Clastic Dikes Research Articles

Hardgrounds, reworked concretion levels and condensed horizons in the Jurassic of western India: their significance for basin analysis

Jurassic sediments in the shallow pericratonic basins of Kachchh and Rajasthan, western India, exhibit numerous signs of reduced sedimentation, omission, erosion and in situ reworking, in combination with synsedimentary cementation. Hardgrounds developed on carbonate shoals in the Bathonian of Rajasthan, whilst reworked concretion levels are characteristic of offshore siliciclastic sediments of the Callovian of Kachchh. A prominent marker horizon, the Oxfordian Dhosa Oolite Member, occurs throughout much of the Kachchh sub-basin and is a highly condensed unit characterized by hardgrounds, intraformational cobbles, reworked concretions, stromatolitic iron crusts, iron oncoids, and shell lags. Hardgrounds, reworked concretion levels, and condensation horizons are interpreted as the preserved relicts of transgressive pulses. Such pulses were possibly controlled by eustatic rise of sea level. Of at least equal importance, however, was a tectonic control which is demonstrated by the presence of small neptunian dykes, boulder beds derived from small submarine cliffs and rapid lateral facies and thickness changes in the Dhosa Oolite Member. These indications of extensional tectonics are thought to be connected to rifting and initial sea floor spreading between Africa and India.

Neptunian dykes along a drowned carbonate platform margin: an indication for recurrent extensional tectonic activity?

ABSTRACTThree successive Mesozoic neptunian dyke generations and related unconformities suggest recurrent extensional fracturing and periods of relative sea‐level rise along the NW Trento Plateau margin in the Southern Alps, Italy. The first neptunian dyke generation was induced by NNW–SSE directed extension of Early Jurassic skeletal oolitic periplatform deposits generating micritic early Middle Liassic neptunian dykes with orthogonal orientation. The second generation of neptunian dykes was possibly caused by marginal extension at the drowned platform edge penetrating Late Jurassic, red pelagic limestones with a pelagic matrix of Albian/Cenomanian age and nearly orthogonal fracture orientation. The third generation of neptunian dykes occurred after a prolonged period of submarine exposure and erosion (Aptian/Albian to Late Maastrichtian) during the rapid burial of the submarine Trento Plateau margin relief. The Late Maastrichtian neptunian dykes were caused by extension of Early to Middle Jurassic oolitic periplatform limestones along steep (inclination > 10°) submarine slopes.Generally successive neptunian dyke generations along drowned carbonate platform margins could be caused by repeated extensional brittle fracturing of lithified periplatform deposits and the filling of micritic matrix derived from overlying pelagic sediment sequences under substantial hydrostatic pressure. This would suggest that recurrent extensional fracturing is continuously recorded by neptunian dyke formation which could be used to indicate extensional tectonic activity at a foundering deep‐marine carbonate platform edge.

Jurassic sedimentary record and tectonic evolution of the northwestern corner of Africa

The history of the opening seaway from the westernmost Tethys to the Central Atlantic is traced by the analysis of the sedimentary facies development in the external Rif basin of Northern Morocco and the geological and seismic data from the Moroccan Atlantic continental margin. In the Rif basin, after the early Sinemurian, sedimentary facies dated by ammonites, foraminifers and brachiopods, indicate a progression of rapid subsidence resulting from extensional tectonic (tilted blocks, escarpment fault breccias, neptunian dykes etc.) from the N and NE to the S and SW. From the Toarcian to the Bajocian, deltas progress from the W and SW into the “Rides sud-rifaines” realm. From the late Bathonian to the Oxfordian, deep-sea fans develop in the external Rif. During the same period, deltaic sediments fill in the Middle Atlas basin of Eastern Morocco and progress into the external Rif. The top of the Jurassic is characterised by carbonate deposits. At the northwestern corner of Africa, the subsidence of the sedimentary basins by rifting is initiated in the late Triassic; however, at the Mazagan transect of the Atlantic continental margin, the tectonic pattern characteristic of a passive continental margin appears clearly only in the early Jurassic. At the foot of the Mazagan escarpment, the sedimentary record shows a foundering of the first bloc during early to middle Lias. A thermal uplift phase is indicated by emersion of the African margin shoulder in late Liassic, and thermal relaxation starts in the middle Jurassic. The morphology of this transect, compared with the conjugate side of the American continent is most easily explained by the uniform sense simple shear model.

Neptunian dykes and associated breccias (Southern Alps, Italy and Switzerland): role of gravity sliding in open and closed systems

ABSTRACTNeptunian dykes and sills of Middle Jurassic pelagic limestone within Lower Jurassic shallow‐water carbonate host rocks occur at many localities in the Southern Alps of Italy and Switzerland, especially on what were the upper slopes of tilted half‐grabens created during the Early Jurassic rifting stage of a passive margin that faced the Middle and Late Jurassic Tethyan Ocean. The host rocks were dilated by cracking, folding, and brecciation during movements of shallow‐based gravity‐driven slides and slumps of semibrittle platform strata, commonly along décollement contacts between layers of different competence. In most places, the network of cavities in the dilated strata connected to the sea floor, and pelagic sediments trickled from above into the open spaces. In other places, the brittle strata were overlain by somewhat impermeable sediments that formed a partial seal. Sudden dilation of the brittle beds resulted in forceful injection of the overlying weakly consolidated or plastic sediments into open spaces. The filling in both open and closed systems was commonly episodic, resulting in complex internal‐sediment stratigraphy and cross‐cutting dykes. Stable isotopic data on internal sediments and early‐formed cement lie within the field of normal sea water, and none of the sedimentological or stable isotopic data supports a subaerial, dissolution (karst) origin for the Jurassic neptunian dykes of this region.

Evolution of the Western Valles Caldera Complex, New Mexico: Evidence from intracaldera sandstones, breccias, and surge deposits

Scientific core drilling in the Pleistocene Valles caldera complex (encompassing the Valles (1.13 Ma) and coaxial Toledo (1.50 Ma) calderas) of north central New Mexico has provided new insight into the origins of sandstones, breccias, and pyroclastic surge deposits interbedded with the thick intracaldera ignimbrite sequence. These rocks were previously interpreted from geothermal drill cuttings as dominantly fluvial in origin. As such, representing significant erosional intervals, they formed much of the basis for subdividing the intracaldera ignimbrite sequence (up to 2000 m in apparent thickness where drilled) into four major units: the lower tuffs (> 1.50 Ma); the Otowi (1.50 Ma) and Tshirege (1.13 Ma) members of the Bandelier Tuff; and a new unit, the upper tuffs, believed to be post‐Bandelier in age (<1.13 Ma). All but the upper tuffs correspond to mapped outflow facies ignimbrite sheets. However, Continental Scientific Drilling Program core holes VC‐2A (completed in 1986) and VC‐2B (completed in 1988), in the Sulphur Springs area of the Valles caldera, have provided continuous cores, revealing for the first time that some intracaldera rocks previously thought to be exclusively clastic actually have multiple origins. Some of these rocks are probably pyroclastic surge deposits; others could be lithic‐rich breccias of various origins incorporated nearly instantaneously in ignimbrites during ash flow eruption and concomitant caldera collapse. These new findings demonstrate the value of continuous core for subsurface characterization and correlation of complex intracaldera lithologies; they also necessitate revision of Nielson and Hulen's (1984) cuttings‐based intracaldera stratigraphic framework. For example, the hematitic S2 “sandstone” was initially interpreted as marking an erosional interval between the Tshirege Member of the Bandelier Tuff and the overlying, petrographically similar upper tuffs. Core from VC‐2A and VC‐2B, however, shows that the S2 cuttings could also represent disaggregated, Permian red bed‐rich, lithic lag breccias or caldera collapse mesobreccias. If this is the case, then most or all of the upper tuffs are actually uppermost Tshirege Member ignimbrites. In similar fashion and upon review of previously applied correlation criteria the “lower tuffs” of the western Valles caldera complex could represent both genuine pre‐Bandelier ignimbrites and those of the lowermost Otowi Member. The core, however, shows that in the Sulphur Springs subsurface the lower tuffs are separated from overlying ignimbrite sheets by prominent erosional and eruptive breaks; they appear to be slightly more mafic than the overlying tuffs and host distinctive pumice lapilli. At this site the lower tuffs almost certainly predate the Otowi Member and are probably correlative with the outflow facies San Diego Canyon ignimbrites (1.78 Ma). Cores from VC‐2A and VC‐2B support earlier interpretation of the S3 “sandstone” as a major marker horizon separating the intracaldera Otowi and Tshirege members of the Bandelier Tuff but clearly shows that this important unit is not, as previously thought, entirely a simple intracaldera epiclastic apron. In VC‐2A the S3 has the superficial appearance of a sandstone but contains abundant blocky shards as well as accretionary and armored lapilli; it is also soft‐sediment‐deformed and invades overlying nonwelded tuff as small clastic dikes. We believe that here the S3 was emplaced by a wet pyroclastic surge. In nearby corehole VC‐2B the S3 consists of a basal, massive, sediment‐gravity‐flow (?) sandstone overlain by sandstone and dacite breccias with accretionary and armored lapilli‐bearing tuffaceous matrices. These deposits are probably caldera collapse mesobreccias that were formed simultaneously with early Tshirege Member ash flow eruptions through or into a Toledo caldera lake.

A clastic dike‐sill assemblage in late Miocene (c. 6 Ma) strata, Annedale, Northern Wairarapa, New Zealand

Abstract A glauconitic, sand‐filled, vertical clastic dike (>10 m high, base unexposed) and two associated sand‐filled sills in late Miocene mudstone (c. 6 Ma) near Annedale, Northern Wairarapa, New Zealand, pinches out upwards and thickens downwards. Relationships suggest that the mudstone remained solid while the glauconitic sand was fluidised during a penecontemporaneous earthquake event and was injected from below. After injection the margins of the clastic dike and sills were cemented by calcium carbonate. The cemented margins of the dike record at least two subsequent periods of shearing which are shown by subhorizontal and vertical slickensides on the dike walls. The clastic dike and the horizontal slickensides of the margin of the dike may have formed during two late Miocene dextral transtensional phases of faulting, whereas the vertical slickensides were probably formed during a short period when the subduction zone became locked. These features suggest that northeast Wairarapa had a similar tect...

Geological evidence for intra-Jurassic faulting in the Wessex Basin and its margins

Geological observations on Jurassic outcrops close to major faults in the Wessex Basin–Mendip area reveal the local presence of ammonite- and brachiopod-bearing sediments penetrating underlying strata. Toarcian and Bajocian neptunian dykes and particularly sills are associated with the Eypemouth Fault and Bajocian sills with the Bride Fault and Mere Fault. In the Mendip area numerous neptunian dykes of Hettangian, Sinemurian, Pliensbachian and Bajocian ages, cross-cutting Carboniferous Limestone, are recorded, typically also associated with major basement faults (e.g. Cranmore and Leighton Faults). These periods of assumed sediment injection are taken as indicating times of displacement along the faults in question. Variations in facies (Hettangian–Sinemurian, Toarcian, Bajocian, uppermost Oxfordian, Kimmeridgian) spatially linked to faults are documented from some areas, and boreholes reveal considerable fault-controlled thickness changes in Hettangian–Sinemurian, Bajocian and Kimmeridgian sediments. The timing of Jurassic faulting in the Wessex Basin–Mendip area thus polarizes into two intervals: Hettangian–Bajocian and latest Oxfordian onwards, correlating with the early rifting phases of the Central and North Atlantic respectively.

Geologic Structures That Affect Coal-Mine Roof Stability

The US Bureau of Mines has investigated several geologic structures that plague underground coal mining. These structures have caused numerous roof falls and, consequently, many miners have been hurt or fatally injured. Hazardous geologic anomalies that can be encountered in the eastern and western US coal fields include slips, hill seams, slickensides, sedimentary dikes, kettlebottoms, and paleochannels. Slips occur throughout the United States and are high-angled fractures in the roof rock. Typically, the rock adjacent to slips is highly slickensided and striated. Hill seams are weathered fracture zones and usually are found in areas where the overburden is shallow. Due to their nature and regional mining conditions, hill seams usually are found only in the southern Appalachian coal basin. Slickensides are lower angled planes of weakness found in coal-mine roof rock throughout the country. Slickensides usually are found in clusters and the adjacent rock is highly polished and striated. Sedimentary dikes (also clastic dikes and clay veins) are the remnants of ancient fissures that have been filled through sedimentary processes, Sedimentary dikes normally are found in coal basins that were slowly subsided. Kettlebottoms are the fossilized remains of trees that once grew in and above the swamp forest and generallymore » are found in coal basins that were rapidly subsided. Paleochannels are the lithified remains of rivers and frequently are found in all coal basins except the Black Warrior.« less

Deformation paths in the shallow levels of an accretionary prism: The Eocene Shimanto belt of southwest Japan

Detailed structural analysis of coherent turbidites in a part of the Eocene Shimanto belt on the Muroto Peninsula, Shikoku Island, Japan, reveals a structural evolution that is typical of the shallow levels of accretionary prisms in style and sequence of deformational fabrics, but anomalous in that it includes both a marked rotation in shortening direction and a major out-of-sequence thrust. Deformation throughout a history of progressive lithification and burial was dominated by subhorizontal shortening and seaward-vergent shear. Structures evolved from predominantly mesoscale thrusts and folds to larger, more brittle faults, and finally to kilometer-scale, amplified fault-bend folds with an associated pressure-solution cleavage. This structural progression is consistent with increasing strength of the rocks, decreasing fluid pressures, and increasing temperature and pressure as the sediments were dewatered, lithified, and accreted to the prism. In contrast to this straightforward structural evolution, the prism-scale deformation of the Eocene Shimanto belt is difficult to reconcile with simple models of prism developmentthat assume orthogonal subduction and seaward-stepping imbrication. A 50° counterclockwise rotation in the shortening direction and a major out-of-sequence (or intraprism) thrust are recorded by two distinct structural domains. In both, the earliest structures are clastic dikes and faults indicative of north-south shortening. One domain is characterized by east-striking, regional-scale folds and cleavage that are transected by an east-northeast-trending cleavage; however, the other is characterized by northeast-striking folds and cleavage that are transected by a slightly more north-striking cleavage. The different deformation histories of the two do-mains require that they be separated by a major intraprism fault. This fault carries tectonic melange containing blocks of pillow basalt and chert, indicating that it rooted in the decollement zone beneath the prism. The tectonostratigraphy and deformation histories of the two domains suggest that both before and after the out-of-sequence fault developed, higher structural levels of the prism experienced significant subhorizontal shortening, whereas lower structural levels bypassed this zone of deformation.

Depositional and Deformational History of the Franciscan Complex, Northernmost California: ABSTRACT

Pervasive extensional shear fractures and curvilinear arrays of clay and silt-filled veins in Franciscan Complex melanges and turbidites formed when Franciscan sediments were unlithified. Sandstone dikes both crosscut and follow fractures. Several scales of extensional faulting account for the juxtaposition of turbidites of different facies and/or with varying degrees of stratal disruption, the formation of sandstone lozenges and pinch-and-swell structures, and the formation of scaly foliation within the matrix of melange units. Within turbidites, the upper laminated portions of beds commonly contain abundant listric microfaults and the more massive lower portions of beds contain sediment-filled vein arrays. Veining and faulting occurred concurrently and resulted in differential extension of upper verses lower portions of beds. The finer sediment in veins reflects both cataclasis and filtering in of clay and silt from vein walls. Most Franciscan rocks record an early pervasive, layer-parallel flattening strain, which may be related to the gravitational collapse of late Mesozoic Franciscan inner trench slope sediments that accompanied accretionary prism expansion resulting from underplating. However, some turbidites record noncoaxial extension that resulted from downslope creep of sediments. At Crescent City, sediment creep resulted in oversteepening of the Franciscan inner trench slope, which, in turn, may have triggered large-scalemore » failure of slope materials resulting in the emplacement of the Crescent City olistostrome. The olistostrome crops out for 12 km along the coast, is up to 600 m thick, is in depositional contact with turbidites, and contains chiefly sandstone, greenstone, chert olistoliths up to 200 m across, and zones of slump-folded turbidites.« less

Franciscan Complex olistostrome at Crescent City, northern California

ABSTRACTAn olistostrome and bounding turbidites are exposed within the late Mesozoic Franciscan Complex along the Crescent City (California) coastline. Facies grade in character from Mutti & Ricci Lucchi (1978) mixed facies B, C and D, to F (the olistostrome), to mixed A, B and E, progressing upwards within the Franciscan stratigraphic section. The facies F unit outcrop is up to 600 m thick and extends 12 km along strike. It consists of oblate to tabular blocks, up to 200 m in maximum dimension, of greenstone, tonalite, radiolarian chert, limestone, phyllite and greywacke dispersed in a scaly argillite matrix. The olistostromal origin of the unit is indicated by depositional contacts with bounding turbidites, by the presence of abundant recycled sedimentary clasts within the unit, and by the presence of sedimentary breccias and associated dismembered, slump‐folded turbidites both within the olistostrome and among subjacent turbidites. Sandstones are chiefly feldspathic litharenites that were very likely derived from the partially dissected, late Mesozoic Sierran‐Klamath magmatic are.Franciscan rocks record an early pervasive, layer‐parallel flattening strain in such features as extensional faults, necking and pinch‐and‐swell structures. Several scales of extensional faulting account for the juxtaposition of turbidites of different facies and/or with varying degrees of stratal disruption, the formation of sandstone lozenges, and the formation of scaly foliation in the olistostrome matrix. The latter resulted from the juxtaposition of lenticles with varying concentrations of silt and clay. These were ultimately derived from the finer divisions of turbidite beds that were incorporated into the olistostrome. The presence of gradational contacts between some sandstone olistoliths and the olistostrome matrix, and of sandstone dykes that intrude fractures and associated drag‐folded turbidite beds indicate that Franciscan sediments were not lithified during their early deformation. These sediments were deposited in either a trench or trench slope basin, and were first deformed either by gravity collapse of the trench slope cover or, less likely, by vertical loading beneath the toe of the accretionary wedge. They later were folded during internal shortening of the growing Franciscan accretionary prism.

Clastic dikes in granite, Fujioka-cho, Aichi Prefecture, Central Japan

Clastic dikes in granite, Fujioka-cho, Aichi Prefecture, Central Japan

The rockwall in rockwall, texas: A study of unusual natural magnetic effects in geoarcheological surveys produced by mineral oxidation

AbstractThe city council of Rockwall, Texas, decided in 1985 to build a park containing an exposure of the “famed” rockwall after which the city and county of Rockwall derive their names. This wall, thought to be man‐made because of it's unusual brick‐like appearance, was first discovered in 1851. As a result of weathering, the wall has no surface exposure and crews working for the city had not been able to find it. Geological studies, published between 1927 and 1950, clearly demonstrate that the rockwall is one of a series of sandstone dikes with a carbonate cement and some unusual material constituents.Geoarchaeological methods, including magnetic and electrical resistivity surveys, and Mössbauer studies were used in identifying the precise location of the rockwall. This was possible because of strongly magnetic secondary minerals produced within the wall by weathering of two primary mineral constituents, siderite and marcasite. the resulting natural magnetic anomaly pattern made it possible to pinpoint the wall's location. This pattern closely resembles those identified in association with historic or prehistoric cultural activity. Our observations may help explain why some archeological excavations, at sites associated with well defined magnetic anomalies, do not yield clear sources for those anomalies, i.e., buried cultural material. These anomalies may result from weathering or from other natural processes, and in some cases interpretation of such ambiguities can be resolved using mineralogical and geochemical data.

Sandstone dykes as evidence of localized transtension in a Transpressive Regime, Bir Zreir Area, eastern Sinai

The Bir Zreir area, NE Sinai, is part of the 40‐km‐wide eastern Sinai shear zone stretching along the western margin of the Gulf of Elat. Sinistral movement along faults comprising this shear zone is consistent with the regional NNW‐SSE compression in which the Dead Sea transform fault was formed. In the Bir Zreir area, sandstone dykes within NE striking fault breccias were developed within the Precambrian crystalline basement. These breccias, up to 200 m in width, resulted from transpression along the NE striking faults in response to sinistral movement along the NNE striking major faults. Paradoxically, the sandstone dykes developed by infilling of transtensional open fissures which share the NE trend of the breccia zones. The source rock for the dykes is Nubian Sandstone unconformably overlying the basement. The depth beneath the unconformity to which the sandstone collapsed is 500–600 m. Local NW‐SE transtension along parts of the NE striking faults enabled the formation of fissures and their infilling by sand and sandstone blocks of various sizes as dykes. The infilling occurred despite the regional NNW‐SSE compression which prevailed in the study area. A kinematic model is developed which combines sinistral movement and clockwise rotation of rigid blocks between the faults with resultant local NW‐SE transtension along the northern segment of the NE trending strike‐slip faults.

Low-temperature deformation mechanisms and their interpretation

Low-temperature deformation is characterized by heterogeneous strain in which the bulk of the material clearly retains its primary texture. Deformation is by grain-scale crystal plasticity, rotation, fracture, and pressure solution, and by transgranular mechanisms that crosscut numerous grains. The important low-temperature crystal-plastic features are twin lamellae, deformation bands, and undulatory extinction. Subgrain formation by recrystallization or crystal-plastic strain of more than 15% marks the upper limit of the low-temperature regime. Grain rotation may produce foliations in soft sediments or rocks. Microscopic to mesoscopic kinks and crenulations of bedding occur in soft clay and shale. Transgranular features include Luders' bands, cooling and desiccation cracks, joints, extension-fracture cleavage, clastic dikes, mineral-filled veins of several types, recrystallization/replacement veins, vein arrays, boudins, faults, stylolites, slickolites, solution cleavages that range from widely spaced to slaty and pencil cleavage. Pressure fringes form adjacent to relatively rigid grains and have fabrics analogous to those in veins. Faults include conjugate fault pairs (Andersonian faults) multiple simultaneous conjugates (Oertel faults), and Riedel shear-zone configurations. The sense of fault displacement is determined from bends, steps, trails, tails, and feather fractures. Superplasticity, especially if aided by diffusion in grain-boundary water, might be important at low temperatures. Fault textures are diagnostic of the environment of deformation but have yet to be uniquely correlated with the presence or absence of earthquakes. Riedel shears and pseudotachylite may form in earthquake source regions, although pseudotachylite is evidently rare in brittle fault zones. The best indicators of stress magnitudes are the critical' resolved shear stress for deformation twinning and the presence of tensile fractures. Strain magnitudes and stress and strain tensor orientations can be determined with a variety of methods that are based on mechanical twins, platy grain orientation, grain center distribution, and fault geometry and slip directions. Different deformation mechanism associations, expressed by the partitioning of the total strain into different mechanisms, are related to the ductility and environment of deformation. Deformation fronts separating different mechanism associations are defined on the basis of changes in the crystal-plastic component of strain.

Stable isotope geochemistry of sphalerite and other mineral matter in coal beds of the Illinois and Forest City basins

Cleat and clastic dikes of Middle Pennsylvanian-age coal beds of the Illinois and Forest City basins of Illinois, Iowa, Missouri, and Kansas locally contain appreciable amounts of sphalerite within a kaolinite-pyrite-sphalerite (+ or - pyrite)-calcite paragenetic sequence. The sphalerite and associated minerals are of interest as a partial record of the history of fluids in the sedimentary basin and as possible indicators of Mississippi Valley-type mineralization. Moreover, zinc from the sphalerite may represent an exploitable by-product of coal mining and combustion.Pyrite is abundant in these coals both as fine-grained disseminated framboids that formed during early diagenesis and as coarser grained cleat fillings, etc., that formed much later. The delta 34 S values of this later pyrite, the sphalerite, and the coal organic matter range from -12 to 19 per mil. Within individual samples, however, pyrite, sphalerite, and organic matter sulfur delta 34 S values are nearly identical. This similarity strongly suggests related origins. The delta 34 S values of pyrite, sphalerite, and organic matter from coals of the Illinois basin and the central and southern Forest City basin range from -12 to 0 per mil. This range is not unique to the sphalerite-bearing coals; pyrite and organic matter from sphalerite-free high sulfur coals have similar delta 34 S values (Price and Shieh, 1979). We believe that sulfur produced by the breakdown of organosulfur compounds in the coal is incorporated into the epigenetic sulfides. In the northern Forest City basin, however, many samples have delta 34 S values between 8 and 19 per mil. These are similar to the delta 34 S values of sphalerite and pyrite from the Upper Mississippi Valley Zn-Pb district which lies northeast of the Forest City basin and north of the Illinois basin. This similarity suggests input from a Mississippi Valley-type hydrothermal system.Cleat kaolinite has delta D values of -36 to -24 per mil and delta 18 O values of 19.2 to 20.2 per mil. Assuming that clearing postdated the lignite rank of coalification implies that kaolinite was deposited between 40 degrees and 100 degrees C. At the lower temperatures, the kaolinite fluid would have had an isotopic composition similar to seawater; at the higher temperatures, it would have had a composition similar to the sphalerite-depositing fluid. Sphalerite was deposited from a fluid that was chemically and isotopically similar to the fluids responsible for the main-stage ores of the Upper Mississippi Valley Zn-Pb district (75 degrees -113 degrees C, approximately 21 equiv wt % NaCl, delta D = -65 to -3ppm, and delta 18 O = -0.9-9.9ppm). The isotopic compositions (delta 13 C = -12.4 to -2.7ppm and delta 18 O = 21.1-25.8ppm) and paragenetic positions of the cleat calcite samples are similar to those of the late-stage calcite of the Upper Mississippi Valley Zn-Pb and Illinois-Kentucky fluorspar districts. Calcite inclusion fluids had delta D (sub H 2 O) values of -80 and -58 per mil, implying a meteoric water source. At probable temperatures of 15 degrees to 75 degrees C, this calcite formed from fluids having calculated delta 18 O (sub H 2 O) values of -9 to 1 per mil, implying slight to moderate exchange with sedimentary rocks. The diagenetic-epigenetic history of fluids recorded by the cleat minerals probably began with a slightly evolved seawater, followed by a metal-bearing deep basin brine, and concluded with a cooler, slightly evolved, meteoric-water fluid.

Clastic limestone dikes from the late Proterozoic Bhima Group, South India

Clastic dikes of the late Proterozoic Bhima Group, South India, intruding the Kasturpalli Glauconitic Member are reported from two localities. These glauconitic sediments of the Rabanpalli Formation form the oldest transgressive sequence of the Bhima Group, which starts with basal conglomerates and grades upward into supermature quartz arenites. These are followed successively by glauconitic fine-grained sediments and shales. This sequence is capped by thick deposits of micritic flaggy limestones of the Adki Formation. The dikes are generally emplaced vertically, but also have small lateral apophyses along the bedding planes of the host rocks. The clastic dikes are chiefly composed of a matrix of micritic calcite along with quartz, clay minerals, etc. In places, crystals of calcite are observed in the form of veins. In one locality the dikes are emplaced in the vicinity of a post-depositional fault. This faulting seems to have triggered the intrusion of the clastic material, which shows a thixotropic behaviour, into the glauconitic sandstones and siltstones.

Geometry and tectonic significance of Albian sedimentary dykes in the Sisteron area, SE France

Sedimentary dykes in the Sisteron area have penetrated pre-existing fracture sets in late Aptian and early Albian strata. Dynamic characteristics of such fractures are revealed by the bifurcation and the branching of dykes. Three-dimensional statistical analysis of the initial orientation of two vertical conjugate sets of dykes indicates their mechanics of formation, as well as the principal compressional stress direction during the development of the sill-dyke regime in the Sisteron area. Another set of sandstone dykes present in the area suggests syn-sedimentary and extensional faulting tectonic activity before the area became involved in compressional tectonics. Several significant dyke-forms clearly show the sense of shear along fractures filled with sand, and enable the stress pattern during dyke formation in the area to be recognized.

Advance and retreat glacigenic deformation at Williams Lake, British Columbia

Glacigenic deformation structures at Williams Lake, British Columbia, occur within stratified Quaternary sediments that both overlie and underlie lodgement till of the last (Fraser = Late Wisconsinan) glaciation. The main structures in sediments below the lodgement till are faults, joints, and clastic dikes produced by glacier overriding during Late Wisconsinan time. The orientation of these structures is closely related to the direction of glacier flow at Williams Lake. Their character and stratigraphic position suggest that the substrate in this area was partly frozen and partly unfrozen when covered by ice during the Fraser Glaciation.A different suite of glacigenic deformation structures is found in sediments above the lodgement till. These include complex folds, load casts, and high-angle faults formed during deglaciation as a result of loss of ice support and collapse. These structures yield information on the local pattern of ice decay, but bear no relationship to the regional direction of Late Wisconsinan glacier flow.

Normal fault geometry related to sediment compaction and burial

Field observations are presented on meso-scale normal faults and sandstone dikes which cross-cut turbidites in the petroliferous Recôncavo Basin situated in Bahia, NE Brazil. Two discrete phases of normal faulting occurred. It is argued that the first was probably active during the first 200 m of burial. Compaction estimates from folded sandstone dikes and the accommodation patterns of shale bedding planes around ball-and-pillow structures were made. First-phase fault profiles were initially reasonably straight but refraction on the fault planes increased due to the differing compaction between sandstone and shale. The later phase of faulting occurred at a greater depth, with steeper fault dips and strong refraction of fault planes at lithological boundaries. This greater fault refraction is probably the result of burial and diagenesis, which increased the difference in rheological properties between sandstone and shale before faulting occurred. Thus, complex superposed fault geometries may be developed by changing rheological properties and stress conditions during progressive burial. It is shown that sandstone dikes were not always intruded vertically. Injected dikes cannot be used in the same manner that Neptunian dikes are conventionally used to estimate tectonic strain, if independent evidence of the original orientation of the dike is not available.