Phanerozoic Sedimentary Rocks Research Articles

Space/ground-borne techniques and petrographic microscopic dissection for geologic mapping in Gabal Ras Abda area, Northeastern Desert, Egypt

BackgroundGabal Ras Abda area as a part of the Red Sea Mountain range, is characterized by inaccessible and rugged terrains. The exposed rock units are hardly followed in the field because of the rigid topography. Thus, the present work proposes and develops an integrated approach to map the exposed rock units and extract the geologic structures using satellite imagery data followed by both field and petrographic verification, saving time, efforts and cost.ResultsTo achieve the target, both the measured spectral signature curves with Landsat-8 and Sentinel-2A data were used to develop and create the most enhanced Band Ratios and Principal Components for lithological discrimination and mapping which were (((Band7 + Band 4)/(Band 7), (Band 2)/(Band 2 + Band 5) and (Band 5) in RGB) and ((Band 9 + Band 11 + Band 12)/(Band 1), (Band 4 − Band 2) and (Band 11/Band 6) + (Band 6) in RGB)) with Principal Component Bands ((PC1, PC2 and PC3 in RGB) and (PC3, PC2 and PC1 in RGB)), respectively. Also, georeferenced Google Earth Pro, panchromatic band of Landsat-8 and ALOS PALSAR Digital Elevation Model images were used to extract the structural lineaments. Geologic, petrographic and field structural studies were emphasized the remote sensing results, indicating that the main rock types cropped out in Ras Abda area from the oldest to the youngest are older granitoids (quartz-diorites, tonalites and granodiorites), Dokhan volcanics (andesites, rhyodacites, rhyolites and their related tuffs), younger gabbros, younger granites (monzogranites, syenogranites and alkali-feldspar granites), post-granite dykes and offshoots (acidic, microgranitic and basic types) and Phanerozoic sedimentary rocks. Also, the study emphasized that the E-W trend is the main structural trend controlling the investigated area followed by WNW-ESE and NE-SW directions.ConclusionsThe results of remote sensing achieved compliance with the geologic, petrographic and structural investigation through distinctly differentiating the different rocks and extracting the lineaments, indicating the accuracy of the remote sensing results and emphasizing their importance and effective role in producing a precise and highly accurate geologic map.Graphical abstract

The Meter Reader: Early magnetic studies in the Lake Superior region

In the mid-19th century, the quality of life in the United States was improving rapidly with widespread electrification of the nation and growth in the industrial revolution. These advances were dependent on the increasing availability of copper for instruments and electric wires and iron ore for the production of iron and steel. Accordingly, reconnaissance geologic investigations in the Lake Superior region of North America that discovered copper and iron-rich rocks in the 1840s rapidly attracted the attention of prospectors and mining companies. However, ore deposits were a challenge to find in this region because of superimposed geologic events and limited rock exposures due to the widespread cover of unconsolidated sediment deposited from Pleistocene glaciation, the abundant lakes, and, locally, the cover of Phanerozoic sedimentary rocks. Limitations to surface geologic mapping were evidenced by the few outcrops of iron formation in the iron ranges and the even fewer ore exposures. As a result, geologic mapping was restricted to locating regions for constructing test shafts and pits and drilling that would lead to identifying favorable mining sites.

Thermal history of the Peel Plateau, northern Canadian Cordillera, resolved through multikinetic apatite fission-track dating

The Peel Plateau of the northern Canadian Cordillera has undergone multiple burial and unroofing events throughout the Phanerozoic, including subsidence and uplift associated with emergence of the Mackenzie Mountains. Episodic unroofing events are evinced by unconformities that represent a period where an unknown amount of material was deposited and eroded. Thermochronological studies have been conducted in neighboring physiographic regions, however no work has been conducted in the Peel Plateau and there is interest to determine whether it shares a similar thermal evolution as the surrounding regions. Apatite fission-track thermochronology (AFT) is a useful method for reconstructing basinal thermal histories as the system is sensitive to upper crustal temperatures, although the kinetics of the thermochronometer are complicated by numerous elemental substitutions within apatite that are known to control the specific temperature window within which fission-tracks anneal. This study uses three strategically positioned AFT samples from Phanerozoic sedimentary rocks to investigate the thermal history of the region from the Devonian to the present. Pooled AFT ages from all samples fail the χ2 test, implying there is a high level of intra-sample age dispersion and potentially indicating the samples contain multiple age populations. To assess whether the data are consistent with multiple kinetic AFT populations, we use grain-specific apatite compositions to derive rmro, a kinetic parameter that accounts for elemental substitutions known to control annealing kinetics of fission-tracks. By plotting AFT ages and track lengths against rmro we identify more-retentive and less-retentive apatite populations within each sample, which serve as independent thermochronometers when incorporated into inverse thermal history modelling. Results from a thermal history model of the Upper Devonian Imperial Formation indicate that the uppermost Paleozoic strata in the Peel Plateau reached maximum burial temperatures (∼165°C–185 °C) in the Triassic and were reheated to lower temperatures (∼90°C–100 °C) under a Cretaceous-Paleocene foreland basin. The temperature regime of the latter burial event is supported by thermal history models of two Cretaceous samples that record peak burial temperatures (∼80°C–90 °C) in the Paleocene to Eocene. This study highlights the capabilities of thermal history modelling from multikinetic AFT data and serves as a reminder that addressing variable annealing kinetics of apatite grains can improve the resolution of thermal models and the associated geologic interpretations.

Mapping the basement architecture using magnetotelluric data across a coastal part of the Borborema structural province, Ceará – Brazil

The Borborema Province (BP) constitutes part of the west Gondwana supercontinent formed by several tectonic collisions during Neoproterozoic, widely known as Brasiliano - Pan-African orogeny. It consists of a complex tectonic mosaic composed mainly by Archean to Paleoproterozoic migmatite-gneiss basement complexes, Proterozoic metasedimentary belts, a large volume of Neoproterozoic granitic intrusions, and Phanerozoic covers that represent ca. 25 % of the total area of BP. This work presents the results from 21 Magnetotelluric (MT), and electromagnetic time-domain (TDEM) soundings interpretations carried out along a 120 km profile in a coastal area of the northeast Borborema Province covered by Phanerozoic sedimentary rocks and sediments. The final geoelectric model allows delineating the architectural geometry of the subsurface and variations in physical properties of different crustal blocks bounded by major electrical discontinuities. The interpretation was based on a 2D and 3D model from the inversion of the MT data. The 2D model has a visually satisfactory agreement with the region's geological framework and a reasonable resolution of the upper crust compared to the 3D model. The geoelectric section allows delineating the variation in physical properties of different upper crust blocks bounded by major electrical discontinuities. The result enables the characterization of 1) the uppermost structures correlated to the Mesozoic to mid-Cenozoic sedimentary formations; 2) covered lithotectonic framework represented by lithotypes and main shear zones related to Ceará Central and Rio Grande do Norte crustal tectonic domains; 3) distinctive anomalies corresponding to the Senador Pompeu, Orós, and Jaguaribe Neoproterozoic main shear zones. Our interpretations agree with the results and interpretation presented by previous MT works in the Borborema Province, which reveals the existence of an important conductive zone, interpreted as a suture zone, spatially related to the Senador Pompeu shear zone and in smaller proportion to the Orós shear zone. The results presented here are a starting point to further MT long-period investigation to understand this complex part of the Borborema Province.

Integration of remote-sensing data for mapping lithological and structural features in the Esh El-Mallaha area, west Gulf of Suez, Egypt

Esh El-Mallaha west of the Gulf of Suez is a potential area for hydrocarbon resources. In this article, Landsat-8 Operational Land Imager (OLI), Shuttle Radar Topography Mission (SRTM), and Advanced Land Observing Satellite (ALOS) Phased Array Type L‐band Synthetic Aperture Radar (PALSAR) data were utilized to identify the geological and structural features of this area which represents a part of the North Eastern Desert (NED), Egypt. The use of band ratios (BR) and principal component analysis (PCA) allowed characterizing the different varieties of rock units. These approaches allowed discriminating the various rock units e.g., metavolcanics, granitoids including the two phases of the older granites and younger granites along with the third phase which represented by elongated ridges cut the rocks and clearly allowed tracing the contacts of different lithological units of the Neoproterozoic basement and Phanerozoic sedimentary rocks. Applying the fused Landsat-8 OLI and ALOS/PALSAR data improved the structural elements that difficult to reveal by optical images. Combination of Landsat-8 OLI, ALOS/PALSAR, and SRTM data effectively display the structural and lithological contacts. The lineament extraction and interpretation indicated that the area is clearly shaped by the NW-SE graben and controlled by the Gulf of Suez tectonic trend. Such structures allowed the accumulation of thick sediments in the faulted basin that represents the promise area for oil, gas, and water resources. Lineament analysis revealed that the major tectonic trends controlling the area are NW-SE and NE-SW directions with less effect of N-S and E-W trends. The NW-SE sinistral and NE-SW dextral lateral movements are recognized. Overall results revealed that the integration of optical and radar remotely sensed data is an effective tool in deciphering geological and structural features in the arid regions.

Diachronous development of Great Unconformities before Neoproterozoic Snowball Earth

The Great Unconformity marks a major gap in the continental geological record, separating Precambrian basement from Phanerozoic sedimentary rocks. However, the timing, magnitude, spatial heterogeneity, and causes of the erosional event(s) and/or depositional hiatus that lead to its development are unknown. We present field relationships from the 1.07-Ga Pikes Peak batholith in Colorado that constrain the position of Cryogenian and Cambrian paleosurfaces below the Great Unconformity. Tavakaiv sandstone injectites with an age of ≥676 ± 26 Ma cut Pikes Peak granite. Injection of quartzose sediment in bulbous bodies indicates near-surface conditions during emplacement. Fractured, weathered wall rock around Tavakaiv bodies and intensely altered basement fragments within unweathered injectites imply still earlier regolith development. These observations provide evidence that the granite was exhumed and resided at the surface prior to sand injection, likely before the 717-Ma Sturtian glaciation for the climate appropriate for regolith formation over an extensive region of the paleolandscape. The 510-Ma Sawatch sandstone directly overlies Tavakaiv-injected Pikes granite and drapes over core stones in Pikes regolith, consistent with limited erosion between 717 and 510 Ma. Zircon (U-Th)/He dates for basement below the Great Unconformity are 975 to 46 Ma and are consistent with exhumation by 717 Ma. Our results provide evidence that most erosion below the Great Unconformity in Colorado occurred before the first Neoproterozoic Snowball Earth and therefore cannot be a product of glacial erosion. We propose that multiple Great Unconformities developed diachronously and represent regional tectonic features rather than a synchronous global phenomenon.

Factors driving mercury variability and background values in a tropical region: The case of western Amazonia

This study provides additional information about mercury sources and their distribution in the Amazônia regolith using a large dataset of samples collected from distinct geology, mineralogy, and tropical weathering effects. Furthermore, the Hg distribution allowed discussing the superficial dynamics and controversial question on the Hg natural or anthropogenic source. The dataset composed of 341 samples, allowed assigning Hg background values for western Amazônia of 305, 125 and 132 ppb, respectively, for lateritic duricrust and oxisol developed on Archean-Proterozoic Amazon Craton and Phanerozoic sedimentary rocks and for sediment along the Solimões and Madeira rivers compartments. The data show that natural weathering, leading to the formation of Fe/Al lateritic duricrusts and oxisols, promoted a very high Hg enrichment (up to 800%). Considering this feature, although Hg is naturally accumulated by weathering processes relative to rocks, the large enrichment may be potentially harmful and confirms studies that pointed out the key role of weathering as an efficient scavenger for metals like Hg in Amazônia. Moreover, it highlights the importance of tropical regions as a natural sink that should be better considered in the Hg cycle and as a possible natural Hg source.

Mineral and geochemical composition of the Onega Ice Lake sediments

An analysis of the mineral-geochemical composition and structure of the Holocene–Pleistocene bottom sediments was performed on Lake Polevskoye, a small lake in the northern Lake Onega area in Russian Karelia, which is considered representative to describe the Late Weichselian Onega Ice Lake sediments. The analysis was accomplished using modern analytical methods, including scanning electron microscopy and ICP-MS, which allowed us to interpret their genesis in a new light. It is assumed that the distribution of rare earth elements (REE) in the bottom sediments of the recent Lake Onega and in the sediments of Onega Ice Lake will improve our understanding of the history of sedimentogenesis. It is apparent in the distribution of REE, their composition and data on the geochemical and mineral composition of the Holocene bottom sediments of Lake Onega and Lake Polevskoye (varved clays) that in their composition of terrigenous matter the material introduced from the north-western part of the catchment area is mainly composed of Archaean and Early Proterozoic crystalline complexes. However, the values of several indicator ratios of elements in the lower part of the of varved clays with shungite interlayers indicate the presence of mixing of clastic material from two sources of different geochemical origin: the north-western part of the catchment area (source of shungite rocks) and the south-eastern part of the catchment area (Phanerozoic sedimentary rocks of the Russian Platform).

NEW RESULTS OF POLYMETALLIC, PGE AND REE MINERALIZATION RESEARCH IN THE SUWAŁKI ANORTHOSITE MASSIF (NE POLAND)

Suwałki Anortosite Massif (SAM) occurs in the crystalline basement of NE Poland within 200 km of the magmatic, Mesoproterozoic AMCG (anorthosite–mangerite–charnockite–granite) rock suite terrane called the Mazury Complex. SAM was discovered as a result of the drilling research of the prominent negative magnetic and gravimetric anomalies. There is an extensive negative anomaly of both potential fields related to the anorthosite massif. Gravimetric anomaly is surrounded by the bands of positive anomalies caused by rocks with elevated densities, such as granitoids, monzondiorites and granodiorites. A negative magnetic anomaly is surrounded by the bands of positive anomalies with significant amplitudes, particularly strongly marked from the south, west and north. Positive magnetic anomalies are associated with the presence of rocks with proven strong magnetic susceptibility due to the content of ferrolites (ilmenite-magnetite rocks) with accompanying Fe-Cu-Ni-Co sulphide mineralization. Fe-Ti-(V) ore deposits in the SAM were discovered in the early 1960s, in the region of Krzemianka and Udryn, but also Jeleniewo and Jezioro Okrągłe, under a thick overburden of Phanerozoic sedimentary rocks within small positive magnetic anomalies. These deposits were documented in about 100 deep boreholes to a depth of 2300 m, and the resources in C1 + C2 category were estimated for about 1.5 billion tons of titanium-magnetite ores with vanadium, mainly in the Krzemianka and Udryn ore fields. The model age obtained by the Re-Os NTIMS method for Fe-Ti-V ores and sulphides from the Krzemianka and Jezioro Okrągłe ore deposits was 1559 ±37 Ma with an initial ratio of 187Os/188Os = 1.16 ±0.06. This age was recognized as the age of the entire Suwałki Massif. Despite many years of research, the deep structure and the form of the massif has not been fully recognized. At present, geophysical and geological 3D modelling of borehole data is carried out using the OasisMontaj (Geosoft) software package. The 3D model is generated in the GeoModeller 3D application (Intrepid Geophysics) in order to recognize the geological correctness and interpretation of magnetic-gravity anomalies of the whole massif and its cover.

Step-like growth of the continental crust in South China: evidence from detrital zircons in Yangtze River sediments

Detrital zircons from nine river sand samples from the upper, middle and lower streams of the Yangtze River (the world's third largest river) and its two largest tributaries, the Han and Jialing rivers have been analyzed for U–Pb–Lu–Hf–O isotope compositions. Zircons from the upper Yangtze River cluster in age groups at 0–100, 200–300, 400–500, 700–1000, 1800–1900 and 2300–2500 Ma, with a marked peak at 41 Ma diagnostic of magmatism on the Tibetan Plateau. Zircons from the middle and lower Yangtze River and its tributaries exhibit broadly similar age groups at 100–300, 400–500, 700–900, 1800–2000 and 2300–2700 Ma, except for the lack of Cenozoic ages.The Yangtze River catchment is covered by thick Phanerozoic sedimentary rocks and so Archean-aged zircons are likely to be under-represented in modern river sands. We therefore applied the approach of Dhuime et al. (2012) to calculate a crustal growth curve for South China, and the resultant curve has two inflections. The older inflection at ~2.8 Ga is slightly younger than that of Dhuime et al. (2012), suggesting the onset of plate tectonics in the Yangtze Craton. The younger inflection at ~1.8 Ga appears to mark the onset of another period of relatively high crustal growth rate in South China. This period of increased crustal growth rate in the Mesoproterozoic is coeval with the breakup of the supercontinent Columbia, suggesting that the Mesoproterozoic crustal growth in South China may be related to supercontinent dispersal.Comparison with global crustal growth curves based on detrital zircons highlights the distinct step-like pattern of crustal growth in South China. This emphasizes the potential of comparing regional with global crustal growth curves, and the changes in the rates of crustal growth in South China appear to have been controlled by changes in regional geodynamics. The maximum δ18O values in the zircons analyzed increase markedly at the end of the Archaean, and these increases therefore accompany the estimated increases in atmospheric oxygen at that time. It is therefore suggested the atmospheric O2 levels were associated with increased crustal weathering, the development of more elevated δ18O sediments and their increasing incorporation in the generation of crustally derived magmas from ca. 2.5 Ga.

Weathering processes, catchment geology and river management impacts on radiogenic (87Sr/86Sr) and stable (δ88/86Sr) strontium isotope compositions of Canadian boreal rivers

Radiogenic (87Sr/86Sr) and stable (δ88/86Sr) strontium isotope compositions spanning a calendar year are reported for rivers from across subarctic Canada that drain contrasting lithologies ranging from Precambrian bedrock (Koksoak, Great Whale and La Grande rivers of Northern Quebec) to carbonate and clastic Phanerozoic sedimentary rocks of the Western Interior Platform (Nelson River, of central and western Canada). The 87Sr/86Sr isotopic compositions of the river waters reflect the underlying geology, with rivers draining the Precambrian Shield having higher 87Sr/86Sr ratios (0.727–0.734) than the Phanerozoic dominated Nelson River (0.713). The stable strontium isotope values (δ88/86Sr) range from 0.26 to 0.39‰, with the values for the Nelson River overlapping those of the other three. Rivers that have not been developed for hydroelectric power show a seasonal variation in the 87Sr/86Sr ratios, whereas those that have been diverted or dammed show little or no seasonal variation due to increased residence time of their water in hydroelectric reservoirs. The three rivers from Northern Quebec show discrete ranges in their 87Sr/86Sr and δ88/86Sr isotope compositions that correlate with the Ca/Sr and Na/Sr ratios of the river water. These correlations are interpreted to reflect differential chemical weathering of felsic versus mafic source rocks and/or of surficial sediment vs bedrock sources.

The initiation and evolution of the River Nile

The Nile is generally regarded as the longest river in the world. Knowledge of the timing of the Nile's initiation as a major river is important to a number of research questions. For example, the timing of the river's establishment as a catchment of continental proportions can be used to document surface uplift of its Ethiopian upland drainage, with implications for constraining rift tectonics. Furthermore, the time of major freshwater input to the Mediterranean is considered to be an important factor in the development of sapropels. Yet the river's initiation as a major drainage is currently constrained no more precisely than Eocene to Pleistocene.Within the modern Nile catchment, voluminous Cenozoic Continental Flood Basalts (CFBs) are unique to the Ethiopian Highlands; thus first detection of their presence in the Nile delta record indicates establishment of the river's drainage at continental proportions at that time. We present the first detailed multiproxy provenance study of Oligocene–Recent Nile delta cone sediments. We demonstrate the presence of Ethiopian CFB detritus in the Nile delta from the start of our studied record (c. 31 Ma) by (1) documenting the presence of zircons with U–Pb ages unique, within the Nile catchment, to the Ethiopian CFBs and (2) using Sr–Nd data to construct a mixing model which indicates a contribution from the CFBs. We thereby show that the Nile river was established as a river of continental proportions by Oligocene times. We use petrography and heavy mineral data to show that previous petrographic provenance studies which proposed a Pleistocene age for first arrival of Ethiopian CFBs in the Nile delta did not take into account the strong diagenetic influence on the samples.We use a range of techniques to show that sediments were derived from Phanerozoic sedimentary rocks that blanket North Africa, Arabian–Nubian Shield basement terranes, and Ethiopian CFB's. We see no significant input from Archaean cratons supplied directly via the White Nile in any of our samples. Whilst there are subtle differences between our Nile delta samples from the Oligocene and Pliocene compared to those from the Miocene and Pleistocene, the overall stability of our signal throughout the delta record, and its similarity to the modern Nile signature, indicates no major change in the Nile's drainage from Oligocene to present day.

Biostratigraphical dating conundrums in the Cambrian and earlier stratigraphy of the Indian subcontinent

Dating rocks by using fossils remains one of the most important stratigraphic tools both for Phanerozoic sedimentary rocks and, increasingly, for older rocks too. Inevitably situations arise in which different types of data offer seemingly contradictory indications of age, of which several examples from Cambrian and earlier rocks of Indian subcontinent are discussed herein. These examples highlight the main kinds of biostratigraphical conundrums and their resolution, their role in moving stratigraphic geology forward, and also surprising ways in which they are misused. The growth of geological knowledge regionally and globally, along with the introduction of additional techniques for dating rock strata, means that the temporal range of the alternative explanations related to particular conundrums has tended to decline with time, although a controversy with alternatives over 1.0 Ga apart is currently active concerning Vindhyan geology. Although important in their own right, the solutions to this and other conundrums must integrate with other types of geological data if age determination is to be satisfactorily concluded and the wider geological and evolutionary implications of biostratigraphical dating are to be realised.

A detrital record of the Nile River and its catchment

This research uses analyses from Nile catchment rivers, wadis, dunes and bedrocks to constrain the geological history of NE Africa and document influences on the composition of sediment reaching the Nile delta. Our data show evolution of the North African crust, highlighting phases in the development of the Arabian–Nubian Shield and amalgamation of Gondwana in Neoproterozoic times. The Saharan Metacraton and Congo Craton in Uganda have a common history of crustal growth, with new crust formation at 3.0 – 3.5 Ga, and crustal melting at c. 2.7 Ga. The Hammamat Formation of the Arabian–Nubian Shield is locally derived and has a maximum depositional age of 635 Ma. By contrast, Phanerozoic sedimentary rocks are derived from more distant sources. The fine-grained (mud) bulk signature of the modern Nile is dominated by input from the Ethiopian Highlands, transported by the Blue Nile and Atbara rivers. Detrital zircons in the Nile trunk are predominantly derived from Phanerozoic cover rocks. Most detritus from the upstream White Nile is trapped in the Sudd marshes and contributes little to the Nile trunk. Therefore, the White Nile downstream is dominated by locally derived Phanerozoic cover. The White Nile proximal to the Gezira Fan is influenced by the fan's Blue Nile signature. Supplementary material : Sample information, analytical methods and data tables are available at https://doi.org/10.6084/m9.figshare.c.3569490

Specific features of the distribution of trace and rare earth elements in recent bottom sediments in the lower course of the Severnaya Dvina River and White Sea

The paper discusses results of the lithogeochemical examination of recent bottom sediments in the lower course of the Severnaya Dvina River and White Sea. It has been established that the average con� centration of several trace elements (Hf, Sc, Co, Y, Ni, V, Cr, Zr, Ba, and others) therein correlates with the content of the silt-pelite fraction. Maximal concentrations of the majority of above elements are confined to the silty-clayey sediments at the Basin/Dvina Bay boundary. They localized near the coastal zone only for some clastophile (Zr, Cr, and others). Typical values of the hydrolyzate module, chemical index of alteration, and Al2O3/SiO2 ratio in the aleuropelitic and pelitic sediments of the Severnaya Dvina River delta, Dvina Bay, and the Dvina Bay Basin boundary suggest that these sediments are confined to sufficiently cold climate set� tings. Data points of sediment composition in discriminant paleotectonic diagrams are scattered over a large field probably due to high contents of the weakly weathered plagioclases, micas, and amphiboles, as well as the hydrogenic process promoting the accumulation of Fe and Mn. The PAASnormalized spectra of rare earth elements (REE) in bottom sediments of the Pinega and Severnaya Dvina rivers, marginal filter of the latter river, Dvina Bay, and the Dvina Bay Basin boundary are similar to the REE distribution in clayey rocks of the ancient platform cover (except for a slight positive Eu anomaly). The REE systematics and distribution pattern of compositional data points of recent bottom sediments in the GdN/YbN-Eu/Eu* and Eu/Eu*- Cr/Th diagrams and values of several indicator ratios of trace elements suggest that the studied rocks were formed by the mixing of clastic materials from geochemically contrast provenances: northwestern provenance (Kola-Karelia geoblock), which is mostly composed of the Archean and Early Proterozoic crystalline com� plexes, and the southeastern provenance (northwestern periphery of the Mezen syncline), which is almost totally composed of Phanerozoic sedimentary rocks. The latter provenance likely played a crucial role in the geochemical signature of recent bottom sediments over a significant area of the White Sea.

The buried southern continuation of the Oaxaca-Juarez terrane boundary and Oaxaca Fault, southern Mexico: Magnetotelluric constraints

Thirty magnetotelluric soundings were made along two NW–SE profiles to the north and south of Oaxaca City in southern Mexico. The profiles crossed the N–S Oaxaca Fault and the Oaxaca-Juarez terrane boundary defined by the Juarez mylonitic complex. Dimensionality analysis of the MT data showed that the subsurface resistivity structure is 2D or 3D. The Oaxaca and correlative Guichicovi terranes consist of ca. 1–1.4 Ga granulitic continental crust overlain by Phanerozoic sedimentary rocks, characterized by high and low resistivities, respectively. The Juarez terrane consists of oceanic Mesozoic metavolcanic and metasedimentary rocks, characterized by a low to medium resistivity layer, that is approximately 10 km thick. The Oaxaca Fault is a Cenozoic aged, normal fault that reactivated the dextral and thrust Juarez mylonitic complex north of Oaxaca City: its location south of Oaxaca City is uncertain. In the southern profile, the MT data show a ca. 20–50 km wide, west-dipping, relatively low resistivity zone material that extends through the entire crust. This is inferred to be the Juarez terrane bounded on either side by the ca. 1–1.4 Ga granulites. The Oaxaca Fault is imaged only by a major electrical resistivity discontinuity (low to the west, high to the east) along both the western border of the Juarez mylonitic complex (northern profile) and the San Miguel de la Cal mountains (southern profile) suggesting continuity.

Metasedimentary rocks from the Bol’shoi Cheremshan Group in the East European Platform basement: Initial mineral composition and petrogeochemical properties

The initial normative mineral composition of some metasedimentary rocks (high-alumina crystalline schists, aluminous gneisses, and eulysites) from the Archean Bol’shoi Cheremshan Group is considered. Based on some data (distribution of data points available for these rocks in diagnostic diagrams, values of several indicator ratios, and initial rock composition), it is assumed that examined rocks were partly formed after highly mature sediments (high-alumina crystalline schists and gneisses), while biotite-garnet-andesine gneisses represent products of graywacke sediments. Elevated concentrations of some indicator elements (Fe, Cr, Ni, V, Zn, Pb, and others) imply that material in the sedimentation paleobasin was likely derived from provenances composed of variable lithologies (ultramafic, mafic, intermediate, and acid rocks). It seems that conditions in this paleobasin were favorable for the development of organic life. It is shown that correlation between some elements (Ti, V vs. Al; Ba, Pb vs. K; Sr vs. Ca; La vs. Na; and others) typical of Phanerozoic sedimentary rocks is also observed in the rocks. The possible contents of organic matter (Corg) and U in primary sediments of metasedimentary rocks of the Bol’shoi Cheremshan Group in the East European Platform basement are reconstructed.

Reply to discussion on ‘Multiple post-Caledonian exhumation episodes across NW Scotland revealed by apatite fission-track analysis'

We thank John Hudson for his welcome comments on the nature and extent of the now-eroded Mesozoic sedimentary cover across parts of the present-day Scottish landmass that is indicated by the apatite fission-track analysis (AFTA) results presented by Holford et al. (2010). As Hudson points out, low-temperature thermochronological techniques, including AFTA, can provide unique insights into the post-Palaeozoic geological evolution of basement terranes that lack significant Phanerozoic cover (as witnessed by studies including Green et al. 2000, 2001; Belton et al. 2004; Weber et al. 2005). As we demonstrated (Holford et al. 2010), these techniques become even more powerful when applied to basement terranes where some sedimentary cover is preserved. Whereas low-temperature thermochronological methods can provide information on only the cooling histories of basement rocks (often reflecting exhumation), the age of a sedimentary unit (however thin it may be) overlying eroded basement defines a time when the basement had been exhumed to the surface and was being buried. Thus, integration of low-temperature thermochronological data from basement rocks and overlying sedimentary units can define more complex thermal histories encompassing both heating and cooling (reflecting episodic burial and exhumation). Application of this approach to basement and cover samples from a number of regions where Phanerozoic sedimentary rocks are sparsely preserved, including NW Scotland, has revealed that some continental regions that have long been thought to …

Hf isotope study of Palaeozoic metaigneous rocks of La pampa province and implications for the occurrence of juvenile early Neoproterozoic (Tonian) magmatism in south-central Argentina

Abstract On a global scale, juvenile Tonian (Early Neoproterozoic) magmatic rocks are associated with the extensional events that lead to the breakup of the Rodinia supercontinent. In Argentina, no geological record is available for this time interval, lasting from 1000 to 850 Ma. We present indirect evidence for the existence of Tonian extension in Argentina, as supported by Hf and Nd isotope determinations on Phanerozoic magmatic and sedimentary rocks. We mainly focus on our own Hf isotope determinations carried out on U–Pb SHRIMP dated zircons from Palaeozoic metaigneous rocks of La Pampa province, south-central Argentina, i.e. metagabbros of Valle Daza, dioritic orthogneiss of Estancia Lote 8, and metadiorite of Estancia El Carancho, having found that these rocks were derived from sources of ca. 920 to ca 880 Ma, with eHf values between +6.83 and + 9.59. Inherited zircons of this age and character identified in these rocks also point to the same source. We also compile additional Hf and Nd studies from previous work on Phanerozoic magmatic and sedimentary rocks. We preliminarily compare the age of the juvenile Tonian sources referred to in our work with that of two extensional events identified in the Sao Francisco craton, Brazil.

Structural and kinematic assemblies in sedimentary rocks of the Phanerozoic cover of the Mid-Russian Dislocation Zone

The Mid-Russian Dislocation Zone is a large within-plate structural element of the East European Platform, which extends for more than 1100 km from the Timan Foredeep to the Orsha Basin. This deep, long-lived zone was formed against a background of changeable geodynamic settings, including (1) Late Paleoproterozoic collision events, (2) Late Riphean-Early Vendian epicontinental rifting, (3) Late Vendian-Early Triassic intraplatform tectogenesis with formation of horst-like uplifts within the zone against the background of general subsidence, and (4) Mesozoic-Cenozoic within-plate reactivation. At the final Kimmerian-Alpine stage of its evolution, the Mid-Russian Zone developed as a left-lateral transpressional structure with penetrative dissipative shear deformation resulting in the general horizontal transfer of Phanerozoic sedimentary rocks. The dislocations were manifested as two dynamically conjugate structural forms: a zone of scattered shearing and a bedding-plane tectonic flow. The dynamic manifestation of the Mid-Russian and the conjugate Belomorian-Dvina zones, which make up a common arcuate structure (in plan view), allowed us to outline the Dvina-Sukhona plate-flow with horizontal mass transfer in the southeastern direction. The tectonics of the Mid-Russian Dislocation Zone is considered in this paper with particular emphasis on the structural and kinematic assemblies in sedimentary rocks of the Phanerozoic cover.