Conglomerate Units Research Articles

Volcanic, sedimentary, and deformation history of Winter Lake greenstone belt, Slave craton, Northwest Territories: preliminary results from the 2019 field season

The Archean Winter Lake greenstone belt (WGB) in the Slave craton, located ~250 km northeast of Yellowknife in the Northwest Territories, has been underexplored relative to other supracrustal belts in the Slave craton, but shows potential for base-metal mineralization. It consists of lower greenschist to upper amphibolite grade mafic to felsic volcanic rocks and sedimentary rocks that are surrounded by ca. 3.3 to 2.9 Ga granitoids of the Central Slave Basement Complex (CSBC). The overall objective of this study is to better understand the origin and geodynamic evolution of the WGB and to evaluate its economic potential. The project will examine the tectonic setting of volcanic and sedimentary rocks of the WGB and their relationships to the CSBC granitoids.
 In 2019, three weeks were spent bedrock mapping near Newbigging Lake and one week near Big Bear Lake (at 1:5,000 and 1:10,000 scales, respectively) to generate a lithostratigraphic scheme for the southern WGB. This mapping evaluated the nature of contacts, previously interpreted as unconformities, between the CSBC, the ca. 2734-2924 Ma Central Slave Cover Group (CSCG) volcanic and sedimentary rocks, and post-volcanic sedimentary and granitoid rocks (Hrabi et al., 1995). Mapping near Big Bear Lake also allowed the examination of a previously mapped ~ 1.3-1.7 km wide rhyolite interpreted to be part of the ca. 3.3 Ga Newbigging Formation (Hrabi et al., 1995). Based on field observations, the formation does not include a rhyolite sensu stricto but rather a succession of weathered mafic to intermediate volcanic and intrusive rocks. This field season, the first of three, also led to the discovery of several semi-massive to massive sulfide showings within the mafic volcanic sequence, and the identification of sulfides (interpreted as remobilised) and multiple younging indicators within the younger conglomerate unit that are necessary for our lithostratigraphy study.
 Future U-Pb geochronological analysis of detrital zircon and Sm-Nd isotope geochemistry of metasedimentary rocks will enable us to determine their sources and age. Together with absolute timing of thermotectonic events, an Archean oceanic and continental crust geodynamic evolution model will be produced. An additional goal of this study is to identify the ore-forming environment for the sulfide mineralization and to generate an ore-deposit model for the mineralisation observed in the WGB.

Novi podaci o stratigrafiji badena okoline Krčedina (severna Srbija, Centralni Paratetis)

Badenian sediments are found for the first time in the surroundings of the Krcedin village. They represent remnants of the Central Paratethys marginal sea. The sediments are represented by a basal conglomerate unit overlain by pure and sandy marl. Foraminifera and rare ostracods are present, limited to the marls. Normal gradation and the prevalence of planktonic foraminifera suggest a rapidly deepening basin.

Timing of deposition in the Dengzhangzi and Guojiadian Basins of the Yanshan fold-thrust belt, North China

ABSTRACT The Triassic-Jurassic stratigraphy in the Yanshan fold-thrust belt is dominated by nonmarine deposits, and the division of this stratigraphy remains controversial among geologists. Age results from biostratigraphic and isotope chronological studies are inconsistent, especially for the Dengzhangzi and Guojiadian Basins in the eastern part of the Yanshan belt. We present four new detrital zircon U–Pb isotope datasets for the Triassic-Jurassic stratigraphic units and one for the Permian unit in the basins. The geochronological results show that the andesitic breccia in the Permian strata was deposited at ca. 273.78 ± 1.68 Ma, the Laohugou Formation was deposited in the Anisian-Ladinian Stage, the andesitic lava in the Shuiquangou Formation erupted at ca. 238.56 ± 0.91 Ma, and the conglomerate in the Guojiadian Formation was deposited from ca. 170 ± 5 to 163.0 ± 2.2 Ma. Field observations and facies analyses demonstrate that the Laohugou, Shuiquangou and Dengzhangzi Formations predominantly consist of subaqueous debris flow deposits, subaerial fluvial deposits, and fan delta deposits while the Guojiadian Formation is generally composed of fluvial and subaerial mass-flow deposits that formed in braided channels. Here, we propose a more precise and comprehensive Triassic-Jurassic stratigraphic division scheme based on age constraints and facies analysis. With this new stratigraphic sequence, we compare the Mesozoic stratigraphic units in the study area with those in other basins in the Yanshan fold-thrust belt in terms of the lithological associations, regional unconformities and zircon U-Pb dating results. This comparison indicates that the Indosinian and Yanshanian movements exerted a far-reaching influence on the development and evolution of the Dengzhangzi and Guojiadian Basins in the study area, forming several important regional unconformities and syntectonic conglomeratic units. These new age data on the stratigraphy are of fundamental significance for understanding the Mesozoic tectonic evolution of Yanshan.

Designing Employee Engagement Framework for a Business Unit of an Indian Multinational Conglomerate

Employee engagement is an area which has been in discussion lately. Many organizations fail to engage their employees as a result they face high attrition resulting to high cost. The level of employee engagement in an organization can be enhanced by identifying the drivers. These drivers can then contribute in developing an employee engagement model and framework which could be of help for the organization in the long run. This research is conducted in one of the business unit of an Indian multinational conglomerate. The purpose of the research was to design an employee engagement framework. For this, the researcher prepared a survey questionnaire using quantitative technique and floated it to all the employees (760) in that business unit. There were total 40 questions out of which 36 were closed ended 5 point Likert scale positive statement and 4 open ended questions.To understand the characteristics of the population size for identifying which all drivers are influencing the employee engagement in the organization the most, the researcher had used Descriptive research. MS Excel and statistical tools were used to analyze the data and to come up to a conclusion. Using the research findings of the research, the researcher framed an employee engagement framework which the company is planning to implement.

A new type of slumping-induced soft-sediment deformation structure: the envelope structure

Abstract The sediments of the Cretaceous Gyeokpori Formation in south-western South Korea accumulated in a lake in which mainly siliciclastic rocks were deposited, with some interbedded volcaniclastics. The nearby volcanic activity resulted in unstable lake margins inducing a dominance of gravity-flow deposits. The high sedimentation rate facilitated soft-sediment deformation on the sloping margin. The deposition of numerous gravity-flow deposits resulted in a vertically heterolithic stratification. The slumps are composed of different lithologies, which is expressed in different types of deformation due to the difference in cohesion between sandy and mussy layers within the slumps. Coarser-grained (cohesionless) slumps tend to show more chaotic deformation of their lamination or layering. The difference in slumping behaviour of the cohesive and non-cohesive examples is explained and modelled. A unique soft-sediment deformation structure is recognized. This structure has not been described before, and we call it ‘envelope structure’. It consists of a conglomerate mass that has become entirely embedded in fine-grained sediment because slope failure took place and the fine-grained material slumped down with the conglomerate ‘at its back’. The cohesive laminated mudstone formed locally slump folds that embedded the non-cohesive overlying conglomerate unit, possibly partly due to the bulldozing effect of the latter. This structure presumably can develop when the density contrast with the underlying and overlying deposits is exceptionally high. The envelope structure should be regarded as a special – and rare – type of a slumping-induced deformation structure.

Geometallurgical Approach for Implications of Ore Blending on Cyanide Leaching and Adsorption Behavior of Witwatersrand Gold Ores, South Africa

Gold production in South Africa is projected to continue its decline in future, and prospects for discovery of new high-grade deposits are limited. Many of the mining companies have resorted to mining and processing low-grade and complex gold ores. Such ores are technically challenging to process, which results in low recovery rates, excessive reagent consumption and high operating costs when compared to free-milling gold ores. In the Witwatersrand mines, options of blending low-grade gold ores with high-grade ores exist. Although it is well known that most of the Witwatersrand gold ores are highly amenable to gold cyanidation, not much is known on the leachability of blended ores, especially the effects of mineralogical and metallurgical variability between different gold ores. In this study, we apply a geometallurgical approach to investigate mineralogical and metallurgical factors that influence the leaching of blended ores in a set of bottle shaker and reactor column tests. Three gold-bearing conglomerate units, so-called reefs, i.e., Carbon Leader Reef, Ventersdorp Contact Reef and the Black Reef, all in the Carletonville goldfield, were sampled. The ores were prepared using a terminator jaw crusher followed by vertical spindle pulverizer (20 kg aliquot) and high-pressure grinding rolls (80 kg aliquot). Mineralogical analysis was conducted using a range of complementary tools such as optical microscopy, QEMSCAN and micro–XCT. The results show that Witwatersrand gold ores are amenable to the process of ore blending. Some of the ores, however, contain impervious inert gangue and reactive ore minerals. Leach solution can only access gold locked in impervious gangue minerals through HPGR-induced pores and/or cracks. The optimum ore blending ratio of the bottle shaker experiments (p80 = − 75 μm) comprises 60% Carbon Leader Reef, 20% Ventersdorp Contact Reef and 20% Black Reef and yields 92% recovered Au over a leach period of 40 h. Blended ores with high carbonaceous material (> 1 wt% carbonaceous material, (Black Reef = 36–60%) yield lower recoveries of 60–69% Au). Ore leaching at the mixed-bed reactor column (− 75 μm and − 5.6/+ 4 mm) yields about 70% over a leach period of two weeks. We therefore suggest that the feasibility of ore blending is strongly controlled by the mineralogy of the constituent ores and that a mixed-bed reactor may be a viable alternative method for leaching of the low-grade Witwatersrand gold ores. Material from certain reefs, such as the Black Reef, has synergistic/antagonistic (nonadditive) blending effects. The overall implication of this study is that ore blending ratios, effects of comminution on mineral liberation, an association of gold with other minerals, and gold adsorption behavior will greatly inform future technology choices in the area of geometallurgy.

Reply to Molina-Garza et al. (2019) “Discussion of: Ortega-Flores et al. (2018) provenance analysis of Oligocene sandstone from the Cerro Pelón area, southern Gulf of Mexico”

ABSTRACTThe origin of the Oligocene turbidites from the Cerro Pelón area in south Gulf Mexico proposed by Ortega-Flores et al. (2018) is in disagree with the interpretations made by Molina-Garza et al. (2019), which main criticism is based on U-Pb ages of detrital zircons from the matrix of a conglomerate unit, which they refer to as ‘Nanchital Conglomerate’, as well as on the presence of limestone, gabbros, and mafic protolith-derived clasts. Molina-Garza et al. (2019) basically interpret the Nanchital Conglomerate as Miocene in age, which was sourced mainly from metamorphic complexes including their sedimentary covers located to the west and south of the Cerro Pelón area. For some reason, Molina-Garza et al. (2019) suppose that the Nanchital Conglomerate should have the same provenance sources that the Oligocene turbidites from Cerro Pelón area, reported by Ortega-Flores et al. (2018). Based on the foregoing, we strongly disagree with Molina-Garza et al. (2019) considering that, from the beginning, they intend to compare two units of different age. Additionally, the scarce data reported from both the matrix and the clasts of the Nanchital Conglomerate are not determinant for interpreting the provenance of this conglomeratic unit and subsequently, to consider the same rock sources from the Oligocene through Miocene time.

Palaeoenvironmental evolution of the late Miocene palaeolake at Zahle (Bekaa Valley, Lebanon)

Neogene non-marine sedimentary rocks of the Bekaa Valley (Lebanon) are microfossil rich and show minimal lateral facies changes and post-depositional diagenetic alteration across continuous exposures. In this paper, we analyze the sedimentary facies and microfossils of three closely-spaced stratigraphic sections near the town of Zahle. The sedimentary sequence is divided, from base to top, into three intervals: (1) a massive conglomerate unit interpreted as alluvial fan deposits, (2) fossiliferous yellowish marls and limestone interpreted as perennial lacustrine deposits, and (3) an organic-rich marlstone intercalated with volcanic ash horizons interpreted as a palustrine interval representing an overall shallowing sequence. Intervals 2 and 3 contain a diverse microfossil assemblage comprising charophytes, terrestrial molluscs, a freshwater bivalve, ostracods as well as fish and mammal remains. The palaeoenvironmental characteristics and evolution of these lacustrine deposits are inferred through facies analysis and through comparison of microfossil tolerances with the ecological requirements of their nearest living relatives. Results suggest that during the Miocene the Bekaa Valley was occupied by a relatively shallow, stable, oligotrophic freshwater or slightly oligohaline lake that evolved to a very shallow eutrophic lake with a dense palustrine vegetation belt. The palaeolake ultimately regressed to the south due to climatic changes and tectonic stresses leaving the lacustrine deposits exposed in the area of Zahle as an erosional surface.

Response to comment on “A high-precision 40Ar/39Ar age for the Nördlinger Ries impact crater, Germany, and implications for the accurate dating of terrestrial impact events” by Schmieder et al. (Geochimica et Cosmochimica Acta 220 (2018) 146–157)

Rocholl et al. (2018) claim that the 40Ar/39Ar age of 14.808 ± [0.038] Ma (2σ; full external error) for the Ries impact, recently obtained from step-heating analysis of tektites (Schmieder et al., 2018, Geochim. Cosmochim Acta 220, 146–157), is “geologically impossible” and violates paleomagnetic constraints. We demonstrate that the moldavite age of Schmieder et al. (2018) is in good agreement with recalculated isotopic ages for ash layers and magnetic (sub-)chrons that define the Langhian stage in the Astronomically Tuned Neogene Time Scale, using updated K decay constants and monitor ages in 40Ar/39Ar geochronology. Moreover, we stress that the normally magnetized basal pelitic sediments of the Ries crater lake, which are separated from the reversely magnetized suevite by a thick unit of post-impact conglomerates and sandstones, may not have been deposited ‘immediately’ after the impact. The paleomagnetic constraints proposed by Pohl (1977, 1978) and Rocholl et al. (2018) are, therefore, questionable.

Comments on the paper by J. Jirásek, L. Sedláčková, M. Sivek, K. Martínek & J. Jureczka "Castle Conglomerate Unit of the Upper Silesian Basin (Czech Republic and Poland): a record of the onset of late Mississippian C2 glaciation?"

Comments on the paper by J. Jirásek, L. Sedláčková, M. Sivek, K. Martínek & J. Jureczka "Castle Conglomerate Unit of the Upper Silesian Basin (Czech Republic and Poland): a record of the onset of late Mississippian C2 glaciation?"

Lithofacies And Depositional Analysis Environment Of West Section Kolok Nan Tuo Village, Sawahlunto City, West Of Sumatera

Research areas were located in the west of Kolok Nan Tuo Village. Geographically this area is located at coordinates 00 ° 36'57,85 '' - 00 ° 37'56,89 '' latitude and 100 ° 42'10,08 '' 100 ° 43'47,28 " BT. The methods used in research is geological mapping. Based on the results of stratigraphic research area is divided into three units consist of: Crystalline Limestone Unit (SBGK) consisting Crystalline Limestone of and mudstone lithofacies, Conglomerate Units (SK) consists of polymic conglomerate and sandstones greywacke lithofacies while claystone Unit (SBL) lithofacies consists of claystone with sedimentary structures is flake. Result of research can interpreted Depositional environment based on type of lithofacies include of grain size, sedimentary structures and content of fossils. SBGK interpretated depositional environment is a basement of basin, SK depositional environment debris unit limestones can be seen from fragments of conglomerates that many there are crystalline limestones and mudstone that make up the alluvial fan (deposition surface) and lithologies clay from sedimentary structures where rock mudstone generally in doposition the current flow that quiet can form sedimentary structures flake and properties of claystone which carbonated an identifier depositional environments sea so can be determined that the environment in the form of neritic environment.

Mineral-petrographic study of greenstone cobbles from Quaternary alluvial deposits and Oligocene conglomerates of the Lemme valley (northwestern Italy): Comparison with analogous Neolithic polished stone implements and archaeometric implications

Rare HP meta-ophiolites of archeologic interest occur in the western Alps, both as small primary outcrops at high altitude located in the Monviso or Voltri massifs and as boulders/pebbles in the derived secondary conglomeratic deposits and/or alluvial beds downhill/downstream, originated after erosion of the former ones. Despite their scarceness, these rocks – in the archaeologic literature still grouped under the term “greenstones” – are of paramount importance for archaeologists, since they were used to produce polished stone tools rediscovered in prehistoric sites spread all over Europe. Tracing the sources of these raw materials is an important issue for reconstructing the migratory fluxes and trade routes of our ancestors. A certain number of “greenstone” geologic samples were retrieved from the alluvial beds of the Lemme valley streams (southeastern Piemonte, Italy) and in the Oligocene conglomerate units of the Tertiary Piemonte Basin, presently exposed in the same valley. Representative samples were analyzed with a classic mineralogical and petrographic approach, which includes XRPD, polarizing microscope and SEM-EDS. These rocks compositions and microstructures were then compared with those of analogous implements found in archaeological sites located nearby and already described in the literature, searching for analogous traits suggesting a common origin. The obtained outcomes show that these secondary deposits of the Lemme valley – originated from the dismantlement of upstream primary outcrops in the Voltri massif (or from a close, analogous palaeo-unit, nowadays completely eroded) – might have represented, during the early-to-middle Neolithic, one of the preferential supply sources of raw “greenstones” for the archaeologic sites of Brignano Frascata, Momperone, Villaromagnano and Rivanazzano.

Origin of exotic clasts in the Central-Southern Apennines: clues to the Cenozoic fold-and-thrust collisional belt in the Central Mediterranean area

AbstractThe Central-Southern Apennines are the result of the collision between Europe and Africa. Despite the volume of existing literature, many problems remain unsolved such as the presence of Tertiary conglomerates containing exotic basement clasts. The lack of basement rocks in the Central-Southern Apennines implies that the origin of these clasts has to be sought in areas where the basement is extensively exposed. These include the Calabro–Peloritani arc and the Sardinia–Corsica block, which in Cenozoic time were connected to the Central-Southern Apennines. In this work we present the results of sedimentary, geochemical and petrographic analyses performed on the exotic basement-derived clasts. These analyses include lithological, major- and minor-element and rare Earth element compositions which are compared to analogous rocks from Calabria and Sardinia basements. Results indicate Eastern Sardinia as the primary source area for the studied conglomeratic units, linking the Central-Southern Apennines sedimentary cover to the Mesozoic carbonates of Eastern Sardinia prior to the opening of Tyrrhenian Sea. The Cilento unit (Campania) was directly fed by an uplifting Cenozoic orogen, and the Filettino, Gavignano (Latium) and Ariano Irpino (Campania) units were produced by the successive reworking of ‘Cilento-like’ sedimentary units. These results may imply that part of the Central-Southern Apennines represented a portion of the European margin of the Tethys.

Provenance and paleo–weathering of Tertiary accretionary prism–forearc sedimentary deposits of the Andaman Archipelago, India

In order to understand the provenance and tectono–sedimentary processes occurring in the Andaman Subduction Zone (ASZ), the Late Cretaceous to Oligocene sedimentary records from the Andaman Islands have been studied. These sedimentary records are considered to have preserved the history of the India–Asia collision, evolution of the Himalayas, climatic development and palaeo–drainage reorganizations on the Indian and Asian plates. About 47 sandstones and mudstones (shales and siltstones) samples were analyzed for whole rock major, trace, and rare earth element compositions. The geochemical results suggest mixing of sediments derived from the mafic igneous sources comprising local ophiolites and volcanic arc of the ASZ and an older Archean to Proterozoic age felsic cratonic source with compositions similar to average granodiorite or upper continental crustal sources. The compositions were dominated by sources of the mafic arc during deposition of the Mithakhari Group, whereas they were controlled by continental sources during deposition of the Andaman Flysch Group. The Hope Town Conglomerate unit of the Mithakhari Group was mainly derived from weathering and erosion of the subaerially exposed local ophiolite thrust sheets, whereas its Namunagarh unit contains significant detritus from volcanic arcs. The Andaman Flysch turbidites were deposited with a greater supply of sediments from first–cycle active continental margin sources probably located in the Tibetan and eastern Myanmar region and recycled quartzose sedimentary sources within the nascent Himalayas. The sediments supplied to both the Mithakhari and the Andaman Flysch Groups were characterized by varying values of CIA, PIA and W. These variable values were either due to non–steady state weathering conditions in the sources or the changing climatic conditions owing to the motion of Indian plate with reference to the equator. The uniformly high CIA and W values in the Andaman Flysch rocks can be related to high precipitation and strong chemical weathering associated with the initiation of the Indian monsoon.

New data on the Lower Mesozoic basal series of the Traras mounts (Tlemcen, northwestern Algeria)

The lower Mesozoic of the Traras mount starts with the so-called Red Series, which lays uncomformably on the folded and granitized Paleozoic basement. This series is overlain by Middle Liassic limestones. On the basis of its lower and upper limits and the lack of dating evidence, former authors attribute a Permian to lower Liassic (Sinemurian) age for this series. Palynological results allow to range it in the upper Triassic probably Norian-Rhetian. The Red Series is studied from five sections that depict important thickness and facies variations from southwest to the northeast, inferring the irregularities of the Hercynian erosional surface. In the Central Traras, this series shows over 130-m-thick micaceous sandy conglomeratic units, deposited within alluvial fans, where the bedrock source corresponds to granite of Nedroma or its equivalent. In the southern and eastern Traras, where the series’ thickness respectively does not exceed 30 m and averages 40–80 m, besides alluvial deposits issued from hercynian basement erosion, it exhibits volcanic evidences interbedded by lacustrine deposits. Vertical and spatial distribution depicts synsedimentary infilling of narrow elongated rift basins, typically half-grabens, bounded on one side by a normal fault or a series of normal faults trending NNE-SSW to NW-SE, inherited from the hercynian event. The study of tectonostratigraphic units permitted to put forward geodynamics of the infilling basins.

Detection of potential methane gas pathways in deep South African gold mines

At a number of gold mines in South Africa, methane gas has been encountered when drilling into faults and/or dykes extending to depths beyond 7000 m. Methane gas has been reported to migrate up through structures from within the basin to the mine working environments (∼3000 m depth) and caused explosions. The Booysens Shale is one of the possible source rocks for hydrocarbons and it forms the footwall to the gold-bearing Ventersdorp Contact Reef (VCR, ∼1.5 m thick). The Booysens Shale lies at depths between 3500 and 4500 m below the surface and can be best described as a base of the divergent clastic wedge, hosting the quartzite and conglomerate units that sub-crop against the VCR towards the west of the gold mining areas. Geometric attributes (dip and dip azimuth) and instantaneous attributes (phase, frequency and envelope) computed for the Booysens Shale and VCR horizons provide insight into structures that extend from the Booysens Shale into the overlying mining level (i.e. VCR). These attributes successfully mapped the structures (faults and dykes) that have displaced both the VCR and Booysens Shale horizons. The edge-detection attributes were particularly useful in delineating faults with throws below the traditional seismic resolution criteria (e.g. a quarter of the dominant wavelength) that were not visible in the conventional seismic interpretation. The structural analysis using seismic attributes gives a new visual representation of geological structures that may be probable conduits for hydrocarbons, particularly methane, migrating from the Booysens Shale to the VCR mining levels.

Lithostratigraphy of the Enon Formation (Uitenhage Group), South Africa

Abstract The Uitenhage Group represents the earliest deposits that filled Mesozoic rift basins in the southern Cape of South Africa during the fragmentation of the supercontinent Gondwana. The sedimentology of the Enon Formation records the development of alluvial systems that drained the region since the onset of Gondwanan rifting, and therefore plays an important role in our understanding of early landscape evolution along the southern African continental margin. The mostly coarse conglomeratic unit was deposited continuously in actively subsiding, but separated, rift basins. As a result, the deposits are diachronous between basins and display highly varied thicknesses of up to well over 2000 m.

High-resolution magnetostratigraphic study of the Paleogene-Neogene strata in the Northern Qaidam Basin: Implications for the growth of the Northeastern Tibetan Plateau

The Cenozoic terrestrial, intermontane Qaidam Basin on the northeastern edge of the Tibetan Plateau contains >12km of sedimentary rocks that potentially document the accommodation of India-Asia convergence and the growth of the plateau. The chronology remains incomplete, hindering cross-basin correlation between lithostratigraphic units and their further interpretation. Here we present a high-resolution magnetostratigraphy spanning >5km of Paleogene-Neogene sequence at Dahonggou in the Northern Qaidam Basin. Based on correlation with the geomagnetic polarity time scale (GPTS), we have dated the section to being between ~52 and ~7Ma. The bottom conglomeratic unit, ranging from >52Ma to ~44Ma, was deposited in high-energy environments (e.g., alluvial fan or braided river), reflecting the earliest deformation and uplift of the basin-bounding Qilian Shan fold-thrust belt in response to India-Asia collision. In addition, we identified two major increases in sedimentation rate at 25–16Ma and after ~9.5Ma and three phases of lesser increases at 52–44Ma, 38–33Ma, and 14.6–12.0Ma. These increases in sedimentation rate are consistent with regional thermochronology and basin analysis studies, which revealed enhanced motion on basin-bounding thrust faults. We argue that these accelerated sedimentation rates indicate pulsed tectonism in the northeastern Tibetan Plateau. The pulse at 25–16Ma may further relate to phases of strong rainfall linked to an intense monsoon at that time.

Cambrian–Ordovician orogenesis in Himalayan equatorial Gondwana

An early Paleozoic tectonic event, the Kurgiakh orogeny, has long been known from the western Tethyan Himalaya, and it is conspicuously recorded by an angular unconformity between Cambrian marine shelf deposits and coarse Ordovician conglomerate, as well as widespread granitic plutons. Although the lateral extent of this event is poorly known, two regions in the central and eastern Himalaya, the Kumaon of India and klippen of Bhutan, contain conglomerate units that may be correlative with this event. In the Kumaon, the Ralam Formation, which contains a basal polymictic conglomerate unit with detrital zircon grains as young as ca. 512 Ma, is overlain by sandstone containing the arthropod walking trace Diplichnites gouldi , an ichnospecies known from Ordovician and younger strata. The late early Cambrian trilobite Redlichia sp., recovered from the underlying Martoli Group, indicates that the conglomerate is post−early Cambrian and, based on stratigraphic analysis, also likely records the Kurgiakh orogeny. In central Bhutan, the lower Paleozoic succession contains conglomerate units with monomictic quartz sandstone clasts and less mature sandstone matrix. These units have also been assigned to a variety of ages, but rocks that apparently underlie these strata contain the Furongian (late Cambrian) zonal trilobite Kaolishania granulosa . Here, we show that detrital zircon ages of a conglomerate clast (no grains younger than = ca. 781 Ma) are distinctly older than those of the matrix, which contains a large peak at 490−500 Ma. Thus, these conglomerate units are also potential lateral correlatives of the Ordovician conglomerate deposits of the western Himalaya. Our paleontological and geochronologic data suggest that the Kumaon and Bhutan conglomerate units, and possible equivalents in Tibet, correlate with those of the western Himalaya. This increases their known along-strike extent by over 1000 km, bringing the total extent to ∼1200 km. Thus, the Kurgiakh orogeny spanned much of the Himalayan orogen along the northern Gondwanan margin and reflected a widespread tectonic reorganization of the peri-Gondwanan realm to Andean-type margins. Recent work indicates that it also likely extended into the Lhasa block and both the North and South China blocks. This event was associated with widespread ca. 500 Ma arc-related plutonism recorded in the Himalaya, and with abundant detrital zircon grains of that age in younger Himalaya strata. Based on current fossil data, the duration of the hiatus associated with the Cambrian-Ordovician boundary, which separates pre- and postdeformational strata, ranges from 22 to 36 m.y. Available geochronological data do not provide greater constraint. If the event was close in age to the depositional age of the Ordovician molasse, then the main phase of tectonic uplift, erosion, and deposition may have been much later than the presumed Cambrian-Ordovician boundary interval age of ca. 485 Ma.

First occurrence of the Salvador Formation in the Jatobá Basin (Pernambuco, Northeast Brazil): Facies characterization and depositional systems

Fan deltas, constituting proximal depositional systems adjacent to boundary faults, are common features associated with rift basins. The Cretaceous fan delta systems of the Salvador Formation, deposited during the rift phase of the Recôncavo-Tucano-Jatobá Basin, were first reported in the Recôncavo Basin and later discovered in the Tucano Basin. Because of the absence of any outcrops in the Jatobá Basin until now, these alluvial fans were interpreted solely through seismic analysis. We report the first revealed outcrops of the Salvador Formation in that basin and characterize their depositional systems as interlayered with the lacustrine Candeias Formation. Based on facies and architecture, the alluvial system can be subdivided into three associations: (1) proximal fan delta, characterized by meter-scale conglomerate bodies with a predominance of boulders and cobbles with thin sandstone layers; (2) distal fan delta, characterized by sheet-like pebble conglomerate and sandstone layers with flame and load structures; and (3) lacustrine, further subdivided into shallow lake facies reddish shales and mudstones with oolitic limestones and deep lake facies grey to green shales with pyrite. Paleocurrent measurements for the proximal fan association show paleoflow direction varying from SW to SE, which is expected for the rift phase alluvial system. The Recôncavo-Tucano-Jatobá rift system has two conglomeratic units, namely the Salvador and Marizal Formations, the former a syn-rift and the latter a post-rift unit. The absence of sedimentary clasts in the conglomerates, very low maturity, the presence of giant clasts, and a visible relationship between boundary faults in the outcrop, define the syn-rift Salvador Formation characteristics. Based on the facies and paleocurrent analyses, the Salvador Formation deposits in Jatobá Basin were interpreted as a deposition of a debris flow-dominated fan delta, indicating the lacustrine setting represented by the Candeias Formation.