Precambrian Bedrock Research Articles

Isolation of radioactive waste in the swedish precambrian bedrock

Safety analyses regarding final disposal of radioactive waste require methods to predict groundwater movements in jointed rocks. This paper summarizes some of the most important results of flow calculations which were undertaken in the Swedish repository safety assessment. It includes several factors that influence the groundwater flow, like temperatures and mechanical deformations.

Cenozoic Provenance and Sediment Cycling: Hamilton Bank, Labrador Shelf

ABSTRACT Within the context of Cenozoic climatic and glacial history, grain size, mineralogy and sand grain surface textures have been used to investigate sources of glacial drift. Components of Precambrian crystalline bedrock, friable, Hadrynian or younger Double Mer sedimentary rocks, offshore Paleozoic sediments and Tertiary-Quaternary mudstones and sands have been identified in the drift. The sand fraction appears to have evolved through a complex history of several sedimentary cycles beginning with chemical, fluvial and glacial erosion of grains from the Double Mer Formation. There then followed, proglacial silica precipitation on grain surfaces, reworking and Pliocene to Pleistocene deposition in a shallow marine delta. In the late Pleistocene the delta was exumed allowing subaereal weat ering, shallow marine-aeolian reworking, glacial modification and quartz crystal overgrowths to further modify the grains.

CONTRIBUIÇÃO AO ESTUDO DO NEOCENOZÓICO DA BAIXADA DA GUANABARA: FORMAÇÃO MACACU

This paper gives a first description of the Lower Quaternary Macacu.Formation, in a contribution to a better understanding of the geologic significance of the Barreiras Group in Southeastern Brazil. The Macacu Formation has been mapped in the surroundings of the Guanabara bay and lies unconformably. over the Precambrian bedrock and the arkosic sands and montmorillonite clays ofthe Pre-Macacu (Tertiary) beds. It consists on a sequence oftabular beds and lenses ofpoorly sorted sandy clayey, clayey sandy and silty clayey incoherent, afossiliferous and seldom stratified materials. The post-depositional weathering seems to be partially responsible for the high mineralogical maturity of the sediments. The sediment characteristics indicate water flows and debris flows as the main processes involved in deposition, in a piedmont, type alluvial fan environment.

Some properties of Norwegian tills

Grain size distribution data for a large number of basal tills from different areas are presented. The Norwegian tills can be divided in two major groups: Tills derived from Precambrian bedrock and tills derived from Cambro‐Silurian sedimentary rocks. Both types are significantly different from Danish tills where unconsolidated sediments are incorporatcd. Chemical and mineralogical composition of the 125–250 μm fraction of tills derived from Precambrian rocks and tills derived from phyllites are presented. These data are used to determine transport distances for the till material.Due to a marked relief and short distances from the ice‐divides to the coast, the tills found in the Norwegian mountains have generally had a short transport and are, unless large amounts of unconsolidated sediments are incorporated, strongly influenced by the underlying bedrock.

CONTRIBUIÇÃO AO ESTUDO DO NEOCENOZÓICO DA BAIXADA DA GUANABARA: FORMAÇÃO MACACU

This paper gives a first description of the Lower Quaternary Macacu.Formation, in a contribution to a better understanding of the geologic significance of the Barreiras Group in Southeastern Brazil. The Macacu Formation has been mapped in the surroundings of the Guanabara bay and lies unconformably. over the Precambrian bedrock and the arkosic sands and montmorillonite clays ofthe Pre-Macacu (Tertiary) beds. It consists on a sequence oftabular beds and lenses ofpoorly sorted sandy clayey, clayey sandy and silty clayey incoherent, afossiliferous and seldom stratified materials. The post-depositional weathering seems to be partially responsible for the high mineralogical maturity of the sediments. The sediment characteristics indicate water flows and debris flows as the main processes involved in deposition, in a piedmont, type alluvial fan environment.

The Slate Islands: a probable complex meteorite impact structure in Lake Superior

Shock metamorphic effects in samples from the Slate Islands, Lake Superior (48°40' N, 87°00' W) suggest that the islands are part of a meteorite impact structure. The islands form the central uplift of a complex crater and are ringed by a submerged trough and annular ridge with a diameter of 30 km. Precambrian bedrock units are locally brecciated and cut by allochthonous breccia dikes. These dikes contain clasts of identifiable country rock and also fragments of a sedimentary unit, possibly Upper Keweenawan in age, which is no longer present in outcrop. The orienta tions of shatter-cones present in the breccia host-rocks indicate the interior of the islands as the approximate shock centre. Microscopic planar features, equivalent to those described from other impact sites, occur in quartz and plagioclase and the level of shock deformation increases towards the interior of the islands. The shock event postdates Keweenawan igneous activity (about 1.1 b.y. old) and, on the basis of the erosion level, may be early Paleozoic in age.

Cretaceous and Cenozoic faulting in eastern North America

Cretaceous and Cenozoic faults in eastern North America provide evidence of modest intraplate tectonic activity in this region. Thrust and gravity faults are found; strike-slip movement is not demonstrable but could have occurred in some cases. Fault movements are dated by offset Cretaceous, Tertiary, and Quaternary sediments, Cretaceous kimberlite, and Pleistocene glacial stria. Cretaceous and Cenozoic displacements range from about 1 mm to several tens of metres. The faults are found in the central Mississippi River valley, the southeastern and northeastern United States, and eastern Canada. Correlations among historic seismicity, fault plane solutions, and the Cretaceous and Cenozoic faulting in the central Mississippi River valley and the southeastern United States suggest a close causal relationship. The quality of the correlation between historic seismicity and Cretaceous and Cenozoic faulting in the northeastern United States and eastern Canada varies considerably from area to area; hence the relation there between Cretaceous and Cenozoic faulting and contemporary seismotectonics is uncertain. Sharp offsets in loosely consolidated sediments, the presence of slickensides and fault gouge, and the involvement of Precambrian or Paleozoic bed rock provide the best evidence that faulting was associated with tectonic earthquakes. The displacement of near-surface deposits implies that surface faulting has occurred, although it has not been documented for modern earthquakes in eastern North America. At least some of the displacements occurred along pre-existing faults. The number and amounts of Cretaceous and Cenozoic movements on each fault have not been determined, but on one fault a displacement of 0.6 m during a short time span suggests that an earthquake of about magnitude 6 was associated with the displacement. The largest Cretaceous and Cenozoic displacements must represent more than one movement. The presence of thrust and gravity faults and the inconsistent trends of the faults imply that stresses that vary spatially and (or) temporally, rather than a single stress field acting steadily throughout the entire North American plate, are responsible for causing this intraplate tectonic activity.

K—Ar dating and Sr-isotope composition of rhyolitic rocks from Lake Mien in southern Sweden

Abstract The circular topographic structure in the Precambrian bedrock of southern Sweden which forms the site of Lake Mien has been interpreted to be either the result of volcanic activity or of a meteorite impact. K—Ar dating of glass and whole-rock samples suggest an age of 110–120 m.y., which cannot be related to any clearly established volcanic activity in southern Sweden during the Cretaceous. Rb and Sr analysis and Sr-isotope composition convincingly indicate that the lava-like rocks and breccias at Lake Mien have been formed by melting of the country rock.

Crater or kettle? A geophysical study

Roseau Lake in northwestern Minnesota occupies a conspicuous, circular depression in a level plain of glacial drift. A detailed gravity survey of the lake was made to determine if the Precambrian bed rock under the thin cover of glacial drift exhibited the low-density, fractured character of an impact crater or if the feature was confined solely to the drift. Although a sharp local gravity minimum was found in the vicinity, it is displaced to the northwest in association with a positive magnetic anomaly. Roseau Lake therefore is interpreted to be an exceptionally large glacial kettle.

The magnetization and densities of precambrian rocks and iron-ores of northern Sweden

In an attempt to improve the interpretation of the geophysical results of an exploration programme in Norrbotten country, northern Sweden, a systematic investigation has been made of the magnetization and densities of the main rock and ore types. The rocks have low porosities and their densities vary predictably according to their composition between 2.6 and 3.1 g cm −3 with an average of 2.73 g cm −3 for the Precambrian bedrock of the area. The high grade magnetite ores have susceptibilities in the 0.5 –1.2 emu cm −3 range but for the normal rock types there is a well defined upper limit at about 2·10−2 emu cm −3 and there is no general systematic variation with the composition. The remanent magnetization in both the rocks and magnetite ores is generally weak with Q-values between 0.1 and 1.0 and shows many features characteristic of viscous remanent magnetization (VRM) including a coincidence of the directions of magnetization with that of the geomagnetic field. In the rocks the field direction is undisturbed but due to the strong demagnetization fields within the magnetite ores the direction of the incucing field is determined by the form and position of the ore deposit. The remanent magnetization generally influences only the amplitudes of the magnetic anomalies observed but not their forms. There is some evidence from the results of this survey that there is a significant difference between the remanence of samples collected from outcrops and from drillcores, suggesting that the weak VRM can be disturbed by drilling processes.

Epifaunal Zonation on an Upper Cretaceous Rocky Coast

In Cenomanian time, the deeply weathered Precambrian bedrock of southern Sweden was transgressed by a shallow sea, resulting in the formation of an archipelago with low islands and peninsulas during early Campanian time. On one of these islands, a high-diversity fauna characteristic of a rocky coast ecosystem is found comprising nearly all levels of the food pyramid from phytoplankton and zooplankton to reptiles. The fauna is dominated by bivalves, especially oysters, and includes the northernmost Upper Cretaceous reef corals and rudists. Three distinct epifaunal zones exist: the lowest zone is characterized by large numbers of serpulids and occupies the underside of the boulders; the next higher zone is found on the vertical sides of the boulders and is characterized by oysters and a large, vertically orientated craniacean brachiopod; and the highest zone occupies the upper part of the vertical sides and the rounded upper surfaces of the boulders and is characterized by spondylid bivalves. All species normally had a clumped distribution. The zonation is interpreted as a result of the combined effect of negative phototropism (serpulids), protection against sedimentation (craniaceans), and tolerance of strong turbulence, occasional exposure, and limited competition by other species for space (spondylids).

Contemporaneous development of mullions and hornblende lineations in divergent directions

A mullion structure is described from a several meter thick amphibolite from the Precambrian bedrock of south-eastern Norway. The mullion axes deviate about 60–700 from the fold axis defined in the amphibolite by the constructed intersection of foliation planes. The fold axis is also reflected in the lineated hornblende grains. The orientation of the mullion axes is parallel to the trace of the amphibolite foliation on the mullioned surface. It is concluded that the mullions were formed on a discordant surface (probably a fracture surface) in responce to movement along the intersecting foliation planes at the same time as the hornblende lineation was developed.

Calcium, magnesium and potassium in mineral soils from the southern half of Finland

210 samples of mineral soils from the southern half of Finland with mainly an acid precambrian bedrock, were analysed for the total contents of Ca, Mg and K, and for the portion of these nutrients which could be exchanged by N NH4OAc (pH 7), dissolved by 0.1 N HCI at room temperature, or released by N HCI at 50° C. The total content of Ca was lowest in samples of heavy clay, 0.78±0.14 % in the surface soils and 0.92±0.10 % in the deeper layers. The mean content in the groups of other soils was at least about 1.1 %. The total content of Mg increased with an increase in the clay content (r = 0.81***). It ranged from 0.6±0.1 % in the sand and fine sand samples to 1.53±0.19 % in the heavy clay soils of the surface layers and to 1.89±0.12 % in those of the deeper layers. Also in the groups of loam and silt soils and of the coarser clay soils, respectively, the Mg content was in the deeper layers higher than in the surface soils. The total content of K also increased with the clay content (r=0.73***) from 1.7±0.1 % in the sand and fine sand soils to 2.74±0.21 % in the heavy clay soils of the surface layers and to 3.10±0.07 % in those of the deeper layers. The portion of exchangeable Ca was relatively high: in the groups of surface soils from more than one tenth to one third of the total amount. The corresponding average amounts released by even the more drastic treatment with acid were not markedly higher. Only a few per cents of total Mg were exchangeable and slightly higher amounts were dissolved by 0.1 N HCI, whereas the treatment with N HCI at 50° C released about half of the total Mg. Exchangeable K and K dissolved by 0,1 N HCI did not exceed 1 % of the total K, except slightly in the heavy clay soils; the average amounts released by N HCI ranged from 5 to 18 % of the total K. The plant availability of these nutrients was discussed.

Hydrogeologic Studies at a Subsurface Radioactive-Waste Management Site in West-Central Canada: ABSTRACT

Nearly all radioactive wastes produced in Canada are stored or disposed of at 2 waste-management sites in shallow Quaternary deposits in south-central Canada. The oldest site and the one which has received most of Canada's radioactive waste is at the Chalk River Nuclear Laboratories (CRNL) in east-central Ontario. The newest site is at the Whiteshell Nuclear Research Establishment (WNRE) in southeastern Manitoba. Both waste-management sites are operated by Atomic Energy of Canada Limited. Hydrogeologic studies were conducted at WNRE to predict the subsurface behavior of radioactive contaminants, which at some future time possibly could enter the groundwater zone. The site is composed of Quaternary deposits of clayey till above a sandy artesian aquifer which overlies the Precambrian bedrock. The bedrock is 50 ft below ground surface. At present there is no significant contamination of the subsurface hydrologic environment. Hydrogeologic information is being used to facilitate the design and operation of the waste-management facilities and to assess long-term storage and disposal capabilities of the area. The hydrogeologic investigations involved 3 main parameters. These are (1) expected residence times of radionuclides which may enter the groundwater flow system, (2) anticipated travel paths and discharge processes, and (3) suitability of the hydrogeologic environment for physical manipulation to achieve greater containment capabilities. Hydrogeologic studies conducted during the past 5 years have involved field and laboratory techniques such as geologic test drilling, mapping of hydraulic head distributions using wells and piezometers, field permeability tests using single well response tests and long- and short-term pumping tests, mapping of natural hydrochemical patterns in the groundwater zone, tritium tracer experiments, groundwater age dating using C14, and mathematical modeling using digital-simulation programs. Comparisons of the results from the studies indicate that we have attained a reasonable level of predictability in our understanding of the hydrogeologic environment in the area. End_of_Article - Last_Page 1593------------

An Early Tertiary Outcrop in North-Central Baffin Island, Northwest Territories, Canada: Environment and Significance

A thin, impure limestone was found in situ on Precambrian bedrock at latitude 70°36.6′ N and longitude 75°20′ W some 26 km northwest of the Barnes Ice Cap. The unit consists of undulating laminations composed of alternating fine- and coarse-grained sediment, which are interpreted as a series of algal mats or algal-laminated sediments. An analysis of enclosed palynomorphs indicates the presence of Ulmus, Taxodiutm, Liriodendron, Carpinus, and Engelhardtia plus other genera. On the basis of the microflora a Paleogene age is assigned to the unit. The climate at that time was warm-temperate and the environment suggested is a freshwater marsh or swamp. The outcrop is restricted to a single hill summit and its location suggests considerable Neogene geomorphological activity, primarily river-cutting associated with vertical movements along the western margin of the Davis Strait/Baffin Bay Tift.

Geophysical Study of Unconsolidated Sediments and Basin Structure in Cache Valley, Utah and Idaho

During the summer of 1969, an integrated geophysical study was completed in Cache Valley near the state line between Utah and Idaho. The survey covered about 30 sq mi and was designed to provide data on unconsolidated sediments and basin structure in a study of the ground-water resources of the area. Gravity, magnetic, resistivity, and seismic reflection methods were employed to obtain the desired information. Detailed gravity data revealed that the west side of Cache Valley is underlain by a trough which is about 5 mi wide and which is filled with as much as 7,000 ft of Quaternary and Tertiary sedimentary units. The magnetic data revealed a probable mafic dike intruded into Paleozoic and Precambrian bedrock in the Lewiston area and possibly a buried volcanic flow unit in the Tertiary sediments. Interpretations of 38 vertical electrical soundings supported by three well logs showed that an area of about 8 sq mi in the center of the surveyed area is underlain by conductive clays (resistivity less than 10 ohm-meters) that are as much as 400 ft thick. Most of the area surveyed is underlain at depths of 100 to 500 ft by more resistive materials (20 to 40 ohmmeters) interpreted to contain a higher percentage of coarse-grained sediments. These sediments may constitute a potential aquifer. Seismic reflection data further confirmed the existence of the massive clays in the central part of the surveyed area, revealed complex sedimentary geology at the valley margins, and defined subtle structures within the valley fill.

Lake Mien — An Astrobleme or a Volcano-Tectonic Structure

Lake Mien forms a depression in the acid Precambrian bedrock in southeastern Sweden. According to the first interpretation, the brecciated and lava-like rocks found in the glacial deposits south of Lake Mien were believed to be of volcanic origin and were described as rhyolites. Recehtly, a meteoritic origin has been proposed from identification of coesite and shock-wave deformation lamellae in quartz grains in the rhyolitic rocks. In the present paper some preliminary results of field investigations in the Mien area are discussed, which in some respects do not support the meteoritic theory. Furthermore, these results establish that the peculiar rocks, described earlier as rhyolites, are not typical volcanic rocks. More detailed investigations, especially of a geophysical and petrographical character, are necessary, however, to draw a final conclusion about the origin of the Mien structure.

Uranium Mineralizations and age Relationships in the Precambrian Bedrock of Central and Southeastern Sweden

Abstract This paper summarizes studies on the genesis of uranium ores in the Precambrian bedrock of Central and southeastern Sweden. The occurrence of uranium in the mineralizations described was detected during the boom period 1950–1960. None of these occurrences is of economic value. Several different paragenetic types occur, the most common probably being the pitchblende-bearing vein fillings and disseminations of uraninite in iron ores of Central Sweden. The well known Co-Ni-Bi-U paragenesis also occurs, as well as iron deposits possibly being mechanical accumulations and containing uraninite and monazite supposedly of detrital origin. Alteration of uranium minerals is light although a thin coating of secondary uranium minerals may occasionally be observed. More frequent is the formation of thucholite and radiogalena. To correlate the uranium mineralizations with major Precambrian events, minerals and rocks with stratigraphic key positions have been dated by the U/Pb and Rb/Sr whole rock method. As a result the age relationships in the Precambrian bedrock of Central and southeastern Sweden are also discussed.

Further Age Measurements on Radioactive Minerals from Sweden

Abstract Age determinations utilizing the U/Pb method have been carried out on radioactive minerals in the Precambrian bedrock of Sweden. As a result, minerals from northern Sweden have been found to have an age of 1775–1845 m.y. but a more precise result could not be obtained. The pegmatites in the Ostersund—Sundsvall area and the pegmatites and aplites in southeastern Sweden are 1380 and 1745 m.y. old respectively. A pegmatite associated with the Karlshamn granite and a granite on Romeleasen each have ages of 1455 m.y. Some previously determined K/Ar ages are discussed and it is concluded that the uplift of the gneiss block in southwestern Sweden occurred during or after a period of igneous activity in western Sweden 935–1000 m.y. ago. The age data also indicate a contemporaneity between the deposition of the Dal formation in western Sweden and the Jotnian sandstones, the former being of marine lithology, the latter representing a terrestrial facies.

The Occurrence of Asphaltite and Thucholite in the Precambrian Bedrock of Sweden

Abstract Carbonaceous matter has frequently been formed in most parts of the Precambrian bedrock of Sweden. Preferentially it is deposited in fracture zones or breccias. Occasionally the organic matter is associated with uranium or thorium and is then called thucholite. In chemical and physical respects the thucholite is different from the rest of the organic material which is called asphaltite. For both types infrared studies, carbon and hydrogen analyses, carbon-13 and carbon-14 measurements have been performed, also two age measurements by the U/Pb method. The result of the work shows that the carbonaceous matter has been transported by groundwater streaming in fractures in the Precambrian bedrock and deposited down to a depth of several hundred metres. This organic material had a composition which varied within certain limits but is essentially represented by asphaltite. The carbonaceous matter is of plant or animal origin and consists of many different organic compounds. The deposition took place dur...