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At least 469 records · Page 26Linked to original sources
USGS1999

From in situ coal to the final coal product: A case study of the Danville Coal Member (Indiana)

A surface coal mine operation and preparation plant in southwestern Indiana was sampled to examine variations in coal quality and coal petrography parameters for the Danville Coal Member of the Dugger Formation (Pennsylvanian-Desmoinesian, Westphalian D). Representative samples from in situ coal, preparation plant feeds, and a final coal product were collected in order to compare coal quality, coal petrography, trace element concentrations, and ash chemistry of the coal to those of the product. Coal quality parameters of the in situ samples and various feeds, coarse refuse, and final product were variable. The quality of the final coal product was best predicted by the coal quality of the clean coal feed (from the middle portions of the seam). Some trace element contents, especially lead and arsenic, varied between the coal feeds and the product. Lead contents increased in the feeds and product compared to the channel sample of the raw coal, possibly due to contamination in the handling process.A surface coal mine operation and preparation plant in southwestern Indiana was sampled to examine variations in coal quality and coal petrography parameters for the Danville Coal Member of the Dugger Formation (Pennsylvanian-Desmoinesian, Westphalian D). Representative samples from in situ coal, preparation plant feeds, and a final coal product were collected in order to compare coal quality, coal petrography, trace element concentrations, and ash chemistry of the coal to those of the product. Coal quality parameters of the in situ samples and various feeds, coarse refuse, and final product were variable. The quality of the final coal product was best predicted by the coal quality of the clean coal feed (from the middle portions of the seam). Some trace element contents, especially lead and arsenic, varied between the coal feeds and the product. Lead contents increased in the feeds and product compared to the channel sample of the raw coal, possibly due to contamination in the handling process.

International Journal of Coal Geology↗
USGS2000

Metalliferous coals of the Westphalian A Joggins Formation, Cumberland basin, Nova Scotia, Canada: Petrology, geochemistry, and palynology

Five coals of Westphalian A (early Middle Pennsylvanian) age were sampled from the Joggins Formation section exposed along Chignecto Bay at Joggins, Nova Scotia. Coal beds along the bay were mined beginning in the early 17th century, yet there have been few detailed published investigation of the coal beds of this classic section. The lowermost coal, the Upper Coal 28 (Upper Fundy), is a high-vitrinite coal with a spore assemblage dominated by arboreous lycopsid spores with tree ferns subdominant. The upper portions of the coal bed have the highest ratio of well-preserved to poorly-preserved telinite of any of the coals investigated. Coal 19 ('clam coal') has 88% total vitrinite but, unlike the Fundy coal bed, the telinite has a poor preservation ratio and half the total vitrinite population comprises gelocollinite and vitrodetrinite. The latter coal bed is directly overlain by a basin-wide limestone bed. The Lower Kimberly coal shows good preservation of vitrinite with relatively abundant telinite among the total vitrinite. The Middle Kimberly coal, which underlies the tetrapod-bearing lycopsid trees found by Lyell and Dawson in 1852, exhibits an upward decrease in arboreous lycopod spores and an upward increase in the tree fern spore Punctatisporites minutus. Telinite preservation increases upwards in the Middle Kimberly but overall is well below the preservation ratio of the Upper Fundy coal bed. The coals all have high sulfur contents, yielding up to 13.7% total sulfur for the lower lithotype of the Upper Fundy coal bed. The Kimberly coals are not only high in total and pyritic sulfur, but also have high concentrations of chalcophile elements. Zinc, ranging up to 15,000 ppm (ash basis), is present as sphalerite in fusain lumens. Arsenic and lead each exceed 6000 ppm (ash basis) in separate lithotypes of the Kimberly coals. Together these data are consistent with elevated pH in planar mires. The source of the elemental enrichment in this presumed continental section is enigmatic. (C) 2000 Elsevier Science B.V. All rights reserved.Five coals of Westphalian A (early Middle Pennsylvanian) age were sampled from the Joggins Formation section exposed along Chigneto Bay at Joggins, Nova Scotia. All the coals were found to have high sulfur contents. Overall, the data obtained are consistent with elevated pH in planar mires.

International Journal of Coal Geology↗
USGS2000

In-situ analysis of solid bitumen in coal: Examples from the Bowen Basin and the Illinois Basin

Solid bitumen and associated vitrinite from selected coals from the Bowen Basin and the Illinois Basin were studied using electron microprobe and micro-FTIR techniques. The coal studied covers a range of vitrinite reflectance from 0.59% to 1.33%. Carbon content in the bitumen is generally lower than in vitrinite in coals with vitrinite reflectance below 0.67%. In coals with reflectance above 0.67%, carbon content of bitumen is higher than in vitrinite, reflecting higher aromaticity due to hydrocarbon generation. Sulfur and iron content are comparable between vitrinite and bitumen. Functional group distribution suggests the presence of two types of bitumen in the Illinois Basin coals. The more aliphatic variety occurring in veins and cleats is interpreted as pre-gas generation bitumen, and the more aromatic variety filling cells and voids in inertinite as post-gas generation bitumen. (C) 2000 Elsevier Science B.V. All rights reserved.Solid bitumen and associated vitrinite from selected coals from the Bowen Basin and the Illinois Basin were studied using electron microprobe and micro-FTIR techniques. The coal studied covers a range of vitrinite reflectance from 0.59% to 1.33%. Carbon content in the bitumen is generally lower than in vitrinite in coals with vitrinite reflectance below 0.67%. In coals with reflectance above 0.67%, carbon content of bitumen is higher than in vitrinite, reflecting higher aromaticity due to hydrocarbon generation. Sulfur and iron content are comparable between vitrinite and bitumen. Functional group distribution suggests the presence of two types of bitumen in the Illinois Basin coals. The more aliphatic variety occurring in veins and cleats is interpreted as pre-gas generation bitumen, and the more aromatic variety filling cells and voids in inertinite as post-gas generation bitumen.

International Journal of Coal Geology↗
USGS2001

Paleomagnetic data bearing on style of Miocene deformation in the Lake Mead area, Southern Nevada

Paleomagnetic and structural data from intermediate to mafic composition lava flows and related dikes in all major blocks of the late Miocene Hamblin-Cleopatra Volcano, which was structurally dismembered during the development of the Lake Mead Fault System (LMFS), provide limits on the magnitude and sense of tilting and vertical axis rotation of crust during extension of this part of the Basin and Range province. Sinistral separation along the fault system dissected the volcano into three major blocks. The eastern, Cleopatra Lobe of the volcano is structurally the most intact section of the volcano. Normal and reverse polarity data from paleomagnetic sites collected along traverses in the Cleopatra Lobe yield an in situ grand mean of Declination (D) = 339??, Inclination (I) = +54??, ??95 = 3.1??, k = 27.2, N = 81 sites. The rocks of the central core of the volcano yield an in situ grand mean of D = 3??, I = + 59??, ??95 = 6.8??, k = 42.5, N = 11 sites (six normal, five reverse polarity). Sites collected within the western Hamblin Lobe of the volcano are exclusively of reverse polarity and yield an overall in situ mean of D = 168??, I = -58??, ??95 = 6.5??. k = 28.9, N = 18 sites. Interpretation of the paleomagnetic data in the context of the structural history of the volcano and surrounding area, considers the possibility of two different types of structural corrections. A stratigraphic tilt correction involves restoring flows to the horizontal using the present strike. This correction assumes no initial, possibly radial, dip of flows of the volcano and is considered invalid. A structural tilt correction to the data assumes that dikes of the radiating swarm associated with the volcano were originally vertical and results in block mean directions of D = 9??, I = +53??, ??95 = 3.1??, k = 27.2, and D = 58??, I = + 78??, ??95 = 6.8, k = 42.5, for the Cleopatra Lobe and the central intrusive core, respectively. The data from the Cleopatra Lobe are slightly discordant, in a clockwise sense, from expected middle- to late-Miocene field directions. The data from the volcano are not consistent with a proposed structural model of uniform, moderate magnitude, statistically significant, counter-clockwise vertical axis rotation of fault-bounded blocks during overall sinsitral displacement along the LMFS. We also analyzed dikes of the northernmost part of the Miocene Wilson Ridge hypabyssal igneous complex, strata of the Triassic Chinle Formation, and basalt flows of the Miocene West End Wash/Callville Mesa volcanic centers. Dikes in the Wilson Ridge pluton and the Triassic strata yield magnetizations with directions suggestive of statistically significant, clockwise, vertical-axis rotations consistent with local, large-magnitude shear of crustal fragments near some of the faults of the LMFS. Late Cenozoic deformation of the Hamblin-Cleopatra volcano area appears to have been non-uniform in scale and magnitude and no single structural model, involving strictly strike-slip faulting, can account for the observed paleomagnetic data. ?? 2001 Elsevier Science Ltd. All rights reserved.

Journal of Structural Geology↗
USGS1997

Initiation and growth of gypsum piercement structures in the Zechstein Basin

The importance of tectonic processes in initiating halite diapirs has become much better understood in recent years. Less well understood is the development of diapiric structures involving rocks composed predominantly of gypsum. Below about 1000 m, gypsum dehydrates to anhydrite, which often obscures primary sedimentary textures. If the strain associated with diapiric rise in the rock induces the transition to anhydrite, obliteration of primary features in the gypsum can be expected. In our study, we infer that the diapiric movement in the Werra Anhydrite member of cycle 1 of the Zechstein Formation of Europe occurred before the initial transition of gypsum to anhydrite based on the presence of pseudomorphs of bedded primary gypsum crystals, the overburden lithologies and depositional environment, and the mechanical properties of gypsum, anhydrite and carbonate rocks. Faulting and differential loading of a shallow overburden were the key components in initiating the gypsum diapirism. The transition to anhydrite occurred after burial and after cessation of diapirism. In comparison, the diapirism of calcium sulfate of the Leine Anhydrite into the Leine Halite members of cycle 3 of the Zechstein Formation probably occurred much later after burial and appears to have been triggered by halite diapirism, which in turn triggered the dehydration reaction, causing the calcium sulfate to become the incompetent phase relative to the halite.

Journal of Structural Geology↗
USGS2002

Proterozoic metamorphism and uplift history of the north-central Laramie Mountains, Wyoming, USA

The Laramie Mountains of south-eastern Wyoming contain two metamorphic domains that are separated by the 1.76 Ga. Laramie Peak shear zone (LPSZ). South of the LPSZ lies the Palmer Canyon block, where apatite U-Pb ages are c. 1745 Ma and the rocks have undergone Proterozoic kyanite-grade Barrovian metamorphism. In contrast, in the Laramie Peak block, north of the shear zone, the U-Pb apatite ages are 2.4-2.1 Ga, the granitic rocks are unmetamorphosed and supracrustal rocks record only low-T amphibolite facies metamorphism that is Archean in age. Peak mineral assemblages in the Palmer Canyon block include (a) quartz-biotite-plagioclase-garnet-staurolite-kyanite in the pelitic schists; (b) quartz-biotite-plagioclase-low-Ca amphiboles-kyanite in Mg-Al-rich schists, and locally (c) hornblende-plagioclase-garnet in amphibolites. All rock types show abundant textural evidence of decompression and retrograde re-equilibration. Notable among the texturally late minerals are cordierite and sapphirine, which occur in coronas around kyanite in Mg-Al-rich schists. Thermobarometry from texturally early and late assemblages for samples from different areas within the Palmer Canyon block define decompression from > 7 kbar to < 3 kbar. The high-pressure regional metamorphism is interpreted to be a response to thrusting associated with the Medicine Bow orogeny at c. 1.78-1.76 Ga. At this time, the north-central Laramie Range was tectonically thickened by as much as 12 km. This crustal thickening extended for more than 60 km north of the Cheyenne belt in southern Wyoming. Late in the orogenic cycle, rocks of the Palmer Canyon block were uplifted and unroofed as the result of transpression along the Laramie Peak shear zone to produce the widespread decompression textures. The Proterozoic tectonic history of the central Laramie Range is similar to exhumation that accompanied late-orogenic oblique convergence in many Phanerozoic orogenic belts.

Journal of Metamorphic Geology↗
USGS2003

Exhumation of high-pressure rocks beneath the Solund Basin, Western Gneiss Region of Norway

The Solund-Hyllestad-Lavik area affords an excellent opportunity to understand the ultrahigh-pressure Scandian orogeny because it contains a near-complete record of ophiolite emplacement, high-pressure metamorphism and large-scale extension. In this area, the Upper Allochthon was intruded by the c. 434 Ma Sogneskollen granodiorite and thrust eastward over the Middle/Lower Allochthon, probably in the Wenlockian. The Middle/Lower Allochthon was subducted to c. 50 km depth and the structurally lower Western Gneiss Complex was subducted to eclogite facies conditions at c. 80 km depth by c. 410-400 Ma. Within < 5-10 Myr, all these units were exhumed by the Nordfjord-Sogn detachment zone, producing shear strains > 100. Exhumation to upper crustal levels was complete by c. 403 Ma. The Solund fault produced the last few km of tectonic exhumation, bringing the near-ultrahigh-pressure rocks to within c. 3 km vertical distance from the low-grade Solund Conglomerate.

Journal of Metamorphic Geology↗
USGS1958

Contact metamorphism adjacent to a teschenite intrusion

Shale adjacent to the upper contact of an annular teschenite intrusion was converted to andalusite hornfels in an aureole 2–4 feet wide. At some points along the contact there is no evidence of anhydrous recrystallization. Rarely, magmatic reaction with small shale xenoliths resulted in formation of cordierite‐sillimanite (?) hornfelses, and locally a very small‐scale movement of magmatic constituents took place across the contact before consolidation of the teschenite. Very severe deuteric alteration of teschenite adjacent to the contact resulted in large losses of Si, Fe, Mg, alkalis, and probably Ca, some of which were recombined in clays and carbonates in the more porous shales and in joints, but analyses of uniformly fine‐grained shales indicate little change in bulk composition. Mineralogical evidence indicates a maximum contact temperature within the range 500°‐600° C, and a maximum intrusion temperature within the range 1000°‐1200° C.

Journal of the Geological Society of Australia↗
USGS1981

U-Th-Pb systematics of some granitoids from the northeastern Yilgarn Block, Western Australia and implications for uranium source rock potential

The Mount Boreas‐type granite and spatially associated syenitic granitoid of Western Australia yield Pb‐Pb ages of 2370 ± 100 Ma and 2760 ± 210 Ma, respectively. Th‐Pb ages, although less precise, are concordant with these ages, and therefore the apparent ages are interpreted to be the crystallisation ages for these two units. U‐Pb ages are variable and for the most part anomalously old, which suggests a Cainozoic uranium loss. However, this loss is generally small (< 3 μ g/g); therefore, neither granitoid in its fresh state provides a good source for nearby calcrete‐hosted uranium deposits. The possibility remains that the Mount Boreas‐type granite that has been completely weathered during the Tertiary could have been a source for the calcrete‐type uranium deposits in W.A. Although the Mount Boreas‐type granite is highly fractionated, it does not bear a strong geochemical imprint of a sedimentary precursor. This feature contrasts it with apparently fresh granitoids from other parts of the world that have lost large amounts of uranium (∼ 20 μ g/g ) and are associated with large roll‐type and other low temperature‐type uranium deposits.

Journal of the Geological Society of Australia↗
USGS2007

High-pressure amphibolite facies dynamic metamorphism and the Mesozoic tectonic evolution of an ancient continental margin, east- central Alaska

Ductilely deformed amphibolite facies tectonites comprise two adjacent terranes in east-central Alaska: the northern, structurally higher Taylor Mountain terrane and the southern, structurally lower Lake George subterrane of the Yukon-Tanana terrane. The pressure, temperature, kinematic and age data are interpreted to indicate that the metamorphism of the Taylor Mountain terrane and Lake George subterrane took place during different phases of a latest Palaeozoic through early Mesozoic shortening episode resulting from closure of an ocean basin now represented by klippen of the Seventymile-Slide Mountain terrane. High- to intermediate-pressure metamorphism of the Taylor Mountain terrane took place within a SW-dipping (present-day coordinates) subduction system. High- to intermediate-pressure metamorphism of the Lake George subterrane and the structural contact zone occurred during NW-directed overthrusting of the Taylor Mountain, Seventymile-Slide Mountain and Nisutlin terranes, and imbrication of the continental margin in Jurassic time. -from Authors

Alaska↗
USGS2007

Metamorphic and structural history of continental crust at a Mesozoic collisional margin, the Ruby terrane, central Alaska

The Ruby terrane is an elongate fragment of continental crustal rocks that is structurally overlain by thrust slices of oceanic crust. Our results from the Kokrines Hills, in the south‐central part of the Ruby terrane, demonstrate that the low‐angle schistose fabric formed under high‐ P /low‐ T conditions, at peak conditions of 10.8‐13.2 kbar and 425‐550° C, consistent with the rare occurrence of glaucophane. White mica 40 Ar/ 39 Ar cooling ages from these blueschists indicate that the metamorphism occurred prior to 144 ± 1 Ma. The blueschist facies assemblages are partially replaced by greenschist facies assemblages in the eastern Kokrines Hills. In contrast, in the central and western Kokrines Hills, upper amphibolite to lower granulite facies metamorphism associated with extensive late Early Cretaceous plutonism has completely overprinted any evidence of an earlier high‐ P/T metamorphic history. Deformation accompanying the plutonism produced recumbent isoclinal folds in the plutonic rocks and pelitic gneisses of the wallrock; decompression reactions in the pelitic gneisses suggest that the deformation occurred during exhumation. Thermochronological data bracket the time of intrusion and cooling below 500° C between 118 ± 3 and 109 ± 1 Ma. Our data from the schists of the Ruby terrane support the general assumption of many authors that the Ruby terrane was subducted beneath an oceanic island arc. This tectonic history is similar to that described for other large continental crustal blocks in northern and central Alaska, in the Brooks Range, Seward Peninsula and Yukon‐Tanana Upland. The current orientation of the Ruby terrane at an oblique angle to these other crustal blocks and to the Cordilleran trend is due to post‐collisional tectonic processes that have greatly modified the original continental margin.

Alaska↗
USGS2007

Organic metamorphism in the Lower Mississippian-Upper Devonian Bakken shales. Part 1: Rock-Eval pyrolysis and vitrinite reflectance

Detailed organic geochemistry has been performed on a large number of Lower Mississippian-Upper Devonian Bakken shales from the North Dakota portion of the Williston Basin, and 28 oils mainly from Mississippian Madison Group rocks from different basinal areas. Here we report results of Rock-Eval pyrolysis and vitrinite reflectance ( R o ) analyses. Variable paleoheat flows in the Williston Basin caused the threshold of intense hydrocarbon generation to occur at different depths in different basinal areas. In higher paleogeothermal gradient basinal areas, this event occurred at depths of 7,650-8,000 ft, and at 10,000 ft or deeper in lower paleogeothermal gradient areas of the Basin. Distinct organic metamorphic imprints in Williston Basin sediments were also caused by extreme, but variable, paleoheat flows in the basin, as well as secondary migration of crude oils from deep basinal source areas. The high paleoheat flows are postulated as being due to a Late Cretaceous - Paleocene aborted rift event. Only a small volume of Bakken shales in restricted areas of the Williston Basin was responsible for the oil found reservoired in Mississippian Madison Group rocks. However, this small shale volume has been responsible for a relatively large amount of crude oil. R o profiles in the Tertiary through Middle Jurassic rocks in the Williston Basin had steep, linear R o versus depth gradients, with strong reversals of R o values occurring in the Lower Jurassic rocks. The lower Mesozoic through Paleozoic rocks of the Basin had strongly suppressed R o values compared to the values in the Tertiary through Middle Jurassic rocks. This was especially true of the R o values in the Bakken shales. This R o suppression was due to a change in organic matter (OM) type from oxygen-rich terrestrially derived OM in the younger rocks, to a hydrogen-rich marine derived OM in the deeper, older rocks. The threshold of intense oil generation (TIHG) occurred in the Bakken shales of the Williston Basin at R o values somewhere between 0.9 and 1.7% (best estimate 0.9), as would be read in oxygen-rich OM. Much higher burial temperatures (and consequently R o values) than usually held to be necessary were required for both the TIHG as well as mainstage hydrocarbon generation in the Bakken shales. These results are most likely applicable in general to source rocks with hydrogen-rich OM. The data of this study have major implications to petroleum exploration as well as to petroleum resource assessment.

Journal of Petroleum Geology↗
USGS2007

Oceanic methane hydrate: The character of the Blake Ridge hydrate stability zone, and the potential for methane extraction

Oceanic methane hydrates are mineral deposits formed from a crystalline 'ice' of methane and water in sea-floor sediments (buried to less than about 1 km) in water depths greater than about 500 m; economic hydrate deposits are probably restricted to water depths of between 1.5 km and 4 km. Gas hydrates increase a sediment's strength both by 'freezing' the sediment and by filling the pore spaces in a manner similar to water-ice in permafrost. Concentrated hydrate deposits may be underlain by significant volumes of methane gas, and these localities are the most favourable sites for methane gas extraction operations. Seismic reflection records indicate that trapped gas may blow-out naturally, causing large-scale seafloor collapse. In this paper, we consider both the physical properties and the structural integrity of the hydrate stability zone and the associated free gas deposits, with special reference to the Blake Ridge area, SE US offshore, in order to help establish a suitable framework for the safe, efficient, and economic recovery of methane from oceanic gas hydrates. We also consider the potential effects of the extraction of methane from hydrate (such as induced sea-floor faulting, gas venting, and gas-pocket collapse). We assess the ambient pressure effect on the production of methane by hydrate dissociation, and attempt to predict the likelihood of spontaneous gas flow in a production situation.Oceanic methane hydrates are mineral sits formed from a crystalline `ice' of methane and water in sea-floor sediments (buried to less than about 1 km) in water depths greater than about 500 m; economic hydrate deposits are probably restricted to water depths of between 1.5 km and 4 km. Gas hydrates increase a sediment's strength both by `freezing' the sediment and by filling the pore spaces in a manner similar to water-ice in permafrost. Concentrated hydrate deposits may be underlain by significant volumes of methane gas, and these localities are the most favourable sites for methane gas extraction operations. Seismic reflection records indicate that trapped gas may blow-out naturally, causing large-scale seafloor collapse. In this paper, we consider both the physical properties and the structural integrity of the hydrate stability zone and the associated free gas deposits, with special reference to the Blake Ridge area, SE US offshore, in order to help establish a suitable framework for the safe, efficient, and economic recovery of methane from oceanic gas hydrates. We also consider the potential effects of the extraction of methane from hydrate (such as induced sea-floor faulting, gas venting, and gas-pocket collapse). We assess the ambient pressure effect on the production of methane by hydrate dissociation, and attempt to predict the likelihood of spontaneous gas flow in a production situation.

Journal of Petroleum Geology↗
USGS2016

Reaction softening by dissolution–precipitation creep in a retrograde greenschist facies ductile shear zone, New Hampshire, USA

We describe strain localization by a mixed process of reaction and microstructural softening in a lower greenschist facies ductile fault zone that transposes and replaces middle to upper amphibolite facies fabrics and mineral assemblages in the host schist of the Littleton Formation near Claremont, New Hampshire. Here, Na-poor muscovite and chlorite progressively replace first staurolite, then garnet, and finally biotite porphyroblasts as the core of the fault zone is approached. Across the transect, higher grade fabric-forming Na-rich muscovite is also progressively replaced by fabric-forming Na-poor muscovite. The mineralogy of the new phyllonitic fault-rock produced is dominated by Na-poor muscovite and chlorite together with late albite porphyroblasts. The replacement of the amphibolite facies porphyroblasts by muscovite and chlorite is pseudomorphic in some samples and shows that the chemical metastability of the porphyroblasts is sufficient to drive replacement. In contrast, element mapping shows that fabric-forming Na-rich muscovite is selectively replaced at high-strain microstructural sites, indicating that strain energy played an important role in activating the dissolution of the compositionally metastable muscovite. The replacement of strong, high-grade porphyroblasts by weaker Na-poor muscovite and chlorite constitutes reaction softening. The crystallization of parallel and contiguous mica in the retrograde foliation at the expense of the earlier and locally crenulated Na-rich muscovite-defined foliation destroys not only the metastable high-grade mineralogy, but also its stronger geometry. This process constitutes both reaction and microstructural softening. The deformation mechanism here was thus one of dissolution–precipitation creep, activated at considerably lower stresses than might be predicted in quartzofeldspathic rocks at the same lower greenschist facies conditions.

New Hampshire↗
USGS2024

Deformation-induced graphitization and muscovite recrystallization in a ductile fault zone

A suite of slate samples collected along a 2 km transect crossing the Lishan fault in central Taiwan were evaluated to assess the role of ductile deformation in natural graphitization at lower greenschist facies metamorphic conditions. The process of natural aromatization, or graphitization, of an organic precursor is well established as a thermally driven process; however, experimental studies have shown that the energy provided by deformation can substantially reduce the activation energy required for graphitization. This study provides a natural example of deformation-induced graphitization. A strain gradient approaching the Lishan fault was established by scanning electron microscope imaging and X-ray diffraction analysis of phyllosilicates and quartz that showed an increase in the strength of slaty cleavage development via dissolution-precipitation processes. Thermal conditions were constrained to be near isothermal using calcite-dolomite geothermometry. Raman spectroscopic results from carbonaceous material, including D1-full width-at-half-maximum (FWHM), G-FWHM, Raman band separation (RBS), and a lesser-known vibrational mode B 2g -FWHM, showed robust linear trends across the same sampling transect. However, the G-FWHM parameter showed a trend opposite of that expected from thermally driven graphitization. The Raman results are interpreted to reflect a strain-driven reduction in graphite crystallite size (decrease in G-FWHM) but improvement in structural ordering in individual coherent domains. A multiple linear regression with an R 2 value of 0.92 predicts the graphite D1-FWHM values from the XRD-derived ratio of muscovite populations and muscovite microstrain, demonstrating the concomitant recrystallization of silicates and carbonaceous material across the strain gradient, despite the disparate processes accommodating the deformation. This study demonstrates the role of deformation in natural graphitization and provides a new perspective on the use of graphite as a geothermometer in strongly deformed greenschist facies rocks.

Journal of Metamorphic Geology↗
USGS2022

Quaternary fans and terraces in the Khumbu Himal south of Mount Everest: their characteristics, age and formation

Large fans and terraces are frequent in the Khumbu Himal within the high Himalayan valleys south of Mt. Everest. These features are composed of massive matrix- and clast-supported diamicts that were formed from both hyperconcentrated flows and coarse-grained debris flows. Cosmogenic radionuclide (CRN) exposure ages for boulders on fans and terraces indicate that periods of fan and terrace formation occurred at c. 16, c. 12, c. 8, c. 4 and c. 1.5 ka, and are broadly coincident with the timing of glaciation in the region. The dating precision is insufficient to resolve whether the surfaces formed before, during or after the correlated glacial advance. However, the sedimentology, and morphostratigraphic and geomorphological relationships suggest that fan and terrace sedimentation in this part of the Himalaya primarily occurs during glacier retreat and is thus paraglacial in origin. Furthermore, modern glacial-lake outburst floods and their associated deposits are common in the Khumbu Himal as the result of glacial retreat during historical times. We therefore suggest that Late Quaternary and Holocene fan and terrace formation and sediment transfer are probably linked to temporal changes in discharge and sediment load caused by glacier oscillations responding to climate change. The timing of major sedimentation events in this region can be correlated with fans and terraces in other parts of the Himalaya, suggesting that major sedimentation throughout the Himalaya is synchronous and tied to regional climatic oscillations. Bedrock incision rates calculated from strath terrace ages average c. 3.9 mm a −1 , suggesting that the overall rate of incision is set by regional uplift.

Imja Khola Valley, Khumbu Himal, Mount Everest↗
USGS2022

Graptolite succession across the Ordovician–Silurian boundary in south-eastern Alaska

The first discovery in North America of a succession of graptolite faunas across the Ordovician–Silurian boundary has been made on Esquibel Island, in south-eastern Alaska, where five graptolite zones are represented in an 18 m interval of shale in the Descon Formation. Despite the thinness of the graptolite zones, no stratigraphical breaks can be inferred from more complete graptolite successions known elsewhere. The Alaskan faunas can be closely correlated with those of the standard British zones—the Dicellograptus anceps Zone (uppermost Ordovician), the Glyptograptus persculptus Zone (lowermost Silurian) and the immediately overlying Akidograptus acuminatus, Orthograptus vesiculosus and Monograptus cyphus Zones of the Lower Silurian. They thus give evidence of the cosmopolitan distribution of graptolite faunas during latest Ordovician and earliest Silurian time.

Alaska↗