Fine-grained Sedimentary Rocks Research Articles

Maximizing Information from Fine-Grained Sedimentary Rocks: An Inclusive Nomenclature for Mudstones

ABSTRACT A simple nomenclature for fine-grained sedimentary rocks is proposed based on sedimentary structures present and abundance of all materials forming more than 10% of rock volume irrespective of their origin (allochthonous, autochthonous, and diagenetic). A (sedimentary rock that comprises > 50% grains 90% clay), silt- and sand-bearing, clay-rich mudstone (50 to 90% clay, 10 to 50% sand, 10 to 50% silt), and calcite ce-ment-, calcareous nannoplankton-, and clay-bearing mudstone (all components 10 to 50%). Textural information can and should be incorporated into this scheme by prefixing the rock name with descriptions such as bioturbated, and thin-bedded. Such a nomenclature is necessary to fully describe variability within fine-grained sedimentary rocks that hitherto have typically been described using selected constituents of varying significance such as total organic carbon, fissility, parting spacing, -ray signature, and lamination. Using the inclusive nomenclature proposed here will improve opportunities to compare all fine-grained sediments, aid interpretation of depositional and diagenetic controls of variability, and enable these rocks to be more easily integrated into basin-scale facies models.

High-Resolution Rapid Elemental Analysis Using an XRF Microscanner

ABSTRACT The Horiba energy-dispersive XRF microscanner is an instrument designed for nondestructive, high-resolution, rapid, semiquantitative chemical mapping under ordinary atmospheric conditions. However, its application to quantitative analysis has not yet been attempted. In this study we developed a nondestructive, rapid, high-resolution, quantitative analytical method for fine-grained rocks using this instrument. Average concentrations and elemental mapping images of a small square region (0.5 mm 0.5 mm to 100 mm 100 mm) can be obtained for major elements (Mg, Al, Si, K, Ca, Ti, Mn, Fe) within 100 seconds per measurement. High-resolution, continuous measurements of slabbed fine-grained sedimentary rock samples are easily achieved. A stratigraphic interval of 1 m can be measured with 1 cm resolution within 10 hours, which is several times faster than the conventional X-ray fluorescence method.

CO 2 migration processes in argillaceous rocks: pressure-driven volume flow and diffusion

The capillary sealing efficiency of fine-grained sedimentary rocks has been investigated by gas (CO 2, N 2) breakthroughexperiments and CO 2 diffusion experiments. Experiments were performed on initially fully water-saturated samples. Absolute (single phase) permeability coefficients ( k abs), determined by steady-state flow tests, ranged between 6 × 10 −22 and 5.5 × 10 −19 m 2. Maximum effective permeabilities to the gas phase ( k eff), measured after gas breakthrough, ranged from 1 × 10 −23 up to 1.1 × 10 −18 m 2. Capillary displacement pressures ( P d) ranged from 0.06 to 6.7 MPa. However, for individual claystone samples, gas breakthrough with subsequent pressure-driven volume flow (Darcy flow) was not observed even at much higher differential pressures (15 MPa). In these instances, molecular diffusion is the dominating transport process. It was shown that for nominal permeability coefficients below 10 −24 m 2, a distinction between pressure-driven volume flow and diffusion processes is no longer possible. Repeated diffusion experiments with CO 2 on the same sample plug yielded varying effective diffusion coefficients ranging from 10 −9 to 10 −11 m 2/s. This variability is taken as an indication of complex mineral matrix interactions affecting the molecular transport properties.

Sedimentology of the lower Karoo Supergroup fluvial strata in the Tuli Basin, South Africa

The Karoo Supergroup in the Tuli Basin (South Africa) consists of a sedimentary sequence (∼450–500 m) composed of four stratigraphic units, namely the informal Basal, Middle and Upper Units, and the formal Clarens Formation. The units were deposited in continental settings from approximately Late Carboniferous to Middle Jurassic. This paper focuses on the ∼60-m-thick Basal Unit, which was examined in terms of sedimentary facies and palaeo-environments based on evidence provided by primary sedimentary structures, palaeo-flow measurements, palaeontological findings, borehole data (59 core descriptions) and stratigraphic relations. Three main facies associations have been identified: (i) gravelstone (breccias and conglomerate-breccias), (ii) sandstone and (iii) fine-grained sedimentary rocks. The coarser facies are interpreted as colluvial fan deposits, possibly associated with glaciogenic diamictites. The sandstone facies association is mainly attributed to channel fills of low sinuosity, braided fluvial systems. The coal-bearing finer-grained facies are interpreted as overbank and thaw-lake deposits, and represent the lower energy correlatives of the sandy channel fills. Sediment aggradation in this fluvio-lacustrine system took place under cold climatic conditions, with floating lake ice likely associated with lacustrine environments. Palaeo-current indicators suggest that the highly weathered, quartz-vein-rich metamorphic rock source of the Basal Unit was situated east–northeast of the study area. The accumulation of the Basal Unit took place within the back-bulge depozone of the Karoo foreland system. In addition to flexural subsidence, the amount of accommodation in this tectonic setting was also possibly modified by extensional tectonism in the later stages of the basin development. Based on sedimentological and biostratigraphic evidence, the coal-bearing fine-grained facies association displays strong similarities with the Vryheid Formation of the main Karoo Basin to the south. The lowermost non-fossiliferous breccias have been correlated before with the Dwyka Group in the main Karoo, and hence the Basal Unit may be regarded as the distal equivalent of the Dwyka and Ecca groups to the south.

Clay mineralogy and geochemistry of early Jurassic sedimentary rocks from the Moezian Platform, northern Bulgaria

AbstractClay minerals in early Jurassic sequences of shales, siltstones and sandstones deposited in non-marine, transitional and shallow marine environments have been examined by X-ray diffraction, electron microscopy and chemical analysis to study the relationship between clay minerals, their environment of deposition and subsequent diagenetic modifications.The inherited clay mineral composition of the fine-grained sediments reflects the influence of climate, relief, source rocks and depositional processes. Inhomogeneous clay mineral assemblages, comprising abundant kaolinite and varying proportions of illite, I-S, chlorite and vermiculite, characterize fine-grained sediments from the non-marine and transitional environments. In shallow marine depositional environments clay mineral assemblages are more uniform, dominated by illite+I-S with minor kaolinite and chlorite.The principal diagenetic process affecting fine-grained sedimentary rocks is the smectite–illite transformation. In sandstones, the authigenic formation of kaolinite, chlorite and illite appears to have been primarily determined by the environment of deposition.

Presence of an iron‐rich nanophase material in the upper layer of the Cretaceous‐Tertiary boundary clay

Abstract— We report new geochemical evidence from ten Cretaceous‐Tertiary (K‐T) boundary sites in North America and Europe, indicating the presence of a material remnant of a large asteroid or comet that struck the Earth at 65.0 Ma. Mössbauer spectroscopic data reveals that a ubiquitous iron‐rich nanophase material exists at the uppermost part of the K‐T boundary layer in the Western Hemisphere and in Europe in marine and continental fine‐grained sedimentary rock. The high surface‐to‐volume ratio of nanophase material suggests that it may be the carrier of the iridium abundance enhancement that marks the K‐T boundary. Even more provocative is the possibility that the discovered nanophase material is, for the most part, composed of the vaporized impactor after the impact‐generated high‐temperature vapor plume rose and cooled above the atmosphere.

Long-period Milankovitch cycles from the Late Triassic and Early Jurassic of eastern North America and their implications for the calibration of the Early Mesozoic time–scale and the long–term behaviour of the planets

During the Late Triassic and Early Jurassic the Newark rift basin of the northeastern US accumulated in excess 5 km of continental, mostly lacustrine strata that show a profound cyclicity previousl...

Experimental investigation of molecular transport and fluid flow in unfaulted and faulted pelitic rocks

Abstract Laboratory experiments have been performed to investigate the fluid transport properties of fine-grained (pelitic) sedimentary rocks with respect to their hydrocarbon sealing efficiency. The experiments comprised the molecular transport (diffusion) of gases and the pressure-driven volume flow (Darcy flow). Diffusion coefficients of methane and nitrogen in water-saturated rock samples were measured at 90°C from an equimolar mixture of the two gases (10 MPa gas pressure). Methane had lower effective diffusion coefficients but higher steady-state molecular transport rates than nitrogen. Permeability coefficients measured with water on selected unfaulted natural samples, under controlled effective stress up to 47 MPa, ranged between 10 −22 and 10 −19 m 2 . The permeability reduction with increasing stress mostly followed an exponential relationship. Permeabilities of faulted mudstones from compressive fault zones ranged from 2 up to 20 × 10 −21 m 2 . No systematic relationship between permeability and microfault frequency within the samples or distance of samples from microfault planes could be identified. Permeability measurements were also performed on macroscopically homogeneous Carboniferous shales from a compressive tectonic stress field. These samples, which showed distinct sonic velocity anisotropies, had permeability coefficients in the nanodarcy (nDarcy; 10 −21 m 2 ) range. Only in one case was a significant permeability anisotropy associated with the sonic velocity anisotropy.

The size distribution of framboidal pyrite in modern sediments: An indicator of redox conditions

Pyrite framboids are densely packed, generally spherical aggregates of submicron-sized pyrite crystals. In this study, a survey was made of framboid size distributions in recently deposited sediments from euxinic (Black Sea; Framvaren Fjord, Norway; Pettaquamscutt River Estuary, Rhode Island, USA), dysoxic (Peru Margin), and oxic (Wallops Island, Virginia, USA; Great Salt Marsh, Delaware, USA) environments. Pyrite framboids in sediments of modern euxinic basins are on average smaller and less variable in size than those of sediments underlying dysoxic or oxic water columns. Down-core trends indicate framboid size distribution is a sediment property fixed very early during anoxic diagenesis, generally within the top few centimeters of burial. Size distributions in modern sediments are comparable with those in ancient sedimentary rocks, evidence that framboid size is preserved through advanced stages of diagenesis and lithification. It is proposed that where secondary pyrite growth is limited, as to preserve primary pyrite textures, framboid size distribution may be used to indicate whether fine-grained sedimentary rocks were deposited under oxic or anoxic conditions. The Crystal Size Distribution Theory relates framboid size to growth time and rate. On the basis of this theory, the characteristic smaller sizes of framboids in sediments of modern euxinic basins reflect shorter average growth times relative to oxic or dysoxic environments. In euxinic environments, framboid nucleation and growth occurs within anoxic water columns, and growth times are, on average, shorter because of hydrodynamic effects than when framboid nucleation and growth occurs within anoxic sediment porewaters underlying oxic water columns. A maximum framboid growth time of 0.4 years is indicated for framboids forming in the water columns of euxinic basins.

Evolution of a Middle Jurassic volcanic arc: stratigraphic, isotopic, and geochemical characteristics of the Harrison Lake Formation, southwestern British Columbia

The Harrison Lake Formation is an Early to Middle Jurassic volcanic-arc assemblage unconformably overlying Triassic oceanic basement in the eastern Coast Belt of southwestern British Columbia. The formation is subdivided into four members including, in ascending order, the Celia Cove Member (conglomerate), the Francis Lake Member (fine-grained strata), the Weaver Lake Member (flows and breccias), and the Echo Island Member (pyroclastic and epiclastic strata). New biostratigraphic constraints pinpoint the initiation of volcanism to late early Toarcian. U–Pb geochronology demonstrates the arc was active until at least late Bajocian–early Bathonian time (166.0 ± 0.4 Ma), and that the timing of arc volcanism strongly overlaps emplacement of both hypabyssal intrusions (Hemlock Valley stock) and deep-seated plutons (Mount Jasper pluton) within and adjacent to the arc. Geochemical data indicate the arc is of medium- to high-K calc-alkaline affinity, and is strongly light rare earth element enriched (LaN/YbN = 1.5 – 2.5). Nd and Sr isotopic data from primary volcanic rocks demonstrate the juvenile nature of the magmatic system, but isotopic data from associated fine-grained sedimentary rocks suggest temporally controlled variations in isotopic composition interpreted to represent two-component mixing between juvenile volcanic detritus and a more evolved detrital component. The succession of facies in the Harrison Lake Formation records initial basin subsidence in the Early Jurassic, initiation of explosive volcanism in the late early Toarcian, a change to effusive volcanism in the early Aalenian, and late-stage explosive volcanism in the late Bajocian. The Harrison Lake Formation contains mesoscopic folds and overturned bedding that are absent in the overlying Callovian Mysterious Creek Formation, strongly suggesting the existence of a regional Bathonian deformational event in the southern Coast Belt.

Remnant roof coal thickness measurement with passive gamma ray instruments in coal mines

The need to leave coal on the roof of a mined-out area occurs on both continuous miner and longwall sections and is derived from considerations of ground control, quality control, machine guidance, or simply good operating practice. Efforts at measuring boundary coal thickness using the nucleonic method is described. Natural gamma devices are the instruments of choice, and the use of gamma-ray backscatter sensing is discussed. The calibration of natural gamma background (NGB) instruments is described, and it is concluded that they cannot be used in areas where NGB radiation is not present. This radiation is ordinarily present in the fine-grained sedimentary rocks that bound many coal seams. >

Introduction to geochemistry of metalliferous black shales

1. Metalliferous black Metalliferous black exhibit a wide range of physical and chemical characteristics. They can be enriched in many different metals, and the metals can be in a variety of geochemical states. Depositional and geological histories of these are diverse. Some black contain relatively high concentrations of organic matter whereas others do not. Because of elevated carbonate contents, some are calcareous or marlstones. Metal enrichments can result from redox reactions mediated by bacteria during early diagenesis and also from those associated with hydrothermal processes after deep burial. However, unifying characteristics of black are sufficiently well established to justify combining a diverse number of fine-grained sedimentary rock types under a common term that reflects enhanced metal content, dark color and an inferred association with organic matter (e.g., Huyck, 1990). Moreover, the term metalliferous black shales is sufficiently accepted that Working Group 254 of the International Geological Correlation Program (IGCP) has that title and focus. This special issue of Chemical Geology brings together reports describing characteris

Porosities and seismic velocities of mudstones from Wairarapa and oil wells of North Island, New Zealand, and their use in determining burial history

Abstract The seismic velocities and porosities of mudstones and other fine-grained sedimentary rocks can be used to estimate burial history. The porosity of exhumed mudstone sequences of Wairarapa has been derived from density measurements, and can be used to estimate prior depth of greatest burial, to within about 100 m, by the linear regression function, greatest depth of burial = ™111 porosity +4975, or the exponential function, greatest depth of burial=™8636 log porosity + 14384. The velocity trend of the exhumed mudstones of southeastern Wairarapa is not as good an indicator of burial history, as it has only a poor fit to the regression function, greatest depth=2666 velocity ™3331. However, fully loaded, fine-grained sedimentary rocks (mainly mudstones) still under lithostatic pressure in North Island oil wells have a good fit to the velocity-depth regression functions, depth = 1026 velocity ™1419 or, depth=6617 log velocity ™1403. This relationship can be used to calculate the thickness of sediment ...

Applications of Mössbauer spectroscopy in the geochemistry of sedimentary rocks

A set of pure iron-rich minerals characteristic of common fine-grained sedimentary rocks has been prepared and their Mossbauer spectra obtained. These include pyrite, carbonates and silicates. Six marine mudrocks were then chosen from a large set for which complete chemical data were available. It proved possible to reproduce mudrock spectra by addition of the pure mineral spectra in proportions indicated by the chemical analyses. Unfortunately, however, overlap of Fe(III) in dioctahedral silicates with Fe(II) in pyrite prevents reliable deconvolution of mudrock spectra to obtain quantitative estimates of component concentrations. Useful information about Fe(II) in other minerals can still be obtained. Mossbauer spectroscopy would appear to have great promise for the study of early reactions in non-marine sediments which are usually low in sulphur. Mineral products are either X-ray amorphous or extremely fine-grained mixtures and very difficult to study by other means.

Influence of microfabric on geomechanical behaviour of tertiary finegrained sedimentary rocks from Central North Island, New Zealand

Tertiary to Early Quaternary fine-grained marine sedimentary rocks (soft rocks) of the Central North Island, New Zealand, suffer from high erosion rates and slope instability. In geomechanical laboratory tests they exhibit a range in behaviour which straddles the division between overconsolidated soils and rocks. Direct application of published classification systems to predict performance has been unsuccessful for New Zealand soft rocks so far. Mineralogical composition does not vary greatly between ages and locations, in contrast to geomechanical parameters. Most constituents are detrital, no cement has been precipitated and post-depositional diagenetic alterations are negligible. Mineralogy does not comprehensively account for the geomechanical performance of these rocks. More direct influence is exerted by their microfabric. Basic building blocks of the fabric are detrital grains, clay microaggregates, microfossils and larger clay aggregates. Grains, microaggregates and fossils are mechanically stable, but larger aggregates exhibit varying degrees of stability. The geometric arrangement of structural elements can be described within a framework of three fabric types, the skeletal, matrix and turbostratic microfabrics, which form an evolutionary sequence related to the loading history of the deposits. Some geomechanical characteristics of the rocks, particularly strength, cohesion and durability, correlate well with the fabric type classification. In all fabrics, contact between structural elements involve clay, and its physico-chemical properties are important for their performance under stress. The fabrics are dynamic and adjust to changing stress conditions by re-arranging their constituents and contact configurations. Their response to loading and slow unloading during regional uplift is quasielastic, resulting in a loss of stress memory. During rapid unloading by erosion or excavation stress relief fractures parallel to topography can develop. Aggregates and connector assemblages form weak links in the fabrics. They are less competent in their response to stress and wetting and drying than other structural elements. Their breakdown decreases effective grainsize, average pore size and permeability, and initiates further disintegration of the fabric.

Exchangeable cations and mineralogy of some selected Nigerian soils

The cation exchange characteristics and mineralogy of six soils collected from southeast Nigeria have been investigated. Four of these soils (Uturu/Akaeze soils) are developed on fine-grained sedimentary rocks and two (Unwana and Mballa soils) on more coarse-grained sands and sandstones. The Uturu/Akaeze soils have a higher CEC and contain more exchangeable bases in both the total soil and particle size fractions separated by mechanical dispersion, not only because they contain more clay but also because interstratified minerals occur as a major component. The Unwana and Mballa soils, on the other hand, are predominantly kaolinitic. The 2–20 μm and 20–200 μm fractions of the soils, obtained after exhaustive mechanical separation, still show the presence of large amounts of clay minerals in the form of aggregated grains. Because of chemical and mineralogical differences, it is suggested that the aggregates in the 20–200 μm fractions of the Uturu/Akaeze Group soils could represent undisaggregated rock fragments inherited from parent material, whereas aggregates in the 2–20 μm fraction are interpreted as being largely pedogenic in origin. The clay mineralogy and exchangeable cations are strongly correlated in the 20–200 μm fraction, but not in the finer fractions. Differences in the exchange characteristics of clay minerals in an aggregated or non-aggregated form may be important in rationalizing the nutrient status of these soils in terms of their mineralogy.

Tourmalinites in the Aphebian Mistassini Group, Quebec

Tourmalinites, or fine-grained sedimentary rocks containing 15–20% or more tourmaline, occur within the Papaskwasati Formation of the Aphebian Mistassini Group in central Quebec. The formation, which is largely composed of fluvial sand-stones and conglomerates with minor argillite beds, was deposited in a restricted embayment close to the transition into marine carbonates. Concentrations of tourmaline are limited to fine-grained sediments and intraclasts, and fine sedimentary structures are preserved within the resulting tourmalinites. This suggests that the tourmalinites formed from preexisting boron-rich muds. It is unlikely that the muds were formed from sea-floor hydrothermal emanations, as no hydrothermal manifestations or spenecontemporaneous vulcanism is known to be associated with this formation. An evaporitic origin for the tourmalinites is consistent with both the sabkha conditions postulated for the origin of nearby carbonates and the limited areal extent of the sedimentary basin.

LITHOSTRATIGRAPHY OF THE LOWER EROMANGA BASIN SEQUENCE IN SOUTH WEST QUEENSLAND

Five fully-cored and wire-line logged stratigraphic bores have been drilled by the Queensland Department of Mines, relatively close to producing oil fields in the Eromanga Basin, south-west Queensland. Correlations between the stratigraphic bores and petroleum wells have established lithologic control in an area where lithostratigraphy is interpreted mainly from wire-line logs. The Eromanga Basin sequence below the Wallumbilla Formation has been investigated, and a uniform lithostratigraphic nomenclature has been applied; in the past, an inconsistent nomenclature system was applied in different petroleum wells.Accumulation of the Eromanga Basin sequence was initiated in the early Jurassic by major epeirogenic downwarping; in the investigation area the pre-Eromanga Basin surface consists mainly of rocks comprising the Thargomindah Shelf and the Cooper Basin. The lower Eromanga Basin sequence in the area onlaps the Thargomindah Shelf and thickens relatively uniformly to the north-west. The sequence comprises mainly Jurassic/Cretaceous terrestrial units in which vertical and lateral distribution is predominantly facies-controlled. These are uniformly overlain by the mainly paralic Cadna-owie Formation, signalling the initiation of a major Cretaceous transgression over the basin.The terrestrial sequence over most of the area comprises alternating coarser and finer-grained sedimentary rocks, reflecting major cyclical changes in the energy of the depositional environment. The Hutton Sandstone, Adori Sandstone and 'Namur Sandstone Member' of the Hooray Sandstone comprise mainly sandstone, and reflect high energy fluvial depositional environments. Lower energy fluvial and lacustrine conditions are reflected by the finer-grained sandstone, siltstone and mudstone of the Birkhead and Westbourne Formations, and 'Murta Member' of the Hooray Sandstone. Similar minor cycles are represented in the 'basal Jurassic' unit. The Algebuckina Sandstone, recognised only in the far south-west of the investigation area, comprises mainly fluvial sandstones.

Migration of C 1-C 8 hydrocarbons in marine sediments

Light hydrocarbon (C 1-C 8) profiles are compared for three wells of varying maturities: two immature DSDP wells (Site 397 near the Canary Islands and Site 530A near the Walvis Ridge in the south-east Atlantic) and a mature well, the East Cameron well in the Texas Gulf Coast. Primary migration of C 1 and C 2 appears to be occurring in all of the sedimentary rocks examined. Primary migration of C 3+ components becomes important only as fine-grained sedimentary rocks enter the catagenetic hydrocarbon generation zone or over short distances in more permeable sections. Lateral migration along bedding planes was more important than vertical migration in sedimentary rocks of all maturities. The lightest (methane, ethane and propane gases) hydrocarbon show greater fractionation than do the C 4-C 8 alkanes which generally show minimal fractionation during the migrational process. Subsurface diffusion coefficients for these p.p.b. quantities of C 2-C 5 alkanes from immature sediments from DSDP Site 530 are estimated to be several orders of magnitude less than values reported in the literature for diffusion of much larger amounts of these compounds from mature water wet sediments into air or sandstones. Since our calculations suggest light hydrocarbons are present in amounts less than their reported solubilities in pure water at 25°C, we postulate that the sediment organic matter has a substantial effect on retarding the movement of these light hydrocarbons.

The Use of Back-Scattered Electron Imaging for the Petrographic Study of Sandstones and Shales

ABSTRACT In this paper, the imaging of mineral grains in sedimentary rocks by atomic-number contrast using back-scattered electrons (BSE) in the SEM is discussed. The analyses of polished thin sections of a sandstone and a shale are used to illustrate the technique and particularly to demonstrate its usefulness in petrographic studies of fine-grained sedimentary rocks. The coupling of energy or wavelength-dispersive X-ray elemental analysis to atomic-number contrast allows minerals to be readily identified and paves the way for detailed geochemical and mineral crystallochemical studies of fine-grained sediments.