Jurassic Carbonate Research Articles

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.

Influence of climate and dolomite composition on dedolomitization: insights from a multi-proxy study in the central Oman Mountains

Abstract Dedolomitization is an important diagenetic process that can yield information on the history of paleo-fluids in a given aquifer or outcrop, and by extension it offers a window into past hydrologic conditions. Dedolomitization is also relevant economically as it can alter the porosity and permeability of carbonates, thus affecting reservoir quality. Despite considerable research, the process of dedolomitization is still not entirely understood. Here, new findings from the central Oman Mountains highlight the importance of dolomite chemistry and fluctuating climate on dedolomitization. Petrographic, mineralogical, and geochemical data from both altered and pristine dolomite hosted in Jurassic carbonate rocks from the Sahtan Group and outcropping at Wadi Mistal in the Jebel Akhdar tectonic window reveal two dolomite bodies with different characteristics, stratabound and fault-related dolostone. The (ferroan) dolomite crystals are larger in the fault-related dolostone, whereas the (non-ferroan) dolomite crystals are smaller in the stratabound dolostone. Petrography reveals a complex suite of alteration textures, including pristine dolomite, recrystallized dolomite, and calcitized dolomite (dedolomite). Iron oxides are present pervasively in the altered rocks, and different alteration textures co-occur in the same sample. Relative to unaltered dolomite in this outcrop, the recrystallized dolomite is characterized by similar positive carbon isotope values but less negative oxygen isotope values, and the calcitized dolomite is characterized by even less negative oxygen isotope values and more negative carbon isotope values. Based on field data and petrography, two phases of alteration are evident, an earlier dedolomitization event affecting the stratabound dolomite only, and a more pervasive Pleistocene-Holocene alteration phase associated with surface weathering that affected both the fault-related dolomite and to a lesser extent the stratabound dolomite. Stable isotope results further suggest that the more recent subaerial weathering phase happened under two alternating climate regimes: dedolomitization was triggered by meteoric fluids that interacted with soil-related carbon during humid climatic period(s), whereas recrystallization of ferroan fault-related dolomite happened during more arid climatic period(s) with less developed soils and lower fluid–rock interaction. These results suggest that weathering of ferroan fault-related dolomite can lead to formation of goethite and recrystallized, non-stoichiometric dolomite (with a reset stable oxygen isotope signature), and sometimes to dedolomitization.

Tectonostratigraphic overview of the Zagros Suture Zone, Kurdistan Region, Northeast Iraq

ABSTRACTThe northwestern segment of the Zagros Orogenic Belt of the Kurdistan Region of Iraq includes the Zagros Suture Zone which is consisting of allochthonous Tethyan Triassic–Eocene thrust sheets. The zone is bounded by the Zagros Main Reverse Fault in the northeast, and the Zagros Thrust Front in the southwest. Parts of this zone’s rocks are represented by stacks of thrust megasheets obducted over the autochthonous Cretaceous–Cenozoic sequence of the Arabian Plate margin. Each sheet represents a specific Tethyan tectonostratigraphic facies, and includes (from older to younger): isolated Triassic carbonate platforms (Avroman Limestone), Jurassic carbonate imbricates (Chia Gara, Sargelu and other formations), radiolarites (Qulqula Group), sedimentary mélange (sedimentary-volcanic units of the Qulqula Group), ophiolites (Mawat and Penjwin ultramafics complexes), and Cenozoic fore-arc volcano-sedimentary sequences (Walash Group). Petrography, facies interpretation and lithostratigraphic correlation of these allochthons along four traverses across the Zagros Suture Zone of the examined area indicate that they evolved during the closure of the Neo-Tethys Ocean. Their stacking pattern and tectonic association resulted from two important events: the Late Cretaceous obduction processes, and the Late Miocene–Pliocene collision, uplift, folding and suturing between the Arabian Plate and the Sanandaj-Sirjan Block of Iran. Based on these field observations and by using the model of the Iranian Zagros evolution, a tectonic scenario is proposed to explain the history and evolution of the Zagros Suture Zone in this area.

Jurassic carbonate depositional systems of the Mt. Catria-Mt. Acuto area (Umbria-Marche Apennines, Italy)

A geologic survey, integrated with sedimentalogic observations nd macrofacies analysis, has been carried out for the Jurassic eposits outcropping in the Mt. Catria-Mt. Acuto area. he study area is located within the northern Umbria-Marche pennines and is characterized by the presence of Jurassic different type sucessions that reflect depositional environment variability. te aim of this paper is to reconstruct the Jurassic paleographic structural etting and the paleotectonic evolution of the area. Parti - cular attention was given to the characterization of the relationships between deposits formed during and after the drowning of the Calcare Massiccio peritidal carbonate platform. In addition, the depo - sitional trend of this carbonate platform was determined by the charac terization of tidal environment sedimentary structures and facies associations. The data obtained allowed for the identification of two isolated, uplifted blocks of the carbonate platform (Corno di Catria and Mt. Acuto), reduced in size, bounded by escarpments of tectonic origin and separated by a narrow Jurassic basinal area (Mt. Catria). Along paleoslopes of structural highs, drowning Calcare Massiccio “B”-type deposits have been identified that differ from Calcare Massiccio “B” deposited on the tops of paleohighs in their mainly lithoclastic facies. The drowning phase, at the tops as well as along the flanks of the blocks, is also marked by unconformity surfaces (drowning unconformities). The different paleostructural sectors have been differentiated during the Lias by a phase of strong extensional tectonics that was responsible for the formation of the tectonic escarpments, subsequently affected by submarine erosion. In the early stages of the Middle Jurassic, tectonic reactivation resulted in formation of some marginal, down-dropped portions of the structural highs, which were subsequently filled by the same cherty sediments that deposited in the Mt. Catria basinal area. In the study area, the depositional environment has remained differentiated throughout the Jurassic as evidenced by the presence of conformable and unconformable boundaries between the Tithonian calpionellid pelagites of the Maiolica Formation and underlying deposits

Geology and geochemistry of the Jurassic carbonate hosted barite deposits, Northeast Zakho, Iraq

Barite deposits located in Marsis locality of about 25km northeast of Zakho City were hosted in Jurassic dolomitic rocks. The barite is extending for about 6km and limited within about 38m thickness of dolomitic rocks. The thickness of the barite veins varies between few centimeters to 2.5m which increases downward. The major mode of occurrence of barite is as vein and cavity-filling. The barite and other minerals of vein and cavity-filling deposits are typical of suite precipitated from low-temperature hydrothermal solutions associated with igneous activity. Carbonate-hosted barite deposits contain 0.237% BaO, 103ppm Zn, and 83 ppm Pb. These rocks are deficit in Fe and F. Most of carbonate rocks are dolomitized, which provide appropriate physical and chemical conditions that permitted the passage of mineral-bearing fluids. These deposits are formed in carbonate platform setting and commonly developed in foreland thrust belt.

Paleomagnetism of Jurassic carbonate rocks from Sardinia: No indication of post-Jurassic internal block rotations

[1] Several paleomagnetic studies on Carboniferous and Permian sedimentary and volcanic rocks from Sardinia and Corsica have recently demonstrated (1) the tectonic coherence between southern Corsica and northern Sardinia and (2) significant rotations between individual crustal blocks within Sardinia itself. The geodynamic significance of these rotations, however, is not clearly understood mainly because of uncertainties in defining their timing and causes. In order to contribute to these issues, a pioneering paleomagnetic study on Jurassic carbonates from the Baronie-Supramonte region of eastern-central Sardinia has been extended regionally and stratigraphically. A total of 280 oriented drill cores were taken from 44 sites of Middle and Late Jurassic age in the Nurra, Baronie-Supramonte, Barbagia-Sarcidano, and Sulcis regions. Despite generally weak remanent magnetization intensities, on the order of less than 1 mA/m, thermal and alternating field demagnetizations were successfully applied to define a characteristic remanent magnetization component in about 60% of the samples. Site mean directions show rather good agreement after correction for bedding tilt and yield Middle and Late Jurassic overall mean directions of D = 269.7° and I = 45.0° (α95 = 8.0°, k = 14, and n = 25 sites) and D = 275.5° and I = 50.7° (α95 = 7.2°, k = 45.3, and n = 10 sites). Positive regional and local fold and reversal tests demonstrate the primary character of the natural magnetic remanence, which is carried by magnetite. These results indicate only insignificant amounts (±10°) of post-Jurassic rotations within the island of Sardinia. The resulting Middle and Late Jurassic paleopoles (latitude (Lat) = 16.5°, longitude (Long) = 299.1°, dp = 6.4°, and dm = 10.1° and Lat = 23.4°, Long = 301.2°, dp = 6.5°, and dm = 9.7°), corrected for the opening of (1) the Liguro-Provençal Basin and (2) the Bay of Biscay using rotation parameters from the literature, fall near the coeval segment of the European apparent polar wander path. These results constrain the timing of large differential block rotations found in Late Carboniferous-Permian rocks to a pre–Middle Jurassic age and lead us to exclude tectonics related to the Alpine orogeny for such rotations.

MIDDLE AND LATE HETTANGIAN RADIOLARIANS FROM THE MT. CAMICIA SUCCESSION (GRAN SASSO, CENTRAL APENNINES - ITALY)

Well-preserved middle and late Hettangian radiolarians have been discovered S-E of Mt. Camicia (Central Apennines) in limestone beds that contain also Hettangian ammonites. These beds are part of an Upper Triassic to Lower Jurassic carbonate succession, which crops out in the eastern part of the Gran Sasso range. This succession includes euxinic deposits (bituminous dolostone) and other pelagic sediments (mudstone and calcarenites) which developed in a basin rimmed by a carbonate platform. In the same time interval, the well-known formations of Dolomia Principale and Calcare Massiccio were forming on this platform. In this paper we examine the radiolarian assemblages collected in three levels of the ammonite bearing beds, and we define two new genera ( Squinabolia and Turritus ) and eight new radiolarian species ( Anaticapitula parvireticulata , Anaticapitula triangularis, Farcus aquilensis, Farcus leonseveroi , Parahsuum vraddense , Squinabolia multispinata, Turritus venturii, Zhamoidellum sphaericus ), and we define twenty four other species, for a total of thirty eight identified species. The ammonites in the lowest level indicate a middle Hettangian age, those from the two upper levels a late Hettangian. In the Western Tethys successions bearing Hettangian radiolarians are rare and have been found together with ammonites only in the Mt. Camicia section described in this paper. This section permits to correlate the radiolarian assemblages with the ammonite zonation.

An open ocean record of the Toarcian oceanic anoxic event

Abstract. Oceanic anoxic events were time intervals in the Mesozoic characterized by widespread distribution of marine organic matter-rich sediments (black shales) and significant perturbations in the global carbon cycle. These perturbations are globally recorded in sediments as carbon isotope excursions irrespective of lithology and depositional environment. During the early Toarcian, black shales were deposited on the epi- and pericontinental shelves of Pangaea, and these sedimentary rocks are associated with a pronounced (ca. 7 ‰) negative (organic) carbon isotope excursion (CIE) which is thought to be the result of a major perturbation in the global carbon cycle. For this reason, the lower Toarcian is thought to represent an oceanic anoxic event (the T-OAE). If the T-OAE was indeed a global event, an isotopic expression of this event should be found beyond the epi- and pericontinental Pangaean localities. To address this issue, the carbon isotope composition of organic matter (δ13Corg of lower Toarcian organic matter-rich cherts from Japan, deposited in the open Panthalassa Ocean, was analysed. The results show the presence of a major (>6 ‰) negative excursion in δ13Corg that, based on radiolarian biostratigraphy, is a correlative of the lower Toarcian negative CIE known from Pangaean epi- and pericontinental strata. A smaller negative excursion in δ13Corg (ca. 2 ‰) is recognized lower in the studied succession. This excursion may, within the current biostratigraphic resolution, represent the excursion recorded in European epicontinental successions close to the Pliensbachian/Toarcian boundary. These results from the open ocean realm suggest, in conjunction with other previously published datasets, that these Early Jurassic carbon cycle perturbations affected the active global reservoirs of the exchangeable carbon cycle (deep marine, shallow marine, atmospheric).

EVIDENCE FROM CARBONATE PLATFORMS BEARING ON CLIMATE, SALINITY, DASYCLADALEAN DIVERSITY, AND MARINE ANOXIC EVENTS DURING THE LATE JURASSIC-EARLY CRETACEOUS GREENHOUSE

Carbonate platforms are sensitive recorders of environmental change through geologic time. Climatically induced changes in sea level or changes in subsidence are expressed through migration of sediment belts that are recorded in the accumulating sedimentary succession. Changes in nutrient levels are recorded in shifts in the biotic community and, if eutrophic levels are reached, platforms may drown due to breakdown in carbonate production (Follmi et al., 1994). Our group has been focusing on the ∼60 Ma Late Jurassic–Early Cretaceous period, which was originally considered to have had uniform greenhouse conditions (Fischer, 1982). Available climate proxy data, however, such as oxygen isotope data (δ18O) from deep sea cores, the paleontologic data (e.g., Frakes et al., 1992), and the 87Sr/86Sr and carbon isotope data of Jurassic and Cretaceous carbonates (Jenkyns and Wilson, 1999), indicate that there were major cooling and warming events that likely affected global ice volume, sea level, and the accumulating sedimentary and microfossil record of carbonate platforms. Numerous carbonate platforms were developed within the tropical–subtropical belt of the circumequatorial Late Jurassic–Early Cretaceous Tethys Ocean. Many of these platforms have comparable shapes, sizes, facies, subsidence rates, and geologic structure to the present-day Bahamas Banks, a commonly used modern-day analogue (e.g., Bosellini, 2002, and references therein). In addition, the outcropping Tethyan platforms provide important clues to the poorly known subsurface Mesozoic carbonates that underpin the modern Bahamas platform. The Tethyan platforms were characterized by high rates of sedimentation (from 100 m/myr; D'Argenio et al., 1999) and exhibit meter-scale shallowing-upward cycles or parasequences generated during high frequency, small sea-level fluctuations within the Milankovitch band (Strasser, 1991; Schulz and Schafer-Neth, 1997; Lehmann et al., 1999; Strasser et al., 1999; Immenhauser et al., 2004; Husinec and …

Innovative Alternative to Full-Field Compositional Modeling-Case Study of the North Kuwait Jurassic Complex

Summary The North Kuwait Jurassic complex (NKJC) consists of six fields with four potential reservoirs in a naturally fractured Jurassic carbonate formation. Current understanding of the complex has led to 12 subdivisions of the area and potentially 48 separate compartments (segments) in the complex. These subdivisions are defined by fault boundaries supported by a combination of variations in fluid composition, initial pressures, and free-water levels estimated from capillary pressure and log saturation data. Multiscenario production forecasts based on integrated full-field modeling were needed in the process of building a field-development plan (FDP) for the NKJC. An integrated asset modeling (IAM) framework is adopted in which multiple separate reservoir models are coupled through global constraints to meet gas-delivery targets. We present the results of a feasibility study to select the optimal modeling strategy for the complex. We discuss available options to simulate multiple reservoirs, which, although isolated based on the current understanding of compartmentalization, need to meet global production targets. We show that using multiple-reservoirs integration through a controller to couple the separate reservoir models through global production targets/limits provides an optimal simulation framework for the NKJC—the flexibility and computational efficiency of multiple segment models and the comprehensiveness of full-field simulation modeling. The solution uses a black-oil delumping technique to obtain compositional wellstreams while running black-oil simulation models. The feasibility study demonstrated that black-oil delumping enables the composition and component molar rates of a producing well from a black-oil reservoir simulation to be reconstituted accurately. Comparison of results obtained from a compositional model to those obtained from a black-oil model using black-oil delumping shows excellent agreement. The adopted simulation framework provided us with all the benefits of a compositional full-field simulation model while adding two advantages (i.e., computational speed and flexibility). Full computational advantages of black-oil modeling are obtained while preserving the reservoir details required for an accurate prediction of well and reservoir behavior.

An overview of the diagenesis of the Upper Jurassic carbonates of Jubaila and Hanifa formations, central Saudi Arabia

The Jubaila Formation (Upper Jurassic) in central Saudi Arabia has been divided into lower, middle, and upper parts purely on lithologic grounds. Each part consists of a major lower unit of lime mudstone and a minor upper unit of grainstone. This persistent change in the limestone facies is interpreted as a reflection of repeated shoaling up in the depositional shelf environment. It is a normal marine carbonate sequence that varies in thickness from 85 to 126 m. In the Hanifa Formation, the lowermost brown ledges in the section comprise a series of coarsening upward sequences which generally terminate in a fossiliferous/peloidal packstone and grainstone and subordinately lime mudstone facies. The middle slope member is yellow, blocky weathered shale and marl. Above this slope member are several thick beds of brown-coated fossiliferous wackestone, packstone, and grainstone with the association of lime mudstone in certain levels. These are fairly resistant ledges due to the occurrence of stromatoporoids. Dedolomitization occurs in the Jubaila Formation in various textural forms which include composite calcite rhombohedra, zonal dedolomitization, regeneration of predolomitization fabric of the limestone, and coarsely crystalline calcite mosaics with or without ferric oxide rhombic zones. Rhombohedral pores commonly occur in intimate association with dolomite, possibly resulting from the leaching of calcitized dolomite rhombohedra. The regional dedolomitization was most likely brought about by calcium sulfate solutions reacting with dolomites. The source of sulfate solutions is the dissolved anhydrite deposits of the Arab–Hith Formations, sometime before their erosion, and it takes place at or near an exposed surface. The Hanifa Formation shows various diagenetic features. These include dolomitization, dedolomitization, micritization, cementation, and recrystallization. Most of the examined samples of the Hanifa carbonates are dolomitized and subsequently dedolomitized as evidenced by the presence of iron-coated dolomite rhombs partially or completely calcitized. Dolomite also occurs in the lime mudstone, wackestone, packstone, and grainstone facies, while leaching of wackestone and packstone and dedolomitization of dolomite and dolomitic limestone followed by recrystallization are common processes.

New Early Jurassic brachiopods from the Western Tethys (Eastern Subbetic, Spain) and their systematic and paleobiogeographic affinities

Two new brachiopod genera ( Alebusirhynchia and Praesphaeroidothyris), five new species ( Alebusirhynchia jorali, Salgirella? goicoecheai, Praesphaeroidothyris cisnerosi, Lychnothyris lancisi, and Cincta peiroi) and a new combination ( Securina plicata) have been recorded and established after the internal and external morphological study of populations attributed to Lower Jurassic carbonate successions of the South-Iberian Paleomargin (SE Spain). The analysis made enables us to detail the diagnostic characters of little-known genera such as Lychnothyris or Securina and to clarify the systematics of the group from new supraspecific definitions, establishing a new, strongly multicostate rhynchonellid genus ( Alebusirhynchia) and a new smooth terebratulid genus ( Praesphaeroidothyris). The palaeobiogeographic signature of these taxa has been analysed, revealing remarkable Mediterranean affinities. Thus, the previously restricted geographical distribution of these faunas can be extended to the westernmost Tethys areas, where the maximum degree of endemism is established for the brachiopod groups in the Subbetic domain around the Sinemurian-Pliensbachian transition.

New data on the Upper Jurassic-Lower Cretaceous limestones from Bihor Mountains: case study of Gârda Seacă-Hodobana region, Romania

This study refers to some problems regarding the lithological succession, facies and carbonate microfacies, and to biostratigraphic markers of the Upper Jurassic-Lower Cretaceous carbonates from Bihor Mountains. Three types of facies (external marginal facies, subtidal and peritidal facies) were separated for the Upper Jurassic, and two (coastal-peritidal and open shelf facies) for the Lower Cretaceous carbonates. The micropaleontological assemblages identified contain species that allow the separation of the two formations of different age: Labyrinthina mirabilis, Kurnubia palastiniensis, Neokilianina rahonensis, Clypeina sulcata (Kimmeridgian-Lower Tithonian) and Parakoskinolina? jourdanensis, Montseciella arabica, Palorbitolina lenticularis, Falsolikanella danilovae (Barremian – Lower Aptian). Field observations and data obtained from studies on thin sections and polished slabs indicate that a large part of the Triassic (Ladinian-Lower Carnian) and Lower Cretaceous limestones from Gârda Seacă-Hodobana region, delimited on the geological maps 1:50 000 scale, sheets 56b (Poiana Horea) and 56d (Avram Iancu) (Bleahu et al., 1980; Dimitrescu et al., 1977), belong in fact to the Upper Jurassic carbonate succession.

Activités volcaniques sous-marines à la limite Jurassique-Crétacé dans le Rif externe (Maroc). Âge et relation avec la sédimentation et la paléogéographie du sillon rifain externe

Following the recent stratigraphic works carried out on the Jurassic-Cretaceous boundary in the external Rif chain (Mesorif area and Bou Haddoud nappe), numerous submarine volcanism traces have been discovered in Upper Oxfordian to Upper Berriasian deposits. These traces display various aspects: volcaniclastic complexes incorporated within Upper Berriasian marls; volcanic lavas and basalt clasts included in the breccias with clay matrix of Upper Oxfordian to Lower Berriasian age, or in brecciated Lower Tothonian calcareous beds of the Early Tithonian. These submarine volcanic activities took place in a carbonate platform environment during the Kimmeridgian to Early Tithonian interval or in a pelagic basin from Late Tithonian onwards. They caused an intense brecciation of Upper Jurassic carbonate formations and a general dismantling of marly calcareous alternations of Upper Tithonian-Lower Berriasian. Therefore, the Upper Tithonian-Lower Berriasian deposits are marked by frequent stratigraphic gaps in many outcrops of Mesorif, Prerif areas and in the Bou Haddoud nappe.

Multistage dolomitization and distribution of dolomitized bodies in Early Jurassic carbonate platforms (Southern Alps, Italy)

AbstractThe Early Jurassic dolomitized carbonates are a hydrocarbon exploration target in Northern Italy. Of these carbonates, the Liassic Albenza Formation platform and the overlying Sedrina Formation shelf were studied to define a pervasive dolomitization model and to shed light on dolomite distribution in the sub‐surface. Field work, as well as analyses of well cores, stable isotopes, trace elements and fluid inclusions, was carried out on the outcropping thrust belt and sub‐surface deformed foreland of the Southern Alps. Petrographic analyses showed a first, pervasive, replacement dolomitization phase (D1) followed by volumetrically less important dolomite cement precipitation phases (D2, D3 and D4). The δ18O values fall between −8·2‰ and 0·1‰ Vienna‐Pee Dee Belemnite with the more depleted samples belonging to dolomite cement‐rich dolostones; the δ13C ranges from 2·6‰ to 3·7‰ Vienna‐Pee Dee Belemnite. Analysis of trace elements showed different Fe and Mn contents in the sub‐surface and outcropping dolostones, and a higher Fe in the younger dolomite cements. An increase in the precipitation temperature (up to 130 °C from fluid inclusion data) and a decrease in diagenetic fluid salinity (from sea water to brackish) are observed from the first pervasive replacement dolomite to the dolomite cement phases. Field observations indicate that, in the Albenza Formation, dolomitization was limited to palaeohighs or faulted platform margins in the Early Jurassic carbonates. The pervasive replacement phase is interpreted based on a ‘compaction model’; the formation fluids expelled from compacting basinal carbonates could have funnelled along faults into permeable palaeohighs. The high homogenization temperature of the dolomite cements and decreased salinities indicate precipitation at great depth with an influx of meteoric water. These data, along with the thermal history, suggest that the dolomite cements precipitated according to the ‘tectonic squeegee’ dolomitization model. The dolomite precipitation temperature was set against the thermal history of the carbonate platform to interpret the timing of dolomite precipitation. The dolomite precipitation temperatures (90 to 100 °C) were reached in the studied formations first in the thrust fold belt (Early Tertiary, 60 Ma), and then in the foreland succession during the Late Tertiary (10 Ma). This observation suggests that the dolomite precipitation fronts moved southwards over time, recording a ‘diagenetic wave’ linked to the migration of the orogenic system. Observations suggest that the porosity increased during the first phase of replacement dolomitization while the dolomite cementation phases partially occluded the pores. The distribution of porous dolomitized bodies is therefore linked to the ‘compaction dolomitization’ model.

Integration of sedimentology and ground‐penetrating radar for high‐resolution imaging of a carbonate platform

AbstractGround‐penetrating radar has not been applied widely to the recognition of ancient carbonate platform geometries. This article reports the results of an integrated study performed on an Upper Jurassic outcrop from the south‐east Paris basin, where coral bioherms laterally change into prograding depositional sequences. Ground‐penetrating radar profiles illustrate the different bedding planes and major erosional unconformities visible at outcrop. A ground‐penetrating radar profile conducted at the base of the cliff displays a palaeotopographic surface on which the outcropping bioherms settled. The excellent penetration depths of the ground‐penetrating radar (20 m with a monostatic 200 MHz antenna) images the carbonate platform geometries, ranging between outcrop workscale (a few metres) and seismic scale (several hundreds of metres). This study supports recent evidence of icehouse conditions and induced sea‐level fluctuations controlling the Upper Jurassic carbonate production.

Sedimentary bodies, forms and occurrences in the Tudorevo and Mirovo glacial deposits of northern Velebit (Croatia)

A small glacier of cirque-valley type existed during the Late Pleistocene Wurm Glacial in the Tudorevo and Mirovo karst valleys of Northern Velebit. Its source was in Tudorevo, and it moved through Dundovic Mirovo and Bilensko Mirovo to Baricevic Dolac, shaping U-valleys around 4 km in length. After melting, glacial deposits remained, composed of chaotic and unsorted till, composed of carbonate sand, debris and sub-rounded clasts, cobbles and blocks of predominantly Middle Jurassic and subordinately Lower Jurassic carbonate rocks. In Dundovic Mirovo and Bilensko Mirovo, where the largest masses of glacial deposits occur, terminal and recessional moraines can be found over the ground moraine, as well as some other features, mostly drumlins (drumlin field), eskers, erratic blocks, kettle holes and striations. Some erratics have been transported for more than 4 km from their primary outcrops, e.g. clasts of Lower Jurassic Toarcian Spotty limestone. A terminal moraine was deposited between Bilensko Mirovo and Baricevic Dolac, perpendicular to the glaciated U-valley and it forms the Bilo hill, the northern and southern foothills of which are composed partly of glaciofluvial deposits. Between Tudorevo and Mirovo, a recessional moraine occurs above the ground moraine. The glacier was subject to polyphase melting and freezing, and the youngest freezing events may be related to cirques in Tudorevo. During melting events, glacier lakes are supposed to have existed, initially in the Baricevic Dolac, later in Mirovo area and finally in Tudorevo. These discharged into the karst underground by percolation through till and by erosion to the karstified underlying Middle Jurassic carbonates.

Upper Jurassic (Lower Kimmeridgian-Olvido) carbonate strata from the La Popa Basin diapirs, NE Mexico

Carbonate strata that occur as blocks encased in gypsum of three different salt diapirs in La Popa Basin contain a single invertebrate fauna indicating a Late Jurassic (Kimmeridgian) age for all sampled fossiliferous blocks. The most biostratigraphically significant species, present in all diapirs, is the gryphaeid oyster Nanogyra virgula. Other species identified in the blocks include the bivalves Astarte gracilicostula, Cercomya durangensis, Myophorella sp., Grammatodon sp. and the nautiloid Cymatoceras? sp. Geologic relations of the entire assemblage of carbonate blocks, including nodular gypsum interbedded with the fossiliferous beds, salt casts in micrite of the blocks, random structural position of blocks within the diapiric evaporite, absence of blocks younger than Early Kimmeridgian in the diapirs, and regional stratigraphic relations, suggest that the carbonate strata were originally interbedded with, or directly overlie, evaporite strata. This inference is corroborated by the presence of Nanogyra virgula (previously reported as Exogyra cf. E. susplicifera) in carbonate strata directly overlying massive gypsum at Potrero Minas Viejas, adjacent to La Popa Basin. Although previously correlated with the Zuloaga Limestone, the fauna in the carbonate blocks of the diapir instead indicate a correlation with carbonate and evaporite strata of the Olvido Formation, and by extension, with siliciclastic strata of La Caja and La Casita formations. The new biostratigraphic data thus indicate that evaporite deposition persisted into the Kimmeridgian and, because the fossiliferous beds are inferred to have overlain a very thick section of halite (>2 km), it is suggested that evaporite deposition was continuous with only brief interludes of carbonate deposition from Callovian to early Kimmeridgian time in La Popa Basin.

3D seismic survey for geothermal exploration at Unterhaching, Munich, Germany

An increasing number of low-enthalpy geothermal power plants are being implemented in the Munich area with holes drilled into the Malm Formation (Upper Jurassic carbonates) to depths greater than 3000 m, where temperatures in excess of 100°C are encountered. Optimal development of geothermal productivity requires exploration of the geological structure, as well as information on the karstification of the Malm. Seismic surveys, including reprocessed older 2D lines from hydrocarbon exploration, new 2D lines filling gaps, and 3D surveys, are increasingly used for geothermal exploration. A geothermal power plant at Unterhaching, near Munich, has been producing heat and electrical power since 2009. A high resolution 3D seismic volume has been acquired over an area of 27 km2 for hydrological modelling purposes at the Gt2 reinjection well, located within the southern suburbs of Munich. First results show preferential hydraulic pathways along reactivated fault zones within the Malm reservoir which were partly active until the Aquitanian (Early Miocene). Additionally, a great number of sinkholes have been identified at the top Malm, located along fault zones which constitute new possible targets for geothermal exploration.

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.