Late Dolomite Research Articles

Facies evolution and sequence chronostratigraphy of a “mid”-Cretaceous shallow-water carbonate succession of the Apulia Carbonate Platform from the northern Murge area (Apulia, southern Italy)

The “mid”-Cretaceous carbonate succession of the Apulia Carbonate Platform cropping out in northern Murge area (Apulia, southern Italy) is composed of shallow-water carbonate rocks and is over 400 m in thickness. This paper focuses on the lithofacies analysis of this carbonate succession, its paleoenvironmental interpretation, and its sequence-chronostratigraphic architecture. Lithofacies analysis permitted to identify deposits which can be grouped into the following three facies belts: (1) terrestrial facies belt formed by: intraclast-supported paleosoils; solution-collapse breccias; (2) restricted facies belt made up of lithofacies deposited in protected peritidal environments; (3) normal-marine facies belt made up of lithofacies formed in moderate- to high-energy subtidal environments. The detailed study both in outcrops and in thin-sections revealed that, at the bed scale, lithofacies are cyclically arranged and form shallowing-upward small-scale depositional sequences comparable to parasequences and/or simple sequences. The following three small-scale sequence types have been distinguished: (1) subtidal sequences mostly made up of lithofacies formed in the normal-marine open subtidal domain; (2) peritidal sequences made up of lithofacies formed in the restricted peritidal domain; (3) peritidal sequences showing a cap formed by paleosoils. Small-scale sequences are not randomly arranged in the compiled succession but form discrete packages, or sets, that alternate in the sedimentary record. The repetition of such small-scale sequence packages in the succession has been the key to recognize large-scale sequences comparable to third-order depositional sequences. Although sedimentological data are often fragmentary due to late dolomitization, four large-scale sequences have been distinguished. The data support a generalized landward-backstepping of facies belts during transgression, which implies a gradual gain of accommodation culminating with the deposition of a package of small-scale sequences formed by normal-marine subtidal deposits. These mark periods of maximum accommodation space and form the maximum-flooding zones of large-scale sequences. A gradual seaward progradation of facies belts is recorded during highstand conditions, which implies a gradual loss of accommodation culminating with the deposition of a package of peritidal small-scale sequences capped by paleosoils or by solution-collapse breccias. The occurrence of terrestrial deposits marks periods of minimum accommodation on the platform and determines the sequence boundary of large-scale sequences. The large-scale sequences identified in this study fit with the main transgressive/regressive cycles published in the sequence-chronostratigraphic chart of European basins. As a consequence, it is interpreted that changes of the sea level recorded at the scale of European basins played an important role in determining the sequence-stratigraphic architecture of the studied succession. In spite of this, the occurrence of solution-collapse breccias, which implies a significant gap in carbonate sedimentation in between Early and Middle Cenomanian times, may also have an alternative interpretation. In particular, this deposit may represent the local fingerprint of the well-known tectonic phase which, during Late Albian-Early/Middle Cenomanian times, determined the subaerial exposure of large parts of Periadriatic carbonate platforms producing a marked regional unconformity.

Post-Early Cretaceous Mississippi Valley-type Zn-Pb Mineralization in the Bongara Area, Northern Peru: Fluid Evolution and Paleo Flow from Fluid Inclusion Evidence

The Bongara area, northern Peru, contains Zn-Pb mineralization with characteristics typical of Mississippi Valley-type (MVT) systems. The northwest–southeast-trending late Triassic–Jurassic carbonate host rocks of the Pucara Group were locally replaced by early (D1) and late (D2) dolomite. Fluid inclusion microthermometric data from dolomite and sphalerite are typical of MVT ore fluids (78° to 187°C; ~15–23 wt.% CaCl 2 equivalent), and do not show apparent covariation between temperature and salinity. Homogenization temperature data from D1 and D2 dolomite show a decrease from northern (mean Th = 138°C) to central (mean Th = 126°C) to southern areas (mean Th = 102°C), suggesting fluid flow from north to south. Assuming a geothermal gradient of ~30°C/km and an average surface temperature of ~20°C, the lower temperature limits might approximate the ambient host-rock temperatures, whereas the upper temperature limits may represent minimum temperatures of a deeper aquifer, probably at a maximum depth of ~6000 m. The lower temperature limits are also similar to the estimated temperatures if dolomitization and mineralization took place in the Late Cretaceous at ~2000–3000 m depth, probably during the formation of the Peruvian fold and thrust belt near the present day Peru–Ecuador border. Fluid inclusion data show that the fluids in each studied location had relatively uniform temperatures and salinities during dolomitization and mineralization. A significant salinity drop and a slight temperature decrease are observed during late calcite precipitation, suggesting mixing of brines with dilute, cooler fluids.

Post-Early Cretaceous Mississippi Valley-type Zn-Pb Mineralization in the Bongara Area, Northern Peru: Fluid Evolution and Paleo Flow from Fluid Inclusion Evidence

Research Article| January 01, 2009 Post-Early Cretaceous Mississippi Valley-type Zn-Pb Mineralization in the Bongara Area, Northern Peru: Fluid Evolution and Paleo Flow from Fluid Inclusion Evidence N.I. Basuki; N.I. Basuki † 1F. Gordon Smith Fluid Inclusion Lab., Department of Geology, University of Toronto, 22 Russell St., Toronto, Ontario M5S 3B1. *Present address: Department of Geology, Institut Teknologi Bandung (ITB), Ganesha 10, Bandung 40132, Jawa Barat, Indonesia. †Corresponding Author: E-mail: basuki@gc.itb.ac.id Search for other works by this author on: GSW Google Scholar E.T.C. Spooner E.T.C. Spooner 1F. Gordon Smith Fluid Inclusion Lab., Department of Geology, University of Toronto, 22 Russell St., Toronto, Ontario M5S 3B1. Search for other works by this author on: GSW Google Scholar Exploration and Mining Geology (2009) 18 (1-4): 25–39. https://doi.org/10.2113/gsemg.18.1-4.25 Article history received: 05 Nov 2007 accepted: 19 Apr 2009 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation N.I. Basuki, E.T.C. Spooner; Post-Early Cretaceous Mississippi Valley-type Zn-Pb Mineralization in the Bongara Area, Northern Peru: Fluid Evolution and Paleo Flow from Fluid Inclusion Evidence. Exploration and Mining Geology 2009;; 18 (1-4): 25–39. doi: https://doi.org/10.2113/gsemg.18.1-4.25 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyExploration and Mining Geology Search Advanced Search Abstract The Bongara area, northern Peru, contains Zn-Pb mineralization with characteristics typical of Mississippi Valley-type (MVT) systems. The northwest–southeast-trending late Triassic–Jurassic carbonate host rocks of the Pucará Group were locally replaced by early (D1) and late (D2) dolomite. Fluid inclusion microthermometric data from dolomite and sphalerite are typical of MVT ore fluids (78° to 187°C; ~15–23 wt.% CaCl2 equivalent), and do not show apparent covariation between temperature and salinity.Homogenization temperature data from D1 and D2 dolomite show a decrease from northern (mean Th = 138°C) to central (mean Th = 126°C) to southern areas (mean Th = 102°C), suggesting fluid flow from north to south. Assuming a geothermal gradient of ~30°C/km and an average surface temperature of ~20°C, the lower temperature limits might approximate the ambient host-rock temperatures, whereas the upper temperature limits may represent minimum temperatures of a deeper aquifer, probably at a maximum depth of ~6000 m. The lower temperature limits are also similar to the estimated temperatures if dolomitization and mineralization took place in the Late Cretaceous at ~2000–3000 m depth, probably during the formation of the Peruvian fold and thrust belt near the present day Peru–Ecuador border.Fluid inclusion data show that the fluids in each studied location had relatively uniform temperatures and salinities during dolomitization and mineralization. A significant salinity drop and a slight temperature decrease are observed during late calcite precipitation, suggesting mixing of brines with dilute, cooler fluids. You do not currently have access to this article.

Dolomitization of the Lower Ordovician Aguathuna Formation carbonates, Port au Port Peninsula, western Newfoundland, Canada: implications for a hydrocarbon reservoir

The Lower Ordovician Aguathuna Formation (∼100 m thick) is formed of shallow-marine carbonates, which constitute the uppermost part of the St. George Group of western Newfoundland. Sedimentation was paused by a major subaerial exposure (St. George Unconformity), which likely developed a significant pore system in the underlying carbonates by meteoric dissolution. The sequence has been affected by multiphase dolomitization that caused complex changes in the rock porosity. The Aguathuna dolomites are classified into three main generations ranging in crystal size between ∼4 µm and 2 mm. The occurrence of fabric-retentive dolomicrites implies that dolomitization likely started during the early stages of diagenesis. Although dolomitization is pervasive in the upper part of the formation and significantly occludes the pores, some intervals in the lower part have higher porosity. The development of lower permeable layers overlain by an impermeable (seal) cap suggests a possible potential diagenetic trap. Unlike sabkha deposits, the Aguathuna carbonates do not have evaporite interlayers. Furthermore, the low Sr contents (∼96 ppm) and the δ18O values of earlier dolomites (–3.3‰ to –6.9‰ VPDB (Vienna Pee Dee Belemnite)) are also difficult to reconcile with a brine origin. The Sr/Ca molar ratios (0.0067–0.0009), calculated for the earliest dolomitizing fluid, suggest a modified seawater origin, likely mixed sea and meteoric waters. The least radiogenic 87Sr/86Sr values of the earliest dolomite are consistent with those of early Ordovician seawater, which supports an early-stage diagenesis. Petrography, geochemistry, and fluid inclusions of the late dolomites suggest precipitation at higher temperatures (∼73–95 °C) in deeper burial environments from hydrothermal solutions.

Isotopic evidence on the diagenetic evolution of coastal sabkha reservoirs from the Solimões Basin, northern Brazil

The isotopic composition of diagenetic carbonates, sulfates, illites and associated diagenetic constituents of the Carboniferous Juruá sandstones, Solimões Basin, largest hydrocarbon reservoirs of northern Brazil, was analyzed in conjunction with petrographic characterization and chemical composition. Eolian Juruá sandstones, deposited in a coastal sabkha of a wide cratonic sag, are the best reservoirs, but diagenetic processes introduced strong and complex heterogeneities, which affect hydrocarbon production. Dolomite and anhydrite cementation, together with compaction, exerts the main control on the quality of the reservoirs. Early dolomite cements precipitated under strong evaporation, while late dolomites are related to thermal decarboxylation. Late anhydrite cements are related to the interbedded evaporites and possibly to the oxidation of dissolved sulfide. Authigenic illite K–Ar ages are related to the voluminous Triassic basic magmatism (around 200 Ma), and to the Jurassic–Cretaceous tectonism (around 150 Ma), which may have also affected the late anhydrite and dolomite–ankerite cementation. This isotopic study revealed the major conditions of the diagenetic processes that strongly affected the Juruá reservoirs, which are essential for the development of models that will optimize the production, as well as decrease the risks in the exploration for new Juruá reservoirs.

Early and late dolomites in the carbonate platform: An example from Middle Devonian carbonates of the Taurus Mountains, south-central Turkey

Early and late dolomites in the carbonate platform: An example from Middle Devonian carbonates of the Taurus Mountains, south-central Turkey

Origin of stylolite related fractures in Atoka Bank Carbonates, Eddy County, New Mexico, U.S.A.

Stylolite-related fractures are rare diagenetic features (1% in volume) in Atoka (Carboniferous) bank carbonates in Eddy County, New Mexico, U.S.A. The fractures are often wedge- to curvilinear-shaped, vertical to subvertical, and their wider ends terminate at the seams of major stylolites. The associated stylolites are centimeters in size, low to medium amplitude (up to 1.5cm) and some of them are fabric selective forming at matrix-grain and matrix-cement boundaries. Fractures are filled with predominantly saddle dolomite and late calcite cements. Fracture size ranges from microscopic to 1cm, and they have a width less than 1 mm. A close coexistence of fractures with stylolites suggests that fractures have formed synchronously with the stylolites. In Atoka carbonates, stylolites were formed by progressive burial that is evidence by bedding-parallel or subparallel orientation and different mineral associations along stylolites and also at near areas. These minerals are commonly late calcite and saddle dolomite. Soon after initial stylolitization, dissolved material and/or formation water and surface morphology of stylolites caused localized stress differentiation around stylolite. Compaction differences of less soluble parts at the both sides of stylolitic-highs or lows such as column/peak or waveform result in breakage of less soluble parts above or below column/peak or wave-form. Later wedge-shaped fractures are occluded by late cements provided by stylolitization or stylolitic-channels carring cement-generating fluids from an external source such as dewatering of adjacent shales. Stable isotope values of saddle dolomite (δ18O=−3.5 to −4.0‰ PDB; δ13C=+0.04 to +0.67‰ PDB) and late calcite (δ18O=−5.4‰ PDB; δ13C=+0.7‰ PDB) suggest precipitation at paleotemperatures of 52–55°C for dolomite and 42°C for late calcite based on oxygen values. The temperatures correspond to burial depths of approximately 1450–1600m for dolomite and 1000 m for calcite under present-day conditions, assuming a geothermal gradient of 22°C/km and a surface temperature of 20°C (well data).

Water circulation near the mixed-water and microbiologic activity of the mesozoic dolomite sequence, and example from the Central Taurus, Turkey

Jurassic to Lower Cretaceous units, locating Central Taurus composed of thick dolomite with massive limestones beds and thinner dolomite beds with intercalated limestone are described. Various dolomite types include: Type I) dolomite formed as dolomicrite as mimetic replacement, Type II) dolosparite as a cloudy centre and clear rim in vugs, Type III) The planar-e texture dolomites are scattered in a micrite matrix, Type IV) Mottled dolomite present as zones of light colored dolomite crystals in a darker groundmass, Type V) Fracture and void filling dolomite (zoned dolomite, overgrowth and saddle dolomite). The investigated dolomites exhibit 0.1 to −2.0‰ PDB in δ18O values relative to their δ13C values (0.38 to 1.59‰ PDB) in the Middle Jurassic dolomites. The petrographic and isotopic characteristics support that dolomitization fluids were variable mixtures of fresh and marine waters related to changes in sea level. The late dolomitization phase was followed by diagenesis in the presence of meteoric fluids. During this period, calcite cements were precipitated within fractures. These calcites have characteristically low δ18O and δ13C values (−3.0 to −4.9 and −0.8 to −5.7‰ PDB, respectively).

Fault‐controlled dolomitization at Swan Hills Simonette oil field (Devonian), deep basin west‐central Alberta, Canada

The partly dolomitized Swan Hills Formation (Middle‐Upper Devonian) in the Simonette oil field of west‐central Alberta underwent a complex diagenetic history, which occurred in environments ranging from near surface to deep (>2500 m) burial. Five petrographically and geochemically distinct dolomites that include both cementing and replacive varieties post‐date stylolites in limestones (depths >500 m). These include early planar varieties and later saddle dolomites. Fluid inclusion data from saddle dolomite cements (Th=137–190 °C) suggest that some precipitated at burial temperatures higher than the temperatures indicated by reflectance data (Tpeak=160 °C). Thus, at least some dolomitizing fluids were ‘hydrothermal’. Fluorescence microscopy identified three populations of primary hydrocarbon‐bearing fluid inclusions and confirms that saddle dolomitization overlapped with Upper Cretaceous oil migration. The source of early dolomitizing fluids probably was Devonian or Mississippian seawater that was mixed with a more 87Sr‐rich fluid. Fabric‐destructive and fabric‐preserving dolostones are over 35 m thick in the Swan Hills buildup and basal platform adjacent to faults, thinning to less than 10 cm thick in the buildup between 5 and 8 km away from the faults. This ‘plume‐like’ geometry suggests that early and late dolomitization events were fault controlled. Late diagenetic fluids were, in part, derived from the crystalline basement or Palaeozoic siliciclastic aquifers, based on 87Sr/86Sr values up to 0·7370 from saddle dolomite, calcite and sphalerite cements, and 206Pb/204Pb of 22·86 from galena samples. Flow of dolomitizing and mineralizing fluids occurred during burial greater than 500 m, both vertically along reactivated faults and laterally in the buildup along units that retained primary and/or secondary porosity.

Petrography and Geochemistry of the Eugui Magnesite Deposit(Western Pyrenees, Spain): Evidence for the Development of a PeculiarZebra Banding by Dolomite Replacement

The Eugui-Asturreta magnesite deposit (Western Pyrenees, Spain) forms a discoidal body with a maximum thickness of 130 m, located within a folded Namurian carbonate sequence. The magnesite rocks are composed of lens-shaped crystals (max 8 cm) arranged in black and white bands (zebra structure). The morphology and textural characteristics of magnesite and the structural relationships between magnesite and dolostone host rocks indicate that magnesite replaced the host dolostone. The magnesite crystals replaced the host dolostones, growing perpendicularly from stylolites and meeting between two adjacent stylolite sets. The portions of the crystals close to the stylolites are black colored because they incorporated carbonaceous matter and clay laminations of the replaced dolostone (incorporative growth), whereas the crystal terminations are white because impurities have been displaced and lie at an intercrystalline position and into the meeting zone (displacive growth). These texture differentiations could be interpreted by changes in the ratios of dolomite dissolution versus magnesite precipitation due to increasing amounts of Ca released during the replacement process. Late dolomite is commonly pseudomorphous after magnesite crystals. Dolostone host rocks and magnesite show similar Mn, Al, and K contents. The host dolostones have a lower average FeO content (0.24 wt %) than magnesite (1.82 wt %) and than later dolomite after magnesite (1.04 wt %). The black magnesite bands with a low percentage of impurities show REE contents and patterns similar to those of the relatively pure dolostone host rocks, suggesting an origin by metasomatic replacement. The white magnesite bands display REE contents similar, but slightly lower in LREE, to those of the host dolostones, as would be expected by magnesite replacing dolomite. REE patterns of dolomite after magnesite are very similar to those of magnesite, suggesting that REE behaved conservatively during the replacment of magnesite by dolomite and that the process has been induced by fluids impoverished in REE and equilibrated with the carbonate sequence. The petrography and geochemistry of the Eugui magnesite indicate that the deposit originated by metasomatic replacement of a dolostone precursor promoted by fluids moving through stylolite and bedding planes. Sedimentary, structural, textural, and geochemical relics of the replaced dolostones are preserved in the magnesite rocks. The peculiar textural characteristics of the magnesite represent replacement features and cannot be interpreted by diagenetic or metamorphic recrystallization of an original marine microcrystalline magnesite deposit.

Subsurface arbuckle group (cambro-ordovician) in the Bowman #4 Well of the Wilburton field in the Arkoma Basin, Oklahoma: Depositional facies, diagenetic signatures, petrophysical aspects, and economic potential

Detailed petrographic and petrophysical analyses were performed on a core section of the Nicor #4 Bowman Well, Arkoma Basin, Oklahoma. The studied interval represents the Cambro-Ordovician Arbuckle Group. Petrographic analyses of core samples distinguish six lithofacies. They are as follows from the bottom to the top of the studied core: (1) Quartzose dolostone, (2) conglomeritic dolostone, (3) brecciated dolostone, (3a) crackle breccia, (3b) dissolution-collapse breccia, (4) oolitic dolostone, (5) stylolitic dolostone, and (6) stromatolitic dolostone. Diagenetic evolution has affected the petrophysical properties of these different lithofacies. The diagenetic history of the selected section records early to late dolomitization, stylolitization, dedolomitization (calcification), and silicification. Petrophysical analyses employing mercury porosimetric techniques determine three distinct petrofacies. These petrofacies are as follow: (1) low porosity and high recovery efficiency, (2) low porosity and low recovery efficiency, and (3) high porosity and high recovery efficiency. Based on petrophysical analyses, the best reservoir candidate is the crackle breccia petrofacies because it has intermediate porosity and high recovery efficiency values, plus this lithofacies is bounded by strata with low porosity and recovery efficiency values (impermeable layer). The Arbuckle Group carbonates were uplifted and subaerially exposed, which, led to the development of an unconformity surface. Consequently, karstification developed and modified various petrophysical properties of the core.

Late Laramide dolomite recrystallization of the Husky Rainbow "A" hydrocarbon Devonian reservoir, northwestern Alberta, Canada: paleomagnetic and geochemical evidence

Late Laramide dolomite recrystallization of the Husky Rainbow "A" hydrocarbon Devonian reservoir, northwestern Alberta, Canada: paleomagnetic and geochemical evidence

Successions of Late Cenozoic Platform Dolomites Distinguished by Texture, Geochemistry, and Crystal Chemistry: Niue, South Pacific

Successions of Late Cenozoic Platform Dolomites Distinguished by Texture, Geochemistry, and Crystal Chemistry: Niue, South Pacific

Landslide at km 56.4 along the Irbid-Amman Highway, Northern Jordan

Abstract During the last 30 years, the northern part of Jordan has been affected by many landslides. In 1992, which was a very wet year, 67 landslides of different volumes occurred. These mainly affected the Late Cretaceous Naur Group, which is well known for being prone to slope movements of many types and sizes. This article describes the Jueidya slide which occurred on the night of 13 August 1992 after a very wet winter at Jueidya village, at station km 56.4 along the under-construction highway between Irbid and Amman. This landslide affected ~250 m of the highway, destroying two houses, causing the evacuation of 10 others, and delaying the opening of the highway for several months. The bedrock of the study area consists of Late Cretaceous marl, limestone, and dolomite of Cenomanian age. These rocks are overlain by younger colluvium, essentially loose materials detached from the bedrock masses by chemical, mechanical, and/or tectonic processes. In addition, the area is partially covered by caliche. Monitoring during the last five years has indicated that the slope is still active, and that the present location of the highway in this portion will continue to be affected by future sliding. This article presents geological and geotechnical studies of the slide area with special emphasis on the geostructures that affect slope stability. A stability analysis for this solution is given, and a comparison between the present as-built road and the recommended solution is also presented.

A Tertiary age from paleomagnetism for mississippi valley-type zinc-lead mineralization in Upper Silesia, Poland

Ore and host rocks of the Middle Triassic Muschelkalk carbonates were sampled at 35 sites in three operating mines and several quarries in the Cracow-Silesia Mississippi Valley-type district of southern Poland. Palcomagnetic analysis was done using alternating field and thermal step demagnetization and saturation isothermal remanence tests. Specimens from three limestone and two early dolomite sites retain a dual-polarity A remanent magnetization component (D = 43 degrees , I = 51 degrees , k = 40, alpha 95 = 12 degrees ), which yields a Middle to Upper Triassic pole. A resides in single domain to pseudosingle domain magnetite in limestone and is probably primary, and in single domain-pseudosingle domain magnetite and hematite in early dolomite and is probably diagenetic. Two limestone sites carry a Tertiary remagnetization and most early dolomite sites carry a partial Tertiary remagnetization, or hybrid remanence, identified by palcomagnetic fold and breccia tests. Late dolomite and Mississippi Valley-type mineralization from 14 sites retain a dual-polarity C remanence component (D = 3 degrees , I = 66 degrees , k = 34, alpha 95 = 7 degrees ), which defines a Tertiary pole position. Conglomerate and breccia tests confirm that the clasts in late dolomite were remagnetized. The Tertiary age for the Mississippi Valley-type ore deposits is within the post-mid-Jurassic to pre-Miocene window permitted by the geologic evidence for mineralization. The age also supports gravity-driven fluid flow models for ore genesis that are associated with the Alpine orogeny. The dual polarities and elongate distribution of site mean directions suggest a 1 to 20 m.y. duration for the late dolomitization and mineralization event.

Stable isotope evidence for the origin of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel

ABSTRACTThe oxygen isotope compositions of diagenetic carbonate minerals from the Lower Jurassic Inmar Formation, southern Israel, have been used to identify porewater types during diagenesis. Changes in porewater composition can be related to major geological events within southern Israel. In particular, saline brines played an important role in late (Pliocene‐Pleistocene) dolomitization of these rocks.Diagenetic carbonates included early siderite (δ18OSMOW=+24.4 to +26.5‰δ13CPDB=−1.1 to +0.8‰), late dolomite, ferroan dolomite and ankerite (δ18OSMOW=+18.4 to +25.8‰; δ13CPDB=−2.1 to +0.2‰), and calcite (δ18OSMOW=+21.3 to +32.6‰; δ13CPDB=−4.2 to + 3.2‰). The petrographic and isotopic results suggest that siderite formed early in the diagenetic history at shallow depths. The dolomitic phases formed at greater depths late in diagenesis. Crystallization of secondary calcite spans early to late diagenesis, consistent with its large range in isotopic values.A strong negative correlation exists between burial depth (temperature) and the oxygen isotopic compositions of the dolomitic cements. In addition, the δ18O values of the dolomitic phases in the northern Negev and Judea Mountains are in isotopic equilibrium with present formation waters. This behaviour suggests that formation of secondary dolomite post‐dates the tectonic activity responsible for the present relief of southern Israel (Upper Miocene to Pliocene) and that the dolomite crystallized from present formation waters. Such is not the case in the Central Negev. In that locality, present formation waters have much lower salinities and δ18O values, indicating invasion of freshwater, and are out of isotopic equilibrium with secondary dolomite. Recharge of the Inmar Formation by meteoric water in the Central Negev occurred in the Pleistocene, and halted formation of dolomite.

Depositional Environments, Diagenesis, and Porosity of Upper Cretaceous Volcanic-Rich Tokio Sandstone Reservoirs, Haynesville Field, Claiborne Parish, Louisiana: ABSTRACT

ABSTRACT Tokio Formation sandstones produce oil from sublitharenitic to litharenitic sandstones in the Haynesville Field. The Tokio interval is approximately 210 feet (64 m) thick, and has been divided into four sandstone zones separated by shales or scoured contacts. In ascending order, the four zones are the RA, S3, S2, and S1. The RA is composed of sublitharenites inferred to have been deposited in delta-front bars and distributary channels. The upper three zones are composed of litharenite and feldspathic litharenite to sublitharenite. The upper sands are interpreted to have been deposited in littoral environments including storm influenced shelf, tidal flats and channels, and barrier island/strand plain. The diagenesis of these sands is strongly related to composition: greater percentages of cements and secondary porosity occur in lithic-rich sandstones. Diagenetic cements in sublitharenites are mainly quartz overgrowths with minor early K-spar overgrowths on plagioclase, early chlorite-rims, and late patchy calcite, pyrite, and rare dolomite and siderite. Diagenesis in lithic-rich sands includes greater amounts of chlorite rim and pore-filling kaolinite cements and less quartz-overgrowth and other cements. The effect of the original mineralogy and diagenetic minerals on wireline logs includes: (1) reduction of SP due to cements, (2) increase in GR response due to K-spar and volcanic detritus, (3) higher resistivity due to carbonate minerals, and (4) increase in irreducible water saturation due to pore-lining and pore-filling clay. Thus, potential reservoir zones with lithic-rich sandstones like the Tokio could be overlooked in many areas around the world.

Unraveling the Multiple Origins of Heterogeneity Within Lower Mississippian Madison Reservoirs: Bighorn Basin, Wyoming and Montana, USA: ABSTRACT

{open_quotes}Fracture-controlled{close_quotes} and {open_quotes}karst-controlled{close_quotes} contributions to reservoir heterogeneity tend to be viewed as non-fabric selective in nature. Given such an outlook, predictions of fracture and karst overprints depend on an awareness of extrinsic controls such as past and present stress-fields, structural curvature, fault proximity, and the positions and movements of paleo-water tables. The hierarchical sequence stratigraphy of the 300 m Madison provides the stratigraphic framework necessary to characterize the vertical distribution of early, fabric-selective platformal dolomite; additionally, this framework assists in discriminating between fabric-selective and non-fabric-selective styles of karst and fracturing. In the case of Madison karst, early meteoric lithification and subtle Mississippian tectonics resulted in a vertically oriented fracture-controlled karst on top of the Madison, yet this non fabric-selective system channeled waters into several fabric-selective, regionally widespread solution collapse zones and cave systems. The horizontally oriented regional dissolution was stratigraphically controlled by soluble evaporitic zones and/or argillaceous aquitards overlying intra-Madison sequence boundaries rather than occurring at various unconfined water-table stillstands. Evaporite solution collapse breccias presently form partial to complete barriers to vertical fluid flow depending on thickness and degree of associated argillaceous influx, while cave-roof {open_quotes}fracture breccias{close_quotes} were preferential sites of late dolomitization within the giant Elk Basin Madison reservoir.more » In the case of Madison fracturing, stratigraphic cycles of several scales provide effective scales of analysis in the quest for true mechanical stratigraphic units defined by common fracture styles.« less

Lithistid sponges from the Late Ordovician Fish Haven Dolomite, Bear River Range, Cache County, Utah

The tricranoclad demosponge Hindia sphaeroidalis Duncan, 1879, is reported as a common silicified sponge in the basal dolomite of the Deep Lakes Member of the Upper Ordovician Fish Haven Formation of northeastern Utah for the first time. A small juvenile orchoclad anthaspidellid, Hudsonospongia? sp., is also the first of that family reported from Fish Haven beds and the Deep Lakes Member. Both taxa are from localities on the eastern slope of Mount Magog, north of Tony Grove Lake, in the Bear River Range, Cache County, east of Logan, Utah.

Jabali, a Zn-Pb-(Ag) carbonate-hosted deposit associated with Late Jurassic rifting in Yemen

The Jabali deposit (3.8 Mt at 16% Zn, 2% Pb and 132 g/t Ag) is hosted by dolomitized platform carbonates of Kimmeridgian age at the southwestern edge of the oil-producing Wadi al Jawf rift basin in northern Yemen. Paleogeographical reconstructions demonstrate that tensional synsedimentary tectonic activity from the Late Jurassic to Early Cretaceous was responsible for the thick accumulation of argillaceous and evaporitic sediments in the subsident rift basin, the unstable margin of which was the site of rapid facies changes, local disconformities and periods of emergence, as well as of dolomitization along the WNW- and NNW-striking boundary fault system. In the Jabali area, the upper part of the Jurassic sequence underwent two stages of dolomitization before emergence and deep karstic erosion. Solution cavities and depressions in the eroded surface were filled by dolomite sand and black pyritic mudstone prior to a last marine transgression of limited extent. Subsequent ore deposition and associated late dolomitization sealed the network of solution cavities, impregnating the dolomite sands and the host dolomites. Sphalerite I and wurtzite, followed by silver-bearing zoned sphalerite II associated with galena, crystallized from a cyclic influx of low-temperature (75-100°C) saline solutions. Lead isotope geochemistry indicates that the lead, zinc and silver probably originated from an Early Proterozoic basement. The dissolved metals were likely derived from the basal aquifer (detrital material of basement origin) of the evaporite-bearing sequence filling the Wadi al Jawf trough. Migrating metalliferous brines from the basin to the uplifted Jabali area, where ore deposition was favoured by a reducing environment, were probably channelled by the boundary fault system during the last stages of synsedimentary tectonic activity.