k bentonites volcanic ash preservation and implications for early to middle devonian volcanism in the acadian orogen eastern north america lower to middle devonian marine strata in the appalachian foreland basin feature up to 80 or more thin k bentonites that represent ancient volcanic ashes the time vs space distribution of k bentonites through the lochkovian to eifelian stages representing 30 m y shows a distinct pattern of clustered multiple beds several scattered beds and thick intervals with no k bentonites four clusters of 7 to 15 individual closely spaced layers occur in the middle lochkovian bald hill k bentonites kalkberg new scotland formations late pragian or early emsian sprout brook k bentonites esopus formation and early eifelian two clusters the tioga middle coarse zone and tioga a g k bentonites onondaga formation detailed examination of these devonian k bentonites shows that in many cases they do not represent a single eruption multilayered beds fossil layers within beds authigenic minerals e g glauconite and phosphate nodules subjacent hardgrounds and an irregular distribution of beds through space and time raise questions about the depositional history and preservation potential of volcanic ash in marine environments and the degree to which the beds represent a primary record of volcanism these and other lines of evidence indicate that postdepositional physical biological and geochemical processes e g sedimentation rate event and background physical processes burrowing have modified the primary record of these water laid ash fall events these factors may lead to preservation of primary ash deposits or to their resedimentation and or partial to complete mixing with background sediments the preservation potential and resulting distribution of the devonian k bentonites can be analyzed across a spectrum of preservational magnafacies in this paper i present a model of ash preservation the model incorporates environmentally related physical biological and chemical processes active in epicontinental seas and marine foreland basins conclusions based on the model indicate that the middle lochkovian early emsian and early eifelian were times of peak volcanic activity in eastern north america related to times of increased tectonism in the acadian orogen

silt aggregates in laminated marine sediment produced by agglutinated foraminifera fabric studies of miocene to holocene hemipelagic and pelagic marine sediments by means of backscattered electron imagery reveal the common presence of exotic aggregates of well to poorly sorted silt size particles new observations of silt aggregates associated with readily identifiable multi chambered agglutinated foraminifera when combined with recent experimental insights into the chamber building activities of these organisms provide a new model for the formation of these aggregates some silt aggregates of well sorted grains appear to represent the collapsed or compacted tests of agglutinated foraminifera aggregates of poorly sorted silt size material probably represent the discarded detritic covers that agglutinated foraminifera erect around themselves while constructing a new chamber these remnant tests and detritic covers are likely to be preserved in settings of low bottom water oxygenation where bioturbation is limited or absent the long stratigraphic range cambrian to recent and diverse habitats hyposaline lagoons to abyssal environments occupied by agglutinated foraminifera suggest that they may be an important but hitherto unrecognized contributor to the production of early fabrics in fine grained marine sediments their presence may provide unique evidence on the concentration of dissolved oxygen in the bottom waters

compositional controls on early diagenetic pathways in fine grained sedimentary rocks implications for predicting unconventional reservoir attributes of mudstones diagenesis significantly impacts mudstone lithofacies processes operating to control diagenetic pathways in mudstones are poorly known compared to analogous processes occurring in other sedimentary rocks selected organic carbon rich mudstones from the kimmeridge clay and monterey formations have been investigated to determine how varying starting compositions influence diagenesis the sampled kimmeridge clay formation mudstones are organized into thin homogenous beds composed mainly of siliciclastic detritus with some constituents derived from water column production e g coccoliths s depleted type ii kerogen as much as 52 6 total organic carbon toc and others from diagenesis e g pyrite carbonate and kaolinite the sampled monterey formation mudstones are organized into thin beds that exhibit pelleted wavy lamination and are predominantly composed of production derived components including diatoms coccoliths and foraminifera in addition to type iis kerogen as much as 16 5 toc and apatite and silica cements during early burial of the studied kimmeridge clay formation mudstones the availability of detrital fe iii and reactive clay minerals caused carbonate and silicate buffering reactions to operate effectively and the pore waters to be fe ii rich these conditions led to pyrite iron poor carbonates and kaolinite cements precipitating preserved organic carbon being s depleted and sweet hydrocarbons being generated in contrast during the diagenesis of the sampled monterey formation mudstones sulfide oxidation coupled with opal dissolution and the reduced availability of both fe iii and reactive siliciclastic detritus meant that the pore waters were poorly buffered and locally acidic these conditions resulted in local carbonate dissolution apatite and silica cements precipitation natural kerogen sulfurization and sour hydrocarbons generation differences in mud composition at deposition significantly influence subsequent diagenesis these differences impact their source rock attributes and mechanical properties

traces in the dark sedimentary processes and facies gradients in the upper shale member of the upper devonian lower mississippian bakken formation williston basin north dakota u s a abstract black organic rich rocks of the upper shale member of the upper devonian lower mississippian bakken formation a world class petroleum source rock in the williston basin of the united states and canada contain a diverse suite of mudstone lithofacies that were deposited in distinct facies belts the succession consists of three discrete facies associations fas these comprise 1 siliceous mudstones 2 quartz and carbonate bearing laminated mudstones and 3 macrofossil debris bearing massive mudstones these fas were deposited in three facies belts that reflect proximal to distal relationships in this mudstone system the macrofossil debris bearing massive mudstones fa 3 occur in the proximal facies belt and contain erosion surfaces some with overlying conodont and phosphate lithoclast lag deposits mudstones with abundant millimeter scale siltstone laminae showing irregular lateral thickness changes and shell debris in the medial facies belt quartz and carbonate bearing laminated mudstones dominate exhibiting sub millimeter thick siltstone layers with variable lateral thicknesses and localized mudstone ripples in the distal siliceous mudstone facies belt radiolarites radiolarian bearing mudstones and quartz and carbonate bearing laminated mudstones dominate overall total organic carbon toc contents range between about 3 and 10 wt with a general proximal to distal decrease in toc content abundant evidence of bioturbation exists in all fas and the lithological and toc variations are paralleled by changes in burrowing style and trace fossil abundance while two horizontal traces and two types of fecal strings are recognized in the proximal facies belt only a single horizontal trace fossil and one type of fecal string characterize mudstones in the distal facies belt radiolarites intercalated into the most distal mudstones are devoid of traces and fecal strings bedload transport processes likely caused by storm induced turbidity currents were active across all facies belts suspended sediment settling from near the ocean surface however most likely played a role in the deposition of some of the mudstones and was probably responsible for deposition of the radiolarites the distribution pattern of high toc sediments in proximal and lower toc deposits in some distal facies is interpreted as a function of higher accumulation rates during radiolarian depositional events leading to a decrease in suspension derived organic carbon in radiolarite laminae the presence of burrows in all fas and nearly all facies in the upper bakken shale member indicates that dysoxic conditions prevailed during its deposition this study shows that in intracratonic high toc mudstone successions such as the upper bakken shale member bed load processes most likely dominated sedimentation and conditions promoted a thriving infaunal benthic community as such deposition of the upper bakken shale member through dynamic processes in an overall dysoxic environment represents an alternative to conventional anoxic depositional models for world class source rocks

early diagenetic pyrite morphology in a mudstone dominated succession the lower jurassic cleveland ironstone formation eastern england abstract diagenetic pyrite in the mudstones and ironstones of the lower jurassic cleveland ironstone formation of eastern england exhibits two distinct morphologies framboidal pyrite commonly associated with organic matter and euhedral pyrite associated with detrital clay pellets these two morphologies are mutually exclusive in occurrence framboidal pyrite is present in clay rich mudstones ooidal ironstones apatite rich units and some silt rich mudstones euhedral pyrite is present in silt rich and sand rich mudstones 34s isotopic analysis of six samples of pyrite suggests that both types of pyrite morphology precipitated during early diagenesis from porewaters with open access to overlying sea water although both probably acted as sites for continued pyrite precipitation during burial it is proposed that framboidal pyrite precipitated from iron dominated porewaters at sites of sulfide supply i e in the region of organic matter as a result of bacterial sulfate reduction where locally sulfide production rates were high enough for porewaters to reach supersaturation with respect to fes euhedral pyrite also precipitated from iron dominated porewaters but sulfide production rates from organic matter was such that fes saturation was not reached at the sites of sulfide production instead euhedral pyrite was precipitated directly from porewater when fes2 saturation was reached the control over pyrite morphology was probably the amount and reactivity of the organic matter within the deposited sediments the sand rich mudstones contained less reactive organic matter due to clastic dilution and deposition in shallower environments with o2 rich bottom waters the ironstones and apatite rich units were deposited under very low sedimentation rates and as a result organic matter contents were very low and iron reduction dominated early diagenesis which inhibited sulfate reduction the presence of minor framboidal pyrite within these units however suggests that sulfide reduction took place in micro environments during early diagenesis

emergence and erosion a model for rill initiation and development soil erosion by overland flow resulting from infiltration excess rainfall generates rill networks on hillslope areas the way in which these networks emerge and develop suggests that hillslope erosion functions as a self organizing dynamic system based upon this argument a model for soil erosion rillgrow 1 has been developed this operates at the spatial scale of raindrops and microtopography in this paper the second generation of the model rillgrow 2 is described and applied to four different soil surfaces results suggest that even at this early stage in its development rillgrow 2 is capable of replicating the success of the earlier model and in some cases of extending them the success of both models suggests that this self organizing view of rill generation may capture some fundamental aspects of the operation of real erosional systems copyright 2000 john wiley sons ltd

clay mineral diagenesis and quartz cementation in mudstones the effects of smectite to illite reaction on rock properties abstract the late cretaceous to early tertiary sequence of the voring and more basins from the norwegian sea has been examined with respect to mineralogy based on 319 cutting samples from five wells a clear relationship between mineralogy and well log data is demonstrated a significant change with respect to velocity porosity and density occurs within the depth interval corresponding to 80 90 c at shallow depths temperatures

cambrian and ordovician metabentonites and their relevance to the origins of associated mudrocks in the northern sector of the lower palaeozoic welsh marginal basin

maximizing information from fine grained sedimentary rocks an inclusive nomenclature for mudstones abstract a simple nomenclature for fine grained sedimentary rocks is proposed based on sedimentary structures present and abundance of all materials forming more than 10 of rock volume irrespective of their origin allochthonous autochthonous and diagenetic a mudstone sedimentary rock that comprises 50 grains 90 clay silt and sand bearing clay rich mudstone 50 to 90 clay 10 to 50 sand 10 to 50 silt and calcite ce ment calcareous nannoplankton and clay bearing mudstone all components 10 to 50 textural information can and should be incorporated into this scheme by prefixing the rock name with descriptions such as bioturbated and thin bedded such a nomenclature is necessary to fully describe variability within fine grained sedimentary rocks that hitherto have typically been described using selected constituents of varying significance such as total organic carbon fissility parting spacing ray signature and lamination using the inclusive nomenclature proposed here will improve opportunities to compare all fine grained sediments aid interpretation of depositional and diagenetic controls of variability and enable these rocks to be more easily integrated into basin scale facies models

quartz types authigenic and detrital in the upper cretaceous eagle ford formation south texas usa abstract lithologic heterogeneity of the eagle ford formation in south texas arises from mixing of extrabasinal grains of siliciclastic composition with intrabasinal grain assemblages composed dominantly of marine carbonate with a lesser component of biosiliceous debris detrital quartz in particular is derived from both extrabasinal and intrabasinal sources posing a challenge for the use of bulk compositional data for mudrock classification extrabasinal detrital quartz supplied along a major axis of siliciclastic influx the woodbine depositional system of east texas is reduced to a minor part of the grain assemblage in south texas petrographic evidence and point count results indicate that around 85 percent of total quartz in these rocks equal to about 12 6 volume percent is authigenic thus significant quantities of authigenic silica are not restricted to siliceous mudrocks but can be found in carbonate rich mudrocks as well formerly opaline skeletons of radiolaria the dominant source of silica for authigenic quartz precipitation are only poorly preserved by replacements including calcite dolomite pyrite and quartz dissolved silica released by dissolution of radiolarians and perhaps also by volcanic glass dissolution is re precipitated in a variety of forms including matrix dispersed microquartz cement fillings within primary intragranular pores and grain replacement of both calcareous and siliceous allochems the mass balance of dissolved silica mobilized from radiolarians and other reactive silicates and the precipitation of authigenic quartz is uncertain because the initial volumes of now dissolved detrital material versus the final volume of authigenic material quartz and other authigenic silicates cannot be determined with accuracy

cherty stringers in the barnett shale are agglutinated foraminifera abstract masses of microcrystalline quartz are abundant within several lithologies in the barnett shale lower mississippian of central texas a typical quartz mass is elongated parallel to bedding and contains a central planar microporous structure that suggests collapse of a formerly hollow spheroidal or tubular body an integrated imaging approach utilizing transmitted polarized light microscopy secondary and back scattered electron imaging cathodoluminescence imaging and x ray mapping reveals these quartz masses to be composed primarily of quartz cemented silt size detrital quartz with a minor admixture of detrital ca plagioclase and dolomite microcrystalline quartz rich masses in the barnett shale are interpreted as agglutinated foraminifera that have been dramatically collapsed during compaction locally a significant portion of the total detrital quartz resides within these biogenic accumulations this study highlights the potential for using these imaging techniques to investigate agglutinated foraminifera in lithified materials that are not amenable to disaggregation and extraction of three dimensional specimens the combined imaging techniques provide an unambiguous view into aspects of skeletal ultrastructure such as particle size and sorting that cannot be readily obtained from either conventional light microscopy or sem these techniques reveal that agglutinated foraminifers are abundant in several lithologies of the barnett shale pointing to the possibility that application in other organic rich shales may reveal these organisms to be more widespread than previously recognized comparative observations in devonian shales of the central eastern united states support this prediction

depositional controls on mudstone lithofacies in a basinal setting implications for the delivery of sedimentary organic matter deep water mudstones from ancient epicontinental settings are significant repositories for organic matter but the detailed temporal variations of and controls on the abundance and type of organic matter om is little studied using micro petrographic and geochemical data from late mississippian mudstones of the widmerpool gulf uk the processes that delivered fine grained sediment to this basin during a glacioeustatic sea level cycle are interpreted from detailed lithofacies analysis seven primary lithofacies are identified from core which show specific and systematic variations in total organic carbon toc content and bulk carbon isotope composition of organic material 13corg during sea level highstands thin bedded carbonate bearing mudstones are the dominant facies deposited contain up to 6 6 toc average 4 6 1 3 and have mean 13corg of 28 5 0 9 during phases of lower sea level thin bedded silt bearing clay rich mudstones with up to 4 1 toc average 2 3 0 8 mean 13corg 28 2 1 0 were interbedded with more organic lean graded silt bearing mudstones and sand bearing silt rich mudstones average toc 1 7 0 6 derived from turbidity currents the latter mean 13corg 26 2 0 7 are closely linked to significant proportions of terrestrial plant material while some rare plant debris and sand bearing mudstones produced from debris flows have more than 7 0 toc and 13corg 26 0 the 13c values of wood fragments ranged from 27 1 to 24 0 and therefore the 13corg is interpreted as a function of the ratio of marine and terrestrial organic matter more negative values in the carbonate bearing and the clay rich mudstones indicate marine planktonic algae whereas the least negative values reflect greater contribution of terrestrial plant material the data suggest that the marine conditions prevailed and supported marine planktonic algae throughout different sea level stages marine om was delivered to the sea floor by continuous hemipelagic settling whereas terrestrial om was delivered by sediment density flows variations in bioproductivity and dilution by siliciclastics influenced the burial rate of marine om organic rich mudstones preserved in these marine basins are potential hydrocarbon source rocks especially as unconventional shale gas reservoirs detailed microtextural and compositional analysis coupled with rigorous geochemical parameters as used in this study are important for the understanding of the source rock potential of basinal mudstones and of fine grained organic rich sediments in general

a compositional classification for grain assemblages in fine grained sediments and sedimentary rocks reply abstract a tripartite compositional classification is proposed for sediments and sedimentary rocks that have grain assemblages with greater than 50 percent of a weight or volume of particles smaller than 62 5 m 4 phi tarl terrigenous argillaceous contains a grain assemblage dominated by more than 75 percent of particles of extrabasinal derivation including grains derived from continental weathering and also volcanogenic debris carl calcareous argillaceous contains less than 75 percent of particles of extrabasinal derivation debris and among its intrabasinal grains contains a preponderance of biogenic carbonate particles including carbonate aggregates sarl siliceous argillaceous contains less than 75 percent of particles of extrabasinal derivation and contains a preponderance of biogenic siliceous particles over carbonate grains these three classes of fine grained particulate sediments and rocks effectively separate materials that have distinct depositional settings and systematic contrasts in organic matter content and minor grain types in the subsurface the grain assemblages that define these classes follow contrasting and predictable diagenetic pathways that have significant implications for the evolution of bulk rock properties and thus assigning a fine grained rock to one of these classes is an important first step for predicting its economic and engineering qualities for purposes of description these three class names can be joined to modifier terms denoting rock texture more precise compositional divisions specific grain types of notable importance and diagenetic features

provenance studies and mudrocks abstract provenance studies based on the mineralogy of sandstones seem to be falling rapidly into disrepute as a result of the increasing emphasis on the effects of diagenesis on the original detrital mineral composition feldspars in sandstones commonly are dissolved altered to clay minerals and albitized by widely occurring diagenetic processes accessory minerals all too frequently are confined in ancient sandstones to ztr mudrocks in contrast have the potential of greater preservation of feldspars and accessory minerals because of their lesser permeabilities few studies have attempted to use the nonclay mineralogy of mudrocks for provenance work and these few have had promising but mixed results mudrocks deserve more attention in provenance studies than they have received

multiproxy approach to understanding the origin of cretaceous pelagic limestone marl alternations dsdp site 391 blake bahama basin limestone marl alternations are usually directly interpreted to reflect cyclic palaeoenvironmental signals however uncertainty in such interpretations stems from the differential diagenesis that most limestone marl alternations have undergone differential diagenesis results in markedly different alterations between limestones and marls and in the loss of comparability of many measurable parameters for an unequivocal interpretation of the origin of rhythmic alternations diagenetically robust parameters or parameters that clearly indicate the degree of diagenetic bias are required the present study uses a multiproxy approach independent biotic sedimentary and geochemical parameters in order to unravel the palaeoenvironmental signal recorded in valanginian early cretaceous limestone marl alternations from the blake bahama basin dsdp site 391 using this approach terrestrial and marine influences can be distinguished changes in nutrient levels estimated and prediagenetic differences in the non carbonate fraction constrained surprisingly no systematic variations in any of these parameters were observed between limestone and marl layers implying that none of these was directly responsible for the formation of the rhythmic alternation hence none of the current models of sedimentary formation of limestone marl rhythmites is applicable here calcareous nannofossils are equally well preserved in limestone and marl layers ruling out their dissolution in marl layers as a source of the calcite cement in the limestone beds sr values of 700 900 p p m indicate that aragonite may have been present in the original pelagic sediment the assumption of fine grained sedimentary aragonite imported from nearby carbonate platforms as the source of the cement would explain a number of otherwise enigmatic features in these rhythmites including the source of the calcite cement observed in the limestones the equally good preservation of calcareous nannofossils in limestones and marls and the higher concentration of calcareous nannofossils in marl layers the study demonstrates that examination of diagenetically inert parameters or parameters in which diagenetic effects can be filtered can yield unexpected results clearly careful analysis of such parameters needs to be undertaken in order to make reliable palaeoenvironmental interpretations from rhythmite successions

importance of volcanic glass alteration to sediment stabilization offshore japan a minor amount ca 1 wt of amorphous silica cement sourced from volcanic glass inhibits consolidation of hemipelagic sediment approaching the nankai trough subduction zone throughout the shikoku basin the distribution and nature of the cement were examined via secondary and backscattered electron imaging the amorphous silica occurs as altered material in contact with volcanic glass coating grains including grain contacts and filling pores based on chemical and petrographic evidence the cement is probably sourced from volcanic glass this is in contrast to a previous suggestion that this silica cement is sourced dominantly from biogenic silica amorphous silica sourced from disseminated volcanic glass shards has the ability to form a thin coating on clay dominated sediment throughout the shikoku basin measured amorphous silica content in hemipelagic sediments suggests that the cementing process is active throughout the shikoku basin at sites separated by 500 km the cementation process may occur in other locations where sediment containing hydrated disseminated volcanic glass is buried sufficiently for heat to facilitate alteration i e central america cascadia and the gulf of alaska

mudstone lithofacies in the kimmeridge clay formation wessex basin southern england implications for the origin and controls of the distribution of mudstones abstract lithofacies analysis has been undertaken on a suite of apparently monotonous organic rich mudstones from the upper jurassic kimmeridge clay formation kcf in the wessex basin southern england using a combination of hand specimen description whole rock geochemistry optical petrography and electron optical petrography five lithofacies have been identified these are clay rich mudstones silt rich mudstones nannoplankton rich mudstones laminated mudstones and concretionary carbonates these facies can be distinguished from one another on the basis of physical sedimentary structures and the relative proportions of allochthonous autochthonous and diagenetic components in each all five of the kcf lithofacies studied were deposited in a marine shelf environment below fair weather wave base but in relatively shallow water close to storm wave base deposition occurred in regions where sand and other coarse clastic allochthonous debris were not being supplied and the bottom waters varied from oxic to anoxic the main factors influencing the development of the five facies were distance from source area primary productivity bottom water anoxia absolute bathymetry clastic dilution and local sediment accumulation rates specifically the clay rich mudstones were deposited in regions farthest down the sediment transport paths commonly in areas by passed by the main sediment supply and beneath surface waters of relatively low primary productivity in contrast the silt rich mudstones were deposited in more proximal areas closer to the source of the sediment in regions where surface productivity and sediment accumulation rates were much higher the nannoplankton rich mudstones accumulated beneath regions where productivity in the surface water layers was fairly high dominated by coccolithophoroid production and the supply of clastic sediment was insufficient to significantly dilute the autochthonous fraction of the sediment unlike the other facies laminated mudstones were deposited in areas where the bottom waters were anoxic surface productivity was enhanced and local sediment accumulation rates were high the concretionary carbonates were precipitated by diagenetic processes in areas where sediment accumulation rates were very low these facies descriptions have enabled a detailed sedimentological study of the kimmeridge clay formation to relate depositional and diagenetic processes directly to the facies present thereby allowing classical facies analyses to be undertaken on apparently homogeneous mudstone successions

occurrence of upper jurassic lower cretaceous black organic rich pelitic sediments as targets for unconventional hydrocarbon exploration in the outer carpathians and adjacent part of the alps our work on the dark pelitic sediments of the polish carpathians and eastern alps shows that these jurassic through lower cretaceous sediments owe their organic content to a combination of global processes such as climatic changes and changes to the carbonate compensation depth ccd and local controls such as basin morphology input of terrestrial organic material and local volcanic activity these sediments developed in basins both floored by oceanic crust as well as within the continental crust north european platform our data show that these anoxic or poorly oxygenated deposits average total organic carbon toc value is around 2 5 wt were laid down in the individual basins at different times from the late jurassic to the barremian and almost continuously up to the early cenomanian a period of 30 to 50 m y and their thickness reached hundreds of meters this long time span made it impossible to distinguish precisely the known aptian and albian oceanic anoxic events oae our data show that sedimentation of dark organic rich deposits was not only controlled by global events such as climatic and ccd changes but also by local ones as a result of differences in their basin morphology and development input of land plant detritus and local volcanic activity as an example of the anoxic succession a detailed description of the black sediments of the proto silesian basin is presented some of these anoxic shales were buried to a depth of a few thousand meters during the folding and overthrusting movements we propose that these shales could represent a unique shale oil and shale gas resource in an intensely structured basin

evidence for high energy events and shallow water deposition in the chattanooga shale devonian central tennessee usa abstract the upper devonian chattanooga shale of central tennessee a classical black shale was deposited in an epicontinental setting west of the appalachian foredeep its finely laminated and highly carbonaceous nature is commonly interpreted to indicate deposition in comparatively deep and stagnant water interbeds of bioturbated greenish gray shale indicating oxygenated bottom waters are commonly ascribed to pycnocline fluctuations however laminated fine sand and silt and hummocky cross stratification hcs at the base of some of these beds indicates interaction of storm waves with the seabed and suggests that greenish gray shale beds are post storm mud drapes other interesting features are inclined undulose erosion surfaces that are conformably overlain by shale beds sets of inclined shale beds that suggest low angle cross bedding and clearly and uniformly developed alignment of clay particles magnetic fabric studies these observations show that the seabed was at times subject to prolonged erosion by bottom currents erosion surfaces agitation and reworking by storm waves hcs and greenish gray shale beds and sediment transport by long lived bottom currents particle alignment the epicontinental sea setting and the presence of hcs and other storm produced features suggest a relatively shallow water depth possibly only a few tens of meters together with abundant evidence of variably strong bottom currents and bioturbation of black and gray shale beds this suggests that abundant planktonic organic matter production rather than stagnant bottom waters are the primary cause for black shale formation

diagenetic origin of quartz silt in mudstones and implications for silica cycling mudstone the most abundant sedimentary rock type1 composed primarily of clay or silt sized particles contains most of the quartz found in sedimentary rocks2 these quartz grains which are chemically and mechanically resistant and therefore preserve their characteristics well have long been considered to be derived from the continental crust1 here we analyse quartz silt from black shales in the eastern usa dating back to the late devonian period about 370 million years ago using backscattered electron and cathodoluminescence imaging and measure oxygen isotopes with an ion probe our results indicate that up to 100 of the quartz silt in our samples does not originate from the continental crust instead it appears to have precipitated early in diagenesis in algal cysts and other pore spaces3 with silica derived from the dissolution of opaline skeletons of planktonic organisms such as radiolaria and diatoms transformation of early diatoms into in situ quartz silt might explain the time gap between the earliest fossil occurrences of diatoms about 120 myr ago4 and molecular evidence for a much earlier appearance between 266 or even 500 myr ago5 6 moreover if many other mudstone successions show similarly high proportions of in situ precipitated rather than detrital quartz silt the sedimentary record in mudstones may have been misinterpreted in the past with consequences for our estimates of palaeoproductivity as well as our perceptions of the dynamics and magnitude of global biogeochemical cycling of silica

open system chemical behavior in deep wilcox group mudstones texas gulf coast usa abstract wilcox group mudstones have been mechanically and geochemically transformed over a temperature range of 20 200 c our research controlled for provenance and age by sampling from five wells parallel to the paleodepositional axis all within the houston delta system across the sampled depths mudstone porosity has been reduced from 25 to

the origin and fate of silt sized carbonate in subsurface miocene oligocene mudstones south texas gulf coast combined petrographic and geochemical data document several kinds of detrital carbonate in subsurface miocene oligocene mudstones of the texas gulf coast in the extreme south of texas in muddy sediments deposited by ancient precursors to the modern rio grande mudstone carbonate is dominated by extrabasinal detritus derived from cretaceous limestones of the edwards plateau further north oligocene mudstones contain carbonate that is mostly syndepositional skeletal material minor amounts of authigenic carbonate are found as replacements of silt sized feldspars in all the mudstones depth related shifts in 18ocalcite and 13ocalcite suggest that small amounts of authigenic carbonate also form by replacement of detrital carbonate recrystallization although this cannot be demonstrated petrographically pervasive carbonate cementation in primary pores in mudstones is not generally observed textural evidence of carbonate loss through pressure dissolution is widespread providing a mechanism for the depth related decline in carbonate content observed in many gulf coast wells carbonate dissolution in mudstones and export of the caco3 implies a massive acid source probably resulting from silicate reactions within the mudstones during burial diagenesis

cathodoluminescent textures and the origin of quartz silt in oligocene mudrocks south texas abstract in subsurface mudrocks of the oligocene frio formation in south texas a substantial proportion of silt size quartz 10 62 m manifests cathodoluminescence cl that varies greatly in intensity within each particle giving rise to textures that differ greatly from grain to grain in contrast most quartz grains of sand size 62 m have cl that is relatively homogeneous within individual grains a substantial percentage of the quartz within silt grains is very weakly luminescent no depth trend in the occurrence of luminescent textures in detrital quartz is observed suggesting that the component of dark possibly low temperature silt size quartz is not related to chemical processes in the present burial setting paleogene rhyolites of west texas are a partial model for the source rocks that supplied sediment to the frio formation phenocrystic quartz in these volcanics has uniform cl similar to that observed in the frio sand fraction whereas complex cl structure seen in the associated groundmass bears many similarities to the cl textures observed in frio silts this observation further supports the idea that cl structure of detrital quartz in the frio is most likely an inherited feature if the quartz with dark cl is of low temperature origin it would help to elucidate the relatively 18o enriched isotopic values reported for mudrock quartz in the frio

from chalcedonic chert to quartz chert diagenesis of chert hosted in a miocene volcanic sedimentary succession central japan abstract chert beds are intercalated in a miocene sedimentary succession of the niu mountains central japan which was deposited in very shallow water the provenance of sediments was a volcanic suite from eruptions during back arc spreading in the sea of japan the cherts accumulated in a small lake underlain by lithic sandstone and lithification was rapid and before deep burial much of the silica appears to be derived from extensive devitrification and weathering of volcanic ash and sediments nearby x ray diffraction analyses and microscopic observations reveal that the cherts consist of various types of chalcedony microcrystalline quartz and megaquartz of very low crystallinity indices transformation of chalcedony to megaquartz was common in the cherts scanning electron microscopic analysis shows that a chainlike alignment of micron size silica spheres forms fibers of chalcedony morphological change and coalescence of the chalcedony fibers result in formation of crystalline quartz with numerous crocks and defects the apparent crystalline quartz in the cherts has inner porous domains and small holes or pipes at the crystal surfaces rotation movement and rearrangement of the spheres seem to be as important as age effects and solubility of silica minerals

texture composition and stratigraphy of volcanic ash beds in lower palaeocene chalk from the north sea central graben area a large number of volcanic ash beds have been identified in danian chalk from the danish north sea sector central graben area the volcanic input was prominent during deposition of the lower part of the danian chalk whereas it was much less during deposition of the upper part of the danian chalk the non carbonate fraction of the ash beds predominantly consists of smectite with some quartz illite and minor amounts of kaolinite chlorite and pyrite and trace amounts of zircon fragments of volcanic glass are common in the ash beds the mineralogical composition of the ash beds varies from bed to bed which makes ash bed correlation a promising high precision correlation tool volcanic provinces that were active in connection with the opening of the north atlantic ocean are possible sources of the volcanic ash the different composition of the ash layers could indicate that they originate from more than one source area

gravity enhanced representation of measured geoid undulations using equivalent sources summary direct global positioning system measurement of geoid undulations on accurately levelled stations usually tens of kilometres apart can be interpolated by taking advantage of denser surveys of free air gravity anomalies covering the same area using either a spherical or a planar earth model a two layer equivalent source is constructed with the deepest masses located under the geoid stations and the shallower ones under the gravity stations in such a way that the effect of the masses fits simultaneously with different precisions the anomalous potential related to the geoid and its vertical gradient or gravity anomaly this poses a linear bayesian problem whose associated system of equations can be solved directly or by iterative procedures the ability of the described method to predict the geoid elevation over the gravity stations is assessed in a synthetic example and in the application to a real case a gravity enhanced geoid is mapped for an area of buenos aires province argentina where local features are put in evidence

discovery of agglutinated benthic foraminifera in devonian black shales and their relevance for the redox state of ancient seas abstract agglutinated foraminifera are benthic organisms that occur in marginal marine to bathyal environments though some taxa can live in oxygen deficient environments they require at least some oxygen in order to persist at the seafloor the discovery that they occur widely in late devonian black shales has a bearing on the boundary conditions required for episodes of extensive carbon sequestration in marine sediments and their connection to atmospheric composition and global climate devonian black shales of the eastern us have been studied extensively to determine the fundamental controls on carbon burial and a range of mechanisms has been proposed finding agglutinated benthic foraminifera in these black shales refocuses the debate about their origin and points to limitations of earlier models

subaqueous sediment gravity flow deposits practical criteria for their field description and classification a new method for the description and classification of subaqueous sediment gravity flow deposits is proposed the classification scheme employs a convenient letter code and divides deposits individual beds into descriptive categories of two hierarchical levels facies and subfacies facies as the higher rank categories are distinguished chiefly on the basis of sediment type i e bed grain size texture a total of 13 facies have been distinguished g gravel gs gravel sand couplet gys gravelly sand s sand sm sand mud couplet ms mud sand couplet tm silt mud couplet mt mud silt couplet m mud mys muddy sand sym sandy mud myg muddy gravel gym gravelly mud subfacies as the lower rank categories are distinguished within the individual facies on the basis of the bed s internal structures the number of subfacies is unlimited and their labelling code includes particular facies symbols see above preceded by lower case letters denoting specific sedimentary structures and their vertical arrangement subfacies thus refer to the bed s intervals or divisions which are labelled as follows m massive unstratified and ungraded g graded unstratified and graded s plane stratified x cross stratified 1 parallel and or cross laminated q liquefied for example subfacies gsg graded to plane stratified gravel are gravel beds that have a lower graded interval and an upper plane stratified interval subfacies xg cross stratified gravel are gravel beds that are cross stratified throughout subfacies sls plane stratified to laminated sand are sand beds that have a lower plane stratified interval and an upper laminated parallel and or cross laminated interval

imaging pores in sedimentary rocks foundation of porosity prediction abstract this review examines the history and current practice of technologies for imaging of pores in sedimentary rocks pores are that portion of the rock volume occupied by components of relative mobility such as water of various salinities microbial life petroleum liquids gases and supercritical fluids through a rock s history the contained pores evolve as the primary detrital assemblage responds chemically and mechanically to changing conditions in the subsurface description and classification of pores based upon their paragenesis depends upon inspection by imaging that allows the pores to be seen and interpreted in the context of historical processes responsible for formation of the pore walls since the late 1960s sample preparation and imaging methodologies that clarified our understanding of pores in their correct historical context have been at the root of important advances in prediction of porosity and related bulk properties in the subsurface these older approaches are now joined by new technologies that serve current efforts to extend predictive capabilities to nm scale pores in the fine grained low permeability rocks that dominate in the deeper parts of sedimentary basins and indeed in the sedimentary record as a whole placing the smallest pores into their proper paragenetic context presents a challenge in the quest to develop predictive models for porosity evolution in fine grained rocks

insights into marls from optical and back scattered electron petrography an example from the outer carpathians poland abstract this paper is a study of grain provenance depositional processes and the relationship of those processes to composition and diagenesis of fine grained rocks based on high resolution petrography the marly successions deposited in siliciclastic flysch and related to the paleotectonics of the northern domain of the tethys ocean outer carpathian basin were examined the following marls were studied upper jurassic marls from the vendryn formation goleszow marls upper cretaceous siliceous fucoid wegierka weglowka frydek jasienica and egocina marls eocene cko budzow zembrzyce and leluchow marls and oligocene sub cergowa and grybow marls these marls are products of the redeposition of shelf sediments into deeper parts of the basin knowledge of their diagenetic history can contribute to the evaluation of the petroleum potential of the polish outer carpathians thin section analysis by optical and electron back scattered microscopy fesem bse highlights the similarity of all these marls with respect to composition grain size sedimentary features and diagenetic changes they differ only in the quantitative proportion of constituents the relative extent of key microfacies and diagenetic effects the diagenetic modifications recognized in the rocks studied are characteristic of the processes operating in the sedimentary environment and during burial diagenesis thin section petrography of the marls reveals fine mostly silt size siliciclastic material and bioclasts supported by matrix siliciclastic particles comprise mostly quartz micas feldspars glauconitic nodules and lithoclasts calcareous and siliceous bioclasts mostly microfossils foram and sponge spicules and a few radiolarians and diatoms and coccospheres are variably preserved they may occur as calcite silica and clay pseudomorphs additional components include peloids and stringers of brown organic matter millimeter scale fabric analysis provides detailed insight into the processes of transport and deposition five microfacies are identified parallel laminated wavy and lenticular laminated homogeneous exhibiting preferred orientation of elongated components bioturbated and bioclast bearing the range of microfacies suggests that the marl sediment was deposited while currents were active the depositional settings identified range from traction reworking by bottom currents supply by nepheloid flow indistinctly bounded laminae possible eolian input laminae a few grains thick and biogenic reworking of sediments back scattered electron images reveal that the matrix consists of coccolithophorid remains with varying abundances of clay silica and carbonate cements five varieties of matrix are distinguished 1 carbonate clay silica bearing 2 carbonate clay supported 3 carbonate silica supported 4 clay silica supported and 5 clay dominated clay appears as rosette and lamellar aggregates silica as nanoquartz microspheres clusters coatings quartz overgrowths and occasionally opal ct as lepispheres and aggregates moss like in appearance and calcite as overgrown coccoliths pseudomicrite micrite and microspar pyrite framboids are present in some samples intrabiotic and intergranular cements are developed also present are partially dissolved coccolith placoliths silica and clay replaced micarb and grains similar in appearance to volcanic glass shards that now consist of calcite and quartz bioclasts may have been a source for calcite and silica cements volcanic glass may have been a source for authigenic silica and clay minerals this is suggested by the close spatial juxtaposition of silica microspheres and authigenic clay cauliflower like aggregates the authigenic origin of the clay in these marls is also suggested by the presence of overgrown coccoliths because recrystallization of shields is reduced when clay is present so that the overgrowths must have been formed in an environment where clay minerals were scarce initially despite diagenetic changes the presence of pyroclasts is still recognizable there are quartz crystal chips shard shaped grains volcanic rock fragments and minerals biotite apatite zircon the marls studied could be described as impure fine grained detrital limestones chalky in type with evidence of redeposition and possible volcanic ash input the marl depositional system within the outer carpathian basin was associated with tectonic movements which initiated chalk type facies and this was particularly widespread in the upper cretaceous and paleocene