Showing papers on "Petrography published in 1976"

Mare basalts: Crystal chemistry, mineralogy, and petrology

Abstract: The paper attempts a synthesis of the major-element chemistry, petrography, mineral chemistry, and crystal chemistry of the mare basalts returned by Apollo and Luna missions A classification of the mare basalts based on major-element chemistry is given, and textural sequences within each major-element group are identified The mineral chemistry and crystal chemistry of each mineral group are considered within the framework of the major-element groups and the textural sequences The various classes of models for the origin of the mare basalts and the nature of their source regions are discussed in the context of the major- and trace-element chemistries and experimental investigations

Petrology of Holocene fluvial sand derived from plutonic source rocks; implications to paleoclimatic interpretation

Abstract: Despite the long and continued interest of sedimentary petrologists in the provenance of ancient sediments, little is known about the detrital modes of Holocene sands of known parentage. As a result it has been impossible to use the "present as a key to the past" for purposes of provenance interpretation. In order to fill this information gap and to aid in further understanding of the provenance problem, first-cycle fluvial sands derived exclusively from granitic plutons in the relatively humid Appalachian Mountains and in the relatively arid Rocky Mountains in the United States have been studied. Characterization of the climatic influence on these sands is based on the results of detailed petrographic modal-analyses of three or five size fractions of 87 sand samples. Distinctive size-dependent compositional trends exist in the modal percents of the four principal constituents of the sands; these are potentially useful in paleoclimatic interpretation. In all size fractions rock fragments are more abundant in the arid west than in the humid east. Also, though rock fragments decrease in abundance with decreasing grain size in both the west and east, the rate of decrease is much higher in the west than in the east. Feldspars increase in abundance with decreasing grain size in both regions, but the quartz/ feldspar ratio is >1 in the east and <1 in the west. This relation holds for all size classes. Accessory, minerals are much less abundant than the feldspars, but show similar trends. Quartz generally decreases in abundance with decreasing grain size in the east. In the west quartz is more abundant in medium sand than in other size fractions. A schematic model is proposed to depict evolution of sand in arid and humid climates from polymineralic coarse-grained source rocks. The model predicts the relative abundances of the four principal detrital ingredients of first-cycle sands in different size fractions as a function of their relative resistance to weathering. This study also establishes the original mineralogic maturity of first-cycle sands. "Tie-lines" are drawn between coexisting sand compositions with similar maturity using a modification of the traditional QFR triangular diagram. In addition, trends toward mineralogic maturity within the framework of such a QFR diagram are defined.

Inferred geologic settings and differentiation in basalts from the Deep‐Sea Drilling Project

Abstract: Petrographic and geochemical studies of basalt glass and related rocks from the Deep-Sea Drilling Project (DSDP) provide a random sampling of magmas which have crystallized to form sea floor basement rocks. The samples range in age from 155 m.y. to less than 4 m.y. and represent a variety of geologic settings within the three major ocean basins. The majority show the mineralogy, textures, and large-ion lithophile (LIL) element-depleted characteristics of modern basalts from mid-ocean spreading ridges (group I). A smaller but compositionally diverse group shows distinctive mineralogy and LIL enrichment (group II). Group II magmas from spreading ridges are very similar to group I magmas in terms of major element variations but are more enriched in LIL elements. Group II magma from seamounts and off-ridge centers tends to be olivine normative and enriched in normative plagioclase, while that from aseismic ridges tends to be iron-enriched and quartz normative. The diversity among DSDP rocks is only slightly greater than that shown at a single well-sampled mid-Atlantic ridge (MAR) site (FAMOUS). There are a few unusual Fe- or Ti-rich samples which so far are unique to the Pacific and Indian oceans. Compositional diversity in the Indian Ocean is especially striking. Preliminary petrogenetic modeling suggests that the samples as a whole are not products of a single high-level fractionation sequence. Glasses which project on a pseudobinary cotectic in the Di-An-Fo system may differ significantly in FeO*/MgO, TiO2, and LIL element enrichment at comparable points on the co-tectic. These liquids probably represent several compositionally distinct mantle-derived partial melts and their fractionation products. Within the data set, individual fractionation sequences can be defined, in which most major elements are adequately explained by simple crystal-melt equilibria. However, even in these sequences, inferred residual liquids tend to show excess LIL element enrichment which is not yet understood. Most DSDP basement rocks can be related back to a spreading ridge enviroment if due account is taken of apparent ‘plume-related‘ geochemical variations along modern ridges. Unusual LIL element-enriched sea floor basalts may be related to areas such as the one at 45°N on the MAR where there is little evidence of structural or bathymetric anomalies associated with a ‘plume’. We see no evidence for geochemical or mineralogical features diagnostic of variations in spreading rate, nor is there any obvious relation between composition and age.

Petrographic Textures of Detrital Polycrystalline Quartz as an Aid to Interpreting Crystalline Source Rocks

Abstract: The internal textures of detrital polycrystalline quartz can be used to distinguish between polycrystalline quartz populations derived from different crystalline source rocks. This is supported by the petrographic examination of the quartz fraction in 65 metamorphic rocks with known metamorphic histories and the detrital polycrystalline quartz populations in 57 samples of medium sand-size Holocene fluvial sand derived from known crystalline source rocks. Empirical observations by metamorphic petrologists have shown that quartz crystal sizes increase systematically with increase in metamorphic grade of the rocks. Petrographic examination of quartz crystals deformed in controlled laboratory experiments have shown quartz responds to stress via a systematic series of processes: deformation, recovery, primary recrystallization, and secondary recrystallization. Deformation is reflected optically by undulose extinction, deformation lamellae and bands, elongated crystal units, and sutured crystal-crystal boundaries. Recovery is recognized optically be segmented extinction (semi-composite extinction) reflecting polygonization of the crystal. Primary recrystallization occurs when strain-free areas surrounded by dislocation tangles form new crys als, and is recognized optically by the presence of small (50 µm), nonundulose crystals. Secondary recrystallization develops large, nonundulose, strain-free polyhedral crystals with smooth crystal-crystal boundaries. Deformation, recovery, and primary recrystallization features characterize quartz in greenschist-grade metamorphic rocks. Detrital quartz grains with these features are, for the lack of a better term, called "unstable", reflecting the transformation from old crystals to new, recrystallized crystals. Detrital polycrystalline quartz with features of secondary recrystallization are called "stable" grains, and are derived from medium- to high-grade metamorphic rocks. Detrital polycrystalline quartz grains derived from intrusive igneous rocks are also characterized by "stable" features. A plot of a polycrystallinity index (the ratio of detrital polycrystalline quartz with more than 3 crystal units per grain to total detrital polycrystalline quartz) vs. an instability index (the ratio of detrital polycrystalline quartz with "unstable" characteristics to total detrital polycrystalline quartz) discriminates between medium sand-size polycrystalline quartz populations in Holocene sands derived from low-, and medium- to high-grade metamorphic source rocks Detrital polycrystalline quartz populations in Holocene sands derived from granitic plutons partly overlap the medium- to high-grade field.

Granitic association of northeastern Sudan

Abstract: Granitic rocks of magmatic origin form about 60 percent of the crystalline basement in the northeastern Sudan. Although divisible on field evidence into older (batholithic) and younger granites and overlying, horizontally disposed rhyolitic volcanic rocks, most units have an isotopic age of 700 m.y.; quantitatively minor intrusive masses have ages of 500 and 100 m.y. Field, geochemical, and, to a lesser extent, petrographic and mineralogic evidence indicates that the Eocambrian granitic rocks represent the cratonization of an island arc. The distribution of ultramafic (ophiolite) masses in northeastern-Africa and western Arabia suggests that several arc systems have been swept together. Paleomagnetic parameters that preclude extensive horizontal movement and closely spaced arc systems, like those of the present-day southwestern Pacific, are envisaged. The later minor magmatic activity was within-plate; a hot-spot origin is thus postulated.

The Petrography and Geochemistry of Some Californian Arkoses with Application to Identifying Gneisses of Metasedimentary Origin

Abstract: Forty marine and continental Eocene and Oligocene arkosic sandstones and interbedded shales from the Santa Ynez Mountains, California and nine continental Holocene sands from the Salton Basin, California have been petrographically and chemically analyzed for 27 elements each. The arkoses and Holocene sands are composed of quartz and feldspar with minor amounts of clay, mica, and rock fragments. The shales are about 65% clay, 15% quartz, and 15% feldspar; Fe, Mg, Zn, K, Hb, Ti, Nb, Cr, and Ni are largely contained in the clay minerals with little detritai mafic rock or minerals. The data indicate a provenance of plutonio igneous and metamorphic rocks with minor volcanics; probably the Mojave Desert region. The composition of the sediments is roughly equivalent to an intermediate igneous rock minus dissolved material, particularly Na and Ca, plus diagenetically added sulfur. The sediments were derived by moderate weathering, in an area of high relief, as is shown by the abundance and relatively unaltered co...

Mid-Infrared Spectral Behavior of Metamorphic Rocks.

Abstract: : Mid-infrared (6 to 40 micrometers) spectra of metamorphic rocks and rock-forming minerals are presented, and their molecular vibration bands identified. While igneous and sedimentary rocks are typically composed of a few major rockforming minerals in different proportions, metamorphic rocks may contain many different mineral components, some of which are vertually unique to the metamorphic environment. As a result, metamorphic rocks display a great range in spectral behavior. However, similar metamorphic facies exhibit similar spectral behavior, and this behavior is readily understandable in terms of rock mineralogy. This understanding is aided by the fact that metamorphic recrystallization is usually accompanied by a sharpening of the spectral features.

The Geochemistry of `Charnockites' and Their Constituent Ferromagnesian Minerals from the Precambrian of South-East Sri Lanka (Ceylon)

Abstract: A chemical and petrographic study of charnockites and pyroxene/hornblende-bearing granulites has been undertaken, and the data used in an interpretation of the petrogenesis of these rocks. Major- and trace-element analyses of fourteen rocks are presented, together with majorelement analyses of seven orthopyroxenes, seven clinopyroxenes, two garnets, and three hornblendes. The compositions of the coexisting ferromagnesian minerals are used to obtain estimates of the likely pressure/temperature conditions of equilibration (5-7 kbar and 700±50°C of the various mineral assemblages.

Actual and anticipated petrographic effects of carbonate undersaturation in shallow seawater

Abstract: In the near future, atmospheric carbon dioxide from the combustion of fossil fuels may cause the surface layers of the ocean to become generally under saturated with calcium carbonate. The hypothetic sedimentary effects of such an event are here discussed in the light of studies in the Skagerrak Bay, North Sea. In this area, near shore sediments in the depth range 0–40 m are affected by present-day carbonate dissolution. Not surprisingly, evidence of inorganic carbonate precipitation is absent. Stages in the dissolution process are microscopic etch patterns on calcareous grains, partially dissolved grains, and finally structural breakdown and complete dissolution of skeletal elements. Mineral phases affected are calcite, aragonite, and a spectrum of magnesian calcites. Dissolution is quantitatively unimportant in living calcareous organisms, but preservation of shells and skeletons as fossils is much impeded. A future change towards similar conditions in all the world's shelf seas would certainly mean an end to the typical warm-sea carbonate sedimentation as it is known today on the Bahama Banks and in other supersaturated regions.

Meteorite-free Apollo 15 crystalline KREEP

Abstract: Electron microprobe and petrographic analyses of an Apollo 15 sample, 15382, from the Apennine Front, have revealed KREEP chemistry and very low concentrations of the characteristic meteorite siderophile elements: Ir, Os, Re, Au, and Ni. It is a crystalline basalt, practically identical in texture and mineral zoning patterns to other lunar high-alumina basalts.

Petrology, structure and origin of the Manicouagan Melt Sheet, Quebec, Canada: A preliminary report Floran ET AL.

Abstract: Chemical, petrographic, isotopic and field studies of igneous-textured rocks within the Manicouagan structure indicate that they are the product of impact melting. Features of the melt sheet include a high degree of textural variation combined with chemical homogeneity. The textures record a complex thermal history and are due primarily to the interaction of clasts with melt. An inverse correlation between clast abundance and matrix grain size allows the melt sheet to be divided into three vertically gradational units of variable thickness. Chemical homogeneity indicates that the impact melting process has the first order effect of homogenizing heterogeneous target material. The process, however, is not totally effective, because significant (although small) deviations from a calculated average composition exist, particularly in the basal clast-laden unit. These differences depend on clast composition and abundance. The results presented here are consistent with proposed models for the generation of terrestrial and lunar impact melts.

Pore space reduction in Cretaceous sandstones through chemical precipitation of clay minerals

Abstract: Volcaniclastic sandstones from the Upper Cretaceous Horsethief Formation, Montana, are characterized by two distinct cement assemblages; (1) Corrensite ± Calcite ± Dolomite, which is restricted to samples from delta distributary channel and distributary mouth bar environments, and (2) Montmorillonite ± Calcite, which characterizes bay-beach, crevasse splay, lagoon, barrier island, and shallow subtidal environments. Petrographic, microprobe, scanning electron microscope, and fluid inclusion analyses indicate the early stage origin of these assemblages as authigenic chemical precipitates. In Horsethief volcaniclastics, pore-space reduction by precipitation of the clay minerals corrensite and montmorillonite reflects the chemistry of early stage pore-fluids, and does not involve syn- or post-depositional introduction of detrital clays.

Composition and origin of Luna 16 aluminous mare basalts

Abstract: The mineralogy and petrology of 58 lithic fragments, 52 of which are basaltic, in the Luna 16 samples were studied, with textures, and mineral and bulk compositions determined by petrographic and electron microprobe techniques These mare basalt fragments differ from other mare basalts in being aluminous (11-20%) which results in plagioclase and (olivine) microphenocrysts; moreover, they have very low-FeO and MgO and high-CaO, FeO/MgO, and normative diopside contents

Significance of Textural and Petrographic Attributes of several Cretaceous Sandstones, Southern Nigeria

Abstract: Textural and petrographic attributes of sandstones of several Cretaceous formations of southern Nigeria sedimentary basin have been evaluated. Sandstones of the first sedimentary cycle (Albian-Santonian) confined mainly to the Abakaliki basin are feldspathic and those of the second sedimentary cycle (Campanian-Eocene) confined mainly to the Anambra-Afikpo basin are quartz arenites. Nearness of the Abakaliki basin to the source region of Cameroun basement complex was the governing factor in the survival and accumulation of feldspars in the sand of the first cycle. Although detritus of the second cycle was derived from the same source region, it underwent intense chemical weathering in a humid climate and during long transportation to the Anambra site of deposition, and thereby attained a high degree of mineralogical maturity. In both cases, duration of erosion played a critical role in the presence or absence of feldspar in sediments. Quartz arenites with low textural index are perhaps indicative of a miogeosynclinal deposition whereas texturally mature quartz arenites are characteristic of cratonic or stable platform. Fragmentation of large polycrystalline quartz grains into finer subindividuals in a dynamic environment is advanced as an adequate mechanism to account for the origin of fine and silt-size particles in a sedimentary system.

A petrographic and mineral chemistry study of the weston, connecticut, chondrite

Abstract: The Weston meteorite is a breccia containing mostly light-colored equilibrated chondritic xenoliths and less abundant highly un-equilibrated chondritic inclusions fixed in a dark grey host of chondrules, mineral and rock fragments. Many of the inclusions show evidence of shock. Unlike most xenolithic chondrites, the Weston host contains a large fraction of considerably more equilibrated silicates than is found in the unequilibrated inclusions, suggesting either that most host silicates retain the mineral chemistry of an equilibrated source indigenous to Weston, or represent a unique fraction which equilibrated separately, prior to final agglomeration. The host silicates are similar in composition to minerals in the common xenoliths, supporting the former possibility that host chondrules and mineral fragments are derived from the xenolithic material, probably by impact fragmentation and melting. Also mixed with Weston is a small but distinct carbonaceous component including the minerals fassaite, Fespinel, forsterite, magnetite and Ca-Al-rich inclusion which are normally associated with carbonaceous chondrites.

Preliminary report on the nature and age of the Blackstones Bank Igneous Centre, western Scotland

Abstract: Over 100 samples from the submarine Black–stones Bank Igneous Centre have been collected by scuba divers. The samples include gabbros, dolerites and metamorphosed sediments but ultramafic rocks, expected in view of the high positive gravity anomaly on the bank, have not so far been sampled. The mode of occurrence, petrography and chemical composition of the samples show many similarities between the Blackstones Bank Centre and the Tertiary igneous centres of NW. Scotland. However, K-Ar age determinations on a basaltic dyke which cuts a gabbro suggest a minimum age for the Blackstones Centre of 70 Ma, while most plutonic rocks in the British Tertiary Province have ages of c . 59 and 53 Ma.

Petrographic and ferromagnetic resonance studies of the Apollo 15 deep drill core.

Abstract: Detailed petrographic grain size, and ferromagnetic resonance studies were performed on a representative suite of samples from the Apollo 15 deep drill core Petrographic analyses of the 90-150 micron size fraction show a subtle upward increase in the ratio of mare to highland components The agglutinate content at the FMR intensity normalized to FeO show that the soils in the core are generally immature to submature The most striking feature shown by the maturity indices is a systematic decrease in maturity from the lunar surface to a depth of about 40 cm Although other mechanisms are possible, the downward decrease in maturity can be attributed to in situ reworking over a time span of 400 my at a 50% probability

Granites and migmatites of the Damara belt, South West Africa. Petrography and melting experiments

Abstract: In the central area of the early paleozoic Damara belt migmatites and some types of granites (sensu lato) crop out. Conventional petrographic data were collected and melting experiments were carried out with 63 samples of such rocks. Based on these data, migmatite genesis can be classified as follows:

Consortium investigation of breccia 67435.

Abstract: The paper presents results of the initial phase of a consortium investigation of lunar rock 67435, a glass coated breccia which contains a unique spinel-troctolite clast. These results include oxygen isotopic data for the host breccia, bulk composition data for the host breccia, glass coating and a large aphanitic clast, and detailed petrographic descriptions of all lithologies encountered.

Petrology of 79215 - Brecciation of a lunar cumulate

Abstract: The paper considers the modal variation and bulk composition, the petrography, and the mineral chemistry of lunar sample 79215, a holocrystalline, nearly monomict breccia that is petrologically and chemically distinct from the majority of lunar highland breccias. The bulk composition of the rock and its REE abundances suggest that the precursor was a plagioclase-olivine cumulate.

Petrology of Late Precambrian-Cambrian Quartzose Sandstones in the Eastern Mojave Desert, Southeastern California

Abstract: Petrographic and radiographic analyses were conducted on oriented samples of Late Precambrian-Cambrian quartzose sandstones collected from five cratonal and five miogeoclinal stratigraphic sections in the eastern Mojave Desert, southeastern California. Twelve petrographic variables were measured in each of 494 samples. Cratonal sandstones are dominantly subarkose, whereas miogeoclinal sandstones are subarkose and quartzarenite in almost equal proportions. Sediments were derived chiefly from granitic rocks, gneiss, and schist as well as older sedimentary and metasedimentary rocks. The cratonal source areas were to the east and southeast. Samples collected from six northern sections within the Cordilleran frontal thrust belt have a higher content of undulatory quartz and silica cement and are partially recrystallized when compared with samples from four southern sections. Polynomial time-trend analysis showed considerable variation in mineral composition with thickness at each stratigraphic section. Areal variation of total quartz was estimated from analysis of variance models. The difference was not highly significant for the Stirling Quartzite. Depositional history was inferred for each formation on the basis of petrography, clasticity and sedimentary structures. The Johnnie Formation fines upward, which suggests a marine transgression or change from open to partially restricted shelf environments. The Stirling Quartzite coarsens upward and consists largely of prograding mature shelf sands or deposits of migrating bars and barrier beaches. The Wood Canyon Formation is dominantly subtidal with local evidence for intertidal deposition.

Petrology and origin of Boulders no. 2 and no. 3, Apollo 17 Station 2

Abstract: The paper presents petrographic and mineralogical data for five samples from Boulder 2 (72315, 72335, 72355, 72375, and 72395) and sample 72435 from Boulder 3 at Apollo 17 Station 2. All the samples were found to consist of a few percent megaclasts set in a fine-grained matrix, which is comprised of microclasts and a poikilitic to subophitic-textured groundmass that crystallized from a melt. The samples have similar modal mineralogies, mineral compositions, and bulk-chemical compositions.

Petrography and Phase Chemistry of Basalts from DSDP Leg 34: Nazca Plate

Abstract: Three sites drilled on the Nazca plate on DSDP Leg 34 recovered fresh basalts quite unlike those recovered from previous legs. The textures of these rocks range from vitrophyric to ophitic, with variolitic intergrowths and altered glass present even in the groundmass of the coarsest unit sampled, and are suggestive of fairly rapid cooling histories. Site 321 basalts are all uniformly fine grained, vesicular, and intersertal. Many samples from all sites are Plagioclase phyric, and Hole 319A basalts appear to be largely Plagioclase and olivine phyric. Fresh relicts of olivine phenocrysts were recognized only in Hole 319A basalts, and olivine crystallization may have persisted into the groundmass in some Hole 319A basalts. The feldspars are K-poor (mainly <1% Or) plagioclases in all Leg 34 samples, and feldspar apparently crystallized in all samples during two or three major stages. The pyroxenes are aluminous augites and pigeonites which crystallized in one or two major stages. The rather strong chemical zonation in both these phases is also indicative of fairly rapid cooling. Site 321 pyroxenes are distinctive with respect to changes in the amounts of components other than Wo-En-Fs with crystallization. Phase chemical and textural data allow the delineation of at least two major cooling units within the depth range of Hole 319A samples, two cooling units represented by Hole 320B samples (out of many reported), and at least two cooling units within the depth range of Site 321 samples, even in the absence of textural evidence for such delineations. INTRODUCTION DSDP Leg 34 sampled parts of the Nazca plate off the western coast of South America (Figure la). Three sites were drilled in an attempt to core deeply into basement rock, but technical problems prevented the desired extensive penetration. The basalts which were recovered from the Nazca plate, however, are among the freshest samples to date from the DSDP expeditions, and their relative freshness facilitates comprehensive petrographic and phase chemical studies. Stratigraphic sections for each of the three sites sampled are presented in Figure lb and show the locations of the samples available for this study. The amount of basalt recovered from each basalt section is indicated by the dark patterns. Hole 319A is located on the western flank of the extinct Galapagos Rise on crustal rocks dated by paleontological study of overlying sediments at about 15 m.y. (Hart et al., 1974). The first basalt was encountered at approximately 98 meters below the water-sediment interface and was continously cored for 59 meters before the hole was abandoned (Figure lb); however, only 25% of the basalt was recovered. Shipboard measurements of densities, seismic velocities, and water contents all suggest rather fresh basalt throughout the cored section (Chapter 3, this volume). On the basis of textural changes, the shipboard scientists divided the basalt section (319A) into six definite cooling units and at least five additional thinner units. The major cooling units are 15.6 and 12.5 meters thick (Figure lb). Rapid cooling of the margins of flow units is suggested by the finegrained variolitic nature of some samples and the abundance of glass fragments recovered from the core catchers; however, no contact zones were actually recovered (Chapter 3, this volume). Preliminary observations by the shipboard scientists suggest that the basalts at this site represent a suite of rapidly cooled, pillowed to massive flows, erupted within a short period of time. Hole 320B cored a series of about nine pillows and thin flows beneath sediment dated at approximately 30 m.y. old. Although the basalts are more altered than those in Hole 319A, they nevertheless are less altered than expected for crustal rocks of this age. Basalt was encountered at about 154 meters below mudline and was cored for about 28 meters with poor recovery (Figure lb). Site 321 penetrated approximately 10 meters of basalt, 5 meters of which was recovered. The basalt section covers at least two cooling units (M. Bass, personal communication, 1974). The oldest overlying sediment is dated at 40 m.y. (Hart et al., 1974). The basalt, which was encountered 125 meters below mudline, was not quite as fresh as that from Hole 319A. The unusual freshness of both Hole 319A and Site 321 basalts (relative to other deep-sea basalts) is one line of evidence which suggests that these rocks may be

Shock metamorphic effects in lunar microcraters

Abstract: Detailed petrographic descriptions and results of electron microprobe analyses are presented for impact glasses as well as shocked and unshocked minerals associated with individual lunar microcraters (diameters of 04 to 44 mm) Rocks of four typical lunar lithologies are studied: anorthosite, anorthositic norite, ophitic basalt, and polymict breccia Textures, mineralogies, and chemical compositions are examined along a radial traverse through each microcrater; ie, across the impact glasses lining the crater wall, the shock-metamorphosed zone immediately underlying the glass liner, and the unshocked host rock The microcraters are discussed in a sequence of increasing mineralogical complexity of the host rock (from anorthosite to polymict breccia) in order to distinguish shock effects among mineral types The shock metamorphic features observed are found to be comparable to those reported in shocked basalt from Lonar Crater, India, and are categorized into five shock-intensity classes with pressures experimentally calibrated

Whole-rock Rb/Sr and zircon U/Pb ages of metamorphic rocks from northern Ellesmere Island, Canadian Arctic Archipelago. II. The Cape Columbia Complex

Abstract: Although not clearly separable on field and petrographic criteria, the gneisses of the Cape Columbia Complex, one of the two major crystalline terrains in the Northern Ellesmere Fold Belt, fall into two Rb/Sr age groups: nine samples define an isochron corresponding to an age of 1083 ± 18 m.y., Sr0 = 0.7057, while six samples show more scatter at 512 ± 90 m.y.,Sr0 = 0.7189. Zircons from two gneisses have 207Pb/206Pb, i.e. minimum, ages of 926 and 980 m.y. These data are interpreted as indicating that the rocks were recrystallized in the amphibolite facies about 1000 m.y. ago; little significance is attached to the younger Rb/Sr age. However, the possibility that the rocks are orthogneisses emplaced about 1000 m.y. ago and subsequently metamorphosed ~500–600 m.y. ago, cannot be excluded. In any event, the Cape Columbia Complex becomes the latest addition to the growing list of occurrences of 900–1200 m.y.-old ('Grenville-Sveco-Norwegian') rocks in the North Atlantic craton and environs.

Upper Smackover Reservoirs, Walker Creek Field Area, Lafayette and Columbia Counties, Arkansas

Abstract: This study has made possible the zonation of two major upper Smackover reservoirs productive at Walker Creek field, Lafayette and Columbia Counties, Arkansas, and a reconstruction of the depositional environments that were present contemporaneously throughout this area. Petrographic examination of etched core plugs, petrographic-microscope studies of thin sections, core descriptions, and consultation with authors of published papers permitted a grouping of upper Smackover carbonate rocks into facies more or less environmentally controlled and similar to those already established in the literature. Minor changes in petrographic groupings were necessitated by the additional data and the numerous core descriptions incorporated in this paper; otherwise, the facies groupings of W. F. Bishop generally have been followed. Distribution of nonskeletal particles in the sea which existed during deposition of the upper Smackover in this area resembled in many aspects that at the present northeastern tip of Yucatan. The presence of discrete sandstone bodies and their carbonate equivalents permitted lithologic markers to be correlated throughout this area of predominantly carbonate deposition, resulting in a unique interpretation of depositional environments. Major zonations of oolite bars have been confirmed by pressure data, and dense dark limestones useful as a diagnostic facies appear to have environmental significance. Certain diagnostic phenomena disclosed in thin-section analysis indicate possible eolianite lithification in the vadose or phreatic zone. The Walker Creek field produces from a structurally controlled stratigraphic trap containing nearly 100 million bbl of oil in place plus 100 Bcf of recoverable gas.