Precambrian Crystalline Bedrock Research Articles

Contribution of bedrock structures to the bedrock surface topography and groundwater flow systems within deep glaciofluvial aquifers in Kurikka, Western Finland

AbstractOne of the key challenges related to glaciofluvial aquifers is understanding how the underlying bedrock structures and the associated bedrock surface topography contributed to the deposition of the glaciofluvial sediments and the generation of groundwater flow pathways. In this study from Western Finland, we present a new digital elevation model of the Precambrian crystalline bedrock surface buried up to 100 m beneath the glaciofluvial sediments along the Kurikka depression. We link bedrock topography to structural anisotropy using additional data from rock outcrops, boreholes and gravity data. Brittle deformation zones are seen in the bedrock‐digital elevation model (DEM) as sharp breaks in the rock surface. These vertical fracture zones contributed to the development of interlinked elongate bedrock depressions and delimit rock blocks with different elevations. A narrow, zigzagging trench, with a stepped floor, follows WNW‐ESE and NE‐SW oriented fracture zones and forms a major hydraulic connection between two major parts of the aquifer. Furthermore, the conductive fracture zones are conduits which connect the shallow glacifluvial aquifer system to deep groundwater in the bedrock. Understanding bedrock structures and buried topography is critical for successful groundwater modelling in crystalline, fractured bedrock.

Hydrogeochemical mechanisms governing the mineralization and elevated fluoride $$\left( {\text{F}^{ - } } \right)$$ F - contents in Precambrian crystalline aquifer groundwater in central Benin, Western Africa

In the central part of Benin (Western Africa), high fluoride $$\left( {{\text{F}}^{ - } } \right)$$ contents have been reported in groundwater from Precambrian crystalline bedrock aquifer which is the main source of drinking water. The hydrogeochemical mechanisms leading to such elevated fluoride concentrations are usually not fully understood. In this context, the objective is to identify the hydrogeochemical processes governing groundwater mineralization and the origin of the high fluoride concentrations. A dataset of 162 groundwater samples was collected from the aquifer consisting of a thin altered bedrock layer (shallow aquifer) and a deep fractured crystalline bedrock (deep aquifer). Geochemical approaches and multivariate statistics have been used to explore the data. Fluoride concentrations vary between 0.00 and 7.19 mg/L in groundwater. Samples collected in the southern part of the investigated area, close to Dassa-Zoume, show the highest concentrations, with more than 75% greater than the guideline value of 1.5 mg/L. The deep fractured aquifer shows higher concentrations of fluoride than the shallow regolith reservoir. Results reveal that groundwater mineralization is derived mainly from the hydrolysis of silicate minerals, but it is also influenced by anthropogenic effects, particularly in the shallow reservoir. However, fluoride has a geogenic origin, primarily from the weathering of silicate minerals, primarily biotite. $${\text{Ca}}^{2 + }/{\text{Na}}^{ + }$$ cation exchanges and $${\text{F}}^{ - }/{\text{OH}}^{ - }$$ anion exchanges between groundwater and the rock matrix also occur as minor processes. Saturation of the water with respect to calcite and the precipitation of this mineral, which reduces calcium activity, also favor the release of fluoride from rocks.

Noble gas residence times of saline waters within crystalline bedrock, Outokumpu Deep Drill Hole, Finland

Noble gas residence times of saline groundwaters from the 2516m deep Outokumpu Deep Drill Hole, located within the Precambrian crystalline bedrock of the Fennoscandian Shield in Finland, are presented. The accumulation of radiogenic (4He, 40Ar) and nucleogenic (21Ne) noble gas isotopes in situ together with the effects of diffusion are considered. Fluid samples were collected from depths between 180 and 2480m below surface, allowing us to compare the modelled values with the measured concentrations along a vertical depth profile. The results show that while the concentrations in the upper part are likely affected by diffusion, there is no indication of diffusive loss at or below 500m depth. Furthermore, no mantle derived gases were found unequivocally. Previous studies have shown that distinct vertical variation occurs both in geochemistry and microbial community structuring along the drill hole, indicating stagnant waters with no significant exchange of fluids between different fracture systems or with surface waters. Therefore in situ accumulation is the most plausible model for the determination of noble gas residence times. The results show that the saline groundwaters in Outokumpu are remarkably old, with most of the samples indicating residence times between ∼20 and 50Ma. Although being first order approximations, the ages of the fluids clearly indicate that their formation must predate more recent events, such as Quaternary glaciations. Isolation within the crust since the Eocene–Miocene epochs has also direct implications to the deep biosphere found at Outokumpu. These ecosystems must have been isolated for a long time and thus very likely rely on energy and carbon sources such as H2 and CO2 from groundwater and adjacent bedrock rather than from the ground surface.

The 9 February 2010 Siheung, Korea, Earthquake Sequence: Repeating Earthquakes in a Stable Continental Region

The 9 February 2010 M w 3.1 Siheung earthquake occurred close to the metropolitan Seoul City in the central Korean peninsula and was widely felt by residents in Seoul. The shock occurred at a depth of 12 km in Precambrian crystalline bedrock with predominantly strike‐slip faulting on a near vertical fault plane striking east‐southeast–west‐northwest. A rupture radius of ∼0.12±0.01 km is estimated for the M w 3.1 shock, with a stress drop of 15.2 MPa. Sixteen small repeating earthquakes, spread in time up to about five and a half years, are detected based on their waveform similarities in a stable continental region setting. These shocks cluster along a trend striking ∼110°, which suggests a nodal plane of the mainshock focal mechanism striking N109° is the likely fault plane. The repeating events around Siheung, Korea, show a recurrence interval of 0.77±0.37 year/event for shocks of magnitude range M w 0.6–3.1. The waveform cross‐correlation detector provided identification of small repeating earthquakes with similar waveforms on an expanded time window, which may suggest the existence of such repeating earthquakes in stable regions worldwide with relatively low seismicity. Such data may lead to improvements in delineating seismogenic features, understanding the earthquake source properties, and evaluating the earthquake hazards associated with infrequent occurrence of such earthquakes in stable regions. Online Material: Figures showing locations from HYPOINVERSE single event location and double‐difference methods, and source time functions determined using empirical Green’s function analysis.

Hydrofracturing water boreholes in hard rock aquifers in Scotland

Hydrofracturing of new public water supply boreholes in Precambrian crystalline bedrock in Scotland has increased borehole yields by at least one order of magnitude, and made the difference between borehole abandonment and success. In many upland rural areas of the UK, low-productivity aquifers are an important resource for small public water supplies. Where a borehole in low-productivity crystalline rocks proves too low yielding for its designed purpose, hydrofracturing is a cost-effective means of enhancing yield.

Palaeoseismicity and De Geer Moraines

Abstract De Geer moraines are small moraine ridges formed partly in crevasses at the base of glacier ice. There may be several different origins of the crevasses. One possibility, proposed by Gerard De Geer who first studied these ridges, was seismic activity in connection with the isostatic rebound during the deglaciation. The present study shows a close connection between occurrences of De Geer moraines in Fennoscandia and seismic activity or fresh-looking fault lines and other features indicating neotectonic movements. This supports De Geer's theory but does not exclude formation in crevasses formed in other ways. There is no real proof that the tectonic movements took place in the Quaternary but the close connection between the moraines and tectonic features is evidence pointing in this direction. If this is correct it has important implications for the stability within cratons of Precambrian crystalline bedrock.

The significance of pre-existing, deeply weathered crystalline rock in interpreting the effects of glaciation in the Minnesota River valley, U.S.A.

AbstractMinnesota is largely underlain by Precambrian crystalline bedrock that was weathered to an average depth of 30 m prior to Late Cretaceous time. The fresh-rock— weathered-rock interface is irregular, with as much as 45 m of relief. Weathering exploited joints, locally isolating meter-sized volumes of rock known as corestones. Variable amounts of residuum were removed through glaciation to leave (1) saprolite overlain by an in-situ Late Cretaceous soil profile; (2) partially eroded saprolite; and (3) undulating fresh rock surfaces (commonly mantled by rounded boulders) that display striae and glacial or fluvial polish.Significant subglacial erosion of fresh bedrock is not required to form smoothly undulating bedrock surfaces with closed depressions; they may also form through removal of weathered bedrock and exposure of the weathering front. Large rounded boulders are not always shaped during transport; they may represent chemically rounded corestones resting at or near the bedrock source.Unambiguous evidence for glacial erosion includes striae and streamlining of bedrock parallel to striae. Polish on rock can be created fluvially, and smoothed grooves and ridges in the rock may be chemically produced. Many rounded boulders found in glacial till and strewn on bedrock surfaces probably originated as corestones.

Radiogenic isotopes of the Estonian and Latvian rapakivi granite suites: new data from the concealed Precambrian of the East European Craton

The Precambrian crystalline bedrock of the Baltic countries is covered under Phanerozoic sedimentary rocks that flank the Fennoscandian Shield in the south. The covered bedrock consists mainly of Palaeoproterozoic medium- to high-grade metamorphic rocks and unmetamorphosed rapakivi granites and related mafic rocks (mainly gabbros and anorthosites). Our UPb zircon data show that small rapakivi granite plutons in Estonia are 1630 Ma old and felsic and mafic rocks from the Riga batholith of Latvia and westernmost Estonia are 1580 Ma old. The Estonian plutons have ϵ Nd (1630 Ma) values ranging from −0.5 to −2.5. The mafic and felsic rocks of the Riga batholith have ϵ Nd (1580 Ma) values between +0.3 and −0.6, except for a pervasively altered silicic volcanic rock on the northern flank of the batholith with an ϵ Nd value of −4.6. Initial 87 Sr 86Sr ratios of the mafic rocks are of the order of 0.7036 to 0.7037 and conform to the evolution of average subcontinental mantle. The Pb isotopic compositions of the felsic and mafic rocks (including the low- ϵ Nd prophyry) are relatively radiogenic with single-stage μ-values of the order of 8.2. The isotopic characteristics of the Estonian and Latvian rapakivi granites are similar to those of the classic rapakivi granites of southern Finland. Our data suggest that the felsic rocks of the Estonian and Latvian rapakivi suites were derived from Palaeoproterozoic (near-chondritic Nd, relatively high-U Pb ) protoliths. The data also imply that the lower crust and upper mantle in this area are devoid of a major Archaean component and that the lithosphere may become more juvenile southward from the Fennoscandian Shield.

The pre‐impact Ordovician Stratigraphy of the tvären bay impact structure, se Sweden

Abstract The Middle Ordovician impact crater in the Tvaren Bay south of Stockholm has a well preserved sequence of sedimentary rocks in an area of Sweden where the Lower Palaeozoic sedimentary rock cover has been eroded away. The crater was formed in the Late Kukrusean by a cosmic body that hit a marine environment The resurge breccia deposited shortly after the impact consists mostly of limestone fragments, but also of fragments from the Precambrian crystalline bedrock. Limestone clasts in the lower part of the breccia were studied by means of visual appearance and microfossil content, and by using conodonts for age determination correlations were made between the pre‐impact limestones of Tvaren and limestones in other parts of Sweden and Estonia. The conodonts range from the Volkhovian Microzarkodina parva Zone to the Upper Kukrusean Prioniodus variabilis Zone. The majority of the species are Llanvirnian‐Llandeilian. The findings show (1) that although the pre‐Volkhovian Lower Ordovician is poorly devel...

Methane venting as a possible mechanism for glacial plucking and fragmentation of Precambrian crystalline bedrock

Abstract A number of processes have been suggested by glaciologists and Quaternary geologists to explain the type of glacial erosion named plucking. These presume that hydraulic jacking, frost-induced wedging or temperature variations in one way or another have loosened joint blocks from the bedrock surface and evacuated and entrained these in the ice. Large mounds of huge angular boulders and caves therein, have been ascribed to splitting by sudden stress release related to neotectonic activity in postglacial time. This paper presents a hypothesis which relates glacial plucking and split bedrock surfaces to violent venting of methane released during isostatic rebound from methane hydrate formed under permafrost and continental ice.

Occurrence of acidic groundwater in precambrian crystalline bedrock aquifers, Southwestern Sweden

The pH and alkalinity of groundwater from 7651 wells drilled in the Precambrian crystalline bedrock of southwestern Sweden has been evaluated. The wells are generally less than 100 m deep. Analytical results were collected from different laboratories and authorities in the region. In areas with thin soil cover or coarse-grained deposits overlying the bedrock, alkalinity is normally less than 100 mg HCO3 L−1. Below the marine limit, where clayey sediments predominate, alkalinity sometimes even exceeds 200 mg HCO3 L−1. When comparing pH and alkalinity of groundwaters from Quaternary deposits with bedrock groundwaters, the latter always have higher pH and alkalinity values. The most acidic bedrock groundwaters are found in small areas close to the city of Goteborg due to additional factors of high acid loadings, high groundwater discharge and thin soil layers.

Influence of the quality of fine fractions on engineering-geological properties of crushed aggregate

Fine fractions of aggregate crushed from Finnish Precambrian crystalline bedrock were studied by determinining the particle size distribution with a Sedigraph particle size analyzer, specific surface area using the nitrogen adsorption method and porosity with a mercury porosimeter. According to abrasion resistance (Los Angeles) and brittleness (Swedish impact) values, all the materials studied were suitable as road aggregate. Roads where material with high specific surface area were used however soon proved to suffer from frost damage. No correlation between specific surface area and values of abrasion and brittleness were found. The specific surface area also proved to depend on the quality rather than the quantity of the fine fractions. Presence of clay minerals tends to increase the sp.s.a. Thus, in addition to the quantity also the quality of fine fractions must be analysed in road aggregate material.

A reservoir analysis of the Denver earthquakes: A case of induced seismicity

Injection of fluid wastes into the fractured Precambrian crystalline bedrock beneath the Rocky Mountain Arsenal near Denver triggered earthquakes in the 1960's. An analysis, based on the assumption that fluid flow in the fractured reservoir can be approximated by flow in a porous medium, is presented. The configuration and hydrologic properties of the reservoir are determined from two lines of evidence: (1) locations of earthquake hypocenters determined by seismic arrays installed at the Arsenal and (2) observed long‐term decline in fluid levels in the injection well. Together these two sets of data indicate that a long, narrow reservoir, aligned in the direction N 60°W, exists. The reservoir is 3.35 km in width, extends 30.5 km to the northwest and infinitely to the southeast, and spans a depth interval from 3.7 to 7.0 km below land surface. It has a transmissivity of 1.08 × 10−5 m2/s and a storage coefficient of 1.0 × 10−5. Computed pressure buildup along the length of the reservoir is compared with the spatial distribution of earthquake epicenters. The comparison shows that earthquakes are confined to that part of the reservoir where the pressure buildup exceeds 32 bars. This critical value is interpreted as the pressure buildup above which earthquakes occur. The migration of earthquake epicenters away from the injection well, a phenomenon noted by previous investigators, can be accounted for by the outward propagation of the critical pressure buildup. The analysis is extended to examining the effects of rapid flow in fractures opened by high injection pressure. The results show that the effect is confined to a small region within 1 km of the injection well. The existence of a critical pressure buildup above which earthquakes occur is completely consistent with the theory on the role of fluid pressure in fault movement as presented by Hubbert and Rubey.

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.