Showing papers on "Petrography published in 1968"

The formation of sedimentary iron minerals

Abstract: An attempt to combine laboratory and field data on recent sedimentary environments to produce mineral stability diagrams suggests that two distinct situations may be recognized. In depositional water, only ferric compounds can be truly stable. In interstitial water, beneath the sediment-water interface, ferrous compounds (pyrite, pyrrhotite, magnetite, siderite, and chamosite) are stable. Anion activity (sulfide and carbonate) appears to be the most important chemical control. The theoretical findings (Pt. 1 by C. D. Curtis) are consistent with interpretation of petrographic observations on Carboniferous and Jurassic ores of the British Isles (Pt. 2 by D. A. Spears).

Sandstone Petrology, Evidence from Luminescence Petrography

Abstract: Using luminescence petrography equipment previously described in the literature, and modifications of this equipment reported here, one can usually distinguish secondary from primary quartz in sandstones. It is thus possible to distinguish solution and overgrowth phenomena even when usual petrographic criteria fail. By luminescence, examples of concavo-convex boundaries, long boundaries, and sutured contacts, all criteria of pressure solution, are revealed to result from optically continuous quartz overgrowths. The luminescence not only prevents ambiguity regarding mechanisms active in the sandstones, but reveals the detrital fabric so that size, shape, and degree of rounding of the detrital grains may be clearly seen. Features relating to prior cycles of sedimentation are also freque tly seen. Fractures and crushing events, long since healed by growth of secondary quartz, are also distinguishable by the luminescence technique; in some immature sandstones such events seem to be very common. Often, the crystal regions revealed by crossed nicols are related to, but are by no means identical with, the fractured fragments, giving evidence of recrystallization with mobility of grain boundaries. Crenulated boundaries also form in the fracturing-healing-recrystallization process resulting in polycrystalline grains showing metamorphic quartzite texture. It is suggested that many of the polycrystalline quartz grains found in sandstones originate in this way either in place or in precursor source rocks. All these features are well documented with photomicrographs.

Sedimentological Evidence for an Emerged Land Mass in the Ligurian Sea during the Palaeogene

Abstract: Petrographic studies, particularly heavy mineral analyses, suggest that the materials of two sandstone flysch formations had related sources. This information supplements other geological evidence that the Ligurian Sea opened by rifting and that there was tectonic rotation of continental blocks in this area during and following the Alpine orogeny.

Potassium-Argon mineral dates on intrusive rocks from the Foveaux Strait area

Abstract: Potassium-argon dates on biotite and hornblende separated from six intrusive rocks from the north shore of Foveaux Strait (between Pahia Peninsula and Bluff) and Ruapuke Island range from 253 m.y, (late Permian) to 133 m.y, (early Cretaceous). The younger dates, however, are discarded because of wide discordance between the biotite and hornblende determinations, and petrographic evidence of alteration. The accepted range of ages is from 250 m.y. to 217 m.y., although the younger of these, corresponding to about middle Triassic, are regarded with caution because of minor discordance between the biotite and hornblende dates in the two rocks concerned. Nevertheless, the possibility that intrusion of some of the rocks extended into the Triassic is recorded and briefly discussed.

Radiogeologic studies in the central part of the sierra nevada batholith, california.

Abstract: Radioactivity in granitic rocks of the central Sierra Nevada varies both regionally and with rock type. It is lowest in the western foothills and increases eastward to the crest of the range. East of the crest radioactivity is generally lower than in the crestal region, but south of Big Pine Creek it increases eastward from low values in the Inconsolable granodiorite to high values (similar to those along the Sierra crest) in the Tinemaha granodiorite of the Poverty Hills. Within any given area radioactivity varies with silica and the alkalis; it is lowest in diorite and gabbro and progressively higher in quartz diorite, granodiorite, and quartz monzonite. On the west slope of the Sierra Nevada, the isotopic ages of the rocks increase westward, opposite to the direction of increase of radioactivity. This relation and preliminary heat flow values (which indicate a westward decrease in heat flow) are consistent with the concept of a vertically fractionated batholith in which the heat sources were concentrated in the upper parts. According to this concept, the oldest rocks have the lowest heat production because they have been eroded to the deepest levels. This concept fits less well with the relations east of the crest, where the isotopic ages of the granitic rocks are even older than those in the western foothills. The picture may be complicated by an original inhomogeneous distribution of the radioactive minerals in the source rocks from which the granitic magmas were derived. The proportions of U, Th, and K are generally constant within any given pluton and in plutons that are compositionally similar and of the same age, but they may be significantly different in plutons that are compositionally and temporally unrelated. Different proportions of the radioactive elements in three pairs of plutons that had been correlated on the basis of petrographic similarity suggested faulty correlations. Recent geologic work and isotopic age dating have shown that the correlations were incorrect in two of the cases. A limited study in fission-track autoradiography suggests the U is mainly contained in biotite in rocks rich in biotite and in the non-magnetic accessory minerals in rocks that contain little or no biotite.

Studies of batholithic feldspars: Sierra Nevada, California

Abstract: Co-existing plagioclase and alkali feldspars of the Sierra Nevada “granites” and plagioclases of the mafic inclusions have been analysed using an ARL EMX electron microprobe analyser. Each Sierran rock type contains co-existing feldspar pairs within specific compositional ranges. Core plagioclase compositions of the mafic inclusions are only slightly higher or lower in anorthite than the host rock plagioclases and cluster between An30 and An40. The chemical inhomogeneity of the Sierran potash feldspars and this effect on the Barth k value prohibits the use of the feldspars as geothermometers for these particular rocks. Results of the electron microprobe, x-ray, and petrographic study and the experimental hydrothermal investigation of the “granites” suggest but do not prove that both the plagioclase composition and the mafic inclusion mineralogy can be explained in terms of a model which considers the inclusions to be the refractory residue left over from the partial melting of crustal material.

A worldwide inventory of features characteristic of rocks associated with presumed meteorite impact structures

Abstract: Worldwide data inventory of rock features associated with meteorite impact structures, including photomicrographs of petrographic features

Uranium deposits in the jackpile sandstone, new mexico.

Abstract: Ultimate sources of uranium are believed to be either fluids from Morrison volcanoes (not demonstrable) or labile constituents in the Jackpile Sandstone. Petrographic studies indicate considerable pre-Dakota diagenesis, which would have made uranium in feldspars, heavy minerals, and volcanic debris available. Deposition of uranium occurred while host sandstone was near the surface during the pre-Dakota hiatus. Sedimentary permeability was the principal factor in distribution of ore and of the organic decomposition products that precipitated it.

The Petrology and Structure of the El Pinal Tonalite, Baja California, Mexico

Abstract: The El Pinal tonalite is one of the many plutons which comprise the southern California batholith of the Peninsular Range province. This pluton crops out on the western slope of Sierra Juarez in northwestern Baja California, Mexico, at elevations ranging from 850 to 1600 m, and is located 45 km from the Pacific coast and 35 km from the international boundary; its outcrop is roughly oval-shaped and covers about 190 square km. The surrounding country rocks include metasedimentary rocks, gabbro, diorite, tonalite, granodiorite, and adamellite. The El Pinal tonalite is very uniform in appearance. It is a pale grayish rock with an average grain diameter of 4 mm, and contains about 4 percent of ubiquitous, dark inclusions which average 30 cm in diameter. The pluton was mapped in three zones, concentric about the approximate geographic center of the body, on the bases of structural and textural features. Rocks of the inner zone are hypidiomorphic granular and exhibit a foliation defined by somewhat inequidimensional, dark inclusions. Textures of the middle zone are hypidiomorphic granular to protoclastic, and foliation is defined by discoidal inclusions and platy biotite grains. Some of the inclusions there are triaxial and, together with hornblende prisms, define a lineation that plunges down the foliation. Rocks of the outer zone are protoclastic and locally cataclastic. In this zone, discoidal inclusions and platy biotite grains define a foliation which is parallel to the contact of the pluton with country rocks. Foliation and lineation are everywhere concentric about the approximate geographic center of the pluton; they are interpreted as primary flow structures. Their dips change gradually and continuously, from gently toward the center of the pluton near its center, to vertical in the middle zone, and to steeply outward in the outer zone. Nowhere is the foliation horizontal. Modes were determined from 142 stained slabs. Twenty of the modes plot within the granodiorite field, and the remainder plot within the tonalite field. Sixteen of the 20 granodiorite samples are from the middle zone. The color indices for the entire pluton increase from an average value of 17 near the center of the pluton to 26 near contacts with country rocks. This may be the result of assimilation of mafic xenoliths stoped near contacts. The concentration of granodiorite in the middle zone is thought to be related to ring fracturing. Seventy-four oriented thin sections were prepared from samples distributed over the entire pluton. From these, poles to {010} and a axes in plagioclase grains were measured for 24 localities. When plotted stereographically, these data indicate that {010} is everywhere parallel to foliation as mapped in the field. Crystallographic a axes form a lineation which everywhere plunges down the dip of foliation. Measurements on etched slabs and thin sections show that most plagioclase grains are essentially biaxial tablets whose longer axis is from 2 to 5 times the length of the shorter axis. The plagioclase fabric is thought to have formed as the result of viscous flow during emplacement of the magma-crystal mixture. Plagioclase grains were presumably elongate parallel to a or c at that time. The total structural pattern of the pluton is best explained by a 360° ring fracture which extended from the top of the magma chamber to the surface of the earth. Although the ring dike is eroded away, its base is thought to correspond to the middle mapped zone, which shows clear evidence of upward convergence of flow lines and planes. Additional geomorphologic, mineralogic, and petrographic data also support the ring-fracture hypothesis.

Petrographic and petrochemical studies of MT Egmont andesites

Abstract: Thin section studies of pebbles and boulders from the Rapanui Formation near Hawera have been made in an attempt to ascertain the principal trends of petrographic variation in Mt Egmont andesites. In addition to the commonly acknowledged hornblende andesites several other varieties of andesite identified included augite andesites completely devoid of hornblende and basaltic andesites containing up to 7% modal olivine. Hypersthene andesites of the type dominating volcanic rocks in the Tongariro National Park area are conspicuously absent. Petrographic and petrochemical relationships of andesites of Mt Egmont indicate that these rocks were formed either by contamination of basaltic magma with sialic materials or by fusion of crustal rocks of dioritic composition. The latter process of origin of andesitic magma is favoured.

Diagenesis of Phosphatic Carbonate Rocks on Remire, Amirantes, Indian Ocean

Abstract: The account is concerned with the diagenetic processes resulting in the phosphatization of a series of calcarenites on the island of Remire, Amirantes. The field relationships, together with details of petrography, mineralogy, and chemistry are discussed. At least four modes of emplacement are involved: (1) as derived phosphate pebbles, with or without carbonates, but with textural evidence of an origin as carbonate sediments; (2) as a primary phosphate sediment, lacking direct evidence of a carbonate origin; (3) as a primary phosphate cement, lining voids within a calcarenite; (4) as a result of in-place phosphatization of a calcarenite. Processes three and four dominate throughout the island. The mechanisms by which these come about are considered. Textural features indicate certain peculiarities in the behavior of the phosphate depositing media. A possible explanation for these is offered. The origin of the phosphates is tentatively linked with the history of the island, suggesting that they are the remnant of sediments deposited during a period of higher sea level.

Petrography and petrology of volcanic rocks in the Mount Jefferson area, High Cascade Range, Oregon

Abstract: The Mount Jefferson area lies along the crest of the Cascade Range in Oregon and is about 25 miles long by 10 miles wide. It includes the major volcanic peaks of Mount Jefferson and Three Fingered Jack. The area is underlain almost entirely by lava flows and local deposits of cinders and breccia; collectively these flows and associated deposits are known as the volcanic rocks of the High Cascades. The rocks, which are undeformed, are of Pliocene to Recent age. They form a high volcanic plateau that is surmounted by the composite cones of Mount Jefferson and Three Fingered Jack and that is locally overlain by Recent cinder cones and intracanyon lava flows. The volcanic rocks are classified primarily by their silica content, determined from the refractive index of fused beads. In general, the rocks increase in silica content with increasing elevation (decreasing age). Basalt (49^-52 percent Si02) contains phenocrysts of olivine and (or) bytownite in a groundmass of plagioclase, clinopyroxene, opaque minerals, and glass. Andesite (52-57 percent Si02) is the most abundant type of rock. Some andesite is porphyritic, the phenocrysts being of bytownite, olivine, and (or) calcic augite. Aphyric rocks and the groundmass of porphyritic rocks consist of plagioclase, clinopyroxene, opaque grains, and glass. Some andesite contains orthopyroxene. Silicic andesite (59-65 percent Si02) contains phenocrysts of labradorite, and, commonly, calcic augite, orthopyroxene, and oxyhornblende. The aphanitic groundmass consists of plagioclase, alkali feldspar, silica minerals, pyroxene, and glass. Dacite and rhyodacite (66-74 percent 8102) contain phenocrysts of labradorite or andesine and, in some, orthopyroxene, salite, and oxyhornblende in an aphanitic groundmass similar to, but more silicic than, that of the silicic andesite. The Recent intracanyon flows are andesite with 54-55 percent Si02 and phenocrysts of plagioclase, olivine, and clinopyroxene in a groundmass of plagioclase, clinopyroxene, glass, and opaque grains. Silica variation diagrams for eight chemically analyzed specimens yield smooth curves. The alkali-lime index of 61 is close to that for other areas in the High Cascades. The Mount Jefferson area rocks contain more alumina and less iron oxide than those of either the Western Cascades or the Koolau Volcanic Series from Oahu, Hawaii. The norms show that the four more silicic rocks are perGl 285-003 O 68 2 G2 CONTRIBUTIONS TO GENERAL GEOLOGY aluminous. Triangular feldspar and quartz-orthoclase-plagioclase diagrams show the paucity of potassium and illustrate the consanguinity of this suite of rocks. Comparison of Mount Jefferson area rocks with the experimental data on the system SiC^-FeO-FeaOs-MgO shows that they very likely crystallized at nearly constant oxygen pressure. The magma forming the volcanic rocks of this area was erupted from a number of vents, probably chiefly from Mount Jefferson and Three Fingered Jack. The Pliocene and Pleistocene lavas represent a differentiated sequence, fed by one or more magma chambers. The Recent intracanyon flows originated from a new, undifferentiated magma. INTRODUCTION SCOPE This report describes the petrography and petrology of a suite of rocks belonging to the volcanic rocks of the High Cascades (Callaghan, 1933), collected from the Mount Jefferson area in Oregon. LOCATION AND GEOGRAPHY The Mount Jefferson area is in Linn, Marion, and Jefferson Counties, Oreg., about 40 miles northwest of Bend and about 55 miles east of Salem (figs. 1 and 2). It is part of the Cascade Range, a mountain chain that trends north and south from northern California across Oregon and Washington into British Columbia. The area from which rock samples were collected, about 25 miles long by 8-12 miles wide, lies along the crest of the range. It includes the major volcanic peaks of Mount Jefferson (alt 10,497 ft; frontispiece) and Three Fingered Jack (alt 7,841 ft; fig. 3). The Cascade Range in the Mount Jefferson area is a north-southtrending ridge whose crest lies at an elevation of 5,500-6,500 feet. The ridge is surmounted by numerous major and minor peaks on its summit and slopes. Spurs lead outward from the crestal ridge and are separated by deep U-shaped valleys which end at steep headwalls against the ridge. Such deep valleys on the west side of the ridge have floors as low as 3,000 feet and thus provide as much as 7,500 feet of relief against Mount Jefferson. PREVIOUS WORK The earliest work on the Mount Jefferson area was a report on the geology of Mount Jefferson by Hodge (1925), who described the general sequence of volcanic rocks and discussed the physiography and glacial history. Thayer (1937) described the petrography and petrology of the rocks from an area extending from Detroit (18 miles west of Mount Jefferson to Olallie Butte (north of the area shown in fig. 2) and Mount Jefferson. Rocks in this area belong to both the volcanic rocks of the Western Cascades and those of the High Cascades and were divided by Thayer VOLCANIC ROCKS, MOUNT JEFFERSON AREA, OREGON G3 124° BRITISH COLUMBIA 122° 120°

The magmatic affinities of some volcanic rocks from the Tweed Shield Volcano, S.E. Queensland — N.E. New South Wales

Abstract: Some basaltic and andesitic rocks from the Tweed shield volcano are tholeiitic in character as evidenced by their chemistry and the nature of their pyroxenes inferred from optical data. A common mineral assemblage in the more basic members is olivine-clinopyroxene-plagioclase-opaques-(glass). Petrographic criteria that characterize tholeiites are often lacking, and some of the more basic lavas appear to link the more typical tholeiites with transitional olivine basalts. Many of the Tweed volcanic rocks are more potassic than similar rocks in other tholeiitic sequences. The likelihood of differing lineages in the shield area is noted.

Petrography of Cenozoic volcanic rocks of Egypt (UAR)

Abstract: The detailed petrographical characters of Cenozoic volcanic rocks of Egypt are given. Two petrographic provinces have been identified: Abu Zaabal-Abu Rawash-Qatrani province and Cairo-Suez-El-Bahnasa-Tahna province. The rocks of the first one are doleritic basalts, while those of the second one are typical basalts. The weathering of the doleritic basalt, as especially observed in quarries, was also studied.

Petrology of the Nipissing Diabase Sill of the Cobalt Area, Ontario, Canada

Abstract: This paper presents a detailed petrographic and mineralogic study of the rocks of the Nipissing diabase sill near Cobalt, Ontario, Canada. The sill has an undulatory form, and the petrology of the sections in the basins differs from that of the arches. Olivine-bearing hypersthene diabase is confined to the basins. Augite, pigeonite (in part inverted), and hypersthene occur in the sill; the systematic manner in which they vary and are related to each other is detailed herein. The diversity of major facies is reasonably explained by gravity control of differentiation accompanied by some lateral movements. Continuous exposures of rock at several mines have facilitated detailed studies.

Catahoula Formation of Western Louisiana and Thin-section Criteria for Fluviatile Depositional Environment

Abstract: Few thin-section studies have been published of post-middle Eocene formations along the Gulf Coast. One purpose of this paper, therefore, is to summarize such a study of the outcropping late Oligocene(?)-early Mioceue(?) Catahoula Formation of western Louisiana. In outcrops, this formation is continental, mainly fluviatile, and in part lacustrine in origin. Thus a second purpose of this study is to enumerate thin-section criteria for identifying strata of similar origin in the subsurface. The Catahoula Formation, where studied, is a 52- to 550-foot-thick sequence of sandstone, siltstone, shale, claystone, and bentonite. A large proportion of the detrital material and matrix is of volcanic origin. Quartz is the most common framework mineral. It is of several types though volcanic quartz is the most abundant. Volcanic rock fragments, bentonite clasts, qnartzite, orthoclase, and plagioclase are next in abundance among framework grains. Lesser amounts of chert, muscovite, biotite, opaques, tourmaline, zircon, epidote, apatite, and other minerals are present. A characteristic of the sandstone is the opaline and montmorillonite cement which imparts a quartzitelike hardness in some outcrops. Thin-section criteria are used for identifying the fluviatile, nonmarine environment of deposition in which the Catahoula formed. These criteria are neither unequivocal nor unique, and some of them are negative. Where these criteria are used collectively, however, they provide strong evidence for the depositional environment of this formation. They include (1) immature textural appearance and scarcity of samples with mature texture, (2) the presence of numerous beds with poorly sorted framework grains, (3) bimodal character of many samples where matrix is allogenic, (4) low degree of grain roundness, (5) low degree of grain sphericity, (6) moderately abundant flat-grain alignments, (7) absence of unabraded marine fossils, (8) presence of fossil wood and leaves, some bone fragments, an fresh-water mollusks, (9) scarcity of glauconite, and (10) general absence of authigenic, primary and secondary cements or grains of calcite, dolomite, and siderite. Although these characteristics may also be found in other types of sediments which are transported in channels and "dumped" (for example, turbidites), other criteria--such as mixed deep-water and shallow-water faunas --can be used to separate fluviatile from turbidity-type deposits. The petrographic characteristics listed here cannot, and should not, be used to the exclusion of other criteria (that is, paleontological).

Volcanic and intrusive structures in the Kila-Warji ring-complex, northern Nigeria

Abstract: The Kila-Warji massif is one of a large number of ring-complexes of Jurassic age which cut the basement rocks of northern Nigeria. It is the only large member of this Nigerian Younger Granite province in which rocks of intermediate composition are dominant over acidic rocks. Structurally the complex presents several features of interest. The southern part——Warji—is a cauldron subsidence structure of volcanic rocks invaded by partly concordant intrusions. In the north the Kila ring-complex consists of a cluster of almost superimposed intrusive ring-structures. The complex lies in the extreme north of the Nigerian Younger Granite province (Fig. 1) and forms a group of low hills rising from an almost level plain developed on basement rocks. Farther north a few small complexes are exposed near the southern fringes of the Plio-Pleistocene sediments of the Chad basin, on the northern flanks of which Younger Granites reappear as large massifs in the Zinder-Goure area of Niger (Black 1963) and at Matsena in Bornu Province (Turner, in press). The Kila-Warji complex, composed mainly of acid and intermediate volcanics, syenites and peralkaline granites, has closer affinities to these northern complexes than to the main part of the Nigerian province, where biotite-granites are the most abundant rock type. Previous work on the area includes a preliminary survey by Raeburn (Falconer & Raeburn 1923) and an account of the petrography of some of the rocks by Beer (1952). Field work for the present study was carried out from October 1965 to January 1966. This paper

Crystallization Behavior of Basalt Magma, Santa Rosa Range, Nevada

Abstract: The Cenozoic volcanic complex in the Santa Rosa Range consists of many widely scattered, independent vents, associated with sequences of flows that reach a maximum thickness of 3000 ft. The predominant rock types are basalt and rhyolite-rhyodacite, but intermediate rock types are abundant and widely distributed. All rocks in the sequence are characterized by a lack of biotite, amphibole, and other hydrous minerals. Olivine is the predominant phenocryst in andesites; olivine and plagioclase are virtually the only phenocrysts in basalts. Xenoliths and xenocrysts are found only in rhyolites, rhyodacites, and dacites. The most primitive rock type in the sequence is a basalt that resembles mid-oceanic tholeiite chemically, and high-alumina basalt from the Oregon plateaus petrographically. In the northern part of the range, basalts, differentiated from this rock type, approach ferro-gabbroic compositions; in the southern part, the major differences between primitive and differentiated basalt are lower CaO and MgO, and much higher alkali percentages in the latter. Siliceous rocks were produced as end products of differentiation in all parts of the range. Petrographic and chemical features suggest that titaniferous magnetite was fractionated in larger amounts in the south than in the north. This very likely prevented iron enrichment in the south, but did not lead to early development of a silica enrichment trend. Apparently labradorite was fractionated in amounts large enough to counterbalance the silica enriching effect of fractionating olivine and magnetite, until a very late stage in magmatic history.

Geochemistry of Some Large Canadian Craters

Abstract: PETROGRAPHIC criteria have been applied to large and complex structures containing rocks of igneous appearance in attempts to prove an impact origin of these structures1,2 Discovery of alkaline and carbonatitic rocks in the Brent crater3, widely accepted as a holotype of “fossil” impact sites, throws serious doubt on petrographic criteria of origin We have therefore examined four large craters on the Canadian shield by geochemical methods The locations and diameters of these structures are as follows–East Clearwater (56° 05′ N, 74° 45′ W), 14 miles; West Clearwater (56° 10′ N, 74° 45′ W), 19 miles; Carswell (58° 27′ N, 109° 30′ W), 24 miles; Manicouagan (51° 25′ N, 68° 30′ W), 38 miles All four structures4 are roughly circular basins having a depth less than 1 per cent of the diameter, and with a concentric central prominence The basin is almost circular at West Clearwater and Carswell quadrate at East Clearwater and octagonal at Manicouagan Rocks of igneous appearance outcrop on the edges of the central prominence and (except at Carswell, in the lowest parts of the basin Volumes of igneous rocks range from less than 001 miles3 at Carswell to more than 20 miles3 at Manicouagan Both dyke-like and sheet-like masses are present–the latter greatly predominant in volume The lower parts of the sheets are rich in inclusions, and locally consist of unusual breccias (suevite) which are also found as dykes

Re-interpretation of a mixed petrographic province — The Sintra intrusive complex (Portugal) and related rocks

Abstract: Sub-alkaline to calc-alkaline granite dominates the high-level plutonic massif of the Serra de Sintra, while two overlapping ring complexes recognised within it consist of four distinct alkaline to sub-alkaline quartz-bearing syenitic units, together with intrusions of a remarkable kaersutite- and (titan?)biotite-bearing theralite heteromorph (Mafraite), contaminated to varying degrees by sialic admixture.

Distribution of Minor Structures in Gulf of Mexico Sediments

Abstract: Minor sedimentary structures are studied by means of X-ray radiography from cores collected from various physiographic provinces of the Gulf of Mexico. The Gulf can be divided into two large petrographic provinces meeting at DeSoto Canyon and Campeche Canyon. The western part is a clastics province and the eastern is a carbonate province. For a study of the sedimentary structures it is necessary to divide both petrographic areas into their proper physiographic provinces. The sedimentary structures and characteristics distinguished are: thin bedding (regular, irregular, lenticular), lamination (parallel, lenticular, irregular), coarse-grained laminae, turbidites (single, multiple, vague), convolute lamination, load casts, degassing structures, mottled, slump, homogeneous sediments, burrowing, mycellium, shells and shell fragments. A number of these characteristics can be found in almost any area but some of them are restricted in occurrence. Cores collected from one province reveal only slight variations in their content of sedimentary structures.

Chemical and modal analyses of some Upper Cretaceous and Paleocene bentonites from western Alberta

Abstract: Twenty-two Upper Cretaceous (Brazeau Formation) and 9 Paleocene (Paskapoo Formation) bentonites from the Foothills of the Rocky Mountains in western Alberta were chemically analyzed by X-ray fluorescence. Petrographic modal analyses were made of the coarse fractions of 11 of the Upper Cretaceous bentonites to compare chemical and mineralogical compositions. Following modal classification schemes of igneous petrologists, the original ashes were predominantly rhyodacites but ranged from dacites to rhyolites. A trend toward increasing acidity is indicated in Late Cretaceous time, reaching a maximum near the Cretaceous-Tertiary boundary. The elemental composition of the bentonites shows no close quantitative relationship to the composition of the coarse crystal fraction. Alteration of the ashes to bentonites has resulted in preferential loss of silica over alumina, and of sodium over potassium and calcium. The chemical composition of the bentonite as a whole is controlled largely by the composition of the dominant clay mineral, montmorillonite, but original differences in composition between ashes are preserved to some degree.

On the Tertiary granitic rocks around Kohu basin, Yamanashi Prefecture

Abstract: The Kohu plutonic body which occurs as a semicircular outcrop in Kohu basin is one of the largest Tertiary granitic bodies in Japan. Its N-S elongation is 45km. and its E-W breadth is 30km. It traverses the boundary between the Shimanto Belt and the Fossa Magna. The granitic rocks composing this plutonic body are divided into 10 types according to their occurrence, petrography, chemistry and mineralogy. These types are named: 1) Hirose, 2) Sanpo, 3) Shiodaira, 4) Yogai, 5) Tonogi, 6) Kanegawa, 7) Kurogane, 8) Ashigawa, 9) Mizugaki-Shosenkyo and 10) Huruyado. The first four types are granodiorite, carrying biotite and hornblende. The next three types are quartz-diorite, with hypersthene and c1inopyroxene (augite or salite). The Ashigawa type is a quartz-diorite without pyroxene. And the last two types are adamellite. These granitic rocks are associated with intrusions of andesite, quartz-porphyry and andesitic pyroclastic rock in the area. New chemical analyses of 32 rock samples of these granitic rocks are given in Table 4. Their chemical compositions, except that of the Asigawa type, are generally similar to the Tertiary granitic rocks in the Green Tuff Region of northeastern Japan and the Shimanto Belt. The Ashigawa type is very low in K2O and high in Na2O and resembling the Tanzawa plutonic body.

Comparative characteristics of jadeite and associated rocks from Polar Ural and Prebalkhash region

Abstract: New additional data on petrography and mineralogy of jadeite and associated rocks from Polar Urals and Prebalkhash region are given. Some questions of the jadeite rock genesis in hyperbasites are also considered. In Pay-Yer massif, Polar Urals, the jadeite rocks are associated with gabbro-pyroxenite complex, garnet-bearing gabbro-amphibolite, metamorphic schists and glaucophane rocks. Jadeitite and albitite form a chain of lens-shaped bodies in hyperbasites and represent metasomatic metagabbro rocks, altered under the conditions of high pressure and high soda potential. Secondary processes of diaphthoresis are strongly revealed. In Kantegir hyperbasite massif, northern Prebalkhash region, jadeitites form drop – like bodies together with numerous inclusions of unaltered granitoids and other metasomatic rocks. Jadeitite and hornblende are the earliest (high temperature) metasomatic rocks. Chemical, optical and X-ray data of jadeitite and accessory and other minerals of jadeite rocks are given. The authors c...

Petrography and Origin of Miliolite Limestone of Western Saurashtra Coast

Abstract: The Miliolite Limestone of the Western Saurashtra coast consists of a succession of mostly high-energy calcarinites, calcirudites, coquinoid bands, and minor carbonate-mud Petrographic examination of these rocks shows that the sediments were formed under shallow, warm, agitated waters of tropical sea shores These are mainly calcareous beach deposits which have been cemented by secondary calcium carbonate precipitated from sea water which was saturated with it There were minor variations in the beach environment which are reflected in the associated sediments The occurrence of these rocks over the Tertiary coastal plains and in the interior of the Deccan Trap country indicates a higher stand of the sea during Pleistocene time than at present

Mineralogical composition of the Nyiragongo rocks

Abstract: The author reviews briefly the application of the AMS calculation of rock analyses developed byAlfred Rittmann to the lavas of the Nyiragongo volcano in North Kivu, Republic of Congo (Kinsasha). In that volcanic field illustrated in Fig. 1, two petrographic provinces can be distinguished: the Nyamuragira area with volcanics of the basanite group (moderate degree of undersaturation) and the Nyiragongo area with those of the leucitite-nephelinite-melilitite group (high degree of undersaturation). In contrast to the Al-rich titanian clinopyroxene of the leucitites and nephelinites, that of the typical nepheline melilitites is poor in Al and Ti with relatively small amount of the acmite component. The melilitites show a significant excess of the alkalies over aluminum. At least a part of this excess is contained in the glassy material found in the rocks. It is suggested that the genesis of the Nyiragongo magma, mainly melilititic in composition, is possibly connected with carbonatic fractions in the magma basin and enrichment of the alkalies by gaseous transfer in the form of carbonates.