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Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

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At least 397 records · Page 22Linked to original sources
USGS2005

The Pinto shear zone; a Laramide synconvergent extensional shear zone in the Mojave Desert region of the southwestern United States

The Pinto shear zone is one of several Late Cretaceous shear zones within the eastern fringe of the Mesozoic magmatic arc of the southwest Cordilleran orogen that developed synchronous with continued plate convergence and backarc shortening. We demonstrate an extensional origin for the shear zone by describing the shear-zone geometry and kinematics, hanging wall deformation style, progressive changes in deformation temperature, and differences in hanging wall and footwall thermal histories. Deformation is constrained between ???74 and 68 Ma by 40Ar/39Ar thermochronology of the exhumed footwall, including multi-diffusion domain modeling of K-feldspar. We discount the interpretations, applied in other areas of the Mojave Desert region, that widespread Late Cretaceous cooling results from refrigeration due to subduction of a shallowly dipping Laramide slab or to erosional denudation, and suggest alternatively that post-intrusion cooling and exhumation by extensional structures are recorded. Widespread crustal melting and magmatism followed by extension and cooling in the Late Cretaceous are most consistent with production of a low-viscosity lower crust during anatexis and/or delamination of mantle lithosphere at the onset of Laramide shallow subduction. ?? 2005 Elsevier Ltd. All rights reserved.

Journal of Structural Geology↗
USGS2006

Influence of deflection on a fold-to-fault progression: field evidence from near Marietta, South Carolina

Four periods of deformation (D1-D4) are recognized in the Lion Park Road borrow pit near Marietta, South Carolina. Although each period is characterized by distinct structures, D3 produced two structural styles (D3a, D3b) resulting from layer-parallel shortening. D3a is characterized by detachment folding at the tip of an underlying thrust. D3b is a fold-to-fault progression that was localized by east-dipping, quartz-filled gash fractures. The fold-to-fault progression demonstrates the influence of a mechanical anisotropy on ramp development. The early stages of D3b were formed by deflection of northwest-directed, layer-parallel shortening and active, down-section propagation of folds and thrusts. Following connection with a splay of basal detachment, later D3b stages resulted from up-section movement that produced kink folding and a throughgoing thrust. This up-section movement deformed and modified the geometries of older, down-section structures. Detailed mesoscopic field observations, integrated with a combination of current thrust fault models, are used to interpret the D3b fold-to-fault progression. ?? 2006 Elsevier Ltd. All rights reserved.

Journal of Structural Geology↗
USGS2009

Structural controls on fractured coal reservoirs in the southern Appalachian Black Warrior foreland basin

Coal is a nearly impermeable rock type for which the production of fluids requires the presence of open fractures. Basin-wide controls on the fractured coal reservoirs of the Black Warrior foreland basin are demonstrated by the variability of maximum production rates from coalbed methane wells. Reservoir behavior depends on distance from the thrust front. Far from the thrust front, normal faults are barriers to fluid migration and compartmentalize the reservoirs. Close to the thrust front, rates are enhanced along some normal faults, and a new trend is developed. The two trends have the geometry of conjugate strike-slip faults with the same ?? 1 direction as the Appalachian fold-thrust belt and are inferred to be the result of late pure-shear deformation of the foreland. Face cleat causes significant permeability anisotropy in some shallow coal seams but does not produce a map-scale production trend. ?? 2008 Elsevier Ltd. All rights reserved.

Journal of Structural Geology↗
USGS2008

Dependence of displacement-length scaling relations for fractures and deformation bands on the volumetric changes across them

Displacement–length data from faults, joints, veins, igneous dikes, shear deformation bands, and compaction bands define two groups. The first group, having a power-law scaling relation with a slope of n = 1 and therefore a linear dependence of maximum displacement and discontinuity length ( D max = γL ), comprises faults and shear (non-compactional or non-dilational) deformation bands. These shearing-mode structures, having shearing strains that predominate over volumetric strains across them, grow under conditions of constant driving stress, with the magnitude of near-tip stress on the same order as the rock's yield strength in shear. The second group, having a power-law scaling relation with a slope of n = 0.5 and therefore a dependence of maximum displacement on the square root of discontinuity length ( D max = αL 0.5 ), comprises joints, veins, igneous dikes, cataclastic deformation bands, and compaction bands. These opening- and closing-mode structures grow under conditions of constant fracture toughness, implying significant amplification of near-tip stress within a zone of small-scale yielding at the discontinuity tip. Volumetric changes accommodated by grain fragmentation, and thus control of propagation by the rock's fracture toughness, are associated with scaling of predominantly dilational and compactional structures with an exponent of n = 0.5.

Journal of Structural Geology↗
USGS2009

Growth trishear model and its application to the Gilbertown graben system, southwest Alabama

Fault-propagation folding associated with an upward propagating fault in the Gilbertown graben system is revealed by well-based 3-D subsurface mapping and dipmeter analysis. The fold is developed in the Selma chalk, which is an oil reservoir along the southern margin of the graben. Area-depth-strain analysis suggests that the Cretaceous strata were growth units, the Jurassic strata were pregrowth units, and the graben system is detached in the Louann Salt. The growth trishear model has been applied in this paper to study the evolution and kinematics of extensional fault-propagation folding. Models indicate that the propagation to slip (p/s) ratio of the underlying fault plays an important role in governing the geometry of the resulting extensional fault-propagation fold. With a greater p/s ratio, the fold is more localized in the vicinity of the propagating fault. The extensional fault-propagation fold in the Gilbertown graben is modeled by both a compactional and a non-compactional growth trishear model. Both models predict a similar geometry of the extensional fault-propagation fold. The trishear model with compaction best predicts the fold geometry. ?? 2008 Elsevier Ltd. All rights reserved.

Journal of Structural Geology↗
USGS2009

Steeply-dipping extension fractures in the Newark basin, New Jersey

Late Triassic and Early Jurassic bedrock in the Newark basin is pervasively fractured as a result of Mesozoic rifting of the east-central North American continental margin. Tectonic rifting imparted systematic sets of steeply-dipping, en ??chelon, Mode I, extension fractures in basin strata including ordinary joints and veins. These fractures are arranged in transitional-tensional arrays resembling normal dip-slip shear zones. They contributed to crustal stretching, sagging, and eventual faulting of basin rift deposits. Extension fractures display progressive linkage and spatial clustering that probably controlled incipient fault growth. They cluster into three prominent strike groups correlated to early, intermediate, and late-stage tectonic events reflecting about 50- 60?? of counterclockwise rotation of incremental stretching directions. Finite strain analyses show that extension fractures allowed the stretching of basin strata by a few percent, and these fractures impart stratigraphic dips up to a few degrees in directions opposing fracture dips. Fracture groups display three-dimensional spatial variability but consistent geometric relations. Younger fractures locally cut across and terminate against older fractures having more complex vein-cement morphologies and bed-normal folds from stratigraphic compaction. A fourth, youngest group of extension fractures occur sporadically and strike about E-W in obliquely inverted crustal blocks. A geometric analysis of overlapping fracture sets shows how fracture groups result from incremental rotation of an extending tectonic plate, and that old fractures can reactivate with oblique slip components in the contemporary, compressive stress regime. ?? 2008 Elsevier Ltd. All rights reserved.

Journal of Structural Geology↗
USGS2010

Internal structure, fault rocks, and inferences regarding deformation, fluid flow, and mineralization in the seismogenic Stillwater normal fault, Dixie Valley, Nevada

Outcrop mapping and fault-rock characterization of the Stillwater normal fault zone in Dixie Valley, Nevada are used to document and interpret ancient hydrothermal fluid flow and its possible relationship to seismic deformation. The fault zone is composed of distinct structural and hydrogeological components. Previous work on the fault rocks is extended to the map scale where a distinctive fault core shows a spectrum of different fault-related breccias. These include predominantly clast-supported breccias with angular clasts that are cut by zones containing breccias with rounded clasts that are also clast supported. These are further cut by breccias that are predominantly matrix supported with angular and rounded clasts. The fault-core breccias are surrounded by a heterogeneously fractured damage zone. Breccias are bounded between major, silicified slip surfaces, forming large pod-like structures, systematically oriented with long axes parallel to slip. Matrix-supported breccias have multiply brecciated, angular and rounded clasts revealing episodic deformation and fluid flow. These breccias have a quartz-rich matrix with microcrystalline anhedral, equant, and pervasively conformable mosaic texture. The breccia pods are interpreted to have formed by decompression boiling and rapid precipitation of hydrothermal fluids whose flow was induced by coseismic, hybrid dilatant-shear deformation and hydraulic connection to a geothermal reservoir. The addition of hydrothermal silica cement localized in the core at the map scale causes fault-zone widening, local sealing, and mechanical heterogeneities that impact the evolution of the fault zone throughout the seismic cycle. ?? 2010.

Journal of Structural Geology↗
USGS2011

Mechanical analysis of fault slip data: Implications for paleostress analysis

Stress inversions are a useful and popular tool for structural geologist and seismologist alike. These methods were first introduced by Wallace (1951) and Bott (1959) and subsequent studies continue to be based on their assumptions: the remote stress tensor is spatially uniform for the rock mass containing the faults and temporally constant over the history of faulting in that region, and the slip on each fault surface has the same direction and sense as the maximum shear stress resolved on that surface from the remote stress tensor. Furthermore, successful implementation requires that slip accumulates on faults of diverse orientation. Many studies employ these methods on isolated faults or on fault systems with limited ranges of orientations, which can lead to erroneous results. We propose a new method that incorporates the effects of mechanical interaction of the entire fault or fault system, and solves the complete mechanical problem rather than employing empirical relationships between slip and stress or strain (or strain rate). The method requires knowledge of the fault geometry and information on at least one slip vector component along portions of the known fault geometry. For example, if throw is known, the strike-slip component can be solved for. We test the method using a single synthetic fault with anisotropic roughness similar to that measured at fault outcrops. While the orientation of remote stress may be determined precisely, the lack of diverse fault orientations introduces a systematic error in the remote stress ratio. We further test the effect of diversity of fault orientations and find that Wallace–Bott type inversions do not perform as well for limited ranges of orientations when compared to the proposed method. Finally, we use published data from the 1999 Chi-Chi, Taiwan, earthquake, and find that the method using surface data only, and surface data with subsurface focal mechanisms, produce similar results. The resulting stress orientations are in good agreement with results from Wallace–Bott inversions. Furthermore, the slip distribution is in general agreement with kinematic slip inversions using coseismic surface deformation. Stress inversion methods using fault slip data can thus be improved upon, significantly in some cases, by solving a mechanical boundary value problem that takes into account the geometry of faults or fault systems. As a bonus, the solution provides the stress, strain, and displacement fields throughout the region and the slip distributions on the faults.

Journal of Structural Geology↗
USGS2012

Observed source parameters for dynamic rupture with non-uniform initial stressand relatively high fracture energy

We have conducted dynamic rupture propagation experiments to establish the relations between in-source stress drop, fracture energy and the resulting particle velocity during slip of an unconfined 2 m long laboratory fault at normal stresses between 4 and 8 MPa. To produce high fracture energy in the source we use a rough fault that has a large slip weakening distance. An artifact of the high fracture energy is that the nucleation zone is large such that precursory slip reduces fault strength over a large fraction of the total fault length prior to dynamic rupture, making the initial stress non-uniform. Shear stress, particle velocity, fault slip and acceleration were recorded coseismically at multiple locations along strike and at small fault-normal distances. Stress drop increases weakly with normal stress. Average slip rate depends linearly on the fault strength loss and on static stress drop, both with a nonzero intercept. A minimum fracture energy of 1.8 J/m 2 and a linear slip weakening distance of 33 μm are inferred from the intercept. The large slip weakening distance also affects the average slip rate which is reduced by in-source energy dissipation from on-fault fracture energy. Because of the low normal stress and small per event slip (∼86 μm), no thermal weakening such as melting or pore fluid pressurization occurs in these experiments. Despite the relatively high fracture energy, and the very low heat production, energy partitioning during these laboratory earthquakes is very similar to typical earthquake source properties. The product of fracture energy and fault area is larger than the radiated energy. Seismic efficiency is low at ∼2%. The ratio of apparent stress to static stress drop is ∼27%, consistent with measured overshoot. The fracture efficiency is ∼33%. The static and dynamic stress drops when extrapolated to crustal stresses are 2–7.3 MPa and in the range of typical earthquake stress drops. As the relatively high fracture energy reduces the slip velocities in these experiments, the extrapolated average particle velocities for crustal stresses are 0.18–0.6 m/s. That these experiments are consistent with typical earthquake source properties suggests, albeit indirectly, that thermal weakening mechanisms such as thermal pressurization and melting which lead to near complete stress drops, dominate earthquake source properties only for exceptional events unless crustal stresses are low.

Journal of Structural Geology↗
USGS2012

Correlation of clayey gouge in a surface exposure of the San Andreas fault with gouge at depth from SAFOD: Implications for the role of serpentinite in fault mechanics

Magnesium-rich clayey gouge similar to that comprising the two actively creeping strands of the San Andreas Fault in drill core from the San Andreas Fault Observatory at Depth (SAFOD) has been identified in a nearby outcrop of serpentinite within the fault zone at Nelson Creek. Each occurrence of the gouge consists of porphyroclasts of serpentinite and sedimentary rocks dispersed in a fine-grained, foliated matrix of Mg-rich smectitic clays. The clay minerals in all three gouges are interpreted to be the product of fluid-assisted, shear-enhanced reactions between quartzofeldspathic wall rocks and serpentinite that was tectonically entrained in the fault from a source in the Coast Range Ophiolite. We infer that the gouge at Nelson Creek connects to one or both of the gouge zones in the SAFOD core, and that similar gouge may occur at depths in between. The special significance of the outcrop is that it preserves the early stages of mineral reactions that are greatly advanced at depth, and it confirms the involvement of serpentinite and the Mg-rich phyllosilicate minerals that replace it in promoting creep along the central San Andreas Fault.

California↗
USGS2014

Deep permeability of the San Andreas Fault from San Andreas Fault Observatory at Depth (SAFOD) core samples

The San Andreas Fault Observatory at Depth (SAFOD) scientific borehole near Parkfield, California crosses two actively creeping shear zones at a depth of 2.7 km. Core samples retrieved from these active strands consist of a foliated, Mg-clay-rich gouge containing porphyroclasts of serpentinite and sedimentary rock. The adjacent damage zone and country rocks are comprised of variably deformed, fine-grained sandstones, siltstones, and mudstones. We conducted laboratory tests to measure the permeability of representative samples from each structural unit at effective confining pressures, P e up to the maximum estimated in situ P e of 120 MPa. Permeability values of intact samples adjacent to the creeping strands ranged from 10 −18 to 10 −21 m 2 at P e = 10 MPa and decreased with applied confining pressure to 10 −20 –10 −22 m 2 at 120 MPa. Values for intact foliated gouge samples (10 −21 –6 × 10 −23 m 2 over the same pressure range) were distinctly lower than those for the surrounding rocks due to their fine-grained, clay-rich character. Permeability of both intact and crushed-and-sieved foliated gouge measured during shearing at P e ≥ 70 MPa ranged from 2 to 4 × 10 −22 m 2 in the direction perpendicular to shearing and was largely insensitive to shear displacement out to a maximum displacement of 10 mm. The weak, actively-deforming foliated gouge zones have ultra-low permeability, making the active strands of the San Andreas Fault effective barriers to cross-fault fluid flow. The low matrix permeability of the San Andreas Fault creeping zones and adjacent rock combined with observations of abundant fractures in the core over a range of scales suggests that fluid flow outside of the actively-deforming gouge zones is probably fracture dominated.

Journal of Structural Geology↗
USGS2014

Comparative mineral chemistry and textures of SAFOD fault gouge and damage-zone rocks

Creep in the San Andreas Fault Observatory at Depth (SAFOD) drillhole is localized to two foliated gouges, the central deforming zone (CDZ) and southwest deforming zone (SDZ). The gouges consist of porphyroclasts of serpentinite and sedimentary rock dispersed in a foliated matrix of Mg-smectite clays that formed as a result of shearing-enhanced reactions between the serpentinite and quartzofeldspathic rocks. The CDZ takes up most of the creep and exhibits differences in mineralogy and texture from the SDZ that are attributable to its higher shearing rate. In addition, a ∼0.2-m-wide sector of the CDZ at its northeastern margin (NE-CDZ) is identical to the SDZ and may represent a gradient in creep rate across the CDZ. The SDZ and NE-CDZ have lower clay contents and larger porphyroclasts than most of the CDZ, and they contain veinlets and strain fringes of calcite in the gouge matrix not seen elsewhere in the CDZ. Matrix clays in the SDZ and NE-CDZ are saponite and corrensite, whereas the rest of the CDZ lacks corrensite. Saponite is younger than corrensite, reflecting clay crystallization under declining temperatures, and clays in the more actively deforming portions of the CDZ have better equilibrated to the lower-temperature conditions.

California↗
USGS2016

Hydrothermal frictional strengths of rock and mineral samples relevant to the creeping section of the San Andreas Fault

We compare frictional strengths in the temperature range 25–250 °C of fault gouge from SAFOD (CDZ and SDZ) with quartzofeldspathic wall rocks typical of the central creeping section of the San Andreas Fault (Great Valley sequence and Franciscan Complex). The Great Valley and Franciscan samples have coefficients of friction, μ > 0.35 at all experimental conditions. Strength is unchanged between 25° and 150 °C, but μ increases at higher temperatures, exceeding 0.50 at 250 °C. Both samples are velocity strengthening at room temperature but show velocity-weakening behavior beginning at 150 °C and stick-slip motion at 250 °C. These rocks, therefore, have the potential for unstable seismic slip at depth. The CDZ gouge, with a high saponite content, is weak (μ = 0.09–0.17) and velocity strengthening in all experiments, and μ decreases at temperatures above 150 °C. Behavior of the SDZ is intermediate between the CDZ and wall rocks: μ < 0.2 and does not vary with temperature. Although saponite is probably not stable at depths greater than ∼3 km, substitution of the frictionally similar minerals talc and Mg-rich chlorite for saponite at higher temperatures could potentially extend the range of low strength and stable slip down to the base of the seismogenic zone.

California↗
USGS2018

Frictional properties and 3-D stress analysis of the southern Alpine Fault, New Zealand

New Zealand's Alpine Fault (AF) ruptures quasi-periodically in large-magnitude earthquakes. Paleoseismological evidence suggests that about half of all recognized AF earthquakes terminated at the boundary between the Central and South Westland sections of the fault. There, fault geometry and the polarity of uplift change. The South Westland AF exhibits oblique-normal fault motion on a structure oriented 052°/82°SE that, for at least 35 km along strike, contains saponite-rich principal slip zone gouges. New hydrothermal friction experiments reveal that the saponite fault gouge is frictionally weak, exhibiting friction coefficients between μ = 0.12 and μ = 0.16 for a range of temperatures ( T = 25–210 °C) and effective normal stresses ( σ n ' = 31.2–93.6 MPa). The saponite gouge is rate-strengthening in all velocity steps performed at velocities between 0.01 and 3.0 μm/s, behavior conducive to aseismic creep. A three-dimensional stress analysis shows that the South Westland AF is favorably oriented with respect to the regional stress field for slip within the frictionally weak saponite fault gouge. Geometrically, the fault is severely misoriented for slip in any fault-forming materials with friction coefficients exceeding μ ∼0.5. The combination of weak gouges prone to aseismic creep, strong asperities , and low resolved shear stress may impede earthquake rupture propagation along the South Westland Alpine Fault.

Journal of Structural Geology↗
USGS2018

Structural evolution of a gold-bearing transtensional zone within the Archean Porcupine-Destor deformation zone, southern Abitibi greenstone belt, eastern Ontario, Canada

The Garrison camp comprises four structurally distinct orogenic gold deposits that formed in different host lithologies during progressive deformation. Detailed field mapping, drill core logging, and geochronological constraints suggest that the 2678 ± 2 Ma Garrison granitic stock played a fundamental rheological role in the location of the four deposits. Initial local shear movement occurred along the southwestern margin of the stock leading to the development of the NW-trending sinistral NE-side-up Buffonta shear zone, which hosts the Buffonta deposit. Subsequently, a transtensional zone formed between the NE-trending sinistral Porcupine-Destor and Munro deformation zones, which host the 903 and Jonpol deposits, respectively. Finally, a local change in shortening orientation from NE to NNW caused westerly directed extension resulting in the formation of the younger gold-bearing veins composing the Garrcon deposit. In situ U-Pb laser ablation-inductively coupled plasma-mass spectrometry performed on monazite grains formed within the shear fabric of the Munro deformation zone indicates that transtension occurred at 2657 ± 15 Ma. Therefore, at least three of the four deposits formed subsequent to crystallization of the Garrison granitic stock. The reported U-Pb dates represent the first direct age constraints on the movement along a gold-bearing deformation zone in the southern Abitibi greenstone belt.

Ontario↗
USGS2000

Trace element geochemistry of altered volcanic ash layers (tonsteins) in late Permian coal-bearing formations of eastern Yunnan and western Guizhou Provinces, China

Trace element compositions were determined (by instrumental neutron activation analysis; INAA) in 30 samples of synsedimentary volcanic ash-derived tonsteins and detrital claystones from coal seams within the late Permian coal-bearing formation of eastern Yunnan and western Guizhou Provinces, China. The characteristics of trace-element geochemistry in the tonsteins can be distinguished from those of detrital claystones because of the former's unique volcanic-ash origin. The detrital claystones are characterized by their relatively high content of V, Ti, Sc, Cr, Co and Ni, relatively low content of Th and U, Th/U ratio, and small negative Eu anomaly (Eu/Eu* 0.63-0.93). Overall, these trace element characteristics are consistent with a mafic source similar to the composition of basalt rocks in the erosional region on the western edge of the study area. In contrast, the tonsteins are low in V, Ti, Sc, Cr, Co and Ni contents and have a high Th/U ratio with a distinct negative Eu anomaly (Eu/Eu* normally in the range of 0.2-0.4), consistent with a silicic magmatic source. Within the group of tonsteins, those from the lower section (P2.1) of the coal-bearing formation are relatively high in Ti, Zr, Hf, Nb, Ta and rare earth elements (REE), as compared to those from the middle and upper sections (P2.2+3). In trace-element discrimination diagrams (scatter plots) of Hf-Ta, Ti-Ta, Ti-V, Hf-Sc, Lu-Hf and Lu-Th, tonsteins from the P2.1 horizon always fall in isolated distribution areas, separate from the tonsteins of the P2.2+3 horizon. These results suggest that the source materials of tonsteins from the two separate horizons were probably derived from volcanic ash falls of two distinctly different natures. Based on a comparison of the concentrations and assemblages of trace elements between various magmatic rocks, the source materials of tonsteins from P2.1 horizon were mostly composed of calc-alkalic, silica-poor volcanic ash (similar to rhyodacitic magma), whereas those from P2.+3 were apparently more siliceous and K-rich (rhyolitic magma). Thus, tonsteins from the two different horizons are characterized by unique geochemical properties, which remain constant over a wide lateral extent. Integration of trace-elemental compositions with mineralogical and textural observations makes possible the establishment of tonstein stratigraphy, thus, facilitating more precise and reliable coal-seam correlations. (C) 2000 Elsevier Science B.V. All rights reserved.Trace element compositions were determined in 30 samples of synsedimentary volcanic ash-derived tonsteins and detrital claystones from coal seams within the late Permian coal-bearing formation of eastern Yunnan and western Guizhou Provinces, China. The characteristics of trace-element geochemistry in the tonsteins can be distinguished from those of detrital claystones because of the former's unique volcanic-ash origin. The detrital claystones are characterized by their relatively high content of V, Ti, Sc, CR, Co and Ni, relatively low content of Th and U, Th/U ratio, and small negative Eu anomaly.

International Journal of Coal Geology↗
USGS2001

Dips, ramps, and rolls- Evidence for paleotopographic and syn-depositional fault control on the Western Kentucky No. 4 coal bed, tradewater formation (Bolsovian) Illinois Basin

The Western Kentucky No. 4 coal is a high-volatile B to high-volatile C bituminous coal that has been heavily mined along the southern margin of the Western Kentucky Coal Field. The seam has a reputation for rolling floor elevation. Elongate trends of floor depressions are referred to as "dips" and "rolls" by miners. Some are relatively narrow and straight to slightly curvilinear in plan view, with generally symmetric to slightly asymmetric cross-sections. Others are broader and asymmetric in section, with sharp dips on one limb and gradual, ramp-like dips on the other. Some limbs change laterally from gradual dip, to sharp dip, to offset of the coal. Lateral changes in the rate of floor elevation dip are often associated with changes in coal thickness, and in underground mines, changes in floor elevation are sometimes associated with roof falls and haulage problems. In order to test if coal thickness changes within floor depressions were associated with changes in palynology, petrography and coal quality, the coal was sampled at a surface mine across a broad. ramp-like depression that showed down-dip coal thickening. Increment samples of coal from a thick (150 cm), down-ramp and thinner (127 cm), up-ramp position at one surface mine correlate well between sample sites (a distance of 60 m) except for a single increment. The anomalous increment (31 cm) in the lower-middle part of the thick coal bed contained 20% more Lycospora orbicula spores. The rolling floor elevations noted in the study mines are inferred to have been formed as a result of pre-peat paleotopographic depressions, syn-depositional faulting, fault-controlled pre-peat paleotopography, and from compaction beneath post-depositional channels and slumps. Although the association of thick coal with linear trends and inferred faults has been used in other basins to infer syn-depositional faulting, changes in palynology within increment samples of the seam along a structural ramp in this study provide subtle evidence of faulting within a specific increment of the coal itself. The sudden increase in L. orbicula (produced by Paralycopodites) in a single increment of a down-ramp sample of the Western Kentucky No. 4 coal records the reestablishment of a rheotrophic mire following a sudden change in edaphic conditions. Paralycopodites was a colonizing lycopod, which in this case became locally abundant after the peat was well established along a fault with obvious growth during peat accumulation. Because many coal-mire plants were susceptible to sudden edaphic changes as might accompany faulting or flooding, changes in palynology would be expected in coals affected by syn-depositional faulting. ?? 2001 Elsevier Science B.V. All rights reserved.

International Journal of Coal Geology↗
USGS2001

Crocoite: An unusual mode of occurrence for lead in coal

What is believed to be a very unusual mode of occurrence for lead in coal has been identified as crocoite (PbCrO4). As part of a larger study on trace elements and mineralogy in the Cretaceous Main Seam in New Zealand, crocoite was found in raw coal samples within the lower part of the coal seam. X-ray diffraction (XRD) and bulk chemical data from a SEM equipped with an energy dispersive X-ray analyser (EDXA) have confirmed the identity of this mineral. This is apparently the first time that crocoite has been reported in coal. Crocoite usually occurs only in the oxidised zone of lead mineral deposits. The occurrence of this mineral in the Main Seam coal implies that the deposit was exposed to an oxidising environment at some stage, most likely after coalification. Published by Elsevier Science B.V.

International Journal of Coal Geology↗