Tithonian Research Articles

Drawing the line: the history of the Jurassic—Cretaceous boundary

The first System boundaries were based on lithostratigraphical criteria — the junction between two formations. As biostratigraphy and chronostratigraphy developed, ammonite faunas were used in the Mesozoic to define stages and form the basis of most zonal schemes. In the uppermost Jurassic and lowermost Cretaceous, this process is complicated by extreme ammonite faunal provincialism; from this developed separate ammonite zonal schemes, separate stages and the recognition of different Jurassic-Cretaceous boundaries, dependent upon the ammonite faunal province. The acceptance of the Tithonian Stage as the Primary Standard for the terminal Jurassic means that the basal Cretaceous boundary will be drawn at the base of the Berriasian Stage. Selection of appropriate boundary stratotype sections is under consideration. Correlations of the southern English non-marine sections with the Tethyan marine limestones have been made using magnetostratigraphy and various microfossil groups; although the conclusions vary somewhat, it is apparent that the majority of the Purbeck Limestone Group of southern England is of Cretaceous age.

Die Barmsteinkalke der Typlokalität nordwestlich Hallein (hohes Tithonium bis tieferes Berriasium; Salzburger Kalkalpen) – Sedimentologie, Mikrofazies, Stratigraphie und Mikropaläontologie: neue Aspekte zur Interpretation der Entwicklungsgeschichte der Ober-Jura-Karbonatplattform un

Die Barmsteinkalke der Typlokalität nordwestlich Hallein (hohes Tithonium bis tieferes Berriasium; Salzburger Kalkalpen) – Sedimentologie, Mikrofazies, Stratigraphie und Mikropaläontologie: neue Aspekte zur Interpretation der Entwicklungsgeschichte der Ober-Jura-Karbonatplattform un

Nouveaux repères biostratigraphiques à terebratulidés (Brachiopodes) dans le Tithonien inférieur ouest-européen

The discovery of a new species of Terebratulid ( Xestosina sera nov. sp.) in the Lower Tithonian allows to specify the biostratigraphical subdivision of the Gigas Zone. This also allows to improve the correlations, when ammonites are lacking, among the Tithonian deposits of Aquitaine, the northern Jura and the eastern border of the Paris Basin. The critical revision of the Xestosina genus gives a nomenclatural setting argued to the index-species of biozones. It constitutes a brachiopods based parallel scale for the Kimmeridgian and Tithonian Stages.

INTEGRATED STRATIGRAPHY FROM THE CONTRADA FORNAZZO SECTION, MONTE INICI, WESTERN SICILY, ITALY: PROPOSED G.S.S.P. FOR THE BASAL BOUNDARY OF THE TITHONIAN STAGE

This paper deals with a definition of the lower boundary stratotype of the Tithonian Stage in the Upper Jurassic succession of Monte Inici, Western Sicily. The upper member of the Rosso Ammonitico Fm. is 27 m thick and shows a typical nodular-calcareous lithofacies; its lower beds have been sampled for biostratigraphic and paleomagnetic purposes. Though the succession is affected by high stratigraphic condensation, the resulting hiatuses have been shown to be below biochronological resolution and thus do not hinder any biostratigraphic definition. The biostratigraphic analysis has been based on the rich ammonite assemblages in which the common genus Hybonoticeras is the index-key for characterizing the Kimmeridgian-Tithonian boundary. Four ammonite biozones have been identified; the basal Tithonian one is defined by the assemblage of Hybonoticeras gr. hybonotum and Haploceras staszycii . The recorded calcareous nannofossil bioevents allow recognition of the V. stradneri and C. mexicana Zones, whose boundary is located a little below the identified Tithonian lower boundary. The paleomagnetic record shows normal polarity in the S. darwini/V. albertinum Zone and mainly reverse polarity in the H. beckeri and H. hybonotum Zones, with three minor normal polarity intervals; the lower boundary of the Tithonian falls in the oldest of these intervals. The integrated multidisciplinary stratigraphic information gathered from the Contrada Fornazzo section defines the lower boundary of the H. hybonotum Zone at the base of Bed 110, and supplies elements of chrono-correlation sufficient to regard this section as a possible G.S.S.P. of the Tithonian Stage.

THE RUSSIAN PLATFORM AS A KEY REGION FOR VOLGIAN/TITHONIAN CORRELATION: A REVIEW OF THE MEDITERRANEAN FAUNAL ELEMENTS AND AMMONITE BIOSTRATIGRAPHY OF THE VOLGIAN STAGE

A review of the ammonite faunas and biostratigraphy of the uppermost Kimmeridgian - lowermost Middle Volgian of Central Russia is presented. The major role of the Sub-mediterranean ammonites in the Volgian assemblages is described, providing additional detail to the existing scheme of Tithonian-Volgian correlations. The base of the Tithonian and the base of the Volgian are coincident, based on the association of Neochetoceras steraspis-Lingulaticeras solenoides . The Klimovi Zone corresponds to most of the Hybonotum Zone. However, the top of the Hybonotum Zone, based on the distribution of Paralingulaticeras , falls within the Sokolovi Zone. The allocation of a Tenuicostata Subzone (with two horizons: neoburgense and puschi ) in the Pseudoscythica Zone is proven. The neoburgense horizon may be correlated with the ciliata and penicillatum horizons of Neuburg and, probably, with the Semiforme Zone. The findings of Lingulaticeras blaschkei , Pseudolissoceras, and Sutneria in the Panderi Zone of the Middle Tithonian suggest that the base of the Middle Volgian may not be younger than the Middle Tithonian Fallauxi Zone.

JURASSIC PALEOGEOGRAPHY OF THE PIENINY AND OUTER CARPATHIAN BASINS

The Jurassic history of the Pieniny/Outer Carpathian basins reflects the evolution of the Circum-Tethyan area, especially its Alpine Tethys part. The Alpine Tethys that is Ligurian, Penninic Oceans and Pieniny/Magura Basin constitute the extension of the Central Atlantic system. The synrift stage lasted in the Pieniny/Magura Basin from late Early Jurassic to Tithonian (the Magura Unit constitutes the southernmost part of the Outer Flysch Carpathians). The Pieniny rift opened during Pliensbachian – Aalenian. The central Atlantic and Alpine Tethys went into a drifting stage during the Middle Jurassic. The Late Jurassic (Oxfordian-Kimmeridgian) history of the Pieniny/Magura Basin reflects strongest facial differentiation within sedimentary basin where mixed siliceous-carbonate sedimentation took place. Greatest deepening effect is indicated by widespread Oxfordian radiolarites, which occur in the all basinal successions, whereas the shallowest zone is completely devoid of siliceous intercalations at that time (sedimentation from Ammonitico Rosso facies up to coral reef limestone). The southern part of the North European Platform, north from the Pieniny/Magura realm, started to be rifted during Late Jurassic time and Silesian Basin in the Outer Western Carpathians and Sinaia Basin in the Eastern Carpathians, with black, mainly redeposited marls have been created. The outer sub-basins were differentiated during the latest (Hauterivian-Barremian) phase of basinal development. The connection of Silesian Basin with Sinaia and Southern Carpathian Severin areas suggests the NW-SE direction of the basinal axis while the orientation of the Pieniny Klippen Belt/Magura Basin was SW-NE so, two Outer Carpathian perpendicular directions are possible within the basins. Major reorganization happened during the Tithonian-Berriasian time. It was reflected by both paleoceanographical and paleoclimatical changes. The Neo-Cimmerian tectonic events as well as main phase of the Outer Carpathian basins opening is connected with this reorganization.

THE PRE-OROGENIC VOLCANO-SEDIMENTARY COVERS OF THE WESTERN TETHYS OCEANIC BASIN: A REVIEW

The records of the Jurassic Western Tethys Ocean are the ophiolitic rocks now scattered in the Tertiary orogenic belts of the Alps, Apennines and Betic Cordillera. These ophiolites, involved in a convergent margin environment, are affected I) by HP/LT metamorphism derived from a subduction process or II) by very low grade overprint, corresponding to the tectonic prism at the margin of the overriding plate. On the whole, they share common characteristics: a- The MORB geochemical signature. b- The ophiolitic successions often “reduced” and thin. c- The volcano-sedimentary covers often directly overlying the serpentinised peridotites. d- The widespread occurrence of cherts as Jurassic pelagic sediment. In the thickest “complete” ophiolitic successions, basalt flows, generally thin, are preceded and followed by ophiolitic breccias. Only the basal portion of the breccias on top of the serpentinites (Levanto Breccias) has a tectonic origin, all other levels have a sedimentary origin. These breccia-basalt assemblages are overlain by thick sequences of Mt. Alpe Cherts and Calpionella Limestones, followed by Palombini Shales. In the reduced (or incomplete) successions, thin breccias and cherts were directly deposited, above the Levanto Breccias (ophicalcites pro parte), and followed by Palombini Shales. This stratigraphic pattern seems to be widespread in the whole Western Tethys ocean. In some sequences, transitional mid-ocean ridge (T-MOR) basalts are present and the ophiolitic rocks are associated with Variscan continental slices and debris, as in the Err-Platta succession (Central Alps) and in some exotic blocks in the flysch of the External Ligurides (Northern Apennines). In the Balagne (Corsica) T-MOR basalts are associated with quartzarenites. These occurrences show that an unroofed mantle and sections of oceanic crust evolved very near to a continental margin. The different radiolarian ages of the cherts deposited before, within, or on top of the MOR basalts allow to infer a minimum time interval for the Western Tethys oceanisation. This interval can be considered between 16 and 21 Ma (from Late Bajocian to Kimmeridgian/Tithonian). If we assume 1cm/yr spreading rate during this time, the basin would have reached about 150-200 km width. The same ages suggest that the ocean opening was diachronous along the Western Tethys basin. Mainly on the basis of the Northern Apennines and Corsica data, it is possible to reconstruct the following evolutive geodynamic, paleogeographic and sedimentary evolution of the Western Tethys ocean basin: 1- Bajocian/Bathonian stage: opening of the Ligurian Northern Apennines oceanic segment and, perhaps, also of the Ligurian, Western and Central Alps ones. 2- Bathonian/Callovian stage: opening of all the segments of the Western Tethys ocean basin. The volcano-sedimentary covers formed during these two stages are constituted by breccias, basalts and siliceous pelagites (cherts). 3- Tithonian/Berriasian: end of the ocean spreading (Tithonian) and beginning of the quiescent stage in the whole basin, marked by the lack of any tectonic activity and by the sedimentation of the Calpionella Limestones and, locally, of mixed siliceous-calcareous deposits (Nisportino-Murlo Fm.). 4- Hauterivian/Santhonian: this is the longest quiescent stage of the basin, dominated by the sedimentation of the Palombini Shales and Limestones. Some siliciclastic deposits are shed from both passive continental margin sides. During the Early Cretaceous, there is also evidence of a rare intraplate magmatism in Southern Tuscany. The Western Tethys ophiolitic successions are similar to those of present day, slow spreading oceans, in particular to those of the Atlantic Ocean (Mutter and Karson, 1992; Tucholke and Linn, 1994). The Galician North Atlantic margin provides a model for the process of mantle denudation. For the oceanic evolution, the model of Tucholke and Linn (1994) is particularly taken in consideration. According to this model, tectonic extension was one major process in the Western Tethys oceanic development.

The Upper Jurassic of Europe: its subdivision and correlation

In the last 40 years, the stratigraphy of the Upper Jurassic of Europe has received much attention and considerable revision; much of the impetus behind this endeavour has stemmed from the work of the International Subcommission on Jurassic Stratigraphy. The Upper Jurassic Series consists of three stages, the Oxfordian, Kimmeridgian and Tithonian which are further subdivided into substages, zones and subzones, primarily on the basis of ammonites. Regional variations between the Mediterranean, Submediterranean and Subboreal provinces are discussed and correlation possibilities indicated. The durations of the Oxfordian, Kimmeridgian and Tithonian Stages are reported to have been 5.3, 3.4 and 6.5 Ma, respectively. This review of the present status of Upper Jurassic stratigraphy aids identification of a number of problems of subdivision and definition of Upper Jurassic stages; in particular these include correlation of the base of the Kimmeridgian and the top of the Tithonian between Submediterranean and Subboreal Europe. Although still primarily based on ammonite stratigraphy, subdivision of the Upper Jurassic is increasingly being refined by the incorporation of other fossil groups; these include both megafossils, such as aptychi, belemnites, bivalves, gastropods, brachiopods, echinoderms, corals, sponges and vertebrates, and microfossils such as foraminifera, radiolaria, ciliata, ostracodes, dinoflagellates, calcareous nannofossils, charophyaceae, dasycladaceae, spores and pollen. Important future developments will depend on the detailed integration of these disparate biostratigraphic data and their precise combination with the abundant new data from sequence stratigraphy, utilising the high degree of stratigraphic resolution offered by certain groups of fossils. This article also contains some notes on the recent results of magnetostratigraphy and sequence chronostratigraphy.

Thermochronology of northern Murihiku Terrane, New Zealand, derived from apatite FT analysis

Murihiku Terrane is the least deformed of a series of tectonostratigraphic terranes that have been amalgamated to form the basement underlying New Zealand. Apatite fission‐track (FT) thermochronology has been applied to the youngest beds (Late Jurassic, Hettangian–Tithonian Stages) of this terrane, located at Port Waikato in western North Island, which are broadly folded. The apatite FT ages are partially annealed, are younger than their stratigraphic ages, and remarkably, decrease in age upsection. Interpretation and modelling of the maximum palaeotemperatures experienced by the sample horizons and the timing of cooling requires a wedge of Cretaceous sedimentary rocks to have accumulated within the axis of the major fold (Kawhia Syncline). Deposition of Murihiku Terrane sequences continued therefore into the Cretaceous and probably until c. 100 Ma. Deposition may have ended as late as 85 ± 10 Ma, when cooling via denudation started. The upper parts of the former succession probably accumulated during folding. In addition, interpretation of the FT data require a marked increase in palaeogeothermal gradient from a subnormal value of 15°C km −1 during the Late Jurassic, up to c. 30°C km −1 at 85 ± 10 Ma. This is attributed to the termination of subduction along the Pacific margin of Gondwanaland, and a transition to a rift‐transform tectonic setting in western New Zealand during Tasman Sea spreading. The latest Cretaceous–Palaeocene eastward tilting of the Murihiku basement at Port Waikato may have been accommodated on the Taranaki Fault or another coast‐parallel fault; the Waikato Fault probably acted as a transfer fault at that time accumulating a significant amount of throw.

MIDDLE JURASSIC TO EARLY CRETACEOUS RADIOLARIA FROM THE OMGON RANGE, WESTERN KAMCHATKA

The Omgon Range (Western Kamchatka) includes intensely deformed tectonic units, some of which consist of volcanogenic rocks and siliceous sediments, whereas others are represented by siliceous sediments and terrigeneous turbidites. All these rocks were considered so far to be not older than Early Cretaceous. Radiolarian assemblages ranging in age from Middle Jurassic to late Early Cretaceous are here reported from siliceous sedimentary rocks in three distinct unit. In particular, Middle Jurassic (Bajocian-Bathonian) radiolarians were collected from red cherts of the lowermost of these units. This is the first finding of Jurassic radiolaria in Kamchatka. A late Tithonian to late Barremian-early Aptian radiolarian assemblage has been yielded by siliceous levels in an overlying unit. An Early Cretaceous radiolarian assemblage has been found in siliceous rocks associated with pillow basalts in a unit lying in a higher tectonic position. The Middle Jurassic radiolarian assemblage shows Boreal affinity; whereas the Late Jurassic-Early Cretaceous (Tithonian-Berriasian) radiolarian assemblages can be related to both Tethyan and Boreal Provinces. The following new species are described: Archaeodictyomitra curta n. sp., Archaeodictyomitra elliptica n. sp., Stichocapsa globosa n. sp., Parvicingula omgonensis n. sp. and Xitus? primitivus n. sp.

JURASSIC PALAEOGEOGRAPHY OF THE TRANSDANUBIAN CENTRAL RANGE (HUNGARY)

The Transdanubian Central Range (TCR) is a flattened range of hills in northern Transdanubia (Hungary), formed mainly by Mesozoic carbonate rocks showing strong facies similarities with the Southern Alps and the Austroalpine domain. The Jurassic system is divided into several formations of predominantly pelagic limestones. Ammonoids are frequent and were collected bed-by-bed in numerous sections, providing an excellent biostratigraphic resolution. The thickness of the Jurassic system is usually small but changes along the strike of the TCR. It reaches a maximum thickness of 500 m in the western part; is very variable (10-400 m) in the central segment (Bakony Mts.) and rather low (less than 100 m) in the east (Gerecse). In the Bakony segment, the thickness variation reflects the strongly dissected topography of the Jurassic sea-floor. Synsedimentary tectonics is dominated by normal faults; tilted blocks and listric faults may be inferred only in the east. Five main steps were identified in the palaeogeographic evolution: 1) Late Hettangian: carbonate oolitic shoals prevail, except for a few sites where non-deposition or neritic sediments occur. 2) Sinemurian and Pliensbachian: tectonic disintegration resulted in an intricate pattern of submarine horsts and intervening basins, with condensed sedimentation or non-deposition on the horsts and thicker, continuous sedimentary sequences in the basins. The submarine topographic highs are surrounded by aprons of redeposited material (scarp breccias, brachiopod coquinas, crinoidal calcarenites, spiculitic cherty limestones), while pure or argillaceous limestones (Rosso Ammonitico) prevail in the distal areas. 3) Early Toarcian: the Tethys-wide anoxic event is superimposed on the previous submarine bottom topography; the resulting black shales and sedimentary Mn-ores are concentrated on the western sides of some horsts. 4) Dogger to Early Malm: radiolarites with heterochronous lower and upper boundaries (Aalenian to Kimmeridgian) prevail, except for the top of some submarine topographic highs. The absence of uppermost Bathonian to Lower Oxfordian carbonates suggests that the whole TCR sunk below the CCD in those times. 5) Latest Jurassic: the uniform deposition of Rosso Ammonitico and Biancone in the Late Kimmeridgian and Tithonian is interrupted only in the Early Tithonian by local intercalations of scarp breccias and coarse biodetrital limestones. This is interpreted as the last manifestation of synsedimentary tectonic movements along the faults bordering the submarine horsts. Based on palaeogeographic similarities and analogies in Jurassic times, the TCR is visualized as the northern foreground of the Trento platform/plateau (lying north of the later Insubric lineament), where the block-tectonic disintegration and differential subsidence started earlier and resulted in a bottom morphology more dissected than in the South Alpine part of this west Tethyan passive margin.

Upper Jurassic (Malm) Shallow-Water Carbonates in the Western Gorski Kotar Area: Facies and Depositional Environments (Western Croatia)

Shallow-water carbonates in the Upper Jurassic of the Gorski Kotar were deposited on a carbonate ramp, in beach-barrier island-lagoonal and peritidal environments. In the continuous sequence, more than 900 m thick, several facies have been found: (A) low-energy shallow-water wackestones/mudstones of the Lower Oxfordian, (B) high-energy shallow-water grainstones of the Middle Oxfordian, (C) low-energy, above the fair-weather wave-base packstones of the uppermost Oxfordian and transition to the Kimmeridgian, (D) shallowing-upward/coarsening-upward units formed through progradation of beach-tidal bars or barriers over the peritidal deposits of the Kimmeridgian and the beginning of the Tithonian and (E) peritidal shallowing-upward units capped by storm tidal deposits of the Tithonian and beginning of the Berriasian. Fossil assemblages adapted to the environmental changes: maximum of their abundance, in the number of the taxa, as well as in the number of individuals, corresponds to the high-energy facies B (Oxfordian), while their minimum corresponds to the peritidal shallowing-upward units of facies E (Tithonian).

Bipolar Molluscs and Their Evolutionary Implications

The phenomenon of bipolarity, one of the major disjunct distribution patterns on the face of the earth, has been investigated repeatedly since the mid-nineteenth century. Running through the many hypotheses that have been put forward to account for its occurrence, it is possible to detect two persistent themes: it is usually interpreted within a dispersal framework, and it is generally believed to be of comparatively recent origin. To many authors, the phenomenon is intimately linked to the Plio-Pleistocene glaciations. Recent palaentological investigations have extablished that bipolarity can now be traced back to at least the Early Jurassic period (i.e. 200 m.y.a.). Here it is well marked in the Pliensbachian stage by a variety of pectinacean bivalve taxa. Further bivalves indicate probable Middle Jurassic examples, but the phenomenon is more clearly seen in the Late Jurassic, especially in the Tithonian stage. At this time, in-oceramid, buchiid and oxytomid bivalve occurrences at northern hemisphere localities such as arctic Canada, N.W. Europe, Siberia, N.E. USSR and Japan can be matched with those in southern South America, Antarctica and Australasia. A striking Early Cretaceous (Aptian-Albian) bipolar pattern for the oxytomid Aucellina may be complemented by several infaunal bivalves, brachiopods and at least one gastropod. There is strong circumstantial evidence to suggest that bipolar molluscs continued to develop through the Cenozoic era. Such is the level of generic and subfamilial differentiation within certain living forms as to suggest that they must be the product of a considerable evolutionary history. It is likely that present-day distribution patterns of prosobranch gastropod groups such as th whelks (Buccinidae), together with certain fissurellids, littorinids, naticids and turrids, can be related to a late Paleogene-early Neogene phase of bipolarity. Many amphitropical taxa, in both the marine and terrestrial realms, have probable late Neogene-Pleistocene origins. It is possible to set the Jurassic and Cretaceous examples of bipolarity within a largely vicariant framework based upon the disintegration of the Pangean supercontinent. In this way the widespread ranges of putative Triassic ancestors were disrupted by tectonic processes in low latitude regions, although it should be emphasized that major climatic and oceanographic changes were almost certainly involved too. Similarly, it is possible to view late Paleogene earl Neogene bipolarity as a vicariant event, but this time with climatic change identified as the single most important agent. Widespread or cosmopolitan distributions are held to have formed during global cool phases (such as the late Eocene-early Miocene) only to be disrupted by global warming (such as in the late early Miocene). It is even possible to view Plio-Pleistocene patterns as, at leat in part, the products of vicariant events caused by rapid temperature and sea level shifts. Clearly, there is an urgent need here for more critical taxonomic data to test these various hypotheses. Phylogenetic studies of groups such as the Mesozoic bivalve superfamily Monotoidea and the Cenozoic Buccinidae, in particular, should constitute future rigorous tests. In so doing, they should also provide much useful information on the relative roles of dispersal and vicariance in promoting global disjunction in marine faunas. Repeated formation of bipolar patterns through geological time may have had important implications for modes of speciation and phenomena such as the origin of taxonomic diversity gradients.

Late Jurassic Paleoclimate Simulation: Paleoecological Implications for Ammonoid Provinciality

During the Kimmeridgian and Tithonian stages (154.7-145.6 Ma) of the Late Jurassic, the Late Permian-early Mesozoic megacontinent Pangea was progressiuely fragmented by two rift systems that propagated westward out of the Tethys Sea and a third more persistent rift system that connected the Boreal and Tethys seas. By the late Tithonian, these major rift systems produced interconnected oceanic seaways that divided Pangea into four continental segments: North America, Eur-Asia, and northern and southern Gondwana

Assessment of Halite-Cemented Reservoir Zones

Summary This paper describes the techniques used to identify the presence anddistribution of presence and distribution of halite-cemented layers in asandstone reservoir. The distribution of these layers in the wells was found bymatching the core data with two independent halite identifiers from the welllogs. Numerical well models were used to assess the dimensions and spatialdistribution of the halitecemented layers. Multiple simulation runs in whichthe spatial distribution, the dimensions, and the vertical permeability werevaried resulted in a permeability were varied resulted in a stochastic modelthat best matched the production history. Gas and water coning are retarded bythe halite-cemented layers if the perforations are properly located.perforations are properly located. Introduction The Azal field is located in the Marib AlJawf basin in the Republic of Yemen(Fig. 1). Of the 20 wells spudded in the field, 13 are oil-production wells andone is a gas-injection well. Production started in May 1988. The reserves areestimated to be 142 × 10(6) bbl [22.6 × 10(6) m3] of oil and 568 × 10(9) scf[16.1 × 10(9) std m3] of gas. The Azal field is an extension northeast of thelarger Alif field, from which it is separated by a half-graben structure. Thefield is 5.6 miles [9 km] long and 0.9 mile [1.5 km] wide. The depth of the topof the reservoir is 6,625 ft [2019 m], which is 3,425 ft [1044 m] below meansea level (MSL). The oil column, with a 39 degrees API [0.830g/cm3]-specific-gravity oil, has an average thickness of 80 ft [24.4 m]. The maximumga-column thickness is 240 ft [73.2 m]. The gas has a high liquid content, andthe underlying aquifer is oversaturated with salt. The Alif field, which hascharacteristics similar to those of the Azal field but a thicker oil column, started production in March 1986. After some time, salt deposition clogged anumber of wells and production facilities. Special measures were necessary toprevent production loss. The salt deposition may have resulted from pressureand temperature changes in the brine produced concurrently with the oil as aresult of water coning. No halite was observed during the formation evaluation;however, no special attention was paid to this aspect at that time. Indicationsof balite were found in the Azal field formation. A detailed core analysisrevealed the existence of diagenetic, postcompactional halite in the Alifmember postcompactional halite in the Alif member (the main hydrocarbon-bearingunit). Furthermore, the production history of the Azal field showed gas andwater production rates lower than those anticipated from the coningcalculations, which indicated that the flow path to the well was prolonged byflow path to the well was prolonged by flow barriers. In this very cleansandstone reservoir with a minor amount of diagenetic minerals, flow barrierscould well be made up of halite-cemented layers. The central issue addressed inthis paper is whether the balitelcemented layers observed in the reservoir willinfluence salt deposition in the wells and production facilities of the Azalfield by retarding water and thus salt flow through the oil column to thewells. This information is important for the planning of well locations andperforation intervals. To investigate this issue, perforation intervals. Toinvestigate this issue, a method was developed to determine the existence ofhalite-cemented zones in wells for which only well logs are available. Thedistribution and dimensions of these zones were studied from a gelogicalviewpoint by looking at the deposition history and from an engineeringviewpoint by examining the production history with numerical well productionhistory with numerical well models. Numerical models also were used to studythe influence of the halite-cemented zones on salt deposition in the wells andon reservoir management. Field Description The Azal field is situated in the Marib AlJawf basin, which trends northwestacross the southwestern corner of the Arabian Peninsula. The basin ischaracterized by a Peninsula. The basin is characterized by a half-grabenstructure divided into several sub-basins, one of which contains the Azalfield. The dominant fault trends are parallel to the basin axis(northwest/southeast). The Azal field is bounded on the southwest by asouth-dipping fault and on the north and east by a dip enclosure. Thestratigraphic time setting of the Mesozoic formations is still subject todiscussion. The term "Alif formation," the main hydrocarbon-bearingformation, refers here to marine, deltaic, and fluviatile sequences below themain evaporite sequence (Main Salt) and above the turbidites (Upper Amran or Lam formation). Palynologic evidence indicates that the deposition of theentire Alif formation occurred during the Tithonian stage. Five geologicalunits can be recognized in the field (Fig. 2). The marine shales of the local Sean member contain turbidite sandstones. A delta shoreline facies forms thetransition zone between the marine shales and the overlying sandstones of the Alif member. The fluviatile and deltaic facies of the Alif member consist ofclean sandstones. The overlying transgressive shale member consists ofsandstones and shales and reflects the beginning of a major transgression. P. 518

The bivalve Aulacomyella from the Early Tithonian (Late Jurassic) of Antarctica

The bivalve Aulacomyella is described formally from Tithonian deposits of the Antarctic Peninsula region for the first time. Two species are recognized. A. willeyi Kelly sp. nov. was collected in situ from the Nordenskjöld Formation, Longing Gap on the east coast of Graham Land, and in clasts reworked into Cretaceous glide block deposits on James Ross Island. A. cf. problematica Furlani is recorded from the Himalia Ridge Formation, Fossil Bluff Group, Alexander Island. These Antarctic records are the first published systematic descriptions of the genus from the Southern Hemisphere. Together with Mexican and Turkish records, they represent the last known occurrences of this genus. Globally the genus is particularly abundant during the Kimmeridgian and Early Tithonian stages and is therefore of value as a biostratigraphic indicator. It was almost certainly an epibyssate suspension feeder, although the precise palaeoecological setting for Aulacomyella is uncertain. It normally occurs in black shales or mudstones. The most likely life habits were either living epifaunally on the sea floor, where it bloomed under episodic dysaerobic conditions in an otherwise anaerobic environment, or pseudoplanktonically infesting floating sea weeds etc. Of these hypotheses the former is preferred here.

The Pliensbachian and Tithonian extinction events

The claim by Raup and Sepkoski1, that an approximately 26-Myr mass extinction periodicity can be recognized on a global scale at family level over the past 250 Myr, has excited much attention and provoked speculation about an ultimate extraterrestrial cause2–10. While the purported periodicity can be challenged on grounds of both the inaccuracy of the geological timescale11 and the method of extinction analysis12, there can be no doubting the reality of at least some of the 12 proposed extinction events, such as those at or near the end of the Permian, Triassic and Cretaceous. Others are less widely acknowledged. Further analysis by Raup and Sepkoski13 confirms the importance of two of these events within the Jurassic, at the end of the Pliensbachian and Tithonian stages. I demonstrate here that marine invertebrate mass extinctions at approximately these times occurred on a regional, not a global scale. They can be related to severe reductions in habitat area provoked either by regression of epicontinental sea or by widespread anoxia.

Microplankton zones of the Savik Formation (Jurassic) Axel Heiberg and Ellesmere Islands, District of Franklin

ABSTRACT Microplankton biostratigraphic zones are established in the thickest exposed section of the Savik Formation in the Sverdrup Basin. This section is located at Vantage Point, Axel Heiberg Island. The ranges of the following key taxa subdivide a sequence that ranges in age from Toarcian to Volgian: Gonyaulacysta jurassica (M. Call -- ?Top of Jur.); G. jurassica var. longicornis (Oxf.); Acanthaulax spp. (M. Call. -- L. Oxf.); Nannoceratopsis pellucida (M. Bath. -- M. Call.); Paragonyaulacysta calloviense n. gen., n. sp. (L. Call.); Pareodinia tripartitus var. obtusus n. sp., n. var. (M. Bath. -- U. Bath.); P. sp. 2 (U. Baj. -- M. Bath.); Nannoceratopsis gracilis (?Toar. -- U. Baj.). The lateral extent of these zones is demonstrated by correlations between the Vantage Point section and sections located on Bjarnasson Island and on Fosheim Peninsula of Ellesmere Island. Of the 34 microplankton species used in the zonation 8 are new. The following new taxa are named and described: Gonyaulacysta jurassica var. brevis n. var.; Paragonyaulacysta calloviense n. gen., n. sp.; Pareodinia tripartitus n. sp.; P. tripartitus var. obtusus n. sp., n. var.; P. tripartitus var. rotundus n. sp., n. var. The genus Pareodinia (Deflandre) is emended to include cysts with 3I archeopyles. Reworked microplankton indicate that the late Middle Jurassic sediments in the centre of the basin were in part derived from earlier Jurassic deposits along the margin. The level of organic metamorphism in samples from southern and western Axel Heiberg Island indicates that the prospects for liquid hydrocarbons in the pre-Cretaceous rocks of that area are minimal. Better prospects are indicated for rocks at least as old as early Jurassic on western Ellesmere Island.

New Foraminiferal Zonation, Upper Mesozoic, Sacramento Valley, California: ABSTRACT

Six new foraminiferal zones have been established in the upper Mesozoic of the Scaramento Valley, California, as the result of detailed micropaleontological work in conjunction with several reconnaissance field mapping projects. The letter designations I, J-1, J-2, K, L, and M are proposed for these microfaunal zones. These designations are a downward continuation of the Upper Cretaceous zones of Goudkoff, who established the A through H Zones in 1945. The rocks on which this new zonation is based range in age from lower Early Cretaceous, Cenomanian stage, through Late Jurassic, Tithonian stage. Micropaleontological work is essential for mapping of this thick clastic sequence, because of the gross lithologic similarity and the lenticular, disconnected, and time-transgress ve nature of the coarse clastic marker beds. This study covers a 150 mile ± section along the regional strike of the outcrop along the western side of the Sacramento Valley, and also extends into the Redding area on the northeastern side of the valley. Over 600 species, many of them new, and approximately 100 genera, including some new, were checklisted or recorded from more than 30 outcrop and well sections in the area. The new zones have been correlated approximately with the European stage classification. The I, J-1, and J-2 Zones range from Cenomanian to Aptian on the basis of both planktonic and benthonic Foraminifera; the K, L, and M Zones range from Barremian to Tithonian on the basis of cosmopolitan benthonic species. Correlations also were made between microfossil localities and northern California megafossil localities which have been equated with the European standard section. Specific criteria for identification of the H Zone were not established by Goudkoff, but detailed study permits both identification and biofacies differentiation within it. The abundant and varied microfaunas of the Middle Cretaceous have made it possible to distinguish the I from the J Zones, separate the closely related J-1 and J-2, and divide the J-1 into three sub-Zones. The K, L, and M Zones are more difficult to differentiate, because of the absence of planktonics and the predominance of many similar Nodosariidae. The M Zone probably can be subdivided if more comprehensive work is done. There is no faunal indication in the outcrop section of any unconformity between the Upper and Lower Cretaceous. Many species range through both Albian and Cenomanian sediments, without apparent interruption. End_of_Article - Last_Page 1080------------

Seven New Genera of Jurassic Ammonites

AbstractWell-defined genera of ammonites from the Toarcian, Bathonian, Oxfordian, Kimeridgian, and Tithonian are diagnosed and named.