Gravity Faults Research Articles

Gravity Fault Subsidence and Beach Ridges Progradation in Quinta-Cassino (RS) Coastal Plain, Brazil

Gravity Fault Subsidence and Beach Ridges Progradation in Quinta-Cassino (RS) Coastal Plain, Brazil

Delineating the main structural outlines and the petrophysical properties of the Albian-upper Cretaceous Reservoirs using seismic and well log data, Shushan Basin, north Western Desert, Egypt

This study concerns with delineating and analyzing the subsurface structural setting of Khalda Oil Field at Shushan Basin as a key factor for evaluating the petrophysical properties and the deliverability of the Bahariya Formation. Shushan Basin is considered one of the most prospective basins in the Western Desert. The oil is trapped within a gentle seismically defined anticline that contains a series of coastal sandstone reservoirs interbedded with a neritic and tidal flat mudstone and a few carbonate interbeds of the Albian-Cenomanian Bahariya Formation forming multiple oil-bearing sandstone reservoirs. A set of borehole logging and seismic data were available, tied, processed, and mapped to delineate the predominant structures in the study area. Results show the implication of the NW–SE, NE–SW, and E–W gravity faulting on the Bahariya and the overlying Abu Roash formations. This set of gravity faults is attributed to the Mesozoic tensional stresses. The depth structure contour maps on the upper surfaces of the Lower and Upper Bahariya members reveal the presence of some NE–SW gravity faults that dissected the Khalda Field into some slightly tilted blocks, shallowing in depth to the south as deepening to the north. Besides, the study area has been affected later in the Jurassic by E–W gravity faults during the Atlantic Ocean opening. This faulting trend was changed later into the NE–SW direction as a result of the movement of North Africa against Laurasia. The petrophysical analysis indicated that the Lower Bahariya sandstone reservoir has a good reservoir quality (16 ≤ ∅ ≤ 25%, 53 ≤ Sw ≤ 59%, 6.0 ≤ Vsh ≤ 32.2%, and good net-pay thickness 18.0–38.0 ft). It is revealed that the drilled wells penetrating the crest of the anticline are prospective while that located in the anticline flanks are water-bearing recommending exploration away from the anticline flanks. By applying this workflow, it is possible to explore for the similar subsurface hydrocarbons-bearing sequences in the Western Desert and North Africa in future exploitation plans.

Facies Architecture and Sedimentary Structures in the drill cores of Uranium Bearing Sediments of Banganapalle Formation of Palnad Sub-basin, Guntur District, Andhra Pradesh

The Banganapalle Formation, the lowest member of the Neoproterozoic Kurnool Group of rocks, resting over the basement granites, has been identified as the host rock for uranium in Koppunuru area in the western part of Palnad sub-basin. The uppermost arenite facies of the Banganapalle Formation is exposed on surface and shows only few bedform indicators like ripple marks, planer laminations etc. Down-hole lithological examinations on course of core drilling in Koppunuru and adjoining areas identified five recognizable lithofacies of Banganapalle Formation, viz. basal conglomerate unit, quartzite-shale intercalated facies, and two quartz arenite facies separated by a grey shale dominated argillaceous facies.
 The polymictic conglomerate, with unsorted grit to pebble size clasts of granite, shale, quartzite, vein quartz and dolerite indicate short distance transportation and derivation from nearby granitoids traversed by quartz reef/dolerite dykes. Cyclic repetitions of arenaceous and argillaceous sediments in Banganapalle lithocolumn above the basal conglomerates point to alternate rhythmic marine transgression and regression regimes. These units can also be discriminated based on their sedimentary texture, bedforms and several soft-sedimentary penecontemporaneous deformational structures (PCD) like load structure, convolute bedding/laminations, and slump structures such as micro-slips, gravity faults and folds. These syn-sedimentary structures clearly indicate perturbation and submergence of the basin contemporaneous to deposition. Cross-beds suggests change in current direction/intensity while bi-directional symmetrical ripples in outcrops of upper arenite facies suggests that the Banganapalle sediments are derived from the basement granitoids exposed to the north as well as upper Cuddapah sediments to its west. Overall, the sedimentary structures, textural and composition variation of the lithounits suggest deposition of these sediments in marginal marine, inter- to supra-tidal flat environment. The porous and permeable nature of the quartz arenite and the basal conglomerates and the presence of available reductants in the form of sulphides and carbonaceous matter make them the best suited loci for fluid movement and precipitation of uranium.

Gravity data as a faulting assessment tool for unconventional reservoirs regional exploration: The Sergipe–Alagoas Basin example

This paper introduces a gravity regional interpretation workflow for mapping the natural fracture system, a challenge in unconventional reservoir exploration. The workflow comprises coupling two gravity data attributes, the tilt angle and the multiscale edge detection (worming), to build a high-resolution image structural framework of a given basin’s basement. That includes a description of the fault directions and fault density, consolidated in a fault distribution map. The proposed workflow was applied to available gravity data of a representative passive-margin basin, the Sergipe-Alagoas Basin, Brazil. The source rocks of Sergipe-Alagoas are organic-rich shale deposited in the principal depocenters during the rift phase of the basin’s evolution. These shaly units are the unconventional reservoirs in the basin. These organic-rich shales are acknowledged as syn-rift unconventional reservoir analogs of all Brazilian continental margin basins and many other passive-margin basins worldwide. The workflow recognized two fault trends in the studied area. A principal NE–SW and a minor NW–SE. 3D seismic interpretation validated the proposed workflow by successfully associating the gravity faults with traditional fault attributes. One advantage of this workflow is that it can be implemented over broad regions at a minimum expense. The proposed method can be used to rank opportunities in unconventional exploration.

HIGH-RESOLUTION GEOMORPHOLOGICAL MAPPING OF THE SHALLOW CONTINENTAL SHELF WEST OF THE KAVALA BAY, NORTH AEGEAN

Prominent geomorphological features of the shallow continental shelf west of the Kavala Bay (Loutra Eleftheron-Nea Peramos) were mapped using the data from a hydrographic survey (June 2014) of 320 nautical miles during which high resolution multibeam bathymetry and seismic-reflection subbottom profiling were carried out simultaneously. A fault zone comprised by a set of two primary sigmoidal gravity faults (recorded lengths and measured offsets: 12 km, 5 km and > 40 m, 25 m, respectively), with distinct expression on the seabed, and three other secondary gravity faults situated southern of the major faults, revealing synsedimentary tectonics, was identified. The striking difference between the texture of the footwall block sediments of the northern major fault and the texture of the sediments occupying the deep hanging wall block of the southern major fault emphasizes the impact of local tectonics on the sedimentary evolution of the study area. Concerning the observed bedforms, the most interesting were the sand dunes occurring at depths from 25 m to 65 m at least and occupying the northeast part of the study area. Their large dimensions and orientation in relation to the coastline position imply as a mechanism for their formation intense bottom-current activity.

Landscape response to progressive tectonic and climatic forcing in NW Borneo: Implications for geological and geomorphic controls on flood hazard

Empirical models have simulated the consequences of uplift and orographic-precipitation on the evolution of orogens whereas the effects of these forcings on ridgelines and consequent topography of natural landscapes remain equivocal. Here we demonstrate the feedback of a terrestrial landscape in NW Borneo subject to uplift and precipitation gradient owing to orographic effect, and leading to less-predictable flooding and irreversible damages to life and property. Disequilibrium in a large catchment recording the lowest rainfall rates in Borneo, and adjacent drainage basins as determined through χ, a proxy for steady–state channel elevation, is shown to result in dynamic migration of water divide from the windward-side of the orogen towards the leeward-side to attain equilibrium. Loss of drainage area in the leeward-side reduces erosion rates with progressive shortening resulting in an unstable landscape with tectonic uplift, gravity faults and debris flows. 14C dating of exhumed cut-and-fill terraces reveal a Mid–Pleistocene age, suggesting tectonic events in the trend of exhumation rates (>7 mm a−1) estimated by thermochronology, and confirmed by morphotectonic and sedimentological analyses. Our study suggests that divide migration leads to lowered erosion rates, channel narrowing, and sediment accretion in intermontane basins on the leeward-side ultimately resulting in enhanced flooding.

Gravity inversion of 2D basement relief using entropic regularization

We have developed a gravity interpretation method for estimating the discontinuous basement relief of a sedimentary basin. The density contrast between the basement and the sediments is assumed to be known, and it could be either constant or vary monotonically with depth. The interpretation model consists of a set of vertical, juxtaposed prisms, whose thicknesses are the parameters to be estimated. We used the entropic regularization that combines the minimization of the first-order entropy measure with the maximization of the zeroth-order entropy measure of the solution vector. We validated the method by applying it to synthetic data produced by a simulated basin bordered by high-angle step faults; we obtained a good definition of the relief, particularly of the discontinuities. We also applied the method to a profile across the Büyük Menderes Valley in West Turkey and obtained a solution exhibiting a gravity fault with large slip on the northern border of the valley. When applied to the interpretation of a discontinuous basement relief, the method has a better performance than the global smoothness method. It is comparable to the weighted smoothness method, but it does not require the a priori knowledge about the maximum basin depth.

THE LANDSLIDE IN THE AREA AGIA-ANNA OF THE SOUTHERN PART OF EUBOEA

On 20th February of 2003, an extensive landslide took place at the southern margins of the Agia-Anna village, in the northern Euboea. It caused a lot of material damages (buildings, cemetery and provincial road) and agony in the people of the village, as well as a lot of problems to their properties. In this paper it is described the phenomenon, the form of the landslide and the reasons which caused it, and the mechanism of the breaking and movement are also dilated. The main and secondary breakings of the earth, are extended in an area of 85-90 acres. Their direction is about N-S and the movements are generally directed to the eastern with a resultant ASA. The breakings depth must be not more than 3-4 meters. Most of them are of the order of 0,50-1,5 m and they are not continuous but interrupted. The breakings are usually in a straight line and are arrangement in steps like gravity faults. The length of a continuous breaking is between of 1 to 30 m. In the area where the breakings are noticed, the morphological dip of the soil, before the landslide had been happened, was of the order of 13 to 14%. The drainage basin in the same area is not big enough to concentrate substantial amount of water. Therefore, additional factors, must have also influenced it. It is estimated that these factors are: a. The imperfect drainage of the rain waters and soil saturation through the fissures, because of small landslides and creeps. b. It is possible that small faults (small metaneogenic gravity faults) with parallel direction to the slope, to have acted as sliding surface. The small faults (gravity faults of some cm) are not visible in the argillo-marlaceous material, but it is rumoured to be, comparatively with the obvious tectonic character of the area. c. The mineral constituents of the argillo-marlaceous neogenic sediments, especially the montmorillonite, chlorite, illite, kaolinite and swelling chlorite, as well as their mechanical features, have contributed substantially to the development of landslide. d. The possibility that the neogenic formations are supplied sideways with water, by the adjoining and underlying carbonate rocks, it is also examined.

４Ｄ震探データを取り込んだ油層シミュレーション

Time-lapse seismic changes can be compared with pressure and/or saturation changes derived from a flow simulator by using petro-elastic model. As the number of seismic data is huge and it has a 3D character, it is expected to be a great help to condition the reservoir model. An (automated) history matching loop comprises the following four stages: 1) parameterization, 2) objective function definition, 3) optimization and 4) loop-stopping criterion. Focusing the objective function, the brief explanation of some element technology is done. With the introduction of time-lapse seismic as an additional constraint, history matching is performed to construct a more reliable reservoir model, which can be used to predict a future oil production.The impact of data integration on production forecasting uncertainty is analyzed. The synthetic model used representative for a gravity fault system, such as East flank of Statfjord field. By changing the constraining data, i. e. time-lapse seismic and production data, its impact on the probability density function (pdf) of the future cumulative oil production (FCOP) can be quantified. The variance of FCOP-pdf is reduced and the maximum pdf value is increased by adding extra information to the reservoir model. For this specific synthetic model, the seismic data gives a reservoir model with a higher probability than the production data. The most probable models are obtained when conditioning the reservoir model with both types of data.

Origin of the salt valleys in the Canyonlands section of the Colorado Plateau: Evaporite-dissolution collapse versus tectonic subsidence

The salt valleys over the axis of the salt-cored anticlines in the Paradox fold and fault belt (Canyonlands, Utah and Colorado) are created by subsidence of the anticline crests. Traditionally, the collapse of the anticlinal crests was attributed to dissolution of the salt walls (diapirs) forming the anticline cores. Recent studies based on scaled physical models and field observations propose that the salt valleys are a result of regional extension and that salt dissolution had only a minor influence in the development of the axial depressions. This paper presents several arguments and lines of evidence that refute the tectonic model and support the salt dissolution subsidence interpretation. The development of contractional structures in salt dissolution experiments led the advocates of the tectonic interpretation to reject the dissolution-induced subsidence explanation. However, these salt dissolution models do not reproduce the karstification of salt walls in a realistic way, since their analog involves removal of salt from the base of the diapirs during the experiments. Additionally, numerous field examples and laboratory models conducted by other authors indicate that brittle subsidence in karst settings is commonly controlled by subvertical gravity faults. Field evidence against the regional extension model includes (1) a thick cap rock at the top of the salt walls, (2) the concentration of subsidence deformation structures along the crest of the anticlines (salt walls), (3) deformational structures not consistent with the proposed NNE extension, like crestal synforms and NE–SW grabens, (4) dissolution-induced subsidence structures controlled by ring faulting, revealing deep-seated dissolution, (5) large blocks foundered several hundred meters into the salt wall, (6) evidence of recent and active dissolution subsidence, and (7) the aseismic nature of the recently active collapse faults. Although underground salt dissolution seems to be the main cause for the generation of the salt valleys, this phenomenon may have been favored by regional extension tectonics that enhance the circulation of groundwater and salt dissolution.

Surface expression of fluid venting at the toe of the Nankai wedge and implications for flow paths

Cold seeps have been observed with manned submersibles at many sites on the Tokai margin. Most sites are found on the hanging wall of major thrust faults, within 1–2 km of their outcrop. Few sites relate to purely erosional features. This relationship between thrusting and venting may be explained either by fluid flow along fault zones, by fluid flow along permeable layers outcropping at the fault scarp, or by diffuse upward flow of fluids released during footwall compaction. We use deep-towed side-scan sonar and sediment sounder profiles acquired at the toe of the Nankai accretionary wedge to identify the context of seepage at a smaller scale. Modes of fluid expulsion inferred from observations in this zone include flow channelling along strata, along thrusts, along gravity faults, and along joint fractures as well as diffuse flow. We then combine visual observation and monitoring of electrochemical potential (Eh) to assess the relative importance of seepage sites. We show that bacterial mats are better visual indicators of active seepage than bivalve beds and that the most active seepage sites correlate with outcrops of coarse turbidites, cemented by authigenic carbonates. These were sampled and have permeability high enough to channel fluid flow (1–4×10−14 m2). We suggest they are the main conduits near the seafloor. However, seepage sites are also observed directly on the outcrop of major thrusts, indicating that they do play a role as fluid conduits. We observe negative polarity seismic reflections on these faults below 1 km depth and wonder about their significance.

Isotopic Composition of Carbonates and Sulphates, Potash Mineralisation and Basin Architecture of the Nagaur-Ganganagar Evaporite Basin (North-Western India) and Their Implications on the Neoproterozoic Exogenic Cycle

The Nagaur-Ganganagar evaporite basin covers a large part of the north western India in Western Rajasthan. The evaporite bearing Hanseran Evaporite Group (HEG) is the northern subsurface facies variant of the exposed Bilara Group of carbonates. While the Bilara Group is underlain by Jodhpur Sandstone (disposed on top of the Malani acid effusives, dated -745 Ma), HEG is also underlain by the same sandstones as documented in numerous deep bore holes. South of Bilara outcrops, near Birmania the facies changes to phosphatic carbonates. As documented by Ilyin (1990), this period of the earth history experienced development of narrow rift basins, many of these are repositories of evaporite deposits. The Nagaur-Ganganagar basin located west of the main NE-SW trending Aravalli Range, came into existence due to rejuvenation of Precambrian lineaments, producing horsts and grabens and extensive acid volcanism. This led to the formation of an extensive, shallow marine basin bounded by Jodhpur-Pokaran-Malani ridge in the south, Aravalli range in the east and Delhi-Sargodha-Lohore subsurface ridge to the north and north east. Salt Range in Pakistan and Hormuz basin in Iran are located in the northern extension of this basin (Dasgupta et al., 1988). The HEG exhibits lateral facies variations and deposition of atleast seven halite cycles alternating with maroon clays, marl, limestone, stromatolitic and foetid smelling OM-rich dolomite and magnesite. The halite (mainly the 2nd halite cycle) is associated with several potash minerals like sylvite, polyhalite, langbeinite sylvinite and carnallite. Analysis of subsurface strata and structural interpretation suggests deepening of the basin towards the central part and in WNW direction due to steep, step-like NNE-SSW gravity faults with their down thrown side deepening progressively towards west. Thickness of the HEG and the number of halite cycle increases from the margin of the basin towards the centre. Two bore holes (P/47 and P/12) near Lakhasar in Bikaner district with continuous and well preserved cores were logged and studied. The dolomite and the interbedded anhydrite were analysed for carbon and sulphur isotopes. Petrographic studies show widespread diagenetic replacement features in the carbonates, sulphates and gangue minerals. Oxygen isotopic data for the dolomite also show evidences of meteoric water alteration which is documented by the co-variance of several C-O isotopic data sets. On the other hand, C and S isotopic curves show an inverse correlation (Fig. 1), peculiar for this time range and conforms to the similar observations made by the earlier workers. Pronounced increase of ~34S (mean = 31.6%o) is accompanied by a change in the 6~3Cc~b values from 2.1 to --8%0. Allowing some compensation for the diagenetic modification of the isotopic values the magnitude of the isotopic shift for both the isotopes is about 7%o. Such an inverse correlation of C and S isotopic curves imply that Neoproterozoic and Phanerozoic exogenous cycles operated on similar modes (Strauss,1993).

ＶＬＦ‐ＥＭ法と放射能探査による１８８８年磐梯山爆裂カルデラ内の断層系調査

VLF-EM method and radioactivity prospecting were carried out in Bandai-san Caldera. The distribution and characteristic of fault system in Bandai-san Caldera occurred in 1888 eruption were made clear.The results of these prospecting show that the fault systems is divided into three directions; north-south direction, northeast direction and east-northeast direction. The most prominent fault identified by this study was the north-south fault located within the central portion of the caldera. The northern extension of this fault appeared to be connected to the eastern scarp of Avalanche valley and the southern extension appeared to be under the peak of Kobandai-san which was destroyed by a volcanic eruption in 1888. Northeast and east-northeast faults cut across the caldera.Considering the results of this study and other geological informations, it may be inferred that the northsouth fault was formed by intrusion of north-south dike. The directions of northeast and east-northeast fault are concordant with the ridge direction of Kobandai-san. The feature suggests that the northeast and the east-northeast fault are gravity fault caused landslide plane.

Fault‐generated mountain fronts in the central apennines (Central Italy): Geomorphological features and seismotectonic implications

AbstractThe morphotectonic framework of the Central Apennines is given by faulted blocks bounded by normal faults, mostly trending NW–SE, NNW–SSE and NE–SW, which cut previous compressive structures. Such a structural setting is consistent with the focal mechanisms of the earthquakes which often occur in this area.In this paper, three lithologically different normal fault‐generated mountain fronts are analysed in order to assess the relations between their geomorphic features and active tectonics. They border the Norcia depression (Sibillini Mts, Umbria), the Amatrice–Campotosto plateau (Laga Mts, Lazio) and the Fucino basin (Marsica Mts, Abruzzi).The Norcia depression is bounded by a N20°W trending normal fault to the east and by a parallel antithetic fault to the west. The main fault has a 1000 m throw and gives rise to a wide fault escarpment, characterized by: (1) sharp slope breaks due to low angle gravity faults; (2) important paleolandslides; and (3) several fault scarplets on the piedmont belt affecting Quaternary deposits.The Amatrice–Campotosto plateau is delimited by the western slope of Mt Gorzano which runs along a N20°W trending normal fault having a 1500m throw. Minor parallel faults dislocate Quaternary landforms. Large‐scale massmovements also occur here.The Fucino basin was struck by the 1915 Avezzano earthquake (I=XI MCS) which produced extensive surface faulting along two parallel NW trending normal fault escarpments on the eastern border of the basin. There is paleoseismic evidence including buried gravity graben in Late Glacial gravels and tectonic dip‐slip striations on Holocene calcitic crusts covering bedrock normal fault planes.These data suggest that active extensional tectonics plays a major role in the slope morphogenesis of the Central Apennines and they indicate the importance of geomorphic analysis in seismic zonation of this area.

Exceptionally long, narrow drumlins formed in subglacial cavities, North Dakota

An array of about 200 exceptionally long and narrow drumlins occurs in north‐central North Dakota. The most prominent of the drumlins is 27 km long, and their average length is between 2 km and 3 km. The drumlin field is closely associated with, and occurs immediately up‐glacier from, an extensive area of ice‐thrust topography. Most of the drumlin ridges have small glacier‐thrust masses at their heads. Internal structures are complex, but typically include large numbers of gravity faults, which dip away from the center of the drumlin ridges. Materials contained in the drumlins were transported from higher pressure areas beneath the glacier, inward toward lower pressure areas (cavities) by flowing, thrusting, squeezing and other processes. The close and systematic association of the long drumlins with nearby areas of ice‐thrust topography indicates a genetic tie; conditions that caused the glacier to form large ice thrusts also contributed to drumlin formation. Major factors involved in the formation of the drumlins were high porewater pressures in interbedded permeable and impermeable materials beneath a thin, swiftly flowing glacier, and the presence of areas of frozen ground near the margin of the glacier.

The ophiolites of northeast India — a subduction zone ophiolite complex of the Indo-Burman orogenic belt

The ophiolites of northeast India are rootless blocks of various dimensions, floating in a matrix which belongs to the upper Cretaceous-Lower Tertiary Disang Group. They consist of diverse igneous, sedimentary and metamorphic rocks, of which ultramafics are the main component. They do not constitute a continuous sheet but are made up of units haphazardly juxtaposed along faults or they consist of lensoid slices interbedded with Disang Group rocks. The ultramafics are interpreted as slices of oceanic crust and upper mantle obducted onto the Indian continental margin. Associated blue schist is indicative of subduction zone tectonics. The occurrences of ultramafics showing intrusive contacts and the presence of intermediate — acid volcanics suggest an island arc — continent type of collision along the Benioff Zone coincident with the ophiolite belt. The tectonic history of the Indo-Burman orogenic belt can be tentatively summarised as follows: (1) Introduction of the subduction zone during Cretaceous times; obduction of oceanic crust and upper mantle and intermixing with deep oceanic sediments. (2) Deposition of the Disang sediments. Formation of an island arc separating the sedimentary basin into eastern and western sub-basins. The Barail sediments were deposited in both basins. The formation of the island arc was accompanied by plutonism and volcanism and by deformation and low grade metamorphism of the lower Disang sediments. (3) Gradual shallowing of the basin consequent upon the collision of the island arc with the easterly subducted India plate. (4) Continent-continent collision and rotation of the down-basin normal faults into low angle reverse faults (thrusts) along which the thrust sheets were piled up. The frontal areas of the thrust sheets were asymmetrically folded. This was followed by asymmetrical folding of the rocks of the foreland areas. (5) Formation of gravity faults and conjugate sets of strike-slip faults in the orogen and the foreland areas. The structures associated with the ophiolite belt are attributed first to the collision of the easterly subducting India plate with an island arc and subsequently with the Burma plate.

Combined Effect of Tectonism, Eustasy, and Sediment Supply on the Depositional-Sequence History of the Western Offshore, South Africa

Interpreted seismic profiles and maps of depositional systems tracts highlight variations in basin-fill architecture and their inferred causes along western offshore South Africa. Sequence-stratigraphic concepts developed by Exxon were used to interpret 10,000 km of seismic data and logs from 31 exploration boreholes within an area of 90,000 km{sup 2}. Lower frequency second-order supercycles (5-10 Ma) are attributed to changes in the rate of subsidence and are expressed either by predominantly progradational or aggradational depositional supersequence. In addition to these long-duration, basin-wide stratigraphic packages, component depositional sequences that formed in response to third order (1-5 Ma) and higher frequency cycles vary with structural setting within the depobasin, namely depocenter, basin flanks, shelf-edge, and slope systems. High-order sequences are attributed to eustatically driven, high-frequency, base-level changes and/or localized tectonic events (i.e. slope failure, gravity faulting). Variations in the areal distribution of lowstand and shelf-margin systems tracts document lateral shifting sediment supplies and the presence of entrenched feeder-canyons linked to incised valley systems on the shelf. Many canyons, however, terminate updip and may be related to slope failure and headward erosion. The tentative correlation of regionally developed unconformities of the west coast based on microfaunal dating of a limited number of marinemore » condensed sections points to a remarkable correspondence with the Exxon global sea-level chart.« less

Basin architecture and thermal maturation in the strike‐slip Deer Lake Basin, Carboniferous of Newfoundland

Abstract The Deer Lake Basin is an entirely non‐marine basin associated with the Cabot fault zone. Structural and stratigraphic evidence strongly suggest dextral strike‐slip movements along the fault zone during Tournaisian‐Visean time. Two elongated, end‐on structural blocks (probable positive flower structures) contain fold axes and second‐order faults oriented obliquely to fault traces bounding the blocks, in a manner implying dextral movements. In one part of the basin, the stratigraphic thickness of a long homoclinal section of later basin‐fill sediment (Deer Lake Group) greatly exceeds the suggested depth to basement based on gravity measurements, a situation common to strike‐slip basins. Formations representing basin fill can be arranged into megasequences (from oldest to youngest: Anguille Group, Wetstone Point and Wigwam Brook Formations, Deer Lake Group, Howley Formation) corresponding to lateral growth stages of the basin.Gravity, magnetic, and seismic data show that depths to basement on either side of the end‐on flower structures are comparable, so that the youngest strata in the basin (Howley Formation) are not underlain by earlier basin fill. These geophysical data, therefore, corroborate the geological conclusion of onlapping stratigraphic relations. The geophysical data suggest participation of basement in Carboniferous gravity faulting and show the location of the subsurface extension of the Taylors Brook Fault in the western part of the Deer Lake Basin.Thermal maturation of the Anguille and Deer Lake Groups, as measured by vitrinite reflectance, clay mineral assemblages, illite crystallinity, and Rock‐Eval pyrolysis, indicate a much higher level of maturation for the Anguille than for the Deer Lake Group. Palaeotemperatures for the Anguille and Deer Lake Groups are estimated to be around 200 and 100oC, respectively, suggesting that Anguille Group rocks are overmature whereas Deer Lake Group strata are within the oil‐generating window. Onlapping stratigraphic relations and areally homogeneous time/temperature effects, however, have created a situation in which the Deer Lake Group and Howley Formation have similar maturation levels.

CONSIDERAÇÕES SOBRE A GEOLOGIA DA REGIÃO ENTRE PORTO NACIONAL E IPUEIRAS, GO

The present study carried out in the area between Porto Nacional and Ipueiras (northern Goias) allowed to recognize several steps of its geologic evolution. 1. The Porto Nacional complex, Morro do Aquiles Formation, and Matanca Suite essemble the oldest rocks of the area. They represent strongly modified mafic/ultramafic, varied granitold, and supracrustal rocks, which were sheared, disrupted, and imbricated by progressive non-coaxial deformation, and metamorphosed and migmatized at high-amphibolite facies conditions. Taking into account the regional information, lhe deformation and metamorphic processes are linked to an event of collision tectonics of two crustal blocks (Brasilia and Araguacema), involving movements towards N-330 and oblique suture with N-30 direction. This event is presumed lo be of Late Archean (Jequie) age. 2. Besides these features, the mentioned rocks were also affected by younger transcurrent ductile shear zones, mainly with N-30 trend, which are interpreted as due lo mass rotation during the Early Proterozoic (transamazonian). 3. The granitoid of the Lajeado Suite (1.8 Ga) intruded those rocks and are viewed as anorogenic bodies. like acid intrusions of the eastem Amazonian region. 4. During the Middle Proterozoic-Ordovician interval, an intricated fault system was formed , mostly trending N-30 (Transbrasiliano Lineament). Also, an ensialic basin could develop and the filling rocks were weakly folded and metamorphosed (Natividade Group). 5. The Parnaiba Basin developed during the Ordovician-Jurassic interval in a stable tectonic setting: only the oldest sedimentary formations (Serra Grande and Pimenteiras ) are present in lhe area . 6. Gravity faults and diabase dykes of Jurassic-Cretaceous age formed during the South American Reactivation event. 7. The Cenozoic has been time of tectonic stability and lhe sediments of this age are correlative to morphogenetic episodes.

ESTUDO DAS CONDIÇÕES HIDROGEOLÓGICAS E HIDROQUÍMICAS DOS AQÜÍFEROS DA REGIÃO DE BIRITINGA - PATAÍBA (BA) UTILIZANDO MÉTODOS GEOFÍSICOS ELÉTRICOS

Drill holes which tapped the aquifers of the Ilhas and Massacara Groups in many places of the Tucano Sedimentary Basin produce water with a high content of dissolved salts. In order to optimize a procedure to determine the two dimensional extent of the salt water occurrence, as well as to contribute to the understanding of possible mechanisms responsible for this salinization, geophysical studies were made in the Biritinga-Paraiba area in the State of Bahia. Twenty eight (28) vertical electrical soundings, electrical logs of 13 wells and chemical analysis of 35 samples of groundwater allowed to determine the structural configuration of the Ilhas-Massacara aquifer system in the region, and to estimate the influence of this configuration on the chemical characteristics of the groundwater. In the study region the system consists of a vertical sequence of sandstone and shale layers dipping SE, which are longitudinally sectioned by gravity faults in a sequence of horsts and grabens. In the horsts where the system contains a high shale-sandstone proportion, the water is more salty. Furthermore, some of the structural blocks have shown an increase of water salinity with depth. These facts seem to indicate that ionic filtering by the shales in the system and a process of mixing and hydrodynamic dispersion along the faults are the main controllers of the characteristics of the groundwater. The results also indicate that under favorable conditions, information about the block structure system and water characteristics can be inferred from surface electrical soundings.