Bedrock Relief Research Articles

An integration of aeromagnetic and electrical resistivity methods in dam site investigation

Aeromagnetic map (Katsina-Ala sheet 272 S.E.) and the electrical resistivity sounding data obtained along eight traverses, from two sites, across the Katsina-Ala river were examined with the principal aim of defining depths to the bedrock, bedrock relief, geologic structures and the nature of the superficial deposit. The purpose of this exercise was the selection of probable axis for a hydroelectric power dam. The aeomagnetic map shows the basement rocks trending roughly NE-SW. This trend correlates with the 40°–45° strike of foliation measurements made on the rock outcrops along the river channel. A network of cross cutting lineaments suspected to be fault/fractures trending approximately NE/SW and NW/SE was delineated from the magnetic map. The depths to the bedrock estimated from depth sounding data at site I generally vary from 1–53.1 m. Depths to the bedrock estimated at site II range from 1.9–19.5 m. The superficial deposit varies from clay to sandy clay. clayey sand (with boulders in places), sand and laterite. The bedrock relief is relatively flat and gently undulating along most of the traverses with an overall dip towards the river channel. Except for the relatively high overburden thicknesses, two traverses at site I and one traverse at site II are probable, axes for a major and minor dam respectively.

Ordovician impact crater at Kärdla, Hiiumaa Island, Estonia

The buried and well preserved Kärdla crater in Estonia (Southern Fennoscandia) (58°59Ń, 22°40É), with a diameter of approximately 4 km, was discovered in 1967. The subsurface structure of the crater has been studied by gravity anomaly measurements, aeromagnetics and by more than 300 boreholes in the crater, on the rim (“ring wall”) and in the surrounding area. Recent boreholes, reaching 800 m, have revealed that the crater is 540 m deep and has 100 m high central peak. Barely visible in the present topography, the crater is filled with Palaeozoic and Quaternary deposits and is mostly low marshy land surrounded by a low ridge along the ring wall. Quaternary landforms, such as eskers and post-glacial beach ridges, also complicate the topography. In the buried sub-Quaternary bedrock relief, the crater occurs as a roundish depression bordered with two, 10–30 m high horseshoe-shaped elevations. The bedrock topography was slightly drumlinized by glacial excavation. The crater formed 455 Ma in composite stratigraphy: middle and lower Ordovician (20 m) and Cambrian (120 m) sedimentary rocks overlying crystalline basement on the bottom of a shallow nearshore sea. The subsurface structure of the crater is well preserved. In vertical section from the top are: (1) post-impact cover of Ordovician sedimentary rocks, 15–100 m thick; (2) allochthonous breccias (filling the lower part of the crater) and beds of fall-out breccias and conglomerates, sandstones and sandy limestones consisting of debris of reworked fall-out breccia and wall rocks surrounding the crater; (3) a body of autochthonous and subautochthonous breccias forming the bottom and the central peak of the crater and also remnants of its rim. Shocked rocks and minerals from autochthonous and allochthonous breccias have been revealed by optical microscopy.

Evidence of rapid Cenozoic uplift of the shoulder escarpment of the Cenozoic West Antarctic rift system and a speculation on possible climate forcing

The Cenzoic West Antarctic rift system, characterized by Cenozoic bimodal alkalic volcanic rocks, extends over a largely ice-covered area, from the Ross Sea nearly to the Bellingshausen Sea. It is bounded on one side by a spectacular 4-to 5-km-high rift-shoulder scarp (maximum bedrock relief 5 to 7 km) from northern Victoria Land-Queen Maud Mountains to the Ellsworth-Whitmore-Horlick Mountains. Jurassic tholeiites crop out with the late Cenozoic volcanic rocks along the section of the Transantarctic Mountains from northern Victoria Land to the Horlick Mountains. The Cenozoic rift shoulder diverges here from the Jurassic tholeiite trend, and the tholeiites are exposed discontinuously along the lower elevation (1-2 km) section of the Transantarctic Mountains to the Weddell Sea. Various lines of evidence, no one of which is independently conclusive, lead the authors (as others have also suggested) to interpret the following. The Transantarctic Mountains part of the rift shoulder (and probably the entire shoulder) has been rising since about 60 Ma, at episodic rates of {approximately}1 km/m.y., most recently since mid-Pliocene time, rather than continuously at the mean rate of 100m/m.y. Uplift rates vary along the scarp, which is cut by transverse faults. The authors speculate that this uplift may have climatically forcedmore » the advance of the Antarctic ice sheet since the most recent warm period. They suggest a possible synergistic relation between episodic tectonism, mountain uplift, and volcanism in the Cenozoic West Antarctic rift system and waxing and waning of the Antarctic ice sheet beginning about earliest Oligocene time.« less

Seismic reflection investigation of Kalamalka Lake: a "fiord lake" on the Interior Plateau of southern British Columbia

A uniboom seismic reflection profile survey has revealed the nature of bedrock relief and the acoustic character of Pleistocene glacial sediment fill beneath Kalamalka Lake in southern British Columbia. Despite its continental interior setting, Kalamalka Lake basin has many attributes of coastal fiords, such as being overdeepened below sea level and having closed bedrock depressions and a thick sediment fill.The bedrock surface beneath Kalamalka Lake has been eroded as much as 417 m below lake level (26 m below sea level) and is characterized by a series of closed, glacially overdeepened depressions. We suggest that the location of the lake basin is structurally controlled but was overdeepened by rapidly flowing ice that drained the interior portions of the Cordilleran ice sheet during repeated Pleistocene glaciations.Up to 272 m of sediment has been deposited beneath Kalamalka Lake. The greatest thickness of the sediment fill (up to 237 m) is a seismically transparent unit that overlies a thin (up to 20 m), discontinuous lower stratified unit and is overlain by a thin (up to 15 m), continuous upper unit that is well stratified. The sedimentological nature of the lower stratified unit is not known but could represent a discontinuous coarse lag. The thick, middle transparent unit is interpreted as a massive silt deposited rapidly in a proglacial lake from suspended-sediment plumes during deglaciation. The thin overlying stratified unit may be correlative with laminated glaciolacustrine "white silt" deposits that outcrop extensively across central and southern British Columbia, suggesting a common history of deglaciation and sedimentation.An ambitious research program focused on seismic stratigraphic definition, coupled with direct drill-core sampling, is needed to take full advantage of the extensive sediment record that exists beneath the large, glacially overdeepened lakes of southern British Columbia.

METHOD FOR RECOGNIZING LOCAL BEDROCK INFLUENCE ON DRIFT PLAINS∗

Analysis of topographic and bedrock surface data from 41 sites 23.3 km2 (9 m2) within Midwestern areas glaciated during the Late Wisconsinan identifies an average drift thickness-maximum bedrock relief transition equation for the presence or absence of bedrock surface influences on topographic detail. For sites with average drift thicknesses between 15 and 35 m, the transition occurs when average drift thicknesses exceed 0.5 maximum bedrock relief + 10 m. This equation may provide a practical tool in searching for buried bedrock valleys.

Reconstruction of the bedrock relief of Antarctica after deglaciation

Abstract Analysis of maps depicting “Magnitude of isostatic uplift of Antarctica after removal of the glacier load”; and “Bedrock of Antarctica after deglaciation”; has produced the following results. The total mass of the crust uplifted due to deglaciation is 6.2 million km3; the mean value of isostatic uplift is 450 m. It was also possible to calculate the mean altitude and area of Antarctica within its geographical and geophysical limits. Comparison of the mean altitude (500 m) of the reconstructed bedrock surface and of the area of Antarctica within its geographical limits (16.62 million km2) with the same morphometric parameters of the other continents shows that its mean altitude would not differ from that of the other continents.

Basal Waitemata Group lithofacies: rapid subsidence in an early Miocene interarc basin, New Zealand

ABSTRACTThe abrupt transition from coastal and shallow shelf sediments to bathyal sediments provides a record of rapid subsidence and deepening of the early Miocene Waitemata basin. Basal shallow marine strata (Kawau Subgroup) accumulated upon a highly dissected surface that overlies deformed Mesozoic metagreywacke. The early Miocene coast was characterized by an embayed and cliffed shoreline with numerous sea stacks and islands. Kawau Subgroup lithofacies, which include pocket beach, shallow shelf and base‐of‐cliff talus deposits, reflect rapidly changing coastline configuration and water depths as the rugged bedrock surface was buried.The response to continued rapid subsidence and transgression in Waitemata basin was a decrease in the supply of coarse clastic sediment. Beach gravels were locally displaced to greater water depths by avalanching down steep bedrock slopes. The first bathyal turbidite facies, which abruptly overlie the shallow‐water Kawau Subgroup, include locally derived sediment gravity flows commonly ponded by remnant bedrock submarine highs. When this local supply of sediment had been exhausted, coarse sediment starvation ensued and bathyal muds accumulated. With the resumption of sediment supply and gradual burial of submarine bedrock relief, submarine fans coalesced and increased in lateral extent.Subsidence of the Waitemata basin to bathyal depths is thought to have occurred in less than a million years. From the above hypothesis, a general model of sedimentation is proposed.

Relationships Between Ice-Sheet Dynamics and Bedrock Relief on Dissected Plateau Areas in Finnish Lapland North of 69° Latitude

With the help of numerous till fabric analyses and other glacial indicators obtained from the study area proper a scheme of ice-flow pattern and ice-thickness variation throughout Weichselian time is put forward which in many respects differs from earlier ones. In the two separate fell areas on the Finnish side of the study area there can be discerned a clear zonal arrangement in an assemblage of residual bedrock forms such as tors, kopjes and inselbergs. This can be linked with areally altering conditions in the ice sheet dynamics. The dependence of ice flows on existing local and regional relief during various glacial phases has been verified by means of quantitative comparisons between known ice movement directions and the effective orientation of the bedrock relief. In the light of results gained from a large number of minor and major test areas it seems clear that this dependence has changed considerably from phase to phase according to thermal conditions on the ice sheet base.

Relationships between Ice-Sheet Dynamics and Bedrock Relief on Dissected Plateau Areas in Finnish Lapland North of 69 degrees Latitude

Relationships between Ice-Sheet Dynamics and Bedrock Relief on Dissected Plateau Areas in Finnish Lapland North of 69 degrees Latitude

Some structural and geomorphological aspects of recent crustal movements in estonia

Comparison of some structure contour of northeastern and southeastern Estonia with the bedrock relief patterns leads to the conclusion that direct morphostructures are infrequent in those areas. The formation of bedrock and contemporary relief has been little effected by differential postglacial and recent crustal movements. Morphotectonic analysis enables us to determine the character of those movements only in a few cases.

Sedimentation in an esker system influenced by bedrock topography near Kingston, Ontario

Late Wisconsinan sand and gravel deposits on the Frontenac Arch, Ontario, record a major meltwater drainage system developed subglacially and subaqueously in one or more glacial lake phases of the Lake Ontario basin during retreat of the Laurentide Ice Sheet. Factors influencing channel location, morphology, and sediment deposition are ice flow direction and the Precambrian bedrock topography, in turn dependent upon bedrock structure and composition. Meltwater drainage across the Frontenac Arch is localized within a broad depression oriented approximately parallel to glacial flow. Sediment deposition within the regional depression follows ice-flow direction despite irregular bedrock relief, indicating formation of the meltwater system and associated sediments in three stages: (i) establishment of a continuous meltwater system subglacially under high hydrostatic pressure with minor erosion of underlying Precambrian bedrock; (ii) deposition of poorly sorted, coarse-grained sediment in cavities or channels associated with irregular bedrock topography; and (iii) deposition of several coalescing subaqueous outwash fans at the ice margin as the glacier receded from the area. The discontinuous nature of the deposits and the association of proximal to distal outwash fan facies within a deposit suggest that esker sedimentation occurred during periodic stabilization of the ice front during deglaciation.

A model for glacial and proglacial sedimentation in the shield terrane of southern Ontario

Zonal stagnation is an important element in the regional style of deglaciation in areas of moderate bedrock relief (50–150 m), such as the shield terrane of southern Ontario. Bedrock topography played a major role in the stagnation process, as ice blocks were trapped within bedrock basins of all sizes. Stagnation occurred subglacially as the ice sheet thinned and trapped basal ice became increasingly isolated from its source. Sediment–landform relationships support this model. Bedrock lakes are floored by highly faulted laminated sediment, often displaying large ice-block casts. The dominant depositional landforms are ice-contact terraces, which flank valley walls. Sediment forming the terraces is variable, depending upon ice-marginal depositional environment.Models for two depositional systems are developed: (1) localized ice-marginal and subglacial sedimentation in upland terrain, and (2) thick proglacial basin-fill sequences developed in major structural valleys. In upland areas, sediment cover is thin and discontinuous and is composed primarily of complex sediment-flow assemblages. Basal till is rare but occurs as two distinct lithofacies representing (1) meltout from beneath wholly stagnant ice, and (2) deposition by basal melting from thin slabs of stagnant, debris-rich ice lodged beneath an active shear zone. Thick proglacial basin-fill sequences exhibit complex facies relationships. Ice-contact lacustrine terraces comprise fining-upward subaqueous outwash sequences, developed as density underflows were funnelled between rock knobs and blocks of stagnant ice. Kettle lakes developed within large basins where ice blocks persisted for the longest period of time.

Geophysical investigations for groundwater in Precambrian terrains: a case history from Ikare, southwestern Nigeria

Magnetic, very low frequency electromagnetic (VLF) and electrical resistivity methods have been applied in mapping buried bedrock relief for groundwater exploration in the Precambrian terrains of Ikare, southwestern Nigeria. Quantitative interpretation of the magnetic data involved initial modelling and Euler deconvolution using elementary magnetic sources to identify applicable geological models in the spatial domain. A depth estimate of the top of the buried valley of 23 m was obtained. The application of power spectral deconvolution yielded an estimated depth of up to 67 m on the bedrock surface. The VLF data indicated the existence of subsurface lineament in the area while obtained resistivity of the sedimentary fill amounted to 260 ohm-m. The results of the three techniques have successfully delineated a buried river channel for groundwater extraction in a crystalline basement terrain.

The structural evolution of George VI sound, Antarctic Peninsula

Abstract George VI Sound, 400 km long and between 25 and 90 km wide, separates Alexander Island from the Antarctic Peninsula. The straight parallel sides of its northern section and sub-parallel normal faulting observed in exposed rock on adjacent parts of Alexander Island and Palmer Land indicate a history of rifting. Subglacial topography, revealed through radar sounding of ice thickness in Palmer Land, shows deep areas which owe their presence to two major fault zones parallel to the main N-S trend. One of these is probably the western escarpment of the Palmer Land plateau and the eastern boundary of the rift system. Bedrock relief suggests that transverse block faulting has differentially raised and lowered the mountains on the eastern side of the sound. A seismic profile through George VI Ice Shelf (which occupies most of George VI Sound) confirms that the “W”-shaped cross-section of the bedrock observed by plumb-line at the northern edge of the ice shelf is also found 250 km further south. The shape may be due entirely to glacial erosion, but it is more likely to be a structural feature subsequently glacially modified. These features are consistent with a model of George VI Sound as a region of Tertiary intra-arc extension within the Mesozoic and Cenozoic magmatic arc of the Antarctic Peninsula and Alexander Island.

GEOLOGIC INTERPRETATION OF ANTARCTICA'S MOUNTAINOUS REGIONS WITH SPACE IMAGERY

This paper demonstrates how ice-surface relief identified on space imagery can be used in mapping selected elements of the geologic structure of the Antarctic Peninsula. The mapping procedure is based on the fact that all major subglacial relief forms appear, albeit in subdued form, on the surface of slow moving and relatively thin ice sheets. Since subglacial bedrock relief reflects geologic structure, particular surface configurations of ice identified on space imagery can be used as indicators of major structural elements. Examples of the procedure, including imagery of ice relief and corresponding cartographic representations of underlying structure are provided for faults and ring structures. Translated from: Antarktika, AN SSSR Mezhduvedomstvennaya komissiya po izucheniyu Antarktiki, Doklady komissii [USSR Academy of Sciences Joint Commission on Antarctic Research, Commission Report], No. 24. Moscow, 1985, pp. 50-55. An article demonstrating the application of these methods in the compilation of a geologic map for a part of the Antarctic Peninsula will appear in a later issue.

A combination of electrical resistivity, seismic refraction, and gravity measurements for groundwater exploration in Sudan

In the savannah belt of central Sudan, near the town of Kosti, a regional geophysical survey has been carried out forming part of a groundwater project. Because of the presence of detectable and significant contrasts in physical properties of the subsoil, integrated use could be made of electrical resistivity, seismic refraction, and gravity methods. In the interpretation of multilayer electrical sounding curves, additional subsurface information such as lithological well descriptions and geophysical well logs is normally a necessity for solving the problems of equivalence. Along a profile in the eastern part of the area studied, where additional subsurface information was scarce, 16 vertical electrical soundings have been made. A preliminary simple mathematical interpretation suggested possibilities for the presence of fresh groundwater in the eastern part of the profile. In order to solve the equivalence problem, seismic refraction work was carried out at some selected places; that yielded additional information on depths to bedrock. These seismic data made possible a unique solution of the electrical sounding curves, from which it could be concluded that all groundwater in the area is saline. Subsequent test drilling confirmed these findings. A regional relative Bouguer anomaly map provided a picture of the general geologic structures and made possible rough estimates of depths to bedrock. In areas where the basement rocks are relatively close to the surface, as is the case with the profile presented, the gravity anomalies cannot be correlated with bedrock relief, because the effect is strongly influenced by lateral density variations within the bedrock itself. This is an example of a case where only an integrated application of several geophysical exploration methods can provide the desired hydrogeologic information in an acceptable balance between reliability and cost.

Glacial geomorphology of the Lambert glacier and its surrounding mountains

Abstract The results of glacial‐geomorphic observations in the area of the Lambert Glacier, the largest outlet glacier in Antarctica, are reviewed. The basic types of glacial forms and bedrock relief are described, and attention is focused on the widespread development of preglacial planation surfaces. The principal stages of landform development are traced.

International studies of ice sheet and bedrock

The main Antarctic ice sheet is so vast that large-scale international programmes are necessary for an adequate study of its dynamics, thermodynamics and past history. Radio echo sounding methods are providing reconnaissance mapping of the ice sheet on a continental scale. The soundings also yield significant information on the internal deformation of the ice sheet, on the flow of ice shelves and the nature of the bottom surface of the ice. This information is being used to amplify and extrapolate detailed results from other groups who are studying the mass balance, ice movement, and past history by deep drilling; it is also of value in isolating critical sites for future studies. Reconnaissance mapping has now shown the presence of sub-ice lakes larger than those reported in 1973. Studies of radio wave reflexions from internal layers which are believed to be former depositional surfaces, have shown that the conformity between these layers and the bedrock relief decreases as ice movement increases. Statistical analyses of bedrock relief and radio wave reflexion parameters are being used to delimit major geological provinces beneath the ice and investigate the structural and lithological transition between the Transantarctic Mountains and East Antarctica. Such studies have recently indicated the presence and structure of two large intracratonic sedimentary basins in eastern Antarctica.

River valley anomalies — One approach to the study of Fennoscandian bedrock relief

The intricate problem of major Fennoscandian landforms has been studied along the following lines: Structural control, erosion surfaces, climatic geomorphological forms, glacial erosion, and river valley anomalies. The last-mentioned method of approach was tried several decades ago and is almost forgotten. It will be discussed here. Of special interest are the river captures at the main water divide. Clear cases are few compared with the great difference in river length on both sides. River gaps form the main group of discussed cases; the most striking ones are in the Swedish Caledonides. Less well known are the water gaps in pre-Cambrian Sweden. Local tectonic movements cannot be excluded as an explanation in most cases, but are not necessarily the explanation. More probable as a general explanation are lost sedimentary rocks, notably Cambro-Silurian. This means a stable shield area, slowly being stripped of a sedimentary cover in relation to varying rock resistance and undulations of the crystalline bas...

Polarization Changes of Radio Signals in Vertical Radio-Echo Sounding of Glaciers

An electromagnetic linearly-polarized signal transmitted through the glacier in a vertical direction, reflected from the bedrock, and received with a receiving antenna, is found to be changed into one either partially or elliptically polarized. The polarization changes are believed to be due mainly to the crystal structure of the glacier and anisotropy caused by the pressure of the upper layers. An analysis of the polarization diagrams obtained is up to the present the main method used for studies of the reflected signal polarization. Evidently using a simple dipole a partially-polarized signal is not distinguishable from one that is elliptically polarized, nor is a nonpolarized signal distinguished from circularly polarized. However, the data recently obtained are of great importance, particularly from studies of glacier crystal structure made in deep core drilling. Possible reasons for the polarization changes of the signal have been analysed. Results of the analyses of the polarization diagrams obtained both at individual points and along extended traverses are discussed. It has been found that the signal reflected from a considerable ice thickness is polarized in such a way that the parallel orientation of the receiving and transmitting dipoles can be disregarded. En route recordings of the signal fluctuations obtained by parallel and orthogonally polarized dipoles are shown. The results of polarization studies are important for practical purposes. For example, bedrock relief sounding carried out with crossed dipoles makes it possible to get rid of interference signals occurring due to scattering from inhomogeneous structures of the upper part of the glacier.