stable carbon and oxygen isotope evidence of cretaceous sea level fluctuations recorded in septarian concretions from pueblo colorado u s a abstract limestone concretions in the blue hill member of the carlile formation middle middle turonian exposed in the lake pueblo state park colorado u s a range in size from 20 cm diameter approximately spherical forms to flattened spheroids 2 m in width and 1 m in height the earliest matrix and septarian calcite cement with 13c and 18o values ranging mostly from 18o oo to 8o oo pdb and 3o oo to 8o oo pdb respectively precipitated from diluted seawater during shallow burial where sulfate reduction largely controlled pore water bicarbonate composition in contrast the latest septarian cements exhibit very depleted 18o values 16o oo with 13c values approaching values of 1o oo to 3o oo reflecting a strong meteoric water component to pore fluids overall 13c and 18o values are inversely covariant concretion matrix calcite and septarian cements precipitated from mixtures of modified marine and meteoric fluids pore water reversals resulted from sea level fluctuations leading up to and associated with the formation of a sequence boundary or possibly the development of parasequences near the top of the blue hill member relative sea level falls caused pore water freshening as the coastal mixing zone migrated seaward similarly modified marine derived pore waters replaced fresher pore fluids accompanying subsequent marine flooding carbonate cements precipitated during this active hydrologic history recorded the isotopic characteristics of their parent fluids negative 18o values of the meteoric end member calcite compositions suggest derivation from high altitude precipitation in the sevier highlands to the west septarian cements in concretions despite their enclosure in relatively impermeable host sediment can act as sensitive indicators of relative sea level fluctuations

quantification of the pore structures of malan loess and the effects on loess permeability and environmental significance shaanxi province china an experimental study the pore microstructures of loess control the porosity and permeability of the loess affecting the patterns of groundwater flow and the transport of contaminants in the present study the pore microstructure of malan loess was investigated quantitatively in samples from five different loess layers specimens were examined via sem and pore microstructure parameters were determined using the digital image analysis method pore structures including pore area ratio par pore size distribution pore shape and pore morphology the effects of the pore structure on loess permeability and the environmental significance of these factors are discussed in this paper the results indicate that the samples of five loess layers in this study are considered to be porous soil according to both the par and porosity values the number of micropores small pores mesopores and macropores decreased significantly the differences in number were mainly due to the significant reduction in the number of round pores indicating that round pores tend to be small in terms of area micropores small pores mesopores and macropores were dominated by elongated and irregular pores the macro and mesopore structural characteristics of loess determined the transfer characteristics of groundwater and pollutants in the loess additionally the 62 area reduction in the area of elongated or irregular macro and mesopores and the decreasing connectivity from top to bottom layers caused by the deformation and destruction of the pores due to the overlying soil reduced the permeability and the water pollutant migration rate with depth

petrology chemistry and diagenesis of calcite concretions in silurian shales from central sweden calcite concretions in the lower silurian rastrites shale from central sweden were formed within 1 m depth of the seafloor micrite 1 4 m was formed initially but recrystallized into microspar 4 50 m and subsequently transformed into pseudospar 50 150 m skeletal calcite in the concretions and in the host shale has been replaced by microspar which is chemically similar to that occurring in the concretion body the 13cpdb values mostly between 2 and 2 suggest that carbonates were mainly derived from seawater some of the carbon within calcite was derived from the oxidation of organic matter recrystallization has resulted in increasingly negative 18 opdb values 7 to 4 3 whereas the 13c was not changed significantly

basin scale diagenetic alteration of shoreface sandstones in the upper cretaceous spring canyon and aberdeen members blackhawk formation book cliffs utah abstract leached zones whitecaps and dolomite cement bodies are volumetrically and areally significant features in shoreface sandstones of the upper cretaceous spring canyon and aberdeen members blackhawk formation book cliffs utah quantitative spatial and stratigraphic data of their distribution together with petrography and geochemistry allow development of a model for their formation within a sequence stratigraphic framework leached zones up to 5 m thick and 15 km in lateral extent are present beneath coals capping parasequences concretionary dolomite cement is present in shoreface sandstones downdip of parasequence capping coals and associated whitecaps these concretions are up to 2 m thick and 6 m in length and are present for up to 7 km downdip of these coals petrographic and field data indicate an early diagenetic timing for whitecap and dolomite cement formation 13c data suggest carbonate derivation from detrital dolomite whereas 18o data suggest that cement precipitated from porewater with a significant meteoric component although later diagenetic re equilibration and recyrstallization cannot be rejected we propose that organic acid generation during early diagenesis of coals led to the leaching of detrital dolomite from underlying sandstones and the formation of whitecaps meteoric water flowed basinward in response to base level lowering and sequence boundary development in the upper part of the aberdeen member remobilizing dolomite into shoreface sandstones dolomite precipitation resulted from the mixing of these meteoric fluids with downdip marine pore fluids and the presence of detrital dolomite in the shoreface sandstones that acted as nucleation sites for dolomite precipitation the geometry and extent of dolomite cement in the spring canyon and aberdeen members is in marked contrast to the much greater spatial extent of such cement in low accommodation space settings such as the stratigraphically younger desert member blackhawk formation the relatively simple aggradational to progradational evolution of the spring canyon and aberdeen member parasequences with only minor sequence boundary development in its upper part resulted in spatially limited meteoric fluid input into shoreface sandstones this resulted in dolomite cementation of shoreface sandstones up to a maximum of 7 km downdip of leached zones in the desert member extensive meteoric fluid migration as a result of major sequence boundary development resulted in dolomite cementation of shoreface sandstones for over 25 km downdip of leached zones this highlights that the degree and extent of early fluid diagenesis can be significantly controlled by the nature of sequence development having clear implications for developing predictive models for porosity permeability evolution in sandstone reservoirs

carbon oxygen and sulphur isotope variations in concretions from the upper lias of n e england carbon oxygen and sulphur isotope data for transects across two pyrite bearmg carbonate concretions and their host sediments from the upper lias of n e england show symmetrical zonation 13cpdb values of the calcite cement 12 9 to 15 4 indicate that most of it originated from organic matter by bacterial reduction of sulphate augmented with marine and to a lesser extent fermentation derived carbonate organic carbon 13cpdb 26 1 to 37 0 reflects the admixture of allochtho nous terrestrial organic matter with marine material and the selective preservation of isotopically light organic material through microbiological degradation two phases of pyrite are present in each concretion the earlier framboidal pyrite formed throughout the sediment prior to concretionary growth and has 34scd values of 22 to 26 indicating formation by open system sulphate reduction the later euhedral phase is more abundant and reaches values of 2 5 to 5 5 at concretion margins this phase of sulphate reduction provided the carbonate source for concretionary growth and occurred in a partially closed system the 13c and 34s data are consistent with mineralogical and chemical evidence which suggest that both concretions formed close to the sediment surface the 18o values of the calcite in one concretion 18opdb 2 3 to 4 8 indicate precipitation in pore waters whose temperature and isotopic composition was close to that of overlying seawater the other concretion is isotopically much lighter 18opdb 8 9 to 9 9 and large 18o differences between concretions in closely spaced horizons imply that local factors control the isotopic composition of pore waters

a study of two mid crustal bright spots from southeast georgia usa summary two high amplitude reflections or bright spots occur on cocorp consortium for continental reflection profiling line georgia 16 at a record time of 5 9s corresponding to nearly 16 km depth they are significant because bright spots often mark great contrasts in subsurface physical properties the first is the surrency bright spot sbs an exceptionally bright antiformal reflection and the second is the reedy creek reflection rcr which lies 5 km to the south and resembles the south half of a diffraction the rcr is also imaged on a crossline georgia 18 as a relatively symmetric antiformal reflection these reflections coincide with the brunswick magnetic anomaly thought to mark the late palaeozoic alleghanian suture the sbs and the rcr are reflections from bodies that lie within the plane of georgia 16 migration and 2 d seismic modelling show the aniformal sbs is from a synformal reflector with a buried focus while the rcr is from a relatively small body the inferred geometry of the sbs reflector with its flat portion corresponding to the deepest part of the reflector argues that it does not represent fluids these results combined with recent evidence that the sbs is a reflection from a high velocity thin layer suggest that the sbs and the rcr might be reflections from ultramafic slices emplaced within the alleghanian suture zone during collision or from mafic sheets intruded into the suture zone during subsequent rifting the brightness of the sbs is attributed to a large impedance contrast constructive interference good data quality and a relatively large reflector size with respect to the fresnel zone and neighbouring reflectors

field relationships and stable isotope geochemistry of concretions from james ross island antarctica abstract early diagenetic precompactional concretions are abundant throughout the cretaceous tertiary marambio group larsen basin antarctica four distinct concretion types are recognised 1 spherical subspherical concretions 2 sheet concretions 3 fossil nucleated concretions and 4 concretionary burrow networks all concretion types have a micritic to microsparry variably non ferroan to ferroan calcite cement stable isotope analyses show a wide spread in both 18o and 13c 13c values are typically negative ranging between 3 38 and 39 15 o pdb usually 16 to 230 18o ranges between 1 28 and 13 81 pdb with most of the values between 5 and 10 the 13c signature is interpreted to represent carbon sourced from sulphate reduction and or methane oxidation with minor input from shell dissolution and is consistent with a shallow burial early diagenetic origin a single mudstone hosted concretion has a 18o composition indicative of precipitation of carbonate from seawater the low 18o signatures in the sandstone and siltstone hosted concretions are possibly due to early diagenetic modification of the pore water composition through volcaniclastic mineral dissolution reprecipitation reactions and perhaps through input of meteoric water concretion distribution is related to a changes in sedimentation rate and b the dominance of diffusion on concretion cementation

time capsule concretions unlocking burial diagenetic processes in the mancos shale using carbonate clumped isotopes abstract septarian carbonate concretions contain carbonate precipitated during progressive growth of the concretion and subsequent fracture filling as such they have been used to track variations in 13c and 18o of pore waters during diagenesis and to define diagenetic zones in clastic rocks however the 18o value of the carbonate is dependent on precipitation temperature and the 18o value of the pore fluid from which it precipitated interpretations must assume one of these parameters both of which are highly variable through time in diagenetic settings carbonate clumped isotopes of the cement can provide independent estimates of temperature of precipitation allowing the pore water 18o to be back calculated here we use this technique on carbonate concretions and fracture fills of the upper cretaceous prairie canyon member mancos shale colorado we sampled concretions from two permeable horizons separated by a 5 m shale layer with one permeable horizon containing concretions with septarian fractures we show cores precipitated at cooler temperatures 31 c 660 m burial depth than the rims 68 c 1980 m burial depth and relate that to the 13ccarbonate values to suggest the concretion core precipitated in the methanogenic zone with increasing input from thermogenically produced co2 the two concretion bearing horizons have different back calculated 18oporewater values mean 2 65 and 1 13 vsmow for cements formed at the same temperature and similar 13c values suggesting the shale layer present between the two horizons acted as a barrier to fluid mixing additionally the 18ocarbonate of the septarian fractures 13 8 vpbd are due to precipitation at high temperatures 102 to 115 c from a fluid with a mean 18oporewater of 0 32 vsmow therefore we can tie in the cementation history of the formation to temporal and spatial variations in 18oporewater

constraints on the structure and tectonic development of the early mesozoic south georgia rift southeastern united states seismic reflection data processing and interpretation seismic reflection data collected by the consortium for continental reflection profiling cocorp over the buried triassic jurassic south georgia rift reprocessed in order to improve the upper 5 s of reflection time provide a window into the regional deep structure of the rift and delineate variations in basin structure and depositional styles located at the southernmost extent of the north american east coast early mesozoic rift system the structure of the south georgia rift basin has previously been known only from scattered drilling and potential field interpretation leaving the internal and boundary as well as the deep structure unknown the reflection data reveal that the south georgia basin consists of a broad complex terrane of isolated synrift grabens with intervening structural highs in contrast with earlier concepts of a single narrow trough or an amorphous sag basin two main subbasin depocenters are observed in west and west central georgia the largest of which is more than 100 km in width and the deepest at least 7 km at its inferred base analysis of the reflection data indicates that the distinct structural and seismic stratigraphic character of the reflection packages is analogous to other better studied extensional provinces e g north american basin and range the reprocessing of the reflection data provides a view of contrasting styles of faulting and subsidence within the rift the structural style of the subbasins varies drastically along strike changing from 1 a narrow asymmetric graben bordered by a planar and more deeply penetrating normal fault to 2 a broad and roughly symmetric graben bordered by a shallower dipping possibly listric fault along which extension may have been accommodated at depth by a low angle detachment or by a more complex system of high angle faults only limited crustal extension is suggested by the narrower graben and planar fault 1 whereas much greater extension would be expected from the wider graben bordered by a listric low angle fault 2 such a dual style of basin formation is predicted by segmented plate models of variable extension along continental margins and intracratonic rifts and may imply a significant change in crustal rheology along the developing rift integration of the interpretation of the regional structure of the south georgia basin with that of the deeper crustal structure as inferred from the 16 s reflection time cocorp data indicates that the synrift basins developed mainly over the upper plate of the alleghanian late paleozoic suture between north america and west africa the major subbasin border faults dip northward in an antithetic relation to the predominantly northward vergence of the suture zone the border faults thus do not appear to have reactivated antecedant structure although the development of the south georgia rift appears to have approximately coincided with early mesozoic thinning of the crust in the southeastern united states no horizontally layered or laminated lower crust is observed as has been associated with other more fully rifted terranes e g north american basin and range continental shelf around british isles however the moho discontinuity appears to have formed as a dynamic feature superimposed on late paleozoic compressional structure probably associated with early mesozoic rifting these findings thus support the interpretation that the south georgia rift developed as a major early mesozoic depocenter and possible incipient spreading center associated with the initial formation of the central atlantic ocean

dolomite abundance in chinese loess deposits a new proxy of monsoon precipitation intensity dolomite of detrital origin in carbonate rich soil can help indicate the intensity of carbonate dissolution and leaching processes using fourier transform infrared spectroscopy we generated a high resolution record of dolomite and calcite contents from eight sections on the chinese loess plateau spanning the last 130 kyr based on the relative abundance of calcite and dolomite four dissolution phases of carbonate minerals related to summer monsoon sm variations are identified as follows 1 coexistence of dolomite and calcite indicating a very weak sm with a mean annual precipitation map 725 mm

deep structure of the u s atlantic continental margin offshore south carolina from coincident ocean bottom and multichannel seismic data we present the results of a combined multichannel seismic reflection mcs and wide angle ocean bottom seismic profile collected in 1988 across the carolina trough on the u s atlantic continental margin inversion of vertical incidence and wide angle travel time data has produced a velocity model of the entire crust across the continent ocean transition the margin consists of three structural elements 1 rifted continental crust comprising 1 4 km of post rift sedimentary rocks overlying a 30 34 km thick subsedimentary crust 2 transitional crust a 70 to 80 km wide zone comprising up to 12 km of postrift sedimentary rocks overlying a 10 to 24 km thick subsedimentary crust and 3 oceanic crust comprising 8 km of sedimentary rocks overlying an 8 km thick crystalline crust the boundary between rifted continental and transitional crust marked by the brunswick magnetic anomaly represents an abrupt change in physical properties with strong lateral increases in seismic velocity density and magnetic susceptibility the transitional crust contains mid crustal seaward dipping reflections observed on the mcs section and has seismic velocities of 6 5 6 9 km s in the midcrust and 7 2 7 5 km s in the lower crust modeling of potential field data shows that transitional crust also produces the prominent margin parallel gravity anomaly and the brunswick and east coast magnetic anomalies these observations support the interpretation that the transitional crust was formed by magmatism during continental breakup the prodigious thickness up to 24 km of igneous material rivals that interpreted on continental margins of the north atlantic e g hatton bank and voring plateau which formed in the vicinity of the iceland hotspot these observations when combined with other transects across the margin confirm previous suggestions that the u s atlantic margin is strongly volcanic and further imply that the magmatism was not the result of a long lived mantle plume

meteoric water diagenesis in late cretaceous canyon fill turbidite reservoirs from the espirito santo basin eastern brazil abstract parameters controlling the diagenetic evolution of passive margin marine turbidites which are important targets of hydrocarbon exploration are poorly constrained in the literature this study aims to unravel the conditions of diagenesis and its impact on the reservoir quality evolution from late cretaceous canyon filling turbiditic sandstones of the onshore portion of espirito santo basin eastern brazil kaolinization 18 o 13 3 to 15 2 d 96 6 to 79 6 and dissolution of framework silicate grains is attributed to meteoric water incursion during eodiagenesis in response to a considerable fall in relative sea level eogenetic alterations also include cementation by siderite average 18 o 7 2 13 c 9 3 and pyrite progressive sediment burial present depths 1530 2027 m resulted in the formation of poikilotopic calcite ferroan dolomite ankerite average 18 o 7 9 13 c 2 9 minor amounts of quartz overgrowths and in partial dickitization of kaolinite isotopic values of calcite and dolomite ankerite follow two trends of co variance of 13 c with decreasing 18 o and increasing temperature from a composition closer to marine 0 one trend goes towards positive 13 c values up to 22 4 for calcite 18 6 for dolomite ankerite indicating increasing input of carbonate from methanogenic fermentation the other trend develops towards negative 13 c values down to 17 2 for calcite 15 for dolomite ankerite suggesting increasing contribution from thermal decarboxylation with increasing temperature and depth despite the presence of various cement types mechanical compaction was more important than cementation in reducing depositional porosity in the onshore urucutuca sandstones

geometrical appearance and spatial arrangement of structural blocks of the malan loess in nw china implications for the formation of loess columns abstract loess as one of the main quaternary deposits covers approximately 6 of the land surface of the earth although loess is loose and fragile loess columns are popular and they can stand stably for hundreds of years thereby forming a spectacular landform the formation of such special column shaped soil structures is puzzling and the underlying fundamentals remain unclear the present study focuses on quantifying and examining the geometrical shape and spatial alignment of structural blocks of the malan loess at different locations in the loess plateau of china the structural blocks under investigation include clay and silt sized particles aggregates fragments lumps and columns which vary in size from microns to tens of meters regardless of their size the structural blocks of the malan loess are found to be similar in shape i e elongated with a length to width ratio of approximately 2 6 the aggregates fragments lumps columns and macropores between aggregates exhibit strong concentration in the vertical or subvertical alignment these phenomena imply that the malan loess is anisotropic and it is composed of a combination of vertically aligned strong units and vertically aligned weak segments based on this vertiloess structure is proposed to denote this combination the vertiloess structure prevents horizontal erosion but favors spalling peeling toppling falling and cracking sliding of vertical loess pieces thereby forming loess columns

the internal structure of carbonate concretions in mudrocks a critical evaluation of the conventional concentric model of concretion growth abstract carbonate concretions have traditionally been viewed as forming concentrically through the progressive addition of carbonate to the outer edge of the growing concretion this conventional model of concretion growth is based mainly upon center to edge textural and geochemical trends that are consistent with concentric growth however a number of recent studies of concretions in mudrocks have documented complex zonation that is not compatible with the conventional concentric model in some cases complexly zoned concretions contain late stage cements that precipitated throughout the concretion body none of the center to edge relationships used to support the conventional concentric model exclude more complex modes of origin as concretions that fit the conventional model may be the exception rather than the rule high resolution petrographic analysis of concretions should always be used to characterize internal structure and mode of growth prior to interpretation of geochemical and textural data

the role of premagmatic rifting in shaping a volcanic continental margin an example from the eastern north american margin both magmatic and tectonic processes contribute to the formation of volcanic continental margins such margins are thought to undergo extension across a narrow zone of lithospheric thinning 100 k

the diagenetic history of some septarian concretions from the kimmeridge clay england abstract large septarian concretions from the kimmeridge clay up to 1 2 m in diameter have centres comprising anhedral calcite microspar passing into margins of radiating fibrous calcite microspar with a pyrite rich zone at the transition septarian veins formed and were lined with brown calcite synchronously with fibrous matrix growth with white calcite precipitated in septarian cavities after concretion growth ceased septarian veins filled only with white calcite formed later at the same time as the outermost calcite microspar crystals were enlarged the concretions were buried in the late jurassic to about 130 m and in the late cretaceous to about 550 m with uplift between oxygen isotopes show that the concretion grew throughout the first burial with septarian veins forming from about 30 m depth onwards later septarian veins formed between about 200 and 500 m during the second burial carbon isotopes show that the compact inner matrix grew in the sulphate reduction zone the end of which is marked by the pyrite enriched zone dissolving shells and possibly minor methanogenic carbonate slowly diluted sulphate reduction zone carbonate during deeper burial during early concretion growth mg and sr were depleted in the pore water during later stages of the first burial mg sr mn and fe all increased especially after concretion growth ceased during the second burial fe mn and mg decreased as calcite precipitated implying relatively closed systems for these elements synchronous formation of septarian fractures and fibrous calcite matrix shows that the kimmeridge clay became overpressured during the later stages of both burials

lithospheric sources of north florida usa tholeiites and implications for the origin of the suwannee terrane abstract sr nd and pb isotopic data for mesozoic tholeiites from six drillholes in northern florida suwannee terrane indicate derivation of the magmas from a source that was slightly incompatible element enriched with respect to bulk earth probably continental lithospheric mantle variably contaminated by more enriched crust initial isotopic compositions of the florida tholeiites show a considerable range 87 sr 86 sr 0 7059 to 0 7097 end 4 6 to 0 1 206 pb 204 pb 18 30 to 18 65 207 pb 204 pb 15 57 to 15 66 and 208 pb 204 pb 38 22 to 38 35 these compositions are distinctly different from those of mesozoic alkali basalts from the subsurface of southwestern florida heatherington a l mueller p a 1991 geochemical evidence for triassic rifting in southwestern florida tectonophysics 188 291 302 it is suggested that the north florida tholeiites were derived from within the continental lithosphere in contrast with mesozoic volcanic rocks from southwestern florida which have been previously suggested to be related to a mantle plume sm nd isotope systematics suggest derivation of the tholeiites from a 1 ga source a similar age to that proposed by pegram pegram w j 1990 development of continental lithospheric mantle as reflected in the chemistry of the mesozoic appalachian tholeiites usa earth and planetary science letters 97 316 331 for the source of tholeiitic basalts within the carolina terrane this supports proterozoic tectonic models which suggest that both the suwannee and carolina terranes had affinities with south american portions of gondwana containing 1 ga crust specifically the sunsas rondonian province nance murphy 1996 mueller p a kozuch m heatherington a l wooden j l offield t w koeppen r p klein t l nutman a p 1996 evidence for mesoproterozoic basement in the carolina terrane and speculations on its origin in nance r d thompson m d eds avalonian and related peri gondwanan terranes of the circum north atlantic gsa special paper 304 pp 207 218

microbial processes controls on the shape and composition of carbonate concretions abstract shape size and mineralogical chemical compositions of carbonate concretions vary considerably the questions to be answered are why do they form at all what processes are involved why do they so often form as regular spheroids why do they sometimes form laterally extensive bands what controls whether the mineral composition is calcite dolomite rhodochrosite or siderite or mixtures of them the questions are addressed and the answers illustrated by selected examples carbonate precipitation is the result of the sedimentary system exploiting the potential energy provided by juxtaposing oxidised detrital input with reducing agents organic matter microbes take advantage of the energy source and are believed to be involved in most of the various processes pore waters in steadily accumulated sediments show a depth related succession of chemical and carbon stable isotope compositions this results from degradation of organic matter proceeding via an orderly succession of processes during burial each mediated by a specific microbial group aerobic oxidation using dissolved oxygen occurs first followed by mn iv reduced to mn ii as the oxidant fe iii reduction and sulphate reduction are probably more significant methanogenesis is the final deepest microbial process the common product of all these reactions is co 2 or dissolved hco 3 which can result in carbonate supersaturation the specific process controls the chemical and isotopic compositions of pore water and leads to a characteristic mineralogy of any carbonate precipitated however the processes may not proceed successively and localised micro environments occur recent work on siderite concretions in very young salt marsh sediments norfolk u k has shown that microbial populations are not homogeneously distributed and that these localise and control concretionary growth concretions in ancient rocks can be described in terms of the same processes observed in modern sediments laterally extensive siderite bands in bullhouse quarry westphalian u k resulted from interaction of two sets of complementary processes shallow manganese and iron reduction coupled with deeper methanogenesis and decarboxylation the model is supported by a mass and charge balance calculation similar complementary processes formed spheroidal concretions gammon shale usa but there is evidence that the outer shell of the concretion formed before the inner core and in the absence of a core the shell was crushed by burial compaction the spheroidal form resulted from diffusion of reactive components controlled by the relative rates of the contributory processes calcite or dolomite analogues of the siderite concretions are formed by similar processes but inhibition of carbonate precipitation makes it impossible to use an equilibrium mass balance model kimmeridge bay uk dolomite ledges are marine analogues to the bullhouse examples for the pyrite rimmed calcite jet rock concretions early jurassic yorkshire u k the rate of production of sulphide from sulphate reduction locally exceeded availability of fe ii to precipitate pyrite outward diffusion of sulphide produced the characteristic spheroidal shape of the concretion because of their developmental pattern bedded concretions are more likely to preserve a sequence of diagenetic cements which record the history of pore water evolution however all represent an expression in ancient rocks of microbiological processes similar to those operating today

basin scale dolomite cementation of shoreface sandstones in response to sea level fall this study documents a link between sea level fall and basin scale dolomite cementation of shoreface sandstones in the upper cretaceous desert member book cliffs utah by integrating sedimentology sequence stratigraphy and sedimentary geochemistry within the desert member shoreface sandstones a number of high frequency sequence boundaries formed as a result of relative sea level falls that led to localized incised valley formation subaerial exposure and meteoric leaching of upper shoreface sandstones shoreface sandstones up to 10 km downdip from these subaerial exposure surfaces contain ferroan dolomite concretion bodies as much as 8 m thick basinward of this smaller as much as 1 m in size less abundant dolomite concretions can be traced at the same horizons some 1 8 o data suggest that cement precipitated from porewater with a significant meteoric component although reequilibration and recyrstallization cannot be eliminated some 1 3 c data suggest carbonate derivation from detrital dolomite leached from beneath organic rich coastal plain strata deposited during high frequency sequence boundary development we propose that during high frequency sequence boundary formation meteoric fluids migrated basinward into shoreface sandstones as a result of the relative fall in sea level the precipitation of dolomite cement in the shoreface sandstones was promoted by the updip dissolution and remobilization of detrital dolomite by the meteroic fluids mixing with marine pore fluids and the presence of detrital dolomite in the shoreface sandstones which acted as nucleation sites for dolomite precipitation this study illustrates the impact of stratigraphic development upon basin scale early diagenetic cementation of siliciclastic succcesions

upper cretaceous vernal delta of utah depositional or paleotectonic feature a conspicuous seaward bulge of the middle to late turonian shoreline of the cretaceous seaway in northeastern utah and southwestern wyoming has been identified by previous authors as the vernal delta strata of the frontier formation and the ferron sandstone member of the mancos shale that form the vernal delta consist largely of fluviodeltaic facies the delta however is not recognizable as a locus of turonian sedimentation there is no isopach thick associated with it the vernal data is a large feature encompassing an area of at least 6250 mi sup 2 16 187 km sup 2 a comparison between the depositional setting and paleogeography of northeastern utah during the late cretaceous and a present day area on the east flank of the andes in colombia indicates strong similarities further comparison suggests that feature the size of the vernal delta could not have been produced by a single river the vernal delta overlies the ancestral uinta mountain uplift an area where cenomanian marine shales were entirely removed by what appears to have been submarine erosion during early turonian time when the shoreline prograded eastward across this area during middle uronian time the sediment load caused the area to subside but at a rate slowermore than rates of subsidence to the north and south this differential subsidence is the cause of the shoreline bulge although it includes deltaic facies the vernal delta is not a delta per se but a feature produced as the result of interaction between sedimentation and gentle tectonic movement of the ancestral uinta mountain uplift less

isostatic compensation for a portion of the southern appalachians evidence from a reconnaissance study using wide angle three component seismic soundings wide angle reflections generated by five controlled blasts and over 110 timed quarry blasts in the southern appalachians were used to test models for isostatic compensation of topography the profiles cross the appalachian gravity gradient and gravity low and sample the highest elevations within the orogen migration of p sv and sh reflections suggests that crustal thickness varies from 35 to 39 km within the coastal plain and 37 39 km within the carolina terrane it increases northwestward from 40 to 45 km across the inner piedmont and then thickens to 50 52 km along the southeastern flank of the blue ridge mountains crustal thickness within the blue ridge mountains ranges from 47 to 56 km receiver functions for broadband stations goga and mync show a similar pattern in crustal thickness assuming airy compensation the correlation between elevation and moho depth suggests a range of 50 150 kg m 3 for the density contrast between the crustal root and mantle the greatest moho depths are associated not with the tallest peaks but rather with the broadest portions of the mountain chain this observation is consistent with regional bending of the lithosphere however the planar basement surface suggests that the root either predates alleghanian thrusting and therefore is unrelated to the present topography or formed in response to some other mechanism bounds on curvature of the basement surface suggest a lower bound of 30 40 km for the effective elastic thickness of the lithosphere this is consistent with previous estimates for the southern appalachians based on analysis of gravity data

offshore extent of gondwanan paleozoic strata in the southeastern united states the suwannee suture zone revisited abstract the recognition of the suwannee terrane in southeastern north america was a critical step in advancing our current understanding of the tectonic evolution of ocean basins unfortunately further clarification of the boundaries of the suwannee terrane has proven difficult due to the thick coastal plain cover limited well log analyses and a paucity of seismic reflection data onshore we present the results from a new compilation and analysis of legacy marine seismic reflection refraction and well data from offshore the southeastern united states which reveals in the subsurface a previously unmapped lower paleozoic sedimentary section spanning the continental shelf from florida to north carolina previously only identified in two deep offshore wells cost ge 1 and transco 1005 1 this paleozoic platform sequence is identified in seismic reflection profiles as a package of low frequency sub horizontal laterally continuous reflectors which are clearly discordant with and distinct from the overlying mesozoic and cenozoic stratigraphy above the post rift unconformity pru the inferred base of the paleozoic sequence is marked by the downward diminution of parallel seismic reflectors as well as an increase in seismic velocities to 6 km s observed on numerous offshore seismic refraction surveys while clearly deformed in some areas these paleozoic strata are generally sub horizontal range in thickness from 4 to 6 km and can be mapped continuously over an area in excess of 130 000 km 2 similar sedimentary rocks have been recognized from onshore exploration wells in florida since the 1930s and were subsequently identified to be part of the suwannee basin within the larger exotic suwannee terrane of gondwanan affinity recognition of the presence and extent of these gondwanan strata offshore implies 1 the inferred position of the suwannee suture zone offshore lies 200 km further north approximately along the boundary between the carolina terrane and the charleston terrane 2 previously identified terranes brunswick charleston suwannee northern florida are likely distinct crustal blocks but their amalgamation predates deposition of the overlying paleozoic section of the suwannee basin 3 collectively this crust represents the gondwanan continent based on the size and presence of a stable platform stratigraphy nominally doubling the size of the last sutured terrane the suwannee terrane

loess is not just the accumulation of dust most regrettably there is much misuse of terminology in connection with the concept of loess and the interpretation of its origin people often speak and write incorrectly about loess accumulation even loess specialists who are aware of the fact that loess as a concept has to satisfy at least 10 criteria loess is not simply dust carried and deposited by wind dust only becomes loess after the passage of a certain amount of time in a given geographical zone i e only through diagenesis in certain ecological environments to state that loess is of aeolian origin is an oversimplification and an incorrect definition because an aeolian origin applies only to the dust from which the loess has been formed we are aware of the fact that airborne dust cannot be transformed into loess in every geographical environment but only under those conditions typical of semi arid grassland or steppe or forested steppe the process primarily occurs where the rate of dust accumulation exceeds sheet wash or weathering soil formation rates if the rate of dust accumulation is less than that of surface erosion or of soil biogenetic processes the dust then develops into soil or through intensive weathering and increased precipitation into loam or clay only part of the aeolian dust transported and deposited in a zone suitable for loess formation remains there permanently and is transformed in situ into loess dust not affected by diagenesis is usually further transported by snowmelt or rainwash and is only transformed into loess after it has been redeposited the loess itself however is easily erodible and its minerals are readily reworked and reaccumulated and given the appropriate conditions it readily undergoes diagenesis again we do not always have sufficient evidence to determine whether a given loess body is of primary or secondary origin traditionally the minerals making up primary loess have been regarded as originating in dust accumulated by aeolian processes secondary loess by contrast is different from typical loess in many ways and it is not unusual to find various loess series in which superimposed dust fractions have been transformed into loess by different processes recently we have observed that the origin of various types of loess is governed by differences in litho ecological conditions rather than by the way in which the mineral material from which the loess is derived dust has been primarily accumulated loessification is therefore determined by environmental conditions

using oxygen isotope ratios to quantitatively assess trapping mechanisms during co2 injection into geological reservoirs the pembina case study abstract carbon capture and storage ccs is considered a viable option for reducing co 2 emissions into the atmosphere from point sources such as coal fired power plants monitoring of co 2 storage sites is widely considered necessary for safety reasons and for verification of injected co 2 in the reservoir the latter is crucially dependent on the ability to determine co 2 trapping mechanisms and to assess pore space saturation of co 2 thus far attempts to determine co 2 pore space saturations at co 2 injection sites have had limited success here we present data from the pembina cardium co 2 monitoring project in alberta canada that demonstrate that changes in the oxygen isotope ratios 18 o of reservoir water can be indicative of the extent of pore space saturation with co 2 the 18 o value of injected co 2 at the injection site was 28 6 v smow and 18 o values of reservoir water at eight observation wells varied between 13 5 and 17 1 v smow before co 2 injection after commencement of co 2 injection the 18 o values of reservoir water at three observation wells increased between 1 1 and 3 9 due to the presence of large quantities of injected co 2 and equilibrium isotope exchange between water and co 2 our calculations revealed that reservoir water fully saturated with co 2 would only result in increases of 18 o h2o values of 0 4 hence the observed larger increases in 18 o values of reservoir water indicate free phase co 2 with estimated pore space saturations ranging from 12 corresponding to a 18 o increase of 1 1 to 64 18 o increase 3 9 of the non oil saturated pore space contributions to oxygen in the system from mineral dissolution were calculated to be less than 0 01 of total oxygen and therefore did not alter the 18 o value of the reservoir water significantly hence we conclude that changes in the 18 o values of reservoir water caused by the presence of injected co 2 can be used as a tracer for co 2 plume migration in the subsurface provided that the injected co 2 is isotopically distinct furthermore we submit that the extent of the changes in the 18 o values of the reservoir water provides a quantitative assessment of co 2 stored in dissolved form solubility trapping assuming no density driven convective overturn and as free phase co 2 structural stratigraphic and hydrodynamic trapping thereby elucidating the trapping mechanisms within the reservoir

characterization of macropore structure of malan loess in nw china based on 3d pipe models constructed by using computed tomography technology abstract malan loess is a grayish yellow or brownish yellow clastic highly porous and brittle late quaternary sediment formed by the accumulation of windblown dust the present day pore structure of malan loess is crucial for understanding the loessification process in history loess strengths and mechanical behavior this study employed a modern computed tomography ct device to scan malan loess samples which were obtained from the east part of the loess plateau of china a sophisticated and efficient workflow for processing the ct images and constructing 3d pore models was established by selecting and programming relevant mathematical algorithms in matlab such as the maximum entropy method medial axis method and node recognition algorithm individual pipes within the malan loess were identified and constructed by partitioning and recombining links in the 3d pore model the macropore structure of malan loess was then depicted using quantitative parameters the parameters derived from 2d images of ct scanning included equivalent radius length and aspect ratio of pores porosity and pore distribution entropy whereas those derived from the constructed 3d structure models included porosity coordination number node density pipe radius length length density dip angle and dip direction the analysis of these parameters revealed that malan loess is a strongly anisotropic geomaterial with a dense and complex network of pores and pipes the pores discovered on horizontal images perpendicular to the vertical direction were round and relatively uniform in shape and size and evenly distributed whereas the pores discovered on vertical images varied in shape and size and were distributed in clusters the pores showed good connectivity in vertical direction and formed vertically aligned pipes but displayed weak connectivity in horizontal directions the pipes in vertical direction were thick long and straight compared with those in horizontal directions these results were in good agreement with both numerical simulation and laboratory permeability tests which indicate that malan loess is more permeable in the vertical direction than in the horizontal directions

geochemistry of early carbonate cements in the cardium formation central alberta abstract authigenic minerals and diagenetic textures in the marine conglomerates and sandstones of the cretaceous cardium formation in the northeast pembina area preserve a complex sequence of diagenetic events textural relationships observed in thin section and under the scanning electron microscope were used to determine the relative timing of diagenetic events in these marine sediments paragenetic sequences are similar for the conglomerate sandstone and siderite nodules in the enclosing shales extensive cementation by siderite and calcite occurred early in the diagenetic history before any significant compaction the geometry and distribution of lithofacies in the cardium may have influenced the diagenesis and internal stratigraphy of the conglomerate overlying shale matrix conglomerate may have trapped upward migrating fluids increased in buoyancy by dissolved co2 produced by decay of organic matter in the enclosing shales these fluids could have infiltrated the upper portion of open matrix conglomerate causing cementation petrographic evidence shows alternating precipitation of siderite and pyrite implying fluctuating activities of dissolved carbonate and hydrogen sulfide probably caused by bacterially mediated processes during early diagenesis early sequences of siderite and pyrite precipitation can be related to zones of iron reduction sulfate reduction and possibly decarboxylation during diagenesis carbon and oxygen isotopic data indicate a systematic change in the isotopic compositions of calcite and siderite from 25 to 3013c and 018o for cements early in the paragenetic sequence to 0 to 513c and 15 18o for cements which are interpreted to occur later in the paragenetic sequence this observation is consiste t with the influence of meteoric water on later stages of cement deposition in the cardium a unit considered to be marine and deposited well offshore on a shallow marine shelf the intrusion of meteoric water far offshore may be related to changes in sea level many variables must be accounted for but it is feasible using the estimated regional dip of the cardium at the time of deposition 0 005 for a 1 m drop in sea level to push the freshwater seawater interface seaward on the order of 100 km

oxygen and carbon isotopic composition of marine carbonate concretions an overview abstract compilation of stable isotope data 18o and 13c for marine calcite siderite and dolomite concretions from a large number of studies reveals consistent and unanticipated isotopic trends although many of the dolomites have 18o values that are compatible with precipitation from unaltered seawater at low temperatures most of the calcites and siderites are far too depleted in 12o for such an origin this is surprising as most of the concretions appear to have formed very early in the diagenetic history of the sediments often within several meters of the sediment water interface the most likely cause of the anomalously depleted oxygen values is a decrease in pore water 18o values during early diagenesis a result perhaps of a significant influx of meteoric water into the marine shelf environment or water mineral interaction e g precipitation of 18o enriched minerals the carbon isotopic field for dolomite is quite different from that for calcite and siderite highly positive 13c values are fairly common only for dolomites this indicates that while dolomite concretions often form in sediments where methanogenesis is rapid precipitation of calcite and siderite concretions is restricted to sediments where either n methanogenesis takes place or where methanogenesis is not extensive enough to generate highly positive pore fluid 13c values this variation in carbon isotopic composition among the different minerals suggests that the rates of organic carbon oxidation and sedimentation may control the mineralogy of carbonate concretions high organic carbon oxidation rates favor dolomite and lower oxidation rates favor calcite or siderite

initial rifting and breakup between nova scotia and morocco insight from new magnetic models1 2 1this article is one of a series of papers published in this cjes special issue on the theme of mesozoic cenozoic geology of the scotian basin 2earth sciences sector contribution 20120024 new models of magnetic data are presented that provide insight into the early stages of rifting and breakup between morocco and nova scotia the margins which began forming during the late triassic rifting and middle jurassic separation of the north american and african plates display considerable variability in deeper crustal structure faulting style and basin geometry along their length the central and northeastern portions of the nova scotia margin show characteristics of a magma poor margin with a high degree of crustal thinning and a complex ocean continent transition zone with little direct evidence of volcanism in contrast the margin to the southwest of nova scotia has clearly recognized characteristics of a volcanic style rifted margin including seaward dipping reflector sdr sequences that are interpreted as rift related volcanic flows overlying basement these sdrs are coincident with a strong linear magnetic anomaly the east coast magnetic anomaly ecma which shares many characteristics with the west african coast magnetic anomaly wacma both magnetic anomalies change character and diminish in amplitude northward along the margins the new models show the expected decrease in magnetic source material towards the northeastern end of the margin and suggest that modest amounts of igneous material emplaced at or near the edge of the thinned continental crust will satisfy the observed anomalies in this magma poor section of the margin these new interpretations and supporting evidence indicate volcanism was a factor along much of the margin during early rifting between nova scotia and morocco resume de nouveaux modeles de donnees magnetiques sont presentes ils donnent des apercus des premieres etapes de distension et de rupture entre le maroc et la nouvelle ecosse les bordures ont commence ase former durant la distension

formation of calcareous nodules in loess paleosol sequences reviews of existing models with a proposed new per evapotranspiration model abstract loess is a product of aeolian deposition during quaternary glaciation cycles loess paleosol sequences are rich in calcareous nodules cns in the literature two models are widely cited for the formation of cns namely per descendum and per ascendum however there has been no direct testing or monitoring to support either of these contradictory models this paper reviews a large number of multidisciplinary literature to evaluate the consistency reliability and rationality of these two models three main conclusions are drawn 1 the causative factors variation of ph value along loess paleosol sequence decrease of co2 partial pressure and reduction of solvent water that are used to support the per descendum model do not completely explain the supersaturation of infiltration solution with caco3 thereby making this model questionable 2 the per ascendum model explains the formation of cns along narrow horizons through upward evaporation however it fails to produce sporadic distributions and irregular shapes of nodules on loess slope faces and the frequent appearance of nodules around plant roots in order to reconcile these deficiencies we conducted an extensive field survey in various parts of shanxi province based on this new set of observations it was concluded that the per ascendum model can be extended to explain all occurrences of cns this extended model is called per evapotranspiration

an alternative early opening scenario for the central atlantic ocean abstract the opening of the central atlantic ocean basin that separated north america from northwest africa is well documented and assumed to have started during the late jurassic however the early evolution and the initial breakup history of pangaea are still debated most of the existing models are based on one or multiple ridge jumps at the middle jurassic leaving the oldest crust on the american side between the east coast magnetic anomaly ecma and the blake spur magnetic anomaly bsma according to these hypotheses the bsma represents the limit of the initial basin and the footprint subsequent to the ridge jump consequently the evolution of the northwest african margin is widely different from the northeast american margin however this setting is in contradiction with the existing observations in this paper we propose an alternative scenario for the continental breakup and the mesozoic spreading history of the central atlantic ocean the new model is based on an analysis of geophysical data including new seismic lines an interpretation of the newly compiled magnetic data and satellite derived gravimetry and recently published results which demonstrate that the opening of the central atlantic ocean started already during the late sinemurian 190 ma based on a new identification of the african conjugate to the ecma and on the extent of salt provinces off morocco and nova scotia the identification of an african conjugate magnetic anomaly to bsma the african blake spur magnetic anomaly absma together with the significant change in basement topography are in good agreement with that initial reconstruction the early opening history for the central atlantic ocean is described in four distinct phases during the first 20 myr after the initial breakup 190 170 ma from late sinemurian to early bajocian oceanic accretion was extremely slow 0 8 cm y at the time of blake spur 170 ma early bajocian a drastic change occurred both in the relative plate motion direction from nnw sse to nw se and in the spreading rate an increase to 1 7 cm y after a small increase between chron m25 154 ma kimmeridgian and chron m22 150 ma tithonian the spreading rate slowed down to about 1 3 cm y and remained fairly constant until chron m0 125 ma barremian aptian boundary in addition kinematic reconstructions illustrate a significant spreading asymmetry during the early history of the central atlantic ocean the accretion rates were higher on the american side and led to the formation of more oceanic crust on this plate we infer that this asymmetry could be related to the fact that the thermal anomaly responsible for the significant magmatism of the central atlantic magmatic province camp was preferentially located below the african plate

some comparisons of the structure and evolution of the southern appalachian ouachita orogen and portions of the trans european suture zone region abstract many aspects of the tectonic history of the appalachian ouachita orogen and the trans european suture region are similar in a tectonic sense they are geographically linked because at the end of the paleozoic one could travel from texas to poland following a continuous orogenic belt much of each of these orogenic belts is however buried by younger strata and thus geophysical studies are a key element of efforts to better understand them many more geophysical data are needed to provide a comprehensive picture of the deep structure but in several areas new models of the lithospheric structure are emerging in this paper we review some of these models and introduce two new ones for the southern appalachians in addition we explore the implications of these models for the evolution of these orogenic belts drawing attention to similarities and differences in their structure and evolution the appalachian and caledonian orogens represent temporally parallel development along irregular late proterozoic continental margins but the crustal structure is different because of different accretionary elements and complications in the geometry of the original margins lateral transport via thrusting can confuse efforts to tie geologic structures identified on the surface with deep structures for example in the appalachian ouachita orogen the thrust related transport distance for major features such as the blue ridge piedmont sheet and the benton uplift is at least 200 km on a crustal scale deformation in the appalachian ouachita orogen varies greatly in style and intensity in the ouachita orogen the late proterozoic continental margin seems to be preserved while in the northern appalachians crustal scale deformation is intense along the us appalachians the major crustal structure variation is the thin dense crust of avalonia abutting the relatively thick crust of laurentia along the ouachita orogen several large crustal blocks can be identified that are unlike anything seen along the appalachians

crustal structure interpreted from magnetic anomalies this review discusses publications during the last quadrennium 1987 1990 that used aeromagnetic data marine magnetic data satellite magnetic data and rock magnetic and petrologic data to provide information on the sources of magnetic anomalies the publications reviewed reflect increased integration of rock magnetic property and petrologic studies with magnetic anomaly interpretation studies particularly in deep crustal magnetization exploration for hydrocarbons and inversion of marine magnetic anomalies interpretations of aeromagnetic data featuring image display techniques and using the horizontal gradient method for locating magnetization boundaries became standard

sedimentological petrophysical and seismic characterization of an upper jurassic shoreface dominated shelf margin the boulonnais northern france abstract the integration of high resolution 25 cm vertical spacing and continuous quantitative data on grain size carbonate content porosity density and p wave velocity from outcrop borehole and core over 1000 samples and a real seismic section allows a calibration of seismic sequence stratigraphic principles this approach has been tested in the boulonnais along the english channel coast in northern france on a succession of upper jurassic sharp based shoreface deposits embedded in offshore marine shales statistical analyses of grain size distributions allow a characterization of hydrodynamic conditions and depositional processes affecting low inclination shelf margins and create a framework for identifying locations of preferential and enhanced carbonate cementation which exerts primary control on reservoir and acoustic properties stratigraphic surfaces and systems tracts are quantified for physical properties and results in a quantitative mechanical sequence stratigraphic model the combination of this model with seismic wavelets and a contrast between this synthetic and a real seismic section shows that key bounding sequence stratigraphic surfaces are in fact discontinuous reflectors because stratal relationships of sharp based shoreface deposits are subtle physical contrasts are generally stratabound and the deposits are typically not very thick the geological recordings of sea level falls and potential reservoirs are easily overlooked or go unseen in seismic sequence stratigraphic analyses by defining key diagnostic seismic criteria for sharp based shoreface deposits this study contributes to the understanding of processes controlling sedimentary fills of epicontinental basins

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

widespread nearshore and shallow marine deposition within the lower jurassic precipice sandstone and evergreen formation in the surat basin australia abstract in the surat basin of eastern australia the lower jurassic precipice sandstone and evergreen formation are a highly prospective reservoir seal pair for notional future carbon capture and storage however the succession remains poorly constrained from a paleodepositional standpoint and this has impacted the capacity to construct predictive reservoir models here we integrate sedimentological ichnological and palynological data from ten cores located across a large region of the northern and central basin to produce conceptual depositional models for the interval our analysis shows that the lower jurassic series consists of fifteen recurring sedimentary facies that are arranged into six facies associations braidplain lower delta plain subaqueous delta delta influenced shoreface tidally influenced shoreline and restricted marine shoals the facies associations occur in the context of large scale fluvio deltaic systems that occupied the basin these results are supported by ichnological indications of marine and brackish water a coastal suite of palynomorphs and marine palynomorphs including rare dinocysts acritarchs and copepod fragments the very low abundance of marine palynomorphs are confined to the upper portion of the evergreen formation and in combination with the sedimentology and ichnology suggest that marine influence increased through time as the basin was filled the elucidation of marine influenced deposition contravenes all but the most recent facies interpretations of the precipice sandstone and evergreen formation and suggests that the paleogeography of the mesozoic of eastern australia needs to be reconsidered importantly the nearshore and shallow marine depositional affinity has important implications for the size orientation and distribution of geobodies when building geologically realistic static reservoir models for dynamic flow simulation

crustal structure across the brunswick magnetic anomaly offshore georgia from coincident ocean bottom and multi channel seismic data we present results from a coincident wide angle and deep penetrating multichannel reflection seismic experiment conducted on the continental shelf of the southeast georgia embayment over 5000 air gun shots were recorded by a 6 km long towed streamer and six ocean bottom seismic instruments along a 250 km north south profile crossing the brunswick magnetic anomaly bma these data indicate a transition in seismic properties across the bma including higher seismic velocities south of the bma particularly in the upper crust 7 15 km depth and a transition from reflective to transparent crust from north to south we interpret this transition to indicate an increased mafic content in crust south of the bma magnetic modeling based on our seismic results indicates that the bma may be explained as an edge effect anomaly of the more mafic upper crust we suggest that the increased mafic content toward the south is due to rift related mafic intrusion an alternative interpretation is discussed in which the seismic transition across the bma is related to an alleghanian suture athough there is no direct indication in the data for the association of the bma and an alleghanian suture along our profile such an association cannot be ruled out the southern portion of the profile crosses the margin hinge zone where we observe seaward dipping reflectors similar to those observed along the northern blake plateau basin and carolina trough hinge zones which have been interpreted as volcanic sequences our seismic data include no evidence for very high velocity 7 0 km s mid crust to lower crust inboard of the hinge zone such as is observed in the carolina trough outboard of the hinge zone this indicates that the volume of emplaced mafic material changes dramatically across the hinge zone and suggests that rift related magmatic processes along the margin were highly focused

the transition from a volcanic to a nonvolcanic rifted margin off eastern canada three multichannel seismic reflection profiles were collected on the rifted continental margin southeast of nova scotia eastern canada the profiles cross the east coast magnetic anomaly ecma which parallels much of the margin of eastern north america south of the grand banks and which is usually associated with the transition from continental to oceanic crust studies to the south of the work reported here suggest that the ecma may be related to the emplacement of large thicknesses of late rift stage or early drift stage igneous material which is characterized by seaward dipping reflections in basement and a high velocity lower crustal layer the seismic data show that seaward dipping reflections sdr continue northward into the study area and support the correlation between the sdr unit and the presence of a well developed ecma magnetic modeling confirms this association although it does not rule out an additional contribution to the magnetic anomaly from an edge effect or suture just north of the study area the ecma diminishes and is no longer well developed the sdr unit also terminates and it is not observed over most of the nova scotian margin if our understanding of the origin of these features is correct then their disappearance marks a transition from a volcanic margin in the south to a nonvolcanic margin in the north the association of the transition with significant changes in the prerift fabric of the adjacent continental crust in the trend of synrift extensional structures and in the width of the zone of thinned continental crust below the margins must be clues to the deeper processes controlling the amount of volcanism produced we suggest that these clues are consistent with small scale convection as a mechanism for delivering large melt volumes to crustal depths during rifting

loess rock sediment or soil what is missing for its definition abstract loess is commonly defined as an accumulation of windblown silt however the complex mechanisms that are responsible for most of the structural characteristics of loess require a more precise explanation the common definition of loess ignores a set of processes that start during and after the subaerial deposition of silt the term loessification has been used by a number of authors to refer to the quasi pedogenic quasi diagenetic processes that result in the typical aggregation of loess however these mechanisms are rarely described in detail depending on the researcher s background loess is classified as sediment soil or rock this review gives an overview on loess definitions through time and evaluates the main concepts related to loess formation several gaps of knowledge are identified that require a number of specific studies related to different aspects of loess formation in various environments we propose to 1 differentiate primary loessification which initializes the formation of loess structure from secondary loessification which takes place subsequently and 2 define loess like sediments as deposits that experienced loessification but were not transported aeolian mainly aeolian processes and loessification are of equal importance to define loess

the nature origin and accumulation of loess abstract loess can be defined simply as a terrestrial clastic sediment composed predominantly of silt size particles which is formed essentially by the accumulation of wind blown dust there are three fundamental requirements for its formation 1 a sustained source of dust 2 adequate wind energy to transport the dust and 3 a suitable accumulation site most loess has been modified to some degree by local reworking bioturbation and syn depositional weathering but a process of loessification is not necessary for a dust deposit to qualify as loess the terms primary loess and secondary loess have previously been used to described wind deposited loess and redeposited loess respectively but it is more accurate to describe material reworked and redeposited by non aeolian processes as loess derived colluvium or loess derived alluvium during the quaternary loess formed in three main situations 1 mid continental shield areas beyond the limits of ice sheets periglacial loess 2 on the margins of high mountain ranges perimontane loess and 3 on the semi arid margins of some lowland deserts peridesert loess peridesert loess is relatively poorly developed partly due to the limited efficiency of silt generating mechanisms in low land deserts compared with glaciation but also important is the tendency in arid areas for dust to be dispersed over a wide area and to be rapidly reworked after deposition particularly in areas which experience frequent climatic changes the remarkably thick loess of china and central asia results from an unusual combination of conditions which has persisted for much of the last 2 3 million years namely rapid uplift of the tibetan plateau and surrounding mountain ranges high rates of sediment production and supply to adjacent basins a strong northwesterly and westerly wind regime and the existence of effective dust traps downwind of the source regions

wetting driven formation of present day loess structure abstract present day loess especially malan loess formed in later quaternary has a characteristic structure composed of vertically aligned strong units and weak segments hypotheses describing how this structure forms inside original loess deposits commonly relate it to wetting drying process we tested this causal relationship by conducting unique experiments on synthetic samples of initial loess deposits fabricated by free fall of loess particles these samples were subjected to a wetting drying cycle and their structural evolutions were documented by close up photography and ct scanning analysis of these records revealed three key stages of structural evolution initiation evenly distributed cracks appear due to wetting inhomogeneitization some cracks grow forming large polygons and development polygon forming cracks grow further cracks within polygons narrow down or heal up these experiments successfully reproduced the characteristic structure of present day loess and led to a discovery that it is the wetting of initial loess that initiates and drives the structural evolution while drying preserves and expands resulting features

imaging crustal structure beneath the southern appalachians with wavefield migration to constrain crustal structures in the southern appalachians and the suture zone with the gondwanan affinity suwannee terrane we applied the 2d grt wavefield migration method to the scattered incident p wavefield recorded by the earthscope southeastern suture of the appalachian margin experiment and adjacent transportable array stations we resolve the root of thickened crust beneath the high topography of the blue ridge mountains and estimate its density contrast with the mantle to be only 104 20 kg m3 a weak velocity contrast across the crustal root moho is observed and may be related to an ongoing crustal delamination event possibly contributing to local tectonic rejuvenation beneath the suwannee terrane we confirm prior observations of a gently south southeastward dipping crustal suture indicating the terminal collision of laurentia and gondwana involved several hundred kilometers of overthrusting

refining the formation and early evolution of the eastern north american margin new insights from multiscale magnetic anomaly analyses to investigate the oceanic lithosphere formation and early seafloor spreading history of the north atlantic ocean we examine multiscale magnetic anomaly data from the jurassic early cretaceous age eastern north american margin enam between 31 40 n we integrate newly acquired sea surface magnetic anomaly and seismic reflection data with publicly available aeromagnetic and composite magnetic anomaly grids satellite derived gravity anomaly and satellite derived and shipboard bathymetry data we evaluate these data sets to 1 refine magnetic anomaly correlations throughout the enam and assign updated ages and chron numbers to m0 m25 and eight pre m25 anomalies 2 identify five correlatable magnetic anomalies between the east coast magnetic anomaly ecma and blake spur magnetic anomaly bsma which may document the earliest atlantic seafloor spreading or syn rift magmatism 3 suggest pre existing margin structure and rifting segmentation may have influenced the seafloor spreading regimes in the atlantic jurassic quiet zone jqz 4 suggest that if the bsma source is oceanic crust the bsma may be m series magnetic anomaly m42 168 5 ma 5 examine the along and across margin variation in seafloor spreading rates and spreading center orientations from the bsma to m25 suggesting asymmetric crustal accretion accommodated the straightening of the ridge from the bend in the ecma to the more linear m25 and 6 observe anomalously high amplitude magnetic anomalies near the hudson fan which may be related to a short lived propagating rift segment that could have helped accommodate the crustal alignment during the early atlantic opening

structural development during flysch basin collapse the fencemaker allochthon east range nevada abstract during mesozoic deformation in north central nevada a thick succession of triassic siliciclastic and hemipelagic carbonate strata that were deposited in a deep water basin shortened against and overrode penecontemporaneous shallow water carbonates and interbedded siliciclastics at the eastern margin of the basin initial rheologic contrasts developed between lithologic units during shallow pretectonic burial localized displacements within fine grained poorly cemented units and along lithologic boundaries changes in theology during low pressure and low temperature deformation led to more brittle conditions possibly due to fluid expulsion and subsequent displacements truncated bedding structures in triassic strata suggest that the w facing slope of the shelf to basin transition localized thrusting collapse of the basinal strata against strata of the basin margin and formation of a thrust at the shelf to basin transition demonstrate the ability of stratigraphic declivities and associated rheologic contrasts to buttress deformation and localize thrusting

origin of low δ18o pre compactional ferroan carbonates in the marine sto formation middle jurassic offshore nw norway siderite delta 18 o pdb 9 3 to 5 7 parts per thousand and fe dolomite ankerite delta 18 o pdb 13 7 to 7 7 parts per thousand occur as texturally early diagenetic pre compactional ce

systematic sequence scale controls on carbonate cementation in a siliciclastic sedimentary basin examples from upper cretaceous shallow marine deposits of utah and coloradao usa whilst the relationship between stratigraphic development and carbonate cementation within siliciclastic succession has been documented through a number case studies these studies have been generally restricted to observations upon individual sequences and or limited sub surface data in this paper long term 5 million years large scale 200 km stratigraphic controls on carbonate cementation patterns are documented from the upper cretaceous panther tongue member blackhawk formation and castlegate sandstone exposed in the book cliffs in utah and colorado usa together these comprise eight progradational wedges of sandstones which interfinger with the mancos shale deposited within the western interior seaway foreland basin petrographic analyses of ferroan dolomite cement bodies within these sandstone wedges show that the ferroan dolomite cements are all early relative to burial diagenesis within the host sandstones stable isotope analyses indicates that a significant meteoric component was present in precipitating fluids and this is consistent with the observation that cements are always present down dip of sequence boundaries and or leached whitecaps beneath coals in addition the lateral distribution of cement bodies increases consistently up succession from less than 5 km in extent in the older sequences to 30 km in extent in the youngest sequences these changes in distribution are in response to the increased progradation and increased and more aerially extensive sequence boundary development in younger sequences the implications of these data are that whilst localized spatial patterns of diagenesis and in particular carbonate cementation are predictable and controlled by the nature and presence of individual stratal surfaces systematic diagenetic alteration patterns are also present at the sedimentary basin scale and controlled by the nature of larger scale stratigraphic development and basin evolution this evolution may be driven by eustatic shifts or through tectonic or climatic driven base level shifts these observations allow an improved insight into the basin scale processes that control the macroscopic diagenetic properties of sedimentary successions and sub surface hydrocarbon reservoirs

coeval changes in grain size and sedimentation rate of eolian loess the chinese loess plateau to determine the temporal spatial relationship between strength of the winds transporting the chinese loess and aridity conditions of the loess source area six loess sections located respectively in the northern middle and southern loess plateau were studied grain size distributions were analyzed on loess paleosol samples that are closely spaced in the sections time scales were established by tuning each of the grain size records to theoretical changes in the earth s orbital parameters results obtained show that during the last two glacial cycles particle size of the loess co varies with apparent sedimentation rates suggesting that variations in winter monsoon intensity and the aridity of the loess source area are tightly coupled this proximal relationship is in striking contrast with the observation of distal eolian clays in north pacific sediments we propose that this phenomenon may result from the proximity of the loess plateau to the source area and from the significant control of the winter monsoon on the expansion retreat changes of the deserts in the northern and northwestern china

crustal structure and deep reflector properties wide angle shear and compressional wave studies of the midcrustal surrency bright spot beneath southeastern georgia three component p sh and sv expanding spread profiles esp common midpoint profiles and sparse three dimensional p wave data were collected over an unusually strong midcrustal reflector the surrency bright spot sbs in southeastern georgia shear wave reflections from the sbs at 10 9 s 16 km depth and possibly from the lower crust at 18 3 s 29 km depth were recorded but required substantial source effort stacking and were too weak for reliable reflectivity measurements reflections on the esps delineate a 1 5 km thick atlantic coastal plain section whose seismic properties vp 2 53 km s vs 1 51 km s vp vs 1 67 are consistent with quartz rich sandstones and siltstones sitting atop a 15 km thick upper crust vp 6 38 km s vs 3 25 km s vp vs 1 96 which in turn overlies a 15 km thick lower crust of slower material vp 6 02 km s vs 3 26 km s vp vs 1 84 the velocity inversion may result from underthrusting of upper crustal rocks during suturing of florida to north america amplitude versus offset analyses combined with an earlier reflection polarity test and waveform modeling indicate that the sbs originates from a thin 80 120 m high impedance layer most likely a mafic dike or tectonically emplaced ultramafic body

lithospheric and asthenospheric contributions to shear wave splitting observations in the southeastern united states we present observations of both null and non null sks splitting from temporary deployments across the southeastern united states in order to evaluate the relative contributions of lithospheric deformation and asthenospheric flow to regional anisotropy data for this study come from four temporary broadband seismic deployments the appalachian seismic transect ast the test experiment for eastern north america teena the south carolina earth physics project scepp and the florida to edmonton array fled in general we find fast directions aligned roughly parallel to absolute plate motion of the north american plate apm within and west of the southern appalachians whereas to the southeast we find a broad area dominated by complex splitting patterns consisting of well constrained null splitting measurements over a range of backazimuths along with a very small number of resolved non null measurements this change in splitting patterns is consistent with a transition from drag induced asthenospheric flow beneath the older sections of the north american continent to vertical or incoherent mantle flow likely in combination with complex lithospheric anisotropy beneath the younger accreted terranes to the southeast in addition to these general patterns we find a number of non null splitting measurements that are not aligned with apm but are instead aligned with prominent magnetic anomalies that may correspond to ancient continental suture zones or faults this would suggest that in these areas a strongly anisotropic but localized lithospheric fabric dominates over any ambient asthenospheric anisotropic signature in areas with generally strong apm parallel splitting this would imply a thick sheared mantle lithosphere whose deformation induced anisotropy is strong enough to overprint the anisotropy induced by apm and is aligned with the shallower crustal structures responsible for generating the observed magnetic anomalies in the southeastern areas dominated by null splitting measurements there may be no strong signature from asthenospheric anisotropy to override but a substantial lithospheric thickness is still required to generate the magnitude of the observed sks splitting 1 s more data are required to verify these results but future datasets including data from usarray may be able to exploit the correlations between null and non null sks splitting measurements and magnetic lineaments to better constrain the provenance of the regional anisotropic signature

loess and fragipans development of polygonal crack network structures in fragipan horizons in loess ground one of the defining features of the fragipan horizon is the presence of a blocky polygonal network structure in loess soils this network structure can be explained by contraction forces due to drying operating after hydrocollapse due to loading and wetting as in the bryant hypothesis for fragipan formation three stages are identified in the formation of a fragipan horizon in loess ground there is a deposition phase in which the aeolian deposition of loess material produces certain ground properties a collapse stage allows the soil structure to deform under the influence of loading and wetting this collapsed material develops internal tensile forces as drying contraction proceeds and these cause the development of a characteristic crack network the crack network can be modelled using a very simple monte carlo approach and the two dimensional structure produced gives a good representation of fragipan cracking the collapse contraction process for fragipan formation offers explanations for the strength and hardness of fragipans the constant depth to fragipan horizons the slaking in water predominance of short range contact bonds and the mineralogical similarities throughout the system the fragipan horizon impedes drainage and this becomes increasingly important as land use becomes more widespread

integration of cocorp deep reflection and magnetic anomaly analysis in the southeastern united states implications for origin of the brunswick and east coast magnetic anomalies integration of magnetic anomaly analysis with cocorp deep reflection data from the southeastern united states provides three new constraints on the interpretation of the brunswick and east coast magnetic anomalies as well as on the reflection data these are as follows 1 the source of the brunswick anomaly lies within the deep crust this anomaly is not caused by a mesozoic rift basin as proposed by some workers 2 a simple seaward dipping high susceptibility slab model can explain both the brunswick and east coast magnetic anomalies the along strike change in character of the two anomalies results largely from a change in azimuth of the source body 3 beneath the southeastern united states this source body dips south lies immediately on the south flank of the prominent southward dipping reflective zone revealed on cocorp surveys and was previously associated with the alleghanian suture between north america and africa these results imply that a dipping highly magnetized zone in the upper plate of the alleghanian suture is responsible for both the brunswick and east coast magnetic anomalies the high susceptibility material responsible for these anomalies might be mafic lower continental or oceanic crust thrust upward during alleghanian continental collision or mafic igneous material intruded into the upper plate of the suture zone during subsequent mesozoic rifting or both the latter hypothesis implies that the alleghanian suture acted as a zone of weakness a repository which was reactivated to control the site of ultimate atlantic rifting and possibly initial sea floor spreading

stable isotopic evidence for mixing between metamorphic fluids and surface derived waters during recent uplift of the southern alps new zealand recent studies have shown that the southern alps of new zealand have active hydrothermal systems driven by tectonic uplift these studies have concentrated on the rapidly exhumed rocks immediately adjacent to the alpine fault we present new stable isotopic evidence that shows that fluid flow and fluid mixing processes thought to be restricted to rocks near the alpine fault also occurred in the low uplift rate region of the southern alps orogen during the kaikouran orogeny 10 ma to present low 18ocalcite values of post metamorphic veins in the eastern southern alps indicate that meteoric waters have penetrated to hot midcrustal levels 350 300 c 5 km depth and mixed with metamorphic fluids in areas far removed from the alpine fault in addition the isotopic values of calcites precipitated in active faults in the mackenzie basin define a trend of increasing 18o decreasing 13c and decreasing temperature and form an isotopic mixing line between fault and vein calcites crosscutting metamorphic rocks and authigenic calcites precipitated in mackenzie basin sediments the isotope data indicate a second phase of mixing between a modified metamorphic fluid and a surficial diagenetic fluid at shallow depths we suggest that mixed metamorphic meteoric hydrothermal systems have developed in the east side of the southern alps during uplift and that active faults have played a critical role in transporting metamorphic fluids outward from the mountain front and into the adjacent intermontane basins

formation and development of the structure of collapsing loess in china abstract gao g 1988 formation and development of the structure of collapsing loess in china eng geol 25 235 245 it is well known that in china the regional variations in microstructure of loess are in phase with the overall tendency for collapsibility gradually decreasing from the northwest to the southeast these differences are consistent with variations in climatic conditions this indicates that the structure of collapsing loess is closely related to the climatic condition and geological environment based on the microstructure analyses it can be assumed that the formation and development of the structure of collapsing loess in china has undergone five stages the loessization stage the formation stage the development stage the degeneration stage and the clayization stage the five stages of loess regularly exist in different regions of the loess plateau this also shows that under the modern climatic condition the collapsing loess in the northwest of the loess plateau in china is still developing however the feature of collapsing loess is gradually disappearing in the southeast of the loess plateau

climatic interpretation of the luochuan and lingtai loess sections china based on changing iron oxide mineralogy and magnetic susceptibility abstract hematite and goethite estimated from diffuse reflectance spectra and magnetic susceptibility ms have been determined from both the luochuan and lingtai loess sections of the loess plateau of central china changing concentrations of these iron oxides and ms were used to determine paleoclimatic change based on a model that relates pedogenic production of hematite and magnetite to precipitation we propose two precipitation phases on the loess plateau during the last 2 6 ma from the wucheng red clay boundary up to paleosol s5 mis 13 precipitation on the loess plateau was moderate probably not frequently exceeding 450 mm yr and highly seasonal from s5 to the present precipitation during interglacial times has gradually increased to the present levels of 600 650 mm yr for the localities examined throughout the pleistocene spanning both glacial and interglacial conditions hematite to goethite ratios display a consistent decrease this decrease suggests that precipitation on the loess plateau has been getting less seasonal and or has been occurring during times when temperature was lower in addition to these long term changes it also appears that iron oxide records from the loess plateau exhibit 400 500 ka cycles from the wucheng red clay boundary to s5 these 400 500 ka cycles may be analogous to similar length cycles recorded in 13 c marine records in the south china sea and may be linked through weathering and silica production as a result of the asian monsoon

low latitude sea surface temperatures for the mid cretaceous and the evolution of planktic foraminifera isotopic data from exceptionally well preserved foraminifera show that the subtropical north atlantic had sea surface temperatures ssts between 30 and 31 c during the late albian and early cenomanian these temperatures were warmer than average modern temperatures in the tropical western atlantic and support previous inferences based on diagenetically suspect material of albian tropical ssts of 30 c this finding is consistent with basic theories of oceanic thermal response in the tropics to the high atmospheric carbon dioxide levels during the middle cretaceous greenhouse however our data do not support the concept of a super tethyan region of hypersaline waters in the western north atlantic during late albian and early cenomanian time because seawater salinities substantially higher than modern ones would require significantly higher ssts than those reported here isotopic analysis of mid cretaceous planktic foraminifera shows that planomalina ticinella and biticinella grew in surface waters and provide isotopic proxies for sst whereas deeper growing species include hedbergella rotalipora and praeglobotruncana none of these taxa display isotopic indications of photosymbiosis suggesting that this ecology did not evolve in planktic foraminifera before the late cretaceous contrary to previous hypotheses none of these species increased their depth habitat during their life cycles unlike many extant planktic foraminifera the rotaliporids may have been preferentially susceptible to extinction during oceanic anoxic events because they lived within or below the thermocline for most of their life cycles

breakup of pangaea and plate kinematics of the central atlantic and atlas regions summary a new central pangaea fit type a is proposed for the late ladinian 230 ma together with a plate motions model for the subsequent phases of rifting continental breakup and initial spreading in the central atlantic this model is based on 1 a reinterpretation of the process of formation of the east coast magnetic anomaly along the eastern margin of north america and the corresponding magnetic anomalies at the conjugate margins of northwest africa and the moroccan meseta 2 an analysis of major rifting events in the central atlantic atlas and central mediterranean and 3 a crustal balancing of the stretched margins of north america moroccan meseta and northwest africa the process of fragmentation of central pangaea can be described by three major phases spanning the time interval from the late ladinian 230 ma to the tithonian 147 7 ma during the first phase from the late ladinian 230 ma to the latest rhaetian 200 ma rifting proceeded along the eastern margin of north america the northwest african margin and the high saharan and tunisian atlas determining the formation of a separate moroccan microplate at the interface between gondwana and laurasia during the second phase from the latest rhaetian 200 ma to the late pliensbachian 185 ma oceanic crust started forming between the east coast and blake spur magnetic anomalies whereas the morrocan meseta simply continued to rift away from north america during this time interval the atlas rift reached its maximum extent finally the third phase encompassing the time interval from the late pliensbachian 185 ma to chron m21 147 7 ma was triggered by the northward jump of the main plate boundary connecting the central atlantic with the tethys area therefore as soon as rifting in the atlas zone ceased plate motion started along complex fault systems between morocco and iberia whereas a rift drift transition occurred in the northern segment of the central atlantic between morocco and the conjugate margin of nova scotia the inversion of the atlas rift and the subsequent formation of the atlas mountain belt occurred during the oligocene early miocene time interval in the central atlantic this event was associated with higher spreading rates of the ridge segments north of the atlantis fz an estimate of 170 km of dextral offset of morocco relative to northwest africa in the central atlantic is required by an analysis of marine magnetic anomalies five plate tectonic reconstructions and a computer animation are proposed to illustrate the late triassic and jurassic process of breakup of pangaea in the central atlantic and atlas regions

crustal structure beneath the florida to edmonton broadband seismometer array 1 ps receiver functions were stacked to determine crustal thickness and vp vs ratios for the 28 stations of the florida to edmonton broadband seismometer array and 6 adjacent permanent stations roots of thickened crust exist beneath the paleozoic appalachian orogen and the proterozoic trans hudson orogen the ratios of surface elevation to root thickness are lower than observed in young active orogens as are average crustal root buoyancies determined from gravity data these results are consistent with global trends and may be explained by increased crustal root density due to metamorphism crustal thickening is also observed beneath the mid continent rift in iowa consistent with compressional inversion of the rift during the grenville orogeny

reprocessing and enhanced interpretation of the initial cocorp southern appalachians traverse abstract reprocessing of the 1978 1980 cocorp southern appalachian seismic reflection data has produced improved images of structures related to the emplacement of the blue ridge inner piedmont allochthon the results enhance and extend the interpretation presented previously that the blue ridge and inner piedmont are allochthonous above a shallow and shallow dipping detachment that can be followed from outcrop at the blue ridge valley and ridge transition to at least beneath the carolina terrane the continuity of reflections in the new images supports the interpretation that the southern appalachian detachment is not rooted on the east side of the inner piedmont but rather projects as a low angle detachment or zone of decoupling to beneath the coastal plain an implication of this geometry is that terranes such as the carolina terrane between autochthonous north america and the alleghanian suture beneath the coastal plain are detached thin flakes

undrained shear behavior of loess saturated with different concentrations of sodium chloride solution abstract a series of ring shear tests was conducted on saturated loess to investigate the effects of nacl concentration in pore water and desalinization on the shear behavior under undrained conditions the loess samples were taken from the ground surface of a frequently active landslide in china were saturated by de aired distilled water with different concentrations of nacl and then were sheared undrained after that the samples were retrieved remolded re set into the shear box and re saturated by passing through de aired distilled water such that the samples were desalinized and then were sheared undrained again through comparing the undrained shear behavior the effects of nacl in the pore water and desalinization on the undrained shear behavior of loess were examined the results showed that the variation of nacl concentration in pore water can strongly affect the shear behavior of saturated loess both the peak shear strength and steady state strength increased with increase of nacl concentration until a certain value after which they decreased with further increase of nacl concentration meanwhile the peak shear strength and steady state strength of the desalinized samples recovered to those of the original sample hence the effects of salinization are reversible these findings may be of practical importance to better understand the repeated occurrence of some irrigation induced loess landslides in china

cracking and water seepage of xiashu loess used as landfill cover under wetting drying cycles this paper intends to investigate the anti seepage characteristics of xiashu loess used as landfill cover under wetting drying cycles a series of tests were performed to systematically evaluate the surface cracks pores the hydraulic conductivity and water seepage of the soil under wetting drying cycles the cracking tests indicated that the cracking intensity factor cif increased with the number of wetting drying cycles after three wetting drying cycles the cif gradually reached a stable value the influence of the compactness of the compacted clay was significant to cracking after four drying wetting cycles the cif of compacted clay with relatively low compactness 1 65 g cm3 w 5 25 was 0 15 and the cif of compacted clay with higher compactness 1 65 g cm3 w 3 21 was only 0 06 with the increase in the number of wetting drying cycles the accumulated pore volume of the soil particles increased and the maximum pore size was nearly unchanged after four drying wetting cycles the proportion and average diameter of macro pore in the soil particles were both increased however the proportion and average diameter of the meso pore slightly decreased furthermore the proportion and average diameter of the micro pores were unchanged after four wetting drying cycles the hydraulic conductivity of the compacted soil was at 8 3 10 7 1 5 10 5cm s which was increased about three orders of magnitude compared with the compacted soil without cracks the hydraulic conductivity of compacted soil increased linearly as cracks increased the hydraulic conductivity of compacted soil also gradually increased to a stable value as the pore volume increased after 1 month of wetting drying cycles the cracks of soil with low compactness had extended to a depth of 15 cm below the soil surface while the cracks of soil with higher compactness only reached a depth of 5 cm

crustal structure in the southern appalachians a comparison of results obtained from broadband data and three component wide angle p and s reflection data abstract data for two broadband stations and three component recordings of wide angle reflections are used to constrain the velocity structure and depth to moho beneath a portion of the southern appalachians broadband data from the u s national seismic network usnsn station goga carolina terrane inner piedmont boundary constrained by average crustal v p derived from wide angle data suggest that crustal thickness for individual back azimuths ranges from 41 to 43 km this is consistent with wide angle estimates of 38 40 km for the carolina terrane and 40 45 km for the inner piedmont average crustal v p v s is 1 72 1 76 compared with 1 74 1 76 derived from sms pmp travel time ratios for the carolina terrane the results for station mync blue ridge mountains again constrained by wide angle data suggest a crustal thickness of 50 52 km for individual back azimuths average v p v s is 1 75 1 77 the range in average v p v s derived from wide angle data is broader 1 74 1 80 indicating variations in average composition across the blue ridge mountains as suggested by gravity data large amplitude direct p and ps arrivals on transverse component receiver functions also indicate significant lateral variations in structure beneath mync comparison of migrated images underscores the differences in resolution for single station broadband data and wide angle data recorded with short aperture arrays ps waveforms for mync show a double peak the earlier peak migrates to depths of 48 5 51 km in agreement with wide angle events interpreted as moho for the later peak migrated depths range from 60 to 65 km compared with 59 68 km for peaks in wide angle reflectivity observed for the blue ridge mountains the two methods yield consistent estimates for crustal thickness that lend support to models that predict a significant crustal root beneath the high elevations of the blue ridge

highly depleted oceanic lithosphere in the rheic ocean implications for paleozoic plate reconstructions the rheic ocean formed at ca 500 ma when several peri gondwanan terranes e g avalonia and carolinia drifted from the northern margin of gondwana and were consumed during the late carboniferous collision between laurussia and gondwana a key event in the formation of pangea several mafic complexes ranging in age from ca 400 330 ma preserve many of the lithotectonic and or chemical characteristics of ophiolites they are characterized by anomalously high end values that are typically either between or above the widely accepted model depleted mantle curves these data indicate derivation from a highly depleted hd mantle and imply that i the mantle source of these complexes displays time integrated depletion in nd relative to sm and ii depletion is the result of an earlier melting event in the mantle from which basalt was extracted the extent of mantle depletion indicates that this melting event occurred in the neoproterozoic possibly up to 500 million years before the rheic ocean formed if so the mantle lithosphere that gave rise to the rheic ocean mafic complexes must have been captured from an adjacent older oceanic tract the transfer of this captured lithosphere to the upper plate enabled it to become preferentially preserved possible mesozoic cenozoic analogues include the capture of the caribbean plate or the scotia plate from the pacific to the atlantic oceanic realm our model implies that virtually all of the oceanic lithosphere generated during the opening phase of the rheic ocean was consumed by subduction during laurentia gondwana convergence

quantifying paleoprecipitation of the luochuan and sanmenxia loess on the chinese loess plateau abstract paleoprecipitation is one of the most important paleoclimatic factors for monsoon studies magnetic properties of the chinese loess paleosol sequences have been demonstrated to be controlled strongly by the mean annual precipitation map in this study temporal variations of maps were systematically investigated for the luochuan lc and sanmenxia smx sequences from the loess unit l1 10 ka to the paleosol unit s5 620 kyr by applying the new climofunction map 8 06 fd hirm 254 where fd and hirm are frequency dependent magnetic susceptibility and hard isothermal remanent magnetization respectively for both the lc and smx sequences our climofunction indicates paleosols have higher maps than loess units and the maximum map value occurs in s5 indicating that the east asian summer monsoon was the strongest during the formation of s5 in agreement with previous studies overall maps of smx are higher than those of lc as smx is in the southeast of lc and receives more precipitation the positive correlation between the reconstructed maps and fd and arm suggests that precipitation is a key factor controlling degree of pedogenesis maps of paleosols from smx display a negative correlation with synchronous insolation maxima and a positive correlation with corresponding insolation minima however at lc maps have a less significant relationship with insolation this may be because the summer monsoon signals are relatively weak in lc and covered by local climate and environmental alterations in interglacials the summer monsoon associated with precipitation is controlled by the corresponding insolation maxima and minima in addition the map gradient between lc and smx is also modulated by the insolation amplitude

estimating crustal thickness using sspmp in regions covered by low velocity sediments imaging the moho beneath the southeastern suture of the appalachian margin experiment sesame array se atlantic coastal plain

geochemistry of early siderite cements from the eocene succession of whitecliff bay hampshire basin u k abstract siderite cements in the eocene of whitecliff bay hampshire basin u k occur in a range of depositional facies and morphological associations most siderites are associated with in situ glaucony or occur beneath conspicuous lithological breaks these associations illustrate the importance of slow deposition for early diagenetic carbonate precipitation siderite in the eocene sediments of whitecliff bay is impure and in many cases comprises zoned crystals with an overall decrease in substitution of ca mg and mn for fe from crystal core to rim trends in ca mg and mn from early to later siderite vary between samples with no evident environmental control siderite concretions without calcite cement occur only beneath lithological breaks where meteoric water may have been introduced precipitation temperatures have been calculated from siderite 18o data and are based on the assumption that precipitation occurred from seawater these temperatures are reasonable for typical microbial siderite precipitated in marine sediment but are slightly low for marine siderite where there may have been meteoric overprinting if the precipitation temperature for siderite cements is assumed from 18o data for time equivalent biogenic carbonate rather than the composition of the precipitating fluid the calculated isotopic composition of the precipitating fluid is generally compatible with the inferred marginal marine depositional environments