Residual Gravity Map Research Articles

A new emergent alpine front chain in central Tunisia (Maktar area): filtering gravimetric data contribution

A residual anomaly gravity map of the Maktar area has been processed via several methods including horizontal gradient, upward continuation, and Euler deconvolution to highlight structures hidden by sedimentary cover. These methods reinforce the existence of several faults inferred from geologic studies. This work outlines deep or near-surface faults that had remained hidden until now. The most important set of these faults is NE–SW trending, with depths in excess of 3,000 m. These faults limit the Northwestern limb of Balouta massif and restrain the Northwestern side of Oued Ousafa syncline. This last fault presents a thrust front which constituted the western continuity of a southern Tunisian thetys margin and a western Zaghouan scar prolongation. E–W striking faults refer to an arched beam of faults limiting Kessera plateau and others situated in the northern part of the Maktar plateau. N–S and NW–SE directions correspond to branches of less-apparent importance. Superposition of local maxima specifies dipping vergence of different proven structures. Results from this work show the importance of gravimetric data analysis in Maktar area and provide new insight into Central Atlas Tunisia structure such as new emergent segment front Alpine chain. The proposed map could be a basis for planning future hydrogeological and petroleum research in this region.

Subsurface geometry and structural evolution of La Gomera island based on gravity data

We hereby present a new Bouguer gravity map of the La Gomera island (Canarian Archipelago), which is analysed and interpreted by means of a 3-dimensional inversion, in order to contribute to the knowledge of the structural setting of this volcanic island and its evolutionary history. A land gravity data set covering the whole island of La Gomera is used in combination with offshore measurements to achieve a better determination of the gravity field in areas near the coasts. The study of this map let us to shed some light on the hypothesis established about the volcanism of this island. Moreover, it shows the information that is hidden from a geological surface exploration, modelling deep sections of the crust in La Gomera, which have been unknown until now. A first interpretation of the Bouguer gravity anomaly is achieved from 1) the residual gravity map calculated by removing a regional component and 2) the total horizontal gradient of the gravity. These residual and derivative maps allow us to identify the horizontal location and borders of the shallowest gravity sources. This provides a useful tool to study the structures associated to the latest periods of the volcanism in the area. Moreover, the information so obtained supports the hypothesis about the migration of volcanic activity towards the south of the island. Subsequently, an inversion process is carried out looking for the 3D-modelisation of the sources of the observed gravity field, which provides a comprehensive view of the structures in volcanic environments. The inversion technique used is based on a genetic algorithm (GA) applied upon a prismatic partition of the subsoil volume, and adopting a priori values of density contrast (positive and negative). The main advantage of this method is that let us to model deep and shallow bodies which exhibit very different geometries and density contrasts. So, results indicate that this inversion strategy can be very effective for characterization of volcanic structures, improving the information from previous geologic and volcanologic studies. The inversion model obtained shows correlation between several sources of the gravity field and the volcanic units associated with the growth of La Gomera Island. The main gravity source of this model is associated with the oldest unit, called the Basal Complex. This unit corresponds to the first submarine growth stage and it is modelled as the most important and deepest high density structure. According to previous geological studies, the following edifice (Old Edifice) was also submarine in its initial phases, later being represented by a wide basaltic shield volcano. The original location and morphology of this Old Edifice is deduced from the distribution of positive density contrasts that appears in the model. Moreover, other gravity field sources are identified and associated to several feeding systems of this stage of the volcanism in La Gomera. The shallowest sections of the model let us recognise the distribution of light material inside the Vallehermoso caldera, surrounded by high density structures. This gives us some insight into the internal structure and morphology of the caldera, pointing to a vertical collapse origin followed by erosion and other destructive processes. Finally, other conclusions are obtained from the correlation found between the sources of the gravity field and the migration of the volcanic activity towards the southern area of the island.

Gravity and magnetic investigations in the Haouz basin, Morocco. Interpretation and mining implications

The Marrakech region is known for its significant mining potential concealed by the Hercynian basement. In order to extend the exploration of this basement beyond its outcrops in the Jebilets and Guemassa massifs, we used magnetic and gravity data from a sub-area in the Haouz basin. Our methodology in this study consists of processing the data, so as to highlight the buried geological contacts and to reveal their dips. The study of the geological contacts is ensured by the upward continuation processing of the residual gravity map at various altitudes, followed by the determination of the horizontal gradient maxima for each level. The progressive migration of these maxima while increasing the upward continuation altitude indicates the dip direction. The structures deduced from gravity are superimposed on the magnetic map in order to emphasize their possible association with magnetic sources. Comparison between gravity anomalies and local geology shows that these anomalies are mainly caused by basement topography undulations. Moreover, lows coincide with granite and thick sedimentary depressions while highs coincide with basement outcrops. The local maxima of the horizontal gradient of the residual gravity data and its upward continuation at various altitudes define a field of features, organized into groups oriented both NE–SW and NW–SE, which correspond to the regional Hercynian structures. Moreover, the main highlighted lineaments coincide perfectly with known fractures such as the North-Atlas and the Guemassa faults whose dip and direction are well documented. These detected features allow us to complete and enrich the structural map of the Haouz basin. A joint detailed analysis of the mapped gravity contacts and the magnetic data was carried out, through the use of Magnetic and Gravity Euler Trends in terms of depth and rooting, taking into account the local geology, to identify potential structures for mining exploration.

Derived gravity field of the seismogenic upper crust of SE Germany and West Bohemia and its comparison with seismicity

The gravity field of the seismogenic upper crust was derived from the Bouguer gravity map by applying the Butterworth high-pass filter in the wave-number domain. The cutoff wavelength of the filter was 110 km, to pass the gravity signals of structures within the 18 km thick seismogenic layer. The derived residual gravity map reveals potential stress concentrating structures, which may cause seismicity provided they lie within the existing zones of weakness. Furthermore we derived a shaded relief map of the horizontal gravity gradient, which highlighted the tectonic lines accompanied by density contrast. The directional analysis of this map shows three dominant strike directions. The most prominent one is the Hercynian NW-SE strike direction represented by the Franconian Line, the Gera-Jachymov Fault Zone and the Elbe Zone. The second dominant strike is the Rhenisch NNE-SSW trending represented by the Upper Rhine Graben Zone, Rheinsberg- Heldburg Line and several Proterozoic volcanic belts in the Tepla-Barrandien Unit. The third pronounced trending of the ENE-WSW direction is represented by the Erzgebirge and Eger Graben gravity low. The N-S trending Rostock-Leipzig-Regensburg Zone(Pritzwalk-Naab Lineament) is not distinctly reflected in the derived gravity maps, although many fault segments have a meridian direction. The relative reactivation potential of some pre-existing fault systems identified in the gravity map was studied with respect to the wide range of the recent stress configuration determined in the West Bohemia/Vogtland region. The resulting diagrams show that the steep NNW-SSE to N-S faults (represented by some segments of the Marianske Lazně Fault Zone) are oriented favourably for reactivation. On the contrary, the orientation of the ENE-WSW faults limiting the Eger Graben (Litoměřice Fault, etc.) is unfavourable for reactivation for all dip values

Analysis of the occurrence and importance of slot development (grykes) within shallow dolomite zones in a selected type area on the Eccles Formation

A better understanding of the geology and dolomite stability of sites to be developed is crucial in the quest for releasing land for development that is deemed safe from a dolomite risk perspective. A geological setting often considered to be particularly problematic is shallow dolomite bedrock. This paper presents an analysis of the occurrence and importance of slot development (grykes) within shallow dolomite in a selected type area. Geophysical exploration forms the initial phase of a ground stability investigation. A Bouguer gravity anomaly contour map was produced from the data set for the type area. The interpretation of the residual gravity map was used as the basis for planning the subsequent phase of investigation, in which further rotary percussion borehole drilling was carried out. Over 750 boreholes and trench-mapping results were available to test the interpretation against. Various attempts showed just how problematic it is to produce a gravity map that represents a good estimated depth to dolomite bedrock. The shallow dolomite areas were firstly delineated using only the residual gravity results and incorporate the gravity high plateau where the average bedrock head is predicted to be shallower than 8 m. An analysis is given of the conditions related to these shallow bedrock areas. Shallow dolomite areas associated with some stratigraphic horizons are notorious for instability, however, to the layperson the presence of rock on a site signifies stable conditions to build on. Upon closer investigation dolomitic rock is found to present problematic founding conditions, which are not always easy to overcome by engineering design.

Conductivity Structure along the Gediz Graben, West Anatolia, Turkey: Tectonic Implications

Groups of grabens in west Anatolia have contrasting E-W and NE-SW orientations and are the subject of debate as to their relative ages and relationships. We investigated the E-W-trending Gediz graben and its neighboring NE-SW-trending Gördes, Demirci, and Selendi grabens, which form an important graben system representative of the region. We studied gravity data from one profile and magnetotelluric (MT) data from two profiles, 73 km and 93 km long. The data supports the hypothesis that the Gediz graben was superimposed onto the (older) NE-SW grabens. 2D gravity and MT modelling revealed an undulating graben floor, varying in depth between 500 and 3000-4000 m (gravity-MT); within the graben two apparent basins 3–4 and 1.5-2.5 km deep (gravity-MT) are separated by a subsurface horst. The residual gravity map appears to indicate the continuation of NE-SW grabens from north of Gediz graben to beyond its southern border. The MT model revealed three main zones of varying thickness within the crust. The britde upper crust comprises two zones: sedimentary fill (apparent resistivity 15-50 ohm.m) and Menderes massif basement (200 ohm.m). The third zone is highly conductive lower crust (10 ohm.m), identified by our MT modeling at an average depth of 10 km. This conductive layer was considered in conjunction with two other regional features, high heat flow values and shallow earthquake focal depths. A heat flow map shows a very high average value of 108 mWm−2 for west Anatolia and 120-300 mWm−2 for the Gediz graben area specifically, compared with the world average of 80 mWm−2. Seismological records showing shallow earthquake focal depths together with the high conductivity zone were taken to indicate a partially melted, viscoelastic lower crust.

The role of deep structures in the distribution of some major ore deposits in Iran, ne of the Zagros thrust zone

Examination of isostatic, crustal thickness, residual gravity, gravimetric geoid, complete Bouguer and earthquake intensity maps of Iran, NE of the Zagros thrust zone suggests the presence of major deep boundaries in this region. To test the model of block structure in Iran, the meridional and latitudinal boundaries constructing a rectangular grid were plotted. A comparison with the mineral distribution maps of Iran indicates that the deep structural boundaries played a role in the genesis and distribution of endogenous ore deposits, NE of the Zagros thrust zone. The endogenous deposits and ore-rich regions are concentrated mainly along or close to deep boundaries (lineaments) or at their intersections. The porphyry copper deposits and other endogenous deposits of the Kerman region are also associated with an upper mantle updoming and a felsic mass manifest in the pattern of residual gravity, crustal thickness, complete Bouguer, and earthquake intensity maps of Iran. One of the most important features is the presence of a very significant meridional lineament band occurring approximately at 56°E longitude. The most important ore deposits are located along or close to this meridional band which is associated with more than eight east–west trending structural boundaries in the upper mantle. The two anomalous areas manifesting the mantle updoming and the felsic mass are also located along this band. Several target areas have been selected for mineral exploration.

Gravity modelling of Lockne, a marine impact structure in Jämtland, central Sweden

The Lockne impact structure in Jämtland (63°00′20″N, 14°49′30″E) formed in the middle Ordovician at approximately 455 Ma. The structure is interpreted as a concentric crater with a total diameter of 13.5 km. The impact took place in a marine environment with a water depth exceeding 200 m. The gravity survey of the structure and its surroundings comprises 467 measurements. The residual gravity map reveals a negative anomaly reaching a maximum of 22 g.u. The gravity modelling of the structure revealed its morphology, with a central uplift surrounded by a moat. The fracture zone grades out and diminishes at an approximate depth of 2 km. The model also shows a slight northwestward tilt (1–2°) of the structure. The anomaly of 22 g.u. is relatively small compared to the ones occurring in other craters of similar size. A good preservation state and a crystalline target normally favour larger negative anomalies than occur in eroded structures of the same size in sedimentary strata. The Lockne structure occurs in crystalline target rock, and is well preserved. Therefore a small density contrast of rocks in the Lockne structure is the main reason for the small anomaly. In Lockne the impact breccia has only 1/10 of the density contrast compared to the average supra-aquatic impact structure. Furthermore the fractured target rock shows 1/4 of the density contrast that one finds in average supra-aquatic impact structures. The interpretation is that the cavity and fractures were filled mainly by calcite, precipitated by the hydrothermal system acting after the impact. The abundant water and the residual heat from the impact gave the opportunity for the hydrothermal system to develop.

Structural interpretation of the Bouguer and aeromagnetic anomalies in central Sinai

A large part of the central Sinai peninsula, covering mainly the El-Tih and Egma plateaux, was evaluated using geophysical data. The Bouguer and aeromagnetic anomaly maps were critically analyzed and correlated to investigate regional tectonism, structural elements and major sedimentary basins characterizing this region. The regional Bouguer anomalies reflect density variations at great depth as well as crustal thickening towards the Gulf of Aqaba. Parameters of the interpreted major and minor faults, which are oriented mainly NNE, NE and NW, were calculated. The boundary faults along the Gulf of Aqaba downthrow by about 6 km to the east, with some of these faults showing surface expression. Residual gravity and magnetic maps reflect a central high anomaly zone trending NE, but its northern part deviates E-W suggesting either a shallow and wide basic/ultrabasic dyke or uplifted basement. Depth calculations of the magnetic and gravity anomaly profiles indicate that the shallow sources have depths down to 1.0 km, however, the depth to the basement surface is ∼3.5 km. Significantly low Bouguer and magnetic anomalies reflect a major sedimentary basin containing a thick layer of clastic Palaeozoic-Mesozoic sediments, which overlie basement rocks, and are topped by subhorizontal non-clastic Tertiary sediments. The results of this paper should be of considerable help in delineating some exploration concepts of hydrocarbon and groundwater resources for developing this part of Sinai.

Internal structure of the Sierra Nevada batholith based on specific gravity and gravity measurements

About 6,000 specific‐gravity (SG) measurements of samples collected from nearly 200 granitic plutons comprising the central Sierra Nevada batholith yield a SG contour map across the batholith from 36.25° to 38° north latitude. With notable exceptions, SG decreases from values generally greater than 2.7 in the west to less than 2.6 over a few small areas of high‐silica, high‐potassium granites near the east edge. A good correlation between measured SG and analyzed weight percent SiO2 enables estimation of average silica variations across the batholith. The average SG is 2.69 corresponding to an average of 68 wt. % SiO2 for the 18,000 km² central part of the batholith.A 1‐km gridded version of the SG measurements has been used to generate a series of synthetic gravity maps, assuming that the SG of rocks at the surface extends unchanged to various depths. The synthetic map computed for a depth of 10 km shows the best correspondence with the isostatic residual gravity map indicating that variations in the observed gravity residuals are largely caused by SG variations of the plutonic rocks exposed at the surface that apparently extend downward to an average depth of about 10 km.Although the 10‐km synthetic gravity map gives the best overall fit to the observed gravity data, comparison of individual anomalies indicates that the bottoms of the plutons as defined by SG variations at the surface are generally shallower along the west edge of the Sierra Nevada (7±2 km) and deeper in the younger and more felsic eastern part (12±3 km). These depths do not necessarily represent a distinct base of the Sierra Nevada batholith. They may indicate the depth below which density homogenization occurs, either by igneous, or possibly, structural processes.

Gravity map of Brazil: 2. Regional and residual isostatic anomalies and their correlation with major tectonic provinces

An isostatic residual gravity map of Brazil has been computed by removing from a 0.5° × 0.5° Bouguer anomaly grid a regional gravity field calculated for compensating masses of surface topography. The coherence function, a statistical measure of the correlation between Bouguer anomaly and topography, was first computed in order to constrain the compensation mechanism within Brazil. Similar to results for North America and Australia, the coherence function of South America has a broad transition between high and low coherence values, suggesting a combination of tectonic provinces with different flexural rigidities and/or loading processes. In view of this result, we have considered, as a first approximation, a model in which the surface topography is the only load acting on a nonrigid lithosphere. A regional gravity field has been computed assuming Airy‐Heiskanen isostasy with compensation at the crust‐mantle boundary. The residual gravity map, which was obtained by removing the computed regional gravity field from the observed Bouguer anomaly, shows a long‐wavelength N‐S trending negative anomaly over most of Brazil. This gravity feature of approximately 3000 km width is the southern continuation of the western North Atlantic negative geoid/gravity anomaly and reaches at least ∼15 mGal in the northern portion of Brazil. Using the upward continued isostatic residual gravity field at 300 km, this long‐wavelength component, which may be dynamically induced, has been removed to first approximation. The final isostatic residual gravity anomaly map depicts anomalies with wavelengths between 100 and 1000 km which correlate with major tectonic provinces. Negative anomalies occur mainly over Paleozoic intracratonic and Cretaceous rift‐type sedimentary basins, and granitic intrusions and along Proterozoic thrust belts. Positive residual anomalies are generally observed over regions affected by igneous activity and volcanism such as in the Amazon basin and the Paraná flood basalt province. Positive anomalies are also associated with overthrust crustal plates which define a suture zone in central Brazil and over sub‐Andean Tertiary foreland basins.

Geophysical exploration for coal-bearing Gondwana basins in the states of West Bengal and Bihar in northeast India

The states of West Bengal and Bihar in northeast India are known to bear ‘A’-grade coal seams within Gondwana basins. An alluvium-covered area to the north of the Ajay river was considered to be prospective for the exploration of coal. The area was surveyed to determine the subsurface depressions in the basement (Archeans) underlying the Gondwana sediments. Gravity and magnetic surveys were mainly carried out over an area of 1900 km 2 with resistivity soundings taken at selected places. Geologically, the alluvium-covered area was known to be composed of rocks of different ages such as Archeans, Gondwana Triassic, Cretaceous (trap) and older alluvium in succession. Gondwana basins are formed over basement depressions or in the downthrown side of the faulted Archeans. Gondwana sediments lying over such basement subbasins are prospective areas for the exploration of coal. The gravity survey interpreted an 8-km wide and 20-km long depression, east of a linear boundary fault, known as the Pachami basin. This basin was drilled at several places and they intersected thick ‘A’-grade coal seams up to a maximum thickness of 150 m at the centre of the basin and at depths ranging from 15 to 887 m within the lower Gondwana sediments. The gravity survey broadly outlined the Pachami basin and several other basement depressions through gravity “low” contour closures. A forward modelling of a profile across the Pachami basin indicated a maximum depth of about 1.3 km to the basement at its deepest part. The basins are more perceptible on a residual gravity map. The magnetic map was vitiated through the presence of fluctuating high amplitude and high wavenumber anomalies due to a thick blanket of trap lying above the Gondwana sediments. Selected resistivity soundings could approximately interpret the shallow-basement depths, and the thickness and depths of the overlying trap. Further survey and drilling for coal is recommended in the area.

A preliminary study of satellite-derived gravity and magnetic anomalies of India and contiguous regions

The residual satellite gravity anomaly map in and around the Indian region has been prepared. These residual anomalies are interpreted for crustal inhomogeneities using the MAGSAT anomalies as one of the constraints. While a strong correlation between these anomalies is observed over the Dharwar, Aravalli, Marwar and Singhbhum regions, inverse correlation prevails over the Eastern Ghats and Cambay basin. The positive gravity and magnetisation (strong correlation) over the stable blocks of the peninsula is related to the alternating layers of basic and ultrabasic materials; the positive gravity and low magnetisation (poor correlation) over the Eastern Ghats and Cambay basin is interpreted as thinning of the crust. The residual gravity map identifies the signatures of west-central Indian lineament and the submarine topographic features/ridges in the Indian Ocean such as the Chagos-Laccadive, 85°E and Ninety East ridges, which are not so obvious in the unprocessed anomaly map at ground level. An attempt is also made to explain the anomaly over the Indo-Ganga-Brahmaputra basin using the ground- and 400-km-level satellite gravity anomaly maps.

Gravity and Magnetic Maps of the Santa Maria Province, California

In this paper the authors display an isostatic residual gravity map, contour interval 5 mGal, and an aeromagnetic map, which is a mosaic of existing data . Two computer-derived maps, also at a scale of 1:250,000 display the locations of the steepest gravity gradients and of the steepest gradients on the pseudogravity transform of the magnetic map. These gradients indicate boundaries between rock masses with differing physical properties. The gradient boundaries, where steep and linear, are presumably cause by faults. The gravity reduction removes the isostatic effects of the continental margin, a 60 mGal gradient across the Santa Maria basin, leaving a maximum basin low of 40-50 mGal caused by the low-density basin rocks and bounded by various steep gradients at the basin margin faults. Offshore gravity lows indicate local sedimentary basins trending north to north-west. The magnetic anomalies are caused primarily by serpentinite in the basement of Franciscan assemblage and form irregular belts trending northwest across the basin and basin margins, terminating against the offshore Hosgri fault. The magnetic patterns west of the Hosgri are more irregular in character and may reflect a different basement. The Hosgri fault appears to extend south beyond Point Arguello (about 2 km offshore)more » according to the gravity map. The right-lateral slip on this fault is thus presumably accommodated on north-dipping thrust faults on the north side of the Santa Barbara Channel and perhaps along the south side of the Santa Maria basin as well. Along the east side of the Hosgri fault, high gravity values indicate local uplift.« less

Concept of effective density: Key to gravity depth determinations for sedimentary basins

The gravity anomaly observed over a layered sedimentary basin is close to that calculated over a basin with the same configuration and depth but filled by homogeneous sediments with density equal to the effective (weighted average) density of the real layered basin. Therefore, it is possible to calculate the effective density contrast of a basin from the residual gravity anomaly over it, if the depth of the basin is known at least at one point. Assuming a mathematical function for density‐depth dependence, an expression for the gravity‐depth dependence using the infinite slab (Bouguer) formula can be developed. This expression makes it possible to calculate the depth of a basin or an isopach map from gravity data. An exponential function and a new hyperbolic density‐depth function and their gravity‐depth functions are analyzed and implemented for determining the depths of basins from gravity data for the San Jacinto graben, California, and the Tucson basin, southern Arizona. The hyperbolic functions are more reliable and realistic than exponential ones. The effective density contrast of sediments filling the Tucson basin and density‐depth dependence were calculated using the infinite‐slab formula from residual gravity data over the deepest borehole, which entered pre‐Eocene bedrock at a depth of 3.66 km. The hyperbolic density‐depth dependence for the Tucson basin is also assumed to be effective for all other basins in the Basin and Range Province, southern Arizona and southwestern New Mexico. The density‐depth function is easily converted into a gravity‐depth function, using the infinite‐slab formula. The residual gravity map of each basin in this Province can then be transformed into a map of basin depth contours, using the hyperbolic gravity‐depth function. Comparison of depths of basins calculated from gravity data with available borehole information (19 wells) showed that the proposed simple and fast method has an error in the range of about 13 percent of the actual depth.

Ore deposits of the western United States in relation to mass distribution in the crust and mantle

Colored, digital residual Bouguer gravity maps of the conterminous United States at different cutoff wavelengths reveal significant compositional heterogeneities in the lithosphere, providing a new tool for the studies of controls of mineralization. Examples are presented showing correlations between main endogenic ore deposits of the western United States and gravity anomalies in the residual Bouguer gravity maps at 250-, 625-, and 1,000-km wavelength cutoffs. Some of the clusters of ore deposits are in or adjacent to regions of gravity lows (for example, the majority of deposits of the Colorado mineral belt). Most of the high-amplitude gravity lows reflect low-density igneous masses emplaced along zones of tectonic weakness which guided the ascent of magmatic fluids and caused major geochemical changes in the lithosphere. Some clusters of ore deposits (for example, the group of Tertiary deposits of the Salt Lake City area, Utah, including Bingham, Tintic, Park City, and others) are along the flanks of major zones of gravity highs. In the Salt Lake City area, the major concentrations of metals occur near the eastern edge of a broad gravity high that correlates with crustal thinning. These relations (and others) between the location of major ore deposits and mass distribution in the crust and upper mantle may serve as general guidelines in mineral exploration on a regional scale.

A new isostatic residual gravity map of the conterminous United States with a discussion on the significance of isostatic residual anomalies

To display more clearly the gravity anomalies caused by geologic bodies in the upper parts of the crust, a new colored isostatic residual gravity map of the conterminous United States has been prepared using the gravity data set compiled for the Gravity Anomaly Map of the United States (Society of Exploration Geophysicists, 1982). The new isostatic residual gravity map is based on an Airy‐Heiskanen model of local compensation, in which the surface load requiring compensation is defined by 5‐min topographic and bathymetric data sets. A colored first‐vertical‐derivative map of isostatic residual gravity further enhances the short‐wavelength anomalies produced by bodies at or near the surface and emphasizes the regional fabrics and trends in the gravity field. For the purpose of displaying gravity anomalies caused by shallow bodies of geologic significance, the nature of the isostatic model and the values of its parameters are of lesser importance than the application of an isostatic correction of some sort. Most isostatic models result in residual gravity maps that appear nearly identical in their main patterns and features. Anomalies on isostatic residual gravity maps should not be casually interpreted in terms of “undercompensation” or “overcompensation” because large‐amplitude anomalies can be produced by crustal bodies in complete local isostatic equilibrium. Many isostatic residual gravity anomalies less than several hundred kilometers wide can be related to known geologic bodies. We present here a classification scheme that attempts to categorize such anomalies on the basis of tectonic environment. In general, highs correlate with intruded or accreted mafic material or with upthrusted crustal sections, whereas lows occur over low‐density sedimentary or volcanic sections, felsic intrusive bodies, or down‐warped crustal sections. Although some longer‐wavelength anomalies, such as the broad gravity high centered over Montana, could be manifestations of density contrasts deep in the mantle, many such anomalies can also be modeled by geologically reasonable density contrasts that are isostatically compensated and confined to depths of less than several hundred kilometers, so that their source bodies need not be deep. The fact that certain of these broader anomalies have well‐defined boundaries which correlate with near‐surface geologic features increases the likelihood that their sources lie entirely within the lithosphere. If so, then the density contrasts required to explain the gravity data imply fundamental anomalies in chemical composition or thermal state for the crustal and upper mantle columns under these regions. We have investigated spectral analysis as a method to quantitatively characterize regional anomaly patterns. Contoured plots of normalized amplitude spectra were prepared for various areas of the isostatic residual gravity field of the United States. These Fourier domain representations show characteristic patterns that can be interpreted in terms of the trends and wavelengths of anomalies and may help to more objectively distinguish geologic basements with different origins or tectonic histories.

A gravity and magnetic interpretation of the structure of the Irish Midlands and its relation to ore genesis

Regional gravity and magnetic data from the Irish Midlands have been studied in an attempt to map the structures within and beneath the Lower Carboniferous Limestones. The magnetic data suggest that the Precambrian crystalline basement is at a depth of 3–4 km and is overlain by Ordovician volcanics of varying magnetization. A Fourier analysis of the gravity field has been used to construct a residual gravity map which represents principally the gravity effect of shallow sources. These sources are probably the topographic variations of dense Ordovician volcanics beneath a less dense succession of Ordovician to Carboniferous sediments. The residual gravity clearly shows the presence of a series of linear volcanic blocks and sedimentary troughs trending NE–SW. The major mineralized zones in the area all occur close to the margins of these troughs. This investigation indicates that the initial position of the Midland Plain was controlled by Caledonide factors, some of which modified the later subsidence which we attribute to Hercynian plate-margin processes. The genesis of the major ore-bodies is probably associated with de-watering of the shale-filled troughs, the subsequent concentration of metal-bearing fluids along the trough margins and their deposition in relatively unloaded carbonate host rocks on the up-standing blocks.

Smackover Platform Sand Bodies: A Bahamas Model: ABSTRACT

Stave Creek field, a potential multimillion-barrel Smackover oil field in Clarke County, Alabama, owes much of its prolific production to an extensive, 100 ft (30 m) thick buildup of porous ooid and peloid grainstones. Seismic profiles and residual gravity maps indicate the field is located on the edge of a large, Jurassic, northeast tilted block of approximately 12 mi2 (31 km2). The block, which formed a shallow platform for Late Jurassic deposition, is bounded on the west by an older component of the Jackson fault, and slopes eastward into the present graben complex of the Gilbertown and West Bend faults. This location placed the platform in a position between the mouth of the Manila embayment and the eastern edge of the Mississippi interior salt basin. Tidal fluctuation between basins and the open marine conditions to the west had a strong influence on sedimentation across the platform. Cores from the area indicate a pronounced westward trend in increased carbonate grain stability and sediment winnowing. Smackover sediments from the deeper east side of the platform are composed primarily of peloidal lime mudstones and wackestones. Those of the central platform are peloidal and onchoidal wackestones and packstones. Those of the western edge form a high-energy facies of well-sorted ooid and onchoidal grainstones, e.g., the Stave Creek field reservoir. A modern analog to the deposition on this Jurassic platform is found along the margins of the Great Bahama Bank, and specifically the north Andros-Joulters Cays area. End_of_Article - Last_Page 1461------------

Gravity Analysis of West-Central Calcasieu Parish, Louisiana: ABSTRACT

A gravity study of about 306 mi2 (793 km2) of west-central Calcasieu Parish, Louisiana, was completed. The data used were from a very old Bouguer map of unknown origin. The data on the map were obscure, so a grid was drawn over the map and the points of intersection and the gravity contour lines were digitized. With the Bouguer values, these points served as input to least squares and model analysis systems. The least squares trend surfaces were calculated and mapped using STAMPEDE, an IMB system modified by Wright State University. The data were processed by an IBM 370 computer and mapped with a Calcomp plotter. The residual map was generated by calculating a first-order surface and subtracting it from the Bouguer map. The similarity between the residual gravity map and the structure contour maps on the Vinton and Edgerly salt domes is striking. Models using densities from five deep wells and an array of vertical prisms with square cross sections were calculated. A theoretical gravity map drawn from the models is almost identical to the residual gravity map. Profiles of the theoretical gravity and the residual gravity are compared. These profiles match exactly over the salt dome areas and show little divergence elsewhere. Gravity studies, no matter the age of the survey, are still a definitive exploration technique, especially in areas of marked density contrasts. End_of_Article - Last_Page 1474------------