the posidonia shale lower toarcian of sw germany an oxygen depleted ecosystem controlled by sea level and palaeoclimate abstract the lower toarcian posidonia shale is famous for its excellently preserved fossils and its high amount of organic matter up to 16 both quality of preservation and accumulation of organic matter have been explained by permanent anoxic bottom water conditions high resolution geochemical sedimentological and palaeoecological investigations of various sections of the posidonia shale in sw germany however indicate that oxygen availability was variable and ranged from short oxygenated periods to longer term anoxia the benthic macrofauna consists of nine fossil communities and was used in combination with geochemical data to reconstruct a time averaged oxygen curve anoxic conditions prevailed during the deposition of the toarcian black shales they were however punctuated by various short periods weeks to years with oxygenated bottom water conditions sedimentological e g distinctiveness of microlamination siliciclastic content and geochemical parameters e g organic matter content isotopic signatures 18o and 13c molecular redox parameters pristane phytane ratio arylisoprenoids exhibit a remarkable covariation and seem to be controlled by sea level fluctuations maximum oxygen depletion and an extreme negative shift of 13corg values 34 occurred during the early falciferum zone this is explained by the recycling of 12c enriched carbon derived from remineralization of organic matter on and within the substrate during low sea level stand and a highly elevated redox boundary including photic zone anoxia the subsequent transgression permitted enhanced water exchange with the tethyan ocean and caused improvement of living conditions at the end of the falciferum zone other important factors controlling the depositional environment are the overall palaeogeographic situation and climate the early jurassic is the latest period before break up of pangaea and probably was ruled by a strong meridional atmospheric circulation system with pronounced seasonal changes of prevailing trade and monsoon wind systems an estuarine circulation with a positive water balance and surface water with slightly reduced salinity in the summer alternated with an anti estuarine circulation and a negative water balance in the winter during the summer months a stratified water column with anoxic conditions below the halocline developed 18o data indicate low salinity in the surface water during the monsoon influenced summer high productivity was then located in the photic zone and the corresponding isotopically light 18o signal was fixed in the calcareous nannoplankton during the winter months a saline circulation system brought oxygen to the benthic environment favouring temporary benthic colonization especially during times of relative sea level high stand

carbonate platform evolution from a bioconstructed platform margin to a sand shoal system devonian guilin south china the depositional organization and architecture of the middle late devonian yangdi rimmed carbonate platform margin in the guilin area of south china were related to oblique extensional faulting in a strike slip setting the platform margin shows two main stages of construction in the late givetian to frasnian with a bioconstructed margin evolving into a sand shoal system in the late givetian the platform margin was rimmed with microbial buildups composed mainly of cyanobacterial colonies mostly renalcis and epiphyton these grew upwards and produced an aggradational locally slightly retrogradational architecture with steep foreslope clinoforms three depositional sequences s3 s5 are recognized in the upper givetian strata which are dominated by extensive microbialites metre scale depositional cyclicity occurs in most facies associations except in the platform margin buildups and upper foreslope facies in the latest givetian at the top of sequence s5 relative platform uplift subaerial exposure and associated rapid basin subsidence probably a block tilting effect caused large scale platform collapse and slope erosion to give local scalloped embayments along the platform margin and the synchronous demise of microbial buildups subsequently sand shoals and banks composed of ooids and peloids and a little later stromatoporoid buildups on the palaeohighs developed along the platform margin from which abundant loose sediment was transported downslope to form gravity flow deposits another strong tectonic episode caused further platform collapse in the early frasnian at the top of sequence s6 leading to large scale breccia release and the death of the stromatoporoid buildups siliceous facies banded cherts and siliceous shales were then deposited extensively in the basin centre as a result of the influx of hydrothermal fluids the platform margin sand shoal bank system possibly with gullies on the slope persisted into the latest frasnian until the restoration of microbial buildups four sequences s6 s9 characterized by abundant sand shoal deposits on the margin and gravity flow and hemipelagic deposits on the slope are distinguished in the frasnian strata smaller scale depositional cyclicity is evident in all facies associations across the platform slope basin transect the distinctive depositional architecture and evolution of this yangdi platform are interpreted as having been controlled mainly by regional tectonics with contributions from eustasy environmental factors oceanographic setting biotic and sedimentary fabrics

paleomagnetism of 1 09 ga lake shore traps keweenaw peninsula michigan new results and implications we report paleomagnetic data from a new section of the 1 09 ga lake shore traps exposed on silver island 10 flows and on the adjacent mainland two flows along the northwestern coastline of the

potbellies wasp waists and superparamagnetism in magnetic hysteresis because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material much can be learned by monitoring that response through what is known as a magnetic hysteresis loop the measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research yet the interpretation of hysteresis loops is not simple it has become apparent that although classic single domain pseudo single domain and multidomain loops described in textbooks occur in natural samples loops are frequently distorted having constricted middles wasp waisted loops or spreading middles and slouching shoulders potbellies such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions the physics of the problem have been understood for nearly half a century yet numerical simulations appropriate to geological materials are almost unavailable in this paper we discuss results of numerical simulations using the simplest of systems the single domain superparamagnetic sd sp system examination of the synthetic hysteresis loops leads to the following observations 1 wasp waisting and potbellies can easily be generated from populations of sd and sp grains 2 wasp waisting requires an sp contribution that saturates quickly resulting in a steep initial slope and potbellies require low initial slopes the sp contribution approaching saturation at higher fields the approach to saturation is dependent on volume hence the cube of grain diameter therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size 3 we were unable to generate potbellies using an sp sd threshold size as large as 30 nm and wasp waists cannot be generated using a threshold size as small as 8 nm the occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm 5 4 simulations using a threshold size of 15 20 nm with populations dominated by sp grain sizes that is with a small number of sd grains produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass 5 simulations and measurements using two populations with distinct coercivity spectra can also generate wasp waisted loops a relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp waisting resulting from sp sd behavior

burial of organic carbon and pyrite sulfur in sediments over phanerozoic time a new theory in present day marine sediments almost all of which are deposited in normal oxygenated seawater rates of burial of organic carbon c and pyrite sulfur s correlate positively and bear a constant ratio to one another c s 3 on a weight basis by contrast calculations based on the isotopic model of garrels and lerman 1981 indicate that at various times during the phanerozoic the worldwide burial ratio must have been considerably different than the present day value this ratio change is caused by the requirement that increases in the worldwide mass of organic carbon must be accompanied by equivalent decreases in the mass of sedimentary pyrite sulfur in order to maintain a roughly constant level of o2 in the atmosphere such apparently contradictory behavior can be explained if the locus of major organic carbon burial has shifted over time from normal marine environments as at present to non marine freshwater or to euxinic environments in the geologic past a shift to predominantly freshwater burial can help explain predicted high c s ratios in permo carboniferous sediments and a shift to euxinic environments can help explain predicted low c s ratios during the early paleozoic it is demonstrated that the three environments today exhibit distinguishably different average c s ratios

application of backscattered electron imaging to the study of source rocks microtextures abstract scanning electron microscope sem used in backscattered electron bse mode is a powerful tool for the sedimentological study of source rocks owing to its spatial and chemical resolutions ease and speed of operating the obtained imaging is dependent on atomic number contrasts between the different constituents of the sample using the bse mode combined with energy dispersive x ray spectrometry minerals and organic matter can be distinguished more clearly than with the light microscope classically used by organic petrologists in order to describe microtextures of organic rich sediments particularly the disposition of the organic matter this technique has been tested on source rocks having various sedimentological and geochemical characteristics accordingly two patterns of organic matter disposition have been recognized laminated and particulate these dispositions can be put back into their sedimentary context by considering the depositional environment and diagenesis the organic matter disposition suggests the existence of organic networks which are suspected to be a major control for hydrocarbon expulsion

microseismicity of the bearn range reactivation of inversion and collision structures at the northern edge of the iberian plate the bearn range located to the north of the axial zone in the western pyrenees is affected by numerous small magnitude seismic events these events overlap an area characterised by specific geological structures which are interpreted to have resulted from multistage extensional and compressional deformation an analysis of surface geology draped over dem together with field investigations allow identification of two main shortening episodes with differing direction of contraction d1 represents the inversion of the north pyrenean basin whose mesozoic infill was detached from highly extended crust and transported southwards over the necking zone and the northern margin of the iberian plate d2 corresponds to the collision stage and is characterised in the study area by backfolding and backthrusting deformation coeval with uplift in the axial part of the chain due to thickening of the iberian plate the microseismicity appears to concentrate along the basal part of the inverted basin units d1 where this initially low angle thrust has been tilted and steepened during collision d2 we propose that local steepening of this ancient inversion structure which should not be named north pyrenean fault provided the suitable dip for extensional solicitation in association with the present uplift of the axial zone whatever the driving mechanism of this uplift could be

early diagenetic alteration of organic matter by sulfate reduction in quaternary sediments from the northeastern arabian sea abstract hemipelagic sediments from the northeastern arabian sea off pakistan were analyzed by geochemical and microscopical methods to study the importance of bacterial degradation processes on the quantity and composition of sedimentary organic matter om on the basis of elemental analysis of total organic carbon toc and total sulfur ts the decay of om by sulfate reducing bacteria were calculated concentrations of total iron reactive iron total nitrogen tn total phosphorus tp and hydrogen indices hi were used to assess organic matter reactivity and redox conditions stable sulfur isotopic compositions provided further information about pathway timing and mechanisms of sulfate reduction microscopical observations indicate that throughout the study area om is amorphous 75 95 in almost all samples with only a small contribution of terrigenous om bacterially mediated sulfate reduction leads to continuously decreasing toc ts ratios from 0 4 m to a burial depth of 2 3 m at this depth the anaerobic oxidation of particulate organic matter ceases sulfate reduction in this narrow interval accounts for the decay of up to 70 of the om which has primarily entered the anoxic domain during the bacterially mediated diagenesis preferential consumption of nitrogen and phosphorus containing organic compounds can be observed and quantified the regeneration of up to 60 of tn and more than 50 of tp can lead to a feedback of nutrients to the seawater this recycling process may contribute to the high productivity in the northeastern arabian sea

triple junctions of bay of biscay and north atlantic new constraints on the kinematic evolution recently surveyed triple junction trajectories in the bay of biscay and newly available magnetic maps constrain the early oceanic evolution of the north atlantic and bay of biscay from chrons m0 to a33o 118 to 80 ma the opening of the bay of biscay is of the scissors opening type with a pole of rotation located near bordeaux lat 44 6 n long 0 3 w and predicted simultaneous compression in the central and eastern pyrenees structural directions west of iberia do not fit the north america iberia motion therefore a second triple junction of the ridge ridge fault type is postulated just south of the first one which propagated north ward from a more southerly position

paleomagnetism of the 1 1 ga coldwell complex ontario canada implications for proterozoic geomagnetic field morphology and plate velocities we report new paleomagnetic data from the 1108 ma intrusive coldwell complex ontario canada to investigate the apparent reversal asymmetry observed in some midcontinent rift mcr rocks the rocks of eastern and central part of the complex are reversely magnetized with a group mean direction of d 114 8 i 63 7 95 3 6 n 30 the corresponding paleomagnetic pole at plat 47 2 n plong 206 5 e a95 4 8 is located close to the paleomagnetic poles from nearly coeval reversely magnetized rocks of the mcr system including the lower lava flows at mamainse point the rocks of western part of the complex are normally magnetized with a group mean direction d 298 0 i 56 9 95 5 8 n 10 that passes the reversal test with respect to the reversed group mean direction our results do not support the previous model in which the complex was emplaced during two periods of reversed geomagnetic field polarity separated by a period of normal polarity and hence encompasses two geomagnetic reversals instead our new data indicate that the coldwell complex records only two polarity intervals separated by a symmetrical reversal at 1102 1105 ma this reversal is likely equivalent to the lowermost reversal recorded at mamainse point and provides further evidence that the apparent reversal asymmetry reflects a plate motion rather than a persistent nondipole field geometry together with a high quality data from the 1098 ma north shore volcanics our data indicate a rapid velocity of laurentia at 25 4 cm yr the fast plate motion may reflect a decreased mantle drag due to vigorous mantle indicated by widespread intraplate magmatism at 1 1 ga

selenium as paleo oceanographic proxy a first assessment abstract selenium se is an essential trace element which with multiple oxidation states and six stable isotopes has been suggested as a potentially powerful paleoenvironmental proxy in this study bulk se concentrations and isotopic compositions were analyzed in a suite of about 120 samples of fine grained marine sedimentary rocks and sediments spanning the entire phanerozoic while the se concentrations vary greatly 0 22 72 ppm the 82 76se values fall in a fairly narrow range from 1 to 1 relative to nist srm3149 with the exception of laminated black shales from the new albany shale formation late devonian which have 82 76se values of up to 2 20 black sea sediments holocene and sedimentary rocks from the alum shale formation late cambrian have se to total organic carbon ratios se toc and 82 76se values close to those found in modern marine plankton 1 72 0 15 10 6 mol mol and 0 42 0 22 for the other sedimentary sequences and sediments the se toc ratios show se enrichment relative to modern marine plankton additional input of isotopically light terrigenous se may explain the se toc and 82 76se data measured in recent arabian sea sediments pleistocene the very high se concentrations in sedimentary sequences that include the cenomanian turonian oceanic anoxic event oae 2 may reflect an enhanced input of volcanogenic se to the oceans as the latter has an isotopic composition not greatly different from marine plankton the volcanogenic source does not impart a distinct signature to the sedimentary se isotope record the lowest average 82 76se values are observed in the oae2 samples from demerara rise and cape verde basin cores 82 76se 0 14 0 45 and could reflect fractionation associated with microbial or chemical reduction of se oxyanions in the euxinic water column in contrast a limiting availability of seawater se during periods of increased organic matter production and burial may be responsible for the elevated 82 76se values and low se toc ratios in the black shales of the new albany shale formation overall our results indicate that to unlock the full proxy potential of marine sedimentary se records we need to gain a much more detailed understanding of the sources chemical speciation isotopic fractionations and cycling of se in the marine environment

magnetism of the keweenawan age chengwatana lava flows northwest wisconsin the chengwatana volcanics are subaerial plateau basalts which appear to be stratigraphically above the north shore volcanic group of the lake superior region sixty five oriented cores were collected at six sites near st croix falls wisconsin magnetic cleaning of the samples was primarily step wise alternating field a f demagnetization to 100 mt thermal demagnetization and hysteresis property measurements indicate the dominant magnetic mineralogy is multidomain magnetite however a stable remanence is present which we attribute to grains with single domain behavior the mean remanent direction is d 287 5 i 45 8 95 8 0 with a mean pole at 186 1 e 30 9 n this result is consistent with the 1 1 ga apparent pole for the north american craton two sites mapped as stratigraphically equivalent show distinct reversed directions both a f and thermal demagnetization isolate the same reversed direction which is interpreted as being primary these particular flows record a short reversal in an otherwise normal sequence this strengthens the evidence for more than one reversal in the middle keweenawan and may be the same short reversal noted at mamainse point

depositional and diagenetic c s fe signatures in early paleozoic normal marine shales the degree of pyritisation dop of cambrian silurian normal marine shales is always less than 0 70 similar to that found previously for sediments of devonian to cretaceous ages this indicates that dop can be used as a paleoenvironmental indicator of deposition from normal marine i e oxygenated bottom waters throughout the phanerozoic record although the range in dop is similar significant variations in normal marine cs ratios can occur relatively low cs ratios 1 5 result from lower reactive iron contents which are also independent of organic carbon variations reactive iron contents and hence normal marine cs ratios are able to vary at a regional or basinal level

chemostratigraphy of the jurassic system applications limitations and implications for palaeoceanography current chemostratigraphical studies of the jurassic system primarily involve the use of one sedimentary component marine organic carbon one divalent transition metal substituted in carbonate manganese and two isotopic tracers strontium isotope ratios 87 sr 86 sr and carbon isotope ratios 13 c carb and 13 c org in carbonate and in organic matter other parameters such as mg ca and sr ca ratios in calcite oxygen isotope ratios 18 o in carbonate sulphur isotope ratios 34 s in carbonate hosted sulphate nitrogen isotope ratios 15 n org in organic matter osmium isotope ratios 187 os 188 os in black shales and neodymium isotope ratios 143 nd 144 nd in various mineral phases are also useful but at present give poor resolution because the database is incomplete or compromised by various factors stratigraphical patterns in total organic carbon toc can be of either local or regional significance depending on the lateral extent of the former nutrient rich and productive water mass divalent manganese follows a similar pattern being concentrated most probably as a very early diagenetic phase only in oxygen depleted waters that typically underlie zones of elevated organic productivity shifts in mg ca and sr ca ratios on the time scale of ammonite subzones seem largely to reflect temperature changes strontium isotope ratios from pristine skeletal calcite provide a global signal 13 c values from carbonates with minimal diagenetic overprint potentially do the same although small spatial differences in palaeo water mass composition may have been locally significant oxygen isotope determinations on carbonate rocks and fossils generally yield values that are too scattered to be stratigraphically useful because they reflect palaeotemperature the evaporation precipitation balance in sea water and the impact of any diagenesis involving an aqueous phase nitrogen isotope ratios in organic matter reflect the chemistry of ancient water masses as affected by nitrate utilization and denitrification and the stratigraphical pattern of this parameter is more likely to correlate only on a regional basis neodymium isotope ratios in sea water are also water mass dependent and greatly affected by regional sources and oceanic current systems preliminary data on sulphur isotope ratios in carbonates and osmium isotope ratios in organic rich shales both potentially offering global correlation indicate that these tracers may be valuable although the records at present are not sufficiently well established to allow high resolution regional correlation in all cases biostratigraphically well dated reference sections against which the relevant geochemical data have been calibrated are required in the first instance to date studies on the stratigraphical distribution of organic carbon have been principally carried out in both northern boreal and southern tethyan europe carbon isotope stratigraphy has been undertaken primarily but not exclusively on bulk pelagic sediments from the alpine mediterranean or tethyan domain and strontium isotope stratigraphy has been undertaken largely on calcitic skeletal material belemnites and oysters from northern and southern europe in many sections including those containing ammonites multi parameter chemostratigraphy can give resolution that exceeds that attainable by classic biostratigraphical means strontium isotope ratios in skeletal calcite are a particularly powerful tool for illustrating changes in sedimentary rate and revealing gaps in the stratigraphical record

septarian concretions from the oxford clay jurassic england uk involvement of original marine and multiple external pore fluids septarian concretions occur at several horizons within the oxford clay formation a marine mudstone containing pristine aragonite and immature biomarker molecules they record the passage of at least four generations of pore fluids the first of marine origin and the last still present in cavities concretion bodies formed cracked and calcite and pyrite precipitated in and around the cracks within the sulphate reduction zone as demonstrated by c o s and sr isotopic composition pore fluid 1 before major compaction sandstone dykes were intruded locally and baryte precipitated followed by coarse calcite cements with isotopically light oxygen and radiogenic strontium indicating the introduction of meteoric derived water pore fluid 2 later coarse celestine within concretions has distinct sulphur isotopic composition and requires a further geographically restricted water source pore fluid 3 celestine bearing concretions contain water in tight cavities whose isotopic composition is close to that of modern precipitation its chemistry shows that it is equilibrating with pre existing minerals implying a relatively recent origin pore fluid 4 the mineralogy of the oxford clay concretions shows that complex results can follow from a simple burial and uplift history and that multiple generations of pore fluids can pass through a low permeability clay

lacustrine shales and oil shales from stellarton basin nova scotia canada organofacies variations and use of polyaromatic hydrocarbons as maturity indicators abstract the post acadian stellarton basin is a small pull apart basin in northern nova scotia the coal brook member which is the main focus of this study contains the thickest and most extensive development of oil shales within the late carboniferous stellarton formation bulk and molecular geochemistry indicate that the organic rich oil shales and black shales contain type i ii kerogen with a mixed assemblage of aquatic and terrestrial organic matter that is dominated by the former biomarker evidence indicates that these sediments were deposited under deep anoxic and fresh to brackish water lake conditions organofacies distinctions were made on the basis of several parameters that are independent of maturity and therefore suggest organic matter changes the organofacies identified are the result of water level fluctuations that are climate controlled all of the aromatic compounds evaluated as maturity parameters were seen to experience alterations in proportion to temperature in the form of either depth or geologic age that can be correlated with maturity these alterations are believed to result from changes in molecular composition maturity parameters based on the alkylbenzothiophenic and alkylnaphthalenic compounds are shown to be effective maturity parameters in lacustrine sequences particularly over the higher maturity ranges where standard maturity parameters based on steranes and hopanes are not effective

sulfur isotopic trends and pathways of iron sulfide formation in upper holocene sediments of the anoxic black sea down core trends for extents of iron sulfidation in upper holocene microlaminated deposits of the black sea abyssal plain imply that most of the pyrite in these sediments is formed in the sulfidic water column and or very close to the sediment water interface sulfur isotopic data for pyrite within microlaminated muds from two localities in the deep basin show striking uniformity between sites and down core with a mean 34scdt value of 37 2 0 9 1 n 18 these isotopic trends support the hypothesis of early pyrite formation isotopic compositions of pyrite sulfur in the microlaminated bottom sediments are similar to those reported for particulate reduced sulfur collected in middepth sediment traps and to 34s values for water column dissolved sulfide within a narrow zone immediately below the o2 h2s interface these three sulfur reservoirs are distinctly different from the isotopic composition of ambient dissolved sulfide of the deep water column 175 m with 34s values for the deep dissolved sulfide reservoir being consistently lower by 2 to 4 these isotopic relationships suggest that iron sulfidation is occurring dominantly within a narrow shallow region at the top of the sulfidic water mass isotopic data from a fine grained turbidite collected in the deep basin and from rapidly accumulating iron monosulfide rich muds on the anoxic upper slope are enriched in34s relative to the microlaminated deposits the enrichments on the basin margin reflect a large component of iron sulfide formed during rapid burial a broad suite of geochemical data including sulfur isotopic results are consistent with an upper slope sediment source for the ubiquitous muddy turbidites of the deep basin the collective effects of reworking transport and redeposition result in a complete conversion of avs to pyrite with little additional iron sulfidation this conclusion is supported by a detailed isotopic mass balance the present isotopic study also highlights fundamental differences in the reactivities of the iron reservoirs distributed throughout the anoxic basin sediments of the upper slope and the genetically linked turbidites of the deep basin show similar extents of iron sulfidation that are significantly lower than those of the microlaminated deposits of the deep basin floor suggesting dramatically shorter timescales of iron reactivity for the microlaminated sediments of the deep basin importantly isotopic patterns from the modern black sea should assist greatly in the recognition and interpretation of ancient anoxia in the stratigraphic record

mineralogy and geochemistry of the peterborough member oxford clay formation jurassic uk element fractionation during mudrock sedimentation x ray diffraction electron microprobe and sem analyses show that the peterborough member of the oxford clay formation is composed mainly of quartz and illite with lesser amounts of chlorite kaolinite k feldspar plagioclase very small amounts of anatase and apatite and variable amounts of calcite and subordinate siderite the geochemistry including major and a range of trace elements are used with the mineralogy to demonstrate the existence of several distinct components which have contributed to variations within the peterborough member these components are 1 quartz zircon and possibly other heavy minerals and k feldspars 2 carbonates 3 phosphatic material 4 organic material and 5 clays geochemical comparisons are made with various average shales demonstrating abnormally low ba cu co mn na and mg in the peterborough member it is suggested that the current mineral assemblage is mostly detrital material with little diagenetic alteration the demonstration of fractionation between hydrodynamically heavy and light grains even in an apparently fine grained sediment such as the oxford clay has disturbing consequences for geochemical studies of sediment source areas where small numbers of small samples are employed

the mesoproterozoic midcontinent rift system lake superior region usa abstract exposures in the lake superior region and associated geophysical evidence show that a 2000 km long rift system developed within the north american craton 1109 1087 ma the age span of most of the volcanic rocks this system is characterized by immense volumes of mafic igneous rocks mostly subaerial plateau basalts generated in two major pulses largely by a hot mantle plume a new ocean basin was nearly formed before rifting ceased perhaps due to the remote effect of the grenville continental collision to the east broad sagging subsidence combined with a system of axial half grabens separated along the length of the rift by accommodation zones provided conditions for the accumulation of as much as 20 km of volcanic rocks and as much as 10 km of post rift clastic sediments both along the rift axis and in basins flanking a central post volcanic horst pre rift mature quartzose sandstones imply little or no uplift prior to the onset of rift volcanism early post rift red bed sediments consist almost entirely of intrabasinally derived volcanic sediment deposited in alluvial fan to fluvial settings the exception is one gray to black carbon bearing lacustrine unit this early sedimentation phase was followed by broad crustal sagging and deposition of progressively more mature red bed fluvial sediments with an extra basinal provenance

the trophic structure of the biota of the peterborough member oxford clay formation jurassic uk the peterborough member of the oxford clay formation is organic rich and contains an abundance of well preserved vertebrate and invertebrate fossils a high nutrient input supported a diverse biota phytoplankton was exceptionally abundant in the surface water and formed the basis for an intricate food web in both surface and bottom waters top predators include some of the largest known mesozoic marine reptiles a giant teleost fish was analogous to modern filter feeding whales and sharks benthic faunas depended on organic matter sinking from surface waters and two parallel food webs may have existed trophic partitioning allowed the higher level predators to become diverse especially the plesiosaurs productivity was high in the surface waters probably high in the mid water column and high on the sea floor at times although benthic diversity may have been reduced due to substrate consistency and or dysoxia bacterial activity within the sediment was also intense the contribution of organic material to the sea floor was high but heterotrophic reworking probably reduced the abundance considerably during early diagenesis a decrease in organic carbon content in the upper callovian and lower oxfordian parts of the oxford clay formation parallels a decreasing abundance of vertebrate fossils

tectonic origin of carbonate depositional sequences in a strike slip setting aptian northern iberia abstract the lower upper aptian transitional series of the aulesti area in the northeastern basque cantabrian region northern iberia shows a depositional sequence arrangement conditioned by local synsedimentary folds and faults three sequence boundaries sb1 sb2 sb3 dated approximately between the deshayesi and nutfieldiensis zones are characterized by erosional surfaces with both palaeokarst and palaeovalley reliefs up to 150 m deep in the downthrow blocks of normal faults one sequence boundary sb2 is also a clear angular unconformity with fossilized folds and faults created by strike slip action facies analyses of the two depositional sequences s1 and s2 reveal general deepening of the sea upward through the sedimentary column only the sequence boundary sb3 could be eustatic according to the haq et al 1987 curve however all sequence boundaries are primarily attributed to local tectonic pulses sudden relaxations of transtensional stresses and associated tectonic uplifts occur after phases of transtensional stress accumulation associated with relative sea level rises the intraplate stress tectonic mechanism of cloetingh et al 1985 is thought to offer the best explanation for the origin of the sequential arrangement seen

marathon dikes rb sr and k ar geochronology of ultrabasic lamprophyres from the vicinity of mckellar harbour northwestern ontario canada the ultrabasic lamprophyre dikes from the vicinity of mckellar harbour northwestern ontario are composed essentially of a mixture of phlogopite olivine primary carbonate spinel and apatite with associated perovskite and melilite petrologically and chemically they have strong affinities with alnoites and magmas derived from upper mantle sources in stable continental regimes rb sr isotopic studies give an isochron age of 1 65 0 12 ga 2 with an initial 87sr 86sr ratio of 0 70167 0 00036 2 this age is considerably older than the neohelikian keweenawan magmatic event associated with the development of the lake superior basin e g 950 1170 ma k ar data indicate a mean whole rock age for the dikes of 1144 ma and a pseudoisochron age of 1120 34 ma 2 these ages are thought to reflect partial degassing of phlogopite and incomplete updating of the rocks under thermal influences induced by the intrusion of the coldwell alkaline complex at 1044 ma ages of carbonatitic and alkaline rock

correction to apparent and true polar wander and the geometry of the geomagnetic field over the last 200 myr 1 in the paper apparent and true polar wander and the geometry of the geomagnetic field over the last 200 myr by jean besse and vincent courtillot journal of geophysical research 107 b11 2300 doi 10 1029 2000jb000050 2002 the data in the top part of table 4 were incorrect the correct table 4 is given here journal of geophysical research vol 108 no b10 2469 doi 10 1029 2003jb002684 2003

erratum to the posidonia shale lower toarcian of sw germany an oxygen depleted ecosystem controlled by sea level and palaeoclimate palaeogeogr palaeoclimatol palaeocol 165 2001 27 52 abstract the lower toarcian posidonia shale is famous for its excellently preserved fossils and its high amount of organic matter up to 16 both quality of preservation and accumulation of organic matter have been explained by permanent anoxic bottom water conditions high resolution geochemical sedimentological and palaeoecological investigations of various sections of the posidonia shale in sw germany however indicate that oxygen availability was variable and ranged from short oxygenated periods to longer term anoxia the benthic macrofauna consists of nine fossil communities and was used in combination with geochemical data to reconstruct a time averaged oxygen curve anoxic conditions prevailed during the deposition of the toarcian black shales they were however punctuated by various short periods weeks to years with oxygenated bottom water conditions sedimentological e g distinctiveness of microlamination siliciclastic content and geochemical parameters e g organic matter content isotopic signatures 18 o and 13 c molecular redox parameters pristane phytane ratio arylisoprenoids exhibit a remarkable covariation and seem to be controlled by sea level fluctuation maximum oxygen depletion and an extreme negative shift of 13 c org values 34 occurred during the early falciferum zone this is explained by the recycling of 12 c enriched carbon derived from remineralization of organic matter on and within the substrate during low sea level stand and a highly elevated redox boundary including photic zone anoxia the subsequent transgression permitted enhanced water exchange with the tethyan ocean and caused improvement of living conditions at the end of the falciferum zone other important factors controlling the depositional environment are the overall palaeogeographic situation and climate the early jurassic is the latest period before break up of pangaea and probably was ruled by a strong meridional atmospheric circulation system with pronounced seasonal changes of prevailing trade and monsoon wind systems an estuarine circulation with a positive water balance and surface water with slightly reduced salinity in the summer alternated with an anti estuarine circulation and a negative water balance in the winter during the summer months a stratified water column with anoxic conditions below the halocline developed 18 o data indicate low salinity in the surface water during the monsoon influenced summer high productivity was then located in the photic zone and the corresponding isotopically light 18 o signal was fixed in the calcareous nannoplankton during the winter months a saline circulation system brought oxygen to the benthic environment favouring temporary benthic colonization especially during times of relative sea level high stand

sulphur isotope geochemistry of pyrite from the upper cretaceous marshybank formation western interior basin abstract variations in the texture and sulphur isotopic composition of pyrite in the upper cretaceous marshybank formation are linked to depositional environment abundant framboidal and lesser euhedral pyrite precipitated in offshore marine rocks moderate quantities of pyrite also crystallized in brackish coastal plain rocks however in contrast to marine pyrite coastal plain pyrite is dominantly euhedral in texture reflecting direct precipitation from a porewater with a relatively low dissolved sulphide concentration in the marine rocks pyrite 34s values range from 35 7 to 27 4 avg 4 8 canyon diablo troilite cdt pyrite within carbonate concretions hosted in these marine rocks has a similar isotopic composition 49 8 to 10 6 cdt however isotopic values are often highly variable within individual concretions as a result of the heterogeneous nature of sulphate reduction and pyrite formation within marine sediments pyrite in coastal plain rocks is characterized by relatively high 34s values 4 2 to 35 5 avg 13 2 cdt while the overlying sideritized conglomerates have the lowest 34s values reported for cretaceous rocks from the western interior basin of north america 49 8 to 41 7 cdt very low 34s values which are only observed in the marine rocks are indicative of microbial sulphate reduction and pyrite formation in a sulphate replete i e open system higher 34s values up to 18 which were obtained for both the marine and coastal plain rocks are indicative of progressive pyrite crystallization in a sulphate limited i e closed system such conditions are expected in marine sediments as burial occurs and in brackish i e low sulphate sediments pyrite with very high 34s values 18 is common in the coastal plain rocks these high values are the result of influx of 34s enriched residual sulphide derived from overlying marine units a minor amount of 34s enriched pyrite is also present within and proximal to marine sedimentary unit h which was deposited at the same time as the coastal plain rocks we hypothesize that porewater containing 34s enriched residual sulphide flowed from the coastal plain and along unit h into the marine sediments thus producing anomalously high 34s pyrite

late precambrian keweenawan asymmetric reversals palaeopoles from the keweenawan 1 200 1 000 myr rocks of the lake superior region form the western and eastern arms of the great logan palaeomagnetic loop 1 5 fig 1a this is defined by both normal n and reversed r poles which are the consequence of at least two geomagnetic reversals4 6 7 the younger one r n has been detected throughout the lake superior region from various rock types 1 6 the older reversal n r is only recorded in the lowermost keweenawan lavas in the southern part of the lake 8 wherever found the reversals depart from the 180 symmetry fig 1a 2 6 due to these asymmetries the logan loop has large segments that are devoid of data and thus the geophysical interpretations of the loop or hairpin critically depend on the cause of the reversal asymmetry 4 four models11 secondary component6 apparent polar wander1 5 wilson s dipole offset5 7 9 and non dipole field configuration1 3 10 have been put forward to explain the asymmetries here we demonstrate that the two dipole field configuration model can explain the reversal asymmetries and discuss the global effects of this model in the light of worldwide late precambrian palaeomagnetic data

palaeomagnetic cleaning strategies abstract cleaning is a term used in palaeomagnetism to refer to laboratory methods designed to demagnetize preferentially the less stable components of magnetization although the diversity of methods suggests that there is a wide choice available this is illusory except in simple cases the wrong cleaning strategy may yield misleading results cleaning methods include thermal magnetic alternating field or a f low temperature chemical shock and microwave cleaning a cleaning strategy involves one or more cleaning methods applied in a specific sequence the design of a strategy is made easier with the a priori knowledge of magnetic mineralogy and granulometry or in other words rock magnetic properties although not a substitute for other rock magnetic experiments the variation of low field susceptibility with temperature k t not only provides information on magnetic mineralogy and granulometry but it also draws attention to chemical alteration that may occur during thermal cleaning apart from the rapid though reversible decrease of k associated with curie temperatures other diagnostic k t features include the low temperature about 140 c peak typical of multidomain md magnetite the high temperature hopkinson peak typical of fine grained magnetite and other magnetic minerals and the classic lepidocrocite maghemite haematite profile which is irreversible examples are given where the remanence caused by lightning can be effectively eliminated only by applying a f or low temperature lt pre treatment such remanence may be an isothermal remanent magnetization irm or a combination of irm and anhysteretic remanent magnetization arm as the unblocking temperature of relatively low coercivity md grains may extend to high temperature it is often desirable to suppress md remanence an example is given of palaeointensity determination with the without lt pre treatment the nature of the lt transition is briefly addressed in the light of a rare double low temperature peak which may reflect both the isotropic point and the verwey transition

albian syndepositional block rotation and its geological consequences basque cantabrian basin western pyrenees stratigraphic structural palaeocurrent and palaeomagnetic analyses of upper albian deep water deposits in and around the deba block northern iberia are presented results indicate an anticlockwise vertical axis rotation of this block by 35 during a maximum time span of c 1 ma late albian intra c auritus ammonite subzone this albian syndepositional block rotation is interpreted to be the consequence of the coeval activity of conjugate major sinistral strike slip faults and minor antithetic dextral strike slip faults which border the deba block on the base of conservative estimations a minimum block rotation rate of 35 ma 1 and a sinistral strike slip rate of 1 2 km ma 1 are calculated as a consequence of the interaction of the rotated deba block with adjacent non rotated blocks its corners experienced coeval transpressive nw and se corners and transtensional deformations sw and possibly ne corners at the transtensional sw corner two domal high reflective seismic structures have been recorded and interpreted as high level magmatic laccoliths these magmatic intrusions triggered the development of a mineralizing hydrothermal system which vented to the late albian seafloor warm to hot hydrocarbon rich fluids vented hydrocarbon was generated from albian organic rich sediments by contact alteration with hydrothermal fluids

progressive oxidation of anoxic and ferruginous deep water during deposition of the terminal ediacaran laobao formation in south china abstract the laobao formation a black chert sequence was deposited in deep water in the terminal ediacaran nanhua basin the chert samples have finely laminated fabric and have no distinct recrystallization or metasomatic relict texture there is an extremely limited detrital component and no obvious hydrothermal influence therefore the chert appears to be an authigenic deep water precipitate the trace elements in the chert were mainly derived from seawater and thus could be a relatively unique proxy for late precambrian seawater conditions redox sensitive trace elements and relevant ratios were used to reconstruct the depositional environment of the chert sequence combined with iron species results of the laobao formation enrichments in redox sensitive trace elements u v mo depleted mn very low th u ratios 7 9 and v v ni 0 6 ratios suggest an overall anoxic and ferruginous conditions over the deposition of the chert the pyrite sulfur isotope compositions are between 15 4 and 11 0 and show a decreasing trend toward the top of the section possibly implying sulfate concentration gradually increased in the terminal ediacaran nanhua basin which is consistent with implication of their pyrite framboid diameter characteristics our data confirm that deep water in the nanhua basin was still anoxic and ferruginous and it was progressively oxidized during the terminal ediacaran period which could create necessary conditions for the emergence and rapid evolution of multicellular metazoans

remagnetization in keweenawan rocks part ii lava flows within the copper harbor conglomerate michigan alternating field and thermal demagnetization studies of a thin lava flow unit within the copper harbor conglomerate michigan native copper district show a westerly increase in chemical remanent magnetization crm overprinting along a 50 km strike length the crm may have formed while secondary minerals including native copper were being generated in the underlying portage lake volcanics and when a rhyolite plug was injected into nearby siltstones of the overlying freda formation the direction of magnetization during copper harbor time as given by the flows is virtually indistinguishable from that found from the portage lake volcanics thus supporting previous geological observations that the two formations are closely associated in time

carbon sulfur iron systematics of the uppermost deep water sediments of the black sea box cores recovered during leg 4 of the 1988 r v knorr black sea oceanographic expedition from deep water regions of the basin were dominated by coccolith rich microlaminated unit 1 sediment and muddy gray turbidite layers both organic carbon oc and pyrite sulfur values for unit 1 display narrow ranges with mean concentrations of 5 3 1 1 1 wt and 1 3 0 3 wt respectively unit 1 is not enriched in pyrite s relative to sediments deposited under oxygenated bottom waters normal marine sediments with comparable oc concentrations carbon sulfur relationships evaluated on a calcium carbonate free basis to avoid spurious correlations resulting from dilution effects demonstrate that oc and pyrite s are essentially decoupled these observations combined with the persistence of elevated pore water sulfide to depth and a strong correlation between pyrite s and the detrital fe component argue strongly for limitation of pyrite formation in unit 1 by the availability of reactive fe unit 1 fe limitation is further indicated by degree of pyritization dop studies a measure of the extent to which the original potentially reactive fe has been transformed to pyrite these studies show sulfidation of reactive fe ranging from 57 to 78 with dop values independent of oc concentration unit 1 dop profiles suggest that the majority of the pyrite is formed in the sulfidic water column and or very close to the sediment water interface pyrite s concentrations of unit 1 when compared with the particulate reduced sulfur fluxes measured in time series sediment traps are compatible with predominantly water column pyrite formation because of the limitations in the supply of reactive fe associated with the comparatively high supply of oc the microlaminated sediment is characterized by c s ratios greater than those typical of holocene oxically deposited sediments the turbidite muds of the deep basin display high reduced s values relative to holocene normal marine sediments in samples with low oc concentrations low c s ratios this reflects pyrite formation under anoxic sulfidic bottom water conditions in a probable upper slope source region as well as during transport and final deposition intermediate dop values for the turbidites in part a product of their rapid rate of deposition reveal that fe limitation is ultimately not a factor and that further pyrite should form during burial however the very rapid rate at which unit 1 pyrite forms suggests fundamental differences between the overall reactivities of the fe phases associated with the microlaminated and turbiditic sediments the signature of water column anoxia with regard to sedimentary pyrite formation is clearly indicated by the high dop values of unit 1 and the comparatively high levels of s associated with the low concentrations of oc of the turbidite muds this agrees with similar conclusions based on studies of ancient sedimentary rocks

isotopic signals from callovian kimmeridgian middle upper jurassic belemnites and bulk organic carbon staffin bay isle of skye scotland abstract the stable isotope data presented here significantly extend and expand upon previous isotopic investigations of the middle to late jurassic interval the belemnite samples collected from the staffin bay and staffin shale formations from the isle of skye scotland yielded oxygen isotope values consistent with callovian kimmeridgian palaeotemperatures of 6 7 20 6 c the carbon isotope data comprise one of the first moderately high resolution investigations of the relationship between terrestrial 13 c org predominantly fossil wood debris and marine 13 c carb belemnites as derived from a geologically coeval record the staffin bay data reveal a broad early to mid oxfordian positive carbon isotope excursion the excursion maximum occurs in the cordatum zone british boreal ammonite zonation although high values persist into the tenuiserratum zone the correspondence between the marine and terrestrial records indicates a strong coupling of the ocean atmosphere system and suggests that the total exchangeable carbon reservoir would have been affected at this time the mid oxfordian negative carbon isotope excursions identified in published tethyan records and commonly attributed to methane release are not recorded in the staffin bay data which may suggest that the tethyan excursions do not represent fluctuations in the global carbon reservoir and that the fidelity of the methane hypothesis should be re evaluated supplementary material isotopic and geochemical data are available at http www geolsoc org uk sup18339

a geochemical investigation of samples from the volgian bazhenov formation western siberia russia abstract in this paper we present analyses of five core samples from the volgian bazhenov formation three different organic facies a b and c are proposed based on microscopical bulk and biomarker analyses facies a and b have much in common including low hopane sterane ratios c 35 c 34 hopane ratios slightly above 1 0 small amounts of tricyclic terpanes and a range of alkylthiophenes in their pyrolysates however facies a and b show some differences for example facies a shows a slight even odd c 22 c 27 n alkane predominance in some of the samples while facies b lacks 28 30 bisnorhopane and shows significantly different n alkane isoprenoid ratios these findings contrast with those of facies c which shows very low organic sulfur an odd even predominance in the higher n alkanes a high c 29 c 27 sterane ratio a very high hopane sterane ratio and is very low in tricyclic terpanes the timing of hydrocarbon generation from facies a together with the type and amount of organic matter appears to be similar to that of the upper jurassic of the norwegian continental shelf it appears that facies a and b have similar types of organic matter with a somewhat higher input of terrestrial debris in the latter and contrastingly in facies c an even greater proportion of terrestrial organic matter

on a tectonic mechanism for regional sealevel variations abstract no satisfactory tectonic explanations have yet been offered for the apparent sealevel fluctuations of about 1 cm 1000 years and a magnitude of up to a few hundred meters that have been proposed by vail et al 1 we propose a mechanism that does appear to be able to explain these changes if horizontal stresses of the order of a few kilobars occur in the lithosphere and if changes in these stress fields occur on geological time scales the proposed model is one of an interaction between these stresses and the deflections of the lithosphere caused by sedimentary loading apparent sealevel changes of up to 100 m can be produced at the flanks of the sedimentary basins by this interaction the mechanism is most effective for young margins that are subject to rapid sediment loading by its nature the tectonic model can explain contemporaneous fluctuations in apparent sealevel in neighbouring depositional environments in principle it implies the possibility of regional correlations in different basinal settings

paleomagnetism and geochemistry of 1144 ma lamprophyre dikes northwestern ontario implications for the north american polar wander and plate velocities

ostracod colour change by thermal alteration offshore ireland and western uk abstract ostracod colour changes from white to amber to red brown to black are principally interpreted to be the result of thermal alteration the changes in colour are thought to be caused by progressive carbon fixation of hydrocarbons derived from the surrounding sediments which invaded the ostracod shell exceptions to this scheme include faunas from organically poor limestones which remain white irrespective of the amount of heating and grey brown or yellow brown specimens coloured by the oxidation of diffuse pyrite thirty four exploration wells have been examined from the irish and western uk sectors and these have shown a progressive darkening of colours which is correlated with both burial depth and distance from igneous intrusions the fastnet basin and southern region of the north celtic sea exhibit higher colour change depth ratios to the porcupine basin and northern region of the north celtic sea basin implying greater palaeoheat flow in the former basins

depositional architecture of a rimmed carbonate platform albian gorbea western pyrenees a lower cretaceous carbonate platform depositional system with a rimmed margin and an adjacent oversteepened slope was analysed in order to determine its depositional architecture and major depositional controls the platform is made up of coral rudist orbitolinid and algal limestones and in a 12 km dip transect there is a gradation from lagoon to platform margin slope and basin environments each characterized by distinctive sedimentological features and facies associations the rimmed platform is an aggradational system developed during approximately 4 2 million years of fluctuating relative sea level rise and it is bounded by unconformities at its base and top internal cyclicity in the construction of the system is evident mainly in platform interior and slope settings the seven recognized sequences average 0 6 million years in duration and are related to minor relative sea level changes carbonate deposition occurred in shallow and deep water settings during periods of high relative sea level reduced rates of sea level rise led to the development of shallowing upward sequences and eventually to the exposure of the shallowest parts of the platform during relative sea level falls during low relative sea level erosion surfaces developed on the slope and gravitational resedimentation occurred at the toe of slope basinwards resedimented units pinch out over distances of a few hundred metres active faults controlled sedimentation at the platform margin promoting the development of steep slopes up to 35 and preventing progradation of the shallow water platform despite high sediment production rates the development of sequences is interpreted to be related to tectonic activity

evolving core conditions ca 2 billion years ago detected by paleosecular variation abstract paleomagnetic data provide one of the few probes available to interrogate early evolution of the core here we apply this probe by examining the latitudinal dependence of paleosecular variation psv data derived from high quality paleomagnetic data collected from proterozoic and neoarchean rocks these data define a neoarchean geomagnetic field that was more dipolar than that during proterozoic times indicating a change in core conditions the signals observed may reflect a change in forcing of the dynamo and an early onset of inner core growth we propose a model that links evolution of the core mantle and crust in three principal phases i before approximately 3 5 ga an entirely liquid core may not have hosted a geodynamo if heat transport was sufficient across the core mantle boundary however a geodynamo could have been generated if so sources in the shallow outer core could have been more important for generating the dynamo relative to deeper convection resulting in a field that was less dipolar than that generated in later times ii cooling of the lower mantle between ca 2 and 3 5 billion years ago was promoted by deep subduction and possibly coincided with inner core growth the geodynamo during this episode was deeply seated producing a highly dipolar surface magnetic field iii after ca 2 billion years ago continued subduction led to large scale core mantle boundary compositional and heat flux heterogeneity with these changes shallow core contributions to the geomagnetic field grew in importance resulting in a less dipolar field

paleomagnetism of alkaline complexes and remagnetization in the kapuskasing structural zone ontario canada the shenango and nemegosenda alkaline complexes of the southern kapuskasing structural zone ksz an uplifted crustal block in northern ontario preserve primary thermoremanent magnetizations with mean directions d 112 i 46 k 13 95 13 n 9 sites and d 304 i 51 k 31 95 10 n 7 sites for shenango and d 124 i 57 k 13 9520 n 4 sites and d 312 i 54 k 17 95 13 n 7 sites for nemegosenda their average paleomagnetic pole is 45 n 189 e dp 6 dm 8 falling on the keweenawan track of the north american apparent polar wander path apwp between 1098 1107 ma reverse polarity r and 1087 1098 ma normal polarity n poles of lake superior lavas and dikes unlike the keweenawan paleopoles which have a reversal asymmetry of 30 the shenango and nemegosenda r and n poles are almost 180 reversed which was found also by lewchuk and symons 1990b this symmetry of r and n poles casts doubt on the previously accepted predominance of asymmetric reversals of the earth s field in keweenawan time the archean shawmere anorthosite is overprinted by a b magnetization with mean direction d 97 i 68 k 12 95 10 n 18 sites the shawmere b paleopole at 38 n 226 e dp 13 dm l6 falls between the carbonatite poles and 1850 ma cratonic poles and the b magnetization seems to be a composite of middle and late proterozoic overprints sites in the western part of the shawmere body close to the carbonatite line of faults linking the various alkaline complexes are severely altered with an abundance of secondary magnetite and hematite virtual geomagnetic poles for these sites fall close to the keweenawan track and have an average pole of 189 53 n dp 15 dm 20 suggesting that the b nrm at these sites is an 1100 ma thermochemical overprint caused by hydrothermal fluids at the time of carbonatite intrusion more easterly sites are less altered their vgps resemble 1850 ma cratonic poles and average to 235 e 30 n dp 15 dm 17 these b natural remanent magnetizations are probably thermochemical remanent magnetizations impressed by middle proterozoic hydrothermal activity associated with intrusion of early alkaline bodies fault movement on the ivanhoe lake fault zone or late stage uplift of the ksz

diagenesis of the kimmeridge clay formation onshore uk study of sedimentology carbonate geochemistry and clay mineralogy of four laterally continuous beds from the outcrop of the kimmeridge clay formation indicates strong lateral variations in the depositional and early diagenetic processes reflecting the basin and swell topography of the depositional basin the mudstones from the swell environment with low sedimentation and subsidence rates are shelly and carbonate rich with thin poorly developed oil shales they were deposited under dysaerobic conditions with consequent poor organic matter preservation early diagenetic processes were limited to the sulphate reduction and methanogenesis zones with the formation of discrete horizons of concretions the shallow burial depths attained are reflected by the immaturity of the organic matter in the mudstones although ordered illite smectites are present sediments from other locations comprise black organic rich mudstones the thick beds and good organic matter preservation indicating deposition under high sedimentation and subsidence rates with anoxic bottom waters in a basinal environment these indurated mudstones contain a few shell fragments and much pyrite indicating intense reaction in the sulphate reduction zone carbonates were precipitated in the methanogenesis and decarboxylation zones as ferroan calcites and dolomites the much greater burial depths are indicated by the higher maturity of the organic matter and by the ordered nature of the illite smectites and the loss of kaolinite

petrographic analyses of organo mineral relationships depositional conditions of the oxford clay formation jurassic uk five samples representing the range of variability of palynofacies and organic geochemical parameters of the organic rich facies of the peterborough member formerly lower oxford clay and one sample from the stewartby member middle oxford clay were studied using optical microscopy transmitted light and fluorescence and scanning electron microscopy backscattered electron mode the organic matter assemblage of the oxford clay formation is dominated by amorphous organic matter of marine origin 75 to 95 whatever their organic content the sediments display a highly microbioturbated texture with edge to face disposition of clay minerals and numerous microburrows signifying that the sea floor was never completely anoxic the presence of abundant organic matter in the sediment has enhanced diagenetic transformations and in particular is related to the formation of pyrite calcite and authigenic quartz the nature of diagenetic changes particularly those involving silica species provides information on the interstitial environment the sediment displays at the micrometric scale numerous microenvironments with varying reducing conditions resulting in different diagenetic products pyrite displays two habits framboidal and euhedral euhedral pyrite which characterizes more anoxic interstitial environments than framboids is found in the most organic rich sediments the abundance of large coccolith rich faecal pellets in the most organic rich sediments is correlated with chemical evidence of intense trophic activity

taphofacies models for epeiric sea environments middle paleozoic examples abstract skeletonized organic remains display various modes of occurrence which may be used to better understand the sedimentary dynamics of strata within which they occur aspects of skeleton biostratinomy and early diagenesis are distributed nonrandomly within the stratigraphic record and therefore are useful in discriminating taphonomic facies for purposes of taphonomic comparisons skeletonized marine taxa are reduced to five generally defined types massive arborescent univalved bivalved and multielement different skeletal types display different attributes which reflect specific responses to physical chemical and or biological processes the taphonomic properties of disarticulation reorientation and sorting fragmentation corrosion and abrasion skeletal dissolution and early diagenesis display patterns which are predictable with respect to a variety of inter related physico chemical parameters these can be quantified through indices which evaluate relative frequencies of occurrence sedimentary events alter the temporal and spatial homogeneity of processes which generate background taphofacies and superimpose event related properties thereby generating composite taphofacies such events produce taphonomic signature which in themselves correspond to specific environmental conditions i e water depth and event intensity indeed background and event related processes interact to generate complex but nonetheless diagnostic facies attributes useful in reconstructing paleoenvironments the comparison of standard indices among beds should allow the recognition of stratigraphic patterns and facilitate the diagnosis of systematic gradients among taphonomic properties deductively derived taphofacies models corroborated by empirical data from paleozoic strata illustrate the distribution of taphonomic properties with respect to environmental energy turbulence background sedimentation rate and sediment oxygenation these parameters also indirectly influence other less obvious factors including bioturbation and sediment chemistry trends in the distribution of taphonomic properties taphonomic gradients permit us to map otherwise unrecognized trends in these environmental parameters across a hypothetical onshore offshore transect within a generalized epeiric sea setting taphofacies models provide the quantifiable basis by which evolution of taphofacies may be gauged and then related to biotic and abiotic phenomena moreover taphofacies models evaluate the preservability of skeletal remains in different environmental settings and therefore provide information important in paleoecological studies

microscopic and sedimentologic evidence for the generation and migration of hydrocarbons in toarcian source rocks of different maturities abstract this study is part of a comprehensive research program on the generation and migration of petroleum from the organic rich posidonia shale lower toarcian jurassic of the hils syncline northern germany four cores of the complete posidonia shale with varying amounts of underlying pliensbachian and overlying aalenian mudstones provided the sample base depending upon their distance from a presumed subjacent heat source the vlotho massif the cores display progressive levels of increasing thermal maturation with r 0 values 0 48 0 68 0 88 and 1 45 which span the limits of the oil generation window primary stratigraphic lithologic and petrographic attributes of the four sections are all very similar each includes a lower marlstone member about 5 m thick overlain by 11 31 m of calcareous clay shale clay minerals calcite pyrite kerogen and quartz are the principal constituents thus there is no evidence that depositional conditions of the posidonia shale varied significantly over the sampled area the omnipresence of framboidal pyrite clearly formed from the activity of sulphate reducing bacteria at or near the depositional interface is strong evidence that uniform reducing anoxic conditions were persistent at all localities this conclusion supports the assumption that initially each section was identical in overall properties and that significant differences observed in geochemistry organic macerals and other properties are directly related to the variable levels of thermal maturation kerogen from the least mature section r o 0 48 is type ii and composed of alginite a alginite b liptodetrinite bituminite and some terrestrial sporinite vitrinite and inertinite macerals with increasing maturation the alginite macerals exhibit a progressive change in morphology and fluorescence properties alginite of the least mature section displays a strong green fluorescence at 0 68 r o alginite macerals exhibit yellow orange fluorescence and are structurally well preserved although geochemical data indicate generation and loss of large amounts of oil at 0 88 r o hydrocarbon generation is almost complete and few primary alginites remain they have been transformed into secondary liptinites designated as metaalginite which have similar fluorescence colours at the highest maturation level 1 45 r o metaalginites are only weakly fluorescent with a shift toward red colours calcite and or bitumen filled bedding plane micro and macrofractures occur exclusively in the organic rich posidonia shale at maturity levels of 0 68 r o and above such features are totally absent in the immature posidonia section and in the adjacent organic poor mudstones this observation indicates that the fractures developed in direct response to overpressuring i e pore pressure exceeding lithologic load related to oil and gas generation in the source rock the fractures undoubtedly provided important expulsion conduits for generated products

tectonic subsidence flexure and global changes of sea level tectonic models for the evolution of passive continental margins predict that following rifting sediments should progressively onlap basement at the edge of a margin as the lithosphere cools and increases its flexural rigidity with age the pattern of modelled onlap is strikingly similar to that used by vail and colleagues to estimate sea level rise through geological time this similarity suggests that major portions of stratigraphical sequences at margins may have a tectonic rather than eustatic control the patterns of onlap used by vail and colleagues may be widespread however because several widely separated passive margins rifted at similar times but they are unlikely to be worldwide

contrasting sulfur geochemistry and fe al and mo al ratios across the last oxic to anoxic transition in the cariaco basin venezuela an abrupt transition from oxic to anoxic sulfidic euxinic marine bottom waters occurred in the cariaco basin in response to increasing productivity resulting from the late pleistocene post glacial rise in sea level and corresponding increase in surface water nutrient availability the microlaminated sediments of the euxinic interval which span the last f14 5 ky suggest a predominance of water column syngenetic pyrite formation based on 1 high pyrite sulfur spy concentrations in the surficial sediment layers 2 values for degree of pyritization dop that generally do not increase appreciably with increasing burial 3 ratios of total iron fet to al that are elevated above the continental baseline recorded in the underlying oxic sediments and 4 spy isotope trends that largely mimic the d 34 shsof the modern water column intermediate dop values in the microlaminated deposits and fet al ratios that are slightly above continental levels indicate an iron reservoir controlled by scavenging during syngenetic pyrite formation in combination with intermediate rates of fe bearing siliciclastic accumulation as predicted from the relative rates of siliciclastic delivery fet al and dop data lie between end member values observed in the modern black sea as viewed broadly fet al and dop relationships in euxinic sediments reflect the balance between syngenetic fe scavenging and temporal and spatial gradients in siliciclastic input pyrite concentrations are generally low in the underlying oxic marine deposits because of limitations in the supply of organic carbon corg however the upper 80 cm of the fe rich corg poor bioturbated sediment show evidence for a strong diffusional hsoverprint from the overlying fe limited euxinic marine environment this post glacial transition manifests in pyrite overprints that are strongly 34 s depleted relative to those in restricted presently euxinic marine settings elsewhere in the world such as the black sea where the sedimentary sequence spanning the last glacial interglacial transition begins with a shift from freshwater to corg poor oxic marine deposition and thus dominantly sulfate diffusion trends for mo al ratios in the microlaminated sediments suggest that mo is enriched by roughly two orders of magnitude above the continental levels recorded in the oxic deposits organic matter plays a role by enhancing hsproduction and or by providing a substrate for mo scavenging significant mo enrichment via diffusion into the upper portion of the bioturbated zone was not observed despite hs rich pore waters as recorded in the heavy iron sulfide overprint we have

preliminary sulphur isotope data of diagenetic and vein sulphides in the lower palaeozoic strata of ireland and southern scotland implications for zn pb ba mineralization genetic models for many irish lower carboniferous zn pb ore deposits invoke a dual supply of sulphide the dominant source is from the bacteriogenic reduction of lower carboniferous seawater sulphate but a significant minor supply is derived from deep seated sources the 34 s range of the latter component varies among the deposits from the lightest range of 15 to 0 at keel to the heaviest of 4 to 14 4 in the navan tatestown area we hypothesize that such sulphur is leached mainly from diagenetic sulphide minerals in the underlying lower palaeozoic sediments 6 s of pyrite in the silurian ordovician moffat shales 17 1 to 0 6 and of sphalerite and galena in lower palaeozoic hosted veins at salterstown 8 7 to 4 5 and wanlockhead 10 3 to 5 1 are consistent with the hypothesis below the navan ore deposit lower palaeozoic shales containing minor diagenetic pyrite with a very wide range in 34 s have been found 34 s is typically heavy 16 but extreme values up to 62 are encountered the general enrichment in 34 s is in accord with the noticeably heavy isotopic composition of deep seated sulphur in the navan orebody and its tatestown satellite our preliminary results therefore suggest that geographical variations in the 34 s range of lower palaeozoic diagenetic write may have contributed to the isotopic variation in the deep seated sulphur among the irish deposits

organic facies variations in the westbury formation rhaetic bristol channel sw england abstract despite their dark colour laminated fabric and dysoxic character the thermally immature shales of the rhaetic westbury formation of the bristol channel area typically have total organic carbon toc values of only 1 0 1 5 and hydrogen indices hi 200 variations in toc and hi are correlated with the amorphous organic matter aom fluorescence but not the aom suggesting that redox related variations in aom preservation control the organic facies the diameter of equidimensional black wood phytoclasts decreases in intervals with higher toc and hi suggesting an association with lower energy depositional conditions such intervals also show a relative increase in light brown wood probably a reflection of more humid onshore conditions and lower phytoclast oxidation at source phytoc values suggest the input of terrestrial om was relatively low and stable during westbury deposition indicating a distal setting removed from fluviodeltaic sediment sources

the geochemistry geochronology and paleomagnetism of dikes and sills associated with the mesoproterozoic midcontinent rift near thunder bay ontario canada abstract the midcontinent rift mcr of north america comprises a series of basaltic sheets flows and intrusive rocks emplaced in the lake superior region during the mesoproterozoic the mafic rocks preserved on the northern flank of lake superior represent the older portions of the rift sequence and offer insights into the early development of the rift new geochronological geochemical and paleomagnetic data are presented for the dikes and sills located in and south of thunder bay ontario three sill suites are recognized within the study area an earlier spatially restricted ultramafic unit termed the riverdale sill the predominant logan sills and nipigon sills in the north of the study area in addition three dike sets are recognized the north east trending pigeon river swarm the north west trending cloud river dikes and the mt mollie dike the geochemical data demonstrate that the majority of sills south of thunder bay are of logan affinity and distinct from those of broadly similar age in the nipigon embayment to the north the pigeon river dikes that intrude the sills are geochemically coherent but distinct from the logan sills and could not be feeders to the sills the new age of 1109 2 4 2 ma for the cloud river dike and its r polarity are consistent with published magnetostratigraphy the mt mollie dike age 1109 3 6 3 ma indicates that it is not coeval with the spatially associated crystal lake gabbro as previously thought the complexity of the dike and sill suites on the northern flank of suggests that the early phases of rifting occurred in distinct and changing stress fields prior to the main extensional rifting preserved in younger rocks to the south the geochemistry and geochronology of the intrusions suggest a long lived and complex magmatic history for the midcontinent rift

oxford clay studies thematic set assembled by j d hudson d m martill the oxford clay formation is one of the classical units in british jurassic stratigraphy it crops out almost continuously in an arc running from the dorset coast to oxford through the midlands to yorkshire it was traditionally subdivided into lower middle and upper divisions now formalized as the peterborough stewartby and weymouth members respectively cox et al 1993 the peterborough member is organic rich the stewartby and weymouth members more calcareous and generally organic poor excellent ammonite faunas throughout the sequence are important in the international bio and chrono stratigraphy of the callovian and oxfordian stages and in correlation between the tethyan and boreal fauna1 realms the peterborough member the clay that burns supports britain9s largest brick industry why study the oxford clay most obviously for its superb fossils to be seen in museums the world over martill hudson 1991 the peterborough member also serves as an example of a potential petroleum source rock besides being an important current industrial resource it has not yielded petroleum because it has never been deeply buried this means that the rocks and fossils preserve much of their original chemistry and thus help bridge the gap between studies of modern sediments and of conventional source rocks in the papers in this thematic set we have tried to make use of these opportunities to show what modern methods and concepts can add to classical geological interpretation we have studied mainly central and eastern england the area of least burial simplest structure best fossil preservation and because of the

pyrite formation in bioturbated clays from the jurassic of britain the dunans clay and upper oxford clay of britain show the importance of bioturbating organisms in controlling the formation of pyrite in the surface layers of normal shales these clays are characterized by low pyrite sulphur content 1 3 low organic carbon content 2 and light 34s values generally lighter than 29 these values are consistent with the formation of pyrite close to the surface where contact with the overlying seawater is maintained by the bioturbating fauna low inputs of organic carbon and its consumption by both aerobic and anaerobic bacteria limits the formation of pyrite it is thought that particularly large apparent fractionations between pyrite and jurassic seawater up to 58 are the result of incorporation of isotopically light sulphate from extensive reoxidation of sulphide

the palaeomagnetism of major middle proterozoic igneous complexes south greenland and the gardar apparent polar wander track abstract the gardar province comprises a major rift related igneous suite emplaced in three pulses at ca 1300 1280 ma ca 1220 ma and 1160 1125 ma this study reports a palaeomagnetic study of the northeastern sector of the present outcrop and incorporates igneous units belonging to the first and last episodes early lavas from the basal part of the rift infill record a reversal of magnetisation and define a palaeomagnetic pole at 207 e 10 n 5 sites dp dm 7 14 this primary magnetisation record has been partially overprinted in the aureoles of large alkaline complexes the latter plutons include the north qoroq intrusion 1291 61 ma yielding a pole position at 203 e 13 n 65 samples from 12 sites dp dm 6 12 the motzfeldt intrusion 1291 31 ma yielding a pole position at 212 e 10 n 25 samples dp dm 6 12 and the narssarssuaq stock yielding a pole position at 207 e 9 n 23 samples dp dm 4 7 the younger south qoroq complex 1160 8 ma yields a pole position at 216 e 42 n 67 samples from 9 sites dp dm 11 16 positive contact tests indicate primary cooling related magnetisations these bodies are predominantly of single polarity equivalent to w directions of magnetisations although reversals are recorded the majority of a suite of ne sw trending dykes emplaced at ca 1154 1143 ma are magnetised in a similar direction d i sol3 51 pole position at 231 e 33 n 18 dykes dp dm 5 7 to the slightly older south qoroq complex together with earlier results from the gardar province these data define a motion of the region from shallow to equatorial and then back to intermediate palaeolatitudes between 1290 and 1140 ma pole positions trace out an apparent polar wander path comprising a small loop followed by a long track this track is the outward path of the great logan palaeomagnetic loop for which data from the keweenawan province of north america ca 1100 1050 ma define the return path the loop was executed between ca 1200 and 1050 ma following a near static interval between ca 1290 and 1200 ma this long static interval was also characterised by constant field polarity asymmetry of geomagnetic field reversals is recognised in early gardar events it is not apparent in younger gardar data although it recurs in keweenawan times

timing of proterozoic magmatism in the gardar province southern greenland the gardar province in southern greenland is believed to have experienced three episodes of rifting all associated with the extrusion of large volumes of alkalic magma a nepheline syenite of the motzfeldt center yields a u pb apatite age of 1 35 0 01 ga an augite syenite from the ilimaussaq complex yields a sm nd pyroxene alkali feldspar magnetite age of 1 13 0 05 ga on the basis of field relationships the eriksfjord formation lavas have been interpreted as the oldest manifestation of gardar magmatism sm nd plagioclase pyroxene wholerock ages of 1 17 0 03 ga and 1 20 0 03 ga for the middle member of the eriksfjord formation however indicate that these lavas may be part of the youngest magmatic activity the available chronological data indicate that the gardar province was active for at least 200 m y from 1 35 to 1 12 ga it is difficult to reconcile this protracted history with plume models for the genesis of such alkaline flood basalt provinces

sequence stratigraphy and systems tracts of a mixed carbonate and siliciclastic platform basin setting the albian of lunada and soba northern spain in the lunada soba carbonate platform basin setting albian of basque cantabrian region northern spain nine type 1 depositional sequences have been identified their boundaries represent times of complete exposure of the platform with accompanying karstification and fluvial erosion relative sea level falls of 15 50 m have been estimated from depths of erosion or from diagenetic effects on platform materials and facies analysis during lowstands deposits of the corresponding systems tract onlapped the platform foreslope or filled incised valleys on the platform top these deposits comprise a variety of facies limestone megabreccias derived from the platform margin allodapic grainstones hemipelagic marls fine grained turbiditic sandstones basinal lutites fine gra ned estuarine sandstones bank bafflestones and grainstones fluvio deltaic pebbly conglomerates and sandstones and shallow water orbitolinid grainstones relative sea level rises were characterized by deposition of monotonous marls in the basin and by the growth of coral rudist mud mounds and accumulations of skeletal grainstones and marls sometimes with back stepping stacking on the platform during highstands hemipelagic marls and prodeltaic and talus delta lutites and sandstones dominated in the basin whereas coral wackestones and orbitolinid grainstones sometimes with prograding clinoforms dominated on the platform transgressive surfaces have erosion or simply omission features such as pebble lags or numerous burrows respectively no well developed condensed sections exis and they are represented by thin beds of marls rich in orbitolinids transgressive and highstand systems tracts are normally difficult to differentiate both in the basin and on the platform but in sequences with an accused wedge shape the presence of back stepping and prograding stacking patterns makes it easier four main types of sequences are documented based on geometrical characteristics three of them are continuous from the platform to the basin tabular wedge shaped with tilted base and wedge shaped with incised base the fourth type scalloped is discontinuous from the platform to the basin it results from the collapse of the platform margin whose debris contributed to form the following sequence and the unveiling of the underlying sequence at the site of collapse most of the sequence boundaries suggest a tectonic influence and three of them show angular unconformities in the basin margin and or the inner platform these angular unconformities show major erosional vacuities at the cores of anticlines on the inner platform they are attributed to phases of accentuated transpression in a major strike slip tectonosedimentary environment related to the opening of bay of biscay a local curve of albian relative sea level changes correlates in part with the eustatic curve of the vail charts