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Bedding Planes (bedding + plane)
Selected AbstractsMiddle Archean ocean ridge hydrothermal metamorphism and alteration recorded in the Cleaverville area, Pilbara Craton, Western AustraliaJOURNAL OF METAMORPHIC GEOLOGY, Issue 7 2007T. SHIBUYA Abstract A hydrothermally metamorphosed greenstone complex, capped by bedded cherts and banded iron formations (BIFs), is exposed in the Cleaverville area, Pilbara Craton, Western Australia. It has been interpreted as an accretionary complex characterized by both a duplex structure and an oceanic plate stratigraphy, and is shown to represent a 3.2 Ga upper oceanic crust. Three metamorphic zones are identified in the basaltic greenstones. The metamorphic grade increases from sub-greenschist facies (zones A and B) to greenschist facies (zone C) under low-pressure conditions. The boundaries between three mineral zones are subparallel to the bedding plane of overlying chert/BIF, and metamorphic temperature increases stratigraphically downward. The zones correspond to the thermal structure of ocean-floor metamorphism, at a mid-ocean ridge. The uppermost greenstone in the study area is more pervasively altered and carbonatized than the modern upper oceanic crust. This indicates the enrichment of CO2 in the metamorphic fluid by which widespread formation of carbonate occurred, compared with a narrow stability region of Ca-Al silicates. It is, therefore, suggested that the Archean hydrothermal alteration played a more important role in fixation of CO2 than present-day ocean-ridge hydrothermal alteration, as an interaction between sea water and oceanic crust. [source] Slope development reconstruction at two sites in the Bohemian Forest MountainsEARTH SURFACE PROCESSES AND LANDFORMS, Issue 4 2010Filip Hartvich Abstract Although the Bohemian Forest is generally considered to be geomorphologically a rather stable region, there are localities which bear proof of a surprisingly high intensity of recent and even present day processes. A multidisciplinary methodology based on the analysis of slope systems was used for researching the present and past dynamics of two hillslopes. Using the results of direct dilatometric monitoring (the slow creep of rock blocks reached a maximum of 1,mm year,1), GPS field mapping, sedimentology, geodetic measurements and DEM analyses, several common rules have been observed for the behaviour of two model localities and a likely polycyclic development established, with recurrence of mass-wasting cycles related to material supply thresholds. Radiometric dating of debris flow activity showed that the cycles span approximately 4000 years. In this area of rather stable bedrock conditions, at least two factors that decrease slope stability need to act together to activate slope processes: i.e. a favourable structural predisposition (dip of major joint or bedding planes) and an increase in relief through either glaciation or river incision. Copyright © 2010 John Wiley & Sons, Ltd. [source] Natural-gradient tracer experiments in epikarst: a test study in the Acqua dei Faggi experimental site, southern ItalyGEOFLUIDS (ELECTRONIC), Issue 3 2008E. PETRELLA Abstract Two natural-gradient tracer experiments were carried out using borehole fluorometers in order to characterize the internal structure of epikarstic horizons and analyze subsurface flow within these high-conductivity layers. The experiments were carried out in a test site in southern Italy where the epikarst is made up of an upper part with pervasive karstification and a lower part without pervasive karstification. Injection and observation boreholes were 6.9 m apart. An initial experiment demonstrated that wider (conduits) and narrower (fractures and bedding planes) openings coexist in a well-connected network within the lower epikarst. The adjusted aperture of the opening network (105 ,m) suggests that conduits are subordinately developed. The lower epikarstic horizon is hydraulically similar to granular porous media and Darcy's law can be applied to describe groundwater flow. A small value of longitudinal dispersivity (0.13 m) shows that variations in the velocity field in the direction of flow are less significant than those typical of carbonate systems at the same experiment scale. A second experiment demonstrated that longitudinal dispersivity (2.42 m) in the upper epikarst is in agreement with findings in other carbonates at the same experiment scale. However, despite the higher dispersivity and more pervasive karstification, the mean tracer velocity (3.7 m day,1) in the upper epikarst is slightly lower than the velocity in the lower epikarst (13.6 m day,1). [source] On the Relation Between Steep Monoclinal Flexure Zones and Steep Hydraulic GradientsGROUND WATER, Issue 5 2007Y. Yechieli Steep hydraulic gradients are found in association with steep monoclinal flexures. However, the physics of the reduction of the hydraulic conductivity, which is responsible for the steep gradients, has seldom been studied. We present results of hydrological and mechanical modeling aiming to study the effect of such steep hydraulic gradients demonstrated in the Judea Group Aquifer system, Israel. The hydrological configuration of steep dips and anisotropy between flows parallel and perpendicular to the bedding planes was simulated using the FEFLOW code. It exhibited a situation whereby part of the flow is oblique to the bedding planes and therefore some steepening of the hydraulic gradients occurred due to actual conductivity reduction. However, this reduction is not enough to account for the steeper gradients observed. The effect of a deep-seated reverse fault under the monocline on the permeability distribution within the structure was examined by numerical mechanical simulations. It exhibited a compressional stress distribution in the steep part of the monocline, which, due to shortening and closure of joints and voids, is presumably responsible for a significant pressure-induced permeability reduction. This process by itself in a layered structure, including interlayering of thin marl layers, could be responsible for the steep hydraulic gradients in the steep part of the monocline. [source] EVALUATION OF THE CONTROLS ON FRACTURING IN RESERVOIR ROCKSJOURNAL OF PETROLEUM GEOLOGY, Issue 4 2005D.C.P. Peacock The style, geometry and distribution of fractures within reservoir rocks can be controlled by numerous factors, including: rock characteristics and diagenesis (lithology, sedimentary structures, bed thickness, mechanical stratigraphy, the mechanics of bedding planes); structural geology (tectonic setting, palaeostresses, subsidence and uplift history, proximity to faults, position in a fold, timing of structural events, mineralisation, the angle between bedding and fractures); and present-day factors, such as orientations of in situ stresses, fluid pressure, perturbation of in situ stresses and depth. The relative timing of events plays a crucial role in determining the geometry and distribution of fractures. For example, open fractures are commonly clustered around faults if the open fractures and faults formed at the same time, but clustering does not tend to occur if the open fractures pre-date or post-date the faults. Understanding these factors requires traditional geological skills, including the analysis of one-dimensional (line-sampling) data from core, borehole images and exposed analogues. This paper reviews the factors that control fractures within reservoir rocks and discusses methods to assess those controls. Examples are presented from Mesozoic limestones in southern England. It is shown that traditional geological skills are of vital importance in determining the rock characteristics, structural and present-day factors that control fractures. [source] TEMPESTITE DEPOSITS ON A STORM-INFLUENCED CARBONATE RAMP: AN EXAMPLE FROM THE PABDEH FORMATION (PALEOGENE), ZAGROS BASIN, SW IRANJOURNAL OF PETROLEUM GEOLOGY, Issue 2 2004H. Mohseni The Pabdeh Formation is part of a thick carbonate-siliciclastic succession in the Zagros Basin of SW Iran which includes carbonate reservoirs of Cretaceous and Cenozoic ages. From field observations and petrographic and facies analysis of exposures in the type section of the Pabdeh Formation, four lithofacies were recognized. These are from oldest to youngest: (i) a mottled, bioturbated bioclastic wackestone/mudstone facies; (ii) a wackestone/packstone facies with horizontal burrows on bedding planes; (iii) a thin-bedded bioclastic wackestone/mudstone facies alternating with thin bioclastic-oolitic-intraclastic intervals; and (iv) a bioclastic foraminiferal / algal / peloidal packstone facies. These observations indicate that facies evolved upwards from deep outer-ramp deposits to inner-ramp deposits within a shoal complex, suggesting progradation of the ramp depositional system. Storm events significantly influenced the ramp system. Storm-generated surges transported sediments from nearshore to the deeper outer-ramp environment where they were deposited as shell-lags, composed mostly of bioclastic packstones, rich in pelagic microfauna with sharp, undulatory erosional basal contacts. The packstones rest on outer ramp mudstones deposited below storm base level. Sedimentary structures in the Pabdeh Formation are those typical of storm deposits, such as hummocky cross-stratification, ripple cross-lamination, ripple marks, escape burrows on the tops of the beds, couplets of fine- and coarse-grained laminae and mixed fauna, as well as intraclasts derived from underlying facies. These distinctive sequences are interpreted to have been generated by waning storm-generated currents. The dominance of fine-grained sediments (medium to fine sand); the lack of large- scale hummocky cross-stratification; the minor amounts of intraclasts derived from underlying facies; the paucity of amalgamated tempestite beds; and the finely-laminated (mm to cm scale) couplets of coarse and fine lamina all suggest a distal tempestite facies. Palaeogeographic reconstruction of the Zagros Basin during the Eocene indicates that the study area was situated in tropical, storm-dominated palaeolatitudes. [source] The first named Ediacaran body fossil, Aspidella TerranovicaPALAEONTOLOGY, Issue 3 2000James G. Gehling Aspidella terranovica Billings, 1872 was first described from the late Neoproterozoic Fermeuse Formation (St. John's Group) on the Avalon Peninsula of eastern Newfoundland, approximately 1km stratigraphically above the famous Ediacaran biota at Mistaken Point, and several kilometres below the base of the Cambrian. Aspidella has been reinterpreted perhaps more than any other Precambrian taxon, and has variously been regarded as a fossil mollusc or ,medusoid', a gas escape structure, a concretion, or a mechanical suction mark. Our studies indicate that Aspidella includes a wide variety of preservational morphs varying from negative hyporeliefs with a raised rim and ridges radiating from a slit (Aspidella -type preservation), to flat discs with a central boss and sharp outer ring (Spriggia preservation), to positive hyporeliefs with concentric ornamentation (Ediacaria preservation). Specimens occur in a continuum of sizes, with preservational styles dependent on the size of the specimen and the grain size of the host lithology; the elongation of specimens is tectonic. Aspidella is confirmed as a body fossil from observations of complex radial and concentric ornamentation, mutually deformed borders in clusters of specimens, and occurrence on the same bedding planes as certain distinctive Ediacaran taxa. Aspidella is indistinguishable from, and has priority over, several of the most common genera of late Neoproterozoic discoidal body fossils worldwide. Similar fossils from Australia are interpreted as holdfasts of frond-like organisms. The density of specimens in the Aspidella beds suggests levels of benthic biomass in the Neoproterozoic that could rival those of modern marine communities. The serial growth forms, PalaeopascichnusIntrites, Neonereites renariusYelovichnus, associated with Aspidella, are interpreted as body fossils of unknown affinities rather than trace fossils. A new, trilobed, Ediacaran body fossil, Triforillonia costellae gen. et sp. nov., is described from the Aspidella beds of the Fermeuse Formation. [source] Karren of Mushroom Mountain (Junzi Shan) in the Eastern Yunnan Ridge, Yunnan, China: Karstological and Tourist AttractionACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 2 2010Martin KNEZ Abstract: Mushroom Mountain karren with predominantly flat tops that formed along the stylolites and bedding planes dictated the special features of this karst landscape. Their features and rock relief clearly reflect the geological conditions and development. The subsoil karren with conical tops dissected by subsoil rock relief were exposed from beneath the soil. Below tree vegetation, the subsoil karren are to a great extent covered by moss and lichen, under which they acquire their characteristic shape. On the bare surface they were reshaped by rainwater that carves flutes and solution pans. The old cave that opens below the top of one of the cones reveals the period before its dissection into hills and cones when this part of the karst aquifer was still deep under the water table. [source] The role of glacitectonic rafting and comminution in the production of subglacial tills: examples from southwest Ireland and AntarcticaBOREAS, Issue 4 2007John F. Hiemstra Sedimentological and structural geological data from two sites in southwest Ireland and Antarctica provide evidence for the formation of subglacial till by the brecciation and crushing of bedrock rafts. Up-sequence transitions, from undisturbed bedrock, to deformed bedrock, to crushed and brecciated bedrock, to massive matrix-supported till with far-travelled erratics, represent a process-form continuum of till production. Initially, bedrock fragments and rafts up to several metres in length are liberated from the substrate by glacitectonic thrusting and plucking. These rafts are then crushed to produce the matrix of a till. Such products are commonly referred to as comminution tills, although the original definition focused primarily on the second phase of the process (crushing of bedrock rafts and fragments) as well as abrasion of bedrock. Data from Ireland and Antarctica indicate that rafting of bedrock is an essential part of the process of till formation. This process is facilitated by weak sedimentary bedrock, which can be displaced along joints and bedding planes to form rafts that are then incorporated into the ,proto-till' prior to being crushed subglacially. Our field data suggest that bedrock failure and displacement of such rafts can occur to depths of 3 m. The occurrence of erratics in the uppermost part of the till demonstrates that the glacier effectively mixes far-travelled and local materials. [source] | ||||||||||