Palynology and genetic sequence stratigraphy of the reservoir rocks (Cenomanian,Bahariya Formation) in the Salam Oil Field,north Western Desert,Egypt

Twenty-eight samples from the Bahariya Formation of the Salam-17 Well in the north Western Desert were palynologically investigated. These samples are of Cenomanian age. Fair diversity and fair to moderately preserved palynomorph assemblage has been recovered. Among them, the dinoflagellate cysts showed very poor diversity and abundance. Four miospore zones have been informally identified in the lower Cenomanian. Various palynofacies criteria, adopted from previous publications (e.g. relative particle abundance data, brown to black wood ratio, equi-dimensional to lath-shaped black wood ratio, average size of phytoclasts and spores/pollen ratio) are applied as alternative indicators to monitor the proximal–distal trends instead of the marine palynomorphs-based parameters. The method can be applied in the Egyptian Western Desert to overcome the rarity and absence of dinoflagellate cysts in the recovered organic residues. The palynofacies study of the section demonstrates a predominantly regressive phase, characterized by deltaic, distributary or tidal channels, interrupted by short-lived marine incursions. The palynofacies trends within the studied succession indicate six genetic sequences informally described as Genetic Stratigraphic Sequences A through F.     

新疆阿吾拉勒山西段下二叠统陆相火山岩岩石地球化学特征、成因及构造背景

新疆阿吾拉勒山西段是伊犁石炭-二叠纪裂谷的重要组成部分,本文对该区下二叠统典型的双峰式玄武岩-流纹岩组合进行了系统的岩相学和岩石地球化学研究。岩石整体高Na2O、高Al2O3、低TiO2、富碱。玄武岩富集大离子亲石元素Ba、K、LREE和P,亏损高场强元素Th、U、Ta、Nb。粗面斑岩和流纹斑岩富集大离子亲石元素Rb、K、LREE、高场强元素Th、Zr、Hf,亏损Ta、Nb、Sr、Ti和P。玄武安山玢岩的蛛网图与粗面斑岩和流纹斑岩较为一致,但少量元素的特征与玄武岩相似。玄武岩浆来源于弱亏损地幔,并受到了下地壳物质的混染,而玄武安山玢岩、粗面斑岩和流纹斑岩则可能来源于地壳物质的部分熔融。双峰式火山岩的形成可能与上地幔玄武岩浆的底侵作用有关。裂谷演化导致的陆相火山活动持续到早二叠世晚期达到顶峰,中二叠世以后,构造环境由拉伸转为挤压,裂谷演化趋于终止。该区石炭纪末-早二叠世的裂谷活动与整个天山地区晚古生代的构造演化背景具有一致性。     

Complex and imitation Tectonites from the Kosciusko Pluton,snowy mountains,New South Wales

Rocks of Upper Precambrian age near Adelaide show evidence of two or more phases of deformation. The first phase has resulted in concentric and similar folds with an associated slaty cleavage. Structures of this phase are overprinted by folds with associated crenulation cleavage. Minor occurrences of later kink folds are also observed. The hypothesis that the first phase folds overprint very large folds not observable in the field is examined. The observed variation in the attitude of first phase folds could also have resulted from large scale inhomogeneities of strain.

1 “Torrens Group” is used in place of the “Torrensian Series” of Mawson and Sprigg (1950) at the suggestion of Daily (1963) since the Torrensian Series has an unwarranted time significance.

2 The scale of folds follows that of Weiss (1957). Macroscopic‐folds larger than a single outcrop. Mesoscopic‐folds on the scale of a hand specimen or single outcrop. Microscopic‐folds on the scale of a thin section.     

Palaeomagnetic results from the Lancer 1 stratigraphic drillhole,Officer Basin,Western Australia,and implications for Rodinia reconstructions

A partial record of the positions of Australia during Middle to Late Neoproterozoic time is provided by palaeomagnetic results for samples from the Lancer 1 stratigraphic drillhole in Western Australia. Lancer 1 was drilled vertically to 1501 m, through essentially horizontal Neoproterozoic strata of the western Officer Basin. We studied 123 samples from 28 intervals of drillcore which were oriented by matching features (fractures, cross-beds, etc.) in the core with oriented acoustic scanner images of the drillhole walls. Three new palaeopoles are reported for red mudstones and sandstones (redbeds) of the Browne (44.5°N, 141.7°E, dp = 5.1°, dm = 9.0°), Hussar (62.2°N, 85.8°E, dp = 7.3°, dm = 14.6°), and Kanpa (74.0°N, 128.8°E, dp = 7.7°, dm = 14.8°) Formations of the ca 830 – 720 Ma Buldya Group (Supersequence 1), which exhibit stable, two-polarity magnetisations carried by fine-grained hematite and magnetite. The overlying ca 610 – 590 Ma Wahlgu Formation glaciogenic diamictite (Supersequence 3) yielded dispersed directions and an imprecise palaeopole that overlaps results from the glaciogenic Elatina Formation and other Late Neoproterozoic rock units. The results help to elaborate the Middle to Late Neoproterozoic apparent polar wander path for Australia and indicate, in agreement with palaeoclimatic data and previous palaeomagnetic studies, that the continent was slow-moving and occupied low latitudes at this time. Assuming that Australia and Laurentia were still joined at ca 780 Ma, comparison of the new Hussar Formation palaeopole with coeval Laurentian data favours AUSMEX, rather than SWEAT or AUSWUS, as the most likely configuration of these two continents in Rodinia. This preliminary study of Lancer 1 demonstrates the utility of acoustic scanner logs for orienting drillcores, as well as the scope for additional sampling and palaeomagnetic studies of Lancer 1, and other oriented drillcores, to yield a more continuous record of Australia's past motions and to provide magnetostratigraphic data for enhancing inter-basin correlations.     

Genesis of the channel iron deposits (CID) of the Pilbara region,Western Australia

Miocene fluvial goethite/hematite channel iron deposits (CID) are part of the Cenozoic Detritals 2 (CzD2), of the Western Australian Pilbara region. They range from gravelly mudstones through granular rocks to intraformational pebble, cobble and rare boulder conglomerates, as infill in numerous meandering palaeochannels in a mature surface that includes Precambrian granitoids, volcanics, metasediments, BIF and ferruginous Palaeogene valley fill. In the Hamersley Province of the Pilbara, the consolidated fine gravels and subordinate interbedded conglomerates, with their leached equivalents, are a major source of export iron ore. This granular ore typically comprises pedogenically derived pelletoids comprising hematite nuclei and goethite cortices (ooids and lesser pisoids), with abundant coarser goethitised wood/charcoal fragments and goethitic peloids, minor clay, and generally minimal porous goethitic matrix, with late-stage episodic solution and partial infill by secondary goethite, silica and siderite (now oxidised) in places. Clay horizons and non-ore polymictic basal and marginal conglomerates are also present. The accretionary pedogenic pelletoids were mostly derived from stripping of a mature ferruginous but apparently well-vegetated surface, developed in the Early to Middle Miocene on a wide variety of susceptible rock types including BIF, basic intrusives and sediments. This deep ferruginisation effectively destroyed most remnants of the original rock textures producing a unique surface, very different to those that produced the underlying CzD1 (Palaeogene) and the overlying CzD3 (Pliocene – Quaternary). The peloids were derived both intraformationally from fragmentation and reworking of desiccated goethite-rich muds, and from the regolith. Tiny wood/charcoal fragments replaced in soil by goethite, and dehydrated to hematite, formed nuclei for many pelletoids. Additionally, abundant small (≤10 mm) fragments of wood/charcoal, now goethite, were probably replaced in situ within the consolidating CID. This profusion of fossil wood, both as pelletoid nuclei and as discrete fragments, suggests major episodic wild fires in heavily vegetated catchments, a point supported by the abundance of kenomagnetite – maghemite developed from goethite in the pelletoids, but less commonly in the peloids. The matrix to the heterogeneous colluvial and intraformational components is essentially goethite, primarily derived from modified chemically precipitated iron hydroxyoxides, resulting from leaching of iron-rich soils in an organic environment, together with goethitic soil-derived alluvial material. Major variations in the granular ore CID after deposition have resulted from intermittent groundwater flow in the channels causing dissolution and reprecipitation of goethite and silica, particularly in the basal CID zones, with surface weathering of eroded exposures playing a role in masking some of these effects. However, significant variations in rock types in both the general CID and the granular ore CID have also resulted from the effects of varied provenance.     

Geochronology,paleomagnetism and magnetic fabric of metamorphic rocks in the northeast Fraser Belt,Western Australia

The first zircon U–Pb SHRIMP dating on high-grade meta-igneous units in the northernmost parts of the Fraser Belt along the southern margin of the Western Australian Yilgarn Craton, reveal crystallisation ages between 1299 ± 10 and 1250 ± 23 Ma. A small number of older xenocrystic zircons, incorporated in some samples, indicate the presence of Late Paleoproterozoic crust in the region. Zircon that crystallised within a melt accumulated in the neck of a boudinaged mafic unit was dated at 1296 ± 4 Ma, indicating that the emplacement of the igneous protoliths took place syntectonically. The anisotropy of magnetic susceptibility of the granulites indicates minimum axes with a mean inclination of 4° towards 130°, corresponding to a nearly vertical southwest–northeast (50–230°) magnetic foliation. This is very close to the structural trend of the Fraser Belt suggesting that the magnetic fabric was acquired syntectonically, during the collision between the Yilgarn and Gawler Cratons. The paleomagnetic data on the granulites overlap with published poles for various 1.2 Ga units in the Albany Belt and the 1.2 Ga Fraser dykes, possibly suggesting that the remanence was acquired during the second stage of the Fraser tectonism. A younger magnetisation component resembles a pole of uncertain age published for Bremer Bay in the Albany Belt.     

Evaluating the provenance of Archean sedimentary rocks of the Diemals Formation (central Yilgarn Craton) using whole-rock chemistry and precise U – Pb zircon chronology

Whole-rock chemistry and precise U – Pb zircon chronology have been used to determine the provenance of Archean greenschist-facies siliciclastic sedimentary rocks of the Diemals Formation in the Marda – Diemals area of the central Yilgarn Craton, Western Australia. Field evidence shows that these siliciclastic rocks are, at least in part, derived from uplift and erosion of underlying greenstones, and this is borne out by the similar La/Sc, Cr/Th and REE chemistry of Diemals Formation siltstones and some sandstones to mafic volcanic rocks of the underlying greenstones. The higher Cr/V and lower Y/Ni of some siltstones is consistent with input from ultramafic and mafic rocks. Diemals Formation sandstones and siltstones cannot be separated in terms of ratios such as Zr/La, and siliciclastic rock chemistry reflects provenance rather than the effects of transport and depositional processes, such as sorting. Chemistry does not support input to Diemals Formation sedimentary rocks from the Marda volcanic complex despite both units being close to each other, and having overlapping maximum depositional and crystallisation ages, respectively. Instead, it is likely that detritus for the two units was deposited in adjacent, physically discrete basins. Some Diemals Formation sandstones are geochemically similar to felsic rocks intruding the underlying greenstone succession, with higher La/Sc and lower Cr/Th, and LREE-enriched patterns with negative Eu anomalies. Support for a genetic relationship is shown by the overlap in the maximum depositional age of these sandstones with the crystallisation age of the geochemically identical Pigeon Rocks Monzogranite. Combined whole-rock chemistry and precise U – Pb zircon chronology indicates that Diemals Formation sedimentary rocks were in large part derived from the underlying mafic volcanic rocks, with progressive unroofing of this succession leading to erosion of felsic intrusive rocks, now represented by sandstones found at various levels in the Diemals Formation.     

Coupled asteroid impacts and banded iron-formations,Fortescue and Hamersley Groups,Pilbara, Western Australia

Asteroid impact spherule layers and tsunami deposits underlying banded iron-formations in the Fortescue and Hamersley Groups have been further investigated to test their potential stratigraphic relationships. This work has included new observations related to the ca 2.63 Ga Jeerinah Impact Layer (JIL) and impact spherules associated with the 4th Shale-Macroband of the Dales Gorge Iron Member (DGS4) of the Brockman Iron Formation. A unit of impact spherules (microkrystite) correlated with the ca 2.63 Ga JIL is observed within a >100 m-thick fragmental-intraclast breccia pile in drill cores near Roy Hill. The sequence represents significant thickening of the impact/tsunami unit relative to the JIL type section at Hesta, as well as relative to the 20–30 m-thick ca 2.63 Ga Carawine Dolomite spherule-bearing mega-breccia. The ca 2.48 Ga-old Dales Gorge Member of the Brockman Iron Formation is underlain by an ?0.5 m-thick rip-up clast breccia located at the top of the ca 2.50 Ga Mt McRae Shale, and is interpreted as a tsunami deposit. We suggest that the presence of impact ejecta and tsunami units stratigraphically beneath a number of banded iron-formations, and units of ferruginous shale in the Pilbara and South Africa may result from a genetic relationship. For example, it could be that under Archean atmospheric conditions, mafic volcanism triggered by large asteroid impacts enriched the oceans in soluble FeO. If so, seasonal microbial and/or photolytic oxidation to ferric oxide could have caused precipitation of Fe₂O₃ and silica. In view of the possible occurrence of depositional gaps and paraconformities between impact ejecta units and overlying ferruginous sediments, these relationships require further testing by isotopic age studies.     

Slumping in the Marra Mamba Supersequence Package in the southern Hamersley Province,Western Australia

Detailed mapping of the Hardey Syncline region at the Marra Mamba Supersequence Package closure, in the southwestern part of the Hamersley Province, has indicated that a hiatus in parts of the Jeerinah Formation was probably caused by submarine slumping during sedimentation. A similar hiatus at the base of the Jeerinah Formation farther west, noted on air photographs, is attributed to the same cause. Removal by slumping, and redeposition westwards, could account for olistostromes which were previously recorded in the Jeerinah Formation at Mt Edith and Mt de Courcey on the southern rim of the Wyloo Dome.