Late Jurassic Carbonate Research Articles

Composition, environment, and economic value of the Permian to Cretaceous coated grains from Zagros and the Persian Gulf

Ooid grainstone/packstone carbonate facies of the Dalan, Kangan, and Arab formations are the main hydrocarbon reservoirs in the Persian Gulf. Based on detailed petrographic and petrophysical analyses, sedimentological and mineralogical features of the Permian to Late Cretaceous carbonate and iron-rich coated grains from Zagros and the Persian Gulf were investigated. Frequent ooids in these formations indicate a high-energy environment and a wave-dominated shallow carbonate platform. Because of wide-spread Permian–Triassic epiric seas and long-lasting periods of arid and hot climate, Permian–Triassic carbonate ooids have more vertical and lateral distribution in comparison with Late Jurassic carbonate ooids. Dolomite, calcite, and anhydrite are the major minerals of carbonate ooid grains. Early Cretaceous iron-rich ooids have been deposited above a major Late Aptian disconformity during the highstand systems tract. Ooid ironstone bulk sample X-ray diffraction (XRD) analysis shows that goethite, siderite, fluorapatite, calcite, quartz, and pyrite are the major phases of ooid ironstone deposits. This layer of iron bearing ooids could have economic importance in its outcrops in the Zagros area. Late Cretaceous iron-rich coated grains (pisoid–oncoids) were deposited above the Sarvak exposure surface during the Late Cretaceous, long-lasting sea-level fall on the Arabian Plate. Boehmite, diaspore, hematite, and goethite are the main minerals of these kind of iron-rich deposits. Late Cretaceous karst-type bauxite deposits have also been reported from different sections of the Zagros area and have economic value for mining.

Three-dimensional morphometric analysis and statistical distribution of the Early Kimmeridgian Hanifa Formation stromatoporoid/coral buildups, central Saudi Arabia

The high porosity and permeability stromatoporoid/coral facies partly characterize some of the most productive supergiant Late Jurassic carbonate reservoirs in the Middle East. Reservoir models often overlook detailed morphology and distributions of this facies due to the limited resolution of subsurface data. 3D morphological architecture and distribution of the Late Jurassic stromatoporoid/coral buildups are poorly understood as not many studies specifically investigate these aspects. This study performs a full three-dimensional outcrop investigation of the stromatoporoid/coral complex part of the Late Jurassic Hanifa reservoir analog in Wadi Birk, Saudi Arabia. We investigated spatial correlation, clustering tendencies, scaling relationships, and comprehensive statistical description over a 1.2 × 1 km area analog to a small-scale sectoral model of a typical Middle Eastern oilfield. This study utilizes integrated datasets including measured sections, drone-based digital outcrop model (DOM), core, ground penetrating radar (GPR), and shallow seismic to perform a full three-dimensional outcrop investigation. We identify and map three levels of the scaling hierarchy of the buildups: S1, S2, and S3. S1-scale buildups (a few meters in length) combine to form S2 clusters (∼35 m in length), which in turn may grow into larger S3 complexes (∼128 m in length). S1-scale buildups are near-circular and isotropic. However, as the S1 buildups cluster and grow into S2 and S3, their circular shape evolves into pseudo-ellipsoids with a strong preferential orientation towards northwest-southeast. The S2 and S3-scale amalgamated buildups are associated with asymmetric flank accumulation towards the northwest. We propose that the orientation of the S2 and S3-scale buildups and associated flanks are attributed to hydrodynamic currents driven by paleo-trade winds. This study also performs a brief static connectivity assessment to demonstrate the subsurface implications of these facies to field development planning. Results show that a vertical 5-spot pattern with a 1 km spacing can, in the best case, only access 14% of the Gross Rock Volume of the build-up facies due to the extreme lateral heterogeneity. Some 84% of the build-ups would be connected via flank and other facies with significantly different reservoir quality. This result underlines the extreme heterogeneity of the late Jurassic shallow-water platform carbonates and the potential for substantial quantities of bypassed hydrocarbons remaining in subsurface analog reservoirs.

Formation of a Late Jurassic carbonate platform on top of the obducted Dinaridic ophiolites deduced from the analysis of carbonate pebbles and ophiolitic detritus in southwestern Serbia

In the Inner Dinarides of southwestern Serbia, Tithonian polymictic carbonate turbidites, deposited in a deep-water foreland basin below overthrust ophiolites, contain Kimmeridgian–Tithonian shallow-water clasts, Triassic open-marine limestone and radiolarite clasts, and chrome spinels of a harzburgitic source (suprasubduction and MOR ophiolites). The results from the component analysis of these Tithonian polymictic carbonate turbidites constrain a Middle to Late Jurassic orogeny in the Western Tethys realm with following geodynamic evolution: (1) The closure of the western part of the Neo-Tethys Ocean caused west- to northwestward-directed ophiolite obduction onto the wider Adriatic shelf from Middle Jurassic times onwards. The former Triassic–Middle Jurassic outer passive continental margin of the Neo-Tethys imbricated and a nappe stack in lower plate (wider Adriatic) position was formed in front of the propagating obducting ophiolites. (2) During a period of relative tectonic quiescence, formation of a Late Jurassic carbonate platform started around the Oxfordian/Kimmeridgian boundary on top of the obducted ophiolites. This detection of a Late Jurassic carbonate platform formed above the obducted Dinaridic ophiolites close an important gap in knowledge about the geodynamic evolution of the Inner Dinarides. (3) From the Kimmeridgian/Tithonian boundary onwards uplift of the imbricated rocks below the obducted ophiolites triggered unroofing. During Tithonian times the obducted ophiolites were transported west-directed along low-angle fault plains near to its present position in the Dinarides. Mountain uplift and unroofing caused the partly erosion of the Late Jurassic carbonate platform, the underlying ophiolites and the Triassic–Jurassic nappe stack consisting of outer shelf sedimentary rocks.

Dispersed Geoheritage Points of the Lagonaki Highland, SW Russia: Contribution to Local Geoheritage Resource

Dispersed geoheritage points, which are small-sized and low-unique localities, manifest local geoheritage and, thus, may be useful for its comprehensive understanding. Field studies in the Lagonaki Highland (Western Caucasus, southwestern Russia) have permitted to find four geoheritage points that are described with argumentation of their importance to scientists, educators, and/or tourists. These points, none of which can be defined as a ‘traditional’ geosite, provide some precious sedimentary, palaeontological, and mineralogical information that improves perception of the local geoheritage landscape dominated by the Late Jurassic carbonate platform deposits. The studied localities are constituents of the local geoheritage resource because of their utility to three main categories of potential users. Scientists may be interested in these dispersed geoheritage points because of the need to collect massive geological data from big areas. Educators and tourists may appreciate these points because of their location along principal roads, i.e., their good accessibility.

Unusual Isolated Large Clasts from the Periphery of the Lagonaki Highland, Western Caucasus: New Evidence of Classification and Origin

Large clasts are in focus of the modern geoscience research, but their broadly-accepted classification is absent and specialists tend to over-emphasize on clasts of coastal zones. New field investigations in the Western Caucasus have permitted the finding of isolated large clasts of carbonate composition that occur above the Early–Middle Jurassic shale sequence and that are covered by modern soils. These clasts can be determined either as large boulders or megaclasts (blocks) depending on the preferred classification. Their shape is chiefly irregular, although smoothened surfaces and rounded angles (resulting from previous karstification) are also registered. These large clasts are located quite far from the natural outcrops of the Late Jurassic carbonates, which are their parent rocks. The origin of these clasts can be linked to collapses of the Pleistocene cliff of a cuesta-type mountain range, which later retreated to its present position. The evidence from the study area implies the size of 1 m or 2 m as a plausible criterion for distinction of boulders and megaclasts, and it also makes claims towards the development of a wider genetic typology of megaclasts.

Coastal cliffs, rock-slope failures and Late Quaternary transgressions of the Black Sea along southern Crimea

Rock-slope failures represent a significant hazard along global coastlines, but their chronology remains poorly documented. Here, we focus on the geomorphology and chronology of giant rockslides affecting the Crimean Mountains along the Black Sea coast. Geomorphic evidence suggests that high (>100 m) limestone cliffs flanking the southern slopes of the Crimean Mountains are scarps of rockslides nested within larger deep-seated gravitational slope deformations (DSGSDs). Such pervasive slope failures originated due to lateral spreading of intensively faulted Late Jurassic carbonate blocks moving atop weak/plastic Late Triassic flysch and tuff layers. By introducing a dating strategy relying on the combination of the uranium-thorium dating (U-Th) of exposed calcareous speleothems covering the landslide scarps with the 36Cl exposure dating of rock walls, we are able to approximate the time interval between the origin of incipient crevices and the final collapse of limestone blocks that exposed the cliff faces. For the three representative large-scale rockslides between the towns of Foros and Yalta, the initiation of the DSGSDs as evidenced by the widening of crevices and the onset of speleothem accumulation was >300 ka BP, but the recent cliff morphology along the coast is the result of Late Pleistocene/Holocene failures spanning ∼20–0.5 ka BP. The exposures of rockslide scarps occurred mostly at ∼20–15, ∼8, ∼5–4 and ∼2–0.5 ka, which substantially coincide with the last major Black Sea transgressions and/or more humid Holocene intervals. Our study suggests that before ultimate fast and/or catastrophic slope failures, the relaxation of rock massifs correlative with karstification, cracks opening, and incipient sliding lasted on the order of 104–105 years. Rapid Late Glacial/Holocene transgressions of the Black Sea likely represented the last impulse for the collapse of limestone blocks and the origin of giant rockslides, simultaneously affecting the majority of the SW coast of the Crimean Peninsula.

Geology and hydrocarbon potential of offshore SE Somalia

Interpretation of a 20 550 line km 2D seismic survey acquired in 2014 by Soma Oil and Gas in the deep water offshore area of SE Somalia has identified three previously undocumented sedimentary provinces – Jubba Deep, Mogadishu Deep and Mid Somalia High – all of which have distinctive geological characteristics. Well and stratigraphic controls are limited, with inferred lithologies largely based on seismic stratigraphic interpretation. The Jubba Deep has a thick Late Cretaceous–Early Tertiary deltaic section deformed by major gravitational collapses in the Paleocene-age Kismaayo Thrust Belt (KTB) and the Pliocene-age Baraawe Thrust Belt (BTB). It is proposed that the KTB has significant hydrocarbon potential in deltaic and pro-delta sands trapped in thrust anticlines and sourced with oil from Mid-Cretaceous mobile shales. The Mogadishu Deep Basin has a thick Mesozoic and Tertiary section but is missing the thick deltaics seen in the Jubba Basin. Volcanics are present in this basin. The Mid Somalia High has a relatively thinner sedimentary section where Cretaceous and Jurassic reservoirs and potential source rocks are at moderate burial depths. An extensive post-rift Mid–Late Jurassic carbonate platform is developed here with potential hydrocarbon targets in interpreted reefs and shoal facies. Supplementary material : Additional seismic examples and map figures are available at https://doi.org/10.6084/m9.figshare.c.3902650

?Kimmeridgian-Tithonian shallow- water platform clasts from mass flows on top of the Vardar/Axios ophiolites

The Late Jurassic to Early Cretaceous sedimentary succession of the Neochorouda Unit lies unconformably on top of the Oreokastro ophiolites of the Vardar/Axios “suture zone” in northern Greece. This succession consists of turbidites and mass flows and provides an upper limit for ophiolite emplacement. New biostratigraphic and microfacies analysis from the clasts in the mass flows were carried out for a better understanding of the Late Jurassic to Early Cretaceous geodynamic history. Microfacies and organism content prove the onset of Late Jurassic carbonate platforms on top of a Middle to Late Jurassic nappe stack striking from the Eastern Alps to the Hellenides. Middle to Late Jurassic nappe stacking towards WNW to NW followed late Early to Middle Jurassic intra-oceanic thrusting in the Western Vardar/Axios (= Neotethys) Ocean and subsequent ophiolite obduction onto the Pelagonian Units forming a thin-skinned orogen on the lower plate. After ophiolite emplacement Kimmeridgian- Tithonian carbonate platforms sealed widespread this tectonic event. Tithonian extension due to mountain uplift resulted in partial erosion of these platforms and new extensional basins were formed. Late Tithonian to earliest Cretaceous erosion of the uplifted nappe stack including the obducted ophiolites resulted in sediment supply into the newly formed basins also east of the Pelagonian Units.

The rebirth of Papua New Guinea

Gas was first discovered by drilling activity in Papua New Guinea in 1956. Nevertheless it was almost 60 years later when the Exxon-operated PNG LNG Project became the first large-scale commercialisation of the country’s prolific gas resources, with export of the first LNG shipment in May 2014. The proven fluvial-deltaic Late Jurassic, early Cretaceous and Tertiary carbonate reservoirs are prolific petroleum producers. When combined with the high-quality liquid-rich nature of the gas and the onshore location, this has resulted in PNG’s LNG projects being some of the lowest cost and most profitable LNG projects globally. The success of the PNG LNG Project along with the substantial identified existing and yet-to-find gas resources has resulted in a recent resurgence of exploration interest in PNG as companies look to expand and capitalise on the country’s developing position as a globally significant LNG supplier. In 2015 Oil Search undertook a whole of country review using its extensive database and in-country knowledge. This study incorporated all well, seismic, surface, remote sensing, production and development data, and has resulted in a detailed understanding of the play distribution and risk ranking, and importantly delineated a number of potentially material new play types in the country. Approximately 4.8 billion barrels of oil equivalent recoverable resources (2P and 2C) have been discovered in PNG to date, of which approximately 85% is gas. The countrywide regional study and common risk segment analysis by Oil Search (2015) established that PNG potentially contains an additional seven billion barrels of oil equivalent resource still to be discovered. The majority of this volume is expected to be gas, estimated to be in the order of 40 trillion cubic feet and 550 million barrels of undiscovered prospective resources. Oil Search has been active in all of PNG’s known petroleum basins since 1929 and built a substantial database. Pool-size distributions and a countrywide prospects and leads inventory suggest discovery of new giant fields is likely and of sufficient scale to support future LNG projects. Six sub-basins contain proven petroleum plays that are predicted to extend into under-explored areas. These areas represent the future petroleum exploration frontier in PNG.

Reinhart A. Gygi (1935–2014): Scientific achievements of a life-long commitment to research in Late Jurassic carbonate sedimentology and biostratigraphy of the northern Swiss Jura Mountains

On November 7th 2014, Reinhart A. Gygi died in his home at the Carl-Spitteler-Strasse 70 in Zurich-Witikon. With him, the scientific community lost the foremost expert of Late Jurassic stratigraphy of the Swiss Jura Mountains, and of Late Jurassic ammonites. During 34 years, he worked as curator at the Natural History Museum Basel, and he was a long-term member of the Swiss Palaeontological Society and the Swiss Geological Society. Reinhart A. Gygi, citizen of Aarau AG and Kappelen BE, was born in Zurich on November 25th 1935. His father was the engineer Hans Adolf Gygi who would later become director of the Holderbank Cement Mills and himself son of the factory founder Adolf Gygi. His mother was Berta Marilies, born Reinhart. Reinhart spent his early years together with his three siblings in the parental home in Moriken-Wildegg. He attended the primary school in Moriken, secondary and high school in Aarau. An inspiring high-school teacher was responsible that Reinhart became enthusiastic about geology. From 1954 on he stayed in Aarau, first in the house of family friends, then in his aunt Clarli’s house. Reinhart finished high school and passed the final exam in March 1956. In the following summer, Reinhart accompanied his father on a business trip to Canada. There he could work as trainee in a mining company and experienced the hard life of a prospecting geologists searching for iron and nickel ores. In October 1956, Reinhart started to study geology in Zurich, until summer 1957 at the ETH, then at the University of Zurich. During his early study years he already acted as an advisor for civil engineering and highway constructions. In 1961 he had completed the mandatory geological mapping, which he did in his ‘‘home range’’ on the Chestenberg just north of Moriken-Wildegg. This resulted in his first two publications (Hofmann and Gygi 1961; Gygi and Trumpy 1966). In 1962, Reinhart started working on his PhD thesis. Professor Rudolf Trumpy had proposed to investigate the Upper Jurassic of the canton Aargau, a subject that initially did not promise to be very rewarding to the student, but fieldwork, which was soon extended to the cantons Solothurn and Schaffhausen, yielded very interesting results. Reinhart had by that time already established himself as an expert of Jurassic stratigraphy of the Jura Mountains (Gygi and Stumm 1965; Gygi 1966a; Gygi et al. 1966). First results of his PhD work were published in 1966 (Gygi 1966b), and Reinhart finished his thesis on the Oxfordian of northern Switzerland and was promoted in the autumn of 1967. Reinhart had planned to become prospecting geologist, but Rudolf Trumpy could persuade him to apply for a research job. On December 1st 1967, he started working as research assistant at the Natural History Museum Basel where he could pursue his studies on the Upper Jurassic of northern Switzerland. The first month of the employment he worked without salary that he compensated with a month of travel abroad in April 1968. From his parents, Reinhart had received a 4WD-truck, a Volvo Laplander, as a present for passing the promotion, and he undertook together with a colleague a longer trip to the Middle East with this vehicle in April 1968. The focus of this trip was rather cultural than geological. Later this truck would also accompany him for fieldwork in many quarries of northern Switzerland. Another stay abroad during his first year at the Natural History Museum of Basel took him in the summer 1968 to Bermuda where he & Walter Etter walter.etter@bs.ch

Detailed 3-D depositional architecture of Late Jurassic carbonate–anhydrite cycles (Brightling Mine, Weald Basin, UK)

Quantifying the geometries of evaporite deposits at a <1 km scale is critical in our understanding of similar ancient depositional systems, but is challenging given evaporite mineral dissolution at surface conditions. A high-resolution stratigraphic study of the basal Purbeck Beds in Brightling Mine, UK, provides insight into the three-dimensional architecture, lateral continuity and vertical heterogeneity within an evaporite seal. We conducted a field mapping study, combined with X-ray diffraction, petrographic microscopy, and δ13C and δ18O isotope analysis. The stratigraphic interval contains five facies. In stratigraphic order, these include supratidal porphyritic nodular evaporite, shallow subtidal peloidal packstone with evaporite and two overlying rhythmic sequences of intertidal microbial laminite, subtidal shale, and subtidal laminar marl, capped by nodular anhydrite. The interpreted environment of deposition is a supratidal sabkha subject to periodic flooding in which intertidal (tidal flat) facies and subtidal (shallow marine) facies laterally passed into the evaporative sabkha. The cycles are interpreted as meter-scale shoaling-upward sequences, likely controlled by localized high-frequency changes in relative sea level and/or sabkha hydrology. Spatial patterns in the geometries of key stratigraphic surfaces reveal a subtle depression towards the central western region of the mine seam. The variation in stratal geometries is interpreted as paleotopography and is a function of individual or composite processes related to dissolution, eolian processes, and coastal erosion. These observations indicate a similar mode of deposition to the modern-day sabkha of the Persian Gulf. We conclude that the dynamic process of evaporite deposition led to subtle stratigraphic heterogeneities and changes in bed thicknesses, but largely continuous lateral bedding at an interwell-scale.

Regression trees for modeling geochemical data—An application to Late Jurassic carbonates (Ammonitico Rosso)

Research based on ancient carbonate geochemical records is often assisted by multivariate statistical analysis, among others, used for data mining. This contribution reports a complementary approach that can be applied to paleoenvironmental research. The choice to use a machine learning method, here regression trees (RT), relied in the ability to learn complex patterns, integrating multiple types of data with different statistical distributions to obtain a knowledge model of geochemical behavior along a paleo-platform.The Late Jurassic epioceanic deposits under scope are represented by six stratigraphic sections located in SE Spain and on the Majorca Island. The used database comprises a total of 1960 data points corresponding to eight variables (stable C and O isotopes, the elements Ca, Mg, Sr, Fe, Mn and skeletal content). This study uses RT models in which the predictive variables are the geochemical proxies, whilst skeletal content is used as a target variable. The resulting model is data driven, explaining variations in the target variable and providing additional information on the relative importance of each variable to each prediction, as well as its corresponding threshold values.The obtained RT revealed a structured distribution of samples, organized either by stratigraphic section or sets of nearby sections. Averaged estimated skeletal abundance confirmed the initial observations of higher skeletal content for the most distal sections with estimated values from 18% to 27%. In contrast, lower skeletal abundance from 5% to 15% is proposed for the remaining sections. The geochemical variable that best discriminates this major trend is δ18O, at a threshold value of −0.2‰, interpreted as evidence for separation of water-mass properties across the studied areas. Other four variables were considered relevant by the obtained decision tree: C isotopes, Ca, Sr and Mn, providing new insights for further differentiation between sets of samples.

Facies heterogeneity of a Kimmeridgian Carbonate Ramp (Jabaloyas, eastern Spain): a combined outcrop and 3D geomodelling analysis.

The Upper Kimmeridgian outcrops exposed around the village of Jabaloyas (NE Spain) have been analyzed to document the facies heterogeneity across the shallow portion of a low angle carbonate ramp. The studied area covers a total surface of 12 square kilometres. Seventeen stratigraphic profiles have provided basic information to carry out the detailed facies analyses presented in this work. The studied unit corresponds to a high-frequency sequence, bounded by discontinuities that can be traced at regional scale. This sequence shows the widespread development of coral-microbial reef buildups with conical to cylindrical shape (up to 19 m thick). The characterization and interpretation of the different sort of inter-reef and post-reef facies resulted in the reconstruction of the sedimentary domains of the studied carbonate ramp, from burrowed mud-dominated skeletal facies (distal middle ramp), to grainy (skeletal, peloidal, oolitic) shoal and backshoal facies (inner ramp). The studied sequence is divided into four sedimentary episodes bounded by sharp facies changes. The lower two episodes include the vertical reef growth during rapid accommodation gain; the upper two episodes are marked by the rapid progradation of the inner ramp facies. The depositional facies and their stacking patterns in the Jabaloyas outcrops are similar to those described in the subsurface Arab D reservoirs. This Late Jurassic carbonate unit forms major hydrocarbon fields in the Middle East. For improving the geological distribution of reservoir heterogeneity in this kind of low angle carbonate ramp system, two 3D models have been created. A Fullfield Model of the overall geometry of the facies belts has allowed the quantification of vertical and lateral extension of the inter-reef and post-reef facies. A Sector Model, which includes individual reef bodies has been used to better simulate reservoir heterogeneity resulting from different diagenetic overprint.

TAXONOMY AND PHYLOGENY OF THE TROCHOLINIDAE (INVOLUTININA)

Exceptionally well-preserved trochospirally coiled Involutinina have been found in Late Triassic (Wallowa Mountains, Oregon, U.S.A.) and Early Jurassic (northern Calcareous Alps, Austria) carbonate rocks. Their remarkable preservation, related to impregnation of the test prior to recrystallization, allowed us to observe features usually obliterated by diagenetic and metamorphic processes. This paper provides a revision of the morphology, structure, systematic position, phylogeny, and stratigraphic range of the family Trocholinidae Kristan-Tollmann. New criteria permitting the distinction between trocholinid genera, even in cases of intense recrystallization, are presented. An identification key and a new, monophyletic tree are proposed for the whole family. The main outcomes of this work are that 1) Neotrocholina Reichel is definitively excluded from the Involutinina; 2) the genera Andersenolina Neagu, Hottingerella Piller, and Septatrocholina BouDagher-Fadel and Banner are placed in synonymy with Coscinoconus Leupold; 3) the genus Ornatoconus di Bari is considered a synonym of Trocholina Schlumberger; 4) the distinction between the genera Auloconus Piller and Lamelliconus Piller is abandoned; 5) five new genera ( Coronaserra n. gen., Frentzenella n. gen., Kristantollmanna n. gen., Papillaconus n. gen., and Wallowaconus n. gen.) and three new species ( Frentzenella frentzeni n. gen., n. sp., Frentzenella perforata n. gen., n. sp., and Wallowaconus oregonensis n. gen., n. sp.) are introduced; 6) the species “ Trocholina intermedia ” Henson is reinstituted; 7) “ Trocholina gigantea ” Pelissie and Peybernes, and “ Semiinvoluta verrucosa ” Tollmann and Kristan-Tollmann are renamed Coscinoconus limognensis nom. nov. and Papillaconus kristani nom. nov., respectively; and 8) new combinations for several species of Trocholinidae are provided. The recognition of two subfamilies (Lamelliconinae and Trocholininae) and twelve genera within the Trocholinidae underlines the high diversity of the family. The rapid diversification and widespread distribution characterizing the group suggest that the potential of trochospirally coiled Involutinina in stratigraphic and paleoenvironmental studies is greatly undervalued.

New age data from the tectonostratigraphic units of the Istranca “Massif” in NW Turkey: a correlation with SE Bulgaria

Abstract The Istranca Crystalline Complex in NW Anatolia and SE Bulgaria includes structural units that differ in lithostratigraphy, metamorphism, age and structural position. They are collectively named as the “Istranca nappes” comprising from bottom to top the Sarpdere, Mahyadağ and Doğanköy Nappes. The Sarpdere Nappe consists of Lower Triassic arkosic metasandstones with slate interlayers, followed by Middle to Upper Triassic carbonates and an alternation of Upper Triassic clastics and carbonates. The Mahyadağ Nappe comprises a low-grade metamorphic Late Paleozoic- Triassic carbonate-siliciclastic sedimentary succession. The Doğanköy Nappe includes Precambrian?-Paleozoic metasediments, intruded by Late Carboniferous-Early Permian calc-alkaline granitoids. Its Triassic cover comprises metaclastics and metacarbonates. The Istranca nappes were juxtaposed at the end of the Triassic and transgressively covered by Lower Jurassic coarse clastics, followed above by Middle to Late Jurassic carbonates, black shales and carbonate-siliciclastic sedimentary succession. The phosphate concretions in black shales yielded radiolarian assemblages indicating Late Bajocian-Early Bathonian, Early Bathonian and Early Kimmeridgian ages. These nappes and their Jurassic cover are unconformably overlain by the Cenomanian-Santonian volcano-sedimentary successions intruded by Santonian-Campanian Dereköy-Demirköy intrusive suite. The preliminary data suggest that the Variscan basements of the Mahyadağ and Sarpdere Nappes were juxtaposed prior to the Triassic and overridden by the Doğanköy Nappe of possible Rhodopean origin from S to N during the Cimmerian compressional events

Pre-discovery seismic modelling and prediction of the Deep Panuke Late Jurassic carbonate bank gas discovery, offshore Nova Scotia

Abstract Deposition of the Abenaki Formation carbonate complex began during the Middle Jurassic, shortly after the interpreted onset of western Atlantic sea-floor spreading, and continued through Late Jurassic time, resulting in thick carbonate deposits along the Jurassic shelf edge. This carbonate complex was the exploration target of 16 wells drilled from 1970 to 1989. Although there were no discoveries in the carbonate objectives, oil was discovered in Cretaceous siliciclastics in structural drape closures overlying the carbonate margin. The Province of Nova Scotia decided to undertake a technical study to further the interest in the carbonate bank objective while the shallow oil play was being developed. Subsequently, in 1989, two studies were undertaken to investigate a porosity play in the Jurassic Baccaro Member of the Abenaki Formation because significant porosity had been encountered in a number of the dry holes that targeted the carbonate margin, particularly the Demascota G-32 well. The first study modelled porosity in several wells utilizing one-dimensional (1D) and wedge models, and amplitude v. offset (AVO) models. The Demascota G-32 well was used to seismically identify and model several types of potential hydrocarbon-bearing prospects along the bank. Modelling indicated that zones of 11–14% porosity with a minimum thickness of 10 m could be detected. However, a thickness of 50 m was required to resolve the top and base of the porosity. The AVO model for the Demascota well demonstrated that an increase in amplitude with offset (AVO effect) in the presence of gas and fracturing (which dramatically decreases Poisson’s ratio) should be detectable and may be useful in identifying prospects in this type of play. The assumption of a fractured reservoir was reasonable due to the number of seismically identified faults at the front of the carbonate bank and the description of fractures in conventional cores taken in the Baccaro Member. The second study involved acquiring a pair of dip and strike seismic lines adjacent to the Demascota well and reprocessing them to preserve relative amplitudes while providing a high-resolution, stacked section, especially at the Baccaro Member level. Interpretation of these lines was undertaken in the AVO domain. An increase in amplitude with offset was observed up-dip of the Demascota G-32 well that indicated a fractured, gas-filled porous zone could exist up-dip of this well. Following these studies, 3D seismic data was acquired over the shallower Panuke Oil Field and similar studies were conducted on that dataset in 1997. As a result, Encana drilled a structural high containing bright amplitude reflectors and encountered over 100 m of net gas pay in vuggy and cavernous limestones and dolomites in 1999. This validated the earlier studies, confirming that these reservoirs could be seen on seismic data and proved that economic gas reservoirs exist in the carbonate bank. These results should encourage exploration along the extensive, sparsely drilled Late Jurassic carbonate margins bounding offshore eastern North America and its conjugates in the Central Atlantic Region.

The ‘Polish Solnhofen’: a long‐awaited alternative?

Here we briefly report on the discovery of a new Fossil‐Lagerstätte locality, Owadów‐Brzezinki quarry (central Poland), which exposes Late Jurassic (Late Tithonian) carbonate sediments with an extremely fossiliferous horizon of lithographic‐type limestones. Numerous specimens of horseshoe crabs were found in association with an enormously rich assemblage of the soft‐shelled bivalves Corbulomima obscura and Mesosaccella sp., the remains of various fishes and marine reptiles, rare ammonites, crustaceans, land insects and pterosaurs. The uniqueness of this new locality lies in its very close stratigraphical relationship to one of the most famous Fossil‐Lagerstätte localities in the world—Solnhofen, in southern Germany, with approximately 2 Ma separating them. Marine and terrestrial creatures lived and died during the Late Jurassic both at Solnhofen (Hybonotum Zone) and in another area (Owadów‐Brzezinki quarry, Zarajskensis Subzone), under closely related environmental conditions. The small palaeogeographical distance separating these two locations enables, for the first time, an effective palaeobiological test of the pace of evolutionary speciation amongst different groups of organisms.

Upper Jurassic Reefs from the Russian Western Caucasus: Implications for the Eastern Black Sea

Exposures of Upper Jurassic reef outcrops in the Russian western Caucasus provide excellent field analogues for possible reef-complex reservoir targets imaged on seismic reflection data from the northern Shatskiy Ridge, eastern Black Sea. The reefs at outcrop can be generally grouped into coral-dominated, siliceous sponge-microbialite and microbialite types. Coral-dominated reefs occur as isolated patchy and massive forms, and can be subdivided into higher-diversity and low-diversity types. The former developed at shallow-water platform margins and in platform interiors, whilst the latter occurred in deeper-water mid-shelf settings. Siliceous sponge-microbialite and microbialite reefs occur as lenses and mounds that were restricted to deeper-water mid-outer shelf environments. The reefs developed on two Late Jurassic carbonate platforms in the Russian western Caucasus: the north Caucasus platform to the north and South Adler platform to the south. These platforms were separated by a deep marine (Greater Caucasus) basin, along the margins of which thick shallow-water coral-dominated reefs formed. The southwestern margin of the north Caucasus platform probably represented a reef barrier-slope-basinal system that was structurally controlled. At the northeastern margin of this platform, in the Laba River region, inner ramp coral-dominated reefs pass northwards into deeper-water siliceous sponge- and microbialite-dominated reefs. The South Adler platform may extend offshore into the eastern Black Sea and the coral-dominated reefs that crop out at its northern margin form potential exploration analogues. The palaeowater depth of the possible isolated reef complexes imaged on the Shatskiy Ridge is unclear. If they were deposited in shallow-water, the coral-dominated reefs examined in the north Caucasus or South Adler carbonate platforms may form suitable analogues. Alternatively, if they were developed in deeper water they may be equivalent to the siliceous sponge and microbialite reefs examined in the Laba River region.

Tithonian–Early Valanginian evolution of deposition along the proto-Caribbean margin of North America recorded in Guaniguanico successions (western Cuba)

In the Guaniguanico Mountains of western Cuba, the Late Jurassic–Early Cretaceous limestones occur in three stratigraphic successions, which have accumulated along the proto-Caribbean margin of North America. The Late Jurassic subsidence and shallow-water carbonate deposition of the Guaniguanico successions have no counterpart on the northeastern Maya block, but some distant similarities with the southeastern Gulf of Mexico may exist. Four facies types have been distinguished in the Tithonian–Lower Valanginian deposits of the Guaniguanico tectonic units. Drowning of the Late Jurassic carbonate bank of the Sierra de los Organos occurred at the Kimmeridgian/Tithonian boundary. During this boundary interval, sedimentation in the west Cuban area and southwestern margin of the Maya block (Mexico) has evolved in a similar way in response to a major second-order transgression. The Lower Tithonian ammonite assemblages of the Guaniguanico successions indicate, in general, the neritic zone. Presence of juvenile gastropods and lack of adult specimens suggest unfavorable environment for these molluscs, probably related to low oxygenation levels. The Early Tithonian transgressive phase terminated about the lower boundary of the Chitinoidella Zone. The Late Tithonian “regressive” phase is weakly marked, whereas the latest Tithonian–earliest Berriasian strata were deposited during a deepening phase. The latter transgressive phase has ended in the Late Berriasian Oblonga Subzone. We correlate the bioturbated pelagic biomicrites of the Tumbitas Member of the Guasasa Formation with a significant fall of the sea level during the latest Berriasian–Early Valanginian. The average sedimentation rate for the Tumbitas Member biomicrites was about three times faster than for the Berriasian Tumbadero Member limestones. Sedimentation rates for the Tumbitas Member and the Valanginian limestones at the DSDP Site 535 in the southeastern Gulf of Mexico were similar. In the Los Organos succession, the Late Valanginian transgressive interval is associated with radiolarian limestones and black chert interbeds in the lower part of the Pons Formation. In the Southern Rosario succession, the pelagic limestones pass into the radiolarian cherts of the Santa Teresa Formation indicating a proximity of CCD during Late Valanginian–Hauterivian times.

Facies architecture of a Late Jurassic carbonate ramp: the Korallenoolith of the Lower Saxony Basin

The sedimentary succession of a Late Jurassic (Oxfordian to basal Kimmeridgian) carbonate ramp is described and interpreted. The study area is located in the central part of the Lower Saxony Basin in NW Germany, which forms part of the Central European Basin. Eight well-exposed and undeformed sections of the study area (Suntel area, Wesergebirge and eastern part of the Wiehengebirge) provide detailed information about lithofacies and lateral thickness variations. Biostratigraphically, the age of these sediments is poorly constrained. Twenty microfacies types are recognized that can be grouped into seven facies associations: (a) strongly bioturbated marlstones deposited near storm wave base (SWB), (b) foraminifera-rich wackestones, (c) wackestones and floatstones with biostromes and (d) bioclastic limestones deposited between SWB and fair-weather wave base (FWWB), (e) oolitic and iron-oolitic limestones and (f) siliciclastic sediments deposited above FWWB, and (g) lagoonal deposits. These facies associations characterize a storm dominated shallow mixed carbonate-siliciclastic ramp. Based on facies changes, quartz content, and gamma ray logs, the Korallenoolith Formation can be subdivided into a lower carbonate-dominated and an upper siliciclastic-dominated part, build up by different scales of small- to large-scale deepening- and shallowing-upward cycles. A preliminary correlation of measured outcrops of this formation is presented.