Showing papers on "Petrography published in 1987"

Unravelling Hinterland and Offshore Palaeogeography from Deep-water Arenites

Abstract: Integration of basin analysis with arenite petrography provides a powerful tool in obtaining palaeogeographic reconstructions. However, several factors need to be considered if ancient geological settings are to be correctly interpreted.

Interpreting the characteristics of coal seams from chemical, physical and petrographic studies of peat deposits

Abstract: Abstract The purpose of this paper is to illustrate some of the kinds of information about coals that are at present being generated from studies of peat deposits and also to point out some possible new areas of research that might be undertaken in the future. Some notable examples of new ideas about coal seam composition or formation that have been generated from studies of modern peat deposits include: (1) discovery of the probable progenitors of certain coal macerals and elucidation of the processes by which they may have formed; (2) evidence that some types of mineral matter may be dissolved out of peat deposits; (3) verification of the role of marine waters in emplacement of sulphur in peat; (4) discovery of the importance of ‘doming’ in peat deposit development; (5) discovery of a new way to form a split in a coal seam, i.e. development of a ‘fire splay’; (6) elucidation of the mechanisms responsible for producing stratification in coal seams; (7) recognition of the world-wide importance of ‘back-barrier’ coal-forming environments. These kinds of discoveries are important in themselves; however, they can also be shown to have many additional practical applications, especially if woven into models to predict the economic characteristics of coal seams.

Crust-magma interactions and the evolution of arc magmas: The San Pedro–Pellado volcanic complex, southern Chilean Andes

Abstract: Field, petrographic, and trace-element data indicate that volcanic rocks of the Quaternary San Pedro–Pellado complex (lat 36°S, Chile) have incorporated a silicic component that may be derived from the crust. Isotope ratios of Sr, Nd, and Pb vary little among the volcanic rocks. This is due primarily to the limited isotopic contrast between the Mesozoic-Tertiary crust and the young magmas. Isotope and element compositions of San Pedro–Pellado rocks are similar to those reported from other localities in the southern volcanic zone of the Chilean Andes. We suggest that crustal assimilation is a significant process in the genesis of differentiated volcanic rocks in the southern volcanic zone. The San Pedro–Pellado example illustrates the importance of considering all types of evidence, not just isotopic data, in the evaluation of crustal contributions to arc magmas.

Petrography, Geochemistry, and Origin of Burial Diagenetic Facies, Siluro-Devonian Helderberg Group (Carbonate Rocks), Central Appalachians

Abstract: Petrographic and geochemical studies of pore-filling cements and replacement products in the Helderberg Group (Upper Silurian-Lower Devonian, central Appalachians) document the diagenetic history of these rocks. Shallow-ramp skeletal limestones and buildups were partially lithified by marine cements. However, most pore-filling cement formed under shallow (<300 m) to deep burial (300-4,000 m) conditions. Regional cathodoluminescent zonation patterns in early, calcite cements indicate meteoric ground waters were involved in shallow burial cementation. Early zoned calcite cements consist of updip nonluminescent cements with thin luminescent laminae in limestone or sandstone, that pass downdip into subzoned dull cements (interlayered bright and dull laminae), and then into nonzoned dull cement in basinward limestone. The regional distribution, stable isotopic compositions, and timing of early zoned cements relative to other diagenetic events suggest that all of the early cements formed synchronously from meteoric ground waters that became progressively more reducing as they flowed downdip (at least 150 km). Helderberg sandstone tongues probably were preferential conduits for meteoric gro nd waters because early calcite cements formed only on scattered skeletal grains in sandstones. Late, void-filling dull cement formed from deep burial pore fluids at burial depths of 300 to 4,000 m. Deep burial pore fluids were dilute to saline Na-Ca-Cl brines (from primary fluid inclusion data) with stable isotopic compositions that probably were similar to isotopic compositions of formation waters from modern oil fields. Void-filling dull calcite cements occluded nearly all remaining porosity before late Paleozoic fracturing and deformation. Latest diagenetic phases include dolomite, fluorite, silica, and fracture-filling dull calcite. This study shows that integrated petrographic and geochemical approaches to diagenetic studies are useful for relating various diagenetic phases and fabrics to burial history and paleoaquifer distribution.

Porosity, Permeability, and Pore Structure of the Tight Mesaverde Sandstone, Piceance Basin, Colorado

Abstract: Special core analyses on 44 tight Mesaverde sandstone samples from the US DOE Multiwell Experiment (MWX) were combined with petrographic investigations to relate the porosity and permeability of the cores to the pore structure of the rocks Core analysis was performed on 1-in (254-cm) -diameter horizontal plug samples with a computerized steady-state-flow measuring device that routinely measures gas flow rates with a resolution of better than 10/sup -6/ std cm/sup 3//s All samples were selected from intervals expected to be gas-productive on the basis of wireline well logs and were taken from the portion of the interval that showed the lowest gamma ray log response The core plugs were measured for dry permeability to gas, relative permeability at various water saturations, porosity to gas, and PV compressibility Petrographic samples were taken directly off the plug ends and were analyzed with both an optical microscope and a scanning electron microscope (SEM) The petrographic study was explicitly directed toward observing the flow paths and pore structure deduced from the core analysis data

Shallow, Subsurface Growth and Burial Alteration of Middle Devonian Calcite Concretions

Abstract: Organic-rich, shale-hosted septarian and nonseptarian calcite concretions, in the Middle Devonian (Givetian) Hamilton Group of central New York State, formed early in the shallow subsurface as indicated by detailed petrography, SEM, X-radiographic, lithologic, geochemical, and isotopic analyses. 13Ccarb values suggest that carbon was derived from microbial breakdown of organic matter in the sulfate reduction zone at subsurface depths of less than 10 m. However, strongly depleted 18O (-7 to -10 PDB) suggests much greater burial depths. The SUP>18O data appear to represent an altered isotopic signature, as indicated by their narrow, depleted range and petrographic evidence for recrystallization of concretion carbonate. Evidence of primary porosity in concretions during growth indicates that early stage diagenetic alteration occurred in an open-marine, phreatic environment that homogenized isotopic and geochemical signatures of metastable carbonate minerals. The association of septarian calcite and bitumen, along with the depleted 18O values, suggests that late-stage diagenesis was a result of migration of warm, basin-derived, connate waters through the open system. Preburial condensation of sediments in younger stratigraphic units within the Hamilton Group, possibly caused by increased winnowing, bioturbation, and erosional events during shallowing of the Hamilton sea, has generated stratigraphic variations in concretion lithology and shape. Within any given unit, however, variation in characteristics of concretions remains a primary function of their timing of growth versus burial compaction. Unlike Cenozoic carbonate concretions in organic-rich marine sediments that possess a dolomitic matrix, no dolomite concretions were discovered in the Hamilton Group. Preliminary data suggest that restriction of calcite as the host mineral for Early to Middle Paleozoic concretions may be controlled by elevated sulfate concentrations that inhibited dolomite precipitation and/or primary secular variations in carbonate mineralogy.

The pseudotachylite from Champagnac in the Rochechouart Meteorite Crater, France

Abstract: As part of a comprehensive study on pseudotachylites from established impact structures, cryptoexplosion structures, and diverse tectonic settings, a vein system containing melt rock and hydrothermal crystallizations from the Champagnac quarry in the basement of the Rochechouart impact structure, northwest Haut Limousin, France was characterised by petrographic and geochemical means. The lack of characteristic shock phenomena in melt clasts (deformation is restricted to kink bands in mica and local planar fracturing in quartz and feldspar), the angular shapes of clasts, and the structural field relations lead to the conclusion that the melt fraction of these vein fillings from the Champagnac quarry represents pseudotachylite produced by friction during lateral shear movements. These veins differ from the impact breccia dyke fillings (Al breccias) reported by Lambert (1981). Bulk chemical analysis of vein fillings and defocussed beam electron probe results obtained on glassy pseudotachylite favor in situ formation of melt and rule out the possibility that it represents injected impact melt. Compositional variations are observed in this vein system and are interpreted to result from varied contributions of mafic and felsic components (mica plus opaque minerals versus quartz and feldspar). Glass analyses indicate that total melting as well as partial melting of parent rock occurred. According to petrographic evidence, the temperature at melt formation is estimated at 700–750°C, and the pressure is estimated to have been less than 1 GPa. The hydrothermal component of vein fillings was generated simultaneously with the pseudotachylite melt and produced distinctive petrographic and geochemical effects within the vein system: (1) sericitization in clasts and adjacent host rock, which altered andesitic feldspar to pure albite; and (2) selective removal of Sr from vein fillings. A Rb-Sr isotope study on vein fillings yielded consistently low Sr and 87Sr contents, but highly varied Rb/Sr ratios and an apparently well-defined isotopic equilibration among seven vein-filling samples. An apparent 217-m.y.-isochron results. It is shown that the Rb-Sr data cannot be explained by mixing processes and are not in agreement with normal radiogenic evolution of the Sr isotope system. Isotopic equilibration by diffusion seems improbable. Therefore the following genetic process for the Champagnac pseudotachylite vein system is suggested: (1) Clastic breccias were formed by impact-induced lateral slip along preexisting planes of weakness. Locally these breccias were transformed into pseudotachylite by friction melting. (2) Circulation of hydrothermal fluids (700°C, high PH2O) led to sericitization of plagioclase of the breccia clasts and immediate host rocks and modified the Sr isotopic system of pseudotachylite. (3) During the subsequent cooling period hydrothermal phases crystallized and the various textures of pseudotachylite matrix and clasts developed, before (4) long-term tectonic activity (crosscutting fractures) and additional hydrothermal modifications took place.

Paleocurrent and Petrographic Analysis of Imbricate Intraclasts in Shallow-Marine Carbonates, Upper Cambrian, Southwestern Virginia

Abstract: The Conococheague Limestone and Copper Ridge Dolomite are coeval Upper Cambrian shallow-marine carbonates exposed in the Valley and Ridge Province of southwestern Virginia. Sheetlike conglomerates of probable storm origin showing well-developed intraclast imbrication are common in these units. Directional and petrographic data obtained from 48 such conglomerates and adjacent strata reflect the landward (Copper Ridge) versus seaward (Conococheague) depositional frameworks in which the two formations originated. Analysis of imbrication azimuths suggests that the Conococheague conglomerates were transported by multidirectional paleocurrents associated with middle- to outer-shelf storm activity. In contrast, the Copper Ridge imbricate intraclasts show much more consistent shoreward movement toward the northwest, probably the result of currents generated by storm waves refracted by shallower, middle-shelf to intertidal environments. At individual sample localities, apparent long-axis alignments of elongated clasts in the conglomerates show inconsistent relationships to transport directions deduced from imbrication, evidently because of high clast concentration and general lack of prolate grains. An overall shore-parallel trend of long axes, more evident in the Copper Ridge, may have developed fr m poststorm-tidal-current and wave activity and could serve as another shallow- versus deep-water discriminant. Paleoflow patterns derived from cross stratification in sandstones above and below the imbricate conglomerates reflect normal, day-to-day, tidal, longshore, and rip-current sedimentation. Petrographically, the intraclast conglomerates show a tripartite grouping strongly related to source sediments and their environments. Group I, found mainly in the Conococheague, formed in platform-margin ooid sand shoals; Group II, common in both formations, originated in subtidal middle-shelf to lower-intertidal inner shelf, peloid sand silt and mud flats; and Group III, present almost exclusively in the Copper Ridge, accumulated in inner-shelf intertidal environments where algal mats and carbonate mud were abundant. Paleocurrent directions determined from imbrication in the petrographic groups also reflect these differences in depositional environments. The importance of paleocurrent data from imbrication in shallow-marine carbonate conglomerates, which are abundant in the geologic record, has not been realized. This information can be synthesized with petrographic data to aid considerably in reconstructing ancient environments.

Mineralogy and Petrology of the Cretaceous-Tertiary Boundary Clay Bed and Adjacent Clay-Rich Rocks, Raton Basin, New Mexico and Colorado

Abstract: The Cretaceous-Tertiary (K-T) boundary is preserved in a sequence of coal-bearing, fluvial rocks in the lower part of the Raton Formation (Late Cretaceous and Paleocene) at several sites in the east-central part of the Raton Basin. The K-T boundary occurs at the top of a kaolinitic claystone layer, commonly referred to as the "boundary clay layer," in an interval of coal and carbonaceous shale. The boundary is defined by the disappearance of certain fossil-pollen taxa. The boundary clay layer also contains shocked quartz grains and abundance anomalies of iridium, chromium, and other elements. Each of these characteristics support the hypothesis of an asteroid impact at the end of the Cretaceous. Small goyazite spheres were also found in the boundary clay bed; however, their origin is ncertain. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and thin-section petrography have shown that the boundary clay bed exhibits unique mineralogic and petrologic characteristics not possessed by other clay-rich beds in coal-bearing rocks of the Raton Basin. The boundary clay bed is composed primarily of well-crystallized kaolinite but also contains subordinate, randomly interstratified, mixed-layer illite/smectite, both of which appear to have been formed by the alteration of vitreous material in a coal-swamp environment. The bulk mineralogy of the boundary clay bed is similar to that of other kaolinite-rich clay beds (tonsteins) that altered in coal swamps from airfall volcanic ash. The alteration process and diagenetic products of the host material in the boundary cl y bed, however, differed from those of the tonsteins, resulting in a unique petrographic fabric. The boundary clay bed is characterized by an overall imbricate fabric that may reflect an original vitreous material that included shards, bubbles, and spheres. The original fallout material of the boundary clay bed first altered to "cabbage-like" microspherules, indicating a halloysite or allophane precursor, and then to well-crystallized kaolinite; the well-crystallized kaolinite is the diagenetic stable phase for these rocks at their maximum burial conditions. In comparison, the vitreous components of the tonsteins were altered directly to coarse vermicular and platey kaolinite. Alteration of the original fallout material in the boundary bed was probably rapid, leading to hydrated, disordered, fine-grained clay minerals that formed into curled spherical or tubular shapes. The facto s that produced these unique fabrics in the boundary clay bed existed at all K-T sites in the Raton Basin and were much different from those that formed the coarse platey and vermicular fabrics of ordinary tonsteins. The distinctive character of the boundary clay may be due to the unique nature and chemistry of the fallout material and (or) physical or chemical conditions existing shortly after its deposition.

Provenance of sand temper in Hohokam Ceramics, Arizona

Abstract: Sand temper compositions of regionally distinct Hohokam pottery sherds were determined by detailed point counts using sedimentary petrographic methods. Different temper compositions from different sites were compared with maps of sand composition zones within the geographic range of the pottery in order to establish the probable provenance of each sherd. A number of probable instances of intraregional pottery exchange were identified. The larger number of sand sources in undecorated vs. decorated pottery at each site suggests that undecorated pottery was made in more places than decorated vessels, and may have been traded differently as well. Petrographic analysis of temper is a useful method for studying exchange of homogeneous pottery in geologically diverse areas, and for investigating prehistoric Hohokam interaction in the Tucson and Red Rock Basins on a scale not possible with traditional archaeological techniques.

Evaluation of ballast materials using petrographic criteria

Abstract: The performance of rock ballast, when subjected to the physical stresses of loading and to the chemical and physical stresses of weathering environment, depends to a great extent on the mineralogical, chemical, textural, and structural properties of the material. Because these properties can readily be identified by petrographic analysis, useful predictions of ballast performance can be made by the experienced petrographer. However, because of the difficulty of assessing these properties quantitatively and the large number of variable factors involved, petrographic evaluation of ballast remains, at this stage, largely a subjective process. Mineralogy is a major factor in determining overall rock hardness and physical durability, chemical weathering potential, composition and quantity of derived fines, and degree of susceptibility to wetting and drying. Rock texture also affects hardness and is important in influencing toughness, relative susceptibility to freeze-thaw degradation, and mechanical stability in track. Most of the standard tests commonly applied to ballast materials essentially provide a measure of a combination of petrographic properties; consequently, the results of the tests can be predicted to within certain limits. Numerous techniques, including the use of microscopes and X-ray diffraction equipment, can be applied in the study of the fines fraction of track samples to determine the nature and source of the material. Three ballast types in use by Canadian Pacific (CP) Rail (Kimberley Float, Walhachin, and Prairie gravels) are discussed to demonstrate the influence of petrographic properties on performance.

Granulite Facies Rocks of Brazil: A Review of their Geologic Setting, Geochronological Evolution, Petrographic and Geochemical Characteristics

Abstract: An attempt to assemble the available data on the granulite facies rocks spread around Brazil is made with a view to discuss their geological setting, geochronological evolution, petrographic and geochemical evolution. The isotopic ages of the granulites fall into different groups ranging from Archaean to Late Proterozoic. The geotectonic evolution of these rock types is discussed in terms of their being either fragments of geochemically differentiated lower crustal layers or eroded segments of modern Cordilleran (Himalayan) type collision belts. The geochemical feature of the rock types demonstrates the non-systematic fractionation of trace elements.

Caracterización del sistema poroso de calizas y dolomías

Abstract: Eight types of carbonate rocks which correspond to well defined petrographic varieties -- micritic limestone, crystalline limestone and crystalline dolomite -- have been selected, and the characteristics of their pore structure studied. Some basic and methodological concepts were previously revised. Pores were directly observed at different levels. Porosity and pore-size distribution have been determined by means of mercury intrusion techniques. As a result, the pore-system for each of the different rock types studied has been characterized and the dependency regarding their petrographic characteristics -- particularly those connected with their mineralogy and degree of crystallinity -- have been analyzed. -- AATA

A quest for a new parameter in petroleum exploration geochemistry

Abstract: Primary Australian terrestrially-derived crudes are characterised by high wax and n-alkane contents. These characteristics, as determined by hydrogenation and hydrous pyrolysis, appear to be unrelated to either the chemical or petrographic compositions of Victorian brown coal lithotypes. Furthermore, since relationships between chemical and petrographic composition are obscure, a re-examination of current concepts which relate these established source rock parameters to liquid hydrocarbon generating potentials is warranted. The content of thermally stable, longer-chain, n-alkyl components in source rocks is introduced as the critical factor in determining whether these rocks have the potential to generate typical Australian waxy crudes or hydrocarbon gases. Modifications to this general concept are required by the thermal stability of directly substituted longer-chain n-alkyl aromatics and hydroaromatics. These appear to be sources of light hydrocarbons and gases, rather than oils. Inherent weaknesses in the experimental techniques of hydrogenation and hydrous pyrolysis have hindered the collection of data, but the concept that n-alkane potential is a critical factor in determining the petroleum-generating potential of immature source rocks is being pursued using techniques modified for the determination of their total heteroatom-bonded n-alkyl contents.

The 'loess' section at Borden, Kent, SE England.

Abstract: A 5.3m profile originally described as loess with a buried interglacial soil is reinterpreted from petrographic evidence and thermoluminescence dating as a Holocene colluvial accretionary soil. Mineralogical analyses of coarse silt (16–63 μm) fractions suggest that most of the silty colluvium was derived from weathered Thanet Beds upslope, though some loess was incorporated during the final depositional phase. Thin sections show that clay illuviation occurred penecontem-poraneously with deposition of colluvium. Thermoluminescence properties suggest partial optical bleaching of the grains during rapid deposition, which is consistent with a Holocene colluvial origin.

Submarine limestones in the nearshore environment off Kuwait, northern Arabian Gulf*

Abstract: Even with careful petrographic and mineralogic characterization of marine limestones, intertidally and subtidally lithified rocks are often difficult to differentiate, thus hindering an accurate delineation of the diagenetic environment. Limestones from water depths of 6 to 8 m off Kuwait vary in petrographic character from oosparite and biosparite (in which the cement is entirely aragonite) to oomicrite and biomicrite (in which at least some of the cement is microcrystalline magnesian calcite). Carbon-14 dates suggest that the oosparite may have lithified at depths shallower than at present (possibly intertidally) during a lower stand of sea-level. In contrast the biosparite, oomicrite and biomicrite appear to be contemporaneous and to have lithified subtidally.