Dolomitic Horizons Research Articles

Clastic-volcaniclastic Rocks in Papaghni Sub-basin: Implications for Syn-sedimentary Felsic Volcanism during the Deposition of Paleoproterozoic Vempalle Carbonate Sequence, Cuddapah Basin, India

Abstract Based on detailed field, petrographic, mineral chemistry and geochemical studies, succinct details on the clastic - volcaniclastic sequence recorded within the dolomite horizon in Paleoproterozoic Vempalle Formation, Cuddapah basin, India are presented. The clastic rocks represented by rudaceous-arenaceous sequence and the volcaniclastic rock represented by an intervening tuffaceous horizon are noticed in the Vempalle dolomite near Tummalapalle area, in southwestern part of the Cuddapah basin. The conglomerate is matrix supported with dominantly chert clasts and the arenaceous unit is represented by fine grained quartz arenite. The tuffaceous unit is finely laminated exhibiting typical; clast matrix texture characteristic of fine grained pyroclastic rocks. Zircon, apatite, rutile and monazite are the accessory phases in the tuff. Geochemically, the felsic tuff is rhyolitic, peraluminous and shows relative enrichment of LREE. The chemical composition of the feldspar falls close to the K-feldspar end member in the Or-Ab-An plot. The clastic - volcaniclastic sequence in the Vempalle carbonate sequence indicate a localised break in carbonate precipitation accompanied by syn-sedimentary felsic volcanism during the evolution of Paleoproterozoic segment in the Cuddapah basin, India.

C, O Isotopic Composition of the Neoproterozoic Pre-, Syn-, and Post-Glacial Carbonates of the Longdor Uplift and the Western Slope of the Aldan Shield (South of the Siberian Platform)

Here we present new data on the C and O isotopic composition in the Neoproterozoic carbonates of the Ballaganakh and Dal’naya Taiga horizons of the southeastern margin of the Patom paleobasin. In the upper part of the Ballaganakh horizon, a negative δ13С anomaly was established. It is comparable in amplitude and stratigraphic position to the Trezona anomaly, which preceded the Marinoan glaciation. The carbonate layers in the glacial deposits of the Nichatka Fm. of the Dal’naya Taiga horizon have moderately positive δ13С values, and the dolomite horizon at the base of the postglacial sequence is characterized by moderately negative values δ13С which is typical for the cap-carbonate sequence associated with the end of the Marinoan glaciation. In the lower part of the postglacial sequence (the Barakun Fm.), there is a gradual increase in the proportion of calcite in the carbonate component of the rocks and a shift in the isotope composition corresponding to the difference in the fractionation coefficients of stable C and O isotopes for dolomite and calcite. This phenomenon, which is also recorded in the postglacial sequences of Namibia and Canada, indicates that the composition of carbonate minerals was determined by a global change in water chemistry, and not by postsedimentary substitution. The overlying carbonate deposits of the Dal’naya Taiga horizon in the epiplatform part of the Patom paleobasin (Sen Fm.) are depleted in 13С compared to their shelf analogs in the Ura uplift. It has been suggested that carbonate accumulation in the epiplatform and shelf facies of the Patom paleobasin occurred asynchronously. The reconstructed trend of variations in the C isotope composition for the Dal’naya Taiga horizon is characterized by positive and low-amplitude negative δ13С anomalies similar to those in the Doushanto Fm. of China.

Stratigraphy and depositional environment of a mixed siliciclastic-carbonate platform and slope succession of the Paleogene Nisai Group, Pakistan

We document the stratigraphy, lithofacies and depositional environment of the lower Eocene-lower Oligocene Nisai Group, based on petrographic and micropaleontological analysis, and field observations.We rename the whole sedimentary succession as the Nisai Group and divide it into three formations, from bottom to top; Jabrai, Torkhezhe Tsah and Nisai formations. The Jabrai Formation comprises a thick succession of olive gray mudstone and shales with subordinate sandstone, limestone and quartzose conglomerate, interpreted as deltaic and littoral deposits. The Torkhezhe Tsah Formation comprises calciturbidites in its lower part and periplatform oozes and basinal shales in upper part. The shallow-marine carbonate material of calciturbidites possibly derived from a lost platform which was a narrow delta-top-platform developed over deltaic sediments of the Jabrai Formation. The uppermost unit, the Nisai Formation, comprises thick bedded limestone with subordinate shale and nodular reefoid limestone, and shows a shallowing-up succession overlying the slope and basinal deposits of the Torkhezhe Tsah Formation. The presence of Cr-rich spinels in sandstone of the Jabrai Formation, ultramafic-mafic and quartzite lithoclasts in calciturbidite beds as well as paleocurrent data, suggest that siliciclastic sediments of the Nisai Group derived from the Zhob Valley Ophiolites. Moreover, the presence of quartz, igneous and metamorphic extraclasts, and dolomitic horizons within calciturbidite beds, indicate deposition of these calciturbidites in response to lowering of sea level.We propose that emplacement of Zhob Valley Ophiolites served as a drainage divide and supplied deltaic sediments to the Jabrai Formation to its north and the Ghazij Group to the south.

Petrography, geochemistry and genesis of dolomites in the upper Cambrian Sanshanzi Formation of the western Ordos Basin, northern China

The Cambrian Sanshanzi Formation is an important dolomite horizon in the Ordos Basin of northern China, and is well exposed in the western part of that basin. This paper focuses on the petrography and geochemistry of dolomites in this formation with an aim to provide a better understanding of their genesis. Reflux seepage, burial, and hydrothermal dolomites are identified in the Sa nshanzi Formation. Reflux seepage dolomites include dolomitic grainstone and powder crystal dolomite, and have high Sr and low Mn contents, a high CaO/MgO ratio and low δ13 C values, and the reflux seepage dolomitization was formed by high-salinity and Mg2+-enriched fluid penetrates downward into the limestone. Burial dolomites are the main type of dolomite distributed through the Sanshanzi Formation. Burial dolomites are composed of fine- to coarse-crystal dolomite and have low Sr and high Mn contents, a low CaO/MgO ratio, and high δ13C values. The features of high salinity, strong reducibility and negative δ18O values indicate the burial dolomites formed under high-temperature and high-pressure conditions, the burial dolomitization was due to the limestone deeply buried during the later diagenetic stage. Hydrothermal dolomites are mainly characterized by saddle dolomite features, a positive δEu anomaly, and low δ18O values, and the hydrothermal dolomitization occurred during Mg2+-rich hydrothermal fluid invaded. The reflux seepage and hydrothermal dolomite developed in local areas, with the hydrothermal dolomite affected by the distribution of faults.

Evaluation of Reservoir Fluids Mobility, Mauddud Reservoir (Albian), Khabbaz Oil Field, Kirkuk Area, NE Iraq

The Albian Mauddud reservoir of the Khabbaz Oil Field is consisting of 170 m alternating shelf carbonates and pervasive dolomite horizons of coarse to fine crystalline mosaic. Core analysis and log measurements reveal the occurrence of three electrofacies units (A, B, and C) with variable petrophysical properties. Unit A with good reservoir quality shows average porosity of 18.8 % and average permeability of 27.5 md. The other two units (B and C) are less attractive and have an average porosity of 9.6 % and 9.2 % consequently. Pore size ranges between macro to meso types and related mainly to vugs, fractures and intercrystalline porosity, especially in the dolomite units. The reservoir fluids saturation, bulk volume, and mobility are evaluated using resistivity logs measurements and porosity logs (Neutron-Density porosities) in addition to other reservoir laboratory data. Calculations and cross data plotting of the related petrophysical parameters were applied to the three units of the Mauddud reservoir in seven wells of the field. It shows an overall good reservoir fluids mobility. Results indicate that the formation water of Khabbaz Oil Feld is a non-movable type especially for the crestal wells which make most of these wells produce water-free hydrocarbon. Variability within well’s hydrocarbon mobility is noticed and related to units and subunits lithology and porosity variation. Other variations seem to be related mainly to permeability, pores geometry and variability of water saturation in addition to the location of well with respect to oil pool within the field structure.

Vendian Hydrogeochemistry of the Siberian Platform

Abstract —The Vendian deposits of the Siberian Platform are represented by four regional horizons (from bottom to top): Nepa, Vilyuchan, Tira, and Danilovka. The TDS of brines in the Nepa and Vilyuchan horizons varies from 170.3 to 470.1 g/L, and the TDS in the overlying Tira and Danilovka horizons, from 84.6 to 583.1 g/L. Brines of Cl Na, Cl Na–Ca, Cl Ca–Na, Cl Ca, and Cl Ca–Mg types have been identified. The pH values of the brines vary from 3.0 to 7.0; acidic and weakly acidic brines (pH = 3.0–5.5) prevail. The redox potential Eh varies from –440 mV (highly reducing conditions) to +130 mV (oxidizing conditions), averaging –176 mV. Areal regional hydrogeochemical zoning is controlled by the outer (on the periphery of the Siberian Platform, where infiltration of meteoric waters took place) and inner feeding areas. The outer feeding areas bear infiltrogenic brines of different ages (group I), mostly of Cl Na composition. They are characterized by TDS ≤ 280 g/L, rNa/rCl ≤ 1.55, Cl/Br ≤ 2512, and average Ca/Cl = 0.3. The average values of the integrated brine metamorphization index S (according to S.L. Shvartsev) range from 50 to 200. In the intraplatform feeding areas, there are several phases of brines that underwent gravitational sinking into the Vendian horizons in the periods when Cambrian salt-bearing basins existed on the platform. These brines were metamorphosed during filtration along the fracture and fault zones. The intrusion of traps in the late Permian–early Triassic was accompanied by hydrothermal activity. The waters of the enclosing Cambrian dolomite horizons were heated to 800–1000 °C; their subsequent interaction with carbonate rocks and evaporites (in particular, chlorides) resulted in extrastrong saturated brines. These brines were also subjected to gravitational sinking into the Vendian reservoirs, which was the main cause of the inverse vertical hydrogeochemical zoning in the hydrogeologic subsalt formation. The brine metamorphism processes and temperature changes in the rock horizons during the intrusion of traps and their subsequent cooling led to a significant alteration of the enclosing rocks under interaction with brines and to salinization of the reservoirs, an increase in the total salinity of the brines, and catagenetic changes in their composition. The brines closest to the maximum saturation with halite were found in reservoir zones with the highest salinization and near faults and boundaries of intrusive bodies. During the pre-Vilyuchan, pre-Nepa, pre-Tira, and pre-Danilovka sedimentation breaks, infiltrogenic waters penetrated into the Vendian sediments. Geochemically, the sedimentogenic, infiltrogenic, and metamorphic strong brines (group II) have a predominantly Cl Ca or Cl Ca–Mg composition with TDS > 350 g/L. They are characterized by low rNa/rCl ratios (on average, 0.3), Cl/Br ≤ 100, high Ca/Cl values (on average, 0.4), and S > 250.

Studium petrologiczno-mineralogiczno-geochemiczne skał w profilu dolomitu głównego w otworze Mo-1

Apetrological, mineralogical and geochemical study of the Main Dolomite in the Mo-1 borehole. A b s t r a c t. Rocks from the Mo-1 well from the Barnowko–Mostno–Buszewo (BMB) oil and gas field have been studied aiming at constructive results for application in exploration for oil and gas. The study focuses on carbonate deposits from the Main Dolomite horizon, commonly diagenetically altered, where the pore space has been filled by dolomite and anhydrite. Standard petrological analysis was conducted, wide fluid inclusion analyses performed and geochemical character of bitumen determined. Based on microscopic fluorescence studies, three types of fluid inclusions have been distinguished in the vertical column of the well: one-phase non-fluorescent (methane), two-phase non-fluorescent (brine), and two-phase fluorescent inclusions (oil). These three types, further studied microthermometrically, display characteristics of fluid fillings present in the basin and closed as the inclusions in minerals.

Woodcutters 55 Years Later: A New Look at an Old Discovery

Abstract The Woodcutters Zn-Pb-Ag deposit in the Rum Jungle district of the Pine Creek orogen in northern Australia was discovered in 1964 and produced 4.6 Mt of ore grading 12.3% Zn, 5.6% Pb, and 83 g/t Ag between 1985 and 1999. Woodcutters, together with several other polymetallic, uranium, and phosphate deposits, is within a Paleoproterozoic sequence of fluviatile and shallow marine sediments deposited in a deepening basin between ~2100 and 2025 Ma around the margins of an Archean granitic and gneissic dome. These sediments were overlain by turbidites and volcaniclastic rocks until the basin was inverted and the sediments and mineral deposits were deformed and metamorphosed at 1860 Ma. Whereas the polymetallic and uranium bodies at Rum Jungle are considered to be syngenetic or syndiagenetic, sulfides in the Woodcutters orebody replace dolomitic horizons in an otherwise carbonaceous unit. This suggests that Woodcutters is similar to Mississippi Valley-type mineralization and rules out affinities with younger sedimentary exhalative-style deposits elsewhere in the Pine Creek orogen. A model is proposed whereby metals were eroded from Archean basement rocks into Paleoproterozoic sandstone aquifers following the Great Oxidation Event, which also liberated sulfur by oxidation of pyrite. Evaporative conditions, as suggested by the widespread occurrence of dolomite and magnesite, may have increased the chloride content of seawater and enhanced its capacity to transport metals. Subsequently, deeply circulating seawater leached metals from the aquifers and ascended up a deep, basin-penetrating fault until it intersected carbonaceous sediments. In this environment, Zn and Pb sulfides were deposited under reducing conditions, while sulfur may have been provided by H2S from organic material. The Woodcutters and other deposits at Rum Jungle show how metals formerly locked up in Archean cratons were delivered by erosion under an oxygenated atmosphere to Paleoproterozoic shorelines, where they were further mobilized and concentrated by a variety of processes.

DOLOMITIZATION AND POROSITY EVALUATION OF HURMALA FORMATION, GARA ANTICLINE, DOHUK AREA, KURDISTAN REGION, IRAQ

The Khurmala Formation (Late Paleocene – Early Eocene) is well exposed at the southern limb of Gara mountain, Dohuk area, Kurdistan Region of north Iraq. The thickness of the Formation is 91 m and generally consists of buff to grey, medium to thick-bedded stromatolitic dolostone alternating with thin beds of buff to dark gray shale or marlstone. Shale interlayers increased in thickness upwards. The studied section has been divided into three units based on shale ratio. The focus of this study is to investigate the type and origin of the dolomite and its impact on the porosity using field observation, petrologic and petrographic investigations with SEM microscopy. The common dolomite types include (1) Unimodal, cryptocrystalline to fine crystalline dolomite of nonplanar mosaic. This type is associated with the relics of the stromatolites and commonly recognized in the upper part. (2) Unimodal fine to medium crystalline, planar-e to Planar-s dolomite mosaic, which is associated with dolomite horizons showing ghosts or relics of benthic foraminifera, algae, ostracods and other bioclasts. The other types are less common and include (3) Porphorotopic dolomite and (4) Bimodal medium to fine, planer-e to planar-s dolomite mosaic. The latter is associated with dolostone horizons display important moldic and vug porosity. Molds, vugs and intercrystalline porosity are noticed in all types but with variable percentages. Fenestral porosity is common in the stromatolitic dolomite of type one. The vertical distribution of the different type of dolomite and its porosities indicate cyclic alternation of the studied Khurmala section which implies effective eustatic control on deposition and dolomite diagenesis. The dolomitization resulted from a possible combined effect of seawater refluxion and sabkha evaporation mechanisms.

Neoproterozoic marine dolomite hardgrounds and their relationship to cap dolomites

Neoproterozoic cap dolomites are unusual and distinctive marker units that occur after large Cryogenian glaciations. Several thin dolomite units that have features resembling cap dolomites occur within the Cryogenian-Ediacaran succession of the Adelaide Geosyncline. While each of these dolomite units has distinctive features, they also share many similarities with cap dolomites including: stratigraphic thickness ranging from several centimetres to several metres; impure dolomicrite composition; graded beds with a sharp top that become more dolomite-rich upwards; and evidence of volume expansion in the form of sheet cracks, tepees or other soft- sediment deformation structures. The carbon isotopic compositions for these thin dolomite horizons have values ranging from ∼−5 to +2 per mil VPDB, which overlap those commonly found in both Sturtian and Marinoan cap dolomites. The generally negative carbon isotope compositions of these dolomites and cap dolomites is most easily explained by mixing of marine and authigenic carbonate.Sedimentological evidence indicates the graded beds within cap dolomites and other dolomite units are a form of submarine hardground or firmground, where dolomicrite is precipitated authigenically at or near the seafloor during periods of little or no sedimentation. Dolomite replacement of clay and other silicates is an important process in the formation of these dolomicrite firmgrounds. Volume expansion structures are a product of this near-surface dolomicrite precipitation, brought about by the high degrees of dolomite supersaturation with a consequent large force of crystallization and unlimited (marine) carbonate source for the precipitating dolomite.We suggest that this deepwater authigenic dolomicrite precipitation is a result of the dominantly anoxic nature of Cryogenian- early Ediacaran oceans and caused by alkalinity-producing anoxic bacterial reactions. When sedimentation rates slow sufficiently (e.g. during transgressions), thin dolomicrite horizons are produced. In this hypothesis, cap dolomites may largely be the result of sediment starvation, combined with a possible increase in carbonate saturation during and following the post-glacial transgressions associated with the Sturtian and Marinoan glacial events. This scenario explains most of the sedimentological features of these unusual carbonates. The widespread development of such authigenic carbonate would have profound implications for the interpretation of Precambrian marine carbon isotopic compositions.

Fluid compartmentalization and dolomitization in the Cambrian and Ordovician successions of the Huron Domain, Michigan Basin

Abstract Strata-bound dolomite occurs in deep-seated Ordovician and Cambrian sediments within the Huron Domain of southern Ontario, Canada. Core samples were analyzed for petrographic, stable and Sr isotopic composition, and fluid inclusion microthermometry to characterize dolomitization and evaluate diagenetic fluid composition. The samples represented a range of host rocks from dolomitized limestones, dolostones, sandy dolostones and sandstones within Ordovician Black River Group and underlying Cambrian formation. The petrographic and geochemical attributes have provided a basis to gain insight on the source fluids that modified these rocks, as well as, the possible timing of formation. Evidence indicates that the formations were subjected to high temperatures (average 115 °C) that cannot be explained by burial history alone. This suggests the occurrence and migration of hydrothermal fluids within the low permeability dolomite horizons, possibly during Paleozoic orogenesis. Dolomite and calcite fracture infill isotopic and fluid inclusion data point to two possibly individual diagenetic fluid systems; i) an earlier Cambrian system that is characterized by a pronounced negative shifts in oxygen and carbon isotopic composition (δ18O average −9.0‰ for dolomite and −13.9‰ for calcite, respectively; and δ13C −3.06‰ for dolomite and −4.82‰ for calcite, respectively), a more radiogenic (Sr87/Sr86 ratios range from 0.70977 to 0.71100), warm (Th values range from 84 to 156 °C for dolomite; average 113.6 °C and 87–141 °C for calcite; average 101.5 °C) and saline signature (salinity range from 23.2 to 27.2 wt% NaCl eq for dolomite; average 24.3 for dolomite and 23.6 wt% NaCl eq for calcite); and ii) a later Ordovician system that is characterized by less negative shifts in both oxygen and carbon isotopes (δ18O average −8.7 ‰for dolomite and −7.5‰ for calcite; and δ13C average + 0.37‰ for dolomite and −0.36‰ for calcite, respectively), hypersaline (salinity range from 22.4 to 30.1 wt% NaCl eq.; average 27.0 for dolomite and 27.5 to 29.7 for calcite; average 29.2), comparable homogenization temperature (Th ranges from 85 to 132 °C for dolomite; average 109.6 °C and 66–153 °C for calcite; average 107.2 °C) and a less radiogenic (Sr87/Sr86 ratios range from 0.70818 to 0.71 000) fluid system. The observation of highly discrete, strata-bound dolomites combined with only trace quantities of saddle dolomite and its associated geochemical signature suggest that diagenesis, as a result of hydrothermal fluids, was neither pervasive in volume or extent within the north western Huron Domain.

Indicator Isotope–Geochemical Characteristics of Sulfides from the Satka Magnesite Ore Field (South Urals Province)

The stable enrichment of pyrite from magnesite ores in δ34S isotope (from 5.4 to 6.9‰) compared with pyrite from the host (sedimentary and igneous) rocks was established in the classical Satka sparry magnesite ore field. Concretionary segregations of fine-grained pyrite in dolomite are depleted in the heavy sulfur isotope (δ34S, from–9.1 to–5.8‰). Pyrite from dolerite is characterized by δ34S values (–1.1 and 1.7‰) close to the meteorite sulfur. The δ34S values in barite from the underlying dolomite horizon vary in the range of 32.3–41.4‰. The high degree of homogeneity of the sulfur isotope composition in pyrite from magnesite is a result of thermochemical sulfate reduction during the syngenetic crystallization of pyrite and magnesite from epigenetic brines, formed during dissolution of evaporite sulfate minerals at the stage of early catagenesis of the Riphean deposits.

Microfractures in bed-parallel veins (beef) as predictors of vertical macrofractures in shale: Vaca Muerta Formation, Agrio Fold-and-Thrust Belt, Argentina

Shales of the Upper Jurassic-Lower Cretaceous Vaca Muerta Formation are the main source rock for petroleum in the Neuquén Basin, Argentina and an important unconventional exploration target. Folded Vaca Muerta Formation is well exposed in the Agrio Fold-and-Thrust belt where an arid climate and rapid erosion reveal relatively unweathered shale strata accessible along creek beds at Arroyo Mulichinco and in 10+ m-tall cliffs at Puesto. Widespread within these organic-rich shales are several cm-thick, prominent bed-parallel veins (BPVs) of fibrous calcite (beef) that are cut by multiple sets of vertical calcite lined or filled fractures having apertures unaffected by near-surface stress release. Similar, and probably contemporaneous fractures are present within horizons of interbedded dolomitic rock. Evidence that vertical fractures in BPVs and dolomitic horizons continue into shale beds suggests that in-depth analysis of vertical fractures within BPVs and dolomitic horizons allows fracture set and orientation identification and size population measurements—primarily aperture distributions—that circumvent some of the limitations of shale outcrops. At Arroyo Mulichinco, four main fracture sets are present separable by orientation and crosscutting relations. An E-W set is oldest, followed by successively younger NE-SW, NW-SE, and N-S sets. At Puesto, the E-W and N-S sets are the most prominent and show opposite cross-cutting relationships (E-W set is youngest) indicating a possible episode of younger E-W fractures. The E-W set shows the highest micro-and macrofracture intensity at both localities. The intensity of N-S micro- and macrofractures is similar at both outcrops away from faults, but macrofracture intensity increases closer to faults. While macrofracture abundance is similar in BPVs and in shale, microfractures having apertures smaller than ∼0.1 mm are mostly absent in shale and dolomitic layers but are abundant cutting BPVs. Thus, microfractures are BPV-bounded and only fractures wider than ∼0.05 mm are tall enough to cut into shale. Nevertheless, using size distributions of microfractures in BPVs that are absent in shale accurately predicts the abundance of macrofractures in nearby shale, either because microfractures in organic shale have annealed, or because of only small differences in fracture strain for fractures of different sizes across different rocks types. Microfractures in readily sampled BPVs may be a practical way to diagnose or predict attributes of macrofractures in adjacent shale.

ABOUT STRATIFICATION FAMENNIAN DEPOSITS SOUTH TIMAN

First Izhma Suite was selected for the South Timan in 1930 N. N. Tikhonovich in the study of oil deposits of the upper Devonian. Part of this Suite, they had included a wide range of rocks exposed on the right Bank of the r. Izhma. Later D. C. Obruchev (1958) as part of the Suite rocks have been identified the remains of testaceous fish Bothriolepis jeremejevi Rohon and Bothriolepis sp. Detailed stratigraphic and paleontologic study of the incision retinue was made So T. I. Kouchnareva (1977), who established her Famennian age. The lower part of the Izhma suites N. N. Tihonovith she took to Ukhta the retinue of the Frasnian stage as gypsum-bearing strata. However, the name of the Suite – Izhma the upper part of the section was saved. Deposits Izhma suites in the new volume it was first compared with padonskimi layers of the Russian platform. Lithology in Izhma the retinue are two parts: the lower part is predominantly clay, and the upper carbonate. In the study of the East wing Ukhta brachyanticline So, T. G. Obukhovska, A. C. Kuzmin (1993) attributed to the Izhma retinue only her upper carbonate part. The separation of the Izhma suites on podsvity was proposed M. N. Moskalenko with co-authors (1999) on the basis of a more careful study of organic remains and features of the lithological composition of the rocks. P. A. Beznosov (2009) studied the fish fauna in the dolomite horizon Izhma suites, lower podsvit transferred to the rank of a Suite called sosnogorskaya. For the upper left them rank podsvity with the same name – Izhma. Thus, twice the changed composition of the Izhma Suite has not resulted in a change of its name. In this article it is proposed to resolve this discrepancy and the upper podsvity transferred to the rank suites by assigning her the name of the place of its location – poznanskaya.

PROGNOSTIC ACCUMULATION ZONES FOR OIL AND NATURAL GAS IN THE CRITERIA FOR THE DISTRIBUTION OF PETROPHYSICAL PARAMETERS IN THE MAIN DOLOMITE IN GORZOW-PNIEWY AREA / PROGNOSTYCZNE STREFY AKUMULACJI DLA ROPY NAFTOWEJ I GAZU ZIEMNEGO W KRYTERIACH ROZKŁADU PARAMETRÓW PETROFIZYCZNYCH DOLOMITU GŁÓWNEGO W OBSZARZE GORZÓW-PNIEWY

Abstract The carbonate reservoirs are anisotropic environments in terms of both the quantitative and qualitative evaluations of pore space. The oil-and-gas-bearing Main Dolomite horizon shows diversified lithology, facial development and thickness resulting in complicated, fluid capacity-fluid filtration system. This system influences both the evaluation and exploration perspectives in the Zechstein Main Dolomite (Ca2) from the Gorzów-Pniewy area. In order to clarify this problem and to determine the hydrocarbon accumulation perspectives, analysis of petrophysical parameters based upon the porosimetric measurements was carried on for the Main Dolomite in the study area, where oil and gas accumulations were discovered. The results of porosimetric measurements clearly indicate the heterogeneity of petrophysical parameters of the Main Dolomite referred to lithologically diversified palaeogeographic zones distinguished in the study area. Such analysis, including the hydrocarbon storage capacity of the Main Dolomite, enabled us to evaluate the possible hydrocarbon accumulation related to generation potential of this horizon.

Three global carbon isotope shifts in the Silurian of Podolia (Ukraine): stratigraphical implications

Podolia is a classical Silurian area in the southwestern part of the Ukraine. Shallow to open shelf rocks cropping out there are usually, except for a few dolomitic horizons, highly fossiliferous and therefore the biostratigraphy of the region has a long successful history. The Ukrainian meeting of the Subcommission on Silurian Stratigraphy of the International Union of Geological Sciences, held in 1983, became an important milestone. Tsegelnyuk et al. summarized different results and views in a field guide published for the meeting (The Silurian of Podolia. The Guide to Excursion. Naukova Dumka, Kiev, 1983) and introduced a practically new stratigraphical terminology, and in some parts of the section, also new age interpretation of the beds. For our paper carbon isotopes were studied in eight sections on the banks of the Dniester River and its tributaries, covering ca 80% of the Silurian succession (Pridoli excluded). Three positive δ 13 C excursions were identified. The first excursion in the Kitaigorod 30 section reaches its peak value of 4.0‰ in the very bottom of the Demshin Subformation. The second shift reaches the peak value of 4.3‰ in the of the Muksha Subformation. The third shift is the most prominent one identified in the Silurian of Podolia - δ 13 C values increase steadily through the Isakovtsy Subformation, reaching 6.9‰ in the top, and a slightly lower value of 6.6‰ occurs in the bottom of the Prigorodok Formation. In the middle Silurian of the World three major positive excursions have been identified: in the early and latest Wenlock and in the late Ludlow. The general character of the carbon isotope trend and stratigraphical positions of the excursions established in this paper demonstrate that in Podolia there occurs the same set of global shifts, which can be used for the improvement of regional and global correlation of Wenlock and Ludlow sections of that area. Some refinements are suggested, but some details need additional study.

A review of the alluvial diamond industry and the gravels of the North West Province, South Africa

The liberalization of the diamond marketing system led to increased diamond prices and this, coupled with the devaluation of the Rand against the US$ during late 2001 and 2002, resulted in a significant increase in onshore alluvial diamond production in South Africa. Inland, alluvial diamond production reached a recent peak of 1.23 million carats in 2003, providing employment for an estimated 25,000 people which then dropped to a disappointing 6,000 people in 2005. The discovery of the Lichtenburg diamond field in 1926 made a huge, short term impact on South Africa’s diamond production but this impact was short-lived due to the global decline in diamond sales that resulted from the Great Depression. In recent years, there have been important advances in the techniques used in the exploration for alluvial diamonds, enhancing the ability to locate new deposits. These include improvements in the remote sensing techniques of aerial photography and satellite image interpretation, as well as various airborne and ground geophysical methods. The extremely low concentrations of diamond in most alluvial deposits and the almost random distribution of the diamonds within the gravels, necessitate the taking of large samples in order to evaluate the commercial potential of a deposit. In the southern, Schweizer-Reneke diamond field, five ages of diamondiferous gravels occur, ranging from late Cretaceous to Pleistocene. Post-depositional modification has resulted in the formation of colluvial and eluvial “ Rooikoppie ” deposits, which were preferentially mined by the artisanal diggers of the previous century. The northern Lichtenburg, and eastern Ventersdorp diamond fields are largely underlain by dolomitic horizons that have undergone at least four phases of karst development. Even though several phases of gravel deposition have occurred, the distribution of the diamondiferous gravels is influenced by weathering features in the dolomite and these now occur as pothole-fill, sinuous, discontinuous, narrow, steep-sided alluvial channels or laterally more continuous, yet thin sheet-like gravels, assumed to be younger reworked material. The pot holes (or sinkholes) have formed excellent trap sites for diamonds and enormous quantities of diamonds have been recovered from some of these.

The stratification and cyclicity of the Dachstein Limestone in Lofer, Leogang and Steinernes Meer (Northern Calcareous Alps, Austria)

The Dachstein Limestone of the Northern Calcareous Alps contains massive limestone beds, capped by dolomitic horizons. Groups of beds form bundles or stratification cycles and spectral analysis gives evidence of at least 3 orders of cycle frequencies. The regularity in the repetition of the limestones and dolomites was first pointed out by Sander (1936) [Sander, B., 1936. Beitrage zur Kenntniss der Anlagerungsgefuege. Mineral. Petrogr. Mitt. 48, 27–139] and the repetition of definite groups by Schwarzacher (1948, 1954) [Schwarzacher, W., 1948. Ueber die sedimentaere Rhytmik des Dachstein Kalkes von Lofer. Verh. Geol. Bundesanst. 1947 (Heft 10–12), 176–188; Schwarzacher, W., 1954. Ueber die Grossrhytmik des Dachstein Kalkes von Lofer. Tschermaks Mineral. Petrogr. Mitt. 4, 44–54]. The facies associated with the change from limestone to dolomite was interpreted by Fischer (1964) [Fischer, A.G., 1964. The Lofer cyclothems of the alpine Triassic. In: Merriam, D.F. (Ed), Symposium on Cyclic Sedimentation. Kansas Geological Survey Bull. 169, 107–146] as an environmental change from lagoonal to peritidal and consequently as evidence of rising and falling sea level. The present paper is based on the study of over 500 aerial photographs from the Loferer and Leoganger Steinberge and also on a detailed section, measured by Paul Enos and E. Samankassou in the Steinernes Meer. It was found that the Dachstein Limestone from Lofer and Leogang comprised about 35 stratification cycles of approximately 20 m thickness. This may correspond to a similar number of cycles in the Steinernes Meer where the spectral analysis gave a cycle thickness of 27 m and 13.6 m. The interpretation of the bundle as being caused by the 100 ka eccentricity cycle is considered tenable.

Laffitteina from the Maastrichtian-Paleocene shallow marine carbonate successions of the Eastern Pontides (NE Turkey): biozonation and microfacies

Microfacies and benthic foraminiferal assemblages from the Maastrichtian-Paleocene shallow-marine carbonate successions of the Eastern Pontides (NE Turkey) are described and illustrated. Microfacies comprise fossiliferous intrasparites, packed biomicrites/biosparites, rudistid biosparites, gastropod-biosparites, miliolid biomicrites/biosparites, coralline algal biosparites and intrabiosparites. Dolomite horizons occur in the Maastrichtian-Danian and Lower-Upper Paleocene transitions. Microfacies and microfossil content indicate deposition in shallow marine to lagoonal environments all within same sedimentary basin. The Shallow Benthic Foraminiferal Zonal (SBZ) Scheme proposed for the Tethyan Paleogene is tested on Eastern Pontian material. It cannot be applied to the Paleocene in the Eastern Pontides due to the presence of the large number of endemic taxa, and variable stratigraphic ranges of some benthic foraminifera. Various species of Laffitteina have been used to produce a biozonation through the Maastrichtian-Thanetian. Laffitteina marsicana FARINACCI and Laffitteina oeztuerki INAN occur in the lower and upper parts of the Upper Maastrichtian, respectively. Laffitteina turcica INAN occurs in the uppermost Maastrichtian. Laffitteina bibensis MARIE appears in the Lower Paleocene (Danian), Laffitteina erki (SIREL) in the Upper Paleocene (Selandian) and they coexist from the Selandian to the lower part of the Thanetian.

Bacterial sulfate reduction and salinity: two controls on dolomite precipitation in Lagoa Vermelha and Brejo do Espinho (Brazil)

The hydrological system of Lagoa Vermelha, a dolomite-precipitating lagoon in Brazil, was investigated using hydrogen and oxygen stable isotopic composition of the water collected during an annual cycle (1996–1997). These data demonstrated that dolomite formed in May–June during high saline conditions. High salinity apparently provides the ions and saturation state necessary for dolomite precipitation. Ion concentrations in the lagoon water indicated an identical timing of dolomite precipitation and demonstrated that dolomite formed at decreased sulfate concentrations. In Brejo do Espinho, a neighbouring lagoon, the ion concentrations in the water column revealed that dolomite precipitates throughout the year, most likely due to its higher salinity than Lagoa Vermelha during the measured period. In Lagoa Vermelha, high δ 34 S of pore water sulfate and high sulfide concentrations correlated with dolomitic horizons, demonstrating the association of bacterial sulfate reduction with dolomite formation. In Brejo do Espinho high δ 34 S of pore water sulfate and high sulfide concentrations occurred throughout the dolomitic sedimentary column. We conclude that elevated salinity and sulfate reduction are the main factors inducing dolomite precipitation in these lagoons, confirming the microbial dolomite formation theory. These results suggest that there may be other settings where sulfate-reducing bacteria induce dolomite precipitation under saline conditions, such as deep-sea sediments or sabkhas, and imply that microbial dolomite may significantly contribute to the sedimentary carbonate budget, particularly in the earliest Earth’s history when anoxic conditions were more prevalent.