Sedimentary Rock Fragments Research Articles

Sedimentary record of a Late Cretaceous volcanic arc in central Patagonia: petrography, geochemistry and provenance of fluvial volcaniclastic deposits of the Bajo Barreal Formation, San Jorge Basin, Argentina

The Upper Cretaceous Bajo Barreal Formation (San Jorge Basin, Argentina) is a fluvial succession mostly composed of channel sandstones interbedded with thicker floodplain deposits dominated by tuffaceous strata. The goal of this contribution is to infer the provenance of the unit through combined petrographical and geochemical data of channel sandstones, primary tuffs and tuffo-psammites (reworked, no-mixed tuffs). Channel sandstones are dominantly litharenites and feldspathic litharenites with abundant participation of volcanic lithic fragments with different textures (porphyritic, eutaxitic, felsitic, pilotaxitic, trachytic and vitric-vitrophyric), pumice and plagioclase. K-feldspar, quartz and sedimentary rock fragments are scarce. QFLu diagram and several provenance indicators including dominance and textural types of rock fragments, K-feldspar/plagioclase ratio and inclusion-free quartz grains indicate a provenance from felsic to intermediate, pyroclastic-rich, arc-related volcanic rocks. This interpretation agrees with the values of various elemental ratios of trace elements such as La/Sc, Th/Sc, Cr/Th, Eu/Eu ∗ and (La/Lu) N and discriminant functions based on major elements. Pyroclastic floodplain strata are commonly pumice-rich, occasionally glass shard-rich, vitric tuffs suggesting an origin from plinian-like eruptions. Subordinate components include volcanic lithic fragments, quartz, orthoclase and plagioclase. Zr/Ti versus Nb/Y and SiO 2 versus K 2O diagrams, as well as a multi-element diagram of selected trace elements shows that the pyroclastic rocks were originated from medium-K content, intermediate, arc related magmatic source. Considering the palaeoflow direction towards the east and south-east and presence of subduction processes along the western margin of Patagonia during the Late Cretaceous, it is interpreted that Bajo Barreal Formation constitutes the sedimentary record (distal facies of volcano-flanking fan or apron) of this coeval volcanic arc. Channel sandstones probably were mostly derived from the Divisadero Formation volcanic rocks because of similar petrographical features and REE pattern. In contrast, pyroclastic tuffaceous floodplain deposits would be derived from vents located over the actual outcrops of the Patagonian Batholith.

Mineralogical-geochemical criteria of diamond potential of kimberlites in the Yubileinaya multiphase Pipe (Yakutia)

Pursuance and refinement of the available criteria for the assessment of diamond potential remain an urgent issue in diamond geology. The universality of mineralogical criteria, which determine the interrelation between the content of high-Cr low-Ca pyropes and high-Cr spinels (xenocrystals) with the diamond potential of kimberlitic breccia, has been confirmed for various diamondiferous provinces of the world [1‐3]. Other compositional criteria of the diamond potential of kimberlites are developed insufficiently as yet. Their application for assessment of the productivity of separate phases of kimberlites, pipes, or fields of kimberlite provinces has certain limitations [4‐6 and others]. Our paper shows for the first time that the complex of mineralogical and petrogeochemical criteria of diamond potential can be used for some kimberlite phases with the Yubileinaya Pipe as an example. The lithological composition of mineralogical‐petrographic varieties of kimberlites in autonomous emplacement phases of the Yubileinaya Pipe was investigated systematically during the mining of various levels of the deposit in the years 1986‐2006. In order to analyze the possible correlation between the productivity and compositional features of separate kimberlite varieties, we used the exploitation and geological exploration data on the diamond content (in conventional units) in kimberlite ores from different horizons of the pipe. Mapping of the southwestern and northeastern flanks of the Yubileinaya Pipe made it possible to outline three autonomous phases of kimberlite emplacement with different textural-structural, mineralogical, and petrogeochemical parameters and diamond potentials: coarse-porphyric micaceous kimberlite, kimberlitic breccia, and autolithic kimberlitic breccia. The central part of the pipe is characterized by a greater diversity of kimberlites. Based on the texturalstructural features and mineral compositions, they are divided into kimberlitic breccias, autolithic kimberlitic breccias, and kimberlites with various (schlieren‐ taxitic, aphyric, and sporadophyric) textures. This zone also includes some blocks of the porphyric kimberlite with the mineral composition similar to that of the coarse-porphyric kimberlite at flanks of the pipe. All these varieties are commercial kimberlite ores with a variable content of the useful component. The upper horizons of the central part are mainly composed of sheeted carbonate breccia with a small amount of the kimberlitic material. Their thickness varies from several meters at the margins of the pipe to 100‐130 m in the central part. The carbonate breccias grade downsection into kimberlite rocks of the “xenolithic belt” characterized by the presence of a great amount (up to 60‐70 vol %) of large xenoliths and “floating reefs” of sedimentary rocks. The large sedimentary xenoliths are cemented by the sporadophyric and schlieren‐taxitic kimberlites and the less common kimberlitic breccias and autolithic kimberlitic breccias. The xenolithic belt encloses the autolithic kimberlitic breccias that make up lower horizons of the pipe. The carbonate breccias (with sedimentary rock fragments up to 90%), sporadophyric and aphyric varieties, and some schlieren‐taxitic kimberlites are noncommercial ores.

Sandstone petrofacies in the northwestern sector of the Iberian Basin

During the most active rifting stages in the northwestern sector of the Iberian Basin (Cameros Basin and Aragonese Branch of the Iberian Range), thick sequences of continental clastic deposits were generated. Sandstone records from Rift cycle 1 (Permo-Triassic) and Rift cycle 2 (Late Jurassic-Early Cretaceous) show similarities in composition. Based on the most recent data, this paper describes sandstone petrofacies developed during both rifting periods. Six petrofacies can be distinguished: two associated with Rift cycle 1 (PT-1 and PT-2) and four with Rift cycle 2 (JC-1 to JC-4). All six petrofacies can be classifi ed as sedimentoclastic or plutoniclastic. Sedimentoclastic petrofacies developed during early rifting stages either through the recycling of pre-rift sediments or signifi cant palaeogeographical changes. These facies comprise a thin succession (<100 m) of clastic deposits with mature quartzose and quartzolithic sandstones containing sedimentary and metasedimentary rock fragments. Carbonate diagenesis is more common than clay mineral diagenesis. Sedimentoclastic petrofacies have been identifi ed in Rift cycle 1 (Saxonian facies, PT-1) and Rift cycle 2 (JC-1 and JC-3; Tithonian and Valanginian, respectively). In the absence of the pre-rift sedimentary cover, metasedimentoclastic petrofacies sometimes develop as a product of the erosion of the low- to medium-grade metamorphic substratum (Petrofacies JC-2, Tithonian-Berriasian). Plutoniclastic petrofacies were generated during periods of high tectonic activity and accompanied by substantial denudation and the erosion of plutonites. Forming thick stratigraphic successions (1000 to 4000 m), these feldspar-rich petrofacies show a rigid framework and clay mineral diagenesis. In Rift cycle 1, plutoniclastic petrofacies (PT-2) are associated with the Buntsandstein. This type of petrofacies also developed in Rift cycle 2 in the Cameros Basin (JC-4) from DS-5 to DS-8 (Hauterivian-Early Albian), and represents the main basin fi ll interval. Sedimentoclastic and plutoniclastic petrofacies can be grouped into three pairs of basic petrofacies. Each pair represents a ‘provenance cycle’ that records a complete clastic cycle within a rifting period. Petrofacies PT-1 and PT-2 represent the ‘provenance cycle’ during Rift-1. In the Cameros Basin, two provenance cycles may be discerned during Rift cycle 2, related both to the Tithonian-Berriasian and the Valanginian-Early Albian megasequences. Tectonics is the main factor controlling petrofacies. Other factors (e.g., maturation during transport, local supply) may modulate the compositional signatures of the petrofacies yet their main character persists and even outlines he hierarchy of the main bounding surfaces between depositional sequences in the intracontinental Iberian Rift Basin.

Texture and petrology of modern river, beach and shelf sands in a volcanic back‐arc setting, northeastern Japan

Abstract The focus in the present study is on characterizing spatial patterns of textural and petrological variabilities, and on evaluating mechanisms influencing the textural and petrological components of modern river, beach and shelf sands in a volcanically active back‐arc tectonic setting. Abashiri Bay and the surrounding area in eastern Hokkaido, Japan, has volcanic source land within a back‐arc region associated with subduction of the Pacific Plate beneath the Okhotsk (North American) Plate. A total of 41 river, beach and shelf sands were obtained for grain‐size and modal composition analyses. Multivariate analytical techniques of hierarchical cluster and principal component analyses were performed on the textural and petrological data for investigating relations among quantitative variables. On the basis of grain‐size data, four sedimentary zones were identified: zone I, palimpsest zone; zone II, relict zone; zone III, modern (proteric) zone; zone IV, coastal sedimentary zone. All sands are feldspatholithic and quartz‐deficient. The framework (quartz, feldspar and rock fragment) modal compositions were also classified into four clusters, A–D. The characteristic components of each cluster are as follows: cluster A, felsic volcanic rock fragments; cluster B, andesitic–basaltic volcanic rock fragments; cluster C, mixed or plagioclase; cluster D, sedimentary rock fragments. Almost all sands in western and central Abashiri Bay belong to cluster A, where the original compositions are influenced by Kutcharo pyroclastic flow deposits. Andesitic–basaltic lava and Neogene volcaniclastic and sedimentary rocks have a major influence on the compositions of shelf sands in eastern Abashiri Bay. The modal compositions are basically controlled by the source lithology. Compositional maturity (percentage of quartz to feldspar and rock fragments; Q/FR%) slightly increased, in order, from river (1.2), zone IV (coastal, 1.7), zone II (relict, 2.2), zone I (palimpsest, 3.6), to zone III (modern proteric, 7.0). Greater maturity in the recycled sediments is indicative of weathering under the sea or abrasion by transportation induced by sea‐level fluctuations, waves, or sea currents. Several controlling factors – (i) source lithological; (ii) mineralogical; (iii) climatic; and (iv) geomorphological controls – might still cause low maturity through all sedimentary zones other than the continental margin sands previously reported.

Provenance of the Hayang Group (Early Cretaceous) in the Yeongyang Subbasin, SE Korea and its bearing on the Cretaceous palaeogeography of SW Japan

The Lower Cretaceous Hayang Group in the Yeongyang Subbasin, SE Korea, was deposited in a forearc basin formed by the subduction of the Palaeo-Pacific (Izanagi) Plate underneath the East Asian continent. It is characterized by an alternating sequence of reddish sandstones and mudstones deposited in fluvial plain environments. Sandstone compositions upsequence reveal that the tectonic regimes of source terrains evolved successively from basement uplift to recycled orogen, to magmatic arc, and back to recycled orogen. Characteristically, sandstones contain abundant plagioclases but rare K-feldspars, except for sandstones derived from basement sources. Sandstones derived from recycled orogens are rich in sedimentary rock fragments, especially chert fragments, which contain radiolarian faunas similar to those of the Jurassic accretionary complex located in Southwest Japan. Minor continental arc volcanism contributed volcanic materials to the basin during the middle stage of the basin filling. During the deposition of the Hayang Group strike-slip fault movements were active due to the oblique subduction of the Izanagi Plate and caused strong pulsed uplift of the trench slope break areas, resulting in supply of accretionary complex-derived sediments. Considering a direct drainage connection between the uplifted accretionary complex in Southwest Japan and the Yeongyang Subbasin, current Cretaceous palaeogeographic reconstructions of the East Asian continental margin need to be reconsidered and it is suggested that the collage of tectonic blocks in Southwest Japan assembled in post-Early Cretaceous time.

Stratigraphy and sedimentology of a chert reservoir at the Precambrian-Cambrian Boundary: the Al Shomou Silicilyte, South Oman Salt Basin

ABSTRACT The Al Shomou Silicilyte is a unique source and reservoir rock found in the South Oman Salt Basin, where up to 400 m thick and several kilometers wide slabs of silicilyte are entrapped in salt domes at depths of 4 to 5 km. Discovered in the early 1990s, the play is characterized by light and sour oil, high overpressures (19.8 kPa/m), and a high-porosity, low-permeability microcrystalline silica matrix rich in organic matter, deposited around the Precambrian-Cambrian boundary. The palaeogeographic setting during the Al Shomou Silicilyte deposition was a restricted, marine intra-cratonic basin, surrounded by carbonate platforms. The basin was most likely segmented into structural highs and lows, with potential relief of more than 200 m. The deposits of the Al Shomou Silicilyte are stratigraphically ‘sandwiched’ by two regionally extensive shale units, the underlying ‘U’-Shale Formation and the overlying Thuleilat Shale Member. The basinal Al Shomou Silicilyte and the time-equivalent platform carbonates were deposited during a TST to HST in sea-level characterised by reduced siliciclastic input. Silicilyte formation was replaced by the regionally extensive lower Thuleilat Shale and Thamoud Carbonate. The Al Shomou Silicilyte is typically organic-rich, finely laminated and consists of 80–90% microcrystalline quartz with a crystal-size of 2–3 microns. Commonly, the silica crystals form sheet-like aggregates with high intercrystalline microporosity (up to 30%). Other authigenic phases in the silicilyte are pyrite, apatite, magnesite and illite/smectite clay minerals. Minor silt- and sand-sized detrital components are mica/illite flakes, K-feldspar, quartz and sedimentary rock fragments. The organic material averages 7% of the bulk rock volume and is finely disseminated and/or concentrated in laminae. No identifiable macro- or microfossils have been found to-date in the silicilyte. The textural and chemical characteristics suggest formation in a reducing, probably anoxic environment below wave base. The 300–400 m thickness and uniform character of the silicilyte indicate relatively stable conditions during deposition. The lack of recognizable biogenic components and only traces of detrital particles, suggest that the silicilyte is mostly composed of chemically formed silica. The microcrystalline silica could well be the result of a rapid inorganic nucleation of silica gel. The large volumes of silica necessary for silicilyte formation require a silica reservoir of reasonable magnitude, i.e. sea water. A model with a stratified water column is proposed, in which the oxic surface waters probably represented the site of organic productivity and carbonate deposition, and the deeper water at the thermocline/chemocline, the site of silica formation and bacterial mat growth. Silica gel formation may well have been linked to the biological cycle, i.e. mediated by sulfate-reducing bacteria (chemoautotrophs). Comparison with other siliceous deposits indicates that there are no known analogues. Models proposed for banded iron formations are probably the closest approximation to the type of processes involved in the formation of the silicilyte. Given its apparent chronostratigraphic position at the Precambrian-Cambrian boundary, it may well represent a global deepwater facies related to key events which are suggestive of extinction and faunal turnover at the Precambrian-Cambrian boundary.

Sand composition in an Iberian passive-margin fluvial course: the Tajo River

The Tajo River, the 10th largest river in Europe, drains part of the western passive margin of Europe that includes multiple tectonic elements of the Iberian plate. Modern fluvial sand composition in the Tajo River drainage basin reflects the nature of the source region, which lies in the central part of the Iberian Peninsula. Four fluvial petrographic provinces (A, B, C and D) can be established in the Tajo drainage basin, corresponding well with the four principal structural units drained: (1) the Iberian Range; (2) the Hesperian massif; (3) The Tertiary Tajo basin; and (4) the Neogene Santarem–Lisboa basin. Province A corresponds to the Tajo River head and is characterized by quartzolithic sedimenticlastic sands (Qm 67F 4Lt 29 and Rs 79Rg 3Rm 18). These sands have been derived from diverse Mesozoic siliciclastics and carbonates of the Iberian Range. Province B appears in the upper reaches of the Tajo River course and is quartzofeldspathic (Qm 57F 34Lt 9) with diverse rock fragments (Rs 34Rg 23Rm 43). Sources are Hercynian granitoids and metasediments and Neogene clastics and minor carbonates of the Tertiary Tajo basin. Province C extends along the middle course of the Tajo River with quartzofeldspathic metamorphiclastic sand modes (Qm 60F 33Lt 7; Rs 4Rg 21Rm 75). The sources are metamorphic rocks intruded by plutonites of the Hesperian Massif. Province D is quartzofeldspathic (Qm 55F 39Lt 6) with a dominance of phaneritic rock fragments (Rs 5Rg 33Rm 62) and corresponds to the lower reaches of the Tajo River, where siliciclastic deposits of the Neogene Santarem–Lisboa basin are the main sources. Sands plot on provenance-discrimination diagram (QmFLt) within the recycled-orogen field (Tajo River head) and continental-block fields (upper, middle and lower course). In addition, we have proven the usefulness of the RsRgRm diagram to discriminate the defined fluvial provinces, originating from heterogeneous parent-rock textures and mineralogy. Climate does not exert any strong influence on the petrogenesis of the Tajo River drainage basin sand, and erosion in the source areas may be described in terms of weathering-limited denudation regime. By contrast, mixing with tributary supplies is the main process that modifies composition in the Tajo River sand. The establishment of fluvial provinces related to the main bedrock structural units reflects the great relevance of tributaries from each province in the generation of the Tajo River sand and the low significance of inherited sandy load from previous provinces. The abundance of granitoid rock fragments (RsRgRm%Rg) in fluvial sand of both tributaries and main channel of the Tajo River drainage basin faithfully represents the relative abundance of granodiorite+monzogranite bedrock exposure in each subbasin. Metasedimentary and metamorphic bedrock outcrop area is overrepresented by metamorphic rock fragments (RsRgRm%Rm) in both sand from tributaries and from the Tajo main trunk river. This is manifested by differences of 18–23% between means of metasedimentary outcrop area and the RsRgRm%Rm mean in the sand from Provinces B and C+D, respectively. Sedimentary (mainly carbonate) outcrop area is underrepresented or not represented by sedimentary rock fragments (RsRgRm%Rs) in sand of the Tajo River. This underrepresentation causes a difference between means of sedimentary outcrop area and the RsRgRm%Rs mean of 21%, 17.1% and 10.7% in sands from Province A, B and C+D, respectively. This fact confirms the rapid loss of these grains during transport because of their labile nature.

Evidence for Multiple Recycling in Neoproterozoic through Pennsylvanian Sedimentary Rocks of the Central Appalachian Basin

Abstract The petrography of sandstones and neodymium (Nd) isotope compositions of mudstones from the central Appalachians indicate changing provenances during the late Neoproterozoic through Pennsylvanian evolution of eastern North America. In contrast, sandstones from all stratigraphic units are dominated by Grenvillian (950–1250 Ma) zircons. Neoproterozoic to Cambrian rift‐related sandstones are mostly feldspathic arenites and contain detrital zircons that are exclusively Grenvillian. The associated mudstones have remarkably uniform initial Nd isotope compositions ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\text...

Mobility of palladium in the surface environment: data from a regional lake sediment survey in northwestern Ontario

This paper describes part of a project to examine geochemical methods in exploration for Pd deposits in northwestern Ontario. Palladium is one of the platinum group elements (PGE) that is mobile in dilute surface waters as hydroxy complexes, a feature that can facilitate its use in exploration. Earlier studies around the Lac des Iles Pd mine have shown that Pd is widely dispersed in solution even from sulphide-poor mineralization ( Hattori &amp; Cameron 2004 ). The region that includes Lac des Iles has been covered by a lake sediment survey by the Ontario Geological Survey ( Dyer &amp; Russell 2002 ). These lake sediment data provide a further means of better understanding the geochemistry of Pd in the surface environment and its application to exploration for PGE deposits. Factor analysis of the lake sediment data reveals three associations: Factor 1, an association of Cr, Co, Ni, Pt and Pd; Factor 2, an association of Pd, S and As; and Factor 3, an association of Cu, Ag and Au. High scores for Factor 1 extend for 10 km down-ice from the mineralized mafic rocks of Archean age at Lac des Iles and reflect the dispersion of clastic material by ice sheets, supplemented by hydromorphic dispersion. High scores for Factor 2 are much more widespread and are spatially associated with Quaternary glaciofluvial deposits, notably eskers. A plot of Pd alone in lake sediments throughout the area largely reflects this association with eskers. The elemental assemblage of Factor 2 originates from the leaching of fragments of Nipigon diabase (Pd) and Sibley sedimentary rocks (S, As), both of Proterozoic age, contained within the eskers. The high permeability of the eskers permits soluble elements to be leached. All three elements, Pd, S and As, form neutral and anionic complexes and are not adsorbed by negatively-charged iron oxyhydroxide colloids and coatings, which restrict the dispersion of elements that dissolve as cations, such as Cu, Ni and Co. Factor 3, Cu, Ag and Au, occurs in areas of Proterozoic bedrock.

Palladium–copper-rich platinum-group element mineralization in Legris Lake mafic–ultramafic complex, Western Superior Province, Canada

The Legris Lake Complex is a northeast-trending, 7.3 km × 3.5 km, mafic-ultramafic intrusive complex located in the western Wabigoon Subprovince of the Archaean Superior Province. It is part of a circular series of mafic-ultramafic complexes, the most notable of which is the Lac des Iles Complex, ca 7 km northwest of the Legris Lake Complex, which hosts the Lac des Iles palladium mine. Four phases of exploration have been conducted on the Legris Lake Complex property since autumn, 1999, defining mineralized zones on surface and at depth, with drill-core intersections of up to 2.04 g/t Pd, 0.41 g/t Pt, 0.71 g/t Au, 0.42% Cu and 0.13% Ni over 9.95 m.The Legris Lake Complex consists of mostly gabbroic rocks but also contains lithologies ranging from anorthosite to wehrlite and a variety of igneous breccias. The gabbroic rocks vary from melanogabbro to porphyritic leucogabbro. Medium-grained, massive, biotite-rich leucogabbro is the predominant exposed variety and probably caps the complex. The north-western margin of the complex (2 km × 600 m), which contains all the known platinum-group element (PGE) mineralization, is characterized by heterolithic breccia with abundant fragments of sedimentary rocks and numerous gabbroic dykes and sills.The PGE mineralization in the Legris Lake Complex has different characteristics from many PGE deposits, where the ore occurs in sulphide-rich bodies at the base of mafic and ultramafic rocks. In the Legris Lake Complex PGE enrichment occurs in sulphide-poor (1-10 vol%, mostly <5 vol%), medium- to coarse-grained, porphyritic leucogabbro (termed 'Main Showing-type') hosted within zones of intense magmatic brecciation. The mineralized leucogabbro is mineralogically highly evolved and exhibits a sill-like form near the stratigraphic top of the complex. The mineralization consists of disseminated to blebby sulphides rimmed by epidote and disseminated magnetite. The mineralized rocks typically contain Cu ranging from 0.2 to 0.4 wt% and Ni from 0.07 to 0.12 wt%, with low Pt/Pd ratios (ca 0.2) and high Cu/Ni ratios (ca 3). The PGE contents display a positive correlation with those of Cu and Ni. The origin of the mineralization is best explained by preferential partitioning of PGE into an immiscible sulphide melt in evolved silicate magma after fractional crystallization of olivine and clinopyroxene. The immiscible separation of sulphide melt may have been aided by incorporation of silica and sulphide from adjacent sedimentary rocks. Formation of magnetite and hydrous minerals in the mineralized zones suggests that the sulphide melt had high oxygen/(oxygen + sulphur) ratios and high contents of volatiles, most probably reflecting a high oxidation state and volatile-rich nature of the parental magmas. This primary magmatic PGE mineralization was followed by minor redistribution of PGE by deuteric hydrothermal fluids released from the parental magma.

Magnetic susceptibility patterns and provenance of ice-rafted material at Feni Drift, Rockall Trough: implications for the history of the British–Irish ice sheet

Piston core ENAM 97-09 from Feni Drift, Atlantic margin west of Ireland provides a record of IRD input variability and provenance during the last ∼250,000 yrs. The IRD patterns can be related to Late Devensian (<30,000 yrs BP) ice advances and retreats at the southwestern sector of the British–Irish ice sheet (BIIS) known from terrestrial investigations. Inferences can be made about the BIIS evolution during earlier periods, which is poorly constrained from the land record. Detrital carbonate and (meta)sedimentary rock fragments represent two major IRD types, which were dominantly derived from Irish inland ice and the Irish Sea ice stream, respectively; additional evidence for the latter source is based on heavy mineral data. Input from other circum-North Atlantic ice sheets, notably the Laurentide ice sheet, is minor compared to British–Irish IRD, at least during periods of maximum expansion of the BIIS. This is consistent with the marginal location of the core site with reference to the main North Atlantic IRD belt. On the other hand, Laurentide IRD presumably accounts for minor IRD peaks at times of reduced ice volume in the British Isles. The main magnetic susceptibility peak during late isotope stage 2/Termination I can be related to IRD input from the Laurentide ice sheet, following early decay of the BIIS. A similar pattern is observed during oxygen isotope stage 6. These observations are consistent with recent provenance studies indicating that only Laurentide IRD carries a strong magnetic susceptibility signal.

Gold-Bearing Breccias of the Rain Mine, Carlin Trend, Nevada

The Rain mine includes two mined-out open pits that contained 36.4 t (1.17 Moz) gold, averaging 1.8 g/t gold, and underground reserves, including underground production, estimated at 4.9 t (157,000 oz) gold averaging 7.7 g/t. Rain orebodies are localized in a breccia complex within the hanging wall of the Rain fault and hosted within the Mississippian Webb Formation immediately overlying the contact with the Devonian Devils Gate Limestone. The ore host includes four texturally and genetically distinct breccia types: (1) crackle breccia; (2) hydrothermal breccia; (3) tuffisite with accretionary lapilli; and (4) collapse breccia. Crackle breccia forms a capping over multistage hydrothermal breccias that are cut by tabular- to pipe-shaped tuffisite dikes, with some containing accretionary lapilli. Pre- and synore hydrothermal breccias formed during at least three episodes of convective fluidization, followed by quartz-sulfide-barite cementation. High-grade gold was deposited as a late phase along the upper portion of the hydrothermal breccia mass and extended into the crackle breccia zone. Collapse breccias occur along the floor of the composite breccia mass and have irregular upper and lower contacts. The lower contact occurs on a dissolution boundary with the Devils Gate Limestone. Matrix-supported, heterolithic, hydrothermal breccias at Rain consist of sedimentary rock fragments composed of sandstone, siltstone, mudstone, limestone, and conglomerate. Some fragments contain as much as 8 percent introduced biotite in veinlets and/or fragment matrix replacements. The veinlets consist of euhedral quartz, biotite, sphalerite, and pyrite. Barite constitutes as much as 60 percent of the hydrothermal breccias in the form of fragments and as a cement to the breccias. Quartz replacement of fragments and as a breccia cement is pervasive. The total sulfide content in unoxidized ores is less than 5 volume percent. The Rain orebody resulted from five interpreted stages of development: (1) structural preparation along the right-lateral oblique Rain fault system and conjugate left-lateral oblique northeast-striking faults; (2) multiple episodes of hydrothermal breccia formation, with high-grade gold deposition immediately following the last brecciation event; (3) late channelized and fluidized rock fragments and fine clays forming tuffisite bodies with accretionary lapilli; (4) postmineral extensional reactivation of structures; and (5) collapse brecciation resulting from postore supergene acidic fluid ponding on and dissolving the upper Devils Gate Limestone. The age of the Rain orebody is poorly constrained. It is older than 22 Ma supergene alunite, but no maximum age constraints other than the Mississippian host rock are known.

Influence of petrographic composition/organic matter distribution of fluvial aquifer sediments on the sorption of hydrophobic contaminants

Quaternary fluvial valley deposits form major groundwater reservoirs for the drinking water supply, which are at the same time favorable urban and industrialized areas where contamination of the aquifers and their sediments occurs frequently. The transport behavior of dissolved hydrophobic organic contaminants (HOC) depends on the sorptive interactions (sorption isotherms, sorption kinetics) with the sediment grains and therefore on the sediment history in terms of source rocks (lithocomponents), petrographic composition, and depositional processes (lithofacies, e.g. grain-size distribution). The geological formations present in the source rock area already determine the variability of lithocomponents and thus the sorption capacities for organic contaminants in the valley (aquifer) sediments since they show distinct differences in organic carbon content (Corg) and nature of organic matter (both influence the sorption behavior of hydrophobic organic compounds). In general, lithocomponents originating from sedimentary rock have higher Corg contents and thus higher sorption capacities compared to metamorphic and magmatic components, and the quartz and feldspar minerals. The petrographic composition of the samples is grain-size-dependent but very similar within a specific lithofacies. Higher fractions of sedimentary rock fragments occur in the gravel-dominated lithofacies, which therefore have higher sorption capacities. In contrast to this, the sand grain-size fractions (sand facies) are, due to decay and transport processes (sediment maturity), enriched in stable lithocomponents such as quartz minerals which consequently lead to lower sorption capacities for HOC. Attention has to be drawn to the impact of strongly sorbing constituents such as Tertiary coal fragments (local source rock area) present in some of the sand samples.

Provenance of early Palaeozoic sandstones, southeastern Australia, Part 1: vertical changes through the Bengal fan-type deposit

Abstract Ordovician strata of the Lachlan Fold Belt in southeastern Australia are dominated by a turbidite succession that in its undeformed state was comparable in size to the modern-day Bengal Fan. The ancient fan was derived from the northern part of the Cambrian Ross–Delamerian orogenic belt that formed at a convergent boundary between plates containing Gondwanaland and part of the palaeo-Pacific Ocean, respectively. Substrate to the submarine fan consisted of mafic volcanic rocks with an immature island arc affinity similar to the Eocene Izu–Mariana–Bonin system. Sandstone compositions, determined from several stratigraphically constrained sections, demonstrate a uniform framework composition dominated by monocrystalline quartz with less abundant feldspar, polycrystalline quartz, metamorphic rock fragments, sedimentary rock fragments and mica. These indicate derivation from a low-grade metamorphic terrain. Up-sequence variation is limited to decreasing feldspar and lithic fragments and increasing quartz, indicating increased weathering and lowering of topography, in the source terrain over time. 25% of counted sandstones in the Late Ordovician interval contain a small component of mafic–intermediate volcanic rock fragments that accompanied a change in palaeocurrent direction from eastwards to northwards. In the Early Silurian, the Ordovician sequence was disrupted by the formation of several major deformation zones. Between these zones, developed several deep-marine sedimentary basins that contain a continuous succession from the Ordovician into the Early Silurian. Early Silurian sandstone is similar in composition to underlying Early Ordovician rocks except that the feldspar component is generally absent. Early Silurian sandstone was derived from uplifted portions of the Ordovician sequence and includes texturally inverted types with sandstone containing a bimodal mixture of coarse well-rounded quartz and fine angular quartz.

Long Term Sorption Kinetics of Phenanthrene in Aquifer Materials

Most aquifer materials are heterogeneous in terms of grain size distribution and petrography. To understand sorption kinetics, homogeneous subfractions, either separated from heterogeneous sands and gravels (lithocomponents) or fragments of fresh rocks, have to be studied. In this paper we present data on long-term sorption kinetics of phenanthrene for homogeneous samples consisting of one type of lithocomponents or fresh rock fragments in different grain sizes. Diffusion rate constants were determined in batch experiments using a numerical model for retarded intraparticle pore diffusion and correlated to grain size and intraparticle porosity of the lithocomponents. Sorption isotherms were nonlinear for all samples investigated (Kleineidam et al. (1)). The numerical model described the sorption kinetics very well for coarse sand and gravels. Tortuosity factors, which were obtained as final fitting factors, agreed with Archie's law predictions based on the intraparticle porosity. The dependency of sorptive uptake on grain size revealed that for smaller grains intrasorbent diffusion may become significant. This is attributed to relatively large particulate organic matter (POM) within the sedimentary rock fragments. Specifically, charcoal and coal particles, which were found in some of the sandstones, controlled the sorptive uptake rates.

Effect of Permo-Carboniferous Climate on Illite-Smectite, Haushi Group, Sultanate of Oman

AbstractThe Late Westphalian to Artinskian Haushi Group in the Sultanate of Oman consists of the glaciogenic Al Khlata Formation and the Gharif Formation which contains marginal marine, coastal plain, and fluvial sediments. The sequence was deposited during a global-warming event following the Permo-Carboniferous glaciation of Gondwana. Because of a varied subsidence history, these sediments range from the surface in the SE to almost 5000 m in the NW of the basin.Mixed-layer illite-smectite (I-S) is an important constituent of the &lt;2 µm size fraction of sandstone and shale samples in both formations at all depths. Different starting compositions lead to three distinct trends of illite layers in I-S versus temperature for different sedimentary environments and paleoclimatic conditions. The starting compositions of I-S at the surface range from an ordered I-S in the Al Khlata Formation to smectite-rich in the Upper+Middle Gharif members.Physical, chemical and environmental factors were investigated as causes for the different starting compositions of I-S. Both formations share an identical burial history, paragenesis, thermal evolution, and source of detrital material. They differ only in environmental conditions during sedimentation. Thus, the variation in starting composition of I-S appears to be best explained by distinct weathering conditions during sedimentation of the three units. In particular, the expected low intensity of chemical weathering during glaciogenic conditions is marked by the presence of higher amounts of unstable volcanic and sedimentary rock fragments in the Al Khlata Formation.

Geochemistry and provenance of the Middle Ordovician Austin Glen Member (Normanskill Formation) and the Taconian Orogeny in New England

The Austin Glen Member of the upper Middle Ordovician Normanskill Formation is a sandstone‐shale flysch succession deposited in the foreland of the Taconian Orogen. Petrographic, major and trace element, and Nd–Pb isotopic data provide substantial constraints on its provenance. Lack of K‐feldspar and paucity of plagioclase, in addition to the dominance of sedimentary rock fragments, indicate that the source was dominated by recycled, sedimentary components. Major and trace element data support this conclusion and indicate that the provenance of both shales and sandstones was the same. No evidence of an ophiolitic or volcanic component was observed. Interpretation of Nd isotopic characteristics are complicated by a partial resetting of the Nd isotope system at about the time of sedimentation but indicate that the provenance of the Austin Glen Member had a long‐term history of light rare earth element (LREE) enrichment (average TDM = 1·8 Ga). Furthermore, Nd isotopic compositions are extremely homogeneous (ɛNd = –8·1 ± 0·6; 1 s.d.; n = 23) at 450 Ma, the approximate depositional age, indicating either a single source or very well‐mixed sources. 207Pb/204Pb ratios are variable but within the range of Pb isotopic compositions typically described as Grenvillian. The range of 207Pb/204Pb is greater than expected for the range of 206Pb/204Pb and suggests an additional component of Pb, possibly introduced during diagenesis. The immediate source of the Austin Glen Member may have been the accretionary prism that developed as older sediments of the Laurentian margin were scraped off the basin floor, incorporated within the accretionary prism and shed into the basin. No evidence indicating the arrival of an undifferentiated island arc or continental fragment during the Taconian Orogeny has been found. The data acquired during this study can be explained almost exclusively by Grenville Province source components but with possible additional contributions from older Laurentian terranes and Late Proterozoic rift volcanics that are not readily quantified but likely to have been minor. Accordingly, we conclude that the Taconian Orogeny in New England involved either: (1) a continental arc that involved exclusively Laurentia; (2) collision of a continental block with identical geochemical characteristics as Laurentia; or (3) essentially no detritus from any exotic colliding block (island arc or continental fragment) reached the foreland basin at the time of Austin Glen deposition.

Provenance of chert in the Permo-Triassic Sydney Basin, Australia: oxygen isotopic evidence

Detrital chert in chert-rich Permo-Triassic sandstones in the Sydney Basin, Australia, has been previously thought to have been derived from marine bedded chert in the New England Fold Belt (NEFB). This marine bedded chert is associated with volcanic, sedimentary, and metasedimentary rocks. Though volcanic detritus is common in these chert-rich sandstones, sedimentary and metasedimentary rock fragments are rare. This rarity raises the question of whether the source of chert grains in these sandstones was, indeed, the marine bedded chert in the NEFB or whether there was an alternative source. Without confirming the origin of chert, the provenance interpretation seems equivocal. Since the conventional petrographic technique proved to be inadequate in determining the origin of chert, it is necessary to use a technique that can discriminate chert of one type from another. The oxygen isotopic composition of chert from different environments shows characteristic δ 18O values and, therefore, is used in this study to differentiate chert based on their isotopic signatures. The oxygen isotopic composition of chert grains in Permo-Triassic sandstones in the Sydney Basin was determined and the δ 18O values range between 12.7 and 20.8%c (SMOW). On the other hand, the isotopic composition of metamorphosed marine chert in the NEFB ranges between 23.0 and 25.2%. Texturally, detrital chert shows volcanic relicts while the bedded marine cherts show foliated texture. Both isotopic composition and texture are consistent with the interpretation that the primary source of the detrital chert was not from the NEFB but from an alternative source: chemically weathered volcanic rock.

Sandstone composition and provenance of the Rotliegend of the NE German Basin

The NE German Basin contains an approximately 2.5 km thick Rotliegend sedimentary section, subdivided into 4 formations. These were, deposited following the cessation of the Late Carboniferous/Early Permian volcanic episode and a ca. 20 Ma period of erosion and non-deposition. Initial deposition was confined to two areas, but with increasing thermal subsidence the basin extended to the south. Petrographic (190 sandstones) and geochemical (48 sandstones and mudstones) analyses suggested that the sands are quartz-rich and are derived largely from cratonic or recycled sources. Volcanic and sedimentary rock fragments form most of the labile grains. The succession was deposited in a relatively stable tectonic environment with sediment input derived largely from the lower and upper Palaeozoic strata of the Variscan hinterland and the Upper Carboniferous and Lower Permian volcanics. Volcanic input increases with younger stratigraphic units. This is interpreted as resulting from the unroofing of a more basic source.

The Rotliegend of the NE German Basin; background and prospectivity

The NE German Basin contains approximately 2.5 km of Rotliegend sediments, subdivided into four formations. These were deposited following the cessation of the Late Carboniferous/Early Permian volcanic episode and a c. 20 Ma period of erosion and non-deposition. Initial deposition was confined to two areas, but with increasing thermal subsidence the basin extended to the south. Petrographic and geochemical analyses suggest that the sediments are derived largely from cratonic or recycled sources. Volcanic and sedimentary rock fragments predominate. The succession was deposited in a relatively stable tectonic environment with sediment input derived largely from the Lower and Upper Palaeozoic strata of the Variscan hinterland and the Late Carboniferous and Lower Permian volcanics. Hydrocarbon prospectivity is confined to two main areas, with traps being predominantly stratigraphic and subtle.