Wide Range Of Lithologies Research Articles

Svalbox Digital Model Database: A geoscientific window into the High Arctic

Abstract Digital outcrop models (DOMs) have revolutionized the way twenty-first century geoscientists work. DOMs are georeferenced three-dimensional (3-D) digital representations of outcrops that facilitate quantitative work on outcrops at various scales. Outcrop digitalization has been traditionally conducted using laser scanners, but in the past decade, it has seen an exponential growth because of efficient and consumer-friendly structure-from-motion (SfM) algorithms concurrent with the rapid development of cost-effective aerial drones with high-resolution onboard cameras. While DOMs are routinely used in geoscientific research, education, and industry, enhanced DOM usage is restricted because raw data (e.g., photographs) and metadata are often incomplete and/or unavailable. In this contribution, we present the Svalbox Digital Model Database (Svalbox DMDb), a database of metadata and openly available data packages for individual DOMs. The Svalbox DMDb is a regional DOM database geographically constrained to the Norwegian High Arctic archipelago of Svalbard at 74°N–81°N and 10°E–35°E. Svalbard offers exceptional-quality, vegetation-free outcrops with a wide range of lithologies and tectono-magmatic styles, including extension, compression, and magmatism. Data and metadata of the systematically digitalized outcrops across Svalbard are shared according to FAIR principles through the Svalbox DMDb. Fully open-access and downloadable DOMs include not just the DOMs themselves, but also the input data, processing reports and projects, and other data products such as footprints and orthomosaics. Rich metadata for each DOM include both the technical and geological parameters (metadata), enabling visualization and integration with regional geoscientific data available through the Norwegian Polar Institute and the Svalbox online portal. The current release of Svalbox DMDb, documented in this contribution, covers 135 DOMs cumulatively covering 114 km of Proterozoic to Cenozoic stratigraphy.

Effects of erosional resistance on bedrock channel occurrence and morphology: Examination of the Seo River catchment in South Korea

In tectonically stable regions, bedrock channels develop along deep and narrow valleys that are usually associated with resistant lithology, which enables the transport capacity of these channels to exceed the upstream sediment supply. However, bedrock channels are not restricted to the reaches with a high transport capacity. They are often located in reaches with a lower transport capacity in wide valleys underlain by erodible bedrock. Even though bedrock channels are distributed over a wide range of lithologies, the effects of erosional resistance on the transport capacity required for bedrock channel formation have not been fully clarified. We investigated the bedrock channels in the Seo River catchment in South Korea to determine the relationship between erosional resistance and the occurrence and morphology of bedrock channels. We found that more than half of the bedrock channel segments were in resistant regions. However, a considerable portion also occurred in erodible regions. Most of the bedrock channels in the resistant regions were steep and narrow, generating a higher unit stream power. Therefore, these channels had long stretches of exposed bedrock with actively eroded features such as well-abraded surfaces. In the erodible regions, most of the bedrock channels were wide and gentle, leading to a lower unit stream power. Thus, the exposed bedrock surface was poorly polished, and the channel morphology preserved more of the underlying bedrock's joint or bedding structures. These results suggest that erosional resistance affects the occurrence of bedrock channels by adjusting the threshold stream power required for bedrock incision. Moreover, lithology strongly regulates the bedrock channel morphology in erodible regions.

Whetstones in Roman Britain: Character, Distribution, Provenance and Industries

AbstractA substantial database of published excavation and other reports has been used to map the character and distribution in Roman Britain of whetstones, those unprepossessing implements essential in the home, farmstead, workshop and barracks for the maintenance of edge-tools and weapons. The quality of the geological identifications in the reports varies considerably, but a wide range of lithologies are reported as put to use: granite, basalts-dolerites, lava, tuff, mica-schist, slates/phyllites, Brownstones, Pennant sandstone, micaceous sandstones, grey sandstones/siltstones, Millstone Grit, Coal Measures, red sandstones, ferruginous sandstones, sarsen, Weald Clay Formation sandstones, sandy limestones, shelly limestones, cementstones, and (Lower) Carboniferous Limestone. On distributional evidence, some of these categories are aliases for alternatively and more familiarly named lithologies. Bringing ‘high-end’ products to the market, the long-running industry based on sandstones from the Weald Clay Formation (Lower Cretaceous) emerges as a British economic feature, evidenced from the Channel coast to the Scottish Borders, and with a recently demonstrated, substantial representation on the Roman near-continent. The distribution maps point to another and more complete British industry, based on the Brownstones (Old Red Sandstone, Devonian) and Pennant sandstone (Upper Carboniferous), outcropping close together in the West Country. A more systematic and geology-based treatment of excavated whetstones in the future is likely to yield yet more insights into the role these artefacts played in the economy of Roman Britain.

Rapid determination of supercritical CO2 and brine relative permeability using an unsteady-state flow method

Relative permeability of supercritical CO2 (scCO2) and brine was determined in reactive and non-reactive rock cores using a combination of unsteady-state methodology and computed tomography. Experiments were conducted using a medical grade CT scanner to determine saturation using a custom Python script. The saturation and differential pressure across the core were then used to derive four empirical constants to calculate relative permeability. This methodology increases temporal efficiency while reducing experimental complexity. Additionally, we show that the method can be used to determine scCO2 relative permeability in a wide range of lithologies and flow rates, and with the ability to account for matrix dissolution during scCO2 flooding. Plain Language Summary: The ability to capture and store carbon dioxide (CO2) is an important component of efforts to slow climate change. Long-term geologic storage of CO2 requires accurate descriptions of injected CO2 in subsurface rock formations to account for stored volumes and develop adequate risk monitoring. Effectively describing the migration of CO2 injected in underground reservoirs requires an accurate description of how CO2 moves and interacts with the rock. Relative permeability, or the description of how efficiently a fluid moves in a porous media while in the presence of other fluids, is the most utilized tool for describing the competitive flow of fluids. In this study, we detail a modified unsteady-state methodology using computed tomography that provides relative permeability quickly and can be used to analyze heterogenous media. The methodology can be used to describe the impacts of dissolution on CO2 transport through reactive cores, which to the authors knowledge, has not been described in the literature previously.

Tafoni and honeycomb structures as indicators of ascending fluid flow and hypogene karstification

Abstract Tafoni and honeycombs remain some of the most enigmatic and puzzling geomorphological phenomena. Their globally widespread occurrence across a wide range of lithologies and environmental conditions suggests that their formation is determined by a factor that overarches variations in these conditions and weathering processes. Based on a study of tafoni and honeycombs in the Crimean Piedmont, this paper demonstrates that the primary factor in their formation is the pre-exposure alteration of rocks along fractures and karst conduits as a result of fluid–rock interactions. Such alteration is commonly induced by ascending flow and is related to hypogene karstification. The morphological expression of cavernous features through removal of the alterite can occur under subsurface conditions, but is more frequent on exposure to atmospheric weathering. The local or regional characteristics of a weathering system are irrelevant or of only secondary importance in determining the localization and morphology of cavernous features. New features do not form on rock surfaces where the alteration zone was totally denuded or never present. The proposed model resolves major issues inherent to previous interpretations of cavernous features. It has general applicability and important implications for geodynamic and palaeohydrogeological reconstructions. Typical tafoni and honeycombs may indicate past events of ascending flow and the potential presence of hypogene karst systems.

A Pleistocene palaeovegetation record from plant wax biomarkers from the Nachukui Formation, West Turkana, Kenya.

Reconstructing vegetation at hominin fossil sites provides us critical information about hominin palaeoenvironments and the potential role of climate in their evolution. Here we reconstruct vegetation from carbon isotopes of plant wax biomarkers in sediments of the Nachukui Formation in the Turkana Basin. Plant wax biomarkers were extracted from samples from a wide range of lithologies that include fluvial-lacustrine sediments and palaeosols, and therefore provide a record of vegetation from diverse depositional environments. Carbon isotope ratios from biomarkers indicate a highly dynamic vegetation structure (ca 5-100% C4 vegetation) from 2.3 to 1.7 Ma, with an overall shift towards more C4 vegetation on the landscape after about 2.1 Ma. The biomarker isotope data indicate ca 25-30% more C4 vegetation on the landscape than carbon isotope data of pedogenic carbonates from the same sequence. Our data show that the environments of early Paranthropus and Homo in this part of the Turkana Basin were primarily mixed C3-C4 to C4-dominated ecosystems. The proportion of C4-based foods in the diet of Paranthropus increases through time, broadly paralleling the increase in C4 vegetation on the landscape, whereas the diet of Homo remains unchanged. Biomarker isotope data associated with the Kokiselei archaeological site complex, which includes the site where the oldest Acheulean stone tools to date were recovered, indicate 61-97% C4 vegetation on the landscape.This article is part of the themed issue 'Major transitions in human evolution'.

Technology Focus: Matrix Stimulation (June 2015)

Technology Focus The technical contributions highlighted this year focus on matrix stimulation of carbonate and unconventional reservoirs. Matrix stimulation encompasses pumping processes used to improve the connection between the wellbore and the reservoir. They are effective in a wide range of lithologies and have been successful in most types of completions—cased and open hole, horizontal and vertical, simple and complex, injectors and producers. Wells are matrix stimulated during completion and to remove production- or injection-induced impairment. The defining characteristic of matrix stimulation is the use of chemical systems to dissolve material in the near-wellbore region pumped below pressures that propagate reservoir fractures. Three papers are presented in this feature, and several others are referenced for the interested reader. One of the papers, SPE 168198, “The First Visualization of Acid Treatments on Carbonates With 3D Nuclear-Magnetic-Resonance Imaging,” provides a detailed description of the application of cutting-edge technology to image acid-induced wormholes in carbonates. While this technology is new, it offers the promise of quantifying the effect of wormholes on near-wellbore flow to a degree that has not been possible until now. (See also SPE 171699, “NMR as a Characterization Tool for Wormholes: The Complete Picture.”) While the industry has focused on fracture stimulation of unconventional reservoirs, SPE 173640, “Monitoring Acid-Stimulation Treatments in Naturally Fractured Reservoirs With Slickline Distributed-Temperature Sensing,” demonstrates the value of matrix stimulation in a shale reservoir. It is a good example with some excellent data on distributed-temperature-sensing profile monitoring, and it shows how naturally fractured reservoirs, which have generally been a challenge for stimulation, can be addressed. Distributed-temperature-sensing technology continues to develop, and a number of papers have been presented in the past year describing new capabilities and interpretation methods. (See, for example, SPE 173686, “Optimization of Matrix Acidizing With Fluids Diversion in Real Time Using Distributed-Temperature Sensing and Coiled Tubing”; SPE 172546, “Setting a New Milestone in Carbonate Matrix Stimulation With Coiled Tubing”; and SPE 171021, “First Installation of Efficient and Accurate Multilaterals-Stimulation Technology in Carbonate Oil Application.”) The stimulation process can also be improved by applying classical concepts from reaction engineering and other disciplines. Paper SPE 171766, “Challenge of Acidizing Horizontal Wells in Tight Carbonate Reservoirs: Weak Acid and Nonacid Alternatives,” is an excellent example. The lesson is to look in both directions, back to the technical basics and forward to embrace new technologies.

Technology Focus: Matrix Stimulation (June 2014)

Technology Focus Matrix stimulation encompasses pumping processes used to improve the connection between the wellbore and the reservoir. They are effective in a wide range of lithologies and have been successful in most types of completions: cased and openhole, horizontal and vertical, simple and complex, injectors and producers. Wells are matrix stimulated during completion and to remove production- or injection-induced impairment. The defining characteristic of matrix stimulation is the use of chemical systems to dissolve material in the near-wellbore region pumped at pressures below those that propagate reservoir fractures. Chemical systems have been developed to attack carbonates (impairment and reservoir material), organic deposits, aluminosilicate minerals (fines), and sulfate scales. They have been used widely below approximately 120°C, but formulating effective systems for higher temperatures is a challenge. Carbonate dissolution is generally extremely rapid at downhole conditions. Current developments are focused on controlling this reaction either with novel organic acids or with gel systems whose rheology changes with the acid reaction. Solvents for organic deposits and sulfate scales react very slowly. The challenge is to speed up the reaction and develop treatment methods with the necessary contact time. The reaction with fines is extremely complex and can lead to precipitation of solids that are more damaging than the minerals themselves. Several promising new chemistries combining chelants and organic acids are being studied to prevent these damaging precipitates.

Raman mapping of coesite inclusions in garnet from the Kokchetav Massif (Northern Kazakhstan)

Coesite inclusions occur in a wide range of lithologies and coesite is therefore a powerful ultrahigh-pressure (UHP) indicator. The transformation of coesite to quartz is evidenced by three optically well identifiable characteristics (e.g. palisade textures, radial crack patterns, polycrystalline quartz pseudomorphs). Under overpressure monomineralic coesite (on an optical basis), lacking the above transformation characteristics may survive. Raman micro-spectroscopy was applied on monomineralic coesite inclusions in garnet porphyroblasts from diamond-bearing garnet–clinozoisite–biotite gneisses of the Barchi-Kol area (Kokchetav Massif, Northern Kazakhstan). These coesite inclusions are euhedral and display a characteristic anisotropic hallo. However, Raman maps and separate spectra of these inclusions display shifted bands for coesite and quartz. Microscopically undetectable, quartz shows on the Raman map as a thin shell around coesite inclusion. Shift of the main coesite band allows to estimate their overpressure: coesite inclusions record 0–2.4 GPa in garnet and zircon. The quartz shell remains under lower pressure 0–1.6 GPa. The possible application of coesite and quartz Raman geobarometers for UHP metamorphic rocks is discussed.

HyMap of Broken Hill — Imaging Spectrometry for Rock and Mineral Abundance Mapping

Imaging spectrometry (hyperspectral imagery) has been acquired for the Broken Hill Block. The data were acquired using the HyMap scanner on behalf of the New South Wales Department of Primary Industries (former Department of Mineral Resources). Interpretation of the imagery for selected test sites demonstrates the capability to map a wide range of lithologies and minerals. Minerals mapped include iron oxides, garnets, micas, clay minerals, and amphiboles. Lithologies mapped include all the lithologies present in the region, and the surficial sediments. HyMap is also able to map certain lithologies critical to mineral exploration in the region, such as gahnite-bearing quartz, manganese-bearing garnetiferous rocks, and sodium-rich micas indicative of likely mineral alteration. The mapping capabilities of the HyMap scanner are demonstrated using selected test sites, but the lithology and mineral mapping has also been extended to cover a whole 1:25 000 map sheet area.It is suggested that the HyMap-derived information will form the vital surficial component in a three-dimensional view of the crust provided by geophysical data including gravity, magnetic, radiometric, and imaging spectrometry data.

Zircon (U-Th)/He Thermochronometry

Research Article| January 01, 2005 Zircon (U-Th)/He Thermochronometry Peter W. Reiners Peter W. Reiners Department of Geology and Geophysics, Yale University, New Haven, Connecticut, 06520, U.S.A., peter.reiners@yale.edu Search for other works by this author on: GSW Google Scholar Reviews in Mineralogy and Geochemistry (2005) 58 (1): 151–179. https://doi.org/10.2138/rmg.2005.58.6 Article history first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Peter W. Reiners; Zircon (U-Th)/He Thermochronometry. Reviews in Mineralogy and Geochemistry 2005;; 58 (1): 151–179. doi: https://doi.org/10.2138/rmg.2005.58.6 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyReviews in Mineralogy and Geochemistry Search Advanced Search A number of features of zircon (ZrSiO4), including high U-Th concentrations, high abundance in a wide range of lithologies, refractory nature under metamorphic and some magmatic conditions, and resistance to physical and chemical weathering, make it highly suitable for geochronology and thermochronology and thus a versatile tool for examining a wide range of earth processes. Like apatite and many other minerals, radioisotopic dating of zircon was first performed using the (U-Th)/He system, but the thermochronologic significance of zircon He ages has emerged only in the last few years. In this chapter, I review the current status of zircon... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

HyMap of Broken Hill – imaging spectrometry for rock and mineral abundance mapping

Imaging spectrometry (hyperspectral imagery) has been acquired for the Broken Hill Block. The data were acquired using the HyMap scanner on behalf of the New South Wales Department of Primary Industries (former Department of Mineral Resources). Interpretation of the imagery for selected test sites demonstrates the capability to map a wide range of lithologies and minerals. Minerals mapped include iron oxides, garnets, micas, clay minerals, and amphiboles. Lithologies mapped include all the lithologies present in the region, and the surficial sediments. HyMap is also able to map certain lithologies critical to mineral exploration in the region, such as gahnite-bearing quartz, manganese-bearing garnetiferous rocks, and sodium-rich micas indicative of likely mineral alteration. The mapping capabilities of the HyMap scanner are demonstrated using selected test sites, but the lithology and mineral mapping has also been extended to cover a whole 1:25 000 map sheet area. It is suggested that the HyMap-derived information will form the vital surficial component in a three-dimensional view of the crust provided by geophysical data including gravity, magnetic, radiometric, and imaging spectrometry data.

Impact glasses in fallout suevites from the Ries impact structure, Germany: An analytical SEM study

Abstract— t‐Impact‐generated glasses from fallout suevite deposits at the Ries impact structure have been investigated using analytical scanning electron microscopy. Approximately 320 analyses of glass clasts were obtained. Four glass types are distinguished on the basis of composition and microtextures. Type 1 glasses correspond to the aerodynamically shaped glass bombs studied previously by many workers. Major oxide concentrations indicate the involvement of granitic rocks, amphibolites, and minor Al‐rich gneisses during melting. Type 2 glasses are chemically heterogeneous, even within individual clasts, with variations of several wt% in most of the major oxides (e.g., 57–70 wt% SiO2). This suggests incomplete mixing of: 1) mineral‐derived melts or 2) whole rock melts from a wide range of lithologies. Aluminium‐rich clinopyroxene and Fe‐Mg‐rich plagioclase quench crystals are present in type 1 and 2 glasses, respectively. Type 3 glasses contain substantial amounts of H2O (˜12–17 wt%), low SiO2 (50–53 wt%), high Al2O3 (17–21 wt%), and high CaO (5–7 wt%) contents. This suggests an origin due to shock melting of part of the sedimentary cover. Type 4 glasses form a ubiquitous component of the suevites. Based on their high SiO2 content (˜85–100 wt%), the only possible protolith are sandstones in the lowermost part of the sedimentary succession. Calcite forms globules within type 1 glasses, with which it develops microtextures indicative of liquid immiscibility. Unequivocal evidence also exists for liquid immiscibility between what are now montmorillonite globules and type 1, 2, and 4 glasses, indicating that montmorillonite was originally an impact melt glass. Clearly, the melt zone at the Ries must have incorporated a substantial fraction of the sedimentary cover, as well as the underlying crystalline basement rocks. Impact melts were derived from different target lithologies and these separate disaggregated melts did not substantially mix in most cases (type 2, 3, and 4 glasses and carbonate melts).

Geologic lithofacies identification using the multiscale character of seismic reflections

A forward acoustic model shows that geologic lithofacies groups can be identified by the character of the wavelet transform of their seismic reflection response even for incident signals with a wavelength much larger than the dominant bed thickness. The same model shows that multiple interbed reflections can be neglected. This allows the use of an analytical relation of the linear reflection response expressed as a convolution between the incident signal and the scaled derivative of the acoustic impedance. The relation is applied to solve the inverse problem for the acoustic impedance, using orthogonal discrete wavelet transform (DWT) and Fourier transform methods; good agreement is obtained between the well log wavelet spectrum and both the forward modeled seismic data and the real seismic data. It is found that the DWT approach is superior, having a better signal-to-noise ratio and more localized deconvolution artifacts. A population of well logs containing a wide range of lithologies and bed thicknesses, which are categorized into lithofacies groups, is used to define the conditional probability of a wavelet transform response given a lithofacies group. These conditional probabilities are used to estimate the lithofacies probability given a seismic wavelet response via a Bayesian inversion.

Nd and Sr isotopic ratios and trace element geochemistry of epidote from the Swiss Molasse Basin as provenance indicators: implications for the reconstruction of the exhumation history of the Central Alps

Epidote is a common heavy mineral in sandstones. Its use as a provenance indicator, however, is limited because it appears in a wide range of lithologies and geological settings. In order to discriminate between mantle- and crust-derived source rocks, we measured isotopic and trace element compositions of detrital epidote from sandstones of the Swiss Molasse Basin. The compositions of the detrital epidotes were compared to epidotes from Penninic metabasites and Austroalpine metagranites from the present-day exposures and to epidote from ophiolitic and metagranitic pebbles from the molasse basin. We found that Nd isotopic ratios of epidote allow a good discrimination between different types of source rocks with εNd values ranging between 6.1 and 8.6 for metabasic lithologies and between −9.7 and −2.8 for metagranitic lithologies. Sr isotope ratios reveal similar trends but do not allow such a clear discrimination. Ba, U, Rb, Nb and Th contents of epidote correlate well with the εNd values, yielding the same provenance signal. According to isotopic and trace element compositions, three types of epidote-bearing sandstones can be distinguished. One received epidote from Lower Austroalpine metagranites, another from Penninic ophiolites and a third type of sandstone received epidote from both sources. The provenance of the detrital epidote has important bearing on the exhumation history of the Central Alps, elucidating the transition between the erosion of Lower Austroalpine felsic lithologies and the onset of erosion of the Penninic lower plate. According to our data, the Penninic lower plate became exposed simultaneously in the hinterland of all molasse fans at 21–20 Ma. At the same time, the relief collapsed, as reflected by a drastic decrease of sediment accumulation rates in the foreland basin. The combination of both events fits well to the geodynamic scenario of large-scale lateral extension processes affecting the Central Alps in post-collisional times.

Carbon isotope stratigraphy of Upper Cambrian (Steptoean Stage) sequences of the eastern Great Basin: Record of a global oceanographic event

A large, global positive excursion in δ 13 C (from ∼0.5 to 4.5‰) during the Late Cambrian Pterocephaliid biomere/Steptoean Stage (Aphelaspis-Elvinia zones) is documented at high stratigraphic resolution in three sections in the eastern Great Basin. The excursion, which we refer to as the Steptoean Positive Carbon Isotope Excursion, or SPICE excursion, began coincident with a world-wide extinction event. The δ 13 C data from the Great Basin reflect global seawater values in a wide range of lithologies, including oolitic grainstones, wackestones, thrombolitic boundstones, and flat-pebble conglomerates. We use a section at Shingle Pass in the southern Egan Range to divide the Pterocephaliid biomere into eight isotope steps that represent equal divisions of the δ 13 C rise and fall (±4‰). This provides a basis for recognition of a revised chronostratigraphic framework for the Pterocephaliid biomere/Steptoean Stage. Strata deposited during the beginning of the SPICE excursion record a major change in the pattern of sedimentation in the eastern Great Basin. This is reflected in a siliciclastic-carbonate transition at Shingle Pass, Nevada, and a carbonate-siliciclastic transition at the House Range and Lawson Cove sections in Utah. A regional siliciclastic influx recognized throughout the Great Basin occurs near the peak of the SPICE excursion. Carbon isotope analyses from cratonal sections in Wyoming provide independent evidence that a major sedimentary hiatus took place on the craton during the time of the SPICE excursion. The correlated changes in δ 13 C, relative sea level, and the marine biota during the SPICE excursion provide remarkably detailed records of a major paleoceanographic event. We speculate that changes in sea level, climate, or tectonics may have triggered the SPICE excursion and coeval extinction event. Subsequent burial of organic carbon caused the increase in δ 13 C and may have led to an interval of global cooling. The results of this study lend confidence to carbon-isotopic studies of pre-Mesozoic rocks.

Geochemistry and Tectonic Evolution of the Orogenic Granitoids Associated with the Andean-Type Siham Arc, Central Arabian Shield

The syntectonic granitoids and the Al Bara batholith trondhjemite- granite suite exposed in the Afif quadrangle, Saudi Arabia, are the major plutonic components of the Andean-type Siham arc, developed during the period from about 750 to 700 Ma along the western margin of the Afif terrane. These plutonic rocks have a wide range of lithologies, from diorite to leucogranite. They are characterized by multiple periods of plutonism and rejuvenation during Pan-African orogeny (c. 900-550 Ma). The syntectonic granitoids and the Al Bara batholith rocks are calc-alkaline, I-type and metaluminous to marginally peraluminous. They are enriched in Rb, Ba, K, La, Ce and Sr relative to Nb, Zr and Ti, suggesting their emplacement along an active continental margin. The Al Bara batholith rocks, with the largest variations in lithology, texture and incompatible elements’ patterns, indicate the greater effects of Pan-African rejuvenation. The post-tectonic Dariyah batholith granites were intruded at 585 ± 8 Ma in the late Proterozoic (650-615 Ma) Murdama group, a back-arc basin filled up with clastic sedimentary rocks with minor limestone and volcanic rocks. The batholith ranges in composition from co-magmatic granodiorite to syenogranite. Biotite granite is the abundant rock type. All lithologies of the batholith are in part porphyritic with lack of chilled contacts. The porphyritic varieties are modeled as mixtures of phenocrysts (presumably restite) and minimum melts. The higher levels of Rb, Na, La, Ce and Sm, the marked depletion of Ba, Sr, and Ti and the strong Eu negative anomaly are all indicative of in-situ fractionation of the Dariyah rocks after intrusion into higher levels of the crust.

Use of Synthetic Aperture Radar (SAR) for Geologic Reconnaissance in Arctic Regions: An Example from  the Arctic National Wildlife Refuge, Alaska

Satellite-based synthetic aperture radar (SAR) can provide an additional remote-sensing tool for regional geologic studies in arctic regions. Although SAR data do not yield direct information on rock type and do not replace traditional optical data, SAR data can provide useful geologic information in arctic regions where the stratigraphic column includes a wide range of lithologies, and bedrock exposures have been reduced to rubble by frost action. For example, in ERS-1 SAR data from the Arctic National Wildlife Refuge (ANWR) of the northeastern Brooks Range, Alaska, carbonate and clastic rocks can give remarkably different radar responses on minimally reprocessed SAR data. The different radar response of different lithologies can be attributed to variations in surface roughness, specifically the size and angularity of scree in talus slopes. Additional postacquisition processing can both remove many of the negative terrain effects common in SAR data and enhance contrasts in bedrock lithology. Because of this ability to discriminate between gross lithologic packages, the ERS-1 SAR data can be used to provide a regional view of ANWR and a detailed look at specific areas. A mosaic of ERS-1 SAR data from all of ANWR provides a synoptic ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received August 30, 1995; revised manuscript received March 11, 1996; final acceptance August 15, 1996. 2Geophysical Institute, University of Alaska, Fairbanks, Alaska 99775. 3Alaska SAR Facility, Geophysical Institute, University of Alaska, Fairbanks, Alaska 99775. This study was supported by sponsors of the Tectonics and Sedimentation Research Group of the University of Alaska, Fairbanks, including ARCO Alaska, BP (Alaska), Chevron, Exxon, Japan National Oil Co., Mobil, Phillips, and UNOCAL. The Alaska SAR facility provided support for Richard Guritz and reprocessing of the ERS-1 SAR data. ERS-1 SAR data were provided by the European Space Agency under a research agreement with NASA. We would like to thank Steve Ahlgren, Lyn McNutt, and Ken Dean for stimulating discussions on SAR and its potential geologic applications in the Arctic, Wes Wallace for discussions on the regional geology of northeastern Alaska and weathering characteristics of the different stratigraphic units, and Wes Wallace, Ken Dean, Bill Stringer, Larry Lane, M. C. Erskine, Jr., H. R. Hopkins, and K. Biddle for helpful reviews of the manuscript.

Focussed Fluid Flow During Subduction

It is clear that substantial dehydration of subducted metabasic ocean crust occurs, but the fate and behaviour of the liberated water has been the subject of much debate. It is the purpose of this study to present new data from subducted oceanic crust in the western Alps which has a bearing on this ongoing debate.The western Alps is an ideal place to examine subduction zone processes, as ophiolitic material metamorphosed to highpressure conditions from 2GPa have been returned to the Earth's surface. Detailed investigations of ophiolites and ocean crust means that the pattern of alteration and 5180 values prior to subduction can be estimated (e.g. Shiffman and Smith, 1988; Gillis and Robinson; 1990) Regional Geology. The ophiolites of the Piemonte zone of the western Alps represent the former ocean floor that once separated the European and Apulian (Italy, etc.) plates. These ophiolitic relics now form the, sandwiched between European and Apulian (also known as Austroalpine) basement. Both basement units and the Piemonte zone contain high -pressure metamorphic assemblages. The samples used in this study come from two areas. At Pfulwe (near Zermatt), a wide range of lithologies are present in an area c. 500 x 500m, including eclogite, chloritoid eclogite, blueschist and epidosite. Common metabasalts have eclogitic mineral assemblages, and other parageneses are cofacial,--with the variation in mineralogy due to variation in bulk composition due to the effects of sea-floor hydrothermal alteration (Barnicoat and BowteU; submitted). Conditions of metamorphism were estimated as 1.7-2.0GPa, 550-600~ by Barnicoat and Fry (1986). Servette (on the south side of the Val d'Aosta) is the site of a disused copper mine, and again, a wide range of lithologies is present. The area is one of eclogite-facies metamorphism, although blueschists dominate the immediate area. Also present near the mine are talc-chloritoid rich rocks and talc-cummingtonite assemblages. Quartz-pyrite rich material represents the ore material. Servette is interpreted as the upflow zone of a mid-ocean-ridge hydrothermal system, now metamorphosed to high-pressure assemblages. Metamorphic conditions have been estimated as 550_+20~ 1.21.6GPa (Martin and Tartarotti; 1986b). Analytical Method. Oxygen extraction was undertaken using standard techniques with CIF3 as reagent. Isotopic analysis was done on Finnigan Mat Delta E and 252 mass spectrometers.

Lower Silurian reefs and post-reef beds of the Attawapiskat Formation, Hudson Bay Platform, northern Ontario

Outcrop exposures along the Attawapiskat River in the Hudson Bay Lowlands of northern Ontario clearly reveal the morphology of Silurian (upper Llandoverian) reefs of the Attawapiskat Formation and the relationships between reef cores, flanking beds, and post-reef beds. These relationships indicate that the reefs had a syndepositional relief of at least 8–10 m. The relief of the reefs is indicated by debris-flow lenses encased within flanking beds, a debris-flow fan at the base of a reef, thin reef-flanking beds truncated against a vertical reef face, penecontemporaneous brachiopod beds on the flanks of reefs, and three large slide blocks that apparently slid over the side of a reef.Reefs of the Attawapiskat Formation are largely limestones with a wide range of lithologies, from stromatoporoid-, coral-, and cement-rich boundstones to alga–cement-rich boundstones. Thick, laterally extensive Nuia grainstone beds, a product of widespread Nuia monocultures in supratidal to intertidal ponds, are present above the reefs. The most important diagenetic processes were early marine cementation (predominantly radial-fibrous calcite), shallow burial diagenesis, and pervasive neomorphism.Schematic reconstructions of the depositional history of the Attawapiskat Formation in outcrop, constructed from observations of outcrop relationships, show a stratigraphic succession that was controlled by relative-sea-level changes. Reefs growth was terminated by a relative-sea-level fall, but subsequent minor relative-sea-level fluctuations resulted in alternating deposition of supratidal to shallow subtidal sediments for a short time before the final retreat of the Silurian seas from the Hudson Bay Platform; only supratidal evaporite facies are present in the remainder of the Silurian section above the Attawapiskat Formation.