Middle Jurassic Age Research Articles

Strontium isotopes in brines and associated rocks from Cretaceous strata in the Mississippi Salt Dome Basin (southeastern Mississippi, U.S.A.)

Sr concentrations and 87 Sr 86 Sr ratios have been determined for brines from formations spanning most of the Cretaceous in two oilfields in the Mississippi Salt Dome Basin and from local salt domes, chalk, sands and shales. The Selma Group is predominantly a fossiliferous chalk whereas the remainder of the stratigraphic sequence consists of sands and shales with only minor carbonate phases. The sampled interval ranges from 2000 to just over 4000 m below land surface. Sr concentrations range from 443 to 1718 mg kg −1 for brines from Baxterville field and from 528 to 1762 mg kg −1 for brines from Soso field. Brine 87 Sr 86 Sr ratios range from 0.7071 to 0.7116 at Baxterville field and from 0.7083 to 0.7119 at Soso field. With the exception of brines from the Selma Chalk, all 87 Sr 86 Sr ratios are more radiogenic than Cretaceous seawater. On plots of 87 Sr 86 Sr vs. Sr concentration, brines from individual formations within a producing field occupy distinct non-overlapping areas with the exception of a single sample. In some cases, individual reservoirs within formations can be distinguished. Brines from formations that are present in both fields, which are ∼ 85 km apart, do not have the same 87 Sr 86 Sr ratio. In each field, brine Sr concentration increases with depth. 87 Sr 86 Sr ratios increase with depth to ∼ 3000 m and then appear to level off. These trends are interpreted to be caused by dissolution or recrystallization of carbonate minerals which contain significant amounts of non-radiogenic Sr, and decomposition of Rb-rich detrital micas and K-feldspar, which are sources of radiogenic 87Sr. Salt and cap rock from domes in southeastern Mississippi have an average 87 Sr 86 Sr ratio of 0.70707, consistent with a Middle Jurassic age for the Louann Salt. In Baxterville field, chalk from the Upper Cretaceous Selma Group has a Cretaceous seawater 87 Sr 86 Sr ratio of 0.7076. 87 Sr 86 Sr ratios range from 0.7091 to 0.7111 for Sr leached from sands and shales from both Baxterville and Soso fields. If these values reflect the 87 Sr 86 Sr values of the easily dissolved interstitial carbonate phases, they indicate that the calcite cements precipitated from sedimentary fluids which were more radiogenic than the seawater of that time, but which were not as evolved as the brines which are now present. Alternatively, they may reflect leaching of radiogenic sources.

An expanded view of Jurassic orogenesis in the western United States Cordillera: Middle Jurassic (pre-Nevadan) regional metamorphism and thrust faulting within an active arc environment, Klamath Mountains, California

Basaltic to basaltic andesitic volcaniclastic rocks and their contemporaneous mafic-ultramafic intrusive complexes delineate a Middle Jurassic arc terrane within the Klamath Mountain province of northern California. Exposures of the supracrustal volcaniclastic rocks are restricted to a single fault-bounded terrane, but the deeper level intrusive complexes were emplaced into most, if not all, the pre-Late Jurassic terranes of the Klamath Mountain region. The pre-Late Jurassic terranes thus constitute the basement of the Middle Jurassic arc. U-Pb zircon analyses of 39 zircon fractions from 12 intrusive complexes plus K-Ar dating of the volcaniclastic strata demonstrate magmatic activity over the interval of ∼177-159 Ma. The active arc and its basement were imbricated by a compressive deformational event, the signature of which included thrust faulting, isoclinal folding, and regional metamorphism. Several diverse lines of evidence, including K-Ar dating of metamorphic rocks, crosscutting relations of dated intrusive complexes to thrust faults, and U-Pb dating of synmetamorphic intrusive complexes, establish a distinctly pre-Nevadan Middle Jurassic age (ongoing at ∼169 Ma and over by at least 161 Ma) for this compressive deformational episode. Outboard and structurally beneath the Middle Jurassic arc and its basement are several terranes that collectively comprise the western Jurassic belt. These terranes were deformed and regionally metamorphosed during the Late Jurassic Nevadan orogeny, which occurred within the time interval of ∼157-150 Ma, as Upper Jurassic plutons with 150- to 142-m.y.-old zircon ages have contact aureoles that overprint the Nevadan fabric, and the ∼157-m.y.-old Rogue Formation was deformed in the Nevadan event. The Middle and Late Jurassic compressive deformational events were thus distinct and separated by as much as 15-20 m.y. The relation between Middle and Late Jurassic magmatism and deformation suggests that the Klamath Mountain province records the evolution of a considerably long-lived arc system that evolved above an east-dipping subduction zone. In addition, we suggest that this are system may represent an oceanic continuation of the long-recognized early Mesozoic arc terrane of the western U.S. Cordillera.

Trace fossils in coal-bearing sequences

In the past decade trace fossils have been recorded extensively from coal-bearing sediments, differing widely in facies, age and location. Westphalian or Stephanian 'coal-measures' in Britain, Europe and Eastern Canada contain an ichnofauna produced by invertebrates and/or vertebrates in upper delta plain sediments. This contrasts with the marine-related lower delta plain ichnofaunas known from Pennsylvanian rocks of the United States and Permian Gondwana 'coal-measures' of South Africa. Deltaic complexes of Middle Jurassic age in the North Sea basin and Upper Cretaceous age in North America contain marine trace fossils and dinosaur footprints in coastal coal-bearing facies. These case histories illustrate the importance of trace fossils both in facies analysis of coal-bearing sequences and in recording the presence of animals rarely known as body fossils in such clastic sediments.

Evolution of a Middle Jurassic back-arc basin, Cedros Island, Baja California: Evidence from a marine volcaniclastic apron

A rifted arc-ophiolite assemblage and over-lying volcaniclastic rocks, all of late Middle Jurassic age, represent a fragment of the frontal-arc margin of a back-arc basin, now exposed on Cedros Island in Baja California (Mexico). Ophiolite generation was immediately followed by progradation of a deep- marine volcaniclastic apron (Gran Cannon Formation) contemporaneous with the growth of island-arc volcanoes. This back-arc apron shows relatively simple, uniform sedimentation patterns that may be recognizable in other back-arc basins isolated from terrigenous sediment influx. The progradational back-arc apron sequence includes, from base to top, (1) the tuff lithofacies—thin-bedded, well-sorted, laterally continuous tuffs that thicken into basement paleo-lows. These were deposited from dilute sediment gravity flows generated by eruptions from a deeply submerged, nascent arc. (2) the lapilli tuff-tuff breccia lithofacies—medium- to very thick-bedded lapilli tuffs and tuff breccias. These were deposited from debris flows on proximal parts of the apron, and high-density turbidites were deposited on more distal parts. This lithofacies records increasing generation and resedimentation of scoriaceous and hyaloclastic debris as the summits of the arc volcanoes grew nearer to sea level. (3) the primary volcanic lithofacies—monolithologic dacite pyroclastic flows were produced during the eruption of differentiated magmas, which climaxed the growth of the island arc. Basalt lava flows were fed from fissures that extended down the apron within the back-arc basin. Pyroclastic rocks of the back-arc apron were blanketed with silty basinal turbidites (epiclastic facies) that record abrupt cessation of volcanism and erosion of the arc within about 10 m.y. of ophiolite generation, reflecting the temporal episodicity of back-arc basins. It is predicted from this study that the frontal-arc side of back-arc spreading centers is preferentially preserved in the geologic record relative to the center of the basin. It is suggested that the hot frontal-arc side of the basin is prone to intra-plate volcanism but is unaffected by faulting subsequent to basin initiation. Several factors lead to slumps occurring more frequently on the arc flank of back-arc basins than in any other tectonic setting, except perhaps intra-arc marine basins.

SEDIMENTARY HISTORY AND PALEOGEOGRAPHY OF LOWER AND MIDDLE JURASSIC ROCKS, CENTRAL SAUDI ARABIA

Eight clastic and carbonate lithofacies of Lower and Middle Jurassic age (the Marrat and Dhruma Formations respectively) have been identified. These lithofacies are mainly shales, shales‐and‐siltstones, siltstones‐and‐shales, sandstones, argillaceous limestones, calcarenitic limestones, calcarenites and dolomites. Intraclasts, pellets, oolites, gypsum, algae, and coral reefs were also found to be dominant among these lithofacies. Furthermore, these beds contained either restricted or diversifed biota, with a few sedimentary structures such as lamination, cross‐bedding and common bioturbation. Thus, it is presumed that the Lower and Middle Jurassic rocks in Central Saudi Arabia were deposited in very shallow (i.e. tidal flat and lagoon), shallow‐neritic, and deep‐marine conditions of the Tethys Sea.During Toarcian (Lower Jurassic) time, the Tethys Sea extended towards Arabia, forming an arcuate shoreline around the Pre‐Jurassic sediments. The greater part of the Arabian Peninsula was under very shallow marine (Tethys) water as is shown by the presence of tidal‐flat and lagoon deposits in northern Oman, Rub al‐Khali, parts of the Aden Protectorate and Yemen, central, eastern and NNW Saudi Arabia, the northern edge of Sinai, NW Jordan, Lebanon, central and northern Syria, and west, central and eastern Iraq and Iran (Fig. 5). On the other hand, east and north of this area, very shallow marine deposits overlaid the shallow‐neritic Tethys deposits.Further expansion of the Tethys Sea transgression occurred during Bajocian‐Bathonian (Middle Jurassic) time, when major parts of the Arabian Peninsula and its neighbouring regions were submerged below shallow‐neritic and deepmarine Tethys conditions (Fig. 6). During this time, shallow‐nentic Tethys conditions extended to include the Oman Mountains, as far west as western Rub al‐Khali, north to include Central Saudi Arabia, running through parts of its northern area and passing under the Saudi Arabia‐Iraq Neutral Zone and reaching SW Iraq, turning west to include part of the area of NW Arabia and reaching NW Jordan. Northern Oman, most of central Rub al‐Khali, eastern Saudi Arabia, western Iran, central and eastern Iraq, and eastern and northern Jordan, were covered by deepmarine Tethys waters (F&. 6). However, simultaneously, SW Arabia, Yemen and the Aden Protectorate display a marked continuous sedimentation of continental and neritic conditions, as neritic deposits narrow toward the north.

Crustal laminations in deep seismic profiles in France and neighbouring areas

Summary. Remarkable crustal features appear on the ECORS profiles carried out in northern France and the Bay of Biscay as well as on the SWAT profiles shot in the western Channel and the Celtic Sea. The most striking one is the occurrence of flat laminations in the lower crust. Dipping events and laminations are also present in the upper and lower crust, especially in the SWAT profiles. They can readily be related to tectonic events, Variscan in age, some of them identified in the field. The flat laminations in the lower crust are at first interpreted as resulting from delamination, shearing, magmatism and metamorphism at the crust-mantle transition during the Variscan orogeny. This interpretation raises some difficulty concerning the space and time correlation of the laminations with the Variscan orogeny. They seem to have been emplaced after the Permian-Triassic infilling of the Plymouth Bay basin and before the early Cretaceous opening of the Bay of Biscay. An early to middle Jurassic age is suggested, a period when large cratonic basins were formed without noticeable extension. Heat flow increase and magmatism are proposed as a second hypothesis for the formation of the lower crust laminations. Choosing between orogenic and non-orogenic causes of these laminations will require further deep seismic profiles together with good velocity determination.

Diagenetic peculiarities of potential Lower Jurassic reservoir sandstones, Troms 1 area, Off Northern Norway, and their tectonic significance

AbstractIn the Troms 1 area, sediments of Early to Middle Jurassic age, ranging from alluvial plain deposits at the base, passing through coastal plain/tidal flat sediments up into high-energy nearshore shallow-marine sands, mark a transgression. The sandstones, classified as mineralogically and texturally mature quartz-arenites, are potential reservoir rocks in the eastern part of the area. The apparent supermaturity, however, is of secondary origin because unstable detrital components were dissolved during diagenesis. The succession of complex diagenetic processes was: (i) mechanical compaction and simultaneous pressure solution, (ii) partial dissolution with corrosion of detrital quartz and dissolution of unstable fragments, (iii) silica cementation, (iv) calcite cementation, (v) partial carbonate dissolution, (vi) kaolinite/Fe-carbonate cementation in the remaining pore space. Porosity and permeability of the sandstones are controlled by the degree of silicification and by dissolution processes. Two dissolution stages led to partial ‘skeletonization’ of the detrital framework and to elimination of unstable detrital grains. The first stage was a basic process leading to corrosion of detrital quartz and creating transitory secondary porosity; the second stage was acidic leading to the present preserved secondary porosity. Diagenetic dissolution channels formed. The degree of diagenetic alteration was much higher than normally observed in sandstones of such burial depth. Hydrothermal solutions rising from deep-seated faults may have led to this unusual alteration and triggered a rift-related type of complex diagenesis.

新潟積成盆地に関する最近の地質学的諸問題

1) In the Niigata Sedimentary Basin and the surrounding areas, the basement rocks of the Green Tuff formations belong to the Ashio and Joetsu Belts in the northern part, and to the Maizuru Belt in the southern part. Recent paleontological researches indicate that the formations of the Ashio Belt are mostly of Triassic to middle Jurassic ages. The Joetsu Belt is divisible into two zones: Tanigawadake and Katashina, the latter being correlatable to the Maizuru Zone.2) The middle Miocene Jonai Group and its correlatives distributing in the three areas (Jonai, Yuzawa and Tanigawadake-Minakami) overlie the rocks of the Joetsu Tectonic Belt. They were formed in the same sedimentary basin. Recently plant and foraminiferal fossils were discovered from them. The common characteristic volcanism was eruption of basaltic magma in a marine environment. The successive unique sedimentation of conglomerate was also a common event throughout the three areas during successive ages.3) The reservoirs of the Katakai gas-field are rhyolites of the Nanatani Green Tuff, which is composed of lavas and pyroclastics of basalt, andesite and rhyolite. Rhyolites are thickest and are petrographically classified into five types (A, B, C, D and E). Rhyolitic hyaloclastites are intercalated between rhyolitic lavas, and seem to develop especially on the rather steep slopes underlain by lavas. They represent possibly the submarine pyroclastic flow deposits.Alteration (silisification, chloritization, albitization and sericitization) is mainly due to hydrothermal action, and partly to diagenesis. Carbonatization is conspicuous, and the paragenesis of the carbonate minerals (calcite, dolomite, siderite and ankerite) is peculiar in different horizons, and might have concerned genetically to the high content of CO2 in gas phase and of HCO3- in formation water.

Kastamonu granitoid belt of northern Turkey: First arc plutonism product related to the subduction of the paleo-Tethys

Many granitoid plutons of Middle Jurassic age, arc plutonic in character, form a belt to the north of the paleo-Tethyan suture zone in northern Anatolia. These plutons are here called the Kastamonu granitoid belt. When they are interpreted in a regional geologic setting, it can be shown that they may be formed by the magmatism related to the northward subduction of paleo-Tethyan oceanic crust.

Petrology of Middle Jurassic Twin Creek Limestone, Lincoln and Sublette Counties, Southwestern Wyoming: ABSTRACT

End_Page 298------------------------------Cluster analysis was used to delineate lithofacies in the Twin Creek Limestone of Middle Jurassic age in the Tunp, Salt River, and Wyoming Ranges in southwestern Wyoming. Subjective appraisal of the petrographic data produced lithofacies similar to that created by cluster analysis. Modern carbonate environments and their ancient analogs were compared with information obtained from field study and petrographic analysis of samples of the Twin Creek Limestone to delineate environments of deposition, paleogeography, and diagenetic history. Six major lithofacies were recognized: (1) carbonate mudstone, (2) packstone-grainstone, (3) fossiliferous wackestone, (4) terrigenous mudstone, (5) sandstone, and (6) carbonate mudstone breccia. These lithofacies were deposited in a variety of environments, including outer shelf platforms (carbonate mudstone and fossiliferous wackestone), oolitic sand belts (packstone-grainstone), open to restricted lagoons (carbonate mudstone, fossiliferous wackestone, and terrigenous mudstone), tidal flats, and supratidal environments (terrigenous mudstone, sandstone, and carbonate mudstone breccia). The Twin Creek epeiric seaway experienced two major transgressions (early Bajocian and late Bathonian-early Callovian) and two regressions (early Bathonian and middle Callovian). Lateral migration of t e adjacent facies occurred in response to these changes in sea level. Eogenetic diagenetic features include minor compaction, micritization, coarse fibrous rim cementation, granular cementation, syntaxial rim cementation, and silicification of carbonates. These features were produced in environments ranging from freshwater phreatic to marine phreatic. Mesogenetic diagenesis was characterized by pressure-solution features and neomorphism. Telogenetic features are limited to calcite vein fillings and oxidation coating on carbonate and detrital grains. End_of_Article - Last_Page 299------------

Correlation of Twin Creek Limestone with Arapien Shale in Arapien Embayment, Utah--Preliminary Appraisal: ABSTRACT

Striking and important stratigraphic patterns have emerged as a result of recent work during which members of the Twin Creek Limestone were correlated with the Arapien Shale, all of Middle Jurassic age. These correlations, determined first on the basis of electric and lithologic logs, are supported by recent palynologic work. Three distinct dinoflagellate assemblages, assigned to the Bajocian(?), Bathonian, and Callovian stages, form the paleontologic basis for these correlations. The Bajocian(?) assemblage is found in rocks of the Sliderock and Rich Members of the Twin Creek Limestone. The Bathonian assemblage is found in units of the Boundary Ridge and Watton Canyon Members of the Twin Creek, and also in units of the lower Arapien Shale (lower Leeds Creek Member of the Twin Creek of Wyoming). The Callovian assemblage is found in rocks of the upper Arapien (upper Leeds Creek and Giraffe Creek Members of the Twin Creek of Wyoming). Isopach maps, based on these correlations, indicate that most of central Utah was the site of a large marine embayment--the Arapien embayment--that was flanked on the west, south, and east by highlands. The maps also suggest that the ancestral Uinta Mountains, a submerged feature, affected sedimentation as early as Bajocian time, and became a significant barrier from the late Bathonian through Callovian. In central Utah, marine carbonates were deposited in the Arapien embayment during deposition of the Gypsum Spring through Watton Canyon Members of the Twin Creek Limestone. During deposition of the Arapien Shale, a major northward regression occurred; the embayment shrank to form a smaller basin--the Arapien basin--that lay directly south of the ancestral Uinta Mountains. Most of the rapien Shale is shallow-water deposits, that formed in the basin under hypersaline conditions. End_of_Article - Last_Page 950------------

A palaeomagnetic study of Jurassic rocks from the Antarctic Peninsula and its implications

Palaeomagnetic measurements have been made on rocks of Jurassic age from the Antarctic Peninsula to determine whether there has been relative movement between the region and East Antarctica. Samples were collected from the Oscar II Coast and the Cole Peninsula. Reliable results have been obtained from Jurassic dykes dated in one area at 174 ± 5 Ma, from an acid pluton of approximately the same age and from lavas of middle Jurassic age. Combining these results a position for the palaeopole was obtained at 48°S 238°E. If a reconstruction of Gondwana consistent with sea-floor spreading, palaeomagnetic and geological data is adopted, such as that of Norton & Sclater (1979), then a well-determined position for the Antarctic Peninsula within the reconstruction is obtained. Space considerations suggest that during the Jurassic the relative positions of the Antarctic Peninsula and the Ellsworth block were not very different from the present day, though palaeomagnetic measurements (Watts & Bramall 1981) indicates that the orientation was different.

Geochronology of Mesozoic-Cenozoic magmatism in central Chile, lat. 31°–36°S

Abstract Radioisotope dates indicate magmatism in central Chile (lat. 32°–36°S) has been episodic and migratory throughout Mesozoic and Cenozoic time. Dated plutonic and metaplutonic rocks appear to cluster into the following age groups: (1) 415-380 m.y., (2) 310-270 m.y., (3) 230-170 m.y., (4) 170-140 m.y., (5) 120-80 m.y., (6) 63-56 m.y., and (7) 24-7 m.y. Rocks of Meso- and Cenozoic age are distributed in remarkably well defined, parallel linear belts which trend N-S and are older away from the Central Andean Cordillera. This pattern suggests that the continental margin has increased in width by as much as 200 km in the last 200 m.y. by the emplacement of successive plutonic belts. Volcanic strata of Lower Middle Jurassic age (7000 m) and Early Cretaceous age (7000–13000 m) occur on the west side of the present Coastal Cordillera and on both flanks of the Andean Cordillera. These rocks, mainly andesites, rhyolites, and volcano-clastic sediments, are intercalated with fossil-bearing marine sediments which provide age control up until the final emergence of the Andean Cordillera in Late Cretaceous time. K-Ar dates indicate this continental volcanism continued through the Late Cretaceous, then ceased during the period between 60 and 30 m.y. This hiatus in volcanism coincides with the absence of plutonic rocks in this age range. In the central Andean Cordillera, volcanism began again about 25 m.y. ago and has persisted with variable intensity to the present. Post-Oligocene volcanic strata are generally nearly conformable above Late Cretaceous volcanic strata, and Miocene regional folding has deformed the entire stratigraphic sequence into broad N-S trending folds. Volcanic strata younger than 4 m.y. are undeformed. The presently active chain of strato-volcanoes appears to be less than 500 000 years old and west of older extinct volcanoes 1–2 m.y. old suggesting a westward shift of the volcanic are during the Pleistocene.

The northwest Australian continental margin

The Northwest Shelf of Australia offers a typical example of a ‘passive’ continental margin. Major intra-cratonic basins of Permian to Middle Jurassic age developed along the present coastline, superimposed to either orthogonally trending Palaeozoic basins or Precambrian basement rocks. In each of these depocentres distinct lithotectonic units can be recognized that are related to phases of rifting and subsequent continental break-up. The pre-break-up rift valley and intra-cratonic basin stages are represented by a very thick Permian to Middle Jurassic series of mainly fluvio-deltaic sediments but with occasional marine incursions. Break-up took place near the end of the Middle Jurassic and was accompanied by large-scale block faultings with associated uplift and sub-areal erosion. Gradually late Jurassic to early Cretaceous marine sediments transgressed over the eroded surface: within the general transgressive episode, late Callovian, late Oxfordian to Kimmeridgian, late Tithonian to early Cretaceous marine incursions may be singled out. Open marine conditions, related to the breakup of Gondwanaland and opening of the Indian Ocean, became widespread during the Albian in the southern part of the Australian Northwest Shelf and during the Cenomanian in the northern part. The deposition of a thick prograding wedge of mainly carbonate sedimentation since the mid-Eocene resulted in a northwesterly regional tilt of the Shelf. Hydrocarbon occurrences are related to the tectonic evolution. Early Triassic, early Middle Jurassic, late Oxfordian-Kimmeridgian and early Cretaceous marine incursions are directly related to the deposition of potential source rocks in restricted basins. A regressive phase led to the deposition of Triassic fluviatile sediments with excellent reservoir potential. Break-up tectonism and subsequent marine transgression provided the relevant trapping mechanism and probably the migration paths for the major gas-condensate discoveries of the Rankin Platform. The prolonged high rate of subsidence and accompanying thick sedimentation have ensured that hydrocarbon generation occurred, despite the low geothermal gradient.

Lower and Middle Jurassic alkaline magmatism in the Egersund sub-Basin, North Sea

Core samples from the Egersund sub-Basin of the North Sea reveal two distinct phases of strongly alkaline (potassic) magmatic activity. The oldest rocks are phlogopite/leucitebearing trachyte dyke(s), dated at 178 Ma (K-Ar method on phlogopites). This Lower Jurassic (Pliensbachian) magmatic activity is related to the early stages of rifting in the North Sea. The other magmatic phase, of Middle Jurassic age, is represented by strongly alkaline (potassic) basalt lavas of apparently subaerial origin, and correlates with extensive volcanism in the Piper/Forties fields in the central part of the North Sea.

New Upper Paleozoic and Lower Mesozoic Stratigraphic Units, Central and Western Brooks Range, Alaska

The sequence of rocks described lies primarily in the DeLong and Endicott Mountains in the western and central parts of the Brooks Range, northern Alaska. New names are given to dominantly siliceous and shaly sedimentary rocks of Mississippian through early Middle Jurassic age. The Kuna Formation (new name) consists dominantly of black bedded chert, shale, limestone, and dolomite within the Lisburne Group; some beds are phosphatic, and mafic extrusive igneous rocks occur in a few areas. Paleontologic data suggest an Early Mississippian to Early or Middle Pennsylvanian age for the Kuna Formation. Because the formation intertongues eastward with light-colored shallow-water carbonate rocks of the Wachsmuth and Alapah Limestones of the Lisburne Group, it is also assigned to t e Lisburne Group. Light-colored shallow-water carbonate rocks of other formations of the Lisburne Group were apparently also deposited on the south, but have been telescoped northward by thrust faulting over the darker Kuna Formation. The formation ranges up to 100 m in thickness. The Etivluk Group (new name) is composed of dominantly siliceous beds of the previously named Siksikpuk Formation and overlying Otuk Formation (new name). New paleontologic data suggest that the Siksikpuk is of Pennsylvanian, Permian, and Early Triassic age. The Otuk Formation consists of up to 100 m rhythmically interbedded chert, silicified limestone, and shale previously assigned in this area to the Shublik Formation. A thin but distinctive zone of early Middle Jurassic organic shale and dolomitic limestone ocally present within the top of the Otuk constitutes the Blankenship Member (new name), which is up to 7 m thick. Paleontologic data indicate that the Otuk Formation ranges in age from late Early Triassic to early Middle Jurassic. These dominantly siliceous stratigraphic units appear to be limited to the allochthons of the western and central Brooks Range; they are not known to be present on the autochthonous complex of the northeastern Brooks Range and subsurface Arctic Slope.

Geochemistry of the Mesozoic bedded cherts of Central Baja California (Vizcaino-Cedros-San Benito): implications for paleogeographic reconstruction of an old oceanic basin

In central Baja California (Vizcaino Peninsula, and Cedros and San Benito Islands) two distinct radiolarian bedded chert sequences of late Triassic and late Jurassic/lowermost Cretaceous age, can be differentiated on lithostratigraphic and geochemical criteria. These bedded chert sequences are part of the conformable sedimentary cover of more or less dismembered ophiolites, which are overthrusted by the San Andrès-Cedros volcanic arc system of middle late Jurassic age. Major and trace elements permit paleogeographic zonation of the late Jurassic/lowermost Cretaceous radiolarites lying conformably upon ophiolites considered as fragments of an oceanic basin floor which developed westward of the San Andrès volcanic arc. Progressive accretion of this oceanic basin floor, along the continental margin is supported by the fact that the more distal radiolarian chert sequences belong to the lowermost structural units of this area.

Sedimentology of Brent Formation (Middle Jurassic), Statfjord Field, Norway-United Kingdom : ABSTRACT

The Brent Formation of Middle Jurassic (Bajocian-Bathonian) age is the primary reservoir in the 3-billion bbl Statfjord field which is located in the northern North Sea on the boundary between Norway and the United Kingdom. Averaging approximately 180 m in thickness, the Brent can be conveniently divided into three major units that are easily identifiable on electric logs. The lower unit is a coarsening-upward sequence that commences with shale and terminates with coarse-grained sandstone. Sandstones in this unit, with porosities as high as 33%, comprise the best part of the reservoir. Interbedded shales, sandstones, siltstones, and coals characterize the middle unit, and the upper unit consists of sandstones with minor thin shales and occasional coals. Analysis of cores from 10 wells in the field and several wells in nearby areas indicates that the Brent Formation represents a lobate, river-dominated deltaic complex morphologically similar to the Lafource delta of the Mississippi River. The lower unit, in ascending order, consists of prodelta, distal bar, distributary mouth bar, and distributary channel deposits; the heterogeneous middle unit comprises a delta plain association of distributary channel, interdistributary bay, splay, and marsh deposits. The upper unit was also deposited in a delta plain setting, but in places exhibits evidence of marine reworking. Facies patterns and trends demonstrate that the deltaic sediments of the Brent Formation in the Statfjord area were derived from a source area to the south-southeast rather han from the East Shetlands Platform to the west. End_of_Article - Last_Page 959------------

Origin of Intraformational Folds in Jurassic Todilto Limestone, Ambrosia Lake Uranium Mining District, McKinley County, New Mexico: ABSTRACT

The Todilto Limestone of Middle Jurassic age in the Ambrosia Lake uranium mining district of McKinley County, New Mexico, is the host formation for numerous small to medium-size uranium deposits in joints, shear zones, and fractures within small to large-scale intraformational folds. These folds probably were formed as a result of differential sediment loading when eolain sand dunes of the overlying Summerville Formation of Middle Jurassic age migrated over soft, chemically precipitated, lime muds of the Todilto shortly after their deposition in a regressive, mixed fresh and saline lacustrine or marginal-marine environment of deposition. Encroachment of Summerville eolian dunes was apparently restricted to relatively narrow beltlike zones trending radially across the Todilto coastline toward the receding Todilto body of water. Intraformational folding is believed to be confined to the pathways of individual eolian dunes or clusters of dunes within the dune belts. During the process of sediment loading by the migrating dunes, layers of Todilto lime mud were differentially compacted, contorted, and dewatered, producing both small, and large-scale plastic deformation structures including convolute laminations, mounds, rolls, folds, and small anticlines and synclines. During the processes of compaction and dewatering, the mud, in localized areas, reached a point of saturation at which sediment plasticity was lost, causing shearing, fracturing, and jointing of the contorted limestone beds. These areas or zones within the limestone became the preferred sites of uranium mineralization because of the induced transmissivity created by sediment rupture during prolonged sediment loading. Along the Todilto coastline adjacent to the eolian dune belts, both interdune and coastal sabkha environments dominated the Summerville on the margins of the Todilto body of water. Sediment in these areas consists mainly of claystone, siltstone, sandy siltstone, and very fine grained sandstone which was apparently derived from the winnowing of the finer grained fraction of sediment from adjacent eolian dune fields during eolian activity. Most of the sabkha sediments were probably carried in airborne suspension to the low-lying, ground-water saturated, coastal areas where they were deposited as relatively uniform blanketlike layers. Deposition of sabkha deposits was apparently slow and uniform over most of the Todilto coastal and interdune areas, and did not cause the formation of othe than small-scale deformation features in underlying Todilto rocks. Large-scale deformational features as well as uranium deposits are notably absent in the Todilto where it is overlain by finer textured sabkha deposits in the Summerville. End_of_Article - Last_Page 560------------

Statvik Field of Norwegian North Sea--Exploration Study of Unconformity Trap: ABSTRACT

The Statvik field is situated in block 34/10 in the Norwegian sector of the North Sea approximately 20 km east of the Statfjord field. The main reservoir is the Middle Jurassic Brent sandstone which is truncated by the late Kimmerian unconformity. the unconformity is overlain by Upper Cretaceous shales. The delta structure is one of the biggest structures at the late Kimmerian unconformity level in this part of the North Sea. In the early exploration phase the crucial question was whether the Jurassic reservoir rocks had been eroded during the late Kimmerian orogenic event. Early generation seismic showed no reflectors below the unconformity. A square km seismic reflection survey was shot in 1974 and interpretation of this data gave indications of a thick Jurassic sequence below the unconformity. The first well was drilled during the summer of 1978. It showed a 165-m thick hydrocarbon column in sandstones of Middle Jurassic age. The predrilling interpretation was largely correct. An active period with continuous drilling followed; by August 1979, 8 wells had been drilled on the structure. During End_Page 906------------------------------ the summer of 1979 a 3-D seismic survey covering 190 km2 was shot. Interpretation and drilling were simultaneous. Before drilling, probable recoverable reserves were calculated to be 1,600 million bbl. After drilling the estimate was revised to 1,200 million bbl. The field has now been declared commercial and a successful exploration period has been terminated. This discovery has opened new possibilities in an area under active exploration. End_of_Article - Last_Page 907------------