Limestone Sequence Research Articles

The clays involved in the 1963 Vajont landslide: Genesis and geomechanical implications

The catastrophic Vajont landslide that occurred on 9 October 1963 caused about 2000 deaths when 270–300 Mm3 of rock and debris slid from the northern slope of Mt. Toc into the newly created artificial reservoir, displacing some huge water waves that flooded the nearby villages. The 1963 slide was an en-block remobilisation of a prehistoric rockslide lying on the southern reservoir bank. Basal sliding took place within a complex lithostratigraphic sequence of cherty–marly limestones and clay interbeds belonging to the Fonzaso Formation of Upper Jurassic age. In this work, we investigate the geological origin and provenance of the Vajont clays according to the results of mineralogical and geochemical analyses performed on several clay samples which were collected from the landslide failure surface and the same limestone sequence in the surrounding area. The sampled clayey materials contained variable amounts of clay minerals (36–96%), calcite (4–64%) and quartz (0–6%). The dominant clay minerals were illite/smectite (I/S) mixed layers with a high illite content (85–50%), which was consistent with the high percentage of K2O (1.17–5.77%). The Vajont clays included in the Fonzaso Fm. can be referred to as K-bentonites and can be interpreted as distinct deposits of volcanoclastic materials (tephra), representing the sedimentary inputs of volcanic ashes that interrupted the “normal” calcareous sedimentation during the late Oxfordian–upper Tithonian (158–145 Ma). The soft clay interbeds played a different mechanical role in the occurrence of the prehistoric rockslide and the 1963 en-block remobilisation. When referring to the ancient slope failures, distinct clay-rich layers that were concentrated within some specific lithostratigraphic intervals of the Jurassic cherty limestone sequence caused a localised decrease in the available shear strength, allowing for relative shear displacements along the dip-slope bedding planes. This caused additional shear/tensile fracturing of limestone beds and the formation of a stepped basal failure surface. During the 1963 remobilisation, the clay layers contributed to the average shear strength decrease of the debris material forming the basal shear zone of the slide, but the main triggering factor was the increase in pore pressures caused by the reservoir-induced groundwater inflow.

Viséan transgression and reworking at Boudouda (NW Benahmed, western Moroccan Meseta)

At Boudouda NW of Benahmed, upper Frasnian goniatite shales are in unconformable contact with massive, partly dolomitized conglomerate boulders of the new Oued Ayada Formation. They represent debris flow deposits of localized channels and contain reworked supposed Emsian reefal organisms and Frasnian to lower/?middle Famennian conodonts. Rare foraminifers prove upper Visean (Upper Asbian) sedimentation. Laterally, a ca. 35 m thick sequence of bioclastic limestones with crinoids, mostly fragmented brachiopods or gastropod-rich forms a second member. Thin-sections show a dominance of variably sorted wacke-, pack- and grainstones with abundant coated grains (initial ooids) and subangular fine quartz sand. Biota include calcareous green algae, foraminifers, and various Algospongia (Aoujgaliidae and Palaeobereselliidae). Throughout the succession, there are reworked Frasnian to middle Tournaisian deeper-water conodonts, which shows that a probably small-sized pelagic platform existed once in the region, which had been uplifted during the Eovariscan 2 tectonic phase. A single, juvenile, originally pyritic goniatite gives limited evidence for an upper Tournaisian/lower Visean goniatite shale, which has no equivalents elsewhere in the Meseta. Based on a rich record of foraminifers (more than 70 taxa) and stratigraphically meaningful other calcareous microfossils, polyphase erosion and re-deposition by storms occurred on a shallow-water, photic zone carbonate ramp in the upper Visean. The Oued Ayada Formation correlates with the transgressive limestones of the basal Melilla Formation in the southern Mdakra Basin and the Bled Mekrach Formation in the NE Rehamna.

Fracture Propagation and Hydraulic Properties of a Coal Floor Subjected to Thick-Seam Longwalling above a Highly Confined Aquifer

The high-pressure and water-rich confined aquifer occurring in the Ordovician limestone sequence poses great threats to the routine production of underground longwall mining. Considering the intense cooperation of mining disturbance and water pressure, water-conducting fractures within a coal seam floor can connect the lower aquifer and upper goaf, and this hydraulic behavior is considered the root of water inrush hazard and water loss or contamination. In this paper, the panel 4301 of the Longquan coal mine serves as the case where the panel works closely above the floor with high water pressure. By the combination of physical and numerical modelling approaches, the variation characteristics of fracture development and volumetric strain of floor rocks subjected to mining disturbance are analyzed. A numerical computation model is constructed based on the volumetric strain-permeability equation obtained by curve fitting, and on such basis, the impacts of different mining parameters on floor rock permeability are studied. The results show that the floor rocks experience fracture generation, extension, and convergence procedures as the workface advances along the longitudinal direction, and fractures appearing in front of the workface are more developed. In the whole process of coal seam extraction, the volumetric strain profile exhibits “Λ” shape and an inverted saddle shape before and after overburden strata collapse. By controlling a single variable, the paper reveals that panel height is of greater impact on floor permeability changes than panel length and panel width.

BRITISH MILITARY CONTRIBUTIONS TO THE GEOLOGY OF MALTA, PART 1: NINETEENTH CENTURY FOUNDATIONS

Malta, an island in the central Mediterranean Sea, was fortified as a base for the Knights Hospitaller 1530–1798 and to provide major harbours for the British Royal Navy after 1813. Men with British military associations (all subsequently to attain some distinction in public and/or academic life) were amongst the many pioneers of Maltese geology who established the essence of its outcrop stratigraphy and structure: a circa 300-metre-thick sequence of near-horizontal mid-Cenozoic fossiliferous limestones punctuated by a ‘blue clay/marl’, cut by a series of major faults and penetrated by several caves and fissures whose infill contained significant remains of Pleistocene vertebrates. Between 1843 and 1856, Lieutenant (later Vice-Admiral) Thomas Abel Brimage Spratt (1811–1888) defined major units in the bedrock sequence, Colonel (later Major-General) Sir William Reid (1791–1858) promoted publication of a geological memoir, and a 1:31,680-scale geological map prepared by the 3rd Earl of Ducie on a Royal Engineers topographical base map was published under Royal Engineer auspices. Mostly between 1860 and 1866, Captain (later Professor) Frederick Wollaston Hutton (1836–1905) and Surgeon (later Deputy Surgeon-General and Professor) Andrew Leith Adams (1827–1882) made field observations that refined earlier interpretations of stratigraphy and structure and generated revised but small-scale maps. They also collected specimens that facilitated specialist identifications of Malta’s fossil faunas, including foraminifera by Thomas Rupert Jones (1819–1911), Professor of Geology at the Royal Military College, Sandhurst. Rock specimens were sent in 1888 by Surgeon-Captain David (later Surgeon-General Sir David) Bruce (1855–1931) and the former engineer Lieutenant (and later Professor) Osbert Chadwick (1844–1913) to the pioneer oceanographer John (later Sir John) Murray (1841–1914). They stimulated Murray’s benchmark study 1889–1890 of Malta’s sedimentary sequence and fossil foraminifera, and their palaeoenvironmental interpretation, plus his compilation of a 1:129,254-scale geological map. These prompted extensive local studies and collection of macrofossil specimens by schoolmaster (later Lieutenant-Colonel) John Henry Cooke (1862–1933). By the end of the century, representative Maltese fossils had been presented for specialist study and identification or description to major museums in England, Scotland and Italy, facilitating improved correlation of Maltese strata with Oligo-Miocene successions elsewhere.

Late Oligocene\u2014Early Miocene shortening in the Thrace Basin, northern Aegean

Late Cenozoic was a period of large-scale extension in the Aegean. The extension is mainly recorded in the metamorphic core complexes with little data from the sedimentary sequences. The exception is the Thrace Basin in the northern Aegean, which has a continuous record of Middle Eocene to Oligocene marine sedimentation. In the Thrace Basin, the Late Oligocene–Early Miocene was characterized by north-northwest (N25°W) shortening leading to the termination of sedimentation and formation of large-scale folds. We studied the stratigraphy and structure of one of these folds, the Korudağ anticline. The Korudağ anticline has formed in the uppermost Eocene–Lower Oligocene siliciclastic turbidites with Early Oligocene (31.6 Ma zircon U–Pb age) acidic tuff beds. The turbidites are underlain by a thin sequence of Upper Eocene pelagic limestone. The Korudağ anticline is an east-northeast (N65°E) trending fault-propagation fold, 9 km wide and 22 km long and with a subhorizontal fold axis. It is asymmetric with shallowly-dipping northern and steeply-dipping southern limbs. Its geometry indicates about 1 km of shortening in a N25°W direction. The folded strata are unconformably overlain by Middle Miocene continental sandstones, which constrain the age of folding. The Korudağ anticline and other large folds in the Thrace Basin predate the inception of the North Anatolian Fault (NAF) by at least 12 myr. The Late Oligocene–Early Miocene (28–17 Ma) shortening in the Thrace Basin and elsewhere in the Balkans forms an interlude between two extensional periods, and is probably linked to changes in the subduction dynamics along the Hellenic trench.

Biosorption Capacity for Uranium and Thorium by Some Microalgae Species

THE BIOSORPTION is an effective technique for the removal of uranium and thorium from some types of phosphate rocks of red sea area. In this study Cystoseira osmundacea, Palmaria elegans and Chondrus crispus, were used for the biosorption of uranium (U), and thorium (Th), from phosphates rocks in Egypt in Qusser and Safaga areas west red sea which characterized of shallow-water marine origin and mainly associated with sequences of white hard limestone, white chalk, yellow marls and hard cherts rocks. The obtained results showed that Cystoseira osmundacea, Palmaria elegans and Chondrus crispus have biosorption capacities of (81%, 89% and 90%), for U and (77%, 88% and 91%) for Th, respectively from phosphate of Safaga .and biosorption capacities of (81%, 84% and 89% ), for U and (79%, 81% and 88% ) for Th, respectively from phosphate of Qusser area. it is preferable to use Chondrus crispus on biosorption due to its high sorption capacity than the other two algae.

Fast Pliocene integration of the Central Anatolian Plateau drainage: Evidence, processes, and driving forces

Abstract Continental sedimentation was widespread across the Central Anatolian Plateau in Miocene–Pliocene time, during the early stages of plateau uplift. Today, however, most sediment produced on the plateau is dispersed by a well-integrated drainage and released into surrounding marine depocenters. Residual long-term (106–107 yr) sediment storage on the plateau is now restricted to a few closed catchments. Lacustrine sedimentation was widespread in the Miocene–Pliocene depocenters. Today, it is also restricted to the residual closed catchments. The present-day association of closed catchments, long-term sediment storage, and lacustrine sedimentation suggests that the Miocene–Pliocene sedimentation also occurred in closed catchments. The termination of sedimentation across the plateau would therefore mark the opening of these closed catchments, their integration, and the formation of the present-day drainage. By combining newly dated volcanic markers with previously dated sedimentary sequences, we show that this drainage integration occurred remarkably rapidly, within 1.5 m.y., at the turn of the Pliocene. The evolution of stream incision documented by these markers and newly obtained 10Be erosion rates allow us to discriminate the respective contributions of three potential processes to drainage integration, namely, the capture of closed catchments by rivers draining the outer slopes of the plateau, the overflow of closed lakes, and the avulsion of closed catchments. Along the southern plateau margin, rivers draining the southern slope of the Central Anatolian Plateau expanded into the plateau interior; however, only a small amount of drainage integration was achieved by this process. Instead, avulsion and/or overflow between closed catchments achieved most of the integration, and these top-down processes left a distinctive sedimentary signal in the form of terminal lacustrine limestone sequences. In the absence of substantial regional climate wetting during the early Pliocene, we propose that two major tectonic events triggered drainage integration, separately or in tandem: the uplift of the Central Anatolian Plateau and the tectonic completion of the Anatolian microplate. Higher surface uplift of the eastern Central Anatolian Plateau relative to the western Central Anatolian Plateau promoted more positive water balances in the eastern catchments, higher water discharge, and larger sediment fluxes. Overflow/avulsion in some of the eastern catchments triggered a chain of avulsions and/or overflows, sparking sweeping integration across the plateau. Around 5 Ma, the inception of the full escape of the Anatolian microplate led to the disruption of the plateau surface by normal and strike-slip faults. Fault scarps partitioned large catchments fed by widely averaged sediment and water influxes into smaller catchments with more contrasted water balances and sediment fluxes. The evolution of the Central Anatolian Plateau shows that top-down processes of integration can outcompete erosion of outer plateau slopes to reintegrate plateau interior drainages, and this is overlooked in current models, in which drainage evolution is dominated by bottom-up integration. Top-down integration has the advantage that it can be driven by more subtle changes in climatic and tectonic boundary conditions than bottom-up integration.

Fracture Properties of Nash Point Limestone and Implications for Fracturing of Layered Carbonate Sequences

Carbonate reservoirs accommodate a significant proportion of global hydrocarbon reserves. However they are often tight and permeability is therefore usually dependent on either flow through existing fractures or through those produced by hydraulic stimulation. Hence, understanding how fracture networks develop in carbonate reservoir rocks is key to efficient and effective production. However, despite their prevalence as reservoir rocks, there is a paucity of data on key fracture properties of carbonate rocks, particularly in more than one orientation. Here, therefore we report measurements of both the tensile strength and fracture toughness of Nash Point limestone in the three principal fracture orientations to determine what effect any mechanical anisotropy might have on fracture propagation. We find Nash Point limestone to be essentially isotropic in terms of both its microstructure and its fracture properties. When comparing the fracture toughness of Nash Point limestone with that of others limestones, we find that fracture toughness decreases with increasing porosity, although this dependency is not as strong as found in other porous rocks. Finally, as many so-called carbonate reservoirs actually comprise layered sequences, we extend our analysis to consider the layered sequence of limestones and shales at Nash Point. We find that the fracture toughness of Nash Point limestone is higher than Nash Point shale but that the fracture energy is lower. We therefore discuss how the implications of fracturing through multi-layered sequences could be explored in future work.

Emplacement of Paleocene-Eocene magmatism under transtensional regime and its evolution to a dynamic equilibrium on the western edge of Colombia

The Cretaceous and Paleogene magmatic arcs of the Central and Western Cordilleras of Colombia have been attributed to the evolution of a subduction system in the Colombian Pacific coast. In this work the distribution and crystallization ages of plutons emplaced between 60 Ma and 53 Ma in the Central and Western Cordilleras are analyzed. From 53 Ma the magmatic arc migrates towards the west of Colombia, installing magmas in a plate edge transitional crust. The crystallization ages analyzed in this work suggest that, within the study area, the plutonic belt is continuous throughout the Western Cordillera. From 40±5 Ma to 26 Ma there was a significant reduction in the convergence velocity of the Farallon plate; as it decreases, also the tectonic loading diminishes resulting in a process of regional stress relaxation. The process of relaxation of the regional stress also occurred in the intra-continental environments producing peneplanization process in the topographic highs of northern Colombia, the reactivation of the piedmont with westwards progradation of sedimentation and the development of a middle- to late-Eocene regional unconformity. In continental shelf environments, the relaxation of the tectonic stress is evidenced by the distribution of reef limestone sequences throughout the Colombian Pacific margin and the Caribbean of Colombia, Ecuador, Panama and Costa Rica, and by a magmatic gap from 33 Ma to 26 Ma. The Paleocene-Eocene magmatic event distributed in the Central and Western Cordilleras took place under a transtensional regime, with the maximum horizontal compressive stress (σ1) oriented SW-NE, product of the oblique convergence between the Farallon and South American plates.

Secondary detachments within carbonates of the Saraburi Group, Triassic Khao Khwang fold and Thrust Belt, Thailand

Permian limestones of the Saraburi Group in Central Thailand, deformed during the Triassic Indosinian orogeny, exhibit detachments formed in response to the following variations in lithology: thin shale beds within a limestone sequence, massive limestone overlaying well-bedded limestone, massive limestone overlying interbedded limestones and shales, within well-bedded limestones, thin shales interbedded with limestones overlying limestones. Where shales are present the decoupling of layers is similar to examples previously described in the literature from outcrop and analogue models. However, more unusually, detachments are also present within limestone units. While detachment folds are typically developed above a basal weak unit (usually salt or shale), in the study area is an example developed entirely within well-bedded limestones. Instead of flow of the weak unit into the core of the fold, room problems in the core of the fold are accommodated by pressure solution parallel to bedding, thrusts, small fault propagation folds, and intense folding. Variations in structural style are largely influenced by the spacing/intensity of bedding, limestone clay content (and the efficiency of pressure solution during deformation), and the presence/absence and thickness of interbedded shales. A ductile detachment zone is developed between well-bedded limestones and overlying massive limestones, the detachment zone separates highly folded sequence dominated by steep dips, from a weakly folded sequence dominated by gentle dips. Pressure solution processes, and variability in pressure solution intensity (related to clay content) play a very strong role in accommodating the difference in deformation styles across the detachment.

Cambrian stratigraphy (Series 2 to Miaolingian) of the El Sahuaral area in central Sonora, Mexico: Biostratigraphic implications

The lower and middle Cambrian strata exposed in the state of Sonora, Mexico, consists of sequences of quartzarenite, limestone, shale and dolomite exposed in several localities, mainly in the northwestern and central regions of Sonora. Approximately 800 m interval of Cambrian marine sedimentary rocks are exposed in the El Sahuaral area, a new Cambrian locality in central Sonora. The sedimentary succession of these marine facies is divided into five lithostratigraphic units previously documented in Mexico: Proveedora, Buelna, Cerro Prieto, El Gavilán and El Tren formations. The Proveedora Formation is the oldest sequence of Cambrian strata in the region and consists of a 168 m thick sequence of bioturbated, fine-grained quartzarenite. The Buelna Formation conformably overlies the Proveedora Formation in the El Sahuaral and adjacent areas and consists of 70 m of interbedded limestone and shale, with abundant fragments of olenellid trilobites from either the Bristolia mohavensis or Bristolia insolens zones (Series 2, Stage 4). The overlying Cerro Prieto Formation is composed of a 70 m thick sequence of recrystallized oolitic limestone with possible fragments of Amecephalus arrojosensis, suggesting the Amecephalus arrojosensis Zone (uppermost Series 2, Stage 4). The El Gavilán Formation conformably overlies the Cerro Prieto Formation with a thickness of 225 m; it is composed of interbedded limestone and shale deposited in a marine open shelf environment, with abundant fauna of the upper Ehmaniella Zone (Altiocculus Subzone), Ptychagnostus praecurrens - Ptychagnostus gibbus Zone (Miaolingian Series, Wuliuan Stage). The El Tren Formation, reported by the first time in central Sonora, conformably overlies the El Gavilán Formation and consists of 250 m thick sequence of dolomitic limestone with unidentified trilobite fragments. The Cambrian deposits of central Sonora present marine facies that vary from the littoral zone to open marine shelf, accumulated in the western part of the North American craton and contains an abundant and diverse biota comprised by cyanobacteria, poriferans, brachiopods, trilobites, and echinoderms.

Stress sensitivity of porosity and permeability of Cobourg limestone

We have analysed the stress sensitivity of porosity and permeability of macroscopically intact limestone samples, from the Middle Ordovician Cobourg formation, Ontario, Canada. This limestone is a potential host formation for geological storage of low- and intermediate-level nuclear waste. Porosity and permeability were measured with gas (helium) on dry cylindrical plugs under defined isostatic stress conditions. Measurements were performed only during the second loading and unloading paths (5–20 MPa) to minimize the impact of fissures induced by sample preparation.All samples have porosity values of less than 2%. The Klinkenberg-corrected permeability coefficients of three adjacent samples drilled from one cubic block in different orientations span one order of magnitude (2.3 × 10−19–2.0 × 10−20 m2). The transport porosity of these samples is dominated by slit-shaped pores and micro-fractures within the clay matrix. Much higher permeability coefficients (1.5 × 10−17–6.2 × 10−18 m2) were measured on another sample containing open stylolites, as identified by X-ray Computed Tomography (micro-CT) and broad ion beam milling-scanning electron microscopy (BIB-SEM). Stress sensitivity coefficients of permeability range from 3.0 × 10−2 to 7.1 × 10−2 MPa−1, those of porosity from 3.9 × 10−3 to 9.8 × 10−3 MPa−1. A loading hysteresis effect was observed with permeability coefficients along the unloading path, up to 47% lower than values on the loading path. This is taken as an indication for creep. Due to the occurrence of stylolites in this limestone sequence, the matrix permeability is not considered as a critical factor for the sealing efficiency.

The Devonian/Carboniferous boundary in Sardinia (Italy)

In Sardinia (Italy) two sections, located in the southeastern part of the island, expose the Devonian/Carboniferous Boundary: the Monte Taccu Section and the Bruncu Bullai section. Both expose limestones from the middle Famennian to the lower Tournaisian Siphonodella jii Zone. The limestone sequence is interrupted by a thin level of shales representing the Hangenberg Event. Data on conodont biostratigraphy and magnetic susceptibility across the Devonian/Carboniferous Boundary are here summarised and reviewed.

The early Famennian (Late Devonian) conodonts natural assemblages from Holy Cross Mountains (Poland)

ABSTRACT Two early Famennian natural assemblages of conodont elements were found in a marly shale bed within a limestone and shale sequence representing the Palmatolepis triangularis conodont zone at the Kowala quarry in the Holy Cross Mountains, Poland. The most complete of these assemblages, identified as Ctenopolygnathus brevilamina, contained nine skeletally preserved elements and four imprints on the rock bedding plane. A peculiar aspect of the apparatus was the disposition of S4 elements with respect to other S-array elements. Moreover, the denticles of S4 elements were unusually robust and slightly twisted towards the caudal side, which may have resulted from a mode of food processing different from those of other conodonts known from natural assemblages. Moreover, the location of S0 was not consistent with the hypothetical arrangements of similar apparatuses proposed by previous authors. The second assemblage, representing an unidentified palmatolepidid, contained scattered two fragmentary elements and four imprints.

Chert nodules in pelagic limestones as paleo-stress indicators: A 3D geomechanical analysis

The objective of this study is to explain the occurrence and paleo-stress significance of 3D joint clustering in chert nodules (inclusions) within a layered pelagic limestone sequence. The difference in stiffness between chert and limestone is about one order of magnitude. Field observations show that fracture localization occurs mostly in chert nodules as opposed to the limestone matrix. We show with a novel three-dimensional geomechanical modelling analysis how the inclusion (ellipsoid) axes ratio influences fracture intensity and propagation within and outside the chert nodules and how the nodules record different deformation phases under different remote stress conditions. From field observations, we recognize two joint sets in the chert nodules: joints parallel and normal to the plane containing the two major axes of the nodule (bedding plane). In the nodules, the normal-to-bedding joints are interpreted to be younger than the parallel-to-bedding ones. The modelling of the 3D Eshelby solution for the stress field inside the chert nodule and in the surrounding matrix is consistent with our field observations and it suggests a strong differential stress during deformation (σminr/σmaxr < 0.3). Chert nodules in a deformed carbonate sequence, therefore, can provide important clues on the paleo-stress conditions, the temporal sequence of events, and fracture distribution heterogeneity.

Pyrometamorphic Rocks in the Molinicos Basin (Betic Cordillera, SE Spain): Insights into the Generation of Cordierite Paralavas

A singular thermal anomaly occurred in the Molinicos Miocene, lacustrine, intramontane basin (Betic Cordillera). This gave place to vitreous vesicular materials (paralavas) and baked rocks (clinker) inside of a sequence of marly diatomites and limestones. The chemical composition of the paralavas (SiO2 = 52–57, Al2O3 ≈ 20, Fe2O3 = 10–20, K2O + Na2O &lt; 2.5, CaO &lt; 4.5, and MgO &lt; 1.5, % in weight), which is very different from typical igneous rocks, and their high-T mineralogy (cordierite, sillimanite, anorthite, mullite, and high-T silica polymorphs) suggest that they formed during a pyrometamorphic event. The occurrence of dry intervals in the lacustrine depositional system, the high Total Organic Carbon contents (&gt;4% in weight) of dark clay layers and the existence of tectonic fractures give the right context for a combustion process. Short-term heating favoured the generation of paralavas, clinker and marbles. Thermodynamic modelling constrains the onset of melting at 870–920 °C for &lt;10 MPa at equilibrium conditions. However, the presence of tridymite and/or cristobalite in clinker and paralavas and the compositional variation in both rock types suggests that the temperature at which first melting occurred ranged between 870 °C and 1260 °C due to melt fractionation processes.

Framboidal chalcopyrite and bornite constrain redox conditions during formation of their host rocks in the copper stratabound mineralization of Picachos, north-central Chile

The Picachos Project is a copper deposit that occurs in manto-type orebodies in north-central Chile and it exhibits common features of Chilean stratabound or manto-type copper deposits: (1) an increase of copper contents from the margins to the centre of the orebodies; (2) sodic and potassic hydrothermal alteration of host rocks with subsequent chloritization and sericite alteration; and (3) its occurrence in sequences of limestones that are intercalated with volcanic rocks. The most salient characteristic of the Picachos Project is the presence of pervasive framboidal pyrite in both the non-mineralized limestones and the host rocks of the copper mineralization. These framboidal pyrites are well preserved, probably due to their close relationship with organic matter, and have no evident textures of overgrowth, recrystallization or dissolution. Moreover, framboidal chalcopyrite and bornite are formed in the external and internal areas, respectively, of the orebodies, sharing common morphological characteristics with framboidal pyrite, and are formed by the replacement of the original pyrite framboids, without changing their shape and size distribution. Representative size distributions of framboidal pyrite, chalcopyrite and bornite are determined using high-resolution X-ray tomography (micro-CT). This study highlights the effectiveness of these analytical techniques to obtain the representative size distribution of framboids in mineralized rocks. The preservation of the original framboidal texture of pyrite makes it possible to infer anoxic to near-dysoxic conditions. The size distributions of framboidal chalcopyrite and bornite are hypothesized to be additional potential paleo-redox indicators, referring to the same formation dynamic as typical framboidal pyrites.

Formation Conditions of Ferromagnesian Metasomatic Carbonates in the Lower Riphean Terrigenous–Carbonate Rocks of the Southern Urals

Ferromagnesian carbonate metasomatites in limestones of the Lower Riphean Suran Formation in the Avzyan ore region (Bashkir meganticlinorium) are represented by large Fe-magnesite deposits (e.g., Ismakaevo deposit) and breunnerite stocks (Bogryashka ore occurrence). Metasomatic zonation is manifested as alterations in the series: limestone → dolomite → Fe-magnesite (breunnerite). Fe-magnesite contains up to 8 mol % FeCO3; breunnerite, from 10 to 40 mol % FeCO3. Metasomatic fluid represented the Ca–Na–Mg chloride brine with Fe admixture and was linked with processes of the remobilization of evaporite brines buried in Lower Riphean rocks. Salinity and homogenization temperature of fluid inclusions are: 20–26 wt % equiv NaCl and 200–240°C, respectively, in magnesites; 10–15 wt % equiv NaCl and 140–190°C, respectively, in breunnerites. Interaction of the fluid with terrigenous rocks in a tectonically active zone of the Mashak riftogenic graben ensured the enrichment of brine with different ligands and the accumulation of MREE and HREE in metasomatic products. Upon the migration of fluid across the limestone sequence and its cooling, Fe-magnesite was formed in the frontal zone (Ismakaevo deposit); breunnerite, in the rear zone adjacent to the terrigenous shales (Bogryashka ore occurrence).

Origin and 87Rb–87Sr age of porewaters in low permeability Ordovician sediments on the eastern flank of the Michigan Basin, Tiverton, Ontario, Canada

Porewater extractions and acid leachates of rock core from a 250 m thick sequence of low-permeability Ordovician-age shales and limestones, on the eastern flank of the Michigan Basin, were analysed for strontium isotope ratios in an attempt to infer porewater ages from observed 87Sr/86Sr enrichments. The porewaters originated as Ordovician seawater, which subsequently mixed with evaporated Silurian seawater infiltrating from above, and, to some extent, with a deep brine—with an enriched 87Sr/86Sr signature—from the underlying crystalline shield or deep basin. The porewater 87Sr/86Sr ratios are more radiogenic than contemporaneous seawater but show no obvious correlation to those leached from the solid rock phases. Accepting that the initial 87Sr/86Sr signatures in porewaters were dominated by Late Silurian brine, potentially with an additional deep brine component, the excess of radiogenic 87Sr appears to represent ingrowth from 87Rb decay over a time span of some 420 million years, approaching the depositional age of the rocks. Similarly, Rb/Sr errochron ages of acid leachates of solid phases, and the calculated initial 87Sr/86Sr isotopic ratios, are consistent with a proposition that the calcites inherited their Sr from Ordovician seawater and were dolomitized shortly afterwards by infiltrating Mg-enriched evaporative brine, indicating long-term conservative behaviour for the enclosing carbonate rocks. The errochron for leachates from (alumino)silicates yields a high initial 87Sr/86Sr, but with an errochron age of about 340 ± 48 Ma, likely owing to variable admixtures of diagenetic illite in the shales. Overall, the data provide evidence for a stable hydrologic regime since Paleozoic time.

Engineering Geology, History and Geography of the Pittsburgh, Pennsylvania Area

ABSTRACTThe City of Pittsburgh, PA is located west of the Appalachian Mountains in the Appalachian Plateaus Province. The relatively flat surface of the plateau is dissected by drainage from the three principal rivers of the region, the Allegheny, Monongahela, and Ohio. The formation of Pittsburgh’s three rivers and drainages has a long history dating back to before the Pleistocene Epoch, linked closely to the advance and retreat of continental glaciation.Western Pennsylvania is associated with the westernmost formation of the Appalachian Mountain chain with deformation in the form of a series of nearly flat-lying, gently warped Paleozoic sedimentary rocks. Rocks cropping out in the region range in age from Devonian to Permian. Pennsylvanian strata are dominated by thin cyclic sequences of sandstone, shale, claystone, coal, and limestone. Most of the geologic hazards present in the region include slope instability, expansive shales and slags, mine subsidence, acid mine drainage, pyritic acid rock and flooding. The region also has an abundance of natural resources including coal, natural gas, oil, salt, limestone, sand and gravel and water.Pittsburgh's strategic location helped shape westward expansion during the formation of the Nation, largely because of the rivers, which served as an inexpensive, yet efficient means of transportation. Infrastructure was always significant in Pittsburgh. However, the existing aging infrastructure are deteriorating. Today, Pittsburgh has transcended the legacy name, “Steel City” and has revitalized itself with nationally-recognized universities and medical centers and a resurgence in natural gas exploration. However, many environmental legacy issues still burden the area.