Lower Jurassic Sequence Research Articles

An integrated workflow for petrophysical characterization, microfacies analysis, and diagenetic attributes of the Lower Jurassic type section in northeastern Africa margin: Implications for subsurface gas prospection

The Jurassic sedimentary sequences are an important source and reservoir rocks in the North Africa margin, with its type section, outcropped at the Gebel El-Maghara massif and some other sites along the southern Tethys. These exposed Jurassic outcrops, of the intercalated continental and marine sediments, allow us to conduct more detailed and integrated studies on the various depositional settings with diverse lithologies as analogous for the subsurface reservoirs. The present study concerns the implication of the mineralogical components and the diagenetic controls on the petrophysical properties and the reservoir characteristics of the Lower Jurassic in Gebel El-Maghara outcrop sequence (from base to top Mashaba, Rajabia, and Shusha Formations) in north Sinai, Egypt. Some field trips were conducted, and samples were collected, to characterize the petrographical and petrophysical characteristics of these deposits using comprehensive standard techniques including studying thin sections, SEM imaging, XRD analysis, and various petrophysical measurements. A workflow is proposed to apply a full set of petrophysical measurements on the collected samples including porosity (∅He), permeability (k), grain and bulk densities (ρg and ρb, respectively), and true formation resistivity factor. In this workflow, we have integrated the petrophysical data with detailed microfacies analysis to reveal the implication of diagenesis on the reservoir quality of the Lower Jurassic reservoirs. The Lower Jurassic sequence is subdivided into five hydraulic flow units (HFUs) that consist of three reservoir rock types (RRTs). These RRTs are composed of three microfacies associations (MFAs). The RRT1 plug samples consist of the mostly clastic MFA1; they are porous, permeable, and have good reservoir quality that is using the flow zone indicator (FZI), and the reservoir quality index (RQI). The lowest reservoir quality is assigned to the RRT samples which are mostly composed of tight carbonates. Based on the petrographical studies, SEM imaging, and the XRD analysis, dissolution and fracturing slightly enhanced the reservoir potentiality of the RRT1 samples, whereas physical compaction, tight cementation, and authigenic clay content (kaolinite, hematite, and goethite) are responsible for the deterioration of the reservoir properties of RRT3 samples and reduction of the RRT2 samples. The study outcrop has the potential to be an excellent analogue for the important subsurface Jurassic gas-bearing deposits on the African's northern margins. Furthermore, it may provide insights into the fluid flow units, mineralogical composition, and diagenetic controls on the unique Lower Jurassic deposits of Gebel El-Maghara for the assessment of subsurface gas-bearing sequences in the nearby Mediterranean and Gulf of Suez from the southern Tethys shorelines.

Age of the basement beneath the Mesozoic Lusitanian Basin revealed by granitic xenoliths from the Papôa volcanic breccia (West Iberia)

The dyke of the Papôa volcanic breccia cross-cutting the Lower Jurassic sequence of the Lusitanian Basin (West Iberia) contains granitic xenoliths. In this study, for the first time, U-Th-Pb zircon analysis of two xenoliths yielded 298±4Ma for biotite granite and of 292±2Ma for two-mica granite, indicating that the pre-Mesozoic basement of the Lusitanian Basin includes Permian intrusions. These ages are close within the margin of error of the age of the Late Carboniferous granites of the Berlengas isle that with the Late Devonian high-grade metamorphic rocks of the Farilhões isles, located northwest of the study area, which form the pre-Mesozoic basement of the Lusitanian Basin. These new geochronological findings enable it to be established that Permo-Carboniferous magmatism lasted at least 13Ma, in this region of the Appalachian-Variscan belt. Furthermore, a comparison with available data from Paleozoic tectonic units of the Appalachian-Variscan belt located both in the Iberian Massif and outside it enables the suggestion to be made that the Lusitanian Basin (Peniche) most probably rests on the South Portuguese Zone, which may also be correlated with the Rhenohercynian Zone present in southwest England, and the Meguma terrane of Nova Scotia.

The Hettangian–Early Pliensbachian Evolution Phase of the Anabar–Lena Sedimentary Basin, Nordvik Facies Region

New data on the structure and formation conditions of the Anabar–Lena sedimentary basin in the Nordvik facies region at the beginning of the Early Jurassic (Hettangian–early Pliensbachian) are presented. Based on a detailed lithofacies analysis, coastal-continental, coastal-marine, and shallow-marine sedimentation conditions were reconstructed. Ichnofacies studies were carried out for the first time: various traces of the vital activity of organisms identified as Scolithos and Cruziana ichnofacies were identified. An analysis of the relationship between individual lithofacies and their associations made it possible to identify a series of different rank cyclites in the section. In the general regressive series, six decameter-scale cyclites corresponding (except the basal, regressive migration regime of cyclogenesis) were established. They are characterized by an asymmetric internal structure, reflecting periods of coastline migration. The reconstructed model of the sedimentary sequence allows one to trace temporal changes in the sedimentation conditions and identify the main events in the sedimentary basin. It is established that the main Early Jurassic evolution phases in the western part of the Anabar–Lena paleobasin are reflected in the structure of the section and are caused by the alternation of transgressive and regressive periods against the background of a gradual attenuation of the regional tectonic activity. The results obtained allow us to significantly refine our understanding of sedimentation evolution in the Anabar–Lena basin. The revealed cyclical nature of the Lower Jurassic sequence can be used for the regional lithostratigraphic correlation.

Origin and formational history of some Pb-Zn deposits from Alborz and Central Iran: Pb isotope constraints

Several Pb-Zn deposits and occurrences within Iran are hosted by Mesozoic–Tertiary-aged sedimentary and igneous rocks. This study reports new Pb isotope analyses for galena from 14 Pb-Zn deposits in the Alborz and Central Iran structural zones. In general, Pb isotope ratios are extremely variable with data plotting between the upper crustal and orogenic curves in a plumbotectonic diagram. The latter may be attributed to Pb inputs from crustal and mantle end-members. Most of the galena samples are characterized by high 207Pb/204Pb ratios, suggesting significant input of Pb from old continental crust or pelagic sediment. Pb isotope data also indicate that some of the deposits, which are hosted by sedimentary rocks in Central Iran and Alborz, have similar Pb isotopic compositions and hence suggest similar source regions. Most of the galenas yield Pb model ‘ages’ that vary between ~140 and ~250 Ma, indicating that mineralization resulted from the extraction of ore-bearing fluids from Upper Triassic–Lower Jurassic sequences. The similarity in Pb isotope ratios for the Pb-Zn deposits located within these zones suggests analogous crustal evolution histories. Our preferred interpretation is that Pb-Zn mineralization within the sedimentary and igneous rocks of the Central Iran and Alborz tectonic regions occurred following a Late Cretaceous–Tertiary accretionary stage of crustal thickening in Iran.

Characterizing long-term CO2–water–rock reaction pathways to identify tracers of CO2 migration during geological storage in a low-salinity, siliciclastic reservoir system

Given the prevailing use of saline reservoirs for geological CO2 storage projects, limited data are available on the geochemical evolution of formation water chemistry during geological CO2 storage in low-salinity formations. The low-salinity (total dissolved solids<3000mg/L) middle to lower Jurassic sequence in Australia's Surat Basin has been characterized as a potential reservoir system for geological CO2 storage, comprising three major siliciclastic formations with distinctly different mineral compositions. Contrasts in the geochemical responses of Jurassic sequence core samples have been identified during short-term CO2–water–rock experiments conducted under CO2 storage conditions (Farquhar et al., in this issue). If persistent, such contrasts may serve as geochemical tracers of CO2 migration within the Surat Basin. Here we use a combined batch experiment and numerical modeling approach to characterize the long-term response of the Jurassic succession to storage and migration of CO2 and to assess reaction pathway sensitivity to CO2 partial pressure. Reservoir system mineralogy was characterized for 66 core samples from Geological Survey of Queensland stratigraphic well Chinchilla 4, and six representative samples were powdered and reacted with synthetic formation water and high-purity CO2 for up to 27days at a range of pressures. Formation water alkalinity offers limited buffering at elevated CO2 pressures and pH rapidly declines resulting in sustained enhancement of mineral dissolution rates. Batch reactor results exhibit regional groundwater-like 87Sr/86Sr values (0.7048–0.7066), less radiogenic than whole-rock results (>0.7085) indicating incongruent dissolution of the reservoir matrix. Carbonate and authigenic clay dissolution are expected to be the primary reaction pathways regulating long-term formation water composition during geological CO2 storage in the Surat Basin, with lesser contributions from dissolution of the clastic matrix.

Sequence stratigraphic analysis of the Lower Jurassic succession, Western Iraq

The Lower Jurassic succession of Western Iraq reflects deposition within various depositional environments. These are fluvial, coastal plain, sabkha, shallow restricted marine, and shallow open marine environments. Several third- and fourth-order cycles were recognized within the studied succession. Facies stacking patterns, cycle thicknesses, and symmetries were the direct result of different effects of the major controlling factors (eustacy, subsidence, rate of carbonate production, and clastic supply) at different locations within the study area of Western Iraq. The different rates of subsidence between the Rutba uplift, Anah Graben, and the unstable shelf area produced different effects on the Lower Jurassic sequence development. In the Rutba uplift where the rate of subsidence was at its minimum, eustacy was the major controlling factor. Tectonic subsidence was the major controlling factor on sequence development of the other sections where high rates of subsidence characterized these areas. The development of the Lower Jurassic basin in Western Iraq was initiated during the early stages of the Neo-Tethys opening. The physiography of this passive margin basin was controlled by the Hail-Rutba Arch and the Anah Graben within the stable shelf area. Facies architecture was controlled mainly by the change in accommodation (eustacy + subsidence). The resultant relative sea level greatly affected the nature of carbonate production and consequently facies stacking pattern.

Jurassic sedimentary features and tectonic settings of southeastern China

Two types of the Jurassic basins are distinguished in SE China based on their geodynamic features: the Late Triassic-Early Jurassic post-orogenic basins and the Middle Jurassic intra-continental extensional basins. The Lower Jurassic sequence shows a change from coarse- to fine-grained accumulation, suggesting a gradually deepening depositional environment from river to shore-lake and to deep-water lake. In contrast, the Middle Jurassic accumulation was changed from claystone to conglomerate along the coastal provinces in SE China, indicative of an initial crustal uplift. The Wuyi Mountains have been a paleogeograghic separating zone since the Middle Jurassic. The Late Jurassic strata are absent in most areas of SE China. A large-scale bimodal intra-continental rift-type volcanism occurred during the Middle Jurassic along a 40–60 km wide and 200 km long area in western Fujian and southern Jiangxi provinces, which is most likely the strongest volcanism in SE China since the Cambrian. The SHRIMP zircon U-Pb analyses on the rhyolite from the Dongkeng basin in the southern Jiangxi area yield a concord U-Pb age of 160±0.5 Ma, providing an upper age limit for the bimodal volcanic eruption. The analyses of the basin features indicate a change of the depositional environment during the interval from Middle Triassic to Late Triassic from a shallow-sea to an intra-continent in SE China in response to the strong collision between the Yangtze and North China Blocks. Sedimentary structures record a southward direction of Early Jurassic paleo-currents, reflecting that their source areas were to the north side. We propose that the Wuyi region was uplifted as early as Middle Jurassic, followed by a wide E-W-trending extended depression and bimodal volcanism in the western foot of the Wuyi Mountains. Presumably the uplift of the Wuyi domain changed the Middle Jurassic paleogeographic outline and formed the transformational tectonic regime from compression to extension as a tectonic response to the Pacific plate subduction.

The biostratigraphy of the Upper Pliensbachian-Toarcian (Lower Jurassic) sequence at Ilminster, Somerset

Abstract. Temporary road sections for the A303 bypass at Ilminster, Somerset, revealed Upper Pliensbachian and Toarcian sediments from an alternating limestone–marl facies, in marked contrast to the limestone-dominated Dorset coast succession. The lithostratigraphy is described, with the standard ammonite zonation providing chronostratigraphical correlation. The uppermost Pliensbachian (Spinatum Chronozone) and much of the Toarcian (Serpentinum, Bifrons, Variabilis, Thouarsense and Pseudoradiosa chronozones) of the classic Dorset coast Lower Jurassic sequence are represented by the Beacon Limestone Formation, formerly the ‘Junction Bed’ (a highly condensed carbonate deposit). The Tenuicostatum Chronozone is largely missing from the sequence. Elsewhere in southern and eastern England this interval is poorly or rarely exposed. The argillaceous units have yielded rich microfaunas and -floras (foraminifera, ostracods, calcareous nannofossils and palynomorphs), the biostratigraphical distributions of which are analysed and discussed in relation to contemporary evolutionary patterns and other UK records. This study provides an insight into the micro-biostratigraphy of the Late Pliensbachian to Toarcian interval for onshore southern England.

The early Jurassic fauna from the Gurumdy and Mynhajir zones of the East Pamirs

The Southeast Pamirs is a unique region with a wide development of Jurassic marine sediments that are characterized by complete different-facies sections and diverse fossils. Of particular interest are Early Jurassic corals, which represent the first finds in mountainous Central Asia and the entire eastern Mediterranean belt. Finds of such fossils are extremely rare in other regions of the world so far. According to [1], Jurassic sediments in the Southeast Pamirs can be divided into three (Gurumdy, Mynhajir, and Istyk) structural‐facies zones. Upper Jurassic fossils are confined to the Gurumdy and Mynhajir zones, which occupy marginal parts of the Southeast Pamirs and make up a horseshoe-shaped framing around the Inner Istyk zone (Istyk Uplift), where sedimentation commenced in the terminal-Early-initialMiddle Jurassic (Toarcian‐Aalenian). The Gurumdy and Mynhajir zones constitute the southwestern and northeastern segments of this horseshoe-shaped zone, respectively (figure). In the Jurassic, both zones were elements of a single trough located along the periphery of the Istyk Uplift inside a sea basin. The Gurumdy segment of the trough experienced intense subsidence and accumulated thick sediments, while the Mynhajir segment subsided less significantly to accumulate thinner sediments of variable composition. The present communication is dedicated to the characteristic of distribution, lithology, age, and fossils of the Lower Jurassic sediments developed in the Gurumdy and Mynhajir zones of the Southeast Pamirs. Different fossil groups from Lower Jurassic sediments have been determined and described by many paleontologists, including G.K. Melnikova (corals); V.P. Makridin, S.A. Melnikova, and A.S. Dagys (brachiopods); L.D. Kiparisova, L.V. Sibiryakova, and T.F. Andreeva (bivalves); and G.Ya. Krymgol’ts, V.I. Dronov, and Yu.S. Repin (ammonites). In the present-day structure of the Gurumdy zone, Jurassic sediments are developed in the river basins draining the Vahan Ridge beginning from upper reaches of the Beik River to the Zorkul Depression. Farther northwestward, they are widespread in the basins of the Mashale, Gurumdy, Irikyak, Alichur, Karademur, Kattamarjanai, and Vatasaif rivers that drain the eastern part of the South Alichur and western part of the North Alichur ridges. In the zone under consideration, the Jurassic rocks are unconformably sandwiched between upon Permian‐Triassic strata and Tertiary sediments. The zone is characterized by the continuous Jurassic section that includes sediments ranging from the basal part of the system to upper Callovian layers. Its lower series is very thick because of the development of reeflike sequences in some areas. The total thickness of the Lower Jurassic rocks amounts to 700 m. The Jurassic sediments of the Mynhajir zone are developed in the East and West Mynhajir mountains separated by the Aksu River and along the northern foothill of the Zougan Ridge, in the Karasu River basin, and at the estuary of the North Boztere, Kaindy, and other rivers. They are unconformably sandwiched between Permian‐Triassic and Tertiary layers. The Mynhajir zone also includes a complete section extending from the basal Jurassic to the Bathonian‐Lower Callovian strata, although the Mynhajir zone differs from the previous zone in more diverse lithology and two times lower thickness of the Lower Jurassic sequence (220‐285 m). Following below is a description of the Jurassic type sections of the Gurumdy and Mynhajir zones with Early Jurassic faunal assemblages. The integral Lower Jurassic section of the Gurumdy zone is compiled on the basis of outcrops on the watershed between the Sedek and Irikyak rivers. The author of the present communication studied it first together with V.P. Novikov (in 1969), E.V. Boiko (1970), and V.I. Dronov (1970s‐1980s).

Depositional facies, environments and sequence stratigraphic interpretation of the Middle Triassic–Lower Cretaceous (pre-Late Albian) succession in Arif El-Naga anticline, northeast Sinai, Egypt

The Middle Triassic–Lower Cretaceous (pre-Late Albian) succession of Arif El-Naga anticline comprises various distinctive facies and environments that are connected with eustatic relative sea-level changes, local/regional tectonism, variable sediment influx and base-level changes. It displays six unconformity-bounded depositional sequences. The Triassic deposits are divided into a lower clastic facies (early Middle Triassic sequence) and an upper carbonate unit (late Middle- and latest Middle/early Late Triassic sequences). The early Middle Triassic sequence consists of sandstone with shale/mudstone interbeds that formed under variable regimes, ranging from braided fluvial, lower shoreface to beach foreshore. The marine part of this sequence marks retrogradational and progradational parasequences of transgressive- and highstand systems tract deposits respectively. Deposition has taken place under warm semi-arid climate and a steady supply of clastics. The late Middle- and latest Middle/early Late Triassic sequences are carbonate facies developed on an extensive shallow marine shelf under dry-warm climate. The late Middle Triassic sequence includes retrogradational shallow subtidal oyster rudstone and progradational lower intertidal lime-mudstone parasequences that define the transgressive- and highstand systems tracts respectively. It terminates with upper intertidal oncolitic packstone with bored upper surface. The next latest Middle/early Late Triassic sequence is marked by lime-mudstone, packstone/grainstone and algal stromatolitic bindstone with minor shale/mudstone. These lower intertidal/shallow subtidal deposits of a transgressive-systems tract are followed upward by progradational highstand lower intertidal lime-mudstone deposits. The overlying Jurassic deposits encompass two different sequences. The Lower Jurassic sequence is made up of intercalating lower intertidal lime-mudstone and wave-dominated beach foreshore sandstone which formed during a short period of rising sea-level with a relative increase in clastic supply. The Middle-Upper Jurassic sequence is represented by cycles of cross-bedded sandstone topped with thin mudstone that accumulated by northerly flowing braided-streams accompanying regional uplift of the Arabo–Nubian shield. It is succeeded by another regressive fluvial sequence of Early Cretaceous age due to a major eustatic sea-level fall. The Lower Cretaceous sequence is dominated by sandy braided-river deposits with minor overbank fines and basal debris flow conglomerate.

The Lower Jurassic in the Troyan Region and the south-eastern part of the Teteven Region, Central Balkanids (brachiopod-, bivalve-, and ammonite stratigraphy and palaeotectonics)

As a result of the biostratigraphic study of brachiopods, bivalves and ammonites made in eight instructive sections it was established that most of the Lower Jurassic rock sequences were more or less incomplete. The sections in the valleys of the rivers Kostina and Cherna Reka, Teteven Region, are most complete (from Hettangian to Toarcian). They are situated in the central, the most deeply sinking block one – called Kostina Step of the Izdremets Complex Graben of the Moesian Early-Middle Jurassic Platform. There, the most deep-sea Early Jurassic distal facies were existing. The section near the village of Gorno Shipkovo, Troyan Region, has more shallow-water character and is situated in the Shipkovo Step, the most deeply sinking step of the southern edge of the Izdremets Graben. There, the Lower Jurassic section begins with the Lower Sinemurian. The Lower Jurassic rock sequences in the Central Stara Planina Mts. between the hamlet of Neshkovtsi and the locality Brousovete, Troyan Region, are connected with the Neshkovtsi Step (the sections near the hamlet of Neshkovtsi) and the Ambaritsa Step (the section near the mountain hut Ambaritsa and in the locality Brousovete). There, the Lower Jurassic sequences are most incomplete and they begin with the Upper Sinemurian (near the hamlet Neshkovtsi) or with the Pliensbachian (in the rest sections). The lithofacies and the thanatocoenoses possess a more coastal character and are referred to the proximal part of the basin. During the Early Jurassic the studied south-eastern part of the Izdremets Graben subsided gradually, so that in the end of the Early Jurassic, when the maximum deepening took place, the depths corresponded to the deep sublittoral (150-200m). At that time the marine transgression extended considerably to the south.

Lower Jurassic Sequence Stratigraphy and Sedimentary Facies on the North Slope of Mount Qomolangma

Abstract The Lower Jurassic is subdivided in ascending order into the Wulong, Kangdui and Yongjia Formations on the north slope of Mount Qomolangma, with a total thickness of 1362 m. They are thought to have been deposited respectively in the environments of the carbonate ramp fault–bounded basins and carbonate platform, with six sedimentary facies and six sub–facies. During the Early Jurassic, the Qomolangma area experienced strong faulting and subsidence, and was of a matured rift basin. The Lower Jurassic consists of eleven 3rd–order sequences, which can be grouped into three 2nd–order sequences and form a large transgressive–regressive cycle. The 3rd–order sequences and the corresponding sea–level changes recognized in the area can be correlated quite well with those set up in the western Tethys, and may have been caused by the eustatic fluctuations, while the 2nd–order sequences seem to be more closely related to the basement subsidence and the variation in sedimentary influx, indicating the evolution of the eastern Neotethys and the movement of the plates on its two sides.

Mesozoic regional tectonics and South Viking Graben formation: evidence for localized thin-skinned detachments during rift development and inversion

A regional thin-skinned model of South Viking Graben development is presented in which Zechstein marginal evaporites and interpreted salt have acted as a key de´collement horizon, along which mechanical decoupling of post-salt Triassic and younger strata, from pre-salt ‘basement’ has occurred. The western margin of the South Viking Graben defines an asymmetrical Late Jurassic half-graben, controlled by episodic extension along a NNE-SSW trending, eastward dipping, fault system. An initial, minor phase of rifting in the Triassic is suggested by the slight westward thickening and associated divergence of interpreted Triassic sequences at depth. The scarcity/absence of Upper Triassic and Lower Jurassic sequences is believed to result from a subsequent phase of uplift which affected the region in the mid-Cimmerian, attributed to the development of a thermal dome at the future site of a Jurassic rift triple-junction to the southwest. The onset of large-scale extension, from the Late Callovian onwards, marked the major phase of half-graben formation and created a regional hanging wall tilt, which locally caused gravity gliding of post-salt section to the WNW, with resultant low-angle extensional faulting upslope and salt-cored buckling downslope. The Late Jurassic half-graben was subsequently tightened by a phase of thin-skinned inversion in the Latest Jurassic-Early Cretaceous, associated with back-steepening of the western margin fault system, with resulting reactivation of the extensional detachment system. The effects of this inversion have been noted in many parts of the northern North Sea. Within the South Viking Graben, shortening estimates vary between 14 and 18% and are expressed on seismic data by localized hanging wall folds and the back-steepening of faults along the western margin fault complex of the graben, together with out of graben thrusting up the hanging wall dip-slope. Variation in both the extension and inversion mechanisms appears to be controlled by interpreted NW-SE transfer systems which compartmentalize the graben into three subgraben, within which differential Zechstein basin development occurred.

Tectonic controls on deposition of Middle Jurassic strata in a retroarc foreland basin, Utah‐Idaho trough, western interior, United States

An electronic supplement of this material may be obtained on a diskette or Anonymous FTP from KOSMOS.AGU.ORG. (LOGIN to AGU’s FTP account using ANONYMOUS as the username and GUEST as the password. Go to the right directory by typing CD APEND. Type LS to see what files are available. Type GET and the name of the file to get it. Finally, type EXIT to leave the system.) (Paper 95TC01448, Tectonic controls on deposition of Middle Jurassic strata in a retroarc foreland basin, Utah‐Idaho trough, western interior, United States, Christian J. Bjerrum and Rebecca J. Dorsey). Diskette may be ordered from American Geophysical Union, 2000 Florida Avenue, N. W., Washington, DC 20009; $15.00. Payment must accompany order.A thick succession of Jurassic nonmarine and marine sedimentary rocks is exposed in a large area from northern Arizona to eastern Idaho and western Wyoming. These sediments accumulated in the Utah‐Idaho trough, a deep elongate cratonal basin whose origin has recently been debated. Detailed stratigraphic analysis, subsidence analysis, and first‐order flexural modeling of these deposits (this study) provide new insights into the timing and mechanisms of subsidence in the Utah‐Idaho trough. Lower and Middle Jurassic strata are divided into six unconformity‐bounded sequences. In contrast to the overall uniform thickness of Lower Jurassic sequences (1 and 2), Middle Jurassic strata (sequences 3 through 6) consist of distinctly westward thickening sedimentary packages in which basal shallow marine deposits onlap eastward onto bounding unconformities. Basal strata of sequences 3 through 6 pass upward into widespread progradational continental deposits that are truncated progressively toward the east (cratonward) by the next unconformity. Decompacted total subsidence curves show that the rate of subsidence in most sections increased sharply at the end of sequence 2 time (J‐2 unconformity; ∼170 Ma). This is interpreted to record the onset of Middle Jurassic deposition in the distal part of a retroarc foreland basin. The unconformities and distinctive stratal geometries may have formed in response to forebulge migration caused by episodic thrusting in the Cordilleran orogen to the west. First‐order flexural modeling was carried out to test the hypothesis of flexural subsidence in the Utah‐Idaho trough. Trial‐and‐error comparisons produce a close match between decompacted stratigraphic profiles and model deflection profiles. The best fit is obtained using an infinite elastic plate (D = 1 × 1024 Nm), a moderate load topography, elevated base level, and an overfilled basin. Using recently published tectonic reconstructions for Nevada and Utah, we find close spatial agreement between a large Middle Jurassic fold‐thrust belt and the supracrustal load inferred from model simulations. Our integrated basin analysis thus supports the interpretation of some previous studies that the Middle Jurassic Utah‐Idaho trough was a retroarc foreland basin that formed east of a belt of regional contractile deformation and crustal thickening in western and central Nevada and southeastern California. Late Jurassic extension and normal faulting in northeastern Nevada may have been related to gravitational collapse of overthickened crust in the Cordilleran orogen. This coincides with a period of slowed subsidence in the Utah‐Idaho trough that began at about 157 Ma.

Lower Jurassic Ostracod Biozonation of the Mochras Borehole

Abstract. The extensive Lower Jurassic sequence at Mochras has yielded a diverse ostracod fauna which includes many new taxa. Poor preservation precludes a complete taxonomic review of this material although four new species (Cytherella praetoarcensis, Ogmoconcha convexa, Lophodentina striata and Oligocythereis? mochrasensis) are erected herein. Changes in the faunal composition and the rates of faunal turnover are briefly described. An ostracod biozonation is proposed for the section, comparison with established Lower Jurassic zonal schemes reveals that the present section is distinct from most other European sequences.

Petroleum Generation and Migration in Lower Jurassic Sequences, Hartford Basin, Connecticut and Massachusetts: ABSTRACT

Petroleum Generation and Migration in Lower Jurassic Sequences, Hartford Basin, Connecticut and Massachusetts: ABSTRACT

Palaeosols of the Lower Jurassic Mishhor and Ardon Formations (‘Laterite Derivative Facies’), Makhtesh Ramon, Israel

ABSTRACTThe Lower Jurassic sequence exposed at Makhtesh Ramon, in the south‐western Negev of Israel, comprises cyclic fluviatile sediments derived almost exclusively from the reworking of an older laterite terrain. A succession of palaeosols, having the characteristics of modern, red‐brown earths, has developed during periods of sub‐aerial exposure upon the fine grained overbank sediments. A marked seasonal palaeoclimate with accompanying poor sub‐surface drainage conditions, and some stagnation of ground water, is reflected by the distribution and mottling patterns of the iron minerals (with varying degrees of hydration), precipitation of illuviated carbonate and sulphate minerals and the development of soil microrelief.A complex history of pedogenesis and ground water alteration is shown by the rapid lateral facies change within this sequence, into one dominated by high‐alumina flint clay and bauxite. Relict pedogenetic and sedimentological textural features of the combined palaeosol/fluviatile sequence, have been identified and mapped within the flint clay/bauxite lithologies. These features include irregular voids, produced by solution of pedogenic sulphate nodules, slickensided curviplanar surfaces of soil microrelief, and depositional bedding. The field relationships of these relicts indicate the overprinting of a younger, chemical weathering event giving rise to the high‐alumina flint clay and bauxites.The geometric shape of the upper bounding surface of the ‘bauxite’ alteration zone, is irregular and has an ‘intrusive’ character with respect to the younger, overlying, unaltered sediments. This, together with abrupt changes in mottling patterns near the contact zone and subtle changes in mineralogy from a bauxitic assemblage at depth to one containing mixed‐layer clay minerals and carbonates nearer the contact, leads to the conclusion that the later stages of chemical weathering occurred within a confined system, where ground water movements were induced by intake of underlying aquifers.

Sedimentology of the Lower Jurassic flint clay bearing Mishhor Formation, Makhtesh Ramon, Israel

ABSTRACTPetrographical and mineralogical studies of the Lower Jurassic sequence exposed at Makhtesh Ramon, have shown the domination of a lateritic suite of epiclastic sediments of pisolite conglomerate and laterite arenite composition. Their subsequent modification by epigenetic processes of chemical weathering has given rise to the formation of flint clay, high alumina flint clay, laterite and mottled and variegated claystone. Within the Nahal Ardon area, both the transported laterite accumulates and the autochthonous lithologies occur interbedded within the same section. In all, a total of ten lithofacies have been defined and mapping of the various units, grouped into allochthonous and autochthonous classes, has established the existence of lateral facies changes between Nahal Ardon, where the sequence rests on a broad undulatory Triassic basement, and the zone to the west where it occurs as infillings of Triassic solution cavities, sink holes and irregular depressions. Vertical profile studies of the lithofacies indicate that the vertical range of ground water movements increased westwards from Nahal Ardon and resulted in the transported laterite accumulates being subjected to varying degrees of chemical weathering. The observed lateral facies changes are a direct consequence of this. Within the ‘karstic’ zone, the laterite accumulates in addition to the overlying younger carbonate lithologies have been converted to flint clay and demonstrate the intraformational genesis of the flint clay, whereby vertical ground water movements are related to recharge of aquifer zones within the underlying Triassic Mohilla Formation.

Lower Jurassic biostratigraphy and ostracods of the Danish Embayment

The Lower Jurassic sequence and ostracods from 36 borings within the Danish Embayment are analysed and described.Hettangian, Sinemurian and Pliensbachian are demonstrated on the basis of extensive ostracod faunas, whilst only weak indications of Aalenian have been found.The following ostracod zones are defined as a basis for a biostratigraphical subdivision of the Lower Jurassic series in the Danish Embayment:The Ogmoconchella adenticulata - Nanacythere (Nanacythere) simplex Zone (Upper Pliensbachian).The Ogmoconchella danica Zone (Upper Sinemurian and Lower Pliensbachian).The Gramannella apostolescui - Kinkelinella (Klinglerella ) foveolata Subzone (Lower Pliensbachian).The Progonoidea reticulata Subzone (lowest Upper Sinemurian, Lias Beta-1a).The Cristacythere betzi - C. crassireticulata Zone (upper Lower Sinemurian).The Ogmoconchella aspinata Zone (Hettangian and lower Lower Sinemurian).The chronostratigraphical subdivision of the Lower Jurassic sequence in the Danish Embayment is presented, based upon the ostracod faunas described here and upon previous investigations of sediments, foraminifera and megafossils.The conditions prevailing in the basin are discussed, with descriptions of the sedimentary and faunal developments, and an evaluation of the relationships with other parts of the Northwest European area.124 ostracod species are described; 23 are described as new, whilst 30 species which cannot be referred to any known species are described with open nomenclature owing to limited or badly preserved material. The remaining 71 species have all been described previously and are more or less well-known. The 124 species fall into 27 genera or subgenera, of which 2 genera (Pseudomacrocy pris and Cristacythere) and 1 subgenus (Nanacythere (Goniocythere)) are established as new.A subdivision is made of the described Lower Jurassic "Procytheridea". The genus Klinglerella Anderson, 1964 is placed as a subgenus under Kinkelinella Martin, 1960. Pleurifera Gramann, 1962 and Progonoidea Gramann, 1962 are regarded as independent genera placed together with Kinkelinella and the new genus Cristacythere.The family Healdiidae is regarded as being represented by only three genera in the Lower Jurassic: Ogmoconcha, Ogmoconchella and Pseudohealdia. The Ontogenetic development of the muscle scar area in Ogmoconchella is shown to be a development from a simple muscle scar of the Pseudohealdia-type to a typical Ogmoconchella-scar.ree genera in the Lower Jurassic: Ogmoconcha, Ogmoconchella and Pseudohea!dia. The Ontogenetic development of the muscle scar area in Ogmoconchella is shown to be a development from a simple muscle scar of the Pseudohea/dia-type to a typical Ogmoconchella-scar.

Sedimentological study of the upper Jurassic sequence of limestone in the pui zone (Romania)

In the points Fundătura and Ciclovina, situated northward of the Pui zone (southern Carpathians, Fig.1), we collected two sections of lithological samples from the Lower Jurassic sequence. Thin-sections effected on these samples formed the subject of a study, the results of which are presented in this paper. The lithological sequence of the Upper Jurassic includes, above the Dogger Formations the following sedimentary terms ( Boldor and Stilla, 1967): 1. (1) Lower?-Middle Callovian, represented by gritty, spathic limestones with abundant siliceous accidents, disposed non-uniformly in the rock mass. 2. (2) Upper Callovian-Lower Oxfordian, composed of marly limestones and fine limestones with Kosmoceras duncani Sow. and Euaspidoceras cf. perarmatum Sow. 3. (3) Upper Oxfordian-Kimmeridgian-Tithonian, comprised in a packet of generally fine, sub-lithographic limestones, with siliceous accidents disposed according to the stratification. In the upper third of this sequence of rosy siliceous limestones, there are distinguished alternances of nodulous limestones. Macrofaunally, the deposits are characterized by the presence of the forms Perisphinctes, Belemnites, Aptychus. Representing a new, transgressive, cycle of sedimentation, there are deposited massive reef limestones with Requienia ammonia Goldf., referred to the Lower Cretaceous in urgonian facies. The section of samples for thin sections included in the Fundătura sector only the upper part of the Malm (Kimmeridgian-Tithonian) deposits, and is concluded by several limestone beds of Early Cretaceous age. Notable in this point is the Fundătura fault which breaks up the Jurassic sequence towards the bottom, and puts it into tectonic contact with the reef limestones of the Lower Cretaceous. In the Ciclovina point, sampling began in the last beds of the Upper Callovian-Lower Oxfordian deposits, continued with the Upper Oxfordian-Kimmeridgian-Tithonian packet of limestones with silica, and comprised also, in its upper part, the first limestone beds of Early Cretaceous age.