ankerite cementation in deeply buried jurassic sandstone reservoirs of the central north sea abstract upper jurassic fulmar formation shelf sandstones of the high temperature high pressure franklin and elgin fields south central graben north sea contain abundant disseminated and concretionary ankerite in contrast most jurassic north sea reservoirs contain only minor amounts of dispersed ankerite disseminated ankerite cement in the franklin and elgin fields has a fairly uniform isotopic composition 18o 10 to 12 5o oo pdb 13c 3 to 5o oo pdb ankerite concretions have 18o values similar to disseminated cements but a wider range of 13c values 1 to 5 5o oo pdb they also have highly variable intergranular volumes which together with the 13c data are interpreted as a combination of pore filling cementation and in situ replacement of comminuted bioclastic debris by ankerite fluid inclusion 18o and paragenetic evidence suggests that ankerite formed during deep burial c 3 5 to 4 5 km 140 170 c after the onset of overpressuring but before hydrocarbon emplacement in the reservoirs the regionally consistent 18o data suggest that ankerite formed via a temperature influenced mechanism and the relatively uniform 13c cement value indicates that organic matter and marine bioclastic carbonate contributed to the dissolved carbon reservoir in constant proportion this can be explained by calcite dissolution in response to ph decrease during thermal breakdown of organic acids such acids were derived from adjacent mudrocks undergoing hydrocarbon maturation and clay mineral transformations and are likely to have been transported in pore fluids with mg2 and fe2 the presence of these cations in solution upon thermal decarboxylation is inferred to have stabilized ankerite at the expense of calcite a relative paucity of ankerite in other fulmar formation reservoirs may reflect different sedimentological compositions less bioclastic debris and or lower burial temperatures less advanced decarboxylation

provenance of neoproterozoic and early paleozoic siliciclastic rocks of the tepla barrandian unit bohemian massif evidence from u pb detrital zircon ages abstract new la icp ms u pb detrital zircon ages from ediacaran and paleozoic siliciclastic rocks are used to constrain provenance and paleogeographic affinities of the tepla barrandian unit tbu in the centre of the bohemian massif central europe czech republic the samples taken span the period from 635 ma to 385 ma and permit recognition of provenance changes that reflect changes in geotectonic regime detrital zircon age spectra of two ediacaran one lower cambrian and three upper ordovician samples resemble the ages known from the nw african proportion of gondwana particularly the trans saharan belt while three rocks from higher lower cambrian to lowermost ordovician strata contain detritus that may have been derived exclusively from local sources the age spectrum of the devonian rock is a combination of the nw gondwanan and local features these new findings in combination with a wide range of published data are in agreement with a neoproterozoic subduction related setting at the margin of gondwana followed by a cambrian early ordovician rifting stage and an ordovician passive margin setting furthermore the data are in favour of a position of the tbu at the gondwanan margin throughout pre variscan times

grain size dependence of sediment composition and environmental bias in provenance studies abstract this article investigates both experimentally and theoretically the compositional changes associated with textural effects and hydraulic sorting during sediment transport and deposition which cause systematic distortion in quantitative provenance analysis environmental bias traditional procedures aimed at eliminating textural noise find limited success the gazzi dickinson method cannot remove hydrodynamic related modal variability multiple size window strategies are time consuming narrow or moving size window strategies represent misleading or impractical short cuts being less convenient options than simply analysing each sample in bulk new concepts introduced here unravel the superposed causes of compositional variability in modern sediments intrasample modal variability fundamentally explained by settling equivalence relationships can be accurately modelled mathematically intersample modal variability principally resulting from selective entrainment can be assessed and removed by a simple principle in absence of provenance changes and environmental bias the weighted average density of terrigenous grains srd index should be equal for each sample and each grain size class of each sample to the weighted average density of source rocks by correcting relative abundances of detrital minerals in proportion to their densities we can restore the appropriate srd index for any provenance and subprovenance type in each sample or grain size class modal variability is effectively reduced by this procedure which can be applied confidently to modern sediments deposited by tractive currents in any environment good results are obtained even for placer sands and finest classes where heavy mineral concentration is strongest such srd correction also successfully compensates for biased narrow window modes thus providing a numerical solution of general validity to the problem of environmental bias in sedimentary petrology after compensating for settling equivalence and selective entrainment effects residual size dependent compositional variability may be provenance related minor in ganga brahmaputra sediments provenance related effects are spectacularly displayed in the nile basin where volcaniclastic silt mixes with basement derived quartzofeldspathic sand and wind blown saharan quartz

heat transfer in magma in situ heat transfer rates in a basaltic magma were measured under typical magma chamber conditions and a numerical model of the experiment was used to estimate magma viscosity the results are of value for assessing methods of thermal energy extraction from magma bodies in the upper crust as well as for modeling the evolutionary track of these systems

mesozoic evolution of the tisza mega unit the south eastern part of the basement of the pannonian basin is made up of variscan crystalline complexes and early mesozoic formations showing striking affinity with the corresponding formations in the southern margin of the european plate this large composite structural unit which is actually an exotic terrane of european plate origin has been named the tisza mega unit based upon relevant data of the pre tertiary basement of southern hungary the reconstruction of the position of the tisza terrane in the early alpine evolutionary stages the process of its separation and break off from the european plate and results of its eo alpine deformations are summarised in the present paper in the variscan and early alpine evolutionary stages the area of the later tisza mega unit was located at the margin of the european plate during variscan orogeny terrane accretion led to intensive deformation and metamorphism in this belt this was followed by transpressional tectonics and the development of molasse basins in the late and post variscan stages and passive margin evolution after the neotethys opening in the middle triassic the separation of the tisza mega unit began with incipient continental rifting along the axis of the later ligurian penninic vahic oceanic branch in the late triassic the end of terrigenous material deposition in the most external zones and a coeval change in fossil assemblage point to the separation of the tisza block from the european plate in the early bathonian significant rotation of the tisza mega unit and coeval paroxysm of alkaline rift type basalt volcanism took place in the early cretaceous in the mid cretaceous due to the northward motion of the adria block and the related closure of the westernmost neotethys basin the extensional regime changed to a compressional one leading to onset of the nappe stacking and low grade regional metamorphism within the tisza microplate in the foreland of the nappe systems flexural basins came into existence that are characterised by flysch type sedimentation in the early tertiary the north eastward motion of the alcapa and tisza dacia blocks led to the formation of the present day heterogeneous basement of the pannonian basin

contrasting p t t paths from the basement of the tisia unit slavonian mts ne croatia application of quantitative phase diagrams and monazite age dating abstract medium grade mica schists and intercalated paragneisses and amphibolites from the basement of the tisia unit slavonian mountains northeastern croatia contain complexly zoned garnets at the kutjevo locality mica schists are characterised by garnets with mn rich cores and ca rich rims mn decreases steadily from core to rim and ca increases abruptly this is in contrast to the paragneisses and amphibolites which contain garnets with smoothly decreasing ca from core to rim quantitative phase diagrams and garnet composition isopleths calculated from bulk rock analyses reveal that the ca poor garnet cores in the mica schists formed during an earlier event at 584 592 c and 6 4 7 8 kbar ca rich rims formed at conditions of 600 660 c and 11 12 kbar calculated using garnet isopleths and mineral thermobarometry the paragneiss and amphibolite provide similar p t information for the later peak event ca 650 c 10 12 kbar but do not preserve a record of the earlier lower p t event and modelling shows that garnet was not stable at these conditions contrary to previous studies on this outcrop and rock type no staurolite was observed and quantitative phase diagrams contoured for h 2 o mode isopleths indicate that the rock did not cross staurolite bearing fields during the retrograde p t path mica schists from the krndija locality contain zoned polyphase garnets phase diagram calculations reveal that ca rich garnet cores formed between 520 and 630 c and 7 8 kbar rims have a lower ca content and formed at considerably reduced pressures together with andalusite and staurolite at ca 530 570 c and 3 4 kbar since both localities were traditionally considered to be part of the same tectono metamorphic unit evidence presented here clearly shows that this cannot be the case emp monazite ages are variscan 350 ma in the krndija mica schists and around pre or early variscan 440 ma in the kutjevo mica schists we therefore propose a more complex internal structure and metamorphic history for this area than previously recognised suggesting a metamorphic evolution of the slavonian mountains that includes a pre variscan and a variscan cycle

conventional and in situ geochronology of the tepla crystalline unit bohemian massif implications for the processes involving monazite formation the tepla crystalline unit tcu western bohemian massif proves highly suitable for studying the effects of differential metamorphic reworking on the u th pb systematics in monazite as the overprint of variscan regional metamorphism onto high grade cadomian paragneisses intensifies progressively towards the northwest although variably hampered by scarcity small size and low uranium contents of monazite isotope dilution thermal ionisation mass spectrometry of monazite from paragneisses from the garnet staurolite and kyanite zones of the tcu gives a narrow 206pb 238u age range from 387 to 382 ma for variscan peak metamorphism these data are supported by 382 373 ma monazite ages derived from electron microprobe analyses inheritance of older components in grains from the central tcu imply major resetting of pre variscan monazite around 380 ma possibly due to widespread garnet growth during variscan metamorphism which led to the consumption of pre variscan high y monazite and subsequent growth of new low y monazite concordant 498 494 ma monazite ages in a migmatitic paragneiss close to the adjacent marianske lazn complex mlc grew in response to metagabbro emplacement in the mlc from 503 to 496 ma and not during either cadomian or variscan regional metamorphism backscatter imaging and electron microprobe analyses reveal that discordant monazite of the migmatite comprises a mix of various age domains that range from ca 540 to 380 ma combined evidence presented here suggests that instead of pb loss by volume diffusion the apparent resetting of the u th pb systematics in monazite rather involves new crystal growth or regrowth by recrystallisation and dissolution reprecipitation

petrology and geochemistry of orthoamphibolites from the variscan metamorphic sequences of the south tisia in croatia an overview with geodynamic implications in the southern pannonian basin the variscan barrovian and overprinted abukuma type progressive metamorphic sequences of the south tisia in slavonian mts sm and mt moslavacka gora mg are interlayered with orthoamphibolites these sequences represent part of a disrupted variscan belt that extends southeastwards of the bohemian massif through the carpathians to the caucasus orthoamphibolites contain hornblende mg hornblende tschermakite pargasite and edenite plagioclase an48 98 mg and an26 35 sm biotite grossular mg and almandine sm enriched garnet diopside mg with accessory ilmenite titanite and clinozoisite based on major and trace element analyses and cipw norms these orthoamphibolites which originally were olivine tholeiites to slightly alkalic basalts can be correlated with tholeiites of consuming plate margins 18o of orthoamphibolites varies from 5 8 to 7 1 the measured 87sr 86sr ratio from 0 70435 to 0 70665 which indicates slight continental crust contamination and the calculated ratio from 0 70262 to 0 70535 indicating an upper mantle origin of the original basaltic melts for associated penecontemporaneous i type granitoids the calculated initial 87sr 86sr ratio also have upper mantle values ranging between 0 70293 and 0 70368 geochemical data for orthoamphibolites and the associated penecontemporaneous i type granites and the occurrence of alpine type ultramafic bodies within the barrovian sequence indicate that they have many features in common with orogenic associations related to recent subduction related settings some characteristic element ratios suggest back arc basin environments and the ce k vs ce diagram suggests a pargasite lherzolite source its partial melting gave primitive basaltic melts of olivine tholeiite to slight alkalic affinities that produced at first differentiated mafic rocks by olivine fractionation future orthoamphibolites and afterwards a strongly differentiated suite of i type granitoids by amphibole fractionation magma emplacement and subsequent afc processes took place in subduction environments preceding the main variscan deformational metamorphic event during which orthoamphibolites were generated this interpretation is compatible with geodynamic modeling of this part of the paleotethys

provenance of the upper triassic siliciclastics of the mecsek mountains and villany hills pannonian basin hungary constraints to the early mesozoic paleogeography of the tisza megaunit the tisza megaunit in the southern pannonian basin formed part of the southern margin of the european plate in the early mesozoic era its exact paleo position and relation to other structural blocks is disputed for a long time detrital zircon u pb dating heavy mineral analysis and petrographical examination of carnian to pliensbachian sandstone members lead to better understanding of the provenance of clastic deposits after the ladinian carnian carbonate to siliciclastic facies shift in the southwestern tisza megaunit investigations allow for constraining its paleogeographic relation to adjacent units the carnian and pliensbachian siliciclastics of the villany hills derive from inside the southwestern tisza megaunit i e the medium grade polymetamorphic rocks of the adjacent slavonian mountains or similar basement fragments the upper triassic clastic deposits of the mecsek mountains most likely derive from variscan felsic plutonic rocks of the local basement or partially from the southern southwestern bohemian massif about 200 ma zircon u pb ages are tentatively interpreted as traces of synsedimentary distal volcanism in the central atlantic magmatic province

paleozoic tectonostratigraphic units of the northwest and central dinarides and the adjoining south tisia in the central dinarides and south tisia different paleozoic complexes occur in four geotectonic zones 1 comparatively autochthonous units located in the cores of disrupted anticlines of the external dinarides 2 allochthonous disrupted units accompanied by more predominant triassic formations in the sava nappe which is thrust onto the northeastern margin of the external dinarides 3 allochthonous disrupted units also together with triassic formations in the pannonian and durmitor nappes of the internal dinarides and 4 polymetamorphic sequences in basement of the pannonian basin and south tisia respectively this paper presents basic geological features for the main paleozoic areas included in these four zones the tectonostratigraphic units of the first two zones were related to the gondwana passive continental margin those of the third zone to the paleotethyan oceanic realm and those of tisia to the active laurussia margin geodynamic evolution of all these paleozoic complexes was related to opening and closure of the rheic and paleotethys oceans rifting processes along north gondwana started in the silurian locally in the cambrian ordovician and were followed by the late silurian devonian opening of the paleotethys subduction processes were active by the end of the devonian and at the beginning of the carboniferous along the laurussia margin they were followed during the westphalian by main variscan deformation during collision of gondwana and laurussia associated metamorphism was very low grade in the paleozoic units of the sava nappe low grade to epidote amphibolite grade within the paleozoic complexes of the pannonian and durmitor nappes in the internal dinarides and poly metamorphic with migmatites and granitoids in south tisia these processes gave rise to a pangea stage with the variscan basement disconformably overlain by late carboniferous and permian sediments

world energy developments 1981 the papers in this issue review significant petroleum and natural exploration and production activities throughout the world with the exception of the soviet union in addition papers on other energy minerals have been included to prepare these papers well data land status production statistics and geophysical activity information was accumulated from numerous sources such as foreign and domestic scouting services foreign and domestic governmental agencies at all levels industry organizations oil companies individual consultants and local representatives 1 fig 1 tab

carbonate cementation in a sequence stratigraphic framework upper cretaceous sandstones book cliffs utah colorado abstract three macroscopic diagenetic features can be recognized in the sandstones of the upper cretaceous desert member of the blackhawk formation and castlegate sandstone of the mesaverde group exposed in the book cliffs utah each of which have distinctive form geometry and stratigraphic distribution diagenetic alterations are 1 leached zones whitecaps up to 10 m thick beneath coal beds 2 large up to 8 m concretionary carbonate cemented bodies in amalgamated shoreface and thin fluvial sandstones and 3 thin up to 2 m laterally extensive carbonate cemented horizons beneath major marine flooding surfaces each feature has distinct petrographic and geochemical signatures and formed through discrete diagenetic processes large isolated carbonate cemented bodies are composed of ferroan dolomite most of which precipitated during early diagenesis field and petrographic data coupled with stable isotope data early cements 13c 2 5 to 3 4o oo vpdb 18o 7 8 to 12 0o oo vpdb 87sr 86sr 0 7078 later cements 13c 3 1 to 5 7o oo vpdb 18o 12 0 to 15 1o oo vpdb 87sr 86sr 0 7093 suggest precipitation from meteoric fluids input into sediments during times of relative sea level fall the source of carbonate for the dolomite cement was dissolution of detrital dolomite from beneath coals by organic acids and subsequent mobilization by meteoric fluids carbonate precipitation in laterally extensive cement horizons appears to have started as a result of hiatus in sediment accumulation during marine flooding events relative sea level rise cement precipitation in these horizons continued through sediment burial as a result of organic matter oxidation reactions in overlying organic rich mudstones the results of this study show a link between sedimentation related to changes in relative sea level and diagenesis leading to the potential for the development of process based predictive models of early diagenesis in depositional successions

synchroneity of climatic change and extinctions in the late triassic although the late triassic was a time of widespread aridity evidence exists for a significant increase in rainfall during the middle to late carnian upper triassic playa lake sediments were interrupted by late carnian fluviatile sandstones with erosive bases and high kao linite illite ratios there was also an increase in the clastic component of marine sequences during this interval middle and upper carnian marine carbonates show an extreme depletion in 13 c values consistent with increased fresh water influx large scale karstic phenomena in limestone areas subaerially exposed during the late triassic are a further indication of increased rainfall important faunal and floral changes occurred during the carnian norian interval marine invertebrate turnover was greatest at the lower middle carnian boundary and terrestrial extinctions were concentrated at the carnian norian boundary the cause of this carnian pluvial episode may have been related to the rifting of pangea through disruption of atmospheric and oceanic circulation patterns eustatic changes or the effects of volcanism associated with rifting a change in surface ocean temperature salinity or ph or habitat loss may have caused the decline of many shallow marine invertebrates at the start of the middle carnian a return to arid conditions at the carnian norian boundary would account for the turnover among terrestrial vertebrates and plants

mesozoic plate tectonic reconstruction of the carpathian region abstract palaeomagnetic palaeobiogeographic and structural comparisons of different parts of the alpine carpathian region suggest that four terranes comprise this area the alcapa tisza dacia and adria terranes these terranes are composed of different mesozoic continental and oceanic fragments that were each assembled during a complex late jurassic cretaceous palaeogene history palaeomagnetic and tectonic data suggest that the carpathians are built up by two major oroclinal bends the alcapa bend has the meliata oceanic unit correlated with the dinaric vardar ophiolite in its core it is composed of the western carpathians eastern alps and southern alcapa units transdanubian range bukk this terrane finds its continuation in the high karst margin of the dinarides further elements of the alcapa terrane are thought to be derived from collided microcontinents czorsztyn in the n and a carbonate unit tisza in the se the tisza dacia bend has the vardar oceanic unit in its core it is composed of the bihor and getic microcontinents this terrane finds its continuation in the serbo macedonian massif of the balkans the bihor getic microcontinent originally laid east of the western carpathians and filled the present carpathian embayment in the late palaeozoic early mesozoic the vardar ocean occupied an intermediate position between the western carpathian austroalpine transdanubian high karst margin and the bihor getic serbo macedonian microcontinent the vardar and pindos oceans were opened in the heart of the mediterranean adriatic microcontinent in the late permian middle triassic vardar subducted by the end of jurassic causing the bihor getic serbo macedonian microcontinent to collide with the internal dinaric western carpathian margin an external penninic vahic ocean tract began opening in the early jurassic separating the austroalpine western carpathian microcontinent and its fauna from the european shelf further east the severin ceahlau magura also began opening in the early jurassic but final separation of the bihor getic ribbon and its fauna from the european shelf did not take place until the late middle jurassic the alcapa and the tisza dacia were bending during the albian maastrichtian the two oroclinal bends were finally opposed and pushed into the gates of the carpathian embayment during the palaeogene and neogene at that time the main n s shortening in distant alpine and hellenic sectors was linked by a broader right lateral shear zone along the former vardar suture

zeolites in petroleum and natural gas reservoirs zeolites have been reported in petroleum and natural gas reservoirs of a wide range of geological ages from many parts of the world such zeolite bearing oil and gas reservoirs are not comparatively large in number and scale however where present in petroleum and natural gas reservoirs zeolites can have profound effects on the quality of the reservoirs affecting rock porosity and permeability zeolite containing reservoir rocks vary widely lithologically including not only porous pelitic sedimentary rocks but also volcanogenic rocks and even fractured intrusive and plutonic rocks several different genetic types of zeolite occurrences have been distinguished and classified e g hay 1966 1986 1995 iijima 1980 1986 as hay and sheppard discussed and summarized in an earlier chapter this volume in addition the organic origin theory for the formation of economic oil and gas resources seems to have been universally accepted including the prominent kerogen decomposition theory summarized by tissot and welte 1978 sedimentary basins filled by very thick sequences of sedimentary strata containing organic rich source rocks and porous reservoir rocks are considered necessary for producing hydrocarbon resources such sedimentary basins develop primarily through rifting or in relation to arc formation at active plate boundaries and they are often accompanied by heating and extensive volcanic activity most types of zeolite occurrence are consequently constrained to burial diagenesis uplift related late stage diagenesis surdam and boles 1979 boles 1993 and hydrothermal alteration as will be discussed in this chapter zeolites in hydrocarbon reservoir rocks occur commonly as cements and grain replacements in the form of disseminated microcrystalline aggregates and occasionally as macroscopic crystals in vugs and veins it was in 1950s that coombs and co workers pioneered the petrologic study of rock forming zeolites through their classic research on burial metamorphism of the triassic geosynclinal deposits in south syncline new

the baltica gondwana suture in central europe evidence from k ar ages of detrital muscovites and biogeographical data

pre variscan geological events in the austrian part of the bohemian massif deduced from u pb zircon ages in an attempt to elucidate the pre variscan evolution history of the various geological units in the austrian part of the bohemian massif we have analysed zircons from 12 rocks mainly orthogneisses by means of shrimp conventional multi grain and single grain u pb isotope dilution mass spectrometry two of the orthogneisses studied represent cadomian metagranitoids that formed at ca 610 ma spitz gneiss and ca 580 ma bittesch gneiss a metagranite from the thaya batholith also gave a cadomian zircon age 567 5 ma traces of neoproterozoic zircon growth were also identified in several other samples underlining the great importance of the cadomian orogeny for the evolution of crust in the southern bohemian massif however important magmatic events also occurred in the early palaeozoic a sample of the gfohl gneiss was recognised as a 488 6 ma old granite a tonalite gneiss from the realm of the south bohemian batholith was dated at 456 3 ma and zircon cores in a moldanubian metagranitic granulite gave similar ages of 440 450 ma this ordovician phase of magmatism in the moldanubian unit is tentatively interpreted as related to the rifting and drift of south armorica from the african gondwana margin the oldest inherited zircons in a migmatite from the south bohemian batholith yielded an age of ca 2 6 ga and many zircon cores in both moravian and moldanubian meta granitoid rocks gave ages around 2 0 ga however rocks from the moldanubian unit show a striking lack of zircon ages between 1 8 and 1 0 ga reflecting an ancestry from armorica and the north african part of gondwana respectively whereas the moravian bittesch gneiss contains many inherited zircons with mesoproterozoic and early palaeoproterozoic ages of ca 1 2 1 5 and 1 65 1 8 ga indicating a derivation from the south american part of gondwana

triassic redbeds in the malaguide complex betic cordillera spain petrography geochemistry and geodynamic implications abstract sandstone petrography and mudstone mineralogy and geochemistry of triassic mudstones and sandstones from continental redbeds of the malaguide complex betic cordillera southern spain provide useful information on provenance palaeoclimate and geodynamics during the early stages of the pangea break up and on their diagenetic evolution the sandstones are quartzarenites to sub litharenites with minor lithic fragments and rare feldspars the mudstone samples show a paas like elemental distribution the samples likely record recycling processes from their metasedimentary basement rocks that significantly affected the weathering indices and monitors cumulative effects including a first cycle of weathering at the source rocks sandstone composition and chemical mineralogical features of mudstones record a provenance derived from continental block and recycled orogen that were weathered under warm and episodically wet climate source areas were located towards the east of the present day malaguide outcrops and were formed by fairly silicic rock types made up mainly of palaezoic metasedimentary rocks similar to those of the paleozoic underlying series with subordinate contributions from magmatic metamorphic sources and a rare supply from mafic metavolcanic rocks clay mineral distribution of mudstones is dominated by illite and illite smectite mixed layer that result from differences in provenance weathering and burial temperature history illite crystallinity values illitization of kaolinite occurrence of typical authigenic minerals and apatite fission track studies coupled with a subsidence analysis of the whole malaguide succession suggest burial depths of at least 4 6 km with temperatures of 140 160 c typical of the burial diagenetic stage and confirm the middle miocene exhumation of the betic internal domain tectonic stack topped by the malaguide complex

marcasite in black shales a mineral proxy for oxygenated bottom waters and intermittent oxidation of carbonaceous muds abstract black shales in the geologic record have long been considered to reflect deposition under anoxic or even euxinic conditions although bottom water anoxia favors deposition of black shales certain biological indicators such as benthic fossils and bioturbation suggest that bottom water conditions may not have been as persistently anoxic as commonly presumed examination of twelve marine carbonaceous shale bearing units ranging in age from middle proterozoic to cretaceous suggests that in addition to paleontological clues early diagenetic marcasite formed in uncompacted surficial sediments is a reliable indicator of oxygen bearing bottom waters marcasite requires low ph values to form and oxidation of already existing sedimentary pyrite provides a plausible chemical scenario to achieve that condition in the studied examples degraded pyrite framboids and corrosion features on pyrite grains indicate partial pyrite destruction in surface sediments and suggest that intermittent reoxidation of earlier formed iron sulfides may indeed have generated the low ph values required for marcasite formation thus observations of textures spatially associated with secondary marcasite growth are consistent with a scenario whereby oxygen in overlying sea water drove downward migration of oxidation fronts pervasive marcasite in black shales from a wide range of occurrences suggests multiple reoxygenation events during the deposition of these shale units and calls for more dynamic ocean circulation than previously assumed the latter consideration may for example prove important for those instances where a black shale is associated with an oceanic anoxic event oae because these are commonly presumed to have been deposited under fully anoxic conditions

sedimentary evolution of the mesozoic continental redbeds using geochemical and mineralogical tools the case of upper triassic to lowermost jurassic monte di gioiosa mudrocks sicily southern italy the continental redbeds from the internal domains of the central western mediterranean chains have an important role in the palaeogeographic and palaeotectonic reconstructions of the alpine circum mediterranean orogen evolution since these redbeds mark the triassic jurassic rift valley stage of tethyan rifting the composition and the sedimentary evolution of the middle triassic to lowermost jurassic continental redbeds of the san marco d alunzio unit peloritani mountains southern italy based on mineralogical and chemical analyses suggests that the studied mudrock sediments share common features with continental redbeds that constitute the internal domains of the alpine mediterranean chains phyllosilicates are the main components in the mudrocks the 10 a minerals illite and micas the i s mixed layers and kaolinite are the most abundant phyllosilicates the amount of illitic layers in i s mixed layers coupled with the illite crystallinity values ic are typical of high degree of diagenesis corresponding to a lithostatic tectonic loading of about 4 5 km the mineralogical assemblage coupled with the a cn k plot suggest post depositional k enrichments palaeoweathering proxies pia and ciw record intense weathering at the source area further the studied sediments are affected by reworking and recycling processes and as consequence it is likely these proxies monitor cumulative effect of weathering the climate in the early jurassic favoured recycling and weathering occurred under hot episodically humid climate with a prolonged dry season the source area is the low grade paleozoic metasedimentary basement mafic supply is minor but not negligible as suggested by provenance proxies