Arkosic Sandstones Research Articles

ABSTRACT: Depth-porosity relationships in Eocene through Miocene arkosic sandstones, San Joaquin basin, California

ABSTRACT: Depth-porosity relationships in Eocene through Miocene arkosic sandstones, San Joaquin basin, California

Assessing the past thermal and chemical history of fluids in crystalline rock by combining fluid inclusion and isotopic investigations of fracture calcite

Fluid inclusion studies combined with the isotope geochemistry of several generations of fracture calcite from the Olkiluoto research site, Finland, has been used to better understand the past thermal and fluid history in the crystalline rock environment. Typically, fracture mineral investigations use O and C isotopes from calcite and an estimate of the isotopic composition of the water that precipitated the calcite to perform δ 18O geothermometry calculations to estimate past temperature conditions. By combining fluid inclusion information with calcite isotopes, one can directly measure the temperature at which the calcite formed and can better determine past fluid compositions. Isotopic, petrologic and fluid inclusion studies at the Olkiluoto research site in Finland were undertaken as part of an investigation within the Finnish nuclear waste disposal program. The study revealed that four fluids were recorded by fracture calcites. From petrologic evidence, the first fluid precipitated crystalline calcite at 151–225°C with a δ 13C signature of −21 to −13.9‰ PDB and a δ 18O signature of 12.3–13.0‰ SMOW. These closed fracture fillings were found at depths greater than 500 m and were formed from a high temperature, low salinity, Na–Cl fluid of possible meteoric water altered by exchange with wallrock or dilute basinal origin. The next fluid precipitated crystalline calcite with clay at 92–210°C with a δ 13C signature of −2.6 to +3.8‰ PDB and a δ 18O signature of 19.4–20.7‰ SMOW. These closed fracture fillings were found at depths less than 500 m and were formed from a moderate to high temperature, low to moderate salinity, Na–Cl fluid, likely of magmatic origin. The last group of calcites to form, record the presence of two distinct fluid types. The platy (a) calcite formed at 95–238°C with a δ 13C signature of −12.2 to −3.8‰ PDB and a δ 18O signature of 14.9–19.6‰ SMOW, from a high temperature, low salinity, Na–Cl fluid of possible magmatic origin. The platy (b) calcite formed at 67–98°C with a δ 13C signature of −13.0 to −6.2‰ PDB and a δ 18O signature of 15.1–20.1‰ SMOW, from a low temperature, high salinity, Ca–Na–Cl fluid of possible basinal brine origin. The two calcites are related through a mixing between the two end members. The source of the fluids for the platy grey (a) calcites could be the olivine diabase dykes and sills that cut through the site. The source of fluids for the platy (b) calcites could be the Jotnian arkosic sandstone formations in the northern part of the site. At the Olkiluoto site, δ 18O geothermometry does not agree with fluid inclusion data. The original source of the water that forms the calcite has the largest effect on the isotopic signature of the calcites formed. Large isotopic shifts are seen in any water by mineral precipitation during cooling under rock–water equilibrium fractionation conditions. Different calcite isotopic signatures are produced depending on whether cooling occurred in an open or closed system. Water–rock interaction, at varying W/R ratios, between a water and a host rock can explain the isotopic shifts in many of the calcites observed. In some cases it is possible to shift the δ 18O of the water by +11.5‰ (SMOW) using a realistic water–rock ratio. This process still does not explain some of the very positive δ 18O values calculated using fluid inclusion data. Several other processes, such as low temperature recrystallization, boiling, kinetic effects and dissolution of calcite from fluid inclusion walls can affect isotopic signatures to varying degrees. The discrepancy between fluid inclusion data and δ 18O geothermometry at the Olkiluoto site was most likely due to poor constraint on the original source of the water.

Using artificial intelligence to predict permeability from petrographic data

Petrographic data collected during thin section analysis can be invaluable for understanding the factors that control permeability distribution. Reliable prediction of permeability is important for reservoir characterization. The petrographic elements (mineralogy, porosity types, cements and clays, and pore morphology) interact with each other uniquely to generate a specific permeability distribution. It is difficult to quantify accurately this interaction and its consequent effect on permeability, emphasizing the non-linear nature of the process. To capture these non-linear interactions, neural networks were used to predict permeability from petrographic data. The neural net was used as a multivariate correlative tool because of its ability to learn the non-linear relationships between multiple input and output variables. The study was conducted on the upper Queen formation called the Shattuck Member (Permian age). The Shattuck Member is composed of very fine-grained arkosic sandstone. The core samples were available from the Sulimar Queen and South Lucky Lake fields located in Chaves County, New Mexico. Nineteen petrographic elements were collected for each permeability value using a combined minipermeameter–petrographic technique. In order to reduce noise and overfitting the permeability model, these petrographic elements were screened, and their control (ranking) with respect to permeability was determined using fuzzy logic. Since the fuzzy logic algorithm provides unbiased ranking, it was used to reduce the dimensionality of the input variables. Based on the fuzzy logic ranking, only the most influential petrographic elements were selected as inputs for permeability prediction. The neural net was trained and tested using data from Well 1–16 in the Sulimar Queen field. Relying on the ranking obtained from the fuzzy logic analysis, the net was trained using the most influential three, five, and ten petrographic elements. A fast algorithm (the scaled conjugate gradient method) was used to optimize the network weight matrix. The net was then successfully used to predict the permeability in the nearby South Lucky Lake field, also in the Shattuck Member. This study underscored various important aspects of using neural networks as non-linear estimators. The neural network learnt the complex relationships between petrographic control and permeability. By predicting permeability in a remotely-located, yet geologically similar field, the generalizing capability of the neural network was also demonstrated. In old fields, where conventional petrographic analysis was routine, this technique may be used to supplement core permeability estimates.

Internally-sourced quartz cement due to externally-derived CO2 in sub-arkosic sandstones, North Sea

The Upper Jurassic sub-arkosic turbidite sandstones of the Magnus oil field, UK North Sea, are dominated by quartz, ankerite and kaolinite cements that all grew immediately prior to, and during, the early stages of oil-filling. The sandstones have no evidence of siliceous bioclasts or pressure dissolution yet contain ≤15% quartz cement with relatively more in the water leg than in the oil leg. Petrographic analysis revealed that the sandstones have undergone major K-feldspar alteration, especially in the water leg. XRF whole-rock data show that sandstones have uniform Si/Si+Al throughout the reservoir indicating that silica is not imported into the sandstone during quartz cementation. Carbon isotope data from the ankerite require an influx of organically derived CO2. Geochemical modelling of a CO2-influx into arkosic sandstone suggests that quartz, ankerite and kaolinite cements are likely to be genetically related. Reaction of K-feldspar with water at low pH will produce quartz and kaolinite. Low pH resulted from CO2-influx and was maintained through ankerite precipitation. The reaction led to bulk loss of potassium (most likely to the surrounding mudrocks). K-feldspar dissolution is thus the result of an influx of externally (source rock) derived CO2 although the silica in the quartz cement is derived internally in the sandstone. While there is no need to invoke large scale-import of silica-laden waters, quartz cementation is the result of fluid influx.

Geochemical modelling of diagenetic reactions in a sub-arkosic sandstone

AbstractA reaction path model was constructed in a bid to simulate diagenesis in the Magnus Sandstone, an Upper Jurassic turbidite reservoir in the Northern North Sea, UKCS. The model, involving a flux of source rock-derived CO2 into an arkosic sandstone, successfully reproduced simultaneous dissolution of detrital K-feldspar and growth of authigenic quartz, ankerite and illite. Generation of CO2 occurred before and during the main phase of oil generation linking source rock maturation with patterns of diagenesis in arkosic sandstones and limiting this type of diagenesis to the earlier stages of oil charging. Independent corroborative evidence for the model is provided by formation water geochemical data, carbon isotope data from ankerite and produced gas phase CO2 and the presence of petroleum inclusions within the mineral cements. The model involves a closed system with respect to relatively insoluble species such as SiO2 and Al2O3 but is an open system with respect to CO2. There are up to seven possible rate-controlling steps including: influx of CO2, dissolution of K-feldspar, precipitation of quartz, ankerite and illite, diffusive transport of SiO2 and Al2O3 from the site of dissolution to the site of precipitation and possibly the rate of influx of Mg2+ and Ca2+. Given the large number of possible controls, and contrary to modern popular belief, the rate of quartz precipitation is thus not always rate limiting.

Was the Late Triassic orogeny in Turkey caused by the collision of an oceanic plateau?

Abstract A belt of Late Triassic deformation and metamorphism (Cimmeride Orogeny) extends east-west for 1100 km in northern Turkey. It is proposed that this was caused by the collision and partial accretion of an Early-Middle Triassic oceanic plateau with the southern continental margin of Laurasia. The upper part of this oceanic plateau is recognized as a thick Lower-Middle Triassic metabasite-marble-phyllite complex, named the Nilüfer Unit, which covers an area of 120 000 km 2 with an estimated volume of mafic rocks of 2 × 10 5 km 3 . The mafic sequence, which has thin stratigraphic intercalations of hemipelagic limestone and shale, shows consistent within-plate geochemical signatures. The Nilüfer Unit has undergone a high-pressure greenschist facies metamorphism, but also includes tectonic slices of eclogite and blueschist with latest Triassic isotopic ages, produced during the attempted subduction of the plateau. The short period for the orogeny (< 15 Ma; Norian-Hettangian) is further evidence for the oceanic plateau origin of the Cimmeride Orogeny. The accretion of the Nilüfer Plateau produced strong uplift and compressional deformation in the hanging wall. A large and thick clastic wedge, fed from the granitic basement of the Laurasia, represented by a thick Upper Triassic arkosic sandstone sequence in northwest Turkey, engulfed the subduction zone and the Nilüfer Plateau.

The petrographic and geomechanical properties of some sandstones from the Newspaper Member of the Natal Group near Durban, South Africa

Sandstones are by far the most important rock type in the Natal Group, which is mainly a terrestrial succession formed during Ordovician–Silurian times. The most notable member of the group is the Newspaper Member from which sample material was obtained. Most of the material sampled was fine to medium grained arkosic sandstone. They are well cemented with generally low porosity. Accordingly, sandstones of the Newspaper Member tend to be very strong and of low deformability. Few of the petrological properties of these sandstones have a notable influence on the mechanical properties. However, increasing clay content tends to lower the unconfined compressive strength whilst increasing quartz content tends to enhance it. On the other hand, many of the mechanical properties are interrelated. Significant or highly significant relationships occur amongst the strength parameters, and between hardness and strength. Young's modulus also has significant relationships with strength and hardness. Another property which has a significant influence on strength and deformability is porosity.

Geological and Isotopic Constraints on the Metallogenic Evolution of the Proterozoic Sediment-Hosted Pb-Zn (Ag) Deposits of Brazil

Integrated studies of seven Proterozoic sediment-hosted, Pb-Zn-Ag sulfide deposits of Brazil, permit the estimation of the age of the hosting sequence and the mineralization, the nature of the sulfur and metal sources, the temperature range of sulfide formation and the environment of deposition. These deposits can be classified into three groups, according to their ages. (a) Archean to Paleoproterozoic: the Boquira deposit, in Bahia state, consists of stratiform massive and disseminated sulfides hosted by parametamorphic sequences of grunnerite-cummingtonite+magnetite that represent a silicate facies of the Boquira Formation (BF). Lead isotope data of galena samples indicate a time span between 2.7 and 2.5 Ga for ore formation, in agreement with the stratigraphic position of the BF. The relatively heavy sulfur isotope compositions for the disseminated and stratiform sulfides (+8.3 to +12.8 ‰ CDT)suggest a sedimentary source for the sulfur. (b) Paleo to Mesoproterozoic: stratiform and stratabound sulfides in association with growth faults are present in the Canoas mine (Ribeira, in Paraná state) and in the Caboclo mineralization (Bahia state). They are hosted by calcsilicates and amphibolites in the Canoas deposit, whereas in the Caboclo area the mineralization is associated with hydrothermally altered dolarenites at the base of the 1.2 Ga Caboclo Formation. The interpreted Pb-Pb age of the Canoas mineralization is coeval with the 1.7 Ga host rocks. Sulfur isotopic data for Canoas sulfides (+1.2 to +16 ‰ CDT) suggest a sea water source for the sulfur. The range between −21.1 and +8.8 ‰ CDT for the Caboclo sulfides could suggest the action of bacterial reduction of seawater sulfates, but this interpretation is not conclusive. (c) Neoproterozoic: stratiform and stratabound sulfide deposits formed during the complex diagenetic history of the host carbonate rocks from the Morro Agudo (Bambui Group), Irecê and Nova Redenção (Una Group), yield heavy sulfur isotope values (+18.9 to +39.4 ‰ CDT). The uniform heavy isotope composition of the barites from these deposits (+25.1 to +40.9 ‰) reflect their origin from Neoproterozoic seawater sulfates. The late-stage, and most important, metallic concentrations represent sulfur scavenged from pre-existing sulfides or from direct reduction of evaporitic sulfate minerals. Lead isotope data from the Bambui Group suggest focused fluid circulation from diverse Proterozoic sediment sources, that probably was responsible for metal transport to the site of sulfide precipitation. (d) Late Proterozoic to Early Paleozoic: lead-zinc sulfides (+pyrite and chalcopyrite) of Santa Maria deposits, in Rio Grande do Sul, form the matrix of arkosic sandstones and conglomerates, and are closely associated with regional faults forming graben structures. Intermediate volcanic rocks are intercalated with the basal siliciclastic members. Lead isotope age of the mineralization (0.59 Ga) is coeval with the host rocks. Sulfur isotopic values between −3.6 and +4.1 are compatible with a deep source for the sulfur. Geological, petrographic and isotopic data of the deposits studied suggest that they were formed during periods of extensional tectonics. Growth faults or reactivated basement structures probably were responsible for localized circulation of metal-bearing fluids within the sedimentary sequences. Sulfides were formed by the reduction of sedimentary sulfates in most cases. Linear structures are important controls for sulfide concentration in these Proterozoic basins.

Petrology and geochemistry of diagenetically altered tuffaceous rocks from the Middle Triassic of central Spain

ABSTRACT Petrographical and geochemical analysis of altered tuffaceous rocks of the Middle Triassic from the Iberian Range in Central Spain shows that pedogenic processes and subsequent diagenesis have strongly modified the original mineralogy and textures. These rocks are related to a magmatic episode that occurred at the Buntsandstein-Muschelkalk unconformity in this region, during the evolution of the Iberian Trough. The tuffaceous rocks accumulated in saline ponds in an open and terminal flood-plain setting. Their alteration is typified by neoformation of K-feldspar and illite. Precipitation of chert and carbonate cements and replacements were also important processes, resulting in intercalated chert beds and nodules. Much of the silica appears to be derived from extensive devitrification and weathering of glassy volcanic ash. X-ray diffraction analysis, microscopic observations, and EDX microanalysis reveal that the chert consists of various types of chalcedony, tridymite, micro and megaquartz, illite, K-feldspar, and small amounts of albite. In some of the nodules dolomite-ankerite is also common, enclosing anhydrite and barite relicts, which are partially silicified. Different levels of ammonium enrichment have been found in each petrofacies type: tuffs and tuffaceous rocks contain from 27 to 326 ppm (mean = 139) of NH4+. Underlying arkosic sandstones with authigenic K-feldspar contain NH4+ ranging from 48 ppm to 191 ppm (mean = 124). The NH4+ was probably derived from decay of organic matter in the evaporitic environment of deposition and its incorporation into feldspar and illite as they formed from unstable volcanic glass, sanidine, plagioclase, and vitric volcanic fragments. This transformation could have occurred via the intermediate phases of smectite and zeolites. The tuffaceous rocks also have a high content of B, ranging from 61 ppm to 170 ppm (mean = 101). However, high levels of B are not positively correlated with high levels of NH4+ in the samples. Both B and NH4+ can be contained in either K-feldspar or illite, but it is not possible to know how much is present in each mineral without separating the minerals. Comparison of spidergrams of the tuffaceous rocks with those in the underlying arkosic sandstones indicates similar patterns with positive anomalies in La, Sm, and Y, and negative in Nb, P, Ti, and Yb, although the tuffaceous rocks are slightly richer in total REE. These data suggest that the distribution of REE must also be controlled by weathering and diagenetic reactions, and cannot be used for discrimination between volcanic and nonvolcanic sandstones.

Infilling history of a Neoproterozoic intracratonic basin: Nd isotope provenance studies of the Uinta Mountain Group, Western United States

Neodymium isotope and selected trace element concentration data were obtained from 27 samples of shales and arkosic sandstones comprising the Neoproterozoic Uinta Mountain Group (UMG) in north-eastern Utah and northwestern Colorado to determine the provenance of siliciclastic detritus deposited in this region and to reconstruct aspects of the Proterozoic geologic history of the southern margin of the Archean Wyoming Province. In the east lobe of the Uinta Mountains, coarse-grained components of the Jesse Ewing Canyon Formation, the oldest recognized formation of the UMG, have measured ϵ Nd values of −23 to −28 and were most probably derived from Archean basement of the Wyoming Province to the north. Fine-grained sediment in the southern portions of the Jesse Ewing Canyon Formation and shales and quartz arenites from overlying undifferentiated UMG all contain at least a component of detritus derived from Proterozoic crust. In the west lobe, the oldest members of the UMG (the Red Castle Lake, Gilbert Peak and Moosehorn Lake Formations) and arkosic arenite comprising the northern portions of the overlying Mount Watson Formation were also derived exclusively from Archean sources. However, as in the east lobe, the more mature sandstones that dominate the southern sections of the Mount Watson Formation have isotopic compositions that require a significant Proterozoic-crust derived detrital component. Our data support a model in which sedimentary rocks with an Archean provenance represent the preserved remnants of alluvial fans marking the northern margin of the basin in which the UMG was deposited. A source for these sediments within the Archean Wyoming Province is consistent with the high light rare-earth element (LREE) and Th abundances of finegrained, low ϵ Nd, sedimentary rocks, given that Late Archean granitic rocks within the Wyoming province have similar trace element characteristics. Sediments with Proterozoic components in both the east and west lobes were apparently deposited from a fluvial system that transported sediment along the basin axis. The Proterozoic detrital component in these sediments originated either through direct erosion of the Colorado Province in the Neoproterozoic, or through erosion of ‘mixed’ Archean and Proterozoic crust located to the east of the UMG basin that originally formed during the Paleoproterozoic accretion of the Colorado Province to the southern margin of the Wyoming craton.

Reservoir architecture of the upper Sherwood Sandstone, Wytch Farm field, southern England

Abstract The Sherwood Sandstone Group reservoir in the Wytch Farm field comprises a c. 150 m thick succession of arkosic sandstones deposited in a variety of fluvial, lacustrine and aeolian depositional systems. These systems show at least three orders of facies variability, which are interpreted to be the depositional response to climatic changes. These comprise a first-order evolutionary trend over the entire Sherwood Sandstone Group from perennial braidplain to ephemeral sheetflood systems to ephemeral lacustrine conditions. This trend culminated in deposition of the Mercia Mudstone Group, and reflects a long-term waning of sand supply and increasing ‘flashiness’ of the fluvial system. This trend is further subdivided into second-order cycles defined by five areally widespread floodplain and lacustrine deposits containing minimal development of fluvial sandstones. These represent widespread, episodic reductions in fluvial sediment supply and rising base level during more ‘humid’ climatic conditions. These horizons form the basis for the reservoir layering scheme. Each floodplain episode is increasingly more mud-rich upwards through the Sherwood section, and the sand-rich fluvial packages between become systematically more ephemeral in character. Third-order cycles are defined by thin (<2 m), but alreally widespread floodplain and lacustrine horizons which are most readily identifiable in the upper half of the Sherwood section. The sandstones between these cycles are composed of aeolian and sheetflood deposits, but are incised by coarse-grained multistorey-multilateral channel deposits. The incisions are interpreted to be the result of fluvial erosion during dry climatic conditions when lake levels fell and the alluvial plain was devegetated. These incised fluvial deposits form the principal producing intervals in the upper part of the reservoir, particularly in the eastern part of the field. Higher frequency stratigraphic cycles are locally expressed by variations in ephemeral lake levels, palaeosol development and episodic development of wind-blown sand patches. At outcrop, the stratigraphically equivalent Otter Sandstone Formation ( c. 100 km to the west) shows comparable evolutionary patterns, albeit with a subtly different facies make-up. The recognition of a hierarchy of climatically driven cycles within the reservoir permits high-resolution correlation and the recognition of subtle, but important, changes in sandbody geometry and connectivity within successive cycles.

Tectonostratigraphy of the Moine Supergroup: a synthesis

The Neoproterozoic (early Riphean) Moine Supergroup crops out extensively in the Scottish Caledonides north of the Great Glen and is characterized by structural complexity and monotonous siliciclastic lithology with a lack of biostratigraphic control. Despite these problems, it has become possible to combine locally defined successions into a regional stratigraphic framework. This permits an evaluation of the tectonic setting of Moine deposition, in a subsiding tract that became the Scottish part of the Laurentian margin when Iapetus opened in the Vendian. Two major rift-basins are inferred, each of half-graben type, controlled by east-facing normal faults. The earlier Morar Group basin received a thick fill of dominantly shallow-marine arkosic sandstones, the Upper Morar Psammite forming a major regressive sequence. The Glenfinnan Group consists of mixed and muddy deposits that are interpreted partly as distal equivalents of the Morar Group, partly as a post-Morar transgressive thermal re-equilibration sequence. The Loch Eil Group, dominated by shallow marine arkosic and siliceous psammites, was deposited in a second, more easterly rift-basin that was subsequently juxtaposed with the Morar basin during Caledonian thrusting. Sediment dispersal in the major sandy sequences was northwards, and there is some evidence that provenance was, at least in part, from a mid-Proterozoic basement terrain to the south. The extension-dominated depositional setting of the Moine Supergroup, and basin geometry, are similar to those inferred for the other major Riphean clastic sequences of Scotland, the ‘Torridonian’ of the foreland and the Grampian Group to the southeast. The latter is followed by Upper Riphean transgressive strata of the Appin Group. Scottish Riphean stratigraphy records a major pre-Iapetan cycle of lithospheric extension and thermal recovery, with no regional scale unconformities yet identified that might constitute evidence for contemporary orogenesis.

Stable isotope geochemistry and diagenetic mineralization associated with the Tono sandstone-type uranium deposit in Japan

The Tono sandstone-type uranium mine area, middle Honsyu, Japan is composed of Miocene lacustrine sedimentary rocks in the lower part (18–22 Ma) and marine facies in the upper part (15–16 Ma). Calcite and pyrite occur as dominant diagenetic alteration products in these Neogene sedimentary rocks. The characteristics of calcite and pyrite differ significantly between lacustrine and marine facies. Abundant pyrite, calcite, organic matter, and small amounts of marcasite or pyrrhotite occur in the lacustrine facies, whereas small amounts of calcite and framboidal pyrite, organic matter and no marcasite or pyrrhotite are found within the marine units. The δ13C values of calcite in the lacustrine deposits are low (−19 to −6‰ PDB) but those in marine formation are high (−11 to +3‰). This implies that the contribution of marine carbonate is larger in upper marine sedimentary rocks, and carbon in calcite in the lower lacustrine formation was derived both from oxidation of organic matter and from dissolved marine inorganic carbon. The δ34S values of framboidal pyrite in the upper marine formation are low (−14 to −8‰ CDT), indicating a small extent of bacterial seawater sulfate reduction, whereas those of euhedral-subhedral pyrite in the lower lignite-bearing arkose sandstone are high (+10 to +43‰), implying a large extent of closed-system bacterial seawater sulfate reduction. The δ34S and δ13C data which deviate from a negative correlation line toward higher δ13C values suggest methanogenic CO2 production. During diagenesis of the lacustrine unit, large amounts of euhedral-subhedral pyrite were formed, facilitated by extensive bacterial reduction of seawater sulfate with concomitant oxidation of organic matter, and by hydrolysis reactions of organic matter, producing CH4 and CO2. Uranium minerals (coffinite and uraninite) were also formed at this stage by the reduction of U6+ to U4+. The conditions of diagenetic alteration within the lacustrine deposits and uranium mineralization is characterized by low Eh in which nearly equal concentrations of CH4 and HCO3 − existed and reduced sulfur species (H2S, HS−) are predominant among aqueous sulfur species, whereas diagenetic alteration of the marine formations was characterized by a predominance of SO4 2− among dissolved sulfur species. Modern groundwater in the lacustrine formation has a low Eh value (−335 mV). Estimated and measured low Eh values of modern and ancient interstitial waters in lacustrine environments indicate that a reducing environment in which U4+ is stable has been maintained since precipitation of uranium minerals.

Petrographic and paragenetic studies of the Agbaja Ironstone Formation, Nupe Basin, Nigeria

The Agbaja Ironstone Formation of the Lokoja district of Central Nigeria occurs within the Upper Cretaceous sedimentary sequence of the Nupe Basin which trends northwest-southeast. Three lithostratigraphical units occur in the basin: the basal Lokoja Formation, overlain by the Patti Formation and the uppermost Agbaja Ironstone Formation. The Lokoja Formation rests unconformably on the Precambrian basement complex and grades upwards from conglomerate to arkosic sandstone. The Patti Formation consists of interbedded sandstone, siltstone and carbonaceous mudstone units. The Agbaja Ironstone Formation is made up of oolitic, pisolitic and argillaceous ironstones. Four petrographic facies are identified within the Agbaja Ironstone Formation: ooidal pack-ironstone, pisoidal pack-ironstone, detrital mud-ironstone and breccia mudironstone. Kaolinite ooids are spherical and fragmented, usually with pseudomorphs of goethite after pyrite at the core. The goethite pisoids are cemented in a kaolinitic to goethitec matrix, are elliptical to subspherical in shape, and are composite in nature. Constituent minerals of the mud-ironstone are kaolinite, quartz and heavy minerals. Paragenetic studies indicate that pyrite, siderite, kaolinite, quartz, mica and heavy minerals; were the primary minerals of the ironstone deposit, whereas secondary minerals; were goethite, hematite, goyazite-crandallite, bolivarite and boehmite. Mineralogical studies provided evidence for a kaolinitic precursor for the ironstone deposit, contrary to the earlier proposed chamositic precursor. In addition, the presence of pseudomorphs of pyrite (in the nuclei and incorporated into the cortex of the ooids) are reliable indicators of a possible accretionary model for the formation of ooids and pisoids prior to ferruginisation. Two ferruginisation periods unrelated to lateritisation are indicated for the ironstone deposit.

Illitization in Permian Arkosic Sandstones Due to Fault-Related and Cross-Formational Fluid Flow, Northern Germany : ABSTRACTS

Illitization in Permian Arkosic Sandstones Due to Fault-Related and Cross-Formational Fluid Flow, Northern Germany : ABSTRACTS

Sequence stratigraphy and plate tectonic significance of the Transvaal succession of southern Africa and its equivalent in Western Australia

The Transvaal succession in the northeastern part of the Kaapvaal Province of southern Africa is the erosional remnant of four, 2.7 to 2.1 Ga unconformity-bounded stratigraphic units, or sequences, each of which is 10 2 to 10 3 m thick and spans 10 7 to 10 8 years. Three of these Phanerozoic-like cratonic sequences occur in the western part of the Kaapvaal Province and in the Pilbara Province of Western Australia; thus, the two provinces once were part of the same continent, Vaalbara. The stratigraphically lowest sequence historically assigned to the Transvaal succession is part of the Ventersdorp Supergroup in Kaapvaal and the upper part of the Fortesque Group in Pilbara. It consists predominantly of arkosic sandstone and basaltic andesite; it probably is ⩽ 2687 ± 2 Ma. The second sequence contains the quartz arenites, dolomites, and banded-iron formations of the Transvaal and Griqualand West successions in the Kaapvaal and the Hamersley Group in the Pilbara. The span of ages (2684 ± 6 to 2432 ± 31 Ma), lithologies, and geographic extent of this sequence imply that the Limpopo orogeny, which welded the Zimbabwe Province to Vaalbara, is ⩽ 2.47 Ga (not 2.67 ± 0.05 Ga). The third sequence (the Pretoria and Postmasburg groups in the Kaapvaal and the lower part of the Wyloo Group in the Pilbara probably is the clastic wedge generated by the Limpopo orogeny. The fourth sequence (dominated by the felsites of the Rooiberg Group) is restricted to northeastern Kaapvaal. Sequence stratigraphy, lithostratigraphy, and lithofacies indicate that the Pilbara rifted from the southern edge of western Kaapvaal. The Penge Iron Formation in northeastern Kaapvaal, the Asbesheuwels Iron Formation in southwestern Kaapvaal and the Brockman Iron Formation in the Pilbara are the same formation, in which cyclothemic units 0.6 to 15.0 m thick extended ⩾ 1200 km.

Groundwater circulation in the Nea Kessani low-temperature geothermal field (NE Greece)

The low-temperature geothermal field of Nea Kessani, located in NE Greece, is characterized by a thermal reservoir made up of arkosic sandstones. The temperature distribution at depth, inferred from exploratory and productive wells, indicates that hot fluids rising from depth enter the arkosic reservoir in a restricted area of the field and flow towards local thermal springs. Well production tests have revealed the presence of hydrogeological boundaries within the arkosic reservoir. The geochemical characteristics of the thermal waters, which have an NaCl/HCO 3 composition and salinity varying between 5 and 6 g/L, indicate that these waters undergo conductive cooling within the reservoir. No admixture of waters from the aquifers in the cover has been observed. The slight chemical differences existing between the thermal waters are probably caused by CO 2, which represents about two thirds by volume of the discharged fluid. This CO 2, as indicated by its isotopic composition, could originate from decomposition of marbles of the Paleozoic basement underlying the arkosic reservoir and may also affect the isotopic composition of the thermal waters, which exhibit an interesting positive oxygen shift. However, such a shift could also be the result of water-rock exchange processes at low temperatures, since the water feeding the field comes from a regional circulation which, as indicated by its deuterium content, has recharge areas on the Rhodope Chain. Alternatively, the shift could be attributed to the contribution of a deep-seated high-temperature geothermal reservoir, but a present there is no evidence of high-temperature resources in the region. A maximum temperature of 110°C has been estimated by quartz geothermometry. The physical, chemical and hydrogeological data available so far have permitted us to formulate a fluid circulation model for the Nea Kessani geothermal field.

Micro-Shock Deformation adjacent to the Surface of Shatter Cones from the Beaverhead Impact Structure, Montana

Petrographic and transmission electron microscopy (TEM) study reveals that planar fractures, pseudotachylite (PST) veinlets, and planar deformation features (PDF) in quartz and K-feldspar grains are developed only within ~2 mm of the surface of shatter cones in Proterozoic arkosic sandstones from the Beaverhead (Montana) impact structure. This unique association reinforces the impact-shock origin attibuted to all these shock-metamorphic features. No high-pressure $SiO_{2}$ polymorphs were found in the PST, but possibly this is because these cones formed far from the center of the impact structure, in a shock-pressure zone below the stability field of the high-pressure polymorphs.

Quarrying and Production of Milling, Implements at Antelope Hill, Arizona

AbstractTo process mesquite, maize, and other substances, aboriginal groups living in the Lower Colorado-Lower Gila rivers region of SW North America used characteristic milling equipment: squared, flat-faced, convex-based metates (lower grindstones) with elongated manos (upper grindstones) and large wooden mortars with long, cylindrical stone pestles. Andesite and sandstone bedrock outcrops where stone was quarried and milling implements produced have been located. The archaeological deposits at the quarries have not been masked by the debris of later quarrying for rotary mills or building blocks, and thus the quarry sites offer an opportunity to study the production of ancient forms of milling implements, and provide insights into the organization of an ancient stone technology. Antelope Hill, a large arkosic sandstone quarry on the Lower Gila River in Arizona is presented as an example of a widespread phenomenon in the region. Analysis of quarry debris in the field and laboratory, experimental replicat...

The controls on the major- and trace-element evolution of shales, siltstones and sandstones of Ordovician to tertiary age in the Wet Mountains region, Colorado, U.S.A.

A vertical section of shales, siltstones and sandstones from Ordovician to Eocene age in a limited geographic area east of the Wet Mountains and Sangre de Cristo Mountains in Colorado have been analyzed for major elements and a variety of trace-element concentrations, including the REE. In addition, the petrography of the sandstones has been determined. The quartz-rich arenites contain considerably lower concentrations of most elements relative to coexisting shales or siltstones at the same outcrop. The arkosic sandstones contain similar to lower concentrations of most elements compared to coexisting shales or siltstones in the same area. Accordingly, the average concentrations of Al 2O 3, Fe 2O 3, MgO, TiO 2, LOI, Rb, Th, Co, Sc, Cr, Cs, Nb, Y and REE are significantly lower in all sandstones relative to all shales and siltstones. The exceptions are SiO 2, Na 2O and Ba concentrations, and the Eu/Eu ★, La Sc and Th Sc ratios as they are higher in the sandstones than the shales and siltstones. There is no significant difference in the MnO, CaO, K 2O and Sr concentrations or the sol La Co , Th Co , La Ni , Cr Th and ( La Lu cn ratios of the average sandstones relative to the shales and siltstones. Many of these differences may be related to the higher quartz and feldspar and lower clay mineral amounts in the sandstones than in the shales and siltstones. For example, average La Sc , Th Sc , Eu Eu ★ and ( La Lu cn ratios of th arkosic sandstones tend to be higher than the coexisting shales and siltstones due to enrichment of the arkoses in feldspar relative to the other minerals (e.g., Eu Eu ★ = 0.82 ± 0.19 in average arkoses in the Trinidad, Vermejo, Raton, Poison Canyon and Cuchara Formations; Eu Eu ★ = 0.63 ± 0.19 in average shales and siltstones in the same formations). In addition, the elemental concentrations and ratios are more variable for most elements in the sandstones than the shales and siltstones. This suggests that the shales are rapidly homogenized near the source. The elemental fractionation in the arkosic sandstones due to enrichment of feldspar relative to other minerals suggests that they are poorer indicators of provenance that the associated shales, whereas, the more homogeneous elemental distributions in the shales and siltstones suggest that they may be better indicators of provenance than the sandstones. Nevertheless, the elemental ratios that are most similar in the sandstones and in the shales and siltstones are consistent with their derivation from similar average intermediate to silicic source rocks.