Stratigraphic Changes Research Articles

A palaeoenvironmental reconstruction based on palynological analyses of Upper Triassic and Lower Jurassic sediments from the Holy Cross Mountains region

ABSTRACT The material for this study was taken from various sites in the Holy Cross Mountains: Studzianna, Huta OP-1 and Przysucha P-3 (bore cores), and Odrowąż (geological outcrop). Palynological and palynofacial analyses were used to reconstruct the Upper Triassic and Lower Jurassic vegetation and palaeoenvironment of that area. Samples from all the sites are characterised by having majority of terrestrial particles. The slides were dominated by translucent phytoclasts and small opaque phytoclasts. Pollen grains and spores were also present. The presence of marine palynomorphs was not confirmed in the samples from any of the sites, dinoflagellate cysts did not occur, and there were no foraminiferal test linings. The Sporomorph EcoGroup (SEG) model (Abbink 1998) was applied to characterise the ecological types of the plant assemblages. The most frequently occurring sporomorphs were assigned to Upland, Lowland and River SEGs. Stratigraphical changes in each of the SEGs indicate differences in climatic conditions. The floristic composition of the Studzianna, Huta and Odrowąż localities inferred from sporomorphs differs somewhat from the composition reconstructed from macroremains.

Time-lapse geophysical technology-based study on overburden strata changes induced by modern coal mining

To study the impact of modern coal mining on overlying strata and its water bearing conditions, integrated time-lapse geophysical prospecting integrating 3D seismic, electrical and ground penetrating radar method were used. Through observing and analyzing the geophysical data variations of all stages of pre-mining, mining and post-mining as well as post-mining deposition stable period, impacts of coal mining on stratigraphic structure and its water bearing were studied and modern coal mining induced stratigraphic change pattern was summarized. The research result shows that the stratigraphic structure and the water bearing of surface layer during modern coal mining have self-healing pattern with mining time; the self-healing capability of near-surface strata is relatively strong while the roof weak; water bearing self-healing of near-surface strata is relatively high while the roof strata adjacent to mined coal beds low. Due to integrated time-lapse geophysical prospecting technology has extra time dimension which makes up the deficiency of static analysis of conventional geophysical methods, it can better highlight the dynamic changes of modern coal mining induced overburden strata and its water bearing conditions.

Major changes in the post-glacial evolution of magmatic compositions and pre-eruptive conditions of Llaima Volcano, Andean Southern Volcanic Zone, Chile

Llaima is one of the most active volcanoes of the Chilean volcanic front with recent explosive eruptions in 2008 and 2009. Understanding how the volcano evolved to its present state is essential for predictions of its future behavior. The post-glacial succession of explosive volcanic eruptions of Llaima stratovolcano started with two caldera-forming eruptions at ∼16 and ∼15 ka, that emplaced two large-volume basaltic-andesitic ignimbrites (unit I). These are overlain by a series of fall deposits (unit II) changing from basaltic-andesitic to dacitic compositions with time. The prominent compositionally zoned, dacitic to andesitic Llaima pumice (unit III) was formed by a large Plinian eruption at ∼10 ka that produced andesitic surge deposits (unit IV) in its terminal phase. The following unit V represents a time interval of ∼8,000 years during which at least 30 basaltic to andesitic ash and lapilli fall deposits with intercalated volcaniclastic sediments and paleosols were emplaced. Bulk rock, mineral, and glass chemical data constrain stratigraphic changes in magma compositions and pre-eruptive conditions that we interpret in terms of four distinct evolutionary phases. Phase 1 (=unit I) magmas have lower large ion lithophile (LIL)/high field strength (HFS) element ratios compared to younger magmas and thus originated from a mantle source less affected by slab-derived fluids. They differentiated in a reservoir at mid-crustal level. During the post-caldera phase 2 (=units II–IV), relatively long residence times between eruptions allowed for increasingly differentiated magmas to form in a reservoir in the middle crust. Fractional crystallization led to volatile enrichment and oversaturation and is the driving force for the large Plinian eruption of the most evolved (unit III) dacite at Llaima, although replenishment by hot andesite probably triggered the eruption. During the subsequent phase 3 (=unit V >3 ka), frequent mafic replenishments at mid-crustal storage levels favored shorter residence times limiting erupted magma compositions to water-undersaturated basaltic andesites and andesites. At around 3 ka, the magma storage level for phase 4 (=unit V <3 ka to present) shifted to the uppermost crust where the hot magmas partly assimilated the granitic country rock. Although water contents of these basaltic andesites were low, the low-pressure storage facilitated water saturation before eruption. The change in magma storage level at 3 ka was responsible for the dramatic increase in eruption frequency compared to the older Llaima history. We suggest that the change from middle to upper crust magma storage is caused by a change in the stress regime below Llaima from transpression to tension.

Abrupt climate change and early lake development – the Lateglacial diatom flora at Hässeldala Port, southeastern Sweden

The fossil diatom record from the Hässeldala Port palaeolake, southeastern Sweden, offers an excellent opportunity to investigate how past climatic shifts influenced catchment conditions and early lake development. The record, dating to between 13 900 and 11 200 cal. a BP, covers a climatically dynamic period, starting with deglaciation followed by oscillations between warmer and colder climate states. The stratigraphical changes in the fossil diatom assemblages show a trend of less open‐water taxa and a successively more complex periphytic community as the lake shallows and the aquatic habitat structure develops. A diatom‐based reconstruction of lake water pH indicates a natural acidification trend early in the record from 13 900 to 12 500 cal. a BP. From 12 500 cal. a BP, coincident with the start of climate cooling, to 11 300 cal. a BP this trend is disrupted and lake waters become more alkaline. A cooler and drier climate most likely resulted in reduced soil organic matter build‐up as well as more frozen ground that impeded hydrological flow and decreased the input of dissolved organic matter and organic acids into the lake system. This study demonstrates the importance of the hydrological system as a link between terrestrial and aquatic ecosystems during early lake ontogeny.

Climate changes for the past 52 ka clarified by total organic carbon concentrations and pollen composition in Lake Biwa, Japan

Total organic carbon (TOC) and total nitrogen (TN) analyses have been performed for three core samples obtained recently from Lake Biwa, which include BIW07-5, BIW07-6, and BIW08-B cores. The TOC and TN data from these cores show similar stratigraphic changes and can be regarded as reliable records of biological productivity occurring in the lake during the past 52 ka. Pollen analysis data are also used to determine climate change during the past 40 ka by reconstructing meteorological parameters such as annual temperature and precipitation. Temporal changes in TOC concentration show detailed climate changes in tens-year intervals. Climate change of orbital scale are well recognized, demonstrating the change from MIS 3 to 1 associated with severe cold periods known as Heinrich events 1 to 5 and warm periods known as Greenland interstadials (GIS 1–13). These climate events in millennial time scale are also well supported by pollen characteristics and by modern analogue analysis. The good concordance of climate change between North Atlantic region and Japan is explained by a hypothesis that the intensity of the Arctic air mass controls the climate in both the North Atlantic and East Asia regions through the migration of the Arctic polar front. Although the average annual mean temperature in MIS 3 was 8.2 ± 0.8 (2σ) °C, it varied largely and frequently among its substages. For example, 9.4 ± 2.0 °C was detected in a warm interstadial (GIS 8), and 6.3 ± 1.5 °C indicated in a cold stadial (Heinrich event 4). The average annual temperature was 4.5 ± 0.3 °C in MIS 2, when the average temperature of the coldest month was −7.3 ± 0.2 °C and that of the warmest month was 16.8 ± 0.2 °C. Through Termination 1, the annual mean temperature suddenly increased more than 5 °C, and the annual mean temperature became 11.2 ± 0.8 °C in early MIS 1, which is much lower than that of the present temperature. This low temperature estimation is attributed to the delay in expansion of evergreen broadleaf trees. The annual mean temperature in the late MIS 1 was as same as that of present meteorological observation.

Estuarine foraminifera record Holocene stratigraphic changes and Holocene climate changes in ENSO and the North American monsoon: Baffin Bay, Texas

During the last Quaternary sea level fall (120–ka), Baffin Bay was formed by the down-cutting of the Los Olmos, San Fernando, and Petronila Creeks of south Texas. When sea level rose, this incised valley was then filled with mixed siliciclastic/carbonate sediments that record coastal environmental change over the past 10ky. Previous sedimentological and seismic analysis shows that Baffin Bay contains atypical depositional environments as a result of its semi-arid climate setting and isolation from the Gulf of Mexico. Three prominent stratigraphic surfaces can be recognized within the bay deposits, and are chronostratigraphically constrained using radiocarbon dates. The purpose of the present study is to use foraminifera to create a separate account of change and to determine if foraminiferal data corroborate sedimentological evidence for sea level and climate fluctuations. Foraminifera were sampled at 20cm intervals from a 14.4m dated core and from surface and subsurface sediments of five cores along a dip transect. Multiple discriminate analysis was used to compare sections of the core by species proportions, and clearly delineates three different foraminiferal communities: deltaic, open-bay, and hypersaline. Breaks between these communities coincide with two of the surfaces observed in the core, one at about 8.0ky and the other around 5.5ky. Rapid sea level rise at the 8.0ky flooding surface corresponds with a shift from a deltaic to an open-bay foraminiferal assemblage, while faunal change across the 5.5ky surface is due to the formation of a large barrier island (Padre Island) and the onset of more arid climate conditions. Prior to the isolation of Baffin Bay at 5.5ky, foraminiferal assemblages do not correspond to climate change records, perhaps because open circulation with the Gulf of Mexico tempered regional climate effects on bay salinity. After 5.5ky, changes in foraminiferal assemblages correspond to independently derived records of the El Niño-Southern Oscillation and North American monsoon. Foraminiferal analysis supports sedimentological interpretations in that assemblages and sediments track climate change.

Provenance of the Cretaceous Athabasca Oil Sands, Canada: Implications for Continental-Scale Sediment Transport

Abstract The McMurray Formation of the Athabasca Oil Sands is one of the largest hydrocarbon resources on Earth and contains an extensive record of Early Cretaceous sedimentation. The provenance of the sandstones that constitute this formation has remained largely unknown. New U-Pb detrital zircon ages (n = 848) record a complicated and variable provenance history that involves several major tectonic regions from across North America. The McMurray Formation contains detrital-zircon signatures indicating sediment sources associated with the Canadian Shield, eastern North America, and the Canadian Cordillera. Vertical stratigraphic changes in the dominant detrital-zircon signatures in the McMurray Formation indicate that provenance of the sediment varied during deposition of the unit. Lowermost deposits in the formation are characterized by zircons with ages of ca. 1800–1900 Ma and ca. 2600–2800 Ma, interpreted as derived from the Canadian Shield. Most of the sediment in the McMurray Formation contains detrital zircons with U-Pb ages of ca. 300–600 Ma and ca. 1000–1200 Ma, interpreted to have been derived originally from Appalachian and Grenville sources in eastern North America. Uppermost samples in the McMurray Formation contain abundant zircons with ages less than 250 Ma, derived from the North American Cordillera. When and how sediments from eastern North America were transported to the Athabasca Oil Sands region of Canada is unclear. We propose the sediments were transported either directly from eastern North America during the Early Cretaceous or recycled during the Early Cretaceous from sub-Cretaceous sedimentary strata in western Canada or the southwestern United States. The presence of Proterozoic and Paleozoic zircons in the Athabasca Oil Sands highlights the importance of Appalachian-derived sediment in the Lower Cretaceous stratigraphic record of western Canada.

Examining the processing differences between P and P-S waves in a Rocky Mountain Foothills model

For the past two decades, most of the shear-wave (S-wave) or converted wave (P-S) acquisitions were performed with P-wave source by making the use of downgoing P-waves converting to upgoing S-waves at the mode conversion boundaries. The processing of converted waves requires studying asymmetric reflection at the conversion point, difference in geometries and conditions of source and receiver, and the partitioning of energy into orthogonally polarised components. Interpretation of P-S sections incorporates the identification of P-S waves, full waveform modeling, correlation with P-wave sections and depth migration. The main applications of P-S wave imaging are to obtain a measure of subsurface S-wave properties relating to rock type and fluid saturation (in addition to the P-wave values), imaging through gas clouds and shale diapers, and imaging interfaces with low P-wave contrast but significant S-wave changes. This study examines the major differences in processing of P and P-S wave surveys and the feasibility of identifying converted mode reflections by P-wave sources in anisotropic media. Two-dimensional synthetic seismograms for a realistic rocky mountain foothills model were studied. A Kirchhoff-based technique that includes anisotropic velocities is used for depth migration of converted waves. The results from depth imaging show that P-S section help in distinguishing amplitude associated with hydrocarbons from those caused by localised stratigraphic changes. In addition, the full waveform elastic modeling is useful in finding an appropriate balance between capturing high-quality P-wave data and P-S data challenges in a survey.

Examining the processing differences between P and P-S waves in a Rocky Mountain Foothills model

For the past two decades, most of the shear-wave (S-wave) or converted wave (P-S) acquisitions were performed with P-wave source by making the use of downgoing P-waves converting to upgoing S-waves at the mode conversion boundaries. The processing of converted waves requires studying asymmetric reflection at the conversion point, difference in geometries and conditions of source and receiver, and the partitioning of energy into orthogonally polarised components. Interpretation of P-S sections incorporates the identification of P-S waves, full waveform modeling, correlation with P-wave sections and depth migration. The main applications of P-S wave imaging are to obtain a measure of subsurface S-wave properties relating to rock type and fluid saturation (in addition to the P-wave values), imaging through gas clouds and shale diapers, and imaging interfaces with low P-wave contrast but significant S-wave changes. This study examines the major differences in processing of P and P-S wave surveys and the feasibility of identifying converted mode reflections by P-wave sources in anisotropic media. Two-dimensional synthetic seismograms for a realistic rocky mountain foothills model were studied. A Kirchhoff-based technique that includes anisotropic velocities is used for depth migration of converted waves. The results from depth imaging show that P-S section help in distinguishing amplitude associated with hydrocarbons from those caused by localised stratigraphic changes. In addition, the full waveform elastic modeling is useful in finding an appropriate balance between capturing high-quality P-wave data and P-S data challenges in a survey.

The Evolution and Effect of the Velocity Model on Various Aspects of Surface Seismic Data Analysis

Abstract Well log data, which are measured in depth, provide detailed information only in the vicinity of the boreholes, while surface seismic data, which are measured in units of time, can reveal important structural and stratigraphic changes beyond well locations. Integrating seismic with well data provides a much more extensive understanding of the subsurface and is key for modelling and developing a subsurface reservoir. Therefore, developing a good velocity model that converts seismic data from the time domain to the depth domain provides an effective means to integrate data from the two domains, depth and time. This study examines the value of additional well and seismic data acquired over the course of four years, and the respective merits of updated velocity models. The Illinois Basin – Decatur Project (IBDP), is a demonstration project funded by the US Department of Energy, and involves injection of CO2 into a highly porous, deep sandstone reservoir several kilometres below the surface. Four velocity models have been created to support a carbon dioxide (CO2) geological storage project located at the Archer Daniels Midland Company (ADM) in Decatur, Illinois, USA. Predicted formation depths from the different velocity models for subsequently drilled wells will be compared. As the CO2 storage project has matured and new data have become available, velocity models have been developed representing the years: 2010, 2011, 2012, and 2013. The models enabled depth predictions of geologic strata in subsequently drilled wells in late 2010 and in 2012. The 2012 model integrated the measurements from three wells and two seismic surveys to update the reservoir model. In 2013, a thorough study comparing well and seismic data identified weaknesses in some measurements, for example, the 3D surface seismic data was negatively influenced by inter-bed multiple reflection events and indicated the need for seismic reprocessing of the 3D data. The evolution of the velocity models has been driven by the progressive availability of new data. Because uncertainty in velocity models occur away from wells used for calibration, velocity model updates with newly acquired data will continue to be beneficial given the demands for accurate estimation of structure and properties in the spaces between wells.

Seismic attribute driven integrated characterization of the Woodford Shale in west-central Oklahoma

The organic-rich, silty Woodford Shale in west-central Oklahoma is a prolific resource play producing gas and liquid hydrocarbons. We calibrated seismic attributes and prestack inversion using well logs and core information within a seismic geomorphologic framework to define the overall basin architecture, major stratigraphic changes, and related variations in lithologies. Core measurements of elastic moduli and total organic content (TOC) indicated that the Woodford Shale can be broken into three elastic petrotypes important to well completion and hydrocarbon enrichment. Upscaling these measurements facilitates regional mapping of petrotypes from prestack seismic inversion of surface data. Seismic attributes highlight rugged topography of the basin floor of the Paleo Woodford Sea, which controls the lateral and vertical distribution of different lithofacies containing variable quantity of TOC as well as quartz, which controls brittleness. Depressions on the basin floor contain TOC-lean cherty lithofacies alternating with TOC-rich lithofacies, resulting in brittle-ductile rock couplets. In contrast, basin floor highs are characterized by overall TOC-rich ductile lithofacies. Seismic attributes illuminate complex post-Woodford tectonic deformation. The Woodford Shale is known to be naturally fractured on outcrop. Image log analysis in other shale plays showed a good correlation between such tectonic features and natural fractures. These features need to be correlated with well trajectories and production data to determine which hypothesized “fracture sets,” if any, improve well performance.

Sediment geochemistry as a provenance indicator: Unravelling the cryptic signatures of polycyclic sources, climate change, tectonism and volcanism

AbstractInterpretation of bulk‐sediment geochemistry is one of several approaches for determining sediment provenance. This study investigates the value added by bulk‐sediment geochemical analysis in interpreting provenance in a passive margin clastic basin, the Upper Jurassic–Lower Cretaceous deltaic sediments of the Scotian Basin. Provenance studies in this basin are challenging because source tectonic terranes are parallel to the basin margin and polycyclic sediment sources are abundant. More than 400 samples of mudstone and sandstone representing the geographical and stratigraphic range of interest were analysed for 57 elements. Diagenetic processes added calcium to many samples and removed potassium in rocks buried below 3 km, thus impacting principal component analysis and published weathering indices. However, multiple geochemical approaches to assessing the degree of weathering showed climatically controlled changes in weathering in the Tithonian and Barremian, and changes in supply from major tectonic events, such as the top‐Aptian uplift in the Labrador rift. Covariance of elements in heavy minerals demonstrates the varying magnitude of polycyclic supply and stratigraphic changes in sources. Geochemical analyses revealed a previously unsuspected Tithonian alkali volcanic sediment source, characterized by high niobium and tantalum. The lack of highly contrasting sources means that geochemistry alone is inadequate to determine sediment provenance. Published discrimination diagrams are of limited value. Statistical analysis of geochemical data is strongly influenced by diagenetic processes, episodic volcanic inputs and polycyclic concentration of resistant heavy minerals in sandstones. Single indicator elements for particular sources are generally lacking. Nevertheless, careful consideration of geochemical variability on a case by case basis, integrated with detrital mineral studies, provides new insights into palaeoclimate, sediment provenance and, hence, regional tectonics. Although there is no simple template for such analysis, this study demonstrates an approach that can be used for other basins.

A model for the demise of large, glacial Lake Ojibway, Ontario and Quebec

Large glacial lakes modulated the return of meltwater to the ocean during deglaciation, and their drainage may have initiated global climate change. Yet few records of their drainage come from observations within their basins. Sediment cores from nine lakes along a 240-km transect from northwestern Quebec to northeastern Ontario cover a portion of former Lake Ojibway and provide a stratigraphy of the terminal phase of this large glacial lake. Magnetic susceptibility, density, grain size, X-ray fluorescence chemistry and X-ray diffraction data were used to characterize stratigraphic changes within the basin. The basal sequence consists of till and rhythmites, with ice-proximal debris flows overlain by varves. The varves thin up-section and become unrecognizable, which indicates decreased deposition rates. This fine-grained sediment forms the matrix of a clay-pebble conglomerate. The clay-pebbles are ice-rafted debris (IRD). The IRD flux was probably constant, whereas the sedimentation rate of the finer-grained matrix decreased. The end of IRD marks the cessation of icebergs in the lake and is the best indication for drainage of the glacial lake. The conglomerate is capped by laminated to massive gray silt deposited after lake drainage and marks the transition to organic-rich, post-glacial lakes. Such sequences place drainage into the broader context of deglaciation.

Diagenesis and quartz cement distribution of low-permeability Upper Triassic–Middle Jurassic reservoir sandstones, Longyearbyen CO2 lab well site in Svalbard, Norway

Reservoir properties of Upper Triassic–Middle Jurassic sandstones, Spitsbergen, are studied as part of a CO2 storage pilot project in Longyearbyen. The reservoir formations show large contrasts in sandstone compositions, with unexpected low permeability despite moderate porosity values. Petrographic analyses were performed to investigate the influence and distribution of diagenesis. It is concluded that, because of various compaction, cementation, and dissolution processes, the sandstone porosity is mainly isolated molds and micropores and associated with fibrous illite and chamosite, explaining the low permeability. Diagenesis and the distribution of quartz cement is influenced by lithofacies and detrital compositions. Mineralogically immature sandstones (De Geerdalen Formation) show a homogeneous distribution of quartz cement overgrowths on quartz grains, distributed interstitial to labile grains and other cements (e.g., late calcite). The main silica source was from the dissolution of adjacent feldspar and labile grains as part of the chemical compaction. In contrast, quartz-dominated sandstones (Knorringfjellet Formation) show a heterogeneous patchy distribution of quartz cement influenced by the sedimentary bioturbation pattern, with silica sourced also from dissolution at clay-rich microstylolites. Phosphatic beds at the base and top of the formation are strongly influenced by marine eogenesis and reworking processes and associated with concentration of iron-rich authigenic minerals. The highest porosity appears in sand-supported conglomerate where moldic clay-mineral ooids contributed to reduce quartz cementation. The stratigraphic change from mineralogical immature (Triassic) to mature (uppermost Triassic–Jurassic) sandstone compositions is detected in wide areas of the Barents Shelf and has considerable implications for the distribution of sandstone reservoir properties.

500 Million years of rift- and unconformity-related Mn mineralization in the Middle East: A geodynamic and sequence stratigraphical approach to the recycling of Mn

The crust along the northern boundary of Gondwana, what is called today Jordan, Israel, Turkey and Egypt, is enriched in Mn. During the geodynamic evolution of this crustal part, lasting for 500million years, the opening of the Paleo- and the Neotethys Oceans was accompanied by the emplacement of several medium- to small-sized Mn deposits in the Middle East region.Manganese concentration was linked in time and space closely to the evolution of triple junctions which mark the break-up of the crust in northern Gondwana and whose linear fault zones deeply penetrated into the upper mantle so that alkaline magmas in the Middle East and East Africa could use these plate boundaries as conduits for their ascent. The first triple junction on the Sinai Peninsula, active around 500Ma ago, marked the beginning of rifting and Mn recycling at the northern edge of Gondwana, the second one in the Afar Region, Ethiopia, denotes factually the end of the first tectono-metallogenetic phase and the onset of another Mn cycle still being in its embryonic state.In the upper crust Mn was recycled by episodic rifting to form Mn deposits during the Cambrian, the late Paleozoic, the Paleogene and the Quaternary in the Middle East. Manganese mineralization is bound to siliciclastic rocks containing Mn oxides and hydrates, e.g., birnessite, pyrolusite, and manganite. Ba, K, Pb, Co and Ni act as chemical qualifiers in these oxidic minerals and allow for a more subtle subdivision of different stages of Mn mineralization (romanechite, cryptomelane, coronadite, asbolane). Concentration of Mn involved deposition in near-shore marine basins under diagenetic, epigenetic hydrothermal, roll-type and supergene conditions. It was mediated by Cu mineralization and microbial processes and, in places, accompanied by baryte concentration.The branches of the Middle East triple junctions were not equally operative through time:(1)The Najd strike-slip zone was active during the Precambrian and Cambrian and was only re-activated after a long period of tectonic quiescence within the Red-Sea Graben(2)The N-trending Dead-Sea Transform Fault (DSTF) was active all the way till the Recent.(3)The third branch of the triple junction, the SW Egypt Rift was episodically active, e.g., during the mid-Permian–Jurassic period, leading to the late Permian Mn concentration. Its NE prolongation, the Palmyra Trough, gave rise to an embryonic Neo-Tethys Ocean and paved the way for Turkey to drift away from the Afro-Arabian Plate leading to a separate Mn province during the Cenozoic.By its drifting away, part of the Mn preconcentrated in the crustal section of Gondwana was also taken away and recycled later north of the Neo-Tethys in what is called today Turkey. During the late Mesozoic, advanced spreading in the Neo-Tethys Ocean generated oceanic crust and added another batch of Mn from the mantle to the crustal Mn cycle in form of Mn-bearing mud on top of the ophiolite sequences in the Troodos and Semail Ophiolites Cenozoic. Mn mineralization resulted from redeposition of an intracrustal Mn repository along the NW prolongation of the Red Sea Rift and along the N prolongation of the DSTF during the Eocene in Turkey and Egypt.Unlike many of the so-called giant Mn deposits located on stable cratons that owe their mineral wealth to their geodynamic persistence and in-situ re-working of Mn, Middle East rift-related Mn concentration was later subjected to considerable dilution by plate motion, leading to a number of Mn occurrences displaced around the former triple junction instead of one unique “giant deposit” on top of it. Mn concentration in the Middle East is confined to megasequence boundaries (AP1/AP2, AP5/AP6, AP10/AP 11), using sequence stratigraphic principles for a temporal subdivision. The crustal sections most productive for Mn concentration are those megasequences typical of maximum regression surfaces or, in other words, where conformities were correlative with unconformities. An important chemical factor of Mn enrichment related to these sequence stratigraphical changes can be microbial activity both in oxic Mn enrichments and anoxic, sulfidic environments.These geodynamic issues, described in terms of sequence stratigraphy and plate tectonics, directly translate into the physico-chemical regime, when the periods of a maximum contrast among the redox conditions fostered the complete sequence of tetra- to bivalent oxidic Mn minerals to evolve. In conclusion, the most fertile areas on the globe to concentrate Mn are crustal sections characterized by hot-spots at a triple-junctions, truncated by surfaces of maximum regression that provoked the strongest contrast of redox conditions and thereby gave rise to the precipitation of a wide range of oxidic Mn minerals. During subduction Mn previously preconcentrated in the oceanic crust will be consumed and, hence, active margins rank lower than passive ones when it comes to Mn accumulation.

Fractionation of metals in the Sa1/2 sediment core from Lake Sarbsko (northern Poland) and its palaeolimnological implications

This article documents and interprets stratigraphical changes in fractionation of Fe, Mn, Mg, K, Pb, Cu and Zn in the Sa1/2 sediment core from a coastal freshwater lake, Lake Sarbsko (northern Poland). The elements were sequentially extracted from the samples to distinguish five geochemical fractions: exchangeable, acid-extractable, reducible, oxidisable and residual. The analyses revealed substantial variations in geochemical partitioning of the elements and showed no correlation between the fractionation patterns and lithology of the sediments. In the sediments of Lake Sarbsko, iron is mainly bound to sulfides. Potassium occurs in its residual form. Magnesium and zinc are associated with carbonates and aluminosilicates, while copper occurs in compounds with organic matter and sulfides. Lead is found in connection with aluminosilicates and, to a lesser extent, with sulfides and organic matter. Manganese is partitioned between the oxidisable, acid-extractable, and exchangeable fractions. Heavy metals and potassium display the overall tendency to reduce the contents of their residual forms towards the top of the depositional sequence. Fe, Mn, Mg and Zn were found to be the most susceptible to post-sedimentary mobilisation.

Investigation of proposed Norse irrigation channels and dams at Garðar/Igaliku, Greenland

It has long been proposed that various features within and beyond the confines of the Greenlandic settlement of Garðar, in modern Igaliku, reflect a medieval irrigation system with interconnected channels and cascading dams associated with Norse colonization and agriculture. These have rarely been investigated archaeologically or sedimentologically. After reviewing available evidence and the confusion surrounding terminology, this paper reports on excavations across three supposed irrigation channels. The lithostratigraphies of sediments located behind some of the proposed dams are also described and discussed. Although one of the excavated channels demonstrates convincing evidence for having been constructed (in part at least, through the presence of a vertical cut and the emplacement of flanking turf embankments), two of the channels are revealed as natural topographic features. It seems that if these were integrated into the Norse irrigation network then this was done without any further requirement for their modification or artificial enhancement. Deposits behind the dams were found to be shallow (~0.5 m deep) but contained sedimentary evidence for the former presence of standing water or waterlogged ground in the form of lake muds and peats. At one location (designated as Dam IX), radiocarbon dating of a stratigraphic change from lake mud to peat—indicating succession from open water to more terrestrial conditions—has been dated to cal. AD 1420–1620 [2σ] and may reflect the reservoir falling into disuse following site abandonment. While doubt is thrown on the existence of one dam identified by previous investigators, two possible new dam/reservoir features are presented.

Stratigraphic changes of Ge/Si, REE+Y and silicon isotopes as insights into the deposition of a Mesoarchaean banded iron formation

In order to determine the origin of silicon (Si) in banded iron formation (BIF), we have undertaken a multi-tracer study combining REE+Y data, Ge/Si ratios and Si isotopes (δ30Si) on stratigraphically resolved layers from a ∼2.95Ga BIF from the Pongola Supergroup, South Africa. Si in both Si-rich and Fe-rich layers has a common origin, represented by a seawater reservoir strongly influenced by continent-derived freshwaters (∼10%) and very limited (<0.1%) high-T hydrothermal fluids as indicated by Eu anomalies and Y/Ho ratios. The coevolution of δ30Si signatures of Si- and Fe-rich layers of the BIF coupled with similar Eu and Y anomalies in both types of layers is in accordance with a common silica precipitation promoted by Si adsorption onto Fe-oxyhydroxides from Archaean seawater. An increase in δ30Si values from −2.27‰ to −0.53‰ stratigraphically upwards in the BIF is inferred to be the result of two successive isotopic fractionation processes during (1) silicon adsorption onto the Fe-oxyhydroxide precursor and (2) silica precipitation at the sediment–water interface from pore fluid triggered by the local silica saturation consecutive to an early diagenetic Si desorption from the precursor Fe-oxyhydroxide. The first fractionation process depleted the parental water in 28Si while the second released 30Si back into the parental water, resulting in an increase of the δ30Si value of the parental water reservoir over time.

Climatic control on the composition of Carboniferous–Permian Gondwana sediments, Khalaspir basin, Bangladesh

Late Paleozoic or Permo-Carboniferous Gondwana successions world-wide are marked by widespread and lengthy glaciation (~67Ma), and multiple transitions from Icehouse to Greenhouse state. Mineralogical and compositional changes in sandstones and mudstones of the Gondwana succession in drill hole GDH-45 from the Khalaspir basin of Bangladesh are interpreted in a climatic framework, using the proxies of the Mineralogical Index of Alteration (MIA) and the Chemical Index of Alteration (CIA). Low MIA (<75) and CIA (<70) values in the lowermost unit (Unit 1) of the Khalaspir Gondwana sequence indicates minimal chemical weathering of the source and imply cold and dry glacial climatic conditions. Values increase progressively in the overlying Unit 2, suggesting warming and deglaciation. Very high MIA (80–100) and CIA (90–100) values in Unit 3 indicate intense chemical weathering associated with warm and humid climatic conditions in a post-glacial setting. These changes are recorded in both sandstones and mudstones, illustrating the value of acquiring data for a variety of size grades. The stratigraphic changes are also well recorded by progressively decreasing K2O/Al2O3 and increasing Ga/Rb ratios, the combination of which may comprise a new geochemical proxy of climate and weathering. Upward increase in the modal Q/(F+L) and Qp/(F+L) ratios are also consistent with change from cool to warm and humid climate in the source region. The change in climatic conditions within the Khalaspir sequence is also well correlated with the Gondwana successions of other continental blocks, and records the climatic fluctuations and extent of climatic impact in Gondwanaland during the Permo-Carboniferous.

Constraining clastic input controls on magnetic susceptibility and trace element anomalies during the Late Devonian punctata Event in the Western Canada Sedimentary Basin

Terra Nova, 24, 301–309, 2012AbstractFactor analyses were applied to trace element and magnetic susceptibility (MS) records of the Late Devonian palaeoceanographic ‘punctata Event’ perturbation in the Western Canada Sedimentary Basin to constrain the extent to which geochemical anomalies were driven by 1) changes in clastic input (Factor 1; 55.1% of total variance) vs. 2) anoxia in the depositional environment (Factor 2; 35.2% of total variance). MS associates only with Factor 1 (r = 0.971) and is significantly correlated (99.9% level) with acid‐insoluble limestone residues and clastic proxies (Al, Si, K, Ti, Zr; r = 0.542–0.639). Based on XRD analyses of a small subset of these residues (N = 7), a multivariate linear regression model accounts for 97.7% of total MS variance as a function of variable admixing of illite, pyrite, quartz and feldspar, in turn variably diluted by stratigraphic changes in the total carbonate content.