Basement Configuration Research Articles

Linking sedimentation in the northern Andes to basement configuration, Mesozoic extension, and Cenozoic shortening: Evidence from detrital zircon U-Pb ages, Eastern Cordillera, Colombia

Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) analyses of 29 samples from the Eastern Cordillera of Colombia reveal the origin of northern Andean basement and patterns of sedimentation during Paleozoic subsidence, Jurassic–Early Cretaceous extension, Late Cretaceous postrift subsidence, and Cenozoic shortening and foreland-basin evolution. U-Pb geochronological results indicate that presumed Precambrian basement is mainly a product of early Paleozoic magmatism (520–420 Ma) potentially linked to subduction and possible collision. Inherited zircons provide evidence for Mesoproterozoic tectonomagmatic events at 1200–1000 Ma during Grenville-age orogenesis. Detrital zircon U-Pb ages for Paleozoic strata show derivation from Andean basement, syn depositional magmatic sources (420–380 Ma), and distal sources of chiefl y Mesoproterozoic basement (1650–900 Ma) in the Amazonian craton (Guyana shield) to the east or in possible continental terranes along the western margin of South America. Sedimentation during Jurassic–Early Cretaceous rifting is expressed in detrital zircon age spectra as Andean basement sources, recycled Paleozoic contributions, and igneous sources of Carboniferous–Permian (310–250 Ma) and Late Triassic–Early Jurassic (220–180 Ma) origin. Detrital zircon provenance during continued Cretaceous extension and postrift thermal subsidence recorded the elimination of Andean basement sources and increased infl uence of craton-derived drainage systems providing mainly Paleoproterozoic and Mesoproterozoic (2050–950 Ma) grains. By Eocene time, zircons from the Guyana shield (1850–1350 Ma) dominated the detrital signal in the easternmost Eastern Cordillera. In contrast, coeval Eocene deposits in the axial Eastern Cordillera contain Late Cretaceous–Paleocene (90–55 Ma), Jurassic (190–150 Ma), and limited Permian–Triassic (280–220 Ma) zircons recording initial uplift and exhumation of principally Mesozoic magmatic-arc rocks to the west in the Central Cordillera. Oligocene–Miocene sandstones of the proximal Llanos foreland basin document uplift-induced exhumation of the Eastern Cordillera fold-thrust belt and recycling of the Paleogene cover succession rich in both arc-derived detritus (dominantly 180– 40 Ma) and shield-derived sediments (mostly 1850–950 Ma). Late Miocene–Pliocene erosion into the underlying Cretaceous section is evidenced by elimination of Mesozoic– Cenozoic zircons and increased proportions of 1650–900 Ma zircons emblematic of Cretaceous strata.

Basement configuration of on-land Kutch basin from seismic refraction studies and modeling of first arrival travel time skips

The Kutch basin is one of the three (Narmada, Cambay and Kutch) major marginal rift basins that are close to each other in mid-western part of the Indian subcontinent. The seismic refraction and wide-angle reflection data were acquired and a first order velocity structure of the Kutch sedimentary basin along Jakhau–Mandvi, Mandvi–Mundra, Mundra–Adesar and Hamirpur–Halvad profiles is derived. The travel time skip phenomenon has been noticed in the travel time plots and record sections indicating presence of low velocity sediments. Derived two-dimensional velocity–depth models revealed a Mesozoic sedimentary sequence sandwiched between Trap and Limestone layers, in some of the profiles. Two thick low velocity layers (that corresponds early to late Mesozoic era) have been identified. These are dipping towards Mandvi along Jakhau–Mandvi profile. The early Mesozoic layer that is thinning towards southeast is completely missing in Mandvi–Mundra profile. It is also noticed that the early Mesozoic Bhuj formation exists in the northern parts of the Mundra–Adesar and Hamirpur–Halvad profiles, where it directly overlies the granitic basement. The derived velocity–depth model suggests that the basement is about 3 km deep near Jakhau and reaches a depth of about 6 km near Mandvi. The layered structure may correspond to the Tertiary, Trap, Late Mesozoic sediments and Mesozoic limestone. The velocity–depth model obtained in Kutch is very similar to earlier derived model for Jamnagar and Dwarka sub-basins of northwestern Saurashtra peninsula suggesting probable continuity/linkage between southern on-land Kutch and, across the Gulf of Kutch to Saurashtra peninunsula. We also conclude that the evolution of Kutch basin, as a peri-cratonic rift basin, is essentially controlled by the four (F1–F4) faults inferred from obvious abrupt changes in layer thickness/velocity along the seismic refraction profiles.

Basement Configuration of the Tomini Basin deduced from Marine Magnetic Interpretation

DOI: 10.17014/ijog.v4i4.86This paper presents the result of marine magnetic survey in Tomini Basin, Central Indonesia. On the basis of marine magnetic modeling, the main structural and geological elements of the basement of Tomini Basin are identified. At the centre of the basin, the up-doming feature points to an elevated magnetic susceptibility value. A geological model indicates that the entire basement of Tomini Basin is characterized by an oceanic-like crust with a basin axis at the centre nearly an east-west direction and suggests a rift-related graben.

Basement Configuration of the Tomini Basin deduced from Marine Magnetic Interpretation

Basement Configuration of the Tomini Basin deduced from Marine Magnetic Interpretation

Present‐Day Geothermal Regime in Tarim Basin, Northwest China

AbstractGeothermal gradient and surface heat flow are key parameters that describe the thermal regime of sedimentary basin, and are so vital for understanding the tectono‐thermal evolution and associated hydrocarbon potential assessment of oil and gas bearing basin. Here we presented the distribution pattern of the updated present‐day geothermal gradient and determined 38 surface heat flow values in the Tarim basin, the biggest sedimentary basins in China, on the basis of formation temperature data from approximately 470 wells and 941 measured thermal conductivities of rocks within this basin. Our results showed that the present‐day geothermal gradient of Tarim basin varied between 17~32°C/km and with a mean of 22.6±3.0°C/km, and the surface heat flow ranged from 26.2 to 65.4 mW/m2 with a mean of 43.0±8.5 mW/m2, suggesting its thermal regime as a cold basin of low temperature and heat flow, compared with other large‐middle scale sedimentary basins in China. Accordingly, this low thermal regime also makes Tarim basin to share the similar thermal regime of other typical craton basins in the world. It is obvious that the geothermal gradient and surface heat flow in uplift areas are usually larger than those in the depressions, indicating the influence of configuration of basement on geothermal field pattern. We also found that the discovered oil and gas fields in Tarim are usually with relatively large geothermal gradient, the cause for this coincidence is not clear but the upward movement and accumulation of hot fluid below is speculated to account for this positive geothermal anomaly. Finally, we summarized the factors that influence the geothermal distribution of basin as deep structure, tectonic evolution and basement configuration of basin, thermal physical properties of rocks and hydrocarbon accumulation as well.

Inversion of Gravity Data for Imaging of a Sediment-basement Interface: A Case Study in the Northwestern Part of Bangladesh

Gravity data collected by the Geological Survey of Bangladesh are processed and interpreted for imaging of a sediment-basement interface over the northwestern part of Bangladesh. The observed gravity data are processed for discriminating gravitational fields contributed subtly from the shallow basement topographic feature with the twelve nodal piecewise cubic polynomial–based finite–element approach. In spectral analysis, the presence of a widely spread shallow basement feature has been detected and interpretation of gravity data using a two-dimensional gravity inversion technique indicates that its depth ranges from 0.041 km to 0.570 km relative to ground surface. In the northern part of the study area, the inferred basement configuration shows a general depression of the basement in the Takurgaon-Panchagar and Lalmonirhat districts and reaches a maximum depth of about 0.570 km. In the Nilphamari district and its southwestern part, the basement occurs at the most shallow depth due to its upliftment. However, the estimated sediment-basement interface depths are compared with the borehole and other geophysical interpretative information and are found to be consistent.

A discussion of structural and thermal control of magnetic anomalies on the mid‐Norwegian margin

ABSTRACTWe discuss the correlation between the depth extent of magnetic sources, the Curie temperature depth and crustal structures on the mid‐Norwegian margin. Spectral methods can be used to estimate the depth extent of magnetic sources, even if the bottom is located in the lower crust, however, only with limited resolution. The bottom of the magnetic surfaces is often regarded to represent the depth to the Curie isotherm. However, comparison with a 3D model based on the interpretation of potential field and seismic reflection data and thermal modelling shows that the depth extent of the magnetic sources is merely controlled by the overall geometry of the crystalline crust and not the temperature distribution. The observed changes in the magnetic field between the inner and outer part of the mid‐Norwegian margin appears not to reflect, as previously assumed, the depth to the Curie temperature but the geometry of the basement and lower crust. Our 3D model of the mid‐Norwegian margin reveals a basement configuration that involves a basement with different petrophysical properties, which can be connected with lithological basement units of onshore Norway.

Basement Configuration of the Tomini Basin deduced from Marine Magnetic Interpretation

Basement Configuration of the Tomini Basin deduced from Marine Magnetic Interpretation

A Rapid Graphical Solution for Determining the Presence of Remnant Magnetization in the Basement Rocks of Western Iraq

A Rapid Graphical Solution for Determining the Presence of Remnant Magnetization in the Basement Rocks of Western Iraq

Regional evolution of a fluviodeltaic cyclic succession in the Marsdenian (late Namurian Stage, Pennsylvanian) of the Central Pennine Basin, UK

SUMMARY Basin-wide analysis of sedimentary facies, isopachytes and palaeocurrents for two late Marsdenian (Pennsylvanian) sedimentary cycles within part of the Millstone Grit Group, has led to a new sequence-stratigraphic interpretation of the relationships between its constituent sandstone units (currently named the Huddersfleld White Rock, Chatsworth Grit, Brooksbottoms Grit, Holcombe Brook Grit, Brown Edge Flags and Redmires Flags). The Bilinguites superbilinguis (R 2c 1) and Verneulites sigma (R 2c 2) marine bands are related to fourth-order marine highstands, and show faunal variations that possibly reflect fifth-order sea-level fluctuations. The lower R 2c 1 cycle consists entirely of relatively deep water (basin floor) mudstone. The overlying R 2c 2 cycle shows an upward regressive passage from similar mudstone through pro-delta and delta-slope deposits to mouthbar and channel sandstones. The latter comprises an ‘eastern inflow’ of northerly provenance, the distribution of which was not influenced by the underlying basement configuration, and a ‘southern inflow’ sourced from the Wales-Brabant High. Falling sea-levels resulted in progressive narrowing of the fluvial pathway within the ‘main sandstone body’ of the ‘eastern inflow’, with a concomitant increase in flow velocities and grainsize. At lowstand, this culminated in the Chatsworth palaeovalley, 25 km wide, the basal surface of which can be correlated into the interfluve areas as a leached palaeosol. ‘Higher sandstone bodies’, where developed, are of two kinds: an earlier set present outside the palaeovalley was formed during regression, and a later set within the palaeovalley was formed as sea level rose. As part of this transgressive systems tract, a Lingula band developed across the flooded Chatsworth palaeovalley and its interfluve margin. The transgression culminated in the highstand of the Cancelloceras cancellatum ( G 1a 1) Marine Band.

Morphology and tectonics of Mahanadi Basin, northeastern continental margin of India from geophysical studies

The bathymetry, total intensity magnetic and satellite free-air gravity data over the eastern continental margin of India between the latitudes 18° 30′N and 20°N within the water depths of 25–2300 m (between Paradip and Gopalpur) were analyzed and interpreted. Onshore aeromagnetic and Bouguer gravity data over the adjacent coastal region are also supplemented in the present analysis. The basement configuration inferred from the bathymetry, magnetic and gravity data resembles a series of coast-parallel structural highs and depressions and their shearing pattern in the Mahanadi offshore Basin. The Chilika offshore lineament and Dhamara offshore lineament constitute the southern and northern boundaries of the offshore basin respectively. In this sector, the inferred Continent Ocean Boundary (COB), around 170 km from the coast, approximately mimics the 2000 m isobath. The width of the inferred offshore basin between the Chilika Offshore Lineament and Dhamara Offshore Lineament is around 240 km and it extends approximately up to 170 km from coast to offshore (perhaps up to the COB). The inferred tectonics of the northeastern continental margin of India suggests pull-apart and sheared/transform mechanisms during the breakup of India from Antarctica. The model studies suggest that the northern part of the 85°E Ridge abuts the coast at Chilika Lake. The shape of 2000 m isobath over the northeastern continental margin of India closely resembles to that of 2000 m isobath off the Lambert Graben of East Antarctica. This inference appears to further support the theory that the eastern continental margin of India and East Antarctica are closely aligned in the pre-breakup tectonic setting of the eastern Gondwanaland.

An integrated geophysical approach for imaging subbasalt sedimentary basins: Case study of Jam River Basin, India

An integrated geophysical strategy comprising deep electrical resistivity and gravity data was devised to image subbasalt sedimentary basins. A 3D gravity inversion was used to determine the basement structure of the Permian sediments underlying the Cretaceous formation of the Jam River Basin in India. The thickness of the Cretaceous formation above the Permian sediments estimated from modeling 60 deep-electric-sounding data points agrees well with drilling information. The gravity effect of mass deficit between the Cretaceous and Permian formations was found using 3D forward modeling and subsequently removed from the Bouguer gravity anomaly along with the regional gravity field. The modified residual gravity field was then subjected to3D inversion to map the variations in depth of the basement beneath the Permian sediments. Inversion of gravity data resulted in two basement ridges, running almost east to west, dividing the basin into three independent depressions. It was found that the Katol and Kondhali faults were active even during post-Cretaceous time and were responsible for the development of the subsurface basement ridges in the basin. The inferred 3D basement configuration of the basin clearly brought out the listric nature of these two faults. Further, the extension of the Godavari Basin into the Deccan syneclise and the fact that the source-rock studies show the presence of hydrocarbons in the Sironcha block in the northern part of the Godavari Basin also shed some light on the hydrocarbon potential of the Jam River Basin.

Modelling and inversion of single-ended refraction data from the shot gathers of multifold deep seismic reflection profiling-an approach for deriving the shallow velocity structure

SUMMARY A multifold crustal-scale deep seismic near-vertical reflection profile generates a large number of single-ended shot gathers, which provide redundant data sets because of overlapping coverage of the shallow refractors. We present an approach for deriving the shallow velocity structure by modelling and inversion of single-ended seismic refraction first arrival traveltime data. We apply this method to a data set acquired with a 12-km long spread with 100 m spacing of shots and receivers, of the Neoproterozoic Marwar basin in the NW Indian shield. The approach is shown to be quite successful for delineating the shallow refractor depths, steep dips and velocities, even in the absence of regular reverse refraction profiles. The study reveals two-layered sedimentary formations, Malani volcanics and a complicated basement configuration of the Marwar basin, and provides a measure of resolution and uncertainty of the estimated model parameters. A seismic section of the near-trace gather is found to be qualitatively consistent with the derived structural features of the basin. The relative highs and lows, observed in the Bouguer gravity profile, further corroborate the derived velocity model. The present approach can be especially useful in offshore areas and elsewhere, where the single-ended multifold seismic profiles are the only available data sets.

Toward a Mineral System Understanding of the Palaeoproterozoic Zinc Belt of northern Australia: Determining Basement Configuration, Basin Shape and Sediment Architecture at 1640 and 1575 Ma.

Airborne magnetics, ground gravity, seismic and wireline log geophysical datasets are integrated with outcrop and drill core stratigraphic, structural, geochronology and palaeomagnetic information to derive three forward model profiles for the Palaeo-Mesoproterozoic rocks of the central and northern Lawn Hill Platform of northern Australia. Each profile depicts current basin geometries and sedimentary architecture. Two of the profiles are subsequently reconstructed to depict sediment architecture at 1640 Ma. and 1575Ma., the times of fluid migration at the Grevillea and Walford Creek Prospects and the Century zinc deposit. Surfaces of chronostratigraphic significance are used to constrain the depositional architecture and bound time-equivalent facies packages. This enables basin scale aquitard and aquifer lithologies to be represented, relationships between these shallow- to deep-burial diagenetic packages and basin scale faults to be depicted and fluid migration scenarios to be tested through numerical mechanical and convective modelling simulations. The two north-south profiles depict an early (1800-1740 Ma.) Leichhardt Superbasin phase of basin-geometries related to E-W extension. This template controlled subsequent extension during the 1740-1680 Ma. Calvert Superbasin. Rocks of the overlying Isa Superbasin can be split into an upper (1640-1575Ma.) and a lower (1680-1640Ma.) basin phase. Rocks of the lower basin phase include the Gun and Loretta Supersequences characterised by facies relationships that show deeper water facies to the south east and east. Rocks of the overlying River-Doom Supersequences define a southward thickening megawedge whose geometry is controlled by growth on the E-W oriented Little Range Fault.

Delineating the northern part of the Socotra Basin, offshore Korea, using marine magnetics

A marine magnetic survey was carried out in and around the northern part of Socotra Basin, offshore Korea (31°42′32″–32°46′29″N and 123°56′26″–125°49′16″E), in order to better delineate its northern and eastern boundaries. Analyses of the observed magnetic field and estimation of the basement depth were used to assess these boundaries. The power spectrum and the three-dimensional analytical signal methods were used for depth estimation and to reconstruct basement configuration. Estimated depths resulting from the power spectrum method range from 1.5 to 6.0 km for deep sources (basement troughs), and from 0.3 to 1.7 km for shallower sources (basement peaks). An isopach map shows that the sedimentary sequence varies from 1.4 to 6.0 km in thickness. Estimated depths from the analytic signal method fluctuate in the range 1.2–6 km. The results of the observed field analysis and depth estimation indicate good agreement with the formerly proposed eastern boundary but disagreement with the northern boundary. The findings suggest either an extension of the Socotra Basin or the existence of other sub-basins possibly interconnected with the study area.

Proposed performance-based laboratory test method for measuring vapour and water permeability of waterproofing membranes under hydrostatic pressure

The satisfactory performance of a sheet membrane waterproofing system, during its useful life, relies solely on the permeability of the waterproofing membrane, when construction and workmanship are correctly implemented. In field conditions, the membranes are subjected to various stresses, such as those brought about by building loads and lateral soil pressure that may strain the membrane, particularly in deep basements, and adversely affect its permeability. In this paper, a new performance-based laboratory test method is proposed to assess the vapour and water permeability of strained waterproofing membranes under hydrostatic pressure. The novelty of the method is that for a given basement configuration (height and depth), the effects of strain arising from tensile stress and compressive pressures acting on the membrane are simulated on the test specimen and the vapour and water permeability is measured in laboratory conditions. The test method was applied to two types of bituminous membranes assuming that each type of membrane was placed in one to six-story high basements. The test results confirmed that at some strain levels that simulate field conditions, to which membranes can be subjected, the strained membranes transmitted vapour under hydrostatic pressure. The tests also revealed that the test apparatus was effective in measuring the vapour and water permeability of the specimens under hydrostatic pressure.

On the use of different methods for estimating magnetic depth

Magnetic depth estimation plays an important role in magnetic interpretation. A complete quantitative interpretation of potential fields data aims to estimate three types of information about sources of geologic interest: the depth, the dimension, and the contrast in the relevant physical property. Such an interpretation suffers from inherent ambiguity. It is impossible to obtain all three types of information simultaneously without other a priori information. In many applications, we are often interested in depth more than either dimension or physical property contrast. Thus, different quick methods have been developed, over half a century, to estimate the magnetic depth. These methods work for simplified source geometries (dimensions) and are independent of the susceptibility contrast. The depths estimated by some method can be used as the final, quantitative solution in some ideal situations—i.e., the anomaly is well isolated and the noise is insignificant or well removed. It is more often that estimated depths provide a good starting point for a genuine structural interpretation (e.g., an interactive modeling or a constrained inversion). In petroleum exploration, for example, the structural surface interpreted from magnetic depth estimates is often the best available approximation to the true crystalline (i.e., metamorphic/igneous) basement configuration. Basement depth (or equivalently, sedimentary thickness) is a primary exploration risk parameter. Estimates of basement depth are directly applicable to basin modeling (e.g., source rock volume estimation) and thermal maturity applications (e.g., source-rock burial-depth). Basement structure inferred from magnetic depth estimates provides insight into the evolution of more recent sedimentary features (e.g., sub/minibasin compartmentalization, salt structure distribution/kinesis, localization of reservoir-bearing structures) in areas where the inherited basement fabric/architecture has affected (either continuously or episodically) basin evolution and development. Examples of the latter are the prolific, passive rifted margins of the southern Atlantic Ocean. A basin's “plumbing” often exploits faults and fractures within the sedimentary …

THE CONFIGURATION OF THE BASEMENT BENEATH THE ZAGROS BASIN

It has long been recognised that the deposition and deformation of the Phanerozoic cover in the Zagros Basin (mountains plus foreland) was strongly influenced by the reactivation of old tectonic fabrics in its basement. Facies boundaries and structures trending north‐south and NW‐SE can be attributed to the reactivation of Pan‐African sutures and Najd faults which are exposed in the Nubian‐Arabian Shield. However, to the east of a projection of the Oman line SWwards into the Rhub Al Khali Basin, cover structures have a NE‐SW trend which is not seen in Arabia. This boundary may overlie a Pan‐African suture between Arabia and India (Somalia or Pakistan).Data including magnetic intensities, geothermal gradients and isopach maps are used here to distinguish old faults which were reactivated in the basement from more recent faults formed in the cover by Zagros shortening. Old faults trending NW‐SE are interpreted as having reactivated episodically since the Permo‐Triassic opening of Neo‐Tethys; perhaps more significantly, the basement faults that reactivated in the East Arabian Block since then trend north‐south. The basement configuration is clarified by extending a modified East Arabian Block across the Zagros to an “East Arabian‐Zagros block” in which the NW trend of the Zagros lies between two syntaxes. This suggests a new tectonic framework for the region. The repeated reactivation of basement faults throughout the East Arabian‐Zagros Block controlled source rocks, traps and seals for the supergiant and giant oil and gas reserves which are present at various stratigraphic levels in different areas.

Bouguer anomaly of the Godavari basin, India and magnetic characteristics of rocks along its coastal margin and continental shelf☆

The Bouguer anomaly map of the Godavari basin has delineated several transverse and median ridges that have divided this basin into several sub-basins. Modelling of a gravity profile in the central part of the basin suggests 5.0 km of Gondwana sediments and high density rocks along the shoulders which may represent upthrusted lower crustal rocks related to the Eastern Ghats orogeny. The total intensity magnetic map of the coastal part of the Godavari basin shows a well-defined magnetic anomaly of approximately 200 nT along the coast which coincides with the Kaza basement ridge. The modelling of this magnetic anomaly indicates 2–2.5 km of sediments over the Kaza ridge and 5.5–6 km thick sediments in the depressions towards the west of this ridge which are constrained from the basement configuration based on seismic sections in the surrounding region. The Kaza ridge, with a susceptibility of 10 −4 SI units, appears to be composed of basic rocks which may be part of the exposed Eastern Ghats towards the west. This magnetic map also shows several high amplitude short wavelength anomalies which are caused by sub-surface basic rocks. These basic rocks are related to the Rajahmundry trap, equivalent to the Deccan traps of late Cretaceous age, and are exposed nearby by. Modelling of the magnetic anomaly across the continental shelf, off the coast of the Godavari basin, suggests an almost 45° inclined contact away from shore at a depth of 2.5 km and having a thickness of 3.5 km. However, this magnetic anomaly could be matched only with a remanent magnetization with declination 310° and inclination −67° which corresponds to the direction of magnetization reported for the Rajmahal traps.

Bouguer anomaly of the Godavari basin, India and magnetic characteristics of rocks along its coastal margin and continental shelf

The Bouguer anomaly map of the Godavari basin has delineated several transverse and median ridges that have divided this basin into several sub-basins. Modelling of a gravity profile in the central part of the basin suggests 5.0 km of Gondwana sediments and high density rocks along the shoulders which may represent upthrusted lower crustal rocks related to the Eastern Ghats orogeny. The total intensity magnetic map of the coastal part of the Godavari basin shows a well-defined magnetic anomaly of approximately 200 nT along the coast which coincides with the Kaza basement ridge. The modelling of this magnetic anomaly indicates 2–2.5 km of sediments over the Kaza ridge and 5.5–6 km thick sediments in the depressions towards the west of this ridge which are constrained from the basement configuration based on seismic sections in the surrounding region. The Kaza ridge, with a susceptibility of 10 −4 SI units, appears to be composed of basic rocks which may be part of the exposed Eastern Ghats towards the west. This magnetic map also shows several high amplitude short wavelength anomalies which are caused by sub-surface basic rocks. These basic rocks are related to the Rajahmundry trap, equivalent to the Deccan traps of late Cretaceous age, and are exposed nearby by. Modelling of the magnetic anomaly across the continental shelf, off the coast of the Godavari basin, suggests an almost 45° inclined contact away from shore at a depth of 2.5 km and having a thickness of 3.5 km. However, this magnetic anomaly could be matched only with a remanent magnetization with declination 310° and inclination −67° which corresponds to the direction of magnetization reported for the Rajmahal traps.