread upper Lower Cretaceous limestone up to 5 km thick over the northern half of Lhasa indicates that southern Tibet could have undergone an extensive backarc subsidence during late Early Cretaceous time. Continuous shallow marine sedimentation through the entire Cretaceous time over much of southern Tibet indicates that southern Tibet was intensely elevated only after the end of Paleogene time, its high topography being the product of the Indo‐Asian collision. Copyright © 2002 John Wiley & Sons, Ltd.

Jurassic foraminifera from the Queen Charlotte Islands, British Columbia, Canada: biostratigraphy, paleoenvironments and paleogeographic implications

Lower and Middle Jurassic (Sinemurian to Callovian) foraminifera were examined from 66 localities of the Queen Charlotte Islands, British Columbia, Canada. Over 200 species of well preserved benthic calcareous and agglutinated taxa belonging to 50 genera were identified. Foraminifera from the Lower Jurassic are represented by an abundant and diverse, Tethyan-derived calcareous assemblage, with Boreally derived agglutinated taxa becoming gradually more dominant in the upper Lower Jurassic to lower Middle Jurassic. Sedimentary strata and their faunal components and position of hiatuses correlate approximately with the Jurassic eustatic sea-level curve. Sea-level changes have resulted in a variety of depositional environments ranging from deep-water euxinic shales with abundant tuff beds, characterized by small pyritized foraminifera, to a well oxygenated outer shelf setting, characterized by a major faunal increase, to shoreface sandstones and conglomerates with absent to rare benthic foraminifera. A gradual shallowing from a slope- to a shelf-dominated deposition, with a magmatic arc provenance, is suggested for the Lower Jurassic, changing in the Middle Jurassic to an inner shelf to upper shoreface setting dominated by sedimentary strata of volcaniclastic origin. A marked faunal turnover is observed in the Lower Jurassic Whiteaves Formation (middle Toarcian) with the extinction of several earlier species and the appearance of a new diverse assemblage of 225 taxa. This turnover is also observed in the ammonite and radiolarian assemblages of the Queen Charlotte Islands. In comparison to the dominantly Tethyan-derived taxa of pre-middle Toarcian age, many of the species from the middle Toarcian are found in northern latitudes such as on the northern slope of Alaska, Arctic Islands and the Northwest Territories. Hence, foraminifera, in conjunction with existing radiolarian and ammonite data, suggest a northward displacement of the Queen Charlotte Islands from low to mid-paleolatitudes by late Early Jurassic (Toarcian) to early Middle Jurassic (Bajocian) time.

Age of Pre-Break-Up Gondwana Magmatism

Extensive outpourings of basalt, and to a lesser extent rhyolite, are closely associated with continental break-up and plume–lithosphere interactions. The Gondwana supercontinent began to fragment during Early–Middle Jurassic times and was associated with the eruption of over three million km3 of dominantly basaltic magma. This intense magmatic episode is recorded in volcanic rocks of the Karoo (Africa), Ferrar (Antarctica) and Chon Aike (South America). K–Ar and Rb–Sr whole rock geochronology has consistently failed to produce reliable ages for these volcanic rocks, but in the last four years, the wider application of single grain 40Ar/39Ar and/or U–Pb geochronology has produced more robust and precise dating of the magmatism. This paper reviews the recent advances in high precision geochronology and provides a full recalibrated 40Ar/39Ar dataset. Application of these methods across the majority of the volcanic provinces indicates that approximately 80% of the volcanic rocks were erupted within a short, 3–4 Myr period at c. 182 Ma. This burst of magmatism occurred in the Karoo province at c. 183 Ma and in the Ferrar provinces at c. 180 Ma, and was dominated by mafic volcanism. This peak in volcanism is coincident with a second order mass extinction event at the end of the Pliensbachian when c. 5% of marine families were wiped out coinciding with widespread oceanic anoxia in the early Toarcian. A prolonged period of silicic volcanism occurred along the proto-Pacific margin, prior to, and during the main phase of break-up. Silicic volcanism was initially coincident with the plume related Karoo-Ferrar provinces, but continued over c. 40 Myr, associated with lithospheric extension and subduction along the proto-Pacific continental margin.

Tectonic significance of the Hellenic-Dinaric ophiolites

Abstract Isotopic and biostratigraphic data show the Hellenic ophiolites (those SE of the Scutari-Pec transform zone in Albania) are of Middle to Late Jurassic age. They are of two contrasting types. The first type of ophiolite is complete, usually harzburgitic, forms allochthonous sheets, commonly develops metamorphic soles, generally includes significant chrome deposits and shows supra-subduction zone (SSZ) trace-element characteristics with an evolution from MORB to IAT and occasionally boninites. The second type is characterized by incomplete ophiolites which lack chrome and mantle tectonite rocks. In contrast to the first type, these are slightly younger, are exposed in situ, show contact metamorphism against the country rock, are similar to MORB and show little influence of a subduction zone. The harzburgitic ophiolites from a western and an eastern outcrop belt. The relationship between the two is unclear, principally because of large, probably strike-slip, offsets in the Vardar zone, best documented in former Yugoslavia. The western Hellenic ophiolites (Pindos, Vourinos, Othris, Evvia ( = Euboea) and Argolis) all formed above a SW-dipping subduction zone in early Middle Jurassic time. They are continuous with a similar belt in the Dinarides of Yugoslavia whose SW margin gradually becomes lherzolitic as it is traced northwards. It is totally lherzolitic at its termination in NW Yugoslavia. This gradual change in ultramafic character is speculatively linked to the changes in spreading rates in a wedge-shaped marginal basin. Emplacement took place in later Middle to Late Jurassic time probably along a low-angle detachment zone that was close to the lithosphere/asthenosphere boundary shortly after spreading ceased. The SW half of the basin was pushed over the NE half, which was subducted to the SW but has left very little trace of its former existence. In the Hellenides the ophiolites were emplaced onto the Pelagonian continent and in the Dinarides onto the Drina-Ivanjica continent. Precise analogues of the complete ophiolites appear to be absent in the present-day major ocean basins — because of their SSZ character — or any island arcs that border the present-day oceans — because their marginal stratigraphy shows no trace of arc sediments or volcanics. The incomplete ophiolites, which appear to be restricted to the Axios zone in Greece, seem to be the expression of transient motion on short extensional segments of a plate margin whose geometry resembled the early stages of the opening of the Gulf of California.

Geology of the Hazelton Volcanic Belt in British Columbia: Implications for the Early to Middle Jurassic evolution of Stikinia

The Hazelton Group is an Early to Middle Jurassic volcanic and sedimentary succession that was deposited on the Stikine Terrane (Stikinia) of the Canadian Cordillera. The group is distributed across the entire width of Stikinia, a distance of about 300 km. Prior to post‐Hazelton contractional deformation, the width across which volcanism had occurred was at least 450 km. A review of chronologic and stratigraphic information corroborates an earlier suggestion that the Hazelton Group formed as a pair of coeval, partly subaerial volcanic chains separated by a subsiding, mainly sedimentary marine basin (Hazelton Trough). The trough developed in response to extension of Stikinia during the latest Triassic and/or earliest Jurassic, and remained the locus of moderate extension during deposition of the Hazelton Group. Volcanic rocks of the group are diverse, comprising subaerially and subaqueously deposited lavas and volcaniclastic rocks of mafic to felsic composition. Calc‐alkaline to tholeiitic characteristics, and depletion of high‐field‐strength elements relative to alkali and alkaline earth elements indicate a subduction‐related origin. The degree of alkalinity (medium‐ to high‐K) is fairly consistent throughout the group. The original width across which volcanism occurred is two to five times greater than widths of modern volcanic arcs. The anomalous width cannot be accounted for by time transgression of a single volcanic arc, because the two volcanic chains were coeval. Extremely low‐angle subduction of a single oceanic plate is also rejected as an explanation because alkalinity does not differ significantly between the two volcanic chains. Genesis of the Hazelton Group above a single subduction zone is therefore considered unlikely. We propose a model in which a pair of oceanic plates were subducted beneath Stikinia from opposite sides, in a manner analagous to the tectonic regime of the Philippine archipelago. Subduction angles of the two downgoing plates are postulated to be 30° to a depth of 150 km, a configuration consistent with a postulated forearc width of about 175 km on each side. Stikinia is thereby considered to have been an 800‐km‐wide microplate on which two island arcs developed. The Hazelton Trough was a back‐arc region common to both volcanic arcs. During deposition of the Hazelton Group, Stikinia was not part of the Intermontane Superterrane, an amalgam composed of more inboard terranes. Rather, it belonged to an oceanic microplate that accreted to the inboard terranes in late Early Jurassic to early Middle Jurassic time, during which Hazelton volcanism waned and ended.

Bowser basin, northern British Columbia: Constraints on the timing of initial subsidence and Stikinia-North America terrane interactions

Clastic strata composing the northern Bowser basin record the accretion of Stikinia to the composite western edge of the North American plate (Cache Creek-Quesnellia-Slide Mountain-Kootenay North America) in early Middle Jurassic time and the concomitant demise of the intervening Cache Creek ocean. Initial flexural subsidence of the northern Bowser basin, resulting from thrust loading of Cache Creek terrane on Stikinia, is represented by an organic-rich shale of Aalenian age (the Abou Formation of the Spatsizi Group). Coarse-grained sediment first appeared in early Bajocian time following uplift and subaerial exposure of Cache Creek rocks in the upper plate. Thus, the inception of the Bowser basin was Aalenian, rather than Bajocian, as believed by earlier workers. Aalenian southwest-vergent thrusting at the composite western edge of North America is also known from southern British Columbia, a coincidence that implicates collision with Stikinia in the south as a cause of that deformation.

Stratigraphic distribution and ecology of European Jurassic bivalves

Documentation of bivalve generic and species diversity and times of first and last appearance through successive Jurassic stages in Europe, together with data on turnover and changes in taxonomic and ecological composition of the faunas, indicate an approximation to the establishment of an equilibrium fauna by early Middle Jurassic times. Subsequently faunal change was slight compared with the early Jurassic. A diversity increase through the Lower into the Middle Jurassic correlates with an increase in the area of epicontinental seas, while a major species extinction in the early Toarcian is bound up with the onset of widespread stagnation associated with a rise of sea level. An increase of the generic extinction rate at the end of the period correlates with a regional marine regression. The mean species longevity is estimated at 15×106 years. The ecological factors thought to control bivalve distribution are reviewed and four ecological associations distinguished: the reefal, lagoonal and nearshore and basinal marine.

Paleomagnetic study of lower Mesozoic diabase dikes and sills of Connecticut and Maryland

Paleomagnetic results from 61 sites on diabase dikes and sills in Connecticut and Maryland yield a mean pole of position of 100.9 °E. 68.6 °N, α95 = 1.6°. These results combined with those from 7S sites on four diabase sills in the Gettysburg Basin of Pennsylvania produce a mean pole position of 101, 6 °E. 65.4 °N, α95 = 1.3 °from 139 VGPs. These rocks arc probably of Early to early Middle Jurassic age by stratigraphic and inferred stratigraphic position. The paleomagnetic results indicate that the Gettysburg and Hartford Basins did not subside simultaneously.The dispersion of virtual geomagnetic poles about the mean pole of the Connecticut, Pennsylvania, and Maryland intrusive rocks is probably the result of 'homogenization' of magnetic directions during low-temperature oxidation of titanomagnetite to titanomaghemite over a period long enough to erase a considerable amount of dispersion due to secular variation.Rotation of Europe against North America in the prerifting configuration causes good convergence of European Jurassic mean poles and the pole of the Connecticut. Maryland, and Pennsylvania intrusive rocks. This is evidence that separation of the two continents had not begun by Early to early Middle Jurassic time, which is in agreement with earlier findings based on sea-floor magnetic anomalies and faunal assemblages.